Source characteristics of the 3 September 2017, North Korean nuclear test (mb = 6.3) inferred from teleseismic forward modeling and regional waveform deconvolution of broadband P and Pn waves

NASA Astrophysics Data System (ADS)

Chaves, E. J.; Lay, T.; Voytan, D. P.

2017-12-01

On 3 September 2017, the Republic of North Korea conducted the sixth and largest declared underground nuclear test at the Punggye-ri test site. Estimates of yield (W) based on magnitude-yield calibrations for other test sites result in a wide range of yield estimates for the North Korean tests, due to uncertainty in the effects of site-specific coupling, likely overburial of the events, and poorly constrained crustal and mantle attenuation for the test site. The event produced good signal-to-noise broadband (BB) teleseismic P wave recordings at hundreds of stations along with high quality regional recordings. When using teleseismic data, robust estimation of W and depth of burial (DOB) must account for the biasing effects of laterally varying upper mantle attenuation (t*) on P waves, so we empirically determine a best choice of average t* by modeling remote observations. We assume a Mueller-Murphy source model for a granite medium to address the coupling issue. We compute synthetic Reduced Velocity Potential (RVP) seismograms for varying combinations of W and DOB for the 2017 event for a simple half-space case to account for possible overburial effects. RVPs are convolved with Futterman, constant operators, corrected for geometric spreading and receiver function, and then compared with teleseismic P wave displacement records from 435 BB seismic stations, pre-stacked in 26 azimuth and distance bins to suppress station effects. Our preliminary results for half-space modeling give high average cross-correlations and low waveform misfit errors between synthetic and observed waveforms for W of 110-130 kt with DOB 700-800 m and a preferred t* = 0.98 s. For the Mueller-Murphy model we find that frequency-dependent absorption band models are not preferred for this test site. Ongoing analysis is exploring effects of receiver crustal layering. Furthermore, we characterize the explosion source time function using the vertical component Pn-waves from regional BB recordings. We correct for attenuation, site and path effects using the lower yield nuclear tests carried out in 2016, 2013 and 2009 as empirical Green's functions. The deconvolved relative source functions exhibit a complex time sequence, with a second peak possibly related to a deviatoric source activated during the large explosion.

High-resolution imaging of the low velocity layer in Alaskan subduction zone with scattered waves and interferometry

NASA Astrophysics Data System (ADS)

Kim, D.; Keranen, K. M.; Abers, G. A.; Kim, Y.; Li, J.; Shillington, D. J.; Brown, L. D.

2017-12-01

The physical factors that control the rupture process of great earthquakes at convergent plate boundaries remain incompletely understood. While recent developments in imaging using the teleseismic wavefield have led to marked advances at wavelengths of a couple kilometers to tens of kilometers, higher resolution imaging of the rupture zone would improve the resolution of imaging and thus provide improved parameter estimation, as the teleseismic wavefield is fundamentally limited by its low frequency content. This study compares and evaluates two seismic imaging techniques using the high-frequency signals from teleseismic coda versus earthquake scattered waves to image the subducting Yakutat oceanic plateau in the Alaska subduction zone. We use earthquakes recorded by the MOOS PASSCAL broadband deployment in southern Alaska. In our first method, we select local earthquakes that lie directly beneath and laterally near the recording array for imaging, and extract body wave information via a simple autocorrelation and stacking. Profiles analogous to seismic reflection profile are constructed using the near-vertically travelling waves. In our second method, we compute teleseismic receiver functions within the 0.02-1.0 Hz frequency band. Both results image interfaces that we associate with the subducting oceanic plate in Alaska-Aleutian system, with greater resolution than commonly used methods with teleseismic sources. Structural details from our results can further our understanding of the conditions and materials that characterize the subduction megathrusts, and the techniques can be employed in other regions along the Alaska-Aleutian system and at other convergent margins with suitable seismic arrays.

Receiver Function Study of the Crustal Structure Beneath the Northern Andes (colombia)

NASA Astrophysics Data System (ADS)

Poveda, E.; Monsalve, G.; Vargas-Jimenez, C. A.

2013-05-01

We have investigated crustal thickness beneath the Northern Andes with the teleseismic receiver function technique. We used teleseismic data recorded by an array of 18 broadband stations deployed by the Colombian Seismological Network, and operated by the Colombian Geological Survey. We used the primary P-to-S conversion and crustal reverberations to estimate crustal thickness and average Vp/Vs ratio; using Wadati diagrams, we also calculated the mean crustal Vp/Vs ratio around stations to further constrain the crustal thickness estimation. In northern Colombia, near the Caribbean coast, the estimated crustal thickness ranges from 25 to 30 km; in the Middle Magdalena Valley, crustal thickness is around 40 km; beneath the northern Central Cordillera, the Moho depth is nearly 40 km; at the Ecuador-Colombia border, beneath the western flank of the Andes, the estimated thickness is about 46 km. Receiver functions at a station at the craton in South East Colombia, near the foothills of the Eastern Cordillera, clearly indicate the presence of the Moho discontinuity at a depth near 36 km. The greatest values of crustal thickness occur beneath a plateau (Altiplano Cundiboyacense) on the Eastern Cordillera, near the location of Bogota, with values around 58 km. Receiver functions in the volcanic areas of the south-western Colombian Andes do not show a systematic signal from the Moho, indicating abrupt changes in Moho geometry. Signals at stations on the Eastern Cordillera near Bogota reveal a highly complex crustal structure, with a combination of sedimentary layers up to 9 km thick, dipping interfaces, low velocity layers, anisotropy and/or lateral heterogeneity that still remain to be evaluated. This complexity obeys to the location of these stations at a region of a highly deformed fold and thrust belt.

Slowness based CCP stacking technique in suppressing crustal multiples

NASA Astrophysics Data System (ADS)

Guan, Z.; Niu, F.

2016-12-01

Common-conversion-point (CCP) stacking of receiver function is a widely used technique to image velocity discontinuities in the mantle, such as the lithosphere-asthenosphere boundary (LAB) in the upper mantle, the 410-km and the 660-km discontinuities in the mantle transition zone. In a layered medium, a teleseismic record can be considered as the summation of the direct arrival and a series of conversions and reflections at boundaries below the station. Receiver functions are an attempt to approximate a Green's function associated with structure beneath the receiver by deconvolving one component of a teleseismic signal from another to remove source signals from seismograms. The CCP technique assumes that receiver functions composed solely of P to S conversions at velocity boundaries, whose depths can be mapped out through their arrival times. The multiple reflections at shallow boundaries with large velocity contrasts, such as the base of unconsolidated sediments and the Moho, can pose significant challenges to the accuracy of CCP imaging. In principle, the P to S conversions and multiples originated from deep and shallow boundaries arrive at a seismic station with incident angles that are, respectively, smaller and larger than that of the direct P wave. Therefore the corresponding slowness can be used to isolate the conversions from multiples, allowing for minimizing multiple-induced artifacts. We developed a refined CCP stacking method that uses relative slowness as a weighting factor to suppress the multiples. We performed extensive numerical tests with synthetic data to seek the best weighting scheme and to verify the robustness of the images. We applied the refined technique to the NECESSArray data, and found that the complicated low velocity structures in the depth range of 200-400 km shown in the CCP images of previous studies are mostly artifacts resulted from crustal multiples.

Shear wave velocity variation across the Taupo Volcanic Zone, New Zealand, from receiver function inversion

USGS Publications Warehouse

Bannister, S.; Bryan, C.J.; Bibby, H.M.

2004-01-01

The Taupo Volcanic Zone (TVZ), New Zealand is a region characterized by very high magma eruption rates and extremely high heat flow, which is manifest in high-temperature geothermal waters. The shear wave velocity structure across the region is inferred using non-linear inversion of receiver functions, which were derived from teleseismic earthquake data. Results from the non-linear inversion, and from forward synthetic modelling, indicate low S velocities at ???6- 16 km depth near the Rotorua and Reporoa calderas. We infer these low-velocity layers to represent the presence of high-level bodies of partial melt associated with the volcanism. Receiver functions at other stations are complicated by reverberations associated with near-surface sedimentary layers. The receiver function data also indicate that the Moho lies between 25 and 30 km, deeper than the 15 ?? 2 km depth previously inferred for the crust-mantle boundary beneath the TVZ. ?? 2004 RAS.

New Lithospheric Model of Taiwan based on the Receiver Function Method

NASA Astrophysics Data System (ADS)

Wang, H.; Zhu, L.; Chen, H.

2008-12-01

Taiwan is situated on the junction area between two subduction systems. The complex orogeny was developed by collision between the Eurasian continental plate and Philippine Sea plate and is still active in the present. Therefore, Taiwan provides unique opportunities for geophysical imaging of the ongoing process underneath. The TAiwan Integrated GEodynamics Research (TAIGER) combined a field program of active and passive seismology, which will undoubtedly be a major step forward in understanding mountain building process. In 2006, we developed a new crustal model of Taiwan from teleseismic waveforms by the receiver function method. We determined lateral variation of Moho discontinuity, crustal thickness (H), and Vp/Vs ratios (Kappa) for each permanent broadband station using all the available teleseismic data collected by BATS (Broadband Array in Taiwan for Seismology) and CWB (Central Weather Bureau). All the broadband stations are distributed uniformly over the whole Taiwan area so that we could delineate the Moho depth contour map. Recently, we concentrated on the three linear temporary arrays of the TAIGER project and obtained three high-resolution images of crustal structure across Taiwan along west-to-east direction from north to south by using the CCP (common-conversion-point) stacking of teleseismic P-to-S converted waves. Sharp impedance contrasts in these images clearly show the relief of each of seismic discontinuities in the crust and upper mantle. The preliminary results show that the Moho depth, 40 to 50 km of central Taiwan is deeper than in other parts of the island, which suggests crustal thickening due to collision. In addition, shallow part of western foothill area show highly acoustic impedance which probably results from thick sediment.

P- and S-wave Receiver Function Imaging with Scattering Kernels

NASA Astrophysics Data System (ADS)

Hansen, S. M.; Schmandt, B.

2017-12-01

Full waveform inversion provides a flexible approach to the seismic parameter estimation problem and can account for the full physics of wave propagation using numeric simulations. However, this approach requires significant computational resources due to the demanding nature of solving the forward and adjoint problems. This issue is particularly acute for temporary passive-source seismic experiments (e.g. PASSCAL) that have traditionally relied on teleseismic earthquakes as sources resulting in a global scale forward problem. Various approximation strategies have been proposed to reduce the computational burden such as hybrid methods that embed a heterogeneous regional scale model in a 1D global model. In this study, we focus specifically on the problem of scattered wave imaging (migration) using both P- and S-wave receiver function data. The proposed method relies on body-wave scattering kernels that are derived from the adjoint data sensitivity kernels which are typically used for full waveform inversion. The forward problem is approximated using ray theory yielding a computationally efficient imaging algorithm that can resolve dipping and discontinuous velocity interfaces in 3D. From the imaging perspective, this approach is closely related to elastic reverse time migration. An energy stable finite-difference method is used to simulate elastic wave propagation in a 2D hypothetical subduction zone model. The resulting synthetic P- and S-wave receiver function datasets are used to validate the imaging method. The kernel images are compared with those generated by the Generalized Radon Transform (GRT) and Common Conversion Point stacking (CCP) methods. These results demonstrate the potential of the kernel imaging approach to constrain lithospheric structure in complex geologic environments with sufficiently dense recordings of teleseismic data. This is demonstrated using a receiver function dataset from the Central California Seismic Experiment which shows several dipping interfaces related to the tectonic assembly of this region. Figure 1. Scattering kernel examples for three receiver function phases. A) direct P-to-s (Ps), B) direct S-to-p and C) free-surface PP-to-s (PPs).

Crustal structure of north Peru from analysis of teleseismic receiver functions

NASA Astrophysics Data System (ADS)

Condori, Cristobal; França, George S.; Tavera, Hernando J.; Albuquerque, Diogo F.; Bishop, Brandon T.; Beck, Susan L.

2017-07-01

In this study, we present results from teleseismic receiver functions, in order to investigate the crustal thickness and Vp/Vs ratio beneath northern Peru. A total number of 981 receiver functions were analyzed, from data recorded by 28 broadband seismic stations from the Peruvian permanent seismic network, the regional temporary SisNort network and one CTBTO station. The Moho depth and average crustal Vp/Vs ratio were determined at each station using the H-k stacking technique to identify the arrival times of primary P to S conversion and crustal reverberations (PpPms, PpSs + PsPms). The results show that the Moho depth correlates well with the surface topography and varies significantly from west to east, showing a shallow depth of around 25 km near the coast, a maximum depth of 55-60 km beneath the Andean Cordillera, and a depth of 35-40 km further to the east in the Amazonian Basin. The bulk crustal Vp/Vs ratio ranges between 1.60 and 1.88 with the mean of 1.75. Higher values between 1.75 and 1.88 are found beneath the Eastern and Western Cordilleras, consistent with a mafic composition in the lower crust. In contrast values vary from 1.60 to 1.75 in the extreme flanks of the Eastern and Western Cordillera indicating a felsic composition. We find a positive relationship between crustal thickness, Vp/Vs ratio, the Bouguer anomaly, and topography. These results are consistent with previous studies in other parts of Peru (central and southern regions) and provide the first crustal thickness estimates for the high cordillera in northern Peru.

Crustal and upper mantle structure of the Hangay Dome, central Mongolia

NASA Astrophysics Data System (ADS)

Stachnik, J. C.; Meltzer, A.; Tsaagan, B.; Munkhuu, U.; Russo, R.; Souza, S.; Martin, P.

2013-12-01

The origin and support of high topography within continental interiors away from active tectonic margins remains a fundamental question in the dynamics and deformation of continents. The Hangay Dome in central Mongolia is one such region that is a broad regional uplift with average elevation of about 2 km, sitting between two large strike-slip faults, the Bulnay Fault to the north and the Gobi-Altay Fault to the south. Both of these faults are seismically active and have experienced M8+ earthquakes as recently as 1957. This portion of the Mongolian Plateau is approximately 300 km south of the Baikal Rift and located at the northern margin of the diffuse-deformation field in Central Asia, adjacent to the Siberian Craton. From previous research, the dynamic support of the Hangay Dome has been attributed to both crustal thickening and low density upper mantle material. However, seismic data leading to these interpretations have been limited to global tomographic models and sparse regional sampling of the wave field leaving the question unresolved. To address this major question in plate tectonic theory, in June 2012 a temporary IRIS/PASSCAL/University of Florida array of 72 seismic stations was deployed around the Hangay Dome to determine lithospheric structure in the region. Preliminary results from the first of two years of data are shown from receiver function analysis, ambient noise surface wave tomography, and teleseismic travel time residual analysis. Using teleseismic waveform records from over 300 earthquakes above M5.5 between 30 and 90 degrees epicentral distance, crustal thickness measurements from H-k stacking of receiver functions range from 42 km to 57 km across the array, with thicker crust beneath the highest topography. At each station the bulk crustal Vp/Vs ratio is also determined with median value for the array of 1.77, perhaps indicating a more mafic composition crust in the region.Teleseismic P-wave travel time residuals generally diminish from south to north across the array consistent with thinning crust, however the depth resolution and magnitude of seismic wavespeed anomalies will be further explored with three-dimensional finite-frequency tomography. Constraints on crustal shear wave velocity from ambient noise surface wave tomography complement both the receiver function analysis and teleseismic tomography. Initial inversions of phase velocity dispersion curves in the central Hangay indicate an average crustal Vs of 3.6 km/s within the Hangay Dome, which translates to an average Vp of 6.4 km/s using Vp/Vs of 1.77. Further refinement of current analysis and an additional year of recording will reveal the first high resolution lithospheric scale model in the region.

High-Resolution Seismic Velocity and Attenuation Models of the Caucasus-Caspian Region

DTIC Science & Technology

2009-09-30

region is part of the Alpine-Himalayan collision belt and is an area of complex structure accompanied by large variations in seismic wave velocities...velocity structure is developed using teleseismic receiver functions and surface waves. Joint inversion of surface wave group dispersion curves...Caucasus and the thinnest is in the Arabian Plate. Thin crust is also observed near the Caspian. The lithospheric mantle in the Greater Caucasus and the

Structure of the Lithosphere and Upper Mantle Across the Arabian Peninsula

DOE Office of Scientific and Technical Information (OSTI.GOV)

Al-Amri, A; Rodgers, A

2007-01-05

Analysis of modern broadband (BB) waveform data allows for the inference of seismic velocity structure of the crust and upper mantle using a variety of techniques. This presentation will report inferences of seismic structure of the Arabian Plate using BB data from various networks. Most data were recorded by the Saudi Arabian National Digital Seismic Network (SANDSN) which consists of 38 (26 BB, 11 SP) stations, mostly located on the Arabian Shield. Additional data were taken from the 1995-7 Saudi Arabian IRIS-PASSCAL Deployment (9 BB stations) and other stations across the Peninsula. Crustal structure, inferred from teleseismic P-wave receiver functions,more » reveals thicker crust in the Arabian Platform (40-45 km) and the interior of the Arabian Shield (35-40 km) and thinner crust along the Red Sea coast. Lithospheric thickness inferred from teleseismic S-wave receiver functions reveals very thin lithosphere (40-80 km) along the Red Sea coast which thickens rapidly toward the interior of the Arabian Shield (100-120 km). We also observe a step of 20-40 km in lithospheric thickness across the Shield-Platform boundary. Seismic velocity structure of the upper mantle inferred from teleseismic P- and S-wave travel time tomography reveals large differences between the Shield and Platform, with the Shield being underlain by slower velocities, {+-}3% for P-waves and {+-}6% for S-waves. Seismic anisotropy was inferred from shear-wave splitting, using teleseismic SKS waveforms. Results reveal a splitting time of approximately 1.4 seconds, with the fast axis slightly east of north. The shear-wave splitting results are consistent across the Peninsula, with a slight clockwise rotation parallel for stations near the Gulf of Aqaba. In summary, these results allow us to make several conclusions about the tectonic evolution and current state of the Arabian Plate. Lithospheric thickness implies that thinning near the Red Sea has accompanied the rupturing of the Arabian-Nubian continental lithosphere. The step in the lithospheric thickness across the Shield-Platform boundary likely reveals a pre-existing difference in the lithospheric structure prior to accretion of the terranes composing the eastern Arabian Shield. Tomographic imaging of upper mantle velocities implies a single large-scale thermal anomaly underlies the Arabian Shield and is associated with Cenozoic uplift and volcanism.« less

Variation in Crustal Structure of the Lesser Caucasus Region from Teleseismic Receiver Functions

NASA Astrophysics Data System (ADS)

Lin, C. M.; Tseng, T. L.; Huang, B. S.; Legendre, C. P.; Karakhanian, A.

2016-12-01

The Caucasus, including the mountains of Greater and Lesser Caucasus, is formed by the continental collision between Arabia and Eurasia. The crustal thickness for this region was mostly constrained by joint analysis of receiver functions and surface waves. Although the thickest value of 52 km was reported under the Lesser Caucasus, the resolution of earlier studies were often limited by sparse array. Large gradient across Moho also makes the definition of Moho difficult. Moreover, higher value of the Vp/Vs ratio is commonly reported in the northeastern Turkey but no estimates had been made for the Caucasus. To further investigate the detail structure around the Lesser Caucasus, we constructed a new seismic network in Georgia and Armenia. We also include other broadband stations to enhance the coverage. The average interval in the Lesser Caucasus is roughly 30 km, much denser than any previous experiments. We selected P-waveforms from teleseismic earthquakes during the operation (January 2012 - June 2016) to calculate receiver functions and then estimate the crustal thickness (H) and Vp/Vs ratio (k) with the H-k stacking technique. Our preliminary results show that Moho depth increases from 40 km under the northeastern Turkey to 50 km beneath northern Georgia, no station with Moho deeper than 50 km under the Lesser Caucasus. The Vp/Vs ratios in the northeastern Anatolian plateau are around 1.8, which is slightly higher than the average of global continents but consistent with the previous estimates. Further to the east, some stations show anomalously higher Vp/Vs ratio in central & southern Armenia that may be associated with Holocene volcanism. In the future, we plan to join locally measured dispersion curves to invert the velocity model without velocity-depth trade-off. We expect to resolve the velocity variations of the crust beneath this region in small scale that may be tied to the continental collision and surface volcanism. Keywords: Caucasus, receiver function, continental collision, volcanic plateau, crustal structure

On the Viability of Using Autonomous Three-Component Nodal Geophones to Calculate Teleseismic Ps Receiver Functions with an Application to the Old Faithful Hydrothermal System and the Cascadia Subduction Zone

NASA Astrophysics Data System (ADS)

Ward, K. M.; Lin, F. C.

2017-12-01

Recent advances in seismic data-acquisition technology paired with an increasing interest from the academic passive source seismological community have opened up new scientific targets and imaging possibilities, often referred to as Large-N experiments (large number of instruments). The success of these and other deployments has motivated individual researchers, as well as the larger seismological community, to invest in the next generation of nodal geophones. Although the new instruments have battery life and bandwidth limitations compared to broadband instruments, the relatively low deployment and procurement cost of these new nodal geophones provides an additional novel tool for researchers. Here, we explore the viability of using autonomous three-component nodal geophones to calculate teleseismic Ps receiver functions by comparison of co-located broadband stations and highlight some potential advantages with a dense nodal array deployed around the Upper Geyser basin in Yellowstone National Park. Two key findings from this example include (1) very dense nodal arrays can be used to image small-scale features in the shallow crust that typical broadband station spacing would alias, and (2) nodal arrays with a larger footprint could be used to image deeper features with greater or equal detail as typical broadband deployments but at a reduced deployment cost. The success of the previous example has motivated a larger 2-D line across the Cascadia subduction zone. In the summer of 2017, we deployed 174 nodal geophones with an average site spacing of 750 m. Synthetic tests with dense station spacing ( 1 km) reveal subtler features of the system that is consistent with our preliminary receiver function results from our Cascadia deployment. With the increasing availability of nodal geophones to individual researchers and the successful demonstration that nodal geophones are a viable instrument for receiver function studies, numerous scientific targets can be investigated at reduced costs or in expanded detail.

Teleseismic Earthquake Signals Observed on an Ice Shelf

NASA Astrophysics Data System (ADS)

Baker, M. G.; Aster, R. C.; Anthony, R. E.; Wiens, D.; Nyblade, A.; Bromirski, P. D.; Stephen, R. A.; Gerstoft, P.

2015-12-01

The West Antarctic Rift System (WARS) is one of Earth's largest continental extension zones. Study of the WARS is complicated by the presence of the West Antarctic Ice Sheet, the Ross Ice Shelf, and the Ross Sea. Recent deployments of broadband seismographs in the POLENET project have allowed passive seismic techniques, such as receiver function analysis and surface wave dispersion, to be widely utilized to infer crustal and mantle velocity structure across much of the WARS and West Antarctica. However, a large sector of the WARS lies beneath the Ross Ice Shelf. In late 2014, 34 broadband seismographs were deployed atop the ice shelf to jointly study deep Earth structure and the dynamics of the ice shelf. Ice shelf conditions present strong challenges to broadband teleseismic imaging: 1) The presence of complicating signals in the microseism through long-period bands due to the influence of ocean gravity waves; 2) The strong velocity contrasts at the ice-water and water-sediment interfaces on either side of the water layer give rise to large amplitude reverberations; 3) The water layer screens S-waves or P-to-S phases originating from below the water layer. We present an initial analysis of the first teleseismic earthquake arrivals collected on the ice shelf at the end of the 2014 field season from a limited subset of these stations.

Lazufre volcanic complex, Chile: attempts to image a large scale magmatic inflation body using regional and teleseismic broadband recordings

NASA Astrophysics Data System (ADS)

Heimann, Sebastian; Trabs, Stefan; Dahm, Torsten

2013-04-01

The Lazufre volcanic area around Lastarria and Cordon del Azufre in the central Andes is the second largest area of presently active volcano related uplift woldwide. A magma reservoir or sill layer, filling in about 10 km depth, is thought to be causative for inflation of an area of more than 1800 km2 (more than four times larger than the total area of the city of Vienna). Uplift rates of up to 3.2 cm/yr were found from InSAR measurements during the past two decades. In 2008, we deployed a network of 17 broadband seismometers in and around the area of active uplift for a period of 2 months. Hundreds of regional and several teleseismic earthquakes were recorded during the experiment. Recorded seismograms show strong anomalies in the region of maximum uplift. We tried to apply (1) tomography with regional events and (2) the receiver function technique with teleseismic events in order to interprete the recorded anomalies. Due to the relatively short dataset and the very heterogeneous structure beneath Lazufre, both techniques could only be applied with limited success. (1) To gain a full tomographic image of the deep underground, ray coverage was not dense enough. However, recorded delay times and amplitude decreases for rays passing through the inflation center carry valuable information. (2) Receiver functions were strongly influenced by 3D structure and could not be intepreted using standard techniques. Nevertheless, careful data selection and comparison with modelling results led to conclusions. In combination, our results allow for some insights into the volcanic complex. We can confirm the presence of molten or partially molten material at a depth of 8 km in the center and 12 km in the outer parts and can derive constraints on the lateral extent of the intrusion. We also find evidence for a huge low velocity zone at greater depth.

Formal integration of controlled-source and passive seismic data: Utilization of the CD-ROM experiment

NASA Astrophysics Data System (ADS)

Rumpfhuber, E.; Keller, G. R.; Velasco, A. A.

2005-12-01

Many large-scale experiments conduct both controlled-source and passive deployments to investigate the lithospheric structure of a targeted region. Many of these studies utilize each data set independently, resulting in different images of the Earth depending on the data set investigated. In general, formal integration of these data sets, such as joint inversions, with other data has not been performed. The CD-ROM experiment, which included both 2-D controlled-source and passive recording along a profile extending from southern Wyoming to northern New Mexico serves as an excellent data set to develop a formal integration strategy between both controlled source and passive experiments. These data are ideal to develop this strategy because: 1) the analysis of refraction/wide-angle reflection data yields Vp structure, and sometimes Vs structure, of the crust and uppermost mantle; 2) analysis of the PmP phase (Moho reflection) yields estimates of the average Vp of the crust for the crust; and 3) receiver functions contain full-crustal reverberations and yield the Vp/Vs ratio, but do not constrain the absolute P and S velocity. Thus, a simple form of integration involves using the Vp/Vs ratio from receiver functions and the average Vp from refraction measurements, to solve for the average Vs of the crust. When refraction/ wide-angle reflection data and several receiver functions nearby are available, an integrated 2-D model can be derived. In receiver functions, the PS conversion gives the S-wave travel-time (ts) through the crust along the raypath traveled from the Moho to the surface. Since the receiver function crustal reverberation gives the Vp/Vs ratio, it is also possible to use the arrival time of the converted phase, PS, to solve for the travel time of the direct teleseismic P-wave through the crust along the ray path. Raytracing can yield the point where the teleseismic wave intersects the Moho. In this approach, the conversion point is essentially a pseudo-shotpoint, thus the converted arrival at the surface can be jointly modeled with refraction data using a 3-D inversion code. Employing the combined CD-ROM data sets, we will be investigating the joint inversion results of controlled source data and receiver functions.

Comparison of seismic waveform inversion results for the rupture history of a finite fault: application to the 1986 North Palm Springs, California, earthquake

USGS Publications Warehouse

Hartzell, S.

1989-01-01

The July 8, 1986, North Palm Strings earthquake is used as a basis for comparison of several different approaches to the solution for the rupture history of a finite fault. The inversion of different waveform data is considered; both teleseismic P waveforms and local strong ground motion records. Linear parametrizations for slip amplitude are compared with nonlinear parametrizations for both slip amplitude and rupture time. Inversions using both synthetic and empirical Green's functions are considered. In general, accurate Green's functions are more readily calculable for the teleseismic problem where simple ray theory and flat-layered velocity structures are usually sufficient. However, uncertainties in the variation in t* with frequency most limit the resolution of teleseismic inversions. A set of empirical Green's functions that are well recorded at teleseismic distances could avoid the uncertainties in attenuation. In the inversion of strong motion data, the accurate calculation of propagation path effects other than attenuation effects is the limiting factor in the resolution of source parameters. -from Author

Imaging and characterizing shallow sedimentary strata using teleseismic arrivals recorded on linear arrays: An example from the Atlantic Coastal Plain of the southeastern U.S.

NASA Astrophysics Data System (ADS)

Pratt, T. L.

2017-12-01

Unconsolidated, near-surface sediments can influence the amplitudes and frequencies of ground shaking during earthquakes. Ideally these effects are accounted for when determining ground motion prediction equations and in hazard estimates summarized in seismic hazard maps. This study explores the use of teleseismic arrivals recorded on linear receiver arrays to estimate the seismic velocities, determine the frequencies of fundamental resonance peaks, and image the major reflectors in the Atlantic Coastal Plain (ACP) and Mississippi Embayment (ME) strata of the central and southeastern United States. These strata have thicknesses as great as 2 km near the coast in the study areas, but become thin and eventually pinch out landward. Spectral ratios relative to bedrock sites were computed from teleseismic arrivals recorded on linear arrays deployed across the sedimentary sequences. The large contrast in properties at the bedrock surface produces a strong fundamental resonance peak in the 0.2 to 4 Hz range. Contour maps of sediment thicknesses derived from drill hole data allow for the theoretical estimation of average velocities by matching the observed frequencies at which resonance peaks occur. The sloping bedrock surface allows for calculation of a depth-varying velocity profile, under the assumption that the velocities at each depth do not change laterally between stations. The spectral ratios can then be converted from frequency to depth, resulting in an image of the subsurface similar to that of a seismic reflection profile but with amplitudes being the spectral ratio caused by a reflector at that depth. The complete data set thus provides an average velocity function for the sedimentary sequence, the frequencies and amplitudes of the major resonance peaks, and a subsurface image of the major reflectors producing resonance peaks. The method is demonstrated using three major receiver arrays crossing the ACP and ME strata that originally were deployed for imaging the crust and mantle, confirming that teleseismic signals can be used to characterize sedimentary strata in the upper km.

Moho map of South America from receiver functions and surface waves

NASA Astrophysics Data System (ADS)

Lloyd, Simon; van der Lee, Suzan; FrançA, George Sand; AssumpçãO, Marcelo; Feng, Mei

2010-11-01

We estimate crustal structure and thickness of South America north of roughly 40°S. To this end, we analyzed receiver functions from 20 relatively new temporary broadband seismic stations deployed across eastern Brazil. In the analysis we include teleseismic and some regional events, particularly for stations that recorded few suitable earthquakes. We first estimate crustal thickness and average Poisson's ratio using two different stacking methods. We then combine the new crustal constraints with results from previous receiver function studies. To interpolate the crustal thickness between the station locations, we jointly invert these Moho point constraints, Rayleigh wave group velocities, and regional S and Rayleigh waveforms for a continuous map of Moho depth. The new tomographic Moho map suggests that Moho depth and Moho relief vary slightly with age within the Precambrian crust. Whether or not a positive correlation between crustal thickness and geologic age is derived from the pre-interpolation point constraints depends strongly on the selected subset of receiver functions. This implies that using only pre-interpolation point constraints (receiver functions) inadequately samples the spatial variation in geologic age. The new Moho map also reveals an anomalously deep Moho beneath the oldest core of the Amazonian Craton.

Lithospheric structure of the Western Alps as seen by full-waveform inversion of CIFALPS teleseismic data

NASA Astrophysics Data System (ADS)

Beller, Stephen; Monteiller, Vadim; Operto, Stéphane; Nolet, Guust; Paul, Anne; Zhao, Liang

2017-04-01

Full-waveform inversion (FWI) is a powerful but constitutionally intensive technique that aims to recover 3D multiparameter images of the subsurface by minimising the waveform difference between the full recorded and modelled seismograms. This method has recently been adapted and successfully applied in lithospheric settings by tackling teleseismic waveform modelling with hybrid methods. For each event, a global scale simulation is performed once and for all to store the wavefield solutions on the edges of the lithospheric target. Then, for each modelling involved in the FWI process, these global scale solutions are injected within the lithospheric medium from the boundaries. We present the results of the application of teleseismic FWI to the data acquired by the CIFALPS experiment that was conducted in the Western Alps to gain new insights its lithospheric structure and geodynamic evolution of the alpine range. Nine teleseismic events were inverted to infer 3D models of density, P-wave velocity and S-wave velocity of the crust and the upper-mantle down to 200 km depth. Our models show clear evidences of continental subduction during the alpine orogeny. They outline a dipping European Moho down to 75 km depth and finely delineate the geometry of the Ivrea body at the suture between European and Adriatic plates. Deeper, in the mantle a slow S-wave velocity anomaly might indicate the location of the European slab detachment. Overall, FWI models give access to new seismic images that fill the resolution gap between smooth tomographic model and sharp receiver function images of the lithosphere and enable integrated interpretations of crustal and upper-mantle structures.

On the sensitivity of teleseismic full-waveform inversion to earth parametrization, initial model and acquisition design

NASA Astrophysics Data System (ADS)

Beller, S.; Monteiller, V.; Combe, L.; Operto, S.; Nolet, G.

2018-02-01

Full-waveform inversion (FWI) is not yet a mature imaging technology for lithospheric imaging from teleseismic data. Therefore, its promise and pitfalls need to be assessed more accurately according to the specifications of teleseismic experiments. Three important issues are related to (1) the choice of the lithospheric parametrization for optimization and visualization, (2) the initial model and (3) the acquisition design, in particular in terms of receiver spread and sampling. These three issues are investigated with a realistic synthetic example inspired by the CIFALPS experiment in the Western Alps. Isotropic elastic FWI is implemented with an adjoint-state formalism and aims to update three parameter classes by minimization of a classical least-squares difference-based misfit function. Three different subsurface parametrizations, combining density (ρ) with P and S wave speeds (Vp and Vs) , P and S impedances (Ip and Is), or elastic moduli (λ and μ) are first discussed based on their radiation patterns before their assessment by FWI. We conclude that the (ρ, λ, μ) parametrization provides the FWI models that best correlate with the true ones after recombining a posteriori the (ρ, λ, μ) optimization parameters into Ip and Is. Owing to the low frequency content of teleseismic data, 1-D reference global models as PREM provide sufficiently accurate initial models for FWI after smoothing that is necessary to remove the imprint of the layering. Two kinds of station deployments are assessed: coarse areal geometry versus dense linear one. We unambiguously conclude that a coarse areal geometry should be favoured as it dramatically increases the penetration in depth of the imaging as well as the horizontal resolution. This results because the areal geometry significantly increases local wavenumber coverage, through a broader sampling of the scattering and dip angles, compared to a linear deployment.

Constraints on the structure of the crust and lithosphere beneath the Azores Islands from teleseismic receiver functions

NASA Astrophysics Data System (ADS)

Spieker, Kathrin; Rondenay, Stéphane; Ramalho, Ricardo; Thomas, Christine; Helffrich, George

2018-05-01

The Azores Archipelago is located near the Mid-Atlantic Ridge (MAR) and consists of nine islands, resting on both sides of the ridge. Various methods including seismic reflection, gravity and passive seismic imaging have previously been used to investigate the crustal thickness beneath the islands. They have yielded thickness estimates that range between roughly 10 and 30 km, but until now models of the more fine-scale crustal structure have been lacking. Pending questions include the thickness of the volcanic edifice beneath the islands and whether crustal intrusions or even underplating can be observed beneath any island. In this study, we use data from nine seismic stations located on the Azores Islands to investigate the crustal structure with teleseismic P-wave receiver functions. Our results indicate that the base of the volcanic edifice is located approximately 1 to 4 km depth beneath the different islands and that the crust-mantle boundary has an average depth of ˜17 km. There is strong evidence for magmatic underplating beneath the island of São Jorge, and indications that the underplating is also present beneath São Miguel and possibly Santa Maria. Additionally, the seismological lithosphere-asthenosphere boundary, defined as a seismic velocity drop in the uppermost mantle, seems to deepen with increasing distance from the MAR. It has a depth of ˜45 km beneath the islands close to the MAR, compared to depths >70 km beneath the more distal islands.

Receiver Functions of the Mangystau Region, Western Kazakhstan

NASA Astrophysics Data System (ADS)

Martinetti, L. B.; Mackey, K. G.

2017-12-01

The Mangystau Region, in southwestern Kazakhstan contains many geographic features such as basins, plateaus, and mountain ranges. However, little has been published in English or Russian, and the region has never been instrumented with broadband seismometers before. From August through September 2016, a seismic noise survey took place where 10 broadband seismic stations were deployed throughout the region for 20 days each by MSU. The sensors recorded various teleseismic events and the data were used to infer crustal thickness. The goal of this study is to determine if reliable receiver functions can be created with 20 days of data recorded, and test what is the best way to use the data to find the thickness of the crust. Since a limited amount of data is available, teleseismic events of magnitude > 5 and that occurred from 30 to 90 degrees from the station were used. To have a better solution, a local calibration event was used to solve the seismic velocity and match the results of a previously done study (5.6 - 6.5 km/s) by a Kazak group, which was then used for H-k stacking. While the work is still in progress, it will add to the knowledge of the area, thus give an insight to the crustal thickness of the overall region. This study can also be used to provide information of the crustal thickness of the northern Caspian basin, near the Caucasus, where another major effort is being conducted.

Evidence for magmatic underplating and partial melt beneath the Canary Islands derived using teleseismic receiver functions

NASA Astrophysics Data System (ADS)

Lodge, A.; Nippress, S. E. J.; Rietbrock, A.; García-Yeguas, A.; Ibáñez, J. M.

2012-12-01

In recent years, an increasing number of studies have focussed on resolving the internal structure of ocean island volcanoes. Traditionally, active source seismic experiments have been used to image the volcano edifice. Here we present results using the analysis of compressional to shear (P to S) converted seismic phases from teleseismic events, recorded by stations involved in an active source experiment "TOM-TEIDEVS" (Ibáñez et al., 2008), on the island of Tenerife, Canary Islands. We supplement this data with receiver function (RF) analysis of seismograms from the Canary Islands of Lanzarote and La Palma, applying the extended-time multitaper frequency domain cross-correlation estimation method (Helffrich, 2006). We use the neighbourhood inversion approach of Sambridge (1999a,b) to model the RFs and our results indicate magmatic underplating exists beneath all three islands, ranging from 2 to 8 km, but showing no clear correlation with the age of the island. Beneath both La Palma and Tenerife, we find localized low velocity zones (LVZs), which we interpret as due to partial melt, supported by their correlation with the location of historical earthquakes (La Palma) and recent earthquakes (Tenerife). For Lanzarote, we do not sample the most recently volcanically active region and find no evidence for a LVZ. Instead, we find a simple gradational velocity structure, with discontinuities at ˜4, 10 and 18 km depth, in line with previous studies.

Anisotropic structure of the mantle wedge beneath the Ryukyu arc from teleseismic receiver function analysis

NASA Astrophysics Data System (ADS)

McCormack, K. A.; Wirth, E. A.; Long, M. D.

2011-12-01

The recycling of oceanic plates back into the mantle through subduction is an important process taking place within our planet. However, many fundamental aspects of subduction systems, such as the dynamics of mantle flow, have yet to be completely understood. Subducting slabs transport water down into the mantle, but how and where that water is released, as well as how it affects mantle flow, is still an open question. In this study, we focus on the Ryukyu subduction zone in southwestern Japan and use anisotropic receiver function analysis to characterize the structure of the mantle wedge. We compute radial and transverse P-to-S receiver functions for eight stations of the broadband F-net array using a multitaper receiver function estimator. We observe coherent P-to-SV converted energy in the radial receiver functions at ~6 sec for most of the stations analyzed consistent with conversions originating at the top of the slab. We also observe conversions on the transverse receiver functions that are consistent with the presence of multiple anisotropic and/or dipping layers. The character of the transverse receiver functions varies significantly along strike, with the northernmost three stations exhibiting markedly different behavior than stations located in the center of the Ryukyu arc. We compute synthetic receiver functions using a forward modeling scheme that can handle dipping interfaces and anisotropic layers to create models for the depths, thicknesses, and strengths of anisotropic layers in the mantle wedge beneath Ryukyu.

A 3-D crustal and uppermost mantle model of the western US from receiver functions and surface wave dispersion derived from ambient noise and teleseismic earthquakes

NASA Astrophysics Data System (ADS)

Shen, W.; Schulte-Pelkum, V.; Ritzwoller, M. H.

2011-12-01

The joint inversion of surface wave dispersion and receiver functions was proven feasible on a station by station basis more than a decade ago. Joint application to a large number of stations across a broad region such as western US is more challenging, however, because of the different resolutions of the two methods. Improvements in resolution in surface wave studies derived from ambient noise and array-based methods applied to earthquake data now allow surface wave dispersion and receiver functions to be inverted simultaneously across much of the Earthscope/USArray Transportable Array (TA), and we have developed a Monte-Carlo procedure for this purpose. As a proof of concept we applied this procedure to a region containing 186 TA stations in the intermountain west, including a variety of tectonic settings such as the Colorado Plateau, the Basin and Range, the Rocky Mountains, and the Great Plains. This work has now been expanded to encompass all TA stations in the western US. Our approach includes three main components. (1) We enlarge the Earthscope Automated Receiver Survey (EARS) receiver function database by adding more events within a quality control procedure. A back-azimuth-independent receiver function and its associated uncertainties are constructed using a harmonic stripping algorithm. (2) Rayleigh wave dispersion curves are generated from the eikonal tomography applied to ambient noise cross-correlation data and Helmoholtz tomography applied to teleseismic surface wave data to yield dispersion maps from 8 sec to 80 sec period. (3) We apply a Metropolis Monte Carlo algorithm to invert for the average velocity structure beneath each station. Simple kriging is applied to interpolate to the discrete results into a continuous 3-D model. This method has now been applied to over 1,000 TA stations in the western US. We show that the receiver functions and surface wave dispersion data can be reconciled beneath more than 80% of the stations using a smooth parameterization of both crustal and uppermost mantle structure. After the inversion, a 3-D model for the crust and uppermost mantle to a depth of 150 km is constructed for this region. Compared with using surface wave data alone, uncertainty in crustal thickness is much lower and as a result, the lower crustal velocity is better constrained given a smaller depth-velocity trade-off. The new 3-D model including Moho depth with attendant uncertainties provides the basis for further analysis on radial anisotropy and geodynamics in the western US, and also forms a starting point for other seismological studies such as body wave tomography and receiver function CCP analysis.

Lithospheric velocity structure of the Anatolian plateau-Caucasus-Caspian region

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gök, R.; Mellors, R. J.; Sandvol, E.

The Anatolian plateau-Caucasus-Caspian region is an area of complex lithospheric structure accompanied by large variations in seismic wave velocities. Despite the complexity of the region, little is known about the detailed lithospheric structure. Using data from 31 new, permanent broadband seismic stations along with results from a previous 29 temporary seismic stations and 3 existing global seismic stations in the region, a 3-D velocity model is developed using joint inversion of teleseismic receiver functions and surface waves. Both group and phase dispersion curves (Love and Rayleigh) were derived from regional and teleseismic events. Additional Rayleigh wave group dispersion curves weremore » determined using ambient noise correlation. Receiver functions were calculated using P arrivals from 789 teleseismic (30°–90°) earthquakes. The stacked receiver functions and surface wave dispersion curves were jointly inverted to yield the absolute shear wave velocity to a depth of 100 km at each station. The depths of major discontinuities (sediment-basement, crust-mantle, and lithosphere-asthenosphere) were inferred from the velocity-depth profiles at the location of each station. Distinct spatial variations in crustal and upper mantle shear velocities were observed. The Kura basin showed slow (~2.7–2.9 km/s) upper crustal (0–11 km) velocities but elevated (~3.8–3.9 km/s) velocities in the lower crust. The Anatolian plateau varied from ~3.1–3.2 in the upper crust to ~3.5–3.7 in the lower crust, while velocities in the Arabian plate (south of the Bitlis suture) were slightly faster (upper crust between 3.3 and 3.4 km/s and lower crust between 3.8 and 3.9 km/s). The depth of the Moho, which was estimated from the shear velocity profiles, was 35 km in the Arabian plate and increased northward to 54 km at the southern edge of the Greater Caucasus. Moho depths in the Kura and at the edge of the Caspian showed more spatial variability but ranged between 35 and 45 km. Upper mantle velocities were slow under the Anatolian plateau but increased to the south under the Arabian plate and to the east (4.3–4.4 km/s) under the Kura basin and Greater Caucasus. The areas of slow mantle coincided with the locations of Holocene volcanoes. Differences between Rayleigh and Love dispersions at long wavelengths reveal a pronounced variation in anisotropy between the Anatolian plateau and the Kura basin.« less

Lithospheric velocity structure of the Anatolian plateau-Caucasus-Caspian region

DOE PAGES

Gök, R.; Mellors, R. J.; Sandvol, E.; ...

2011-05-07

The Anatolian plateau-Caucasus-Caspian region is an area of complex lithospheric structure accompanied by large variations in seismic wave velocities. Despite the complexity of the region, little is known about the detailed lithospheric structure. Using data from 31 new, permanent broadband seismic stations along with results from a previous 29 temporary seismic stations and 3 existing global seismic stations in the region, a 3-D velocity model is developed using joint inversion of teleseismic receiver functions and surface waves. Both group and phase dispersion curves (Love and Rayleigh) were derived from regional and teleseismic events. Additional Rayleigh wave group dispersion curves weremore » determined using ambient noise correlation. Receiver functions were calculated using P arrivals from 789 teleseismic (30°–90°) earthquakes. The stacked receiver functions and surface wave dispersion curves were jointly inverted to yield the absolute shear wave velocity to a depth of 100 km at each station. The depths of major discontinuities (sediment-basement, crust-mantle, and lithosphere-asthenosphere) were inferred from the velocity-depth profiles at the location of each station. Distinct spatial variations in crustal and upper mantle shear velocities were observed. The Kura basin showed slow (~2.7–2.9 km/s) upper crustal (0–11 km) velocities but elevated (~3.8–3.9 km/s) velocities in the lower crust. The Anatolian plateau varied from ~3.1–3.2 in the upper crust to ~3.5–3.7 in the lower crust, while velocities in the Arabian plate (south of the Bitlis suture) were slightly faster (upper crust between 3.3 and 3.4 km/s and lower crust between 3.8 and 3.9 km/s). The depth of the Moho, which was estimated from the shear velocity profiles, was 35 km in the Arabian plate and increased northward to 54 km at the southern edge of the Greater Caucasus. Moho depths in the Kura and at the edge of the Caspian showed more spatial variability but ranged between 35 and 45 km. Upper mantle velocities were slow under the Anatolian plateau but increased to the south under the Arabian plate and to the east (4.3–4.4 km/s) under the Kura basin and Greater Caucasus. The areas of slow mantle coincided with the locations of Holocene volcanoes. Differences between Rayleigh and Love dispersions at long wavelengths reveal a pronounced variation in anisotropy between the Anatolian plateau and the Kura basin.« less

Integrating shear velocity observations of the Hudson Bay

NASA Astrophysics Data System (ADS)

Porritt, R. W.; Miller, M. S.; Darbyshire, F. A.

2013-12-01

Hudson Bay is the core of the Laurentia craton of North America. This region contains some of the thickest lithosphere globally, reaching 250-300 km depth. Previous studies have shown that much of this region is composed of amalgamated proto-continents including the Western Churchill and Superior provinces and that much of the structure of these constituents has been retained since the Trans-Hudson Orogen at 1.8 Ga. Using the Hudson Bay Lithospheric Experiment (HuBLE) and other permanent and POLARIS broadband seismic data, we image the region with S to P receiver functions, joint inversion of P to S receiver functions with surface waves, and teleseismic S and P wave travel-times. The receiver function imaging reveals a persistent mid-lithospheric layer at ~80 km depth under all stations, but a variable lithospheric thickness. The teleseismic S delay times show a pattern of early arrivals around the center of the network, beneath Hudson Bay where the lithosphere is thickest, while the P delay times are early in the Superior province relative to the Western Churchill province. This suggests higher Vp/Vs ratios in the Superior province, which is evidence that stacked oceanic plates formed this province. The relatively flat Moho imaged by earlier receiver function studies and the lower mantle Vp/Vs of the Western Churchill province provides evidence of formation by plume head extraction. The joint inversion shows an LAB that is typically a broad discontinuity spanning ~20-30 km at ~220 km depth suggesting a primarily thermal boundary zone. The mid-lithospheric layer is composed of increasing velocity from the ~40 km depth Moho defined by H-k stacking of PRFs to a broad, constant velocity lithospheric lid spanning 80-200 km depth. We suggest this mid-lithospheric layer represents the mantle lithosphere of the proto-continents prior to collision and the lid formed due to post-collisional cooling. The integration of these seismic datasets furthers our understanding of plate tectonic and non-tectonic processes during the Archean formation of Laurentia craton.

Crustal and Upper Mantle Structure Beneath the Canary Islands From Teleseismic Receiver Functions.

NASA Astrophysics Data System (ADS)

Lodge, A.; Nippress, S. E.; Rietbrock, A.

2007-12-01

The Canary Islands are situated in the North Atlantic Ocean, <200km west of Morocco, Africa. The islands are volcanic ocean islands, associated with the classic hot spot characteristic combination of bathymetric, gravity and geoid anomalies. However, unlike the classic hot spot location of Hawaii, the archipelago is located on a slow moving plate, showing more similarities to the Cape Verde Islands, but unlike both Hawaii and Cape Verde, the Canary Islands are close to the continental shelf. The aims of this work are to provide seismic constraints on the structure beneath the Canary Islands to determine whether this structure indicates a clear age progression across the archipelago as observed at Cape Verde and to determine whether deeper structure may illuminate the source of the hot spot features. To take a transect through the Canary Islands using receiver function analysis, we re-analysed broadband data from the MIDSEA project station (available through IRIS), CDLV on Lanzarote (1999-2001), but apply the multiple- taper spectral correlation estimate for receiver function calculation. We also analysed broadband data from the IRIS Network station of TBT from La Palma (1993-1996). Additionally we also use data from a short period seismic network consisting of 150 short period stations installed for 2 weeks as part of the TOM-TEIDEVS project on the island of Tenerife. Only 1 teleseismic event suitable for receiver function analysis was recorded during this period. Initially an average of all events was to be used for modelling, but significant differences in receiver function shape between different areas of the island, suggested separate stacks for different regions was more appropriate. Initial forward modelling for the average azimuthal stack for CDLV, supports earlier receiver function work that indicates a crust thickened up to ~20km depth, but no evidence of a continental like structure. The average azimuthal stack for TBT shows few details, but when events are grouped by back azimuth and stacked, significant differences in shape are observed. Comparison of the tangential components for different back azimuths, suggests the existence of dipping and/or anisotropic layers. Forward modelling for a stack of data from the caldera in Tenerife, indicates no crustal thickening. This suggests a relationship between crustal thickening and age across the archipelago. Further analysis and application of grid search methods will reveal the structure beneath each of the islands in more detail and indicate whether a thermal or a compositional origin is more appropriate for the islands.

Estimating crustal thickness and Vp/Vs ratio with joint constraints of receiver function and gravity data

NASA Astrophysics Data System (ADS)

Shi, Lei; Guo, Lianghui; Ma, Yawei; Li, Yonghua; Wang, Weilai

2018-05-01

The technique of teleseismic receiver function H-κ stacking is popular for estimating the crustal thickness and Vp/Vs ratio. However, it has large uncertainty or ambiguity when the Moho multiples in receiver function are not easy to be identified. We present an improved technique to estimate the crustal thickness and Vp/Vs ratio by joint constraints of receiver function and gravity data. The complete Bouguer gravity anomalies, composed of the anomalies due to the relief of the Moho interface and the heterogeneous density distribution within the crust, are associated with the crustal thickness, density and Vp/Vs ratio. According to their relationship formulae presented by Lowry and Pérez-Gussinyé, we invert the complete Bouguer gravity anomalies by using a common algorithm of likelihood estimation to obtain the crustal thickness and Vp/Vs ratio, and then utilize them to constrain the receiver function H-κ stacking result. We verified the improved technique on three synthetic crustal models and evaluated the influence of selected parameters, the results of which demonstrated that the novel technique could reduce the ambiguity and enhance the accuracy of estimation. Real data test at two given stations in the NE margin of Tibetan Plateau illustrated that the improved technique provided reliable estimations of crustal thickness and Vp/Vs ratio.

Constraining Crustal Anisotropy by Receiver Functions at the Deep Continental Drilling Site KTB in Southern Germany

NASA Astrophysics Data System (ADS)

Bianchi, Irene; Qorbani, Ehsan; Bokelmann, Götz

2016-04-01

As one of the rare observational tools for studying deformation and stress within the Earth, seismic anisotropy has been one of the focuses of geophysical studies over the last decade. In order to unravel the anisotropic properties of the crust, the teleseismic receiver functions (RF) methodology has started to be widely applied recently. Such effects of anisotropy on RF were illustrated in theoretical studies, showing the strong backazimuthal dependence of RF on the 3D characteristics of the media sampled by the waves. The use of teleseismic RF has the advantage of not being affected by a heterogeneous depth distribution of local earthquakes, since teleseismic rays sample the entire crust beneath the stations. The application of this technique however, needs to be critically assessed using a suitable field test. To test the technique, we need a crustal block where the underground structure is reasonably well-known, e.g., where there is extensive knowledge from local seismic experiments and drilling. A field experiment has thus been carried out around the KTB (Kontinental Tiefbohrung) site in the Oberpfalz area in Southeastern Germany, in order to compare with previous results from deep drilling, and high-frequency seismic experiments around the drill site. The investigated region has been studied extensively by local geophysical experiments, and geological studies. The deep borehole was placed into gneiss rocks of the Zone Erbendorf-Vohenstrauss. The drilling activity lasted from 1987 to 1994, and descended down to a depth of 9101 meters, sampling an alternating sequence of paragneiss and amphibolite, with metamorphism of upper amphibolite facies conditions, and ductile deformation produced a strong foliation of the rocks. The application of the RFs reveals strong seismic anisotropy in the upper crust related to the so-called Erbendorf body. The SKS shear-wave splitting method has been applied as well, revealing coherent results for the whole region with exception of the southernmost station, for which the seismic waves show larger delays. We use the RF observations to test the effect of crustal anisotropy on the SKS records, which sample entire crust and upper mantle.

Mantle Attenuation Estimated from Regional and Teleseismic P-waves of Deep Earthquakes and Surface Explosions

NASA Astrophysics Data System (ADS)

Ichinose, G.; Woods, M.; Dwyer, J.

2014-03-01

We estimated the network-averaged mantle attenuation t*(total) of 0.5 s beneath the North Korea test site (NKTS) by use of P-wave spectra and normalized spectral stacks from the 25 May 2009 declared nuclear test (mb 4.5; IDC). This value was checked using P-waves from seven deep (580-600 km) earthquakes (4.8 < M w < 5.5) in the Jilin-Heilongjiang, China region that borders with Russia and North Korea. These earthquakes are 200-300 km from the NKTS, within 200 km of the Global Seismic Network seismic station in Mudanjiang, China (MDJ) and the International Monitoring System primary arrays at Ussuriysk, Russia (USRK) and Wonju, Republic of Korea (KSRS). With the deep earthquakes, we split the t*(total) ray path into two segments: a t*(u), that represents the attenuation of the up-going ray from the deep hypocenters to the local-regional receivers, and t*(d), that represents the attenuation along the down-going ray to teleseismic receivers. The sum of t*(u) and t*(d) should be equal to t*(total), because they both share coincident ray paths. We estimated the upper-mantle attenuation t*(u) of 0.1 s at stations MDJ, USRK, and KSRS from individual and stacks of normalized P-wave spectra. We then estimated the average lower-mantle attenuation t*(d) of 0.4 s using stacked teleseismic P-wave spectra. We finally estimated a network average t*(total) of 0.5 s from the stacked teleseismic P-wave spectra from the 2009 nuclear test, which confirms the equality with the sum of t*(u) and t*(d). We included constraints on seismic moment, depth, and radiation pattern by using results from a moment tensor analysis and corner frequencies from modeling of P-wave spectra recorded at local distances. We also avoided finite-faulting effects by excluding earthquakes with complex source time functions. We assumed ω2 source models for earthquakes and explosions. The mantle attenuation beneath the NKTS is clearly different when compared with the network-averaged t* of 0.75 s for the western US and is similar to values of approximately 0.5 s for the Semipalatinsk test site within the 0.5-2 Hz range.

A fast and robust method for moment tensor and depth determination of shallow seismic events in CTBT related studies.

NASA Astrophysics Data System (ADS)

Baker, Ben; Stachnik, Joshua; Rozhkov, Mikhail

2017-04-01

International Data Center is required to conduct expert technical analysis and special studies to improve event parameters and assist State Parties in identifying the source of specific event according to the protocol to the Protocol to the Comprehensive Nuclear Test Ban Treaty. Determination of seismic event source mechanism and its depth is closely related to these tasks. It is typically done through a strategic linearized inversion of the waveforms for a complete or subset of source parameters, or similarly defined grid search through precomputed Greens Functions created for particular source models. In this presentation we demonstrate preliminary results obtained with the latter approach from an improved software design. In this development we tried to be compliant with different modes of CTBT monitoring regime and cover wide range of source-receiver distances (regional to teleseismic), resolve shallow source depths, provide full moment tensor solution based on body and surface waves recordings, be fast to satisfy both on-demand studies and automatic processing and properly incorporate observed waveforms and any uncertainties a priori as well as accurately estimate posteriori uncertainties. Posterior distributions of moment tensor parameters show narrow peaks where a significant number of reliable surface wave observations are available. For earthquake examples, fault orientation (strike, dip, and rake) posterior distributions also provide results consistent with published catalogues. Inclusion of observations on horizontal components will provide further constraints. In addition, the calculation of teleseismic P wave Green's Functions are improved through prior analysis to determine an appropriate attenuation parameter for each source-receiver path. Implemented HDF5 based Green's Functions pre-packaging allows much greater flexibility in utilizing different software packages and methods for computation. Further additions will have the rapid use of Instaseis/AXISEM full waveform synthetics added to a pre-computed GF archive. Along with traditional post processing analysis of waveform misfits through several objective functions and variance reduction, we follow a probabilistic approach to assess the robustness of moment tensor solution. In a course of this project full moment tensor and depth estimates are determined for DPRK events and shallow earthquakes using a new implementation of teleseismic P waves waveform fitting. A full grid search over the entire moment tensor space is used to appropriately sample all possible solutions. A recent method by Tape & Tape (2012) to discretize the complete moment tensor space from a geometric perspective is used. Probabilistic uncertainty estimates on the moment tensor parameters provide robustness to solution.

Crustal Structure of Iraq from Receiver Functions and Surface Wave Dispersion

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gok, R; Mahdi, H; Al-Shukri, H

2006-08-31

We report the crustal structure of Iraq, located in the northeastern Arabian plate, estimated by joint inversion of P-wave receiver functions and surface wave group velocity dispersion. Receiver functions were computed from teleseismic recordings at two temporary broadband seismic stations in Mosul (MSL) and Baghdad (BHD), separated by approximately 360 km. Group velocity dispersion curves at the sites were derived from continental-scale tomography of Pasyanos (2006). The inversion results show that the crustal thicknesses are 39 km at MSL and 43 km at BHD. Both sites reveal low velocity surface layers consistent with sedimentary thickness of about 3 km atmore » station MSL and 7 km at BHD, agreeing well with the existing models. Ignoring the sediments, the crustal velocities and thicknesses are remarkably similar between the two stations, suggesting that the crustal structure of the proto-Arabian Platform in northern Iraq was uniform before subsidence and deposition of the sediments in the Cenozoic. Deeper low velocity sediments at BHD are expected to result in higher ground motions for earthquakes.« less

Rupture process of 2016, 25 January earthquake, Alboran Sea (South Spain, Mw= 6.4) and aftershocks series

NASA Astrophysics Data System (ADS)

Buforn, E.; Pro, C.; del Fresno, C.; Cantavella, J.; Sanz de Galdeano, C.; Udias, A.

2016-12-01

We have studied the rupture process of the 25 January 2016 earthquake (Mw =6.4) occurred in South Spain in the Alboran Sea. Main shock, foreshock and largest aftershocks (Mw =4.5) have been relocated using the NonLinLoc algorithm. Results obtained show a NE-SW distribution of foci at shallow depth (less than 15 km). For main shock, focal mechanism has been obtained from slip inversion over the rupture plane of teleseismic data, corresponding to left-lateral strike-slip motion. The rupture starts at 7 km depth and it propagates upward with a complex source time function. In order to obtain a more detailed source time function and to validate the results obtained from teleseismic data, we have used the Empirical Green Functions method (EGF) at regional distances. Finally, results of the directivity effect from teleseismic Rayleigh waves and the EGF method, are consistent with a rupture propagation to the NE. These results are interpreted in terms of the main geological features in the region.

Crustal and mantle shear velocity structure of Costa Rica and Nicaragua from ambient noise and teleseismic Rayleigh wave tomography

NASA Astrophysics Data System (ADS)

Harmon, Nicholas; de la Cruz, Mariela Salas; Rychert, Catherine Ann; Abers, Geoffrey; Fischer, Karen

2013-11-01

The Costa Rica-Nicaragua subduction zone shows systematic along strike variation in arc chemistry, geology, tectonics and seismic velocity and attenuation, presenting global extremes within a few hundred kilometres. In this study, we use teleseismic and ambient noise derived surface wave tomography to produce a 3-D shear velocity model of the region. We use the 48 stations of the TUCAN array, and up to 94 events for the teleseismic Rayleigh wave inversion, and 18 months of continuous data for cross correlation to estimate Green's functions from ambient noise. In the shallow crust (0-15 km) we observe low-shear velocities directly beneath the arc volcanoes (<3 km s-1) and higher velocities in the backarc of Nicaragua. The anomalies below the volcanoes are likely caused by heated crust, intruded by magma. We estimate crustal thickness by picking the depth to the 4 km s-1 velocity contour. We infer >40-km-thick crust beneath the Costa Rican arc and the Nicaraguan Highlands, thinned crust (˜20 km) beneath the Nicaraguan Depression, and increasing crustal thickness in the backarc region, consistent with receiver function studies. The region of thinned, seismically slow and likely weakened crust beneath the arc in Nicaragua is not localizing deformation associated with oblique subduction. At mantle depths (55-120 km depth) we observe lower shear velocities (up to 3 per cent) beneath the Nicaraguan arc and backarc than beneath Costa Rica. Our low-shear velocity anomaly beneath Nicaragua is in the same location as a low-shear velocity anomaly and displaced towards the backarc from the high VP/VS anomaly observed in body wave tomography. The lower shear velocity beneath Nicaragua may indicate higher melt content in the mantle perhaps due to higher volatile flux from the slab or higher temperature. Finally, we observe a linear high-velocity region at depths >120 km parallel to the trench, which is consistent with the subducting slab.

Multifrequency measurements of core-diffracted P waves (Pdiff) for global waveform tomography

NASA Astrophysics Data System (ADS)

Hosseini, Kasra; Sigloch, Karin

2015-10-01

The lower third of the mantle is sampled extensively by body waves that diffract around the earth's core (Pdiff and Sdiff phases), which could deliver highly resolved tomographic images of this poorly understood region. But core-diffracted waves-especially Pdiff waves-are not often used in tomography because they are difficult to model adequately. Our aim is to make core-diffracted body waves usable for global waveform tomography, across their entire frequency range. Here we present the data processing part of this effort. A method is demonstrated that routinely calculates finite-frequency traveltimes of Pdiff waves by cross-correlating large quantities of waveform data with synthetic seismograms, in frequency passbands ranging from 30.0 to 2.7 s dominant period. Green's functions for 1857 earthquakes, typically comprising thousands of seismograms, are calculated by theoretically exact wave propagation through a spherically symmetric earth model, up to 1 Hz dominant period. Out of 418 226 candidates, 165 651 (39.6 per cent) source-receiver pairs yielded at least one successful passband measurement of a Pdiff traveltime anomaly, for a total of 479 559 traveltimes in the eight passbands considered. Measurements of teleseismic P waves yielded 448 178 usable source-receiver paths from 613 057 candidates (73.1 per cent success rate), for a total of 2 306 755 usable teleseismic dT in eight passbands. Observed and predicted characteristics of Pdiff traveltimes are discussed and compared to teleseismic P for this very large data set. Pdiff measurements are noise-limited due to severe wave attenuation with epicentral distance and frequency. Measurement success drops from 40-60 per cent at 80° distance, to 5-10 per cent at 140°. Frequency has a 2-3 times stronger influence on measurement success for Pdiff than for P. The fewest usable dT measurements are obtained in the microseismic noise band, whereas the fewest usable teleseismic P measurements occur at the highest frequencies. dT anomalies are larger for Pdiff than for P, and frequency dependence of dT due to 3-D heterogeneity (rather than just diffraction) is larger for Pdiff as well. Projecting the Pdiff traveltime anomalies on their core-grazing segments, we retrieve well-known, large-scale structural heterogeneities of the lowermost mantle, such as the two Large Low Shear Velocity Provinces, an Ultra-Low Velocity Zone west of Hawaii, and subducted slab accumulations under East Asia and Central America.

Fine-scale crustal structure of the Azores Islands from teleseismic receiver functions

NASA Astrophysics Data System (ADS)

Spieker, K.; Rondenay, S.; Ramalho, R. S.; Thomas, C.; Helffrich, G. R.

2016-12-01

The Azores plateau is located near the Mid-Atlantic Ridge (MAR) and consists of nine islands, most of which lie east of the MAR. Various methods including seismic reflection, gravity, and passive seismic imaging have been used to investigate the crustal thickness beneath the islands. They have yielded thickness estimates that range between roughly 10 km and 30 km, but until now models of the fine-scale crustal structure have been lacking. A comparison of the crustal structure beneath the islands that lie west and east of the MAR might give further constraints on the evolution of the islands. For example, geochemical studies carried out across the region predict the existence of volcanic interfaces that should be detected seismically within the shallow crust of some of the islands. In this study, we use data from ten seismic stations located on the Azores Islands to investigate the crustal structure with teleseismic P-wave receiver functions. We query our resulting receiver functions for signals associated with the volcanic edifice, the crust-mantle boundary, and potential underplated layers beneath the various islands. The islands west of the MAR have a crustal structure comprising two discontinuities - an upper one at 1-2 km depth marking the base of the volcanic edifice, and a lower one at 10 km depth that we interpret as crust-mantle boundary. The islands east of the MAR can be subdivided into two groups. The central islands that are closer to the MAR exhibit a crustal structure similar to that of the western islands, with a volcanic edifice reaching a depth of 2 km and an average crust-mantle boundary at around 12 km depth. The easternmost islands, located on the oldest lithosphere, exhibit a more complex crustal structure with evidence for a mid-crustal interface and an underplated layer, yielding an effective crust-mantle boundary at >15 km depth. The difference in structure between proximal and distal islands might be related to the age of the plate at the time of emplacement of the islands, with an older plate providing conditions that are more favourable for basaltic underplating.

Seismic structure beneath Mt Vesuvius from receiver function analysis and local earthquakes tomography: evidences for location and geometry of the magma chamber

NASA Astrophysics Data System (ADS)

Agostinetti, N. Piana; Chiarabba, C.

2008-12-01

The recognition and localization of magmatic fluids are pre-requisites for evaluating the volcano hazard of the highly urbanized area of Mt Vesuvius. Here we show evidence and constraints for the volumetric estimation of magmatic fluids underneath this sleeping volcano. We use Receiver Functions for teleseismic data recorded at a temporary broad-band station installed on the volcano to constrain the S-wave velocity structure in the crust. Receiver Functions are analysed and inverted using the Neighbourhood Algorithm approach. The 1-D S-velocity profile is jointly interpreted and discussed with a new Vp and Vp/Vs image obtained by applying double difference tomographic techniques to local earthquakes. Seismologic data define the geometry of an axial, cylindrical high Vp, high Vs body consisting of a shallow solidified materials, probably the remnants of the caldera, and ultramafic rocks paving the crustal magma chamber. Between these two anomalies, we find a small region where the shear wave velocity drops, revealing the presence of magma at relatively shallow depths. The volume of fluids (30 km3) is sufficient to contribute future explosive eruptions.

Receiver Function Imaging of Crustal and Lithospheric Structure Beneath the Jalisco Block and Western Michoacan, Mexico.

NASA Astrophysics Data System (ADS)

Reyes Alfaro, G.; Cruz-Atienza, V. M.; Perez-Campos, X.; Reyes Dávila, G. A.

2014-12-01

We used a receiver function technique for imaging western Mexico, a unique area with several active seismic and volcanic zones like the triple junction of Rivera, Cocos and North American plates and the Colima volcano complex (CVC), the most active in Mexico. Clear images of the distribution of the crust and the lithosphere-asthenosphere boundary are obtained using P-to-S receiver functions (RF) from around ~80 broadband stations recorded by the Mapping the Rivera Subduction Zone (MARS), the Colima Volcano Deep Seismic Experiment (CODEX) and a local network (RESCO) that allowed us to considerably increase the teleseismic database used in the project. For imaging, we constructed several 2-D profiles of depth transformed RFs to delineate the seismic discontinuities of the region. Low seismic velocities associated with the Michoacan-Guanajuato and the Mascota-Ayutla-Tapalpa volcanic fields are also observed. Most impressive, a large and well delineated magma body 100 km underneath CVC is recognized along a surely related depression of the moho discontinuity just above it. We bring more tools for a better understanding of the deep processes that ultimately control eruptive behavior in the region.

Lateral Moho variations and the geometry of Main Himalayan Thrust beneath Nepal Himalayan orogen revealed by teleseismic receiver functions

NASA Astrophysics Data System (ADS)

He, Ping; Lei, Jianshe; Yuan, Xiaohui; Xu, Xiwei; Xu, Qiang; Liu, Zhikun; Mi, Qi; Zhou, Lianqing

2018-05-01

The lateral Moho variations and the geometry of the Main Himalayan Thrust under the Nepal Himalayan orogen are investigated to determine a new crustal model using a large number of high-quality receiver functions recorded by the HIMNT and HiCLIMB portable seismic networks. Our new model shows an evident and complicated lateral Moho depth variation of 8-16 km in the east-west direction, which is related to the surface tectonic features. These results suggest a non-uniformed crustal deformation, resulted from the splitting and/or tearing of the Indian plate during the northward subduction. Our migrated receiver function images illustrate a discernible ramp structure of the Main Himalayan Thrust with an abrupt downward bending close to the hypocenter of the 2015 Gorkha Mw 7.8 earthquake. The distribution of the aftershocks coincides with the present decollement structure. Integrating previous magnetotelluric soundings and tomographic results, our results suggest that the ramp-shaped structure within the Main Himalayan Thrust could enhance stress concentration leading to the nucleation of the large earthquake. Our new crustal model provides new clues to the formation of the Himalayan orogen.

Teleseismic constraints on the geological environment of deep episodic slow earthquakes in subduction zone forearcs: A review

NASA Astrophysics Data System (ADS)

Audet, Pascal; Kim, YoungHee

2016-02-01

More than a decade after the discovery of deep episodic slow slip and tremor, or slow earthquakes, at subduction zones, much research has been carried out to investigate the structural and seismic properties of the environment in which they occur. Slow earthquakes generally occur on the megathrust fault some distance downdip of the great earthquake seismogenic zone in the vicinity of the mantle wedge corner, where three major structural elements are in contact: the subducting oceanic crust, the overriding forearc crust and the continental mantle. In this region, thermo-petrological models predict significant fluid production from the dehydrating oceanic crust and mantle due to prograde metamorphic reactions, and their consumption by hydrating the mantle wedge. These fluids are expected to affect the dynamic stability of the megathrust fault and enable slow slip by increasing pore-fluid pressure and/or reducing friction in fault gouges. Resolving the fine-scale structure of the deep megathrust fault and the in situ distribution of fluids where slow earthquakes occur is challenging, and most advances have been made using teleseismic scattering techniques (e.g., receiver functions). In this paper we review the teleseismic structure of six well-studied subduction zones (three hot, i.e., Cascadia, southwest Japan, central Mexico, and three cool, i.e., Costa Rica, Alaska, and Hikurangi) that exhibit slow earthquake processes and discuss the evidence of structural and geological controls on the slow earthquake behavior. We conclude that changes in the mechanical properties of geological materials downdip of the seismogenic zone play a dominant role in controlling slow earthquake behavior, and that near-lithostatic pore-fluid pressures near the megathrust fault may be a necessary but insufficient condition for their occurrence.

Lateral variations of the Guerrero-Oaxaca subduction zone (Mexico) derived from weak seismicity (Mb3.5+) detected on a single array at teleseismic distance

NASA Astrophysics Data System (ADS)

Letort, Jean; Retailleau, Lise; Boué, Pierre; Radiguet, Mathilde; Gardonio, Blandine; Cotton, Fabrice; Campillo, Michel

2018-05-01

Detections of pP and sP phase arrivals (the so-called depth phases) at teleseismic distance provide one of the best ways to estimate earthquake focal depth, as the P-pP and the P-sP delays are strongly dependent on the depth. Based on a new processing workflow and using a single seismic array at teleseismic distance, we can estimate the depth of clusters of small events down to magnitude Mb 3.5. Our method provides a direct view of the relative variations of the seismicity depth from an active area. This study focuses on the application of this new methodology to study the lateral variations of the Guerrero subduction zone (Mexico) using the Eielson seismic array in Alaska (USA). After denoising the signals, 1232 Mb 3.5 + events were detected, with clear P, pP, sP and PcP arrivals. A high-resolution view of the lateral variations of the depth of the seismicity of the Guerero-Oaxaca area is thus obtained. The seismicity is shown to be mainly clustered along the interface, coherently following the geometry of the plate as constrained by the receiver-function analysis along the Meso America Subduction Experiment profile. From this study, the hypothesis of tears on the western part of Guerrero and the eastern part of Oaxaca are strongly confirmed by dramatic lateral changes in the depth of the earthquake clusters. The presence of these two tears might explain the observed lateral variations in seismicity, which is correlated with the boundaries of the slow slip events.

Demonstration of improved seismic source inversion method of tele-seismic body wave

NASA Astrophysics Data System (ADS)

Yagi, Y.; Okuwaki, R.

2017-12-01

Seismic rupture inversion of tele-seismic body wave has been widely applied to studies of large earthquakes. In general, tele-seismic body wave contains information of overall rupture process of large earthquake, while the tele-seismic body wave is inappropriate for analyzing a detailed rupture process of M6 7 class earthquake. Recently, the quality and quantity of tele-seismic data and the inversion method has been greatly improved. Improved data and method enable us to study a detailed rupture process of M6 7 class earthquake even if we use only tele-seismic body wave. In this study, we demonstrate the ability of the improved data and method through analyses of the 2016 Rieti, Italy earthquake (Mw 6.2) and the 2016 Kumamoto, Japan earthquake (Mw 7.0) that have been well investigated by using the InSAR data set and the field observations. We assumed the rupture occurring on a single fault plane model inferred from the moment tensor solutions and the aftershock distribution. We constructed spatiotemporal discretized slip-rate functions with patches arranged as closely as possible. We performed inversions using several fault models and found that the spatiotemporal location of large slip-rate area was robust. In the 2016 Kumamoto, Japan earthquake, the slip-rate distribution shows that the rupture propagated to southwest during the first 5 s. At 5 s after the origin time, the main rupture started to propagate toward northeast. First episode and second episode correspond to rupture propagation along the Hinagu fault and the Futagawa fault, respectively. In the 2016 Rieti, Italy earthquake, the slip-rate distribution shows that the rupture propagated to up-dip direction during the first 2 s, and then rupture propagated toward northwest. From both analyses, we propose that the spatiotemporal slip-rate distribution estimated by improved inversion method of tele-seismic body wave has enough information to study a detailed rupture process of M6 7 class earthquake.

Receiver function deconvolution using transdimensional hierarchical Bayesian inference

NASA Astrophysics Data System (ADS)

Kolb, J. M.; Lekić, V.

2014-06-01

Teleseismic waves can convert from shear to compressional (Sp) or compressional to shear (Ps) across impedance contrasts in the subsurface. Deconvolving the parent waveforms (P for Ps or S for Sp) from the daughter waveforms (S for Ps or P for Sp) generates receiver functions which can be used to analyse velocity structure beneath the receiver. Though a variety of deconvolution techniques have been developed, they are all adversely affected by background and signal-generated noise. In order to take into account the unknown noise characteristics, we propose a method based on transdimensional hierarchical Bayesian inference in which both the noise magnitude and noise spectral character are parameters in calculating the likelihood probability distribution. We use a reversible-jump implementation of a Markov chain Monte Carlo algorithm to find an ensemble of receiver functions whose relative fits to the data have been calculated while simultaneously inferring the values of the noise parameters. Our noise parametrization is determined from pre-event noise so that it approximates observed noise characteristics. We test the algorithm on synthetic waveforms contaminated with noise generated from a covariance matrix obtained from observed noise. We show that the method retrieves easily interpretable receiver functions even in the presence of high noise levels. We also show that we can obtain useful estimates of noise amplitude and frequency content. Analysis of the ensemble solutions produced by our method can be used to quantify the uncertainties associated with individual receiver functions as well as with individual features within them, providing an objective way for deciding which features warrant geological interpretation. This method should make possible more robust inferences on subsurface structure using receiver function analysis, especially in areas of poor data coverage or under noisy station conditions.

The Lithospheric Structure Beneath Canary Islands from Receiver Function Analysis

NASA Astrophysics Data System (ADS)

Martinez-Arevalo, C.; Mancilla, F.; Helffrich, G. R.; Garcia, A.

2009-12-01

The Canary Archipelago is located a few hundred kilometers off the western Moroccan coast, extending 450 km west-to-east. It is composed of seven main islands. All but one have been active in the last million years. The origin of the Canary Islands is not well established and local and regional geology features cannot be completely explained by the current models. The main aim of this study is to provide new data that help us to understand and constrain the archipelago's origin and tectonic evolution. The crustal structure under each station is obtained applying P-receiver function technique to the teleseismic P arrivals recorded by the broadband seismic network installed at the Canary Island by the Instituto Geográfico Nacional (IGN) and two temporary stations (MIDSEA and IRIS). We computed receiver functions using the Extended-Time Multitaper Frequency Domain Cross-Correlation Receiver Function (ET-MTRF) method. The results show that the crust is thicker, around 22 km, in the eastern islands (Fuerteventura and Lanzarote) than in the western ones (El Hierro, La Palma, Tenerife), around 17 km, with the exception of La Gomera island. This island, located in the west, exhibits similar crustal structure to Fuerteventura and Lanzarote. A discontinuity at 70-80 km, possibly the LAB (Lithosphere Asthenosphere Boundary) is clearly observed in all the stations. It appears that Moho depths do not track the LAB discontinuity.

Regional teleseismic body-wave tomography with component-differential finite-frequency sensitivity kernels

NASA Astrophysics Data System (ADS)

Yu, Y.; Shen, Y.; Chen, Y. J.

2015-12-01

By using ray theory in conjunction with the Born approximation, Dahlen et al. [2000] computed 3-D sensitivity kernels for finite-frequency seismic traveltimes. A series of studies have been conducted based on this theory to model the mantle velocity structure [e.g., Hung et al., 2004; Montelli et al., 2004; Ren and Shen, 2008; Yang et al., 2009; Liang et al., 2011; Tang et al., 2014]. One of the simplifications in the calculation of the kernels is the paraxial assumption, which may not be strictly valid near the receiver, the region of interest in regional teleseismic tomography. In this study, we improve the accuracy of traveltime sensitivity kernels of the first P arrival by eliminating the paraxial approximation. For calculation efficiency, the traveltime table built by the Fast Marching Method (FMM) is used to calculate both the wave vector and the geometrical spreading at every grid in the whole volume. The improved kernels maintain the sign, but with different amplitudes at different locations. We also find that when the directivity of the scattered wave is being taken into consideration, the differential sensitivity kernel of traveltimes measured at the vertical and radial component of the same receiver concentrates beneath the receiver, which can be used to invert for the structure inside the Earth. Compared with conventional teleseismic tomography, which uses the differential traveltimes between two stations in an array, this method is not affected by instrument response and timing errors, and reduces the uncertainty caused by the finite dimension of the model in regional tomography. In addition, the cross-dependence of P traveltimes to S-wave velocity anomaly is significant and sensitive to the structure beneath the receiver. So with the component-differential finite-frequency sensitivity kernel, the anomaly of both P-wave and S-wave velocity and Vp/Vs ratio can be achieved at the same time.

Neural Network Classification of Receiver Functions as a Step Towards Automatic Crustal Parameter Determination

NASA Astrophysics Data System (ADS)

Jemberie, A.; Dugda, M. T.; Reusch, D.; Nyblade, A.

2006-12-01

Neural networks are decision making mathematical/engineering tools, which if trained properly, can do jobs automatically (and objectively) that normally require particular expertise and/or tedious repetition. Here we explore two techniques from the field of artificial neural networks (ANNs) that seek to reduce the time requirements and increase the objectivity of quality control (QC) and Event Identification (EI) on seismic datasets. We explore to apply the multiplayer Feed Forward (FF) Artificial Neural Networks (ANN) and Self- Organizing Maps (SOM) in combination with Hk stacking of receiver functions in an attempt to test the extent of the usefulness of automatic classification of receiver functions for crustal parameter determination. Feed- forward ANNs (FFNNs) are a supervised classification tool while self-organizing maps (SOMs) are able to provide unsupervised classification of large, complex geophysical data sets into a fixed number of distinct generalized patterns or modes. Hk stacking is a methodology that is used to stack receiver functions based on the relative arrival times of P-to-S converted phase and next two reverberations to determine crustal thickness H and Vp-to-Vs ratio (k). We use receiver functions from teleseismic events recorded by the 2000- 2002 Ethiopia Broadband Seismic Experiment. Preliminary results of applying FFNN neural network and Hk stacking of receiver functions for automatic receiver functions classification as a step towards an effort of automatic crustal parameter determination look encouraging. After training a FFNN neural network, the network could classify the best receiver functions from bad ones with a success rate of about 75 to 95%. Applying H? stacking on the receiver functions classified by this FFNN as the best receiver functions, we could obtain crustal thickness and Vp/Vs ratio of 31±4 km and 1.75±0.05, respectively, for the crust beneath station ARBA in the Main Ethiopian Rift. To make comparison, we applied Hk stacking on the receiver functions which we ourselves classified as the best set and found that the crustal thickness and Vp/Vs ratio are 31±2 km and 1.75±0.02, respectively.

Measurement of seismometer orientation using the tangential P-wave receiver function based on harmonic decomposition

NASA Astrophysics Data System (ADS)

Lim, Hobin; Kim, YoungHee; Song, Teh-Ru Alex; Shen, Xuzhang

2018-03-01

Accurate determination of the seismometer orientation is a prerequisite for seismic studies including, but not limited to seismic anisotropy. While borehole seismometers on land produce seismic waveform data somewhat free of human-induced noise, they might have a drawback of an uncertain orientation. This study calculates a harmonic decomposition of teleseismic receiver functions from the P and PP phases and determines the orientation of a seismometer by minimizing a constant term in a harmonic expansion of tangential receiver functions in backazimuth near and at 0 s. This method normalizes the effect of seismic sources and determines the orientation of a seismometer without having to assume for an isotropic medium. Compared to the method of minimizing the amplitudes of a mean of the tangential receiver functions near and at 0 s, the method yields more accurate orientations in cases where the backazimuthal coverage of earthquake sources (even in the case of ocean bottom seismometers) is uneven and incomplete. We apply this method to data from the Korean seismic network (52 broad-band velocity seismometers, 30 of which are borehole sensors) to estimate the sensor orientation in the period of 2005-2016. We also track temporal changes in the sensor orientation through the change in the polarity and the amplitude of the tangential receiver function. Six borehole stations are confirmed to experience a significant orientation change (10°-180°) over the period of 10 yr. We demonstrate the usefulness of our method by estimating the orientation of ocean bottom sensors, which are known to have high noise level during the relatively short deployment period.

Structure of the subduction system in southern Peru from seismic array data

NASA Astrophysics Data System (ADS)

Phillips, Kristin; Clayton, Robert W.; Davis, Paul; Tavera, Hernando; Guy, Richard; Skinner, Steven; Stubailo, Igor; Audin, Laurence; Aguilar, Victor

2012-11-01

The subduction zone in southern Peru is imaged using converted phases from teleseismic P, PP, and PKP waves and Pwave tomography using local and teleseismic events with a linear array of 50 broadband seismic stations spanning 300 km from the coast to near Lake Titicaca. The slab dips at 30° and can be observed to a depth of over 200 km. The Moho is seen as a continuous interface along the profile, and the crustal thickness in the back-arc region (the Altiplano) is 75 km thick, which is sufficient to isostatically support the Andes, as evidenced by the gravity. The shallow crust has zones of negative impedance at a depth of 20 km, which is likely the result of volcanism. At the midcrustal level of 40 km, there is a continuous structure with a positive impedance contrast, which we interpret as the western extent of the Brazilian Craton as it underthrusts to the west.Vp/Vs ratios estimated from receiver function stacks show average values for this region with a few areas of elevated Vp/Vs near the volcanic arc and at a few points in the Altiplano. The results support a model of crustal thickening in which the margin crust is underthrust by the Brazilian Shield.

The Moho discontinuity beneath Taiwan orogenic zone inferred from receiver function analysis

NASA Astrophysics Data System (ADS)

Chang, H.; Chen, C.; Liang, W.

2013-12-01

We determine the depth variations of the Moho discontinuity beneath Taiwan from receiver function analysis. Taiwan is a young (~6.5 Ma) orogenic zone as a consequence of oblique collision between the Philippine Sea Plate and the Eurasian Plate. In northeastern Taiwan, the Philippine Sea Plate subducts northwestward under the Eurasian Plate along the Ryukyu Trench; in southern Taiwan, the Eurasian Plate subducts eastward beneath the Philippine Sea Plate along the Manila Trench. Recent tomographic models of Taiwan reveal P-wave velocity variations of the lithospheric structure that provide important constraints on the orogenic processes in this region. However, the depth variations of the Moho discontinuity, a key observation for better understanding crustal deformation, remain elusive. In this study, we aim to delineate the Moho depth variations by analyzing seismic converted phases indicative of the presence of discontinuity structure. We analyze waveform data from teleseismic events recorded at the Broadband Array in Taiwan for Seismology (BATS). Preliminary results of receiver functions beneath BATS stations in eastern Taiwan show that more than one converted phase (P-to-S) are likely present in crustal depths, suggesting possible multiple crustal layering, which may complicate the detection of the Moho. We further carry out synthetic experiments to explore possible crustal structures that reconcile our observations.

Exploring the Llaima Volcano Using Receiver Functions

NASA Astrophysics Data System (ADS)

Bishop, J. W.; Biryol, C.; Lees, J. M.

2016-12-01

The Llaima volcano in Chile is one of the most active volcanos in the Southern Andes, erupting at least 50 times since 1640. To understand the eruption dynamics behind these frequent paroxysms, it is important to identify the depth and extent of the magma chamber beneath the volcano. Furthermore, it is also important to identify structural controls on the magma storage regions and volcanic plumbing system, such as fault and fracture zones. To probe these questions, a dense, 26 station broadband seismic array was deployed around the Llaima volcano for 3 months (January to March, 2015). Additionally, broadband seismic data from 7 stations in the nearby Observatorio Volcanológico de Los Andes del Sur (OVDAS) seismic network was also obtained for this period. Teleseismic receiver functions were calculated from this combined data using an iterative deconvolution technique. Receiver function stacks (both H-K and CCP) yield seismic images of the deep structure beneath the volcano. Initial results depict two low velocity layers at approximately 4km and 12km. Furthermore, Moho calculations are 5-8 km deeper than expected from regional models, but a shallow ( 40 km) region is detected beneath the volcano peak. A large high Vp/Vs ratio anomaly (Vp/Vs > 0.185) is discernable to the east of the main peak of the volcano.

Imaging the deep structures of the convergent plates along the Ecuadorian subduction zone through receiver function analysis

NASA Astrophysics Data System (ADS)

Galve, A.; Charvis, P.; Regnier, M. M.; Font, Y.; Nocquet, J. M.; Segovia, M.

2017-12-01

The Ecuadorian subduction zone was affected by several large M>7.5 earthquakes. While we have low resolution on the 1942, 1958 earthquakes rupture zones extension, the 2016 Pedernales earthquake, that occurs at the same location than the 1942 earthquake, give strong constraints on the deep limit of the seismogenic zone. This downdip limit is caused by the onset of plasticity at a critical temperature (> 350-450 °C for crustal materials, or serpentinized mantle wedge, and eventually > 700 °C for dry mantle). However we still don't know exactly where is the upper plate Moho and therefore what controls the downdip limit of Ecuadorian large earthquakes seismogenic zone. For several years Géoazur and IG-EPN have maintained permanent and temporary networks (ADN and JUAN projects) along the margin to register the subduction zone seismological activity. Although Ecuador is not a good place to perform receiver function due to its position with respect to the worldwide teleseismic sources, the very long time deployment compensate this issue. We performed a frequency dependent receiver function analysis to derive (1) the thickness of the downgoing plate, (2) the interplate depth and (3) the upper plate Moho. These constraints give the frame to interpretation on the seismogenic zone of the 2016 Pedernales earthquake.

Crustal shear wave velocity structure in the northeastern Tibet based on the Neighbourhood algorithm inversion of receiver functions

NASA Astrophysics Data System (ADS)

Wu, Zhenbo; Xu, Tao; Liang, Chuntao; Wu, Chenglong; Liu, Zhiqiang

2018-03-01

The northeastern (NE) Tibet records and represents the far-field deformation response of the collision between the Indian and Eurasian plates in the Cenozoic time. Over the past two decades, studies have revealed the existence of thickened crust in the NE Tibet, but the thickening mechanism is still in debate. We deployed a passive-source seismic profile with 22 temporary broad-band seismic stations in the NE Tibet to investigate the crustal shear wave velocity structure in this region. We selected 288 teleseismic events located in the west Pacific subduction zone near Japan with similar ray path to calculate P-wave receiver functions. Neighbourhood algorithm method is applied to invert the shear wave velocity beneath stations. The inversion result shows a low-velocity zone (LVZ) is roughly confined to the Songpan-Ganzi block and Kunlun mountains and extends to the southern margin of Gonghe basin. Considering the low P-wave velocity revealed by the wide-angle reflection-refraction seismic experiment and high ratio of Vp/Vs based on H-κ grid searching of the receiver functions in this profile, LVZ may be attributed to partial melting induced by temperature change. This observation appears to be consistent with the crustal ductile deformation in this region derived from other geophysical investigations.

Receiver function stacks: initial steps for seismic imaging of Cotopaxi volcano, Ecuador

NASA Astrophysics Data System (ADS)

Bishop, J. W.; Lees, J. M.; Ruiz, M. C.

2017-12-01

Cotopaxi volcano is a large, andesitic stratovolcano located within 50 km of the the Ecuadorean capital of Quito. Cotopaxi most recently erupted for the first time in 73 years during August 2015. This eruptive cycle (VEI = 1) featured phreatic explosions and ejection of an ash column 9 km above the volcano edifice. Following this event, ash covered approximately 500 km2 of the surrounding area. Analysis of Multi-GAS data suggests that this eruption was fed from a shallow source. However, stratigraphic evidence surveying the last 800 years of Cotopaxi's activity suggests that there may be a deep magmatic source. To establish a geophysical framework for Cotopaxi's activity, receiver functions were calculated from well recorded earthquakes detected from April 2015 to December 2015 at 9 permanent broadband seismic stations around the volcano. These events were located, and phase arrivals were manually picked. Radial teleseismic receiver functions were then calculated using an iterative deconvolution technique with a Gaussian width of 2.5. A maximum of 200 iterations was allowed in each deconvolution. Iterations were stopped when either the maximum iteration number was reached or the percent change fell beneath a pre-determined tolerance. Receiver functions were then visually inspected for anomalous pulses before the initial P arrival or later peaks larger than the initial P-wave correlated pulse, which were also discarded. Using this data, initial crustal thickness and slab depth estimates beneath the volcano were obtained. Estimates of crustal Vp/Vs ratio for the region were also calculated.

Crustal properties of the northern Scandinavian mountains and Fennoscandian shield from analysis of teleseismic Receiver Functions

NASA Astrophysics Data System (ADS)

Mansour, Walid Ben; England, Richard W.; Fishwick, Stewart; Moorkamp, Max

2018-04-01

The presence of high mountains along passive margins is not unusual, as shown by their presence in several regions (Scandinavia, Greenland, East US, SW Africa, Brazil, West India and SE Australia). However, the origin of this topography is not well understood. The mountain range between the Scandinavian passive margin and the Fennoscandian shield is a good example. A simple Airy isostatic model would predict a compensating root beneath the mountains but existing seismic measurements of variations in crustal thickness do not provide evidence of a root of sufficient size to produce the necessary compensation. In order to better constrain the physical properties of the crust in northern Scandinavia two broadband seismic networks were deployed between 2007 and 2009 and between 2013 and 2014. A new map of crustal thickness has been produced from P-receiver function analysis of teleseismic data recorded at 31 seismic stations. The map shows an increase in crustal thickness from the Atlantic coast (38.7 +/- 1.8 km) to the Gulf of Bothnia (43.5 +/- 2.4 km). This gradient in thickness demonstrates that the Moho topography does not mirror the variation in surface topography in this region. Thus, classical Airy isostatic models cannot explain how the surface topography is supported. New maps showing variation in Poisson's ratio and Moho sharpness together with forward and inverse modelling provide new information about the contrasting properties of the Fennoscandian shield and crust reworked by the Caledonian orogeny. A sharp Moho transition (R > 1) and low value of Vs (3.5 +/- 0.2 km.s-1) are observed beneath the orogen. The shield is characterised by a gradual transition across the Moho (R < 1) and Vs of (3.8 +/- 0.1 km.s-1) which is more typical of average continental crust. These observations are explained by a Fennoscandian shield underplated with a thick layer of high velocity, high density material. It is proposed that this layer has been removed or reworked beneath the orogen.

Constraining the crustal root geometry beneath the Rif Cordillera (North Morocco)

NASA Astrophysics Data System (ADS)

Diaz, Jordi; Gil, Alba; Carbonell, Ramon; Gallart, Josep; Harnafi, Mimoun

2016-04-01

The analyses of wide-angle reflections of controlled source experiments and receiver functions calculated from teleseismic events provide consistent constraints of an over-thickened crust beneath the Rif Cordillera (North Morocco). Regarding active source data, we investigate now offline arrivals of Moho-reflected phases recorded in RIFSIS project to get new estimations of 3D crustal thickness variations beneath North Morocco. Additional constrains on the onshore-offshore transition are derived from onland recording of marine airgun shots from the coeval Gassis-Topomed profiles. A regional crustal thickness map is computed from all these results. In parallel, we use natural seismicity data collected throughout TopoIberia and PICASSO experiments, and from a new RIFSIS deployment, to obtain teleseismic receiver functions and explore the crustal thickness variations with a H-κ grid-search approach. The use of a larger dataset including new stations covering the complex areas beneath the Rif Cordillera allow us to improve the resolution of previous contributions, revealing abrupt crustal changes beneath the region. A gridded surface is built up by interpolating the Moho depths inferred for each seismic station, then compared with the map from controlled source experiments. A remarkably consistent image is observed in both maps, derived from completely independent data and methods. Both approaches document a large modest root, exceeding 50 km depth in the central part of the Rif, in contrast with the rather small topographic elevations. This large crustal thickness, consistent with the available Bouguer anomaly data, favor models proposing that the high velocity slab imaged by seismic tomography beneath the Alboran Sea is still attached to the lithosphere beneath the Rif, hence pulling down the lithosphere and thickening the crust. The thickened area corresponds to a quiet seismic zone located between the western Morocco arcuate seismic zone, the deep seismicity area beneath western Alboran Sea and the superficial seismicity in Alhoceima area. Therefore, the presence of a crustal root seems to play also a major role in the seismicity distribution in northern Morocco.

Ps mantle transition zone imaging beneath the Colorado Rocky Mountains: Evidence for an upwelling hydrous mantle

NASA Astrophysics Data System (ADS)

Zhang, Zhu; Dueker, Kenneth G.; Huang, Hsin-Hua

2018-06-01

We analyze teleseismic P-to-S conversions for high-resolution imaging of the mantle transition zone beneath the Colorado Rocky Mountains using data from a dense PASSCAL seismic broadband deployment. A total of 6,021 P-to-S converted receiver functions are constructed using a multi-channel minimum-phase deconvolution method and migrated using the common converted point technique with the 3-D teleseismic P- and S-wave tomography models of Schmandt and Humphreys (2010). The image finds that the average depths of the 410-km discontinuity (the 410) and 660-km discontinuity (the 660) at 408 ± 1.9 km and 649 ± 1.6 km respectively. The peak-to-peak topography of both discontinuities is 33 km and 27 km respectively. Additionally, prominent negative polarity phases are imaged both above and below the 410. To quantify the mean properties of the low-velocity layers about 410 km, we utilize double gradient layer models parameterization to fit the mean receiver function waveform. This waveform fitting is accomplished as a grid-search using anelastic synthetic seismograms. The best-fitting model reveals that the olivine-wadsleyite phase transformation width is 21 km, which is significantly larger than anhydrous mineral physics prediction (4-10 km) (Smyth and Frost, 2002). The findings of a wide olivine-wadsleyite phase transformation and the negative polarity phases above and below the 410, suggest that the mantle, at least in the 350-450 km depth range, is significantly hydrated. Furthermore, a conspicuous negative polarity phase below the 660 is imaged in high velocity region, we speculate the low velocity layer is due to dehydration flux melting in an area of convective downwelling. Our interpretation of these results, in tandem with the tomographic image of a Farallon slab segment at 800 km beneath the region (Schmandt and Humphreys, 2010), is that hydrous and upwelling mantle contributes to the high-standing Colorado Rocky Mountains.

Lithospheric Structure Beneath the Hangay Dome, Central Mongolia

NASA Astrophysics Data System (ADS)

Stachnik, J. C.; Meltzer, A.; Souza, S.; Munkhuu, U.; Tsaagan, B.; Russo, R. M.

2014-12-01

The Mongolian Plateau is a broad regional uplift positioned between the Siberian Craton to the north and the far northern edge of the India-Asia collision to the south. Within this intracontinental setting of high topography, the Hangay Dome in central Mongolia reaches elevations of 4 km and contains intermittent basaltic magmatism over the last 30 Ma. The relationship between high topography, magmatism, and geodynamic processes remains largely unsolved although processes ranging from lithospheric delamination to mantle plume effects have been proposed. A temporary array of seismic stations was deployed around the Hangay Dome to determine lithospheric structure. Preliminary results are shown from receiver function analysis, ambient noise tomography, and teleseismic P-wave tomography. Crustal thickness measurements from H-k stacking of receiver functions range from 42 km to 57 km across the array, with thicker crust beneath the highest topography. The bulk crustal Vp/Vs ratio ranges from 1.71 to 1.9 with a median value for the array of 1.77, perhaps indicating a variable crustal composition with some regions having a more mafic crust. The stacked receiver functions are also combined with ambient noise phase velocity dispersion measurements in a joint inversion for shear velocity profiles at each station which reveals crustal thickness estimates consistent with the H-k stacks while also determining the shear velocity step at the Moho. Teleseismic P-wave travel time residuals ranging between +/-1 second are inverted for a 3D P-wave velocity model using finite-frequency kernels. Notable features include 1) a low velocity anomaly (-3%) in the upper 200 km beneath the eastern part of the Hangay Dome near the Orkhon River Valley, , 2) a steeply dipping low velocity anomaly to the north of the Hangay Dome, perhaps related to the nearby Baikal Rift, and 3) generally higher velocities in the upper 200 km surrounding the high topography. To first order, the high topography of the Hangay Dome appears to be largely supported by thickened crust. However, lower P-wave velocities in the upper mantle beneath the dome are observed. The relative contributions of crustal thickness and upper mantle structure for support of topography and their relationship to magmatism will be determined with further refinement of the models.

Using a large-n nodal array to search for remote dynamic triggering in a region of induced seismicity in northern Oklahoma.

NASA Astrophysics Data System (ADS)

Peña-Castro, A. F.; Dougherty, S. L.; Harrington, R. M.; Cochran, E. S.

2017-12-01

Oklahoma has recently experienced a large increase in seismicity that has been linked to injection of large volumes of wastewater into deep disposal wells, a by-product of oil and gas production. Recent studies have shown that areas with active fluid injection and induced seismicity, such as Oklahoma, may be susceptible to dynamic triggering during passage of seismic waves from large, remote earthquakes. In spring 2016, the 1833-station LArge-n Seismic Survey in Oklahoma (LASSO) array was deployed for 30 days to examine an area of active seismicity in Gran County, located in northern Oklahoma. Here we use the LASSO array to look for dynamic triggering caused by teleseismic earthquakes with magnitudes between Mw 6-8 that produce Peak-Ground-Velocities (PGVs) exceeding 10 μm/s at the LASSO array, consistent with PGV values seen to have triggered seismicity at other locations. We focus on examining seismicity around the shallow Mw7.8 event in Ecuador on 04/16/2016 which generated the largest PGV at LASSO (250 µm/s). To establish if earthquake rates change during or following the passage of the teleseismic surface waves, we develop a catalog of earthquakes around the time of each teleseismic event. We first create a preliminary catalogue using a Short-Term Average/Long-Term Average (STA/LTA) detection algorithm window spanning +/- 24 hours around each teleseism,requiring detection at a minimum of 110 LASSO stations to identify an event. Next, we enhance the STA/LTA catalog with manual detections for a period of +/- 1.5 hours around the time of the teleseismic P-wave arrival to explore if triggering occurs that is not detected by the automated procedure. All detected events are then located using standard location techniques. Any observed seismicity rate changes following the teleseismic arrivals will be examined compared to the short-term background rates to determine whether they are statistically significant. If triggering is observed, focal mechanisms will be determined to estimate fault plane orientations and resolve triggering stresses on receiver fault planes. Our preliminary results for the Mw 7.8 Ecuador event suggest there may be delayed triggering that starts roughly 4 hours after the teleseismic phase arrivals, with event rates increasing from 0-5 to 15-25 events per hour.

Crustal structure beneath two seismic stations in the Sunda-Banda arc transition zone derived from receiver function analysis

DOE Office of Scientific and Technical Information (OSTI.GOV)

Syuhada, E-mail: hadda9@gmail.com; Research Centre for Physics - Indonesian Institute of Sciences; Hananto, Nugroho D.

2015-04-24

We analyzed receiver functions to estimate the crustal thickness and velocity structure beneath two stations of Geofon (GE) network in the Sunda-Banda arc transition zone. The stations are located in two different tectonic regimes: Sumbawa Island (station PLAI) and Timor Island (station SOEI) representing the oceanic and continental characters, respectively. We analyzed teleseismic events of 80 earthquakes to calculate the receiver functions using the time-domain iterative deconvolution technique. We employed 2D grid search (H-κ) algorithm based on the Moho interaction phases to estimate crustal thickness and Vp/Vs ratio. We also derived the S-wave velocity variation with depth beneath both stationsmore » by inverting the receiver functions. We obtained that beneath station PLAI the crustal thickness is about 27.8 km with Vp/Vs ratio 2.01. As station SOEI is covered by very thick low-velocity sediment causing unstable solution for the inversion, we modified the initial velocity model by adding the sediment thickness estimated using high frequency content of receiver functions in H-κ stacking process. We obtained the crustal thickness is about 37 km with VP/Vs ratio 2.2 beneath station SOEI. We suggest that the high Vp/Vs in station PLAI may indicate the presence of fluid ascending from the subducted plate to the volcanic arc, whereas the high Vp/Vs in station SOEI could be due to the presence of sediment and rich mafic composition in the upper crust and possibly related to the serpentinization process in the lower crust. We also suggest that the difference in velocity models and crustal thicknesses between stations PLAI and SOEI are consistent with their contrasting tectonic environments.« less

Mapping Shear Zones, Faults, and Crustal Deformation Fabric With Receiver Functions

NASA Astrophysics Data System (ADS)

Schulte-Pelkum, V.; Mahan, K. H.

2014-12-01

Dipping faults, shear zones, and pervasive anisotropic crustal fabric due to deformation are all capable of generating strong near-station mode conversions of teleseismic body waves, even for weak (a few percent) velocity anisotropy. These conversions can be found using the receiver function technique. Dipping foliation and dipping isotropic velocity contrasts can occur in isolation or together in deformed crust. Both generate receiver function arrivals that have a characteristic periodicity with azimuth. Different fixed azimuthal phase shifts between radial and tangential component receiver functions distinguish dipping or tilted structure and fabric from horizontal axis anisotropy. We demonstrate a method that uses these characteristics to map geologically relevant information such as strike and depth of foliation of dipping isotropic velocity contrasts and of horizontal symmetry axis anisotropy contrasts. The method uses waveforms without matching them via forward modeling, which makes choices such as slow versus fast axis symmetry and isotropic dip versus anisotropic axis tilt unnecessary. It also does not use shear wave splitting of the converted waves, which is more difficult to isolate. We show results from the continental U.S. and Canada and from the collision zones in the Himalaya and Tibetan Plateau and Taiwan. We discuss interpretation of our results in the light of recent laboratory measurements of deformed crustal rocks and contributions to the seismic signal from individual minerals such as micas, amphiboles, and quartz. Our observations are connected to geological ground truth by using structural maps and sample anisotropy determined using electron backscatter diffraction from exhumed deep crust in the Athabasca granulite province to predict the seismic signal from present-day deep crust. We also discuss the reconciliation of measurements from anisotropic receiver functions, surface waves, and split shear waves.

Comprehensive Analysis of Broadband Seismic Data in Las Vegas Valley

NASA Astrophysics Data System (ADS)

Tkalcic, H.; Rodgers, A.; Snelson, C.; McEwan, D.

2003-12-01

The city of Las Vegas is one of the fastest growing metropolitan areas in the world. Its urban area is located in a relatively broad sedimentary basin in the Basin and Range Province. Acknowledging that Las Vegas of 2003 is drastically different from Las Vegas of a decade ago, our objectives are to understand and predict ground motions and evaluate the effects of possible future earthquakes and nuclear tests at Nevada Test Site (NTS) on buildings in Las Vegas. A model of the basin depth was derived from gravity data in an independent study, while a model of compressional velocity structure of the basin was derived from seismic refraction studies. We are using strong motion accelerometers regional data, as well as newly acquired broadband teleseismic data to evaluate these models, and predict ground motions at the surface. Delay times of about a dozen analyzed teleseismic P-waves show variation of up to 0.5 seconds across relatively short distances (15 km or less), providing some valuable information on basin shape and thickness. Teleseismic P-waves have favorable signal-to-noise for low frequencies (0.1 to 1.0 Hz). This provides complementary site response measurements to those obtained from regional earthquakes and explosions. Our results indicate a clear difference in site response between hard-rock and basin stations, with amplification reaching factor 5 for the basin stations. The measured P and S wave energies for the recorded data also corelate well with the existing basin depth model, providing additional constraint in modeling the basin shape and structure. We use time domain deconvolution receiver functions to constrain the position of basin boundaries and main crustal discontinuities. Finally, we simulate low frequency (f < 1 Hz) theoretical ground motion in Las Vegas Valley by an elastic finite difference code. Preliminary results show that we can predict relative amplification, as well as some of the complexity in the waveforms, even without invoking complex (and computationaly expensive) three-dimensional structural models. This work is in progress.

Seismic tomography of northeastern Tibetan Plateau from body wave arrival times and surface wave dispersion data

NASA Astrophysics Data System (ADS)

Fang, H.; Yao, H.; Zhang, H.

2017-12-01

Reliable crustal and upper mantle structure is important to understand expansion of material from the Tibetan plateau to its northeastern margin. Previous studies have used either ambient noise tomography or body wave traveltime tomography to obtain the crustal velocity models in northeastern Tibetan Plateau. However, clear differences appear in these models obtained using different datasets. Here we propose to jointly invert local and teleseismic body wave arrival times and surface wave dispersion data from ambient noise cross correlation to obtain a unified P and S wavespeed model of the crust and upper mantle in NE Tibetan Plateau. Following Fang et al. (2016), we adopt the direct inversion strategy for surface wave data (Fang et al., 2015), which eliminates the need to construct the phase/group velocity maps and allows the straightforward incorporation of surface wave dispersion data into the body wave inversion framework. For body wave data including both local and teleseismic arrival times, we use the fast marching method (Rawlinson et al., 2004) in order to trace multiple seismic phases simultaneously. The joint inversion method takes advantage of the complementary strengths of different data types, with local body wave data constraining more on the P than S wavespeed in the crust, surface wave data most sensitive to S wavespeed in the crust and upper mantle, teleseismic body wave data resolving the upper mantle structure. A series of synthetic tests will be used to show the robustness and superiority of the joint inversion method. Besides, the inverted model will be validated by waveform simulation and comparison with other studies, like receiver function imaging. The resultant P and S wavespeed models, as well as the derived Vp/Vs model, will be essential to understand the regional tectonics of the northeastern Tibetan Plateau, and to address the related geodynamic questions of the Tibetan Plateau formation and expansion.

Resolution and Trade-offs in Finite Fault Inversions for Large Earthquakes Using Teleseismic Signals (Invited)

NASA Astrophysics Data System (ADS)

Lay, T.; Ammon, C. J.

2010-12-01

An unusually large number of widely distributed great earthquakes have occurred in the past six years, with extensive data sets of teleseismic broadband seismic recordings being available in near-real time for each event. Numerous research groups have implemented finite-fault inversions that utilize the rapidly accessible teleseismic recordings, and slip models are regularly determined and posted on websites for all major events. The source inversion validation project has already demonstrated that for events of all sizes there is often significant variability in models for a given earthquake. Some of these differences can be attributed to variations in data sets and procedures used for including signals with very different bandwidth and signal characteristics into joint inversions. Some differences can also be attributed to choice of velocity structure and data weighting. However, our experience is that some of the primary causes of solution variability involve rupture model parameterization and imposed kinematic constraints such as rupture velocity and subfault source time function description. In some cases it is viable to rapidly perform separate procedures such as teleseismic array back-projection or surface wave directivity analysis to reduce the uncertainties associated with rupture velocity, and it is possible to explore a range of subfault source parameterizations to place some constraints on which model features are robust. In general, many such tests are performed, but not fully described, with single model solutions being posted or published, with limited insight into solution confidence being conveyed. Using signals from recent great earthquakes in the Kuril Islands, Solomon Islands, Peru, Chile and Samoa, we explore issues of uncertainty and robustness of solutions that can be rapidly obtained by inversion of teleseismic signals. Formalizing uncertainty estimates remains a formidable undertaking and some aspects of that challenge will be addressed.

Rupture history of the 2008 Mw 7.9 Wenchuan, China, earthquake: Evaluation of separate and joint inversions of geodetic, teleseismic, and strong-motion data

USGS Publications Warehouse

Hartzell, Stephen; Mendoza, Carlos; Ramírez-Guzmán, Leonardo; Zeng, Yuesha; Mooney, Walter

2013-01-01

An extensive data set of teleseismic and strong-motion waveforms and geodetic offsets is used to study the rupture history of the 2008 Wenchuan, China, earthquake. A linear multiple-time-window approach is used to parameterize the rupture. Because of the complexity of the Wenchuan faulting, three separate planes are used to represent the rupturing surfaces. This earthquake clearly demonstrates the strengths and limitations of geodetic, teleseismic, and strong-motion data sets. Geodetic data (static offsets) are valuable for determining the distribution of shallower slip but are insensitive to deeper faulting and reveal nothing about the timing of slip. Teleseismic data in the distance range 30°–90° generally involve no modeling difficulties because of simple ray paths and can distinguish shallow from deep slip. Teleseismic data, however, cannot distinguish between different slip scenarios when multiple fault planes are involved because steep takeoff angles lead to ambiguity in timing. Local strong-motion data, on the other hand, are ideal for determining the direction of rupture from directivity but can easily be over modeled with inaccurate Green’s functions, leading to misinterpretation of the slip distribution. We show that all three data sets are required to give an accurate description of the Wenchuan rupture. The moment is estimated to be approximately 1.0 × 1021 N · m with the slip characterized by multiple large patches with slips up to 10 m. Rupture initiates on the southern end of the Pengguan fault and proceeds unilaterally to the northeast. Upon reaching the cross-cutting Xiaoyudong fault, rupture of the adjacent Beichuan fault starts at this juncture and proceeds bilaterally to the northeast and southwest.

Seismic Velocity Assessment In The Kachchh Region, India, From Multiple Waveform Functionals

NASA Astrophysics Data System (ADS)

Ghosh, R.; Sen, M. K.; Mandal, P.; Pulliam, J.; Agrawal, M.

2014-12-01

The primary goal of this study is to estimate well constrained crust and upper mantle seismic velocity structure in the Kachchh region of Gujarat, India - an area of active interest for earthquake monitoring purposes. Several models based on 'stand-alone' surface wave dispersion and receiver function modeling exist in this area. Here we jointly model the receiver function, surface wave dispersion and, S and shear-coupled PL wavetrains using broadband seismograms of deep (150-700 km), moderate to-large magnitude (5.5-6.8) earthquakes recorded teleseismically at semi-permanent seismograph stations in the Kachchh region, Gujarat, India. While surface wave dispersion and receiver function modeling is computationally fast, full waveform modeling makes use of reflectivity synthetic seismograms. An objective function that measures misfit between all three data is minimized using a very fast simulated annealing (VFSA) approach. Surface wave and receiver function data help reduce the model search space which is explored extensively for detailed waveform fitting. Our estimated crustal and lithospheric thicknesses in this region vary from 32 to 41 km and 70 to 80 km, respectively, while crustal P and S velocities from surface to Moho discontinuity vary from 4.7 to 7.0 km/s and 2.7 to 4.1 km/s, respectively. Our modeling clearly reveals a zone of crustal as well as an asthenospheric upwarping underlying the Kachchh rift zone relative to the surrounding unrifted area. We believe that this feature plays a key role in the seismogenesis of lower crustal earthquakes occurring in the region through the emanation of volatile CO2 into the hypocentral zones liberating from the crystallization of carbonatite melts in the asthenosphere. Such a crust-mantle structure might be related to the plume-lithosphere interaction during the Deccan/Reunion plume episode (~65 Ma).

A study of upper mantle discontinuities beneath the Korean Peninsula using teleseismic receiver functions

NASA Astrophysics Data System (ADS)

Lee, S.; Park, Y.; Kim, K.; Rhie, J.

2010-12-01

The study on the topography of the upper mantle discontinuities helps us to understand the complex interactions between the subducting slabs and upper mantle discontinuities. To investigate the depth variation of the upper mantle discontinuities beneath the Korean Peninsula and surrounding regions, we applied the common conversion point stacking of the P-to-s receiver functions. The broadband seismic networks in South Korea and Japan were used to produce the high-resolution receiver function images of the region. The 410- and 660-km discontinuities (hereafter referred to as the 410 and the 660) are clearly imaged and their depth variations show interesting features, especially for the 660. In this region, the subducting Pacific slab bends to flatten over the 660 and several tomographic images indicate that the stagnant slab is extending to the west under China. If the depth of the 660 is affected by the temperature, the broad depression of the 660 is expected and several SS precursor studies support this idea. However, our observation shows that the 660 is locally depressed and its pattern is spatially changing. While the depressed 660 due to the Pacific slab is clearly imaged at lower latitudes (< 37°N), there is no evidence of the depressed 660 to the north. It indicates that the effect of the Pacific slab on the depth variation of the 660 is changing significantly in our study area.

Receiver Function Analyses of Uturuncu Volcano, Bolivia and Lastarria/Cordon Del Azufre Volcanoes, Chile

NASA Astrophysics Data System (ADS)

Mcfarlin, H. L.; Christensen, D. H.; Thompson, G.; McNutt, S. R.; Ryan, J. C.; Ward, K. M.; Zandt, G.; West, M. E.

2014-12-01

Uturuncu Volcano and a zone between Lastarria and Cordon del Azufre Volcanoes (also calledLazufre), have seen much attention lately because of significant and rapid inflation of one to twocentimeters per year over large areas. Uturuncu is located near the Bolivian-Chilean border, andLazufre is located near the Chilean-Argentine border. The PLUTONS Project deployed 28broadband seismic stations around Uturuncu Volcano, from April 2009 to Octobor 2012, and alsodeployed 9 stations around Lastarria and Cordon del Azufre volcanoes, from November, 2011 toApril 2013. Teleseismic receiver functions were generated using the time-domain iterativedeconvolution algorithm of Ligorria and Ammon (1999) for each volcanic area. These receiverfunctions were used to better constrain the depths of magma bodies under Uturuncu and Lazufre,as well as the ultra low velocity layer within the Altiplano-Puna Magma Body (APMB). Thelow velocity zone under Uturuncu is shown to have a top around 10 km depth b.s.l and isgenerally around 20 km thick with regional variations. Tomographic inversion shows a well resolved,near vertical, high Vp/Vs anomaly directly beneath Uturuncu that correlates well with adisruption in the receiver function results; which is inferred to be a magmatic intrusion causing alocal thickening of the APMB. Preliminary results at Lazufre show the top of a low velocityzone around 5-10 km b.s.l with a thickness of 15-30 km.

Evidence for the contemporary magmatic system beneath Long Valley Caldera from local earthquake tomography and receiver function analysis

USGS Publications Warehouse

Seccia, D.; Chiarabba, C.; De Gori, P.; Bianchi, I.; Hill, D.P.

2011-01-01

We present a new P wave and S wave velocity model for the upper crust beneath Long Valley Caldera obtained using local earthquake tomography and receiver function analysis. We computed the tomographic model using both a graded inversion scheme and a traditional approach. We complement the tomographic I/P model with a teleseismic receiver function model based on data from broadband seismic stations (MLAC and MKV) located on the SE and SW margins of the resurgent dome inside the caldera. The inversions resolve (1) a shallow, high-velocity P wave anomaly associated with the structural uplift of a resurgent dome; (2) an elongated, WNW striking low-velocity anomaly (8%–10 % reduction in I/P) at a depth of 6 km (4 km below mean sea level) beneath the southern section of the resurgent dome; and (3) a broad, low-velocity volume (–5% reduction in I/P and as much as 40% reduction in I/S) in the depth interval 8–14 km (6–12 km below mean sea level) beneath the central section of the caldera. The two low-velocity volumes partially overlap the geodetically inferred inflation sources that drove uplift of the resurgent dome associated with caldera unrest between 1980 and 2000, and they likely reflect the ascent path for magma or magmatic fluids into the upper crust beneath the caldera.

Kinematic Modeling of the 2011, Mw7.1 Van, Eastern Turkey Earthquake Using Teleseismic, Regional and GPS Data

NASA Astrophysics Data System (ADS)

Konca, A.

2013-12-01

A kinematic model for the Mw7.1 2011 Van Earthquake was obtained using regional, teleseismic and GPS data. One issue regarding regional data is that 1D Green's functions may not be appropriate due to complications in the upper mantle and crust that affects the Pnl waveforms. In order to resolve whether the 1D Green's function is appropriate, an aftershock of the main event was also modeled, which is then used as a criterion in the selection of the regional stations. The GPS data itself is not sufficient to obtain a slip model, but helps constrain the slip distribution. The slip distribution is up-dip and bilateral with more slip toward west, where the maximum slip reaches 4 meters. The rupture velocity is about 1.5 km/s.

Migration of scattered teleseismic body waves

NASA Astrophysics Data System (ADS)

Bostock, M. G.; Rondenay, S.

1999-06-01

The retrieval of near-receiver mantle structure from scattered waves associated with teleseismic P and S and recorded on three-component, linear seismic arrays is considered in the context of inverse scattering theory. A Ray + Born formulation is proposed which admits linearization of the forward problem and economy in the computation of the elastic wave Green's function. The high-frequency approximation further simplifies the problem by enabling (1) the use of an earth-flattened, 1-D reference model, (2) a reduction in computations to 2-D through the assumption of 2.5-D experimental geometry, and (3) band-diagonalization of the Hessian matrix in the inverse formulation. The final expressions are in a form reminiscent of the classical diffraction stack of seismic migration. Implementation of this procedure demands an accurate estimate of the scattered wave contribution to the impulse response, and thus requires the removal of both the reference wavefield and the source time signature from the raw record sections. An approximate separation of direct and scattered waves is achieved through application of the inverse free-surface transfer operator to individual station records and a Karhunen-Loeve transform to the resulting record sections. This procedure takes the full displacement field to a wave vector space wherein the first principal component of the incident wave-type section is identified with the direct wave and is used as an estimate of the source time function. The scattered displacement field is reconstituted from the remaining principal components using the forward free-surface transfer operator, and may be reduced to a scattering impulse response upon deconvolution of the source estimate. An example employing pseudo-spectral synthetic seismograms demonstrates an application of the methodology.

Three-dimensional full waveform inversion of short-period teleseismic wavefields based upon the SEM-DSM hybrid method

NASA Astrophysics Data System (ADS)

Monteiller, Vadim; Chevrot, Sébastien; Komatitsch, Dimitri; Wang, Yi

2015-08-01

We present a method for high-resolution imaging of lithospheric structures based on full waveform inversion of teleseismic waveforms. We model the propagation of seismic waves using our recently developed direct solution method/spectral-element method hybrid technique, which allows us to simulate the propagation of short-period teleseismic waves through a regional 3-D model. We implement an iterative quasi-Newton method based upon the L-BFGS algorithm, where the gradient of the misfit function is computed using the adjoint-state method. Compared to gradient or conjugate-gradient methods, the L-BFGS algorithm has a much faster convergence rate. We illustrate the potential of this method on a synthetic test case that consists of a crustal model with a crustal discontinuity at 25 km depth and a sharp Moho jump. This model contains short- and long-wavelength heterogeneities along the lateral and vertical directions. The iterative inversion starts from a smooth 1-D model derived from the IASP91 reference Earth model. We invert both radial and vertical component waveforms, starting from long-period signals filtered at 10 s and gradually decreasing the cut-off period down to 1.25 s. This multiscale algorithm quickly converges towards a model that is very close to the true model, in contrast to inversions involving short-period waveforms only, which always get trapped into a local minimum of the cost function.

Exploring Moho sharpness in Northeastern North China Craton with frequency-dependence analysis of Ps receiver function

NASA Astrophysics Data System (ADS)

Zhang, P.; Yao, H.; Chen, L.; WANG, X.; Fang, L.

2017-12-01

The North China Craton (NCC), one of the oldest cratons in the world, has attracted wide attention in Earth Science for decades because of the unusual Mesozoic destruction of its cratonic lithosphere. Understanding the deep processes and mechanism of this craton destruction demands detailed knowledge about the deep structure of this region. In this study, we calculate P-wave receiver functions (RFs) with two-year teleseismic records from the North China Seismic Array ( 200 stations) deployed in the northeastern NCC. We observe both diffused and concentered PpPs signals from the Moho in RF waveforms, which indicates heterogeneous Moho sharpness variations in the study region. Synthetic Ps phases generated from broad positive velocity gradients at the depth of the Moho (referred as Pms) show a clear frequency dependence nature, which in turn is required to constrain the sharpness of the velocity gradient. Practically, characterizing such a frequency dependence feature in real data is challenging, because of low signal-to-noise ratio, contaminations by multiples generated from shallow structure, distorted signal stacking especially in double-peak Pms signals, etc. We attempt to address these issues by, firstly, utilizing a high-resolution Moho depth model of this region to predict theoretical delay times of Pms that facilitate more accurate Pms identifications. The Moho depth model is derived by wave-equation based poststack depth migration on both Ps phase and surface-reflected multiples in RFs in our previous study (Zhang et al., submitted to JGR). Second, we select data from a major back azimuth range of 100° - 220° that includes 70% teleseismic events due to the uneven data coverage and to avoid azimuthal influence as well. Finally, we apply an adaptive cross-correlation stacking of Pms signals in RFs for each station within different frequency bands. High-quality Pms signals at different frequencies will be selected after careful visual inspection and adaptive cross-correlation stacking. At last, we will model the stacked Pms signals within different frequency bands to obtain the final sharpness of crust-mantle boundary, which may shed new lights on understanding the mechanism of cratonic reactivation and destruction in the NCC.

The effects of core-reflected waves on finite fault inversion with teleseismic body wave data

NASA Astrophysics Data System (ADS)

Qian, Y.; Ni, S.; Wei, S.

2016-12-01

Reliable estimation of rupture processes for a large earthquake is valuable for post-seismic rescue, tsunami alert, seismotectonic studies, as well as earthquake physics. Finite-fault inversion has been widely accepted to reconstruct the spatial-temporal distribution of rupture processes, which can be obtained by individual or jointly inversion of seismic, geodetic and tsunami data sets. Among the above observations, teleseismic (30° 90°) body waves, usually P and SH waves, have been used extensively in such inversions because their propagation are well understood and readily available for large earthquakes with good coverages of slowness and azimuth. However, finite fault inversion methods usually assume turning P and SH waves without inclusion of core-reflected waves when calculating the synthetic waveforms, which may result in systematic error in finite-fault inversions. For the core-reflected SH wave ScS, it is expected to be strong due to total reflection from Core-Mantle-Boundary. Moreover, the time interval between direct S and ScS could be smaller than the duration of large earthquakes for large epicentral distances. In order to improve the accuracy of finite fault inversion with teleseismic body waves, we develop a procedure named multitel3 to compute Greens' functions that contain both turning waves (P, pP, sP, S, sS et al.) and core-reflected phases (PcP and ScS) and apply it to finite fault inversions. This ray-based method can rapidly calculate teleseismic body wave synthetics with flexibility for path calibration of 3D mantle structure. The new Green's function is plugged into finite fault inversion package to replace the original Green's function with only turning P and SH waves. With the 2008 Mw7.9 Wenchuan earthquake as example, a series of numerical tests conducted on synthetic data are used to assess the performance of our approach. We also explore this new procedure's stability when there are discrepancies between the parameters of input model and the priori information of inverse model, such as strike, dip of finite fault and so on. With the quantified code, we apply it to study rupture process of the 2016 Mw7.8 Sumatra earthquake.

Crustal anisotropy across northern Japan from receiver functions.

PubMed

Bianchi, I; Bokelmann, G; Shiomi, K

2015-07-01

Northern Japan is a tectonically active area, with the presence of several volcanoes, and with frequent earthquakes among which the destructive M w  = 8.9-9.0 Tohoku-oki occurred on 11 March 2011. Tectonic activity leaves an imprint on the crustal structures, on both the upper and the lower layers. To investigate the crust in northern Japan, we construct a receiver function data set using teleseismic events recorded at 58 seismic stations belonging to the Japanese National (Hi-net) network. We isolate the signals, in the receiver function wavelet, that witness the presence of anisotropic structures at depth, with the aim of mapping the variation of anisotropy across the northern part of the island. This study focuses on the relation among anisotropy detected in the crust, stresses induced by plate convergence across the subduction zone, and the intrinsic characteristics of the rocks. Our results show how a simple velocity model with two anisotropic layers reproduces the observed data at the stations. We observe a negligible or small amount of signal related to anisotropy in the eastern part of the study area (i.e., the outer arc) for both upper and lower crust. Distinct anisotropic features are observed at the stations on the western part of the study area (i.e., the inner arc) for both upper and lower crust. The symmetry axes are mostly E-W oriented. Deviation from the E-W orientation is observed close to the volcanic areas, where the higher geothermal gradient might influence the deformation processes.

Lithospheric Velocity Structure of the Anatolain plateau-Caucasus-Caspian Regions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gok, R; Mellors, R J; Sandvol, E

Anatolian Plateau-Caucasus-Caspian region is an area of complex structure accompanied by large variations in seismic wave velocities. Despite the complexity of the region little is known about the detailed lithospheric structure. Using data from 29 new broadband seismic stations in the region, a unified velocity structure is developed using teleseismic receiver functions and surface waves. Love and Rayleigh surface waves dispersion curves have been derived from event-based analysis and ambient-noise correlation. We jointly inverted the receiver functions with the surface wave dispersion curves to determine absolute shear wave velocity and important discontinuities such as sedimentary layer, Moho, lithospheric-asthenospheric boundary. Wemore » combined these new station results with Eastern Turkey Seismic Experiment results (29 stations). Caspian Sea and Kura basin underlained by one of the thickest sediments in the world. Therefore, short-period surface waves are observed to be very slow. The strong crustal multiples in receiver functions and the slow velocities in upper crust indicate the presence of thick sedimentary unit (up to 20 km). Crustal thickness varies from 34 to 52 km in the region. The thickest crust is in Lesser Caucasus and the thinnest is in the Arabian Plate. The lithospheric mantle in the Greater Caucasus and the Kura depression is faster than the Anatolian Plateau and Lesser Caucasus. This possibly indicates the presence of cold lithosphere. The lower crust is slowest in the northeastern part of the Anatolian Plateau where Holocene volcanoes are located.« less

Flexible kinematic earthquake rupture inversion of tele-seismic waveforms: Application to the 2013 Balochistan, Pakistan earthquake

NASA Astrophysics Data System (ADS)

Shimizu, K.; Yagi, Y.; Okuwaki, R.; Kasahara, A.

2017-12-01

The kinematic earthquake rupture models are useful to derive statistics and scaling properties of the large and great earthquakes. However, the kinematic rupture models for the same earthquake are often different from one another. Such sensitivity of the modeling prevents us to understand the statistics and scaling properties of the earthquakes. Yagi and Fukahata (2011) introduces the uncertainty of Green's function into the tele-seismic waveform inversion, and shows that the stable spatiotemporal distribution of slip-rate can be obtained by using an empirical Bayesian scheme. One of the unsolved problems in the inversion rises from the modeling error originated from an uncertainty of a fault-model setting. Green's function near the nodal plane of focal mechanism is known to be sensitive to the slight change of the assumed fault geometry, and thus the spatiotemporal distribution of slip-rate should be distorted by the modeling error originated from the uncertainty of the fault model. We propose a new method accounting for the complexity in the fault geometry by additionally solving the focal mechanism on each space knot. Since a solution of finite source inversion gets unstable with an increasing of flexibility of the model, we try to estimate a stable spatiotemporal distribution of focal mechanism in the framework of Yagi and Fukahata (2011). We applied the proposed method to the 52 tele-seismic P-waveforms of the 2013 Balochistan, Pakistan earthquake. The inverted-potency distribution shows unilateral rupture propagation toward southwest of the epicenter, and the spatial variation of the focal mechanisms shares the same pattern as the fault-curvature along the tectonic fabric. On the other hand, the broad pattern of rupture process, including the direction of rupture propagation, cannot be reproduced by an inversion analysis under the assumption that the faulting occurred on a single flat plane. These results show that the modeling error caused by simplifying the fault model is non-negligible in the tele-seismic waveform inversion of the 2013 Balochistan, Pakistan earthquake.

Test of high-resolution 3D P-wave velocity model of Poland by back-azimuthal sections of teleseismic receiver function

NASA Astrophysics Data System (ADS)

Wilde-Piorko, Monika; Polkowski, Marcin; Grad, Marek

2015-04-01

Geological and seismic structure under area of Poland is well studied by over one hundred thousand boreholes, over thirty deep seismic refraction and wide angle reflection profiles and by vertical seismic profiling, magnetic, gravity, magnetotelluric and thermal methods. Compilation of these studies allowed to create a high-resolution 3D P-wave velocity model down to 60 km depth in the area of Poland (Polkowski et al. 2014). Model also provides details about the geometry of main layers of sediments (Tertiary and Quaternary, Cretaceous, Jurassic, Triassic, Permian, old Paleozoic), consolidated/crystalline crust (upper, middle and lower) and uppermost mantle. This model gives an unique opportunity for calculation synthetic receiver function and compering it with observed receiver function calculated for permanent and temporary seismic stations. Modified ray-tracing method (Langston, 1977) can be used directly to calculate the response of the structure with dipping interfaces to the incoming plane wave with fixed slowness and back-azimuth. So, 3D P-wave velocity model has been interpolated to 2.5D P-wave velocity model beneath each seismic station and back-azimuthal sections of components of receiver function have been calculated. Vp/Vs ratio is assumed to be 1.8, 1.67, 1.73, 1.77 and 1.8 in the sediments, upper/middle/lower consolidated/crystalline crust and uppermost mantle, respectively. Densities were calculated with combined formulas of Berteussen (1977) and Gardner et al. (1974). Additionally, to test a visibility of the lithosphere-asthenosphere boundary phases at receiver function sections models have been extended to 250 km depth based on P4-mantle model (Wilde-Piórko et al., 2010). National Science Centre Poland provided financial support for this work by NCN grant DEC-2011/02/A/ST10/00284 and by NCN grant UMO-2011/01/B/ST10/06653.

Kinematic Slip Model for 12 May 2008 Wenchuan-Beichuan Mw 7.9 Earthquake from Joint Inversion of ALOS, Envisat, and Teleseismic Data

NASA Technical Reports Server (NTRS)

Fielding, Eric; Sladen, Anthony; Avouac, Jean-Philippe; Li, Zhenhong; Ryder, Isabelle; Burgmann, Roland

2008-01-01

The presentations explores kinematics of the Wenchaun-Beichuan earthquake using data from ALOS, Envisat, and teleseismic recordings. Topics include geomorphic mapping, ALOS PALSAR range offsets, ALOS PALSAR interferometry, Envisat IM interferometry, Envisat ScanSAR, Joint GPS-InSAR inversion, and joint GPS-teleseismic inversion (static and kinematic).

Source process of the Sikkim earthquake 18th September, 2011, inferred from teleseismic body-wave inversion.

NASA Astrophysics Data System (ADS)

Earnest, A.; Sunil, T. C.

2014-12-01

The recent earthquake of Mw 6.9 occurred on September 18, 2011 in Sikkim-Nepal border region. The hypocenter parameters determined by the Indian Meteorological Department shows that the epicentre is at 27.7°N, 88.2°E and focal depth of 58Km, located closed to the north-western terminus of Tista lineament. The reported aftershocks are linearly distributed in between Tista and Golapara lineament. The microscopic and geomorphologic studies infer a dextral strike-slip faulting, possibly along a NW-SE oriented fault. Landslides caused by this earthquake are distributed along Tista lineament . On the basis of the aftershock distribution, Kumar et al. (2012), have suggested possible NW orientation of the causative fault plane. The epicentral region of Sikkim bordered by Nepal, Bhutan and Tibet, comprises a segment of relatively lower level seismicity in the 2500km stretch of the active Himalayan Belt. The north Sikkim earthquake was felt in most parts of Sikkim and eastern Nepal; it killed more than 100 people and caused damage to buildings, roads and communication infrastructure. Through this study we focus on the earthquake source parameters and the kinematic rupture process of this particular event. We used tele-seismic body waveformsto determine the rupture pattern of earthquake. The seismic-rupture pattern are generally complex, and the result could be interpreted in terms of a distribution of asperities and barriers on the particular fault plane (Kikuchi and Kanamori, 1991).The methodology we adopted is based on the teleseismic body wave inversion methodology by Kikuchi and Kanamori (1982, 1986 and 1991). We used tele-seismic P-wave records observed at teleseismic distances between 50° and 90° with a good signal to noise ratio. Teleseismic distances in the range between 50° and 90° were used, in order to avoid upper mantle and core triplications and to limit the path length within the crust. Synthetic waveforms were generated gives a better fit with triangular source time function duration, in order to determine the components of the moment tensor and the focal depth of the main shock. we will discussing the average stress drop and the possible mechanisms on the depth of the event at a region well known for events beyond moho transsion zone.

Seismic Migration Imaging of the Crust and Upper Mantle Discontinuity Structure beneath Southern Taiwan

NASA Astrophysics Data System (ADS)

Liu, Y.-S.; Kuo, B.-Y.

2009-04-01

Taiwan is located in the convergent plate boundary zone where the Philippine Sea plate has obliquely collided on the Asian continental margin, initiating the arc-continent collision and subsequent mountain-building in Taiwan. Receiver function has been a powerful tool to image seismic velocity discontinuity structure in the crust and upper mantle which can help illuminate the deep dynamic process of active Taiwan orogeny. In this study, we adopt backprojection migration processing of teleseismic receiver functions to investigate the crust and upper mantle discontinuities beneath southern Taiwan, using the data from Southern Taiwan Transect Seismic Array (STTA), broadband stations of Central Weather Bureau (CWB), Broadband Array in Taiwan for Seismology (BATS), and Taiwan Integrated Geodynamics Research (TAIGER). This composite east-west trending linear array has the aperture of about 150 km with the station spacing of ~5-10 km. Superior to the common midpoint (CMP) stack approach, the migration can properly image the dipping, curved, or laterally-varying topography of discontinuous interfaces which very likely exist under the complicated tectonic setting of Taiwan. We first conduct synthetic experiments to test the depth and lateral resolution of migration images based on the WKBJ synthetic waveforms calculated from available source and receiver distributions. We will next construct the 2-D migration image under the array to reveal the topographic variation of the Moho and lithosphere discontinuities beneath southern Taiwan.

Assessment of teleseismically-determined source parameters for the April 25, 2015 MW 7.9 Gorkha, Nepal earthquake and the May 12, 2015 MW 7.2 aftershock

NASA Astrophysics Data System (ADS)

Lay, Thorne; Ye, Lingling; Koper, Keith D.; Kanamori, Hiroo

2017-09-01

On April 25, 2015 a major (MW 7.9) thrust earthquake ruptured the deeper portion of the seismogenic plate boundary beneath Nepal along which India is underthrusting Eurasia. An MW 7.2 aftershock on May 12, 2015 extended the eastern, down-dip edge of the rupture. These destructive events caused about 9000 fatalities and 23,000 injuries. The overall rupture zone is about 170 km long and 40-80 km wide. This region of the plate boundary previously experienced a large earthquake in 1833, and in 1934 a larger MS 8.0 event located to the east ruptured all the way to the surface. The Main Himalayan Thrust (MHT) on which slip occurred in 2015 has a very low dip angle of 6°, and the depth of the mainshock slip distribution is very shallow, extending from 7 to 18 km. The shallow dip and depth present challenges for resolving faulting characteristics using teleseismic data. We analyze global teleseismic signals for the mainshock and aftershock to estimate source parameters, evaluating the stability of various procedures used for remotely characterizing kinematics of such shallow faulting. Back-projection and finite-fault slip inversion are used to assess the spatio-temporal rupture history and evidence for frequency-dependent radiation along dip. Slip zone width constraints from near-field geodetic observations are imposed on the preferred models to overcome some limitations of purely teleseismic methods. Radiated energy, stress drop and moment rate functions are determined for both events.

Automatic picker of P & S first arrivals and robust event locator

NASA Astrophysics Data System (ADS)

Pinsky, V.; Polozov, A.; Hofstetter, A.

2003-12-01

We report on further development of automatic all distances location procedure designed for a regional network. The procedure generalizes the previous "loca l" (R < 500 km) and "regional" (500 < R < 2000 km) routines and comprises: a) preliminary data processing (filtering and de-spiking), b) phase identificatio n, c) P, S first arrival picking, d) preliminary location and e) robust grid-search optimization procedure. Innovations concern phase identification, automa tic picking and teleseismic location. A platform free flexible Java interface was recently created, allowing easy parameter tuning and on/off switching to t he full-scale manual picking mode. Identification of the regional P and S phase is provided by choosing between the two largest peaks in the envelope curve. For automatic on-time estimation we utilize now ratio of two STAs, calculated in two consecutive and equal time windows (instead of previously used Akike Information Criterion). "Teleseismic " location is split in two stages: preliminary and final one. The preliminary part estimates azimuth and apparent velocity by fitting a plane wave to the P automatic pickings. The apparent velocity criterion is used to decide about strategy of the following computations: teleseismic or regional. The preliminary estimates of azimuth and apparent velocity provide starting value for the final teleseismic and regional location. Apparent velocity is used to get first a pproximation distance to the source on the basis of the P, Pn, Pg travel-timetables. The distance estimate together with the preliminary azimuth estimate provides first approximations of the source latitude and longitude via sine and cosine theorems formulated for the spherical triangle. Final location is based on robust grid-search optimization procedure, weighting the number of pickings that simultaneously fit the model travel times. The grid covers initial location and becomes finer while approaching true hypocenter. The target function is a sum of the bell-shaped characteristic functions, used to emphasize true pickings and eliminate outliers. The final solution is a grid point that provides maximum to the target function. The procedure was applied to a list of ML > 4 earthquakes recorded by the Israel Seismic Network (ISN) in the 1999-2002 time period. Most of them are badly constrained relative the network. However, the results of location with average normalized error relative bulletin solutions e=dr/R of 5% were obtained, in each of the distance ranges. The first version of the procedure was incorporated in the national Early Warning System in 2001. Recently, we started to send automatic Early Warn ing reports, to the EMSC Real Time Bulletin. Initially reported some teleseismic location discrepancies have been eliminated by introduction of station corrections.

Seismic velocity structure of the crust and upper mantle beneath the Texas-Gulf of Mexico margin from joint inversion of Ps and Sp receiver functions and surface wave dispersion

NASA Astrophysics Data System (ADS)

Agrawal, M.; Pulliam, J.; Sen, M. K.

2013-12-01

The seismic structure beneath Texas Gulf Coast Plain (GCP) is determined via velocity analysis of stacked common conversion point (CCP) Ps and Sp receiver functions and surface wave dispersion. The GCP is a portion of a ocean-continental transition zone, or 'passive margin', where seismic imaging of lithospheric Earth structure via passive seismic techniques has been rare. Seismic data from a temporary array of 22 broadband stations, spaced 16-20 km apart, on a ~380-km-long profile from Matagorda Island, a barrier island in the Gulf of Mexico, to Johnson City, Texas were employed to construct a coherent image of the crust and uppermost mantle. CCP stacking was applied to data from teleseismic earthquakes to enhance the signal-to-noise ratios of converted phases, such as Ps phases. An inaccurate velocity model, used for time-to-depth conversion in CCP stacking, may produce higher errors, especially in a region of substantial lateral velocity variations. An accurate velocity model is therefore essential to constructing high quality depth-domain images. To find accurate velocity P- and S-wave models, we applied a joint modeling approach that searches for best-fitting models via simulated annealing. This joint inversion approach, which we call 'multi objective optimization in seismology' (MOOS), simultaneously models Ps receiver functions, Sp receiver functions and group velocity surface wave dispersion curves after assigning relative weights for each objective function. Weights are computed from the standard deviations of the data. Statistical tools such as the posterior parameter correlation matrix and posterior probability density (PPD) function are used to evaluate the constraints that each data type places on model parameters. They allow us to identify portions of the model that are well or poorly constrained.

Empirical transfer functions: Application to the determination of outermost core velocity structure using teleseismic SmKS phases

NASA Astrophysics Data System (ADS)

Eaton, D. W.; Alexandrakis, C.

2007-05-01

Teleseismic SmKS waves propagate as S-waves in the mantle and compressional (K) waves in the core, with m-1 underside bounce points at the core-mantle boundary. For long-period or broadband recordings at epicentral distances of 115-135°, higher-order SmKS waves (3 ≤ m < ∞) are not often discernible as distinct pulses. Instead, they are typically manifested as a weakly dispersive waveform that lags SKKS by ~ 12-32s. In a ray-theoretical representation of this process, there is a strong geometrical similarity between the coalescence of SmKS turning waves to form a composite arrival and the interference of mantle S waves to form teleseismic Love waves. SmKS waves can thus be viewed as a type of pseudo-interface wave, the dispersive properties of which depend strongly on the fine-scale velocity structure of the outermost core. In order to analyze SmKS arrivals, we have developed an empirical transfer-function (ETF) technique that uses SKKS as a reference phase. An ETF is a wave-shaping filter that transforms the observed SKKS pulse into the observed SmKS pulse. We obtain this filter by windowing the respective pulses and applying frequency- domain Wiener deconvolution. Each ETF contains SmKS-SKKS differential arrival-time, phase-shift and relative-amplitude information; it also implicitly removes the source-time function and instrument response, thus facilitating the merging of results from different stations and events. Here, we apply this approach to global observations of SmKS phases and invert the results to yield a new velocity model for the outermost core region.

Illuminating sesmic discontinuities with receiver functions from a dense array in Mexico City

NASA Astrophysics Data System (ADS)

Perez-Campos, X.; Rodríguez-Domínguez, M. Á.; González-López, A.; Espindola, V. H.; Quintanar, L.; Ramirez-Guzman, L.

2017-12-01

Mexico City, with close to 10 million inhabitants, has grown over a sedimentary basin, from an old dried lake. This has been a big factor in amplifying the seismic waves from large subduction earthquakes, located > 300 km away on the Pacific coast, which represents a significant hazard. For this reason, it is of great interest to improve the knowledge of the seismic structure of the city and its details on spatial variations to reduce the uncertainty in ground motion modeling. In May 2017, such array started its way in Mexico City. It consists of 18 broadband stations, that record in place for 3-5 days, moving then to a new location. In total, the city will be covered with 343 recording sites. In this work, we present preliminary results of receiver functions obtained in such array and in permanent stations of the Seismic Network of the Valley of Mexico. Despite the few teleseismic events, the small spacing between stations ( 500 m) allows identification of converted Ps phases from the Moho discontinuity, as well as other converted phases, which might be related to subtle changes in the vertical and lateral seismic structure. This Project was funded by the Secretaria de Ciencia, Tecnología e Innovación (SECITI) of Mexico City. Project SECITI/073/2016.

Variations in the crustal structure beneath western Turkey

NASA Astrophysics Data System (ADS)

Saunders, Paul; Priestley, Keith; Taymaz, Tuncay

1998-08-01

We use teleseismic receiver functions to investigate the crustal structure at two locations in western Turkey using seismic data recorded on small arrays of temporary broad-band seismographs. The results from these analyses are compared with receiver function results from the GDSN station ANTO on the Anatolian Plateau in central Turkey. The crust is ~ 30 km thick in the region of western Turkey where active normal faulting reveals present-day extension in the upper crust and alkali-basaltic volcanism reveals recent extension within the subcrustal lithosphere The crust is ~ 34 km thick further east where crustal extension is still evident but less pronounced. In the Anatolian Plateau, which is not currently extending, the crust is ~ 38 km thick. The level of extension estimated from these measurements of crustal thickness implies a β -factor of ~ 1.2. This value agrees with the amount of extension estimated in the upper crust from the integrated seismic strain rate (β -factor of ~ 1.3), from surface faulting(β -factor of ~ 1.25) and from the amount of extension in the subcrustal lithosphere estimated from the volcanism (β -factor < 2), all indicating that the extension is approximately uniformly distributed vertically throughout the lithosphere. The Moho transition in this region appears to thin slightly as the degree of extension increases westwards.

Fine crustal and uppermost mantle S-wave velocity structure beneath the Tengchong volcanic area inferred from receiver function and surface-wave dispersion: constraints on magma chamber distribution

NASA Astrophysics Data System (ADS)

Li, Mengkui; Zhang, Shuangxi; Wu, Tengfei; Hua, Yujin; Zhang, Bo

2018-03-01

The Tengchong volcanic area is located in the southeastern margin of the collision zone between the Indian and Eurasian Plates. It is one of the youngest intraplate volcano groups in mainland China. Imaging the S-wave velocity structure of the crustal and uppermost mantle beneath the Tengchong volcanic area is an important means of improving our understanding of its volcanic activity and seismicity. In this study, we analyze teleseismic data from nine broadband seismic stations in the Tengchong Earthquake Monitoring Network. We then image the crustal and uppermost mantle S-wave velocity structure by joint analysis of receiver functions and surface-wave dispersion. The results reveal widely distributed low-velocity zones. We find four possible magma chambers in the upper-to-middle crust and one in the uppermost mantle. The chamber in the uppermost mantle locates in the depth range from 55 to 70 km. The four magma chambers in the crust occur at different depths, ranging from the depth of 7 to 25 km in general. They may be the heat sources for the high geothermal activity at the surface. Based on the fine crustal and uppermost mantle S-wave velocity structure, we propose a model for the distribution of the magma chambers.

Three-Dimensional Passive-Source Reverse-Time Migration of Converted Waves: The Method

NASA Astrophysics Data System (ADS)

Li, Jiahang; Shen, Yang; Zhang, Wei

2018-02-01

At seismic discontinuities in the crust and mantle, part of the compressional wave energy converts to shear wave, and vice versa. These converted waves have been widely used in receiver function (RF) studies to image discontinuity structures in the Earth. While generally successful, the conventional RF method has its limitations and is suited mostly to flat or gently dipping structures. Among the efforts to overcome the limitations of the conventional RF method is the development of the wave-theory-based, passive-source reverse-time migration (PS-RTM) for imaging complex seismic discontinuities and scatters. To date, PS-RTM has been implemented only in 2D in the Cartesian coordinate for local problems and thus has limited applicability. In this paper, we introduce a 3D PS-RTM approach in the spherical coordinate, which is better suited for regional and global problems. New computational procedures are developed to reduce artifacts and enhance migrated images, including back-propagating the main arrival and the coda containing the converted waves separately, using a modified Helmholtz decomposition operator to separate the P and S modes in the back-propagated wavefields, and applying an imaging condition that maintains a consistent polarity for a given velocity contrast. Our new approach allows us to use migration velocity models with realistic velocity discontinuities, improving accuracy of the migrated images. We present several synthetic experiments to demonstrate the method, using regional and teleseismic sources. The results show that both regional and teleseismic sources can illuminate complex structures and this method is well suited for imaging dipping interfaces and sharp lateral changes in discontinuity structures.

Lithospheric structure below seismic stations in Cuba from the joint inversion of Rayleigh surface waves dispersion and receiver functions

NASA Astrophysics Data System (ADS)

González, O'Leary; Moreno, Bladimir; Romanelli, Fabio; Panza, Giuliano F.

2012-05-01

The joint inversion of Rayleigh wave group velocity dispersion and receiver functions has been used to study the crust and upper mantle structure at eight seismic stations in Cuba. Receiver functions have been computed from teleseismic recordings of earthquakes at epicentral (angular) distances in the range from 30° to 90° and Rayleigh wave group velocity dispersion relations have been taken from earlier surface wave tomographic studies in the Caribbean area. The thickest crust (˜30 km) below Cuban stations is found at Cascorro (CCC) and Maisí (MAS) whereas the thinnest crust (˜18 km) is found at stations Río Carpintero (RCC) and Guantánamo Bay (GTBY), in the southeastern part of Cuba; this result is in agreement with the southward gradual thinning of the crust revealed by previous studies. In the crystalline crust, the S-wave velocity varies between ˜2.8 and ˜3.9 km s-1 and, at the crust-mantle transition zone, the shear wave velocity varies from ˜4.0 and ˜4.3 km s-1. The lithospheric thickness varies from ˜65 km, in the youngest lithosphere, to ˜150 km in the northeastern part of the Cuban island, below Maisí (MAS) and Moa (MOA) stations. Evidence of a subducted slab possibly belonging to the Caribbean plate is present below the stations Las Mercedes (LMG), RCC and GTBY whereas earlier subducted slabs could explain the results obtained below the Soroa (SOR), Manicaragua (MGV) and Cascorro (CCC) station.

Forward Modeling of Receiver Functions to Determine Crustal Structure of the Eastern Limb in TheBushveld Complex, South Africa

NASA Astrophysics Data System (ADS)

Loza, E.; Ramirez, C.; Nyblade, A.; Durrheim, R. J.; Raveloson, A.

2016-12-01

The Bushveld Igneous Complex contains the largest layered mafic intrusion on Earth, about the size of England, and has been exploited for metals such as platinum since the 1950s. Several igneous bodies within and around the complex have been dated from 2.06 Ga, possibly representing a single massive magmatic event. The Rustenburg Layered Suite of the Bushveld Igneous Complex intruded into the Transvaal sedimentary sequence, with associated volcanic rocks of the Rooiberg Group forming the roof and part of the floor. The purpose of this study is to determine whether the Rustenburg Layered Suite is a continuous bowl-shaped formation or if it is made up of two separate dipping sheets that crop out in the western and eastern limbs. If the intrusion is connected at depth, then the Moho (crust-mantle boundary) would most likely be depressed due to the weight of the 7-8km of mafic material injected into the crust. Seismic stations were installed in the eastern and northern Bushveld in 2015 to collect teleseismic data. The use of receiver functions derived from seismic data collected since 2015 has helped determine the subsurface crustal structure of the Bushveld. Receiver functions have been used to trace the contact between the high-density mafic lower zone and the low-density Transvaal sediments. The new data gathered show the Moho boundary at about 47 km, and a 5.0 Gaussian width shows a backswing consistent with a mafic-sedimentary boundary at 8km.

Crustal thickness and vP/vS ratio in Shanxi Graben, China

NASA Astrophysics Data System (ADS)

Shi, Yutao; Gao, Yuan; Jing, Honglin

2014-12-01

Shanxi Graben is in the middle part of the North China Craton, from south to north. With the teleseismic data recorded by Regional Seismograph Networks and the temporary ZBnet-W Seismic Array around east part of Shanxi Graben, we measured the crustal thickness and v P/ v S ratio beneath each station using the H-κ stack of receiver functions. The observed crustal thickness shows obvious lateral variation, increasing gradually from east to west in the Shanxi Graben. Beneath the Shanxi Graben the crust is relatively thicker than both sides of the south and the north. In addition, the v P/ v S ratio in the north of study zone is higher than that in the south. The highest v P/ v S ratio exists in the crust of the Xinding basin and the Datong basin. Our study also suggests that high velocity ratio might result from the strong activities of the magmation and volcanism.

Sp and Ps Receiver Function Imaging of the Cenozoic and Precambrian US

NASA Astrophysics Data System (ADS)

Keenan, James; Thurner, Sally; Levander, Alan

2013-04-01

Using teleseismic USArray data we have made Ps and Sp receiver function common conversion point stacked image volumes that extend from the Pacific coast to approximately the Mississippi River. We have used iterative time-domain deconvolution, water-level frequency-domain deconvolution, and least squares inverse filtering to form receiver functions in various frequency bands (Ps: 1.0 and, 0.5 Hz, Sp: 0.2 and 0.1 Hz). The receiver functions were stacked to give an image volume for each frequency band using a hybrid velocity model made by combining Crust2.0 (Bassin et al., 2000) and finite-frequency P and S wave tomography models (Schmandt and Humphreys, 2010; and Schmandt, unpublished). We contrast the lithospheric and asthenospheric structure of the western U.S., modified by Cenozoic tectonism, with that of the Precambrian central U.S. Here we describe 2 notable features: (1) In the Sp image volumes the upper mantle beneath the western U.S. differs dramatically from that to the east of the Rocky Mountain front. In the western U.S. the lithosphere is either thin, or highly variable in thickness (40-140 km) with neither the lithosphere nor asthenosphere having much internal structure (e.g., Levander and Miller, 2012). In contrast, east of the Rocky Mountain front the lithosphere steadily deepens to > 150 km and shows relatively strong internal layering. Individual positive and negative conversions are coherent over 100's of kilometers, suggesting the thrust stacking model of cratonic formation. (2) Beneath parts of the Archean Wyoming Province (Henstock et al, 1998; Snelson et al., 1998; Gorman et al., 2002; Mahan et al, 2012), much of the Great Plains and part of the Midwest lies a vast variable thickness (up to ~25 km) high velocity crustal layer. This layer lies roughly north of the Grenville Front, underlying much of the Yavapai-Mazatzal Province east of the Rockies, parts of the Superior Province, and possibly parts of the Trans-Hudson province.

Lithosphere and Asthenosphere Structure of the Western Mediterranean and Northwest Africa from Rayleigh Wave tomography and Ps Receiver Functions

NASA Astrophysics Data System (ADS)

Palomeras, Imma; Thurner, Sally; Levander, Alan; Bezada, Maximiliano; Villasenor, Antonio; Humphreys, Eugene; Carbonell, Ramon; Gallart, Josep

2013-04-01

Since Cenozoic times the Western Mediterranean has been affected by complex subduction and slab rollback, during African-European convergence. The deformed region occupies a wide area from the Atlas mountains in northwest Africa to the southern Iberian Massif in Spain. Evolutionary models of the Western Mediterranean invoke extensive slab rollback and compression, as well as likely upper mantle delamination/convective drip scenarios during formation of the Alboran domain, the Betics, Rif, and Atlas Mountains. We report on a multidisciplinary, international investigation of the Alboran System and surrounding areas. In this study we have analyzed teleseismic data from the roughly 240 temporary and permanent broadband seismographs operated in this region by more than a dozen different cooperating research groups. Here we present combined results from Rayleigh wave tomography and Ps receiver functions. Receiver functions were made in 3 frequency bands (2 Hz, 1 Hz, 0.5 Hz) using iterative time-domain and water-level frequency-domain methods. We measured Rayleigh phase velocities using the two-plane-wave method and finite-frequency kernels to remove complications due to multi-pathing and to improve lateral resolution, respectively. The resulting 3D shear velocity model was used to create 3D image volumes of the Ps receiver functions. The RF and tomography images are consistent with one another and withteleseismic body wave tomography (Bezada et al., submitted) Our results show high velocities from ~70 km to 230 km depth in an elliptical area just west of the Gibraltar straits which is interpreted as a near vertical slab beneath the Alboran Domain and the adjacent Spanish continental margin. The surface wave results map out the top of a 600+ km deep nearly vertical slab seen in the P body wave tomography. The RF images suggest that the top of this slab is still attached to the Alboran domain Moho beneath Gibraltar, a complex region where lower crustal velocities (

Receiver function structure beneath a broad-band seismic station in south Sumatra

NASA Astrophysics Data System (ADS)

MacPherson, K. A.; Hidayat, D.; Goh, S.

2010-12-01

We estimated the one-dimensional velocity structure beneath a broad-band station in south Sumatra by the forward modeling and inversion of receiver functions. Station PMBI belongs to the GEOFON seismic network maintained by GFZ-Potsdam, and at a longitude of 104.77° and latitude of -2.93°, sits atop the south Sumatran basin. This station is of interest to researchers at the Earth Observatory of Singapore, as data from it and other stations in Sumatra and Singapore will be incorporated into a regional velocity model for use in seismic hazard analyses. Three-component records from 193 events at teleseismic distances and Mw ≥ 5.0 were examined for this study and 67 records were deemed to have sufficient signal to noise characteristics to be retained for analysis. Observations are primarily from source zones in the Bougainville trench with back-azimuths to the east-south-east, the Japan and Kurile trenches with back-azimuths to the northeast, and a scattering of observations from other azimuths. Due to the level of noise present in even the higher-quality records, the usual frequency-domain deconvolution method of computing receiver functions was ineffective, and a time-domain iterative deconvolution was employed to obtain usable wave forms. Receiver functions with similar back-azimuths were stacked in order to improve their signal to noise ratios. The resulting wave forms are relatively complex, with significant energy being present in the tangential components, indicating heterogeneity in the underlying structure. A dip analysis was undertaken but no clear pattern was observed. However, it is apparent that polarities of the tangential components were generally reversed for records that sample the Sunda trench. Forward modeling of the receiver functions indicates the presence of a near-surface low-velocity layer (Vp≈1.9 km/s) and a Moho depth of ~31 km. Details of the crustal structure were investigated by employing time-domain inversions of the receiver functions. General features of those velocity models providing a good fit to the waveform include an approximately one kilometer thick near-surface low-velocity zone, a high-velocity layer over a velocity inversion at mid-crustal depths, and a crust-mantle transition at depths between 30 km and 34 km.

Passive seismic experiment and receiver functions analysis to determine crustal structure at the contact of the northern Dinarides and southwestern Pannonian Basin

NASA Astrophysics Data System (ADS)

Šumanovac, Franjo; Hegedűs, Endre; Orešković, Jasna; Kolar, Saša; Kovács, Attila C.; Dudjak, Darko; Kovács, István J.

2016-06-01

Passive seismic experiment was carried out at the SW contact of the Dinarides and Pannonian basin to determine the crustal structure and velocity discontinuities. The aim of the experiment was to define the relationship between the Adriatic microplate and the Pannonian segment as a part of the European plate. Most of the temporary seismic stations were deployed in Croatia along the Alp07 profile-a part of the active-source ALP 2002 project. About 300-km-long profile stretches from Istra peninsula to the Drava river, in a WSW-ESE direction. Teleseismic events recorded on 13 temporary seismic stations along the profile were analysed by P-receiver function method. Two types of characteristic receiver functions (RF) have been identified, belonging to Dinaridic and Pannonian crusts as defined on the Alp07 profile, while in transitional zone there are both types. Three major crustal discontinuities can be identified for the Dinaridic type: sedimentary basement, intracrustal discontinuity and Mohorovičić discontinuity, whereas the Pannonian type revealed only two discontinuities. The intracrustal discontinuity was not observed in the Pannonian type, thus pointing to a single-layered crust in the Pannonian basin. Two interpretation methods were applied: forward modelling of the receiver functions and H-κ stacking method, and the results were compared with the active-source seismic data at deep refraction profile Alp07. The receiver function modelling has given reliable results of the Moho depths that are in accordance with the seismic refraction results at the end of the Alp07 profile, that is in the area of Pannonian crust characterized by simple crustal structure and low seismic velocities (Vp between 5.9 and 6.2 km s-1). In the Dinarides and its peripheral parts, receiver function modelling regularly gives greater Moho depths, up to +15 per cent, due to more complex crustal structure. The depths of the Moho calculated by the H-κ stacking method vary within wide limits (±13 km), due to band limited data of short-period stations. The results at five stations have to be rejected because of huge deviations in comparison with all previous results, while at the other seven stations the Moho depths vary within ±15 per cent around the Moho discontinuity of the Alp07 profile.

An Improved Method for Seismic Event Depth and Moment Tensor Determination: CTBT Related Application

NASA Astrophysics Data System (ADS)

Stachnik, J.; Rozhkov, M.; Baker, B.

2016-12-01

According to the Protocol to CTBT, International Data Center is required to conduct expert technical analysis and special studies to improve event parameters and assist State Parties in identifying the source of specific event. Determination of seismic event source mechanism and its depth is a part of these tasks. It is typically done through a strategic linearized inversion of the waveforms for a complete or subset of source parameters, or similarly defined grid search through precomputed Greens Functions created for particular source models. We show preliminary results using the latter approach from an improved software design and applied on a moderately powered computer. In this development we tried to be compliant with different modes of CTBT monitoring regime and cover wide range of source-receiver distances (regional to teleseismic), resolve shallow source depths, provide full moment tensor solution based on body and surface waves recordings, be fast to satisfy both on-demand studies and automatic processing and properly incorporate observed waveforms and any uncertainties a priori as well as accurately estimate posteriori uncertainties. Implemented HDF5 based Green's Functions pre-packaging allows much greater flexibility in utilizing different software packages and methods for computation. Further additions will have the rapid use of Instaseis/AXISEM full waveform synthetics added to a pre-computed GF archive. Along with traditional post processing analysis of waveform misfits through several objective functions and variance reduction, we follow a probabilistic approach to assess the robustness of moment tensor solution. In a course of this project full moment tensor and depth estimates are determined for DPRK 2009, 2013 and 2016 events and shallow earthquakes using a new implementation of waveform fitting of teleseismic P waves. A full grid search over the entire moment tensor space is used to appropriately sample all possible solutions. A recent method by Tape & Tape (2012) to discretize the complete moment tensor space from a geometric perspective is used. Moment tensors for DPRK events show isotropic percentages greater than 50%. Depth estimates for the DPRK events range from 1.0-1.4 km. Probabilistic uncertainty estimates on the moment tensor parameters provide robustness to solution.

Mushy Magma beneath Yellowstone

NASA Astrophysics Data System (ADS)

Chu, R.; Helmberger, D. V.; Sun, D.; Jackson, J. M.; Zhu, L.

2009-12-01

A recent prospective on the Yellowstone Caldera discounts its explosive potential based on inferences from tomographic studies on regional earthquake data which suggests a high degree of crystallization of the underlying magma body. In this study, we analyzed P-wave receiver functions recorded by broadband stations above the caldera from 100 teleseismic earthquakes between January and November 2008. After applying a number of waveform modeling tools, we obtained much lower seismic velocities than previous estimates, 2.3 km/sec (Vp) and 1.1 km/sec (Vs), with a thickness of 3.6 km in the upper crust. This shallow low velocity zone is severe enough to cause difficulties with seismic tool applications. In particular, seismologists expect teleseismic P-waves to arrive with motions up and away or down and back. Many of the observations recorded by the Yellowstone Intermountain Seismic Array, however, violate this assumption. We show that many of the first P-wave arrivals observed at seismic stations on the edge of the caldera do not travel through the magma body but have taken longer but faster paths around the edge or wrap-around phases. Three stations near the trailing edge have reversal radial-component motions, while stations near the leading edge do not. Adding our constraints on geometry, we conclude that this relatively shallow magma body has a volume of over 4,300 km3. We estimate the magma body by assuming a fluid-saturated porous material consisting of granite and a mixture of rhyolite melt and supercritical water and CO2 at temperatures of 800 oC and pressure at 5 km (0.1 GPa).Theoretical calculations of seismic wave speed suggests that the magma body beneath the Yellowstone Caldera has a porosity of 32% filled with 92% rhyolite melt and 8% water-CO2 by volume.

Teleseismic body waves from dynamically rupturing shallow thrust faults: Are they opaque for surface-reflected phases?

USGS Publications Warehouse

Smith, D.E.; Aagaard, Brad T.; Heaton, T.H.

2005-01-01

We investigate whether a shallow-dipping thrust fault is prone to waveslip interactions via surface-reflected waves affecting the dynamic slip. If so, can these interactions create faults that are opaque to radiated energy? Furthermore, in this case of a shallow-dipping thrust fault, can incorrectly assuming a transparent fault while using dislocation theory lead to underestimates of seismic moment? Slip time histories are generated in three-dimensional dynamic rupture simulations while allowing for varying degrees of wave-slip interaction controlled by fault-friction models. Based on the slip time histories, P and SH seismograms are calculated for stations at teleseismic distances. The overburdening pressure caused by gravity eliminates mode I opening except at the tip of the fault near the surface; hence, mode I opening has no effect on the teleseismic signal. Normalizing by a Haskell-like traditional kinematic rupture, we find teleseismic peak-to-peak displacement amplitudes are approximately 1.0 for both P and SH waves, except for the unrealistic case of zero sliding friction. Zero sliding friction has peak-to-peak amplitudes of 1.6 for P and 2.0 for SH waves; the fault slip oscillates about its equilibrium value, resulting in a large nonzero (0.08 Hz) spectral peak not seen in other ruptures. These results indicate wave-slip interactions associated with surface-reflected phases in real earthquakes should have little to no effect on teleseismic motions. Thus, Haskell-like kinematic dislocation theory (transparent fault conditions) can be safety used to simulate teleseismic waveforms in the Earth.

Crustal structure beneath the Blue Mountains terranes and cratonic North America, eastern Oregon, and Idaho, from teleseismic receiver functions

NASA Astrophysics Data System (ADS)

Christian Stanciu, A.; Russo, Raymond M.; Mocanu, Victor I.; Bremner, Paul M.; Hongsresawat, Sutatcha; Torpey, Megan E.; VanDecar, John C.; Foster, David A.; Hole, John A.

2016-07-01

We present new images of lithospheric structure obtained from P-to-S conversions defined by receiver functions at the 85 broadband seismic stations of the EarthScope IDaho-ORegon experiment. We resolve the crustal thickness beneath the Blue Mountains province and the former western margin of cratonic North America, the geometry of the western Idaho shear zone (WISZ), and the boundary between the Grouse Creek and Farmington provinces. We calculated P-to-S receiver functions using the iterative time domain deconvolution method, and we used the H-k grid search method and common conversion point stacking to image the lithospheric structure. Moho depths beneath the Blue Mountains terranes range from 24 to 34 km, whereas the crust is 32-40 km thick beneath the Idaho batholith and the regions of extended crust of east-central Idaho. The Blue Mountains group Olds Ferry terrane is characterized by the thinnest crust in the study area, 24 km thick. There is a clear break in the continuity of the Moho across the WISZ, with depths increasing from 28 km west of the shear zone to 36 km just east of its surface expression. The presence of a strong midcrustal converting interface at 18 km depth beneath the Idaho batholith extending 20 km east of the WISZ indicates tectonic wedging in this region. A north striking 7 km offset in Moho depth, thinning to the east, is present beneath the Lost River Range and Pahsimeroi Valley; we identify this sharp offset as the boundary that juxtaposes the Archean Grouse Creek block with the Paleoproterozoic Farmington zone.

Spatial Distribution of Seismic Anisotropy in the Crust in the Northeast Front Zone of Tibetan Plateau

NASA Astrophysics Data System (ADS)

Gao, Y.; Wang, Q.; SHI, Y.

2017-12-01

There are orogenic belts and strong deformation in northeastern zone of Tibetan Plateau. The media in crust and in the upper mantle are seismic anisotropic there. This study uses seismic records by permanent seismic stations and portable seismic arrays, and adopts analysis techniques on body waves to obtain spatial anisotropic distribution in northeastern front zone of Tibetan Plateau. With seismic records of small local earthquakes, we study shear-wave splitting in the upper crust. The polarization of fast shear wave (PFS) can be obtained, and PFS is considered parallel to the strike of the cracks, as well as the direction of maximum horizontal compressive stress. However, the result shows the strong influence from tectonics, such as faults. It suggests multiple-influence including stress and fault. Spatial distribution of seismic anisotropy in study zone presents the effect in short range. PFS at the station on the strike-slip fault is quite different to PFS at station just hundreds of meters away from the fault. With seismic records of teleseismic waveforms, we obtained seismic anisotropy in the whole crust by receiver functions. The PFS directions from Pms receiver functions show consistency, generally in WNW. The time-delay of slow S phases is significant. With seismic records of SKS, PKS and SKKS phases, we can detect seismic anisotropy in the upper mantle by splitting analysis. The fast directions of these phases also show consistency, generally in WNW, similar to those of receiver functions, but larger time-delays. It suggests significant seismic anisotropy in the crust and crustal deformation is coherent to that in the upper mantle.Seismic anisotropy in the upper crust, in the whole crust and in the upper mantle are discussed both in difference and tectonic implications [Grateful to the support by NSFC Project 41474032].

Transition zone structure beneath Ethiopia from 3-D fast marching pseudo-migration stacking

NASA Astrophysics Data System (ADS)

Benoit, M. H.; Lopez, A.; Levin, V.

2008-12-01

Several models for the origin of the Afar hotspot have been put forth over the last decade, but much ambiguity remains as to whether the hotspot tectonism found there is due to a shallow or deeply seated feature. Additionally, there has been much debate as to whether the hotspot owes its existence to a 'classic' mantle plume feature or if it is part of the African Superplume complex. To further understand the origin of the hotspot, we employ a new receiver function stacking method that incorporates a fast-marching three- dimensional ray tracing algorithm to improve upon existing studies of the mantle transition zone structure. Using teleseismic data from the Ethiopia Broadband Seismic Experiment and the EAGLE (Ethiopia Afar Grand Lithospheric Experiment) experiment, we stack receiver functions using a three-dimensional pseudo- migration technique to examine topography on the 410 and 660 km discontinuities. Previous methods of receiver function pseudo-migration incorporated ray tracing methods that were not able to ray trace through highly complicated 3-D structure, or the ray tracing techniques only produced 3-D time perturbations associated 1-D rays in a 3-D velocity medium. These previous techniques yielded confusing and incomplete results for when applied to the exceedingly complicated mantle structure beneath Ethiopia. Indeed, comparisons of the 1-D versus 3-D ray tracing techniques show that the 1-D technique mislocated structure laterally in the mantle by over 100 km. Preliminary results using our new technique show a shallower then average 410 km discontinuity and a deeper than average 660 km discontinuity over much of the region, suggested that the hotspot has a deep seated origin.

Clustering P-Wave Receiver Functions To Constrain Subsurface Seismic Structure

NASA Astrophysics Data System (ADS)

Chai, C.; Larmat, C. S.; Maceira, M.; Ammon, C. J.; He, R.; Zhang, H.

2017-12-01

The acquisition of high-quality data from permanent and temporary dense seismic networks provides the opportunity to apply statistical and machine learning techniques to a broad range of geophysical observations. Lekic and Romanowicz (2011) used clustering analysis on tomographic velocity models of the western United States to perform tectonic regionalization and the velocity-profile clusters agree well with known geomorphic provinces. A complementary and somewhat less restrictive approach is to apply cluster analysis directly to geophysical observations. In this presentation, we apply clustering analysis to teleseismic P-wave receiver functions (RFs) continuing efforts of Larmat et al. (2015) and Maceira et al. (2015). These earlier studies validated the approach with surface waves and stacked EARS RFs from the USArray stations. In this study, we experiment with both the K-means and hierarchical clustering algorithms. We also test different distance metrics defined in the vector space of RFs following Lekic and Romanowicz (2011). We cluster data from two distinct data sets. The first, corresponding to the western US, was by smoothing/interpolation of receiver-function wavefield (Chai et al. 2015). Spatial coherence and agreement with geologic region increase with this simpler, spatially smoothed set of observations. The second data set is composed of RFs for more than 800 stations of the China Digital Seismic Network (CSN). Preliminary results show a first order agreement between clusters and tectonic region and each region cluster includes a distinct Ps arrival, which probably reflects differences in crustal thickness. Regionalization remains an important step to characterize a model prior to application of full waveform and/or stochastic imaging techniques because of the computational expense of these types of studies. Machine learning techniques can provide valuable information that can be used to design and characterize formal geophysical inversion, providing information on spatial variability in the subsurface geology.

Lithospheric Architecture Beneath Hudson Bay

NASA Astrophysics Data System (ADS)

Porritt, R. W.; Miller, M. S.; Darbyshire, F. A.

2015-12-01

Hudson Bay overlies some of the thickest Precambrian lithosphere on Earth, whose internal structures contain important clues to the earliest workings of plate formation. The terminal collision, the Trans-Hudson Orogen, brought together the Western Churchill craton to the northwest and the Superior craton to the southeast. These two Archean cratons along with the Paleo-Proterozoic Trans-Hudson internides, form the core of the North American craton. We use S to P converted wave imaging and absolute shear velocity information from a joint inversion of P to S receiver functions, new ambient noise derived phase velocities, and teleseismic phase velocities to investigate this region and determine both the thickness of the lithosphere and the presence of internal discontinuities. The lithosphere under central Hudson Bay approaches 􏰂350 km thick but is thinner (􏰂200-250 km) around the periphery of the Bay. Furthermore, the amplitude of the lithosphere-asthenosphere boundary (LAB) conversion from the S receiver functions is unusually large for a craton, suggesting a large thermal contrast across the LAB, which we interpret as direct evidence of the thermal insulation effect of continents on the asthenosphere. Within the lithosphere, midlithospheric discontinuities, significantly shallower than the base of the lithosphere, are often imaged, suggesting the mechanisms that form these layers are common. Lacking time-history information, we infer that these discontinuities reflect reactivation of formation structures during deformation of the craton.

Lithospheric architecture beneath Hudson Bay

NASA Astrophysics Data System (ADS)

Porritt, Robert W.; Miller, Meghan S.; Darbyshire, Fiona A.

2015-07-01

Hudson Bay overlies some of the thickest Precambrian lithosphere on Earth, whose internal structures contain important clues to the earliest workings of plate formation. The terminal collision, the Trans-Hudson Orogen, brought together the Western Churchill craton to the northwest and the Superior craton to the southeast. These two Archean cratons along with the Paleo-Proterozoic Trans-Hudson internides, form the core of the North American craton. We use S to P converted wave imaging and absolute shear velocity information from a joint inversion of P to S receiver functions, new ambient noise derived phase velocities, and teleseismic phase velocities to investigate this region and determine both the thickness of the lithosphere and the presence of internal discontinuities. The lithosphere under central Hudson Bay approaches ˜350 km thick but is thinner (˜200-250 km) around the periphery of the Bay. Furthermore, the amplitude of the LAB conversion from the S receiver functions is unusually large for a craton, suggesting a large thermal contrast across the LAB, which we interpret as direct evidence of the thermal insulation effect of continents on the asthenosphere. Within the lithosphere, midlithospheric discontinuities, significantly shallower than the base of the lithosphere, are often imaged, suggesting the mechanisms that form these layers are common. Lacking time-history information, we infer that these discontinuities reflect reactivation of formation structures during deformation of the craton.

Multi-Mode 3D Kirchhoff Migration of Receiver Functions at Continental Scale With Applications to USArray

NASA Astrophysics Data System (ADS)

Millet, F.; Bodin, T.; Rondenay, S.

2017-12-01

The teleseismic scattered seismic wavefield contains valuable information about heterogeneities and discontinuities inside the Earth. By using fast Receiver Function (RF) migration techniques such as classic Common Conversion Point (CCP) stacks, one can easily interpret structural features down to a few hundred kilometers in the mantle. However, strong simplifying 1D assumptions limit the scope of these methods to structures that are relatively planar and sub-horizontal at local-to-regional scales, such as the Lithosphere-Asthenosphere Boundary and the Mantle Transition Zone discontinuities. Other more robust 2D and 2.5D methods rely on fewer assumptions but require considerable, sometime prohibitive, computation time. Following the ideas of Cheng (2017), we have implemented a simple fully 3D Prestack Kirchhoff RF migration scheme which uses the FM3D fast Eikonal solver to compute travel times and scattering angles. The method accounts for 3D elastic point scattering and includes free surface multiples, resulting in enhanced images of laterally varying dipping structures, such as subducted slabs. The method is tested for subduction structures using 2.5D synthetics generated with Raysum and 3D synthetics generated with specfem3D. Results show that dip angles, depths and lateral variations can be recovered almost perfectly. The approach is ideally suited for applications to dense regional datasets, including those collected across the Cascadia and Alaska subduction zones by USArray.

Crustal and Upper Mantle S-velocity Structure From Receiver Functions Analisys Around Terra Nova Bay Base, Antartica

NASA Astrophysics Data System (ADS)

Piana Agostinetti, N.; Amato, A.; Cattaneo, M.; de Gori, P.; di Bona, M.

In the framework of the italian PNRA (Progetto Nazionale di Ricerche in Antartide), we have started to re-analize teleseismic waveforms recorded, using three-components seismometers (equipped with 5 seconds sensors, Lennartz 3D-5s), during five summer campaings, from 1993 to 2000. Seismic stations were deployed around Terra Nova Bay (TNB) italian base, from the sea to reach the interior of the Transantartic Moun- tains (TAM), the most striking example of nocontractional mountain belt. During the last campaingn (1999-2000) seismic stations were deployed deep into Northern Vic- toria Land to reach Rennik and Lillie Glaciers Area and George V coast region, the northest part of TAM. Our main goals were: to compute, using frequency-domanin deconvolution method by Di Bona [1998], Receiver Functions covering all the area around TNB italian antartic base; to map of Moho-depth and intercrustal S-waves ve- locity discontinuity from 1-D velocity model computed using Sambridge's inversion scheme [Sambridge,1999]; to analize new teleseimic waveforms recorded near TNB base: continuos recording, from 1999 to present, permits more accurate modelling S-velocity crustal structure in this critical area situated at the edge of the ipothetic rift [Stern and ten Brik, 1989; Stump and Fitzgerald, 1992; ten Brik et al., 1997]; to present final results from BACKTAM expedition.

Discovering geothermal supercritical fluids: a new frontier for seismic exploration.

PubMed

Piana Agostinetti, Nicola; Licciardi, Andrea; Piccinini, Davide; Mazzarini, Francesco; Musumeci, Giovanni; Saccorotti, Gilberto; Chiarabba, Claudio

2017-11-06

Exploiting supercritical geothermal resources represents a frontier for the next generation of geothermal electrical power plant, as the heat capacity of supercritical fluids (SCF),which directly impacts on energy production, is much higher than that of fluids at subcritical conditions. Reconnaissance and location of intensively permeable and productive horizons at depth is the present limit for the development of SCF geothermal plants. We use, for the first time, teleseismic converted waves (i.e. receiver function) for discovering those horizons in the crust. Thanks to the capability of receiver function to map buried anisotropic materials, the SCF-bearing horizon is seen as the 4km-depth abrupt termination of a shallow, thick, ultra-high (>30%) anisotropic rock volume, in the center of the Larderello geothermal field. The SCF-bearing horizon develops within the granites of the geothermal field, bounding at depth the vapor-filled heavily-fractured rock matrix that hosts the shallow steam-dominated geothermal reservoirs. The sharp termination at depth of the anisotropic behavior of granites, coinciding with a 2 km-thick stripe of seismicity and diffuse fracturing, points out the sudden change in compressibility of the fluid filling the fractures and is a key-evidence of deep fluids that locally traversed the supercritical conditions. The presence of SCF and fracture permeability in nominally ductile granitic rocks open new scenarios for the understanding of magmatic systems and for geothermal exploitation.

Differential energy radiation from two earthquakes in Japan with identical Mw: The Kyushu 1996 and Tottori 2000 earthquakes

USGS Publications Warehouse

Choy, G.L.; Boatwright, J.

2009-01-01

We examine two closely located earthquakes in Japan that had identical moment magnitudes Mw but significantly different energy magnitudes Me. We use teleseismic data from the Global Seismograph Network and strong-motion data from the National Research Institute for Earth Science and Disaster Prevention's K-Net to analyze the 19 October 1996 Kyushu earthquake (Mw 6.7, Me 6.6) and the 6 October 2000 Tottori earthquake (Mw 6.7, Me 7.4). To obtain regional estimates of radiated energy ES we apply a spectral technique to regional (<200 km) waveforms that are dominated by S and Lg waves. For the thrust-fault Kyushu earthquake, we estimate an average regional attenuation Q(f) 230f0:65. For the strike-slip Tottori earthquake, the average regional attenuation is Q(f) 180f0:6. These attenuation functions are similar to those derived from studies of both California and Japan earthquakes. The regional estimate of ES for the Kyushu earthquake, 3:8 ?? 1014 J, is significantly smaller than that for the Tottori earthquake, ES 1:3 ?? 1015 J. These estimates correspond well with the teleseismic estimates of 3:9 ?? 1014 J and 1:8 ?? 1015 J, respectively. The apparent stress (Ta = ??Es/M0 with ?? equal to rigidity) for the Kyushu earthquake is 4 times smaller than the apparent stress for the Tottori earthquake. In terms of the fault maturity model, the significantly greater release of energy by the strike-slip Tottori earthquake can be related to strong deformation in an immature intraplate setting. The relatively lower energy release of the thrust-fault Kyushu earthquake can be related to rupture on mature faults at a subduction environment. The consistence between teleseismic and regional estimates of ES is particularly significant as teleseismic data for computing ES are routinely available for all large earthquakes whereas often there are no near-field data.

The effects of core-reflected waves on finite fault inversions with teleseismic body wave data

NASA Astrophysics Data System (ADS)

Qian, Yunyi; Ni, Sidao; Wei, Shengji; Almeida, Rafael; Zhang, Han

2017-11-01

Teleseismic body waves are essential for imaging rupture processes of large earthquakes. Earthquake source parameters are usually characterized by waveform analyses such as finite fault inversions using only turning (direct) P and SH waves without considering the reflected phases from the core-mantle boundary (CMB). However, core-reflected waves such as ScS usually have amplitudes comparable to direct S waves due to the total reflection from the CMB and might interfere with the S waves used for inversion, especially at large epicentral distances for long duration earthquakes. In order to understand how core-reflected waves affect teleseismic body wave inversion results, we develop a procedure named Multitel3 to compute Green's functions that contain turning waves (direct P, pP, sP, direct S, sS and reverberations in the crust) and core-reflected waves (PcP, pPcP, sPcP, ScS, sScS and associated reflected phases from the CMB). This ray-based method can efficiently generate synthetic seismograms for turning and core-reflected waves independently, with the flexibility to take into account the 3-D Earth structure effect on the timing between these phases. The performance of this approach is assessed through a series of numerical inversion tests on synthetic waveforms of the 2008 Mw7.9 Wenchuan earthquake and the 2015 Mw7.8 Nepal earthquake. We also compare this improved method with the turning-wave only inversions and explore the stability of the new procedure when there are uncertainties in a priori information (such as fault geometry and epicentre location) or arrival time of core-reflected phases. Finally, a finite fault inversion of the 2005 Mw8.7 Nias-Simeulue earthquake is carried out using the improved Green's functions. Using enhanced Green's functions yields better inversion results as expected. While the finite source inversion with conventional P and SH waves is able to recover large-scale characteristics of the earthquake source, by adding PcP and ScS phases, the inverted slip model and moment rate function better match previous results incorporating field observations, geodetic and seismic data.

Rotational motions for teleseismic surface waves

NASA Astrophysics Data System (ADS)

Lin, Chin-Jen; Huang, Han-Pang; Pham, Nguyen Dinh; Liu, Chun-Chi; Chi, Wu-Cheng; Lee, William H. K.

2011-08-01

We report the findings for the first teleseismic six degree-of-freedom (6-DOF) measurements including three components of rotational motions recorded by a sensitive rotation-rate sensor (model R-1, made by eentec) and three components of translational motions recorded by a traditional seismometer (STS-2) at the NACB station in Taiwan. The consistent observations in waveforms of rotational motions and translational motions in sections of Rayleigh and Love waves are presented in reference to the analytical solution for these waves in a half space of Poisson solid. We show that additional information (e.g., Rayleigh wave phase velocity, shear wave velocity of the surface layer) might be exploited from six degree-of-freedom recordings of teleseismic events at only one station. We also find significant errors in the translational records of these teleseismic surface waves due to the sensitivity of inertial translation sensors (seismometers) to rotational motions. The result suggests that the effects of such errors need to be counted in surface wave inversions commonly used to derive earthquake source parameters and Earth structure.

Preliminary study of crust-upper mantle structure of the Tibetan Plateau by using broadband teleseismic body waveforms

NASA Astrophysics Data System (ADS)

Zhu, Lu-Pei; Zeng, Rong-Sheng; Wu, Francis T.; Owens, Thomas J.; Randall, George E.

1993-05-01

As part of a joint Sino-U.S. research project to study the deep structure of the Tibetan Plateau, 11 broadband digital seismic recorders were deployed on the Plateau for one year of passive seismic recording. In this report we use teleseimic P waveforms to study the seismic velocity structure of crust and upper mantle under three stations by receiver function inversion. The receiver function is obtained by first rotating two horizontal components of seismic records into radial and tangential components and then deconvolving the vertical component from them. The receiver function depends only on the structure near the station because the source and path effects have been removed by the deconvolution. To suppress noise, receiver functions calculated from events clustered in a small range of back-azimuths and epicentral distances are stacked. Using a matrix formalism describing the propagation of elastic waves in laterally homogeneous stratified medium, a synthetic receiver function and differential receiver functions for the parameters in each layer can be calculated to establish a linearized inversion for one-dimensional velocity structure. Preliminary results of three stations, Wen-quan, Golmud and Xigatze (Coded as WNDO, TUNL and XIGA), located in central, northern and southern Plateau are given in this paper. The receiver functions of all three stations show clear P-S converted phases. The time delays of these converted phases relative to direct P arrivals are: WNDO 7.9s (for NE direction) and 8.3s (for SE direction), TUNL 8.2s, XIGA 9.0s. Such long time delays indicate the great thickness of crust under the Plateau. The differences between receiver function of these three station shows the tectonic difference between southern and north-central Plateau. The waveforms of the receiver functions for WNDO and TUNL are very simple, while the receiver function of XIGA has an additional midcrustal converted phase. The S wave velocity structures at these three stations are estimated from inversions of the receiver function. The crustal shear wave velocities at WNDO and TUNL are vertically homogeneous, with value between 3.5 3.6 km/s down to Moho. This value in the lower crust is lower than the normal value for the lower crust of continents, which is consistent with the observed strong Sn attenuation in this region. The velocity structure at XIGA shows a velocity discontinuity at depth of 20 km and high velocity value of 4.0 km/s in the midcrust between 20 30 km depth. Similar results are obtained from a DSS profile in southern Tibet. The velocity under XIGA decreases below a depth of 30 km, reaching the lowest value of 3.2 km/s between 50 55 km. depth. This may imply that the Indian crust underthrusts the low part of Tibetan crust in the southern Plateau, forming a “double crust”. The crustal thickness at each of these sites is: WNDO, 68 km; TUNL, 70 km; XI-GA, 80 km.

Crustal Properties Across the Mid-Continent Rift via Transfer Function Analysis

NASA Astrophysics Data System (ADS)

Frederiksen, A. W.; Tyomkin, Y.; Campbell, R.; van der Lee, S.; Zhang, H.

2015-12-01

The Mid-Continent Rift (MCR), a failed Proterozoic rift structure in central North America, is a dominant feature of North American gravity maps. The rift underwent a combination of extension, magmatism, and later compression, and it is difficult to predict how these events affected the overall crustal thickness and bulk composition in the vicinity of the rift axis, though the associated gravity high indicates that large-volume mafic magmatism took place. The Superior Province Rifting Earthscope Experiment (SPREE) project instrumented the MCR with Flexible Array broadband seismographs from 2011 through 2013 in Minnesota and Wisconsin, along two lines crossing the rift axis as well as a line following the axis. We examine teleseismic P-coda data from SPREE and nearby Transportable Array instruments using a new technique: transfer-function analysis. In this approach, possible models of crustal structure are used to generate a predicted transfer function relating the radial and vertical components of the P coda at a particular site. The transfer function then allows generation of a misfit (between the true radial component and a synthetic radial component predicted from the vertical trace) without the need to perform receiver-function deconvolution, thus avoiding the deconvolution problems encountered with receiver functions in sedimentary basins. We use the transfer-function approach to perform a grid search over three crustal properties: crustal thickness, crustal P/S velocity ratio, and the thickness of an overlying sedimentary basin. Results for our SPREE/TA data set indicate that the crust is significantly thickened along the rift axis, with maximum thicknesses approaching 50 km; the crust is thinner (ca. 40 km) outside of the rift zone. The crustal thickness structure is particularly complex beneath southeastern Minnesota, where very strong Moho topography is present, as well as up to 2 km of sediment; further north, the Moho is smoother and the basin is not present. P/S ratio varies along the rift axis, suggesting a higher mafic component (higher ratio) in southern Minnesota. The complexity we see along the MCR is consistent with the results obtained by Zhang et al. (this conference) using receiver function analysis.

3-D Shear Velocity Structure of Costa Rica and Nicaragua from Teleseismic and Ambient Noise Rayleigh Wave Tomography

NASA Astrophysics Data System (ADS)

Harmon, N.; Salas, M.; Rychert, C. A.; Fischer, K. M.; Abers, G. A.

2012-12-01

The Costa Rica-Nicaragua subduction zone shows systematic along strike variation in arc chemistry, geology and seismic velocity and attenuation, presenting global extremes within a few hundred kilometres. In this study we use teleseismic and ambient noise derived surface wave tomography to produce a 3-D shear velocity model of the region. We use the 48 stations of the TUCAN array, and up to 96 events for the teleseismic Rayleigh wave inversion, and 20 months of continuous data for cross correlation to estimate Green's functions from ambient noise. In the shallow crust (0-15 km) we observe low shear velocities directly beneath the arc volcanos (< 3 km/s) with higher velocities in the back arc of Nicaragua. The anomalies are likely caused by heated crust, possibly intruded by magma. We observe > 40 km thick crust beneath the Costa Rican arc and the Nicaraguan Highlands, with thinned crust (~20 km) beneath the Nicaraguan Depression, with increasing crustal thickness in the back arc region. At mantle depths (55-120 km depth) we observe lower shear velocities (~2%) beneath the Nicaraguan arc and back arc relative to Costa Rica. This is well-correlated with a Vp/Vs anomaly beneath Nicaragua. The lower shear velocity beneath Nicaragua may indicate higher melt content in the mantle perhaps due to higher volatile flux from the slab. Finally, we observe a linear high velocity region at depths > 120 km parallel to the trench, which is consistent with the subducting slab.

Comparison of strong-motion spectra with teleseismic spectra for three magnitude 8 subduction-zone earthquakes

NASA Astrophysics Data System (ADS)

Houston, Heidi; Kanamori, Hiroo

1990-08-01

A comparison of strong-motion spectra and teleseismic spectra was made for three Mw 7.8 to 8.0 earthquakes: the 1985 Michoacan (Mexico) earthquake, the 1985 Valparaiso (Chile) earthquake, and the 1983 Akita-Oki (Japan) earthquake. The decay of spectral amplitude with the distance from the station was determined, considering different measures of distance from a finite fault, and it was found to be different for these three events. The results can be used to establish empirical relations between the observed spectra and the half-space responses depending on the distance and the site condition, making it possible to estimate strong motions from source spectra determined from teleseismic records.

Tracing Geophysical Indicators of Fluid-Induced Serpentinization in the Pampean Flat Slab Subduction Region of Chile

NASA Astrophysics Data System (ADS)

Bourke, J. R.; Nikulin, A.; Park, J. J.

2016-12-01

An activity gap in the Andean volcanic arc in the Pampean section of the subduction zone in Chile ( 28°-33°S) marks a section of flat-slab subduction. Past studies connected this change in geometry to the collision and subduction of the Juan Fernandez Ridge and the resulting migration of both the thrust front and magmatism eastward to the Sierras Pampeanas. The fate of fluids released from the subducting Nazca slab remains uncertain and the degree of their interaction with the basal layer of the continental lithosphere is poorly understood. We present initial results of a receiver-function investigation and forward-modeling effort at station GO03 operated by the Chilean National Seismic Network. Receiver function analysis of 75 well-recorded teleseismic earthquake events recorded at GO03 allow us to constrain the position of the subducting Nazca slab and to address the physical properties of the interplate contact zone. Critically, our analysis indicates presence of a highly-anisotropic zone of low velocities directly above the subucting Nazca slab. We point out a remarkable similarity in geophysical characteristics between the observed seismic anomaly at GO03 and a volume of proposed serpentinization in an area of sub-horizontal subduction above the Juan de Fuca slab in Cascadia. This interpretation is further supported by forward-modeling receiver functions at GO03 relying on a velocity model that incorporates a serpentinized interplate region. The newly-identified low-velocity highly-anisotropic layer may extend beyond the GO03 area and act as a mineral reservoir that captures and, possibly, transports fluids derived from the dehydrating Nazca Plate as it subducts below South America. It is likely that there is a relationship between this feature and the lack of volcanic activity in the Pampean flat slab region. Figure Caption: A) Backazimuth sweep of receiver functions recorded at station GO03 with predicted phase arrivals plotted for 55 km, 65 km, 75 km and 85 km. B) Depth-migrated receiver functions for station GO03 relying on AK-135 velocity model and local seismicity (Mw>4.5) plotted within 15km of a 100km profile centered on GO03 along the dominant direction of subduction (74°).

New Insights on the Structure of the Cascadia Subduction Zone from Amphibious Seismic Data

NASA Astrophysics Data System (ADS)

Janiszewski, Helen Anne

A new onshore-offshore seismic dataset from the Cascadia subduction zone was used to characterize mantle lithosphere structure from the ridge to the volcanic arc, and plate interface structure offshore within the seismogenic zone. The Cascadia Initiative (CI) covered the Juan de Fuca plate offshore the northwest coast of the United States with an ocean bottom seismometer (OBS) array for four years; this was complemented by a simultaneous onshore seismic array. Teleseismic data recorded by this array allows the unprecedented imaging of an entire tectonic plate from its creation at the ridge through subduction initiation and back beyond the volcanic arc along the entire strike of the Cascadia subduction zone. Higher frequency active source seismic data also provides constraints on the crustal structure along the plate interface offshore. Two seismic datasets were used to image the plate interface structure along a line extending 100 km offshore central Washington. These are wide-angle reflections from ship-to-shore seismic data from the Ridge-To-Trench seismic cruise and receiver functions calculated from a densely spaced CI OBS focus array in a similar region. Active source seismic observations are consistent with reflections from the plate interface offshore indicating the presence of a P-wave velocity discontinuity. Until recently, there has been limited success in using the receiver function technique on OBS data. I avoid these traditional challenges by using OBS constructed with shielding deployed in shallow water on the continental shelf. These data have quieter horizontals and avoid water- and sediment-multiple contamination at the examined frequencies. The receiver functions are consistently modeled with a velocity structure that has a low velocity zone (LVZ) with elevated P to S-wave velocity ratios at the plate interface. A similar LVZ structure has been observed onshore and interpreted as a combination of elevated pore-fluid pressures or metasediments. This new offshore result indicates that the structure may persist updip indicating the plate interface may be weak. To focus more broadly on the entire subduction system, I calculate phase velocities from teleseismic Rayleigh waves from 20-100 s period across the entire onshore-offshore array. The shear-wave velocity model calculated from these data can provide constrains on the thermal structure of the lithosphere both prior to and during subduction of the Juan de Fuca plate. Using OBS data in this period band requires removal of tilt and compliance noise, two types of water-induced noise that affect long period data. To facilitate these corrections on large seismic arrays such as the CI, an automated quality control routine was developed for selecting noise windows for the calculation of the required transfer functions. These corrections typically involve either averaging out transient signals, which requires the assumption of stationarity of the noise over the long periods of time, or laborious hand selection of noise segments. This new method calculates transfer functions based on daily time series that exclude transient signals, but allows for the investigation of long-term variation over the course of an instrument's deployment. I interpret these new shoreline-crossing phase velocity maps in terms of the tectonics associated with the Cascadia subduction system. Major findings include that oceanic plate cooling models do not explain the velocities observed beneath the Juan de Fuca plate, that slow velocities in the forearc appear to be more prevalent in areas modeled to have experienced high slip in past Cascadia megathrust earthquakes, and along strike variations in phase velocity reflect variations in arc structure and backarc tectonics.

Constraining the Source of the M w 8.1 Chiapas, Mexico Earthquake of 8 September 2017 Using Teleseismic and Tsunami Observations

NASA Astrophysics Data System (ADS)

Heidarzadeh, Mohammad; Ishibe, Takeo; Harada, Tomoya

2018-04-01

The September 2017 Chiapas (Mexico) normal-faulting intraplate earthquake (M w 8.1) occurred within the Tehuantepec seismic gap offshore Mexico. We constrained the finite-fault slip model of this great earthquake using teleseismic and tsunami observations. First, teleseismic body-wave inversions were conducted for both steep (NP-1) and low-angle (NP-2) nodal planes for rupture velocities (V r) of 1.5-4.0 km/s. Teleseismic inversion guided us to NP-1 as the actual fault plane, but was not conclusive about the best V r. Tsunami simulations also confirmed that NP-1 is favored over NP-2 and guided the V r = 2.5 km/s as the best source model. Our model has a maximum and average slips of 13.1 and 3.7 m, respectively, over a 130 km × 80 km fault plane. Coulomb stress transfer analysis revealed that the probability for the occurrence of a future large thrust interplate earthquake at offshore of the Tehuantepec seismic gap had been increased following the 2017 Chiapas normal-faulting intraplate earthquake.

Thunder-induced ground motions: 1. Observations

NASA Astrophysics Data System (ADS)

Lin, Ting-L.; Langston, Charles A.

2009-04-01

Acoustic pressure from thunder and its induced ground motions were investigated using a small array consisting of five three-component short-period surface seismometers, a three-component borehole seismometer, and five infrasound microphones. We used the array to constrain wave parameters of the incident acoustic and seismic waves. The incident slowness differences between acoustic pressure and ground motions suggest that ground reverberations were first initiated somewhat away from the array. Using slowness inferred from ground motions is preferable to obtain the seismic source parameters. We propose a source equalization procedure for acoustic/seismic deconvolution to generate the time domain transfer function, a procedure similar to that of obtaining teleseismic earthquake receiver functions. The time domain transfer function removes the incident pressure time history from the seismogram. An additional vertical-to-radial ground motion transfer function was used to identify the Rayleigh wave propagation mode of induced seismic waves complementing that found using the particle motions and amplitude variations in the borehole. The initial motions obtained by the time domain transfer functions suggest a low Poisson's ratio for the near-surface layer. The acoustic-to-seismic transfer functions show a consistent reverberation series at frequencies near 5 Hz. This gives an empirical measure of site resonance that depends on the ratio of the layer velocity to layer thickness for earthquake P and S waves. The time domain transfer function approach by transferring a spectral division into the time domain provides an alternative method for studying acoustic-to-seismic coupling.

Crustal and Upper Mantle Investigations Using Receiver Functions and Tomographic Inversion in the Southern Puna Plateau Region of the Central Andes

NASA Astrophysics Data System (ADS)

Heit, B.; Yuan, X.; Bianchi, M.; Jakovlev, A.; Kumar, P.; Kay, S. M.; Sandvol, E. A.; Alonso, R.; Coira, B.; Comte, D.; Brown, L. D.; Kind, R.

2011-12-01

We present here the results obtained using the data form our passive seismic array in the southern Puna plateau between 25°S to 28°S latitude in Argentina and Chile. In first instance we have been able to calculate P and S receiver functions in order to investigate the Moho thickness and other seismic discontinuities in the study area. The RF data shows that the northern Puna plateau has a thicker crust and that the Moho topography is more irregular along strike. The seismic structure and thickness of the continental crust and the lithospheric mantle beneath the southern Puna plateau reveals that the LAB is deeper to the north of the array suggesting lithospheric removal towards the south. Later we performed a joint inversion of teleseismic and regional tomographic data in order to study the distribution of velocity anomalies that could help us to better understand the evolution of the Andean elevated plateau and the role of lithosphere-asthenosphere interactions in this region. Low velocities are observed in correlation with young volcanic centers (e.g. Ojos del Salado, Cerro Blanco, Galan) and agree very well with the position of crustal lineaments in the region. This is suggesting a close relationship between magmatism and lithospheric structures at crustal scale coniciding with the presence of hot asthenospheric material at the base of the crust probably induced by lithospheric foundering.

Subsurface signature of North Anatolian Fault Zone and its relation with old sutures: New insight from receiver function analysis.

NASA Astrophysics Data System (ADS)

Özacar, Arda A.; Abgarmi, Bizhan

2017-04-01

The North Anatolian Fault Zone (NAFZ) is an active continental transform plate boundary that accommodates the westward extrusion of the Anatolian plate. The central segment of NAFZ displays northward convex surface trace which coincides partly with the Paleo-Tethyan suture formed during the early Cenozoic. The depth extent and detailed structure of the actively deforming crust along the NAF is still under much debate and processes responsible from rapid uplift are enigmatic. In this study, over five thousand high quality P receiver functions are computed using teleseismic earthquakes recorded by permanent stations of national agencies and temporary North Anatolian Fault Passive Seismic experiment (2005-2008). In order to map the crustal thickness and Vp/Vs variations accurately, the study area is divided into grids with 20 km spacing and along each grid line Moho phase and its multiples are picked through constructed common conversion point (CCP) profiles. According to our results, nature of discontinuities and crustal thickness display sharp changes across the main strand of NAFZ supporting a lithospheric scale faulting that offsets Moho discontinuity. In the southern block, crust is relatively thin in the west ( 35 km) and becomes thicker gradually towards east ( 40 km). In contrast, the northern block displays a strong lateral change in crustal thickness reaching up to 10 km across a narrow roughly N-S oriented zone which is interpreted as the subsurface signature of the ambiguous boundary between Istanbul Block and Pontides located further west at the surface.

Estimating Seismic Moment From Broadband P-Waves for Tsunami Warnings.

NASA Astrophysics Data System (ADS)

Hirshorn, B. F.

2006-12-01

The Richard H. Hagemeyer Pacific Tsunami Warning Center (PTWC), located in Ewa Beach, Oahu, Hawaii, is responsible for issuing local, regional, and distant tsunami warnings to Hawaii, and for issuing regional and distant tsunami warnings to the rest of the Pacific Basin, exclusive of the US West Coast. The PTWC must provide these tsunami warnings as soon as technologically possible, based entirely on estimates of a potentially tsunamigenic earthquake's source parameters. We calculate the broadband P-wave moment magnitude, Mwp, from the P or pP wave velocity seismograms [Tsuboi et al., 1995, 1999]. This method appears to work well for regional and teleseismic events [ Tsuboi et al (1999], Whitmore et al (2002), Hirshorn et al (2004) ]. Following Tsuboi, [1995], we consider the displacement record of the P-wave portion of the broadband seismograms as an approximate source time function and integrate this record to obtain the moment rate function, Mo(t), and the moment magnitude [Hanks and Kanamori, 1972] as a function of time, Mw(t). We present results for Mwp for local, regional, and teleseismic broad band recordings for earthquakes in the Mw 5 to 9.3 range. As large Hawaii events are rare, we tested this local case using other Pacific events in the magnitude 5.0 to 7.5 range recorded by nearby stations. Signals were excluded, however, if the epicentral distance was so small (generally less than 1 degree) that there was contamination by the S-wave too closely following the P-waves. Scatter plots of Mwp against the Harvard Mw for these events shows that Mwp does predict Mw well from seismograms recorded at local, regional, and teleseismic distances. For some complex earthquakes, eg. the Mw 8.4(HRV) Peru earthquake of June 21, 2001, Mwp underestimates Mw if the first moment release is not the largest. Our estimates of Mwp for the Mw 9.3 Summatra-Andaman Island's earthquake of December 26, 2004 and for the Mw 8.7 (HRV) Summatra event of March 28, 2005, were Mwp 8.1, Mwp 8.7 respectively, from p-waves recorded at 15 - 90 degrees from each hypocenter.

Extended fault inversion with random slipmaps: a resolution test for the 2012 Mw 7.6 Nicoya, Costa Rica earthquake

NASA Astrophysics Data System (ADS)

López-Comino, José Ángel; Stich, Daniel; Ferreira, Ana M. G.; Morales, Jose

2015-09-01

Inversions for the full slip distribution of earthquakes provide detailed models of earthquake sources, but stability and non-uniqueness of the inversions is a major concern. The problem is underdetermined in any realistic setting, and significantly different slip distributions may translate to fairly similar seismograms. In such circumstances, inverting for a single best model may become overly dependent on the details of the procedure. Instead, we propose to perform extended fault inversion trough falsification. We generate a representative set of heterogeneous slipmaps, compute their forward predictions, and falsify inappropriate trial models that do not reproduce the data within a reasonable level of mismodelling. The remainder of surviving trial models forms our set of coequal solutions. The solution set may contain only members with similar slip distributions, or else uncover some fundamental ambiguity such as, for example, different patterns of main slip patches. For a feasibility study, we use teleseismic body wave recordings from the 2012 September 5 Nicoya, Costa Rica earthquake, although the inversion strategy can be applied to any type of seismic, geodetic or tsunami data for which we can handle the forward problem. We generate 10 000 pseudo-random, heterogeneous slip distributions assuming a von Karman autocorrelation function, keeping the rake angle, rupture velocity and slip velocity function fixed. The slip distribution of the 2012 Nicoya earthquake turns out to be relatively well constrained from 50 teleseismic waveforms. Two hundred fifty-two slip models with normalized L1-fit within 5 per cent from the global minimum from our solution set. They consistently show a single dominant slip patch around the hypocentre. Uncertainties are related to the details of the slip maximum, including the amount of peak slip (2-3.5 m), as well as the characteristics of peripheral slip below 1 m. Synthetic tests suggest that slip patterns such as Nicoya may be a fortunate case, while it may be more difficult to unambiguously reconstruct more distributed slip from teleseismic data.

Constraining shallow seismic event depth via synthetic modeling for Expert Technical Analysis at the IDC

NASA Astrophysics Data System (ADS)

Stachnik, J.; Rozhkov, M.; Baker, B.; Bobrov, D.; Friberg, P. A.

2015-12-01

Depth of event is an important criterion of seismic event screening at the International Data Center, CTBTO. However, a thorough determination of the event depth can be conducted mostly through special analysis because the IDC's Event Definition Criteria is based, in particular, on depth estimation uncertainties. This causes a large number of events in the Reviewed Event Bulletin to have depth constrained to the surface. When the true origin depth is greater than that reasonable for a nuclear test (3 km based on existing observations), this may result in a heavier workload to manually distinguish between shallow and deep events. Also, IDC depth criterion is not applicable to the events with the small t(pP-P) travel time difference, which is the case of the nuclear test. Since the shape of the first few seconds of signal of very shallow events is very sensitive to the presence of the depth phase, cross correlation between observed and theoretic seismogram can provide an estimate for the depth of the event, and so provide an expansion to the screening process. We exercised this approach mostly with events at teleseismic and partially regional distances. We found that such approach can be very efficient for the seismic event screening process, with certain caveats related mostly to the poorly defined crustal models at source and receiver which can shift the depth estimate. We used adjustable t* teleseismic attenuation model for synthetics since this characteristic is not determined for most of the rays we studied. We studied a wide set of historical records of nuclear explosions, including so called Peaceful Nuclear Explosions (PNE) with presumably known depths, and recent DPRK nuclear tests. The teleseismic synthetic approach is based on the stationary phase approximation with Robert Herrmann's hudson96 program, and the regional modelling was done with the generalized ray technique by Vlastislav Cerveny modified to the complex source topography.

Global teleseismic earthquake relocation with improved travel times and procedures for depth determination

USGS Publications Warehouse

Robert, Engdah E.; Van Hilst, R. D.; Buland, Raymond P.

1998-01-01

We relocate nearly 100, 000 events that occurred during the period 1964 to 1995 and are well-constrained teleseismically by arrival-time data reported to the International Seismological Centre (ISC) and to the U. S. Geological Survey's National Earthquake Information Center (NEIC). Hypocenter determination is significantly improved by using, in addition to regional and teleseismic P and S phases, the arrival times of PKiKP, PKPdf, and the teleseismic depth phases pP, pwP, and sP in the relocation procedure. A global probability model developed for later-arriving phases is used to independently identify the depth phases. The relocations are compared to hypocenters reported in the ISC and NEIC catalogs and by other sources. Differences in our epicenters with respect to ISC and NEIC estimates are generally small and regionally systematic due to the combined effects of the observing station network and plate geometry regionally, differences in upper mantle travel times between the reference earth models used, and the use of later-arriving phases. Focal depths are improved substantially over most other independent estimates, demonstrating (for example) how regional structures such as downgoing slabs can severely bias depth estimation when only regional and teleseismic P arrivals are used to determine the hypocenter. The new data base, which is complete to about Mw 5. 2 and includes all events for which moment-tensor solutions are available, has immediate application to high-resolution definition of Wadati-Benioff Zones (WBZs) worldwide, regional and global tomographic imaging, and other studies of earth structure.

Receiver function analysis applied to refraction survey data

NASA Astrophysics Data System (ADS)

Subaru, T.; Kyosuke, O.; Hitoshi, M.

2008-12-01

For the estimation of the thickness of oceanic crust or petrophysical investigation of subsurface material, refraction or reflection seismic exploration is one of the methods frequently practiced. These explorations use four-component (x,y,z component of acceleration and pressure) seismometer, but only compressional wave or vertical component of seismometers tends to be used in the analyses. Hence, it is needed to use shear wave or lateral component of seismograms for more precise investigation to estimate the thickness of oceanic crust. Receiver function is a function at a place that can be used to estimate the depth of velocity interfaces by receiving waves from teleseismic signal including shear wave. Receiver function analysis uses both vertical and horizontal components of seismograms and deconvolves the horizontal with the vertical to estimate the spectral difference of P-S converted waves arriving after the direct P wave. Once the phase information of the receiver function is obtained, then one can estimate the depth of the velocity interface. This analysis has advantage in the estimation of the depth of velocity interface including Mohorovicic discontinuity using two components of seismograms when P-to-S converted waves are generated at the interface. Our study presents results of the preliminary study using synthetic seismograms. First, we use three types of geological models that are composed of a single sediment layer, a crust layer, and a sloped Moho, respectively, for underground sources. The receiver function can estimate the depth and shape of Moho interface precisely for the three models. Second, We applied this method to synthetic refraction survey data generated not by earthquakes but by artificial sources on the ground or sea surface. Compressional seismic waves propagate under the velocity interface and radiate converted shear waves as well as at the other deep underground layer interfaces. However, the receiver function analysis applied to the second model cannot clearly estimate the velocity interface behind S-P converted wave or multi-reflected waves in a sediment layer. One of the causes is that the incidence angles of upcoming waves are too large compared to the underground source model due to the slanted interface. As a result, incident converted shear waves have non-negligible energy contaminating the vertical component of seismometers. Therefore, recorded refraction waves need to be transformed from depth-lateral coordinate into radial-tangential coordinate, and then Ps converted waves can be observed clearly. Finally, we applied the receiver function analysis to a more realistic model. This model has not only similar sloping Mohorovicic discontinuity and surface source locations as second model but the surface water layer. Receivers are aligned on the sea bottom (OBS; Ocean Bottom Seismometer survey case) Due to intricately bounced reflections, simulated seismic section becomes more complex than the other previously-mentioned models. In spite of the complexity in the seismic records, we could pick up the refraction waves from Moho interface, after stacking more than 20 receiver functions independently produced from each shot gather. After these processing, the receiver function analysis is justified as a method to estimate the depths of velocity interfaces and would be the applicable method for refraction wave analysis. The further study will be conducted for more realistic model that contain inhomogeneous sediment model, for example, and finally used in the inversion of the depth of velocity interfaces like Moho.

AIMBAT: A Python/Matplotlib Tool for Measuring Teleseismic Arrival Times

NASA Astrophysics Data System (ADS)

Lou, X.; van der Lee, S.; Lloyd, S.

2013-12-01

Python is an open-source, platform-independent, and object-oriented scripting language. It became more popular in the seismologist community since the appearance of ObsPy (Beyreuther et al. 2010, Megies et al. 2011), which provides a powerful framework for seismic data access and processing. This study introduces a new Python-based tool named AIMBAT (Automated and Interactive Measurement of Body-wave Arrival Times) for measuring teleseismic body-wave arrival times on large-scale seismic event data (Lou et al. 2013). Compared to ObsPy, AIMBAT is a lighter tool that is more focused on a particular aspect of seismic data processing. It originates from the widely used MCCC (Multi-Channel Cross-Correlation) method developed by VanDecar and Crosson (1990). On top of the original MCCC procedure, AIMBAT is automated in initial phase picking and is interactive in quality control. The core cross-correlation function is implemented in Fortran to boost up performance in addition to Python. The GUI (graphical user interface) of AIMBAT depends on Matplotlib's GUI-neutral widgets and event-handling API. A number of sorting and (de)selecting options are designed to facilitate the quality control of seismograms. By using AIMBAT, both relative and absolute teleseismic body-wave arrival times are measured. AIMBAT significantly improves efficiency and quality of the measurements. User interaction is needed only to pick the target phase arrival and to set a time window on the array stack. The package is easy to install and use, open-source, and is publicly available. Graphical user interface of AIMBAT.

Seismic character of the crust and upper mantle beneath the Sierra Nevada

NASA Astrophysics Data System (ADS)

Frassetto, A.; Gilbert, H.; Zandt, G.; Owens, T. J.; Jones, C.

2008-12-01

Recent geophysical studies of the Southern Sierra Nevada suggest that the removal of a gravitationally unstable, eclogitic residue links to recent volcanism and uplift in the Eastern Sierra. The Sierra Nevada EarthScope Project (SNEP) investigates the extent of this process beneath Central and Northern Sierra Nevada. We present receiver functions, which provide estimates of crustal thickness and Vp/Vs and image the response of the crust and upper mantle to lithospheric removal. For completeness this study combines data from the 2005-2007 SNEP broadband experiment, EarthScope's BigFoot Array, regional backbone stations, and earlier PASSCAL deployments. We analyze transects of teleseismic receiver functions generated using a common-conversion-point stacking algorithm. These identify a narrow, "bright" conversion from the Moho at depths of ~25-35 km along the crest of the Eastern Sierra and adjacent Basin and Range northward to the Cascade Arc. Trade-off analysis using the primary conversion and reverberations shows a high Vp/Vs (~1.9) throughout the Eastern Sierra, which may relate to partial melt present in the lower crust. To the west the crust-mantle boundary vanishes beneath the western foothills. However, low frequency receiver functions do image the crust-mantle boundary exceeding 50 km depth along the foothills to the west and south of Yosemite National Park. Unusually deep, intraplate earthquakes (Ryan et al., this session) occur in the center of this region. The frequency dependence of the Moho conversion implies a gradational increase from crust to mantle wavespeeds over a significant depth interval. The transition from a sharp to gradational Moho probably relates to the change from a delaminated granitic crust to crust with an intact, dense, eclogitic residue. The spatial correlation and focal mechanisms of the deep earthquakes suggest that a segment of this still intact residue is currently delaminating.

3D Structure of Iran and Surrounding Areas From The Simultaneous Inversion of Complementary Geophysical Observations

NASA Astrophysics Data System (ADS)

Ammon, C. J.; Maceira, M.; Cleveland, M.

2010-12-01

We present a three-dimensional seismic-structure model of the Arabian-Eurasian collision zone obtained via simultaneous, joint inversion of surface-wave dispersion measurements, teleseismic P-wave receiver functions, and gravity observations. We use a simple, approximate relationship between density and seismic velocities so that the three data sets may be combined in a single inversion. The sensitivity of the different data sets are well known: surface waves provide information on the smooth variations in elastic properties, receiver functions provide information on abrupt velocity contrasts, and gravity measurements provide information on broad-wavenumber shallow density variations and long-wavenumber components of deeper density structures. The combination of the data provides improved resolution of shallow-structure variations, which in turn help produce the smooth features at depth with less contamination from the strong heterogeneity often observed in the upper crust. We also explore geologically based smoothness constraints to help resolve sharp features in the underlying shallow 3D structure. Our focus is on the region surrounding Iran from east Turkey and Iraq in the west, to Pakistan and Afghanistan in the east. We use Bouguer gravity anomalies derived from the global gravity model extracted from the GRACE satellite mission. Surface-wave dispersion velocities in the period range between 7 and 150 s are taken from previously published tomographic maps for the region. Preliminary results show expected strong variations in the Caspian region as well as the deep sediment regions of the Persian Gulf. Regions constrained with receiver-function information generally show sharper crust-mantle boundary structure than that obtained by inversion of the surface waves alone (with thin layers and smoothing constraints). Final results of the simultaneous inversion will help us to better understand one of the most prominent examples of continental collision. Such models also provide an important starting model for time-consuming and fully 3D inversions.

Crustal structure of the Gulf of Aden southern margin: Evidence from receiver functions on Socotra Island (Yemen)

NASA Astrophysics Data System (ADS)

Ahmed, Abdulhakim; Leroy, Sylvie; Keir, Derek; Korostelev, Félicie; Khanbari, Khaled; Rolandone, Frédérique; Stuart, Graham; Obrebski, Mathias

2014-12-01

Breakup of continents in magma-poor setting occurs primarily by faulting and plate thinning. Spatial and temporal variations in these processes can be influenced by the pre-rift basement structure as well as by early syn-rift segmentation of the rift. In order to better understand crustal deformation and influence of pre-rift architecture on breakup we use receiver functions from teleseismic recordings from Socotra which is part of the subaerial Oligo-Miocene age southern margin of the Gulf of Aden. We determine variations in crustal thickness and elastic properties, from which we interpret the degree of extension related thinning and crustal composition. Our computed receiver functions show an average crustal thickness of ~ 28 km for central Socotra, which decreases westward along the margin to an average of ~ 21 km. In addition, the crust thins with proximity to the continent-ocean transition to ~ 16 km in the northwest. Assuming an initial pre-rift crustal thickness of 35 km (undeformed Arabian plate), we estimate a stretching factor in the range of ~ 2.1-2.4 beneath Socotra. Our results show considerable differences between the crustal structure of Socotra's eastern and western sides on either side of the Hadibo transfer zone; the east displays a clear intracrustal conversion phase and thick crust when compared with the western part. The majority of measurements across Socotra show Vp/Vs ratios of between 1.70 and 1.77 and are broadly consistent with the Vp/Vs values expected from the granitic and carbonate rock type exposed at the surface. Our results strongly suggest that intrusion of mafic rock is absent or minimal, providing evidence that mechanical thinning accommodated the majority of crustal extension. From our observations we interpret that the western part of Socotra corresponds to the necking zone of a classic magma-poor continental margin, while the eastern part corresponds to the proximal domain.

Improved Characterization of Far-Regional and Near-Teleseismic Phases Observed in Central Asia

DTIC Science & Technology

2010-07-02

Pn/P travel-time residuals as a function of epicentral distance. To generate this figure, we retrieved International Seismic Centre (ISC) bulletins...spectral frequency-wave number methods (e.g., Capon, 1969), multiple signal characteristic ( MUSIC ; Stoica and Nehorai, 1989), cross-correlation (Tibuleac...root and cross-correlation implementations. Methods such as MUSIC do not suffer these limitations and can perform well on far-regional arrivals

Accurate relative location estimates for the North Korean nuclear tests using empirical slowness corrections

NASA Astrophysics Data System (ADS)

Gibbons, S. J.; Pabian, F.; Näsholm, S. P.; Kværna, T.; Mykkeltveit, S.

2017-01-01

Declared North Korean nuclear tests in 2006, 2009, 2013 and 2016 were observed seismically at regional and teleseismic distances. Waveform similarity allows the events to be located relatively with far greater accuracy than the absolute locations can be determined from seismic data alone. There is now significant redundancy in the data given the large number of regional and teleseismic stations that have recorded multiple events, and relative location estimates can be confirmed independently by performing calculations on many mutually exclusive sets of measurements. Using a 1-D global velocity model, the distances between the events estimated using teleseismic P phases are found to be approximately 25 per cent shorter than the distances between events estimated using regional Pn phases. The 2009, 2013 and 2016 events all take place within 1 km of each other and the discrepancy between the regional and teleseismic relative location estimates is no more than about 150 m. The discrepancy is much more significant when estimating the location of the more distant 2006 event relative to the later explosions with regional and teleseismic estimates varying by many hundreds of metres. The relative location of the 2006 event is challenging given the smaller number of observing stations, the lower signal-to-noise ratio and significant waveform dissimilarity at some regional stations. The 2006 event is however highly significant in constraining the absolute locations in the terrain at the Punggye-ri test-site in relation to observed surface infrastructure. For each seismic arrival used to estimate the relative locations, we define a slowness scaling factor which multiplies the gradient of seismic traveltime versus distance, evaluated at the source, relative to the applied 1-D velocity model. A procedure for estimating correction terms which reduce the double-difference time residual vector norms is presented together with a discussion of the associated uncertainty. The modified velocity gradients reduce the residuals, the relative location uncertainties and the sensitivity to the combination of stations used. The traveltime gradients appear to be overestimated for the regional phases, and teleseismic relative location estimates are likely to be more accurate despite an apparent lower precision. Calibrations for regional phases are essential given that smaller magnitude events are likely not to be recorded teleseismically. We discuss the implications for the absolute event locations. Placing the 2006 event under a local maximum of overburden at 41.293°N, 129.105°E would imply a location of 41.299°N, 129.075°E for the January 2016 event, providing almost optimal overburden for the later four events.

Illuminating heterogeneous anisotropic upper mantle: testing a new anisotropic teleseismic body-wave tomography code - part II: Inversion mode

NASA Astrophysics Data System (ADS)

Munzarova, Helena; Plomerova, Jaroslava; Kissling, Edi

2015-04-01

Considering only isotropic wave propagation and neglecting anisotropy in teleseismic tomography studies is a simplification obviously incongruous with current understanding of the mantle-lithosphere plate dynamics. Furthermore, in solely isotropic high-resolution tomography results, potentially significant artefacts (i.e., amplitude and/or geometry distortions of 3D velocity heterogeneities) may result from such neglect. Therefore, we have undertaken to develop a code for anisotropic teleseismic tomography (AniTomo), which will allow us to invert the relative P-wave travel time residuals simultaneously for coupled isotropic-anisotropic P-wave velocity models of the upper mantle. To accomplish that, we have modified frequently-used isotropic teleseismic tomography code Telinv (e.g., Weiland et al., JGR, 1995; Lippitsch, JGR, 2003; Karousova et al., GJI, 2013). Apart from isotropic velocity heterogeneities, a weak hexagonal anisotropy is assumed as well to be responsible for the observed P-wave travel-time residuals. Moreover, no limitations to orientation of the symmetry axis are prescribed in the code. We allow a search for anisotropy oriented generally in 3D, which represents a unique approach among recent trials that otherwise incorporate only azimuthal anisotopy into the body-wave tomography. The presented code for retrieving anisotropy in 3D thus enables its direct applications to datasets from tectonically diverse regions. In this contribution, we outline the theoretical background of the AniTomo anisotropic tomography code. We parameterize the mantle lithosphere and asthenosphere with an orthogonal grid of nodes with various values of isotropic velocities, as well as of strength and orientation of anisotropy in 3D, which is defined by azimuth and inclination of either fast or slow symmetry axis of the hexagonal approximation of the media. Careful testing of the new code on synthetics, concentrating on code functionality, strength and weaknesses, is a necessary step before AniTomo is applied to real datasets. We examine various aspects coming along with anisotropic tomography such as setting a starting anisotropic model and parameters controlling the inversion, and particularly influence of a ray coverage on resolvability of individual anisotropic parameters. Synthetic testing also allows investigation of the well-known trade-off between effects of P-wave anisotropy and isotropic heterogeneities. Therefore, the target synthetic models are designed to represent schematically different heterogeneous anisotropic structures of the upper mantle. Testing inversion mode of the AniTomo code, considering an azimuthally quasi-equal distribution of rays and teleseismic P-wave incidences, shows that a separation of seismic anisotropy and isotropic velocity heterogeneities is plausible and that the correct orientation of the symmetry axes in a model can be found within three iterations for well-tuned damping factors.

The Rupture Characteristic of 1999 Izmit Sequence Using IRIS Data

NASA Astrophysics Data System (ADS)

Konca, A. O.; Helmberger, D. V.; Ji, C.; Tan, Y.

2003-12-01

The standard source studies use teleseismic data (30° to 90° ) to analyze earthquakes. Therefore, only a limited portion of the focal sphere is involved in source determinations. Furthermore, the locations and origin times of events remain incompatible with local determinations. Here, we attempt to resolve such issues by using IRIS data at all distances, leading to more accurate and detailed rupture properties and accurate relative locations. The 1999 Izmit earthquake sequence is chosen to test our method. The challenge of using data outside the conventional teleseismic distance range is that the arrival times and waveforms are affected more by the Earth structure. We overcome this difficulty by calibrating the path effects for the mainshock using the simpler aftershocks. Therefore, it is crucial to determine the source parameters of the aftershock. We constructed a Green's function library from a regionalized 1-D model and performed a grid search to establish the depth and fault parameters based on waveform matching for the Pnl waves between the synthetics and data, allowing the synthetics in each station to shift separately to account for the path effect. Our results show that the earthquake depth was around 7 km, rather than 19 km from local observatory (Kandilli) and 15 km from the Harvard's CMT solution. The best focal mechanism has a strike of 263° , a dip of 65° , and a rake of 180° , which is very close to the Harvard's CMT solution. The waveform fits of this aftershock is then used as a criterion to select useful source-station paths. A path with a cross-correlation value above 90% between data and synthetics is defined as a "good path" and can be used for studying the Izmit and Duzce earthquakes. We find that the stations in Central Europe and some of the Greek Islands are "good paths", while the stations in Northeast Africa and Italy cannot be used. The time shifts that give the best cross-correlation values are used to calibrate the picks of the Izmit and Duzce events. We realize that this is a very objective way to pick arrival times. However, our preliminary inversions using teleseismic data for Duzce and Izmit events show that handpicked P and S arrival times of the same station from two very close events are not always well correlated. Obviously, how we pick the arrival time governs the rupture pattern and rupture velocity. Therefore, our methodology brings a more objective approach to pick the travel times. To the end, we will invert for the source history of the Duzce and Izmit earthquakes with the regional data and compare with the inversion result using teleseismic data. Moreover, predictions of the teleseismic data, using the solution from the inversion using regional phases will be presented.

Imaging Crustal Structure of East Central United States using receiver function and implications for the accretion of juvenile crust

NASA Astrophysics Data System (ADS)

Deng, S.; Levander, A.

2017-12-01

Almost half of the North American continental plate is formed by the juvenile terrane accretion between 1.8-1.0 Ga, therefore, the suturing process of juvenile crust in East Central United States, not receiving as much attention probably due to low station coverage before the deployment of US transportable array, is of great importance to better understand the evolution of North American Plate. The Yavapai province is formed by the accretion of juvenile crust during 1.8-1.7 Ga. The northeastern part of Yavapai province is accreted to the Superior province along the Spirit Lake Tectonic Zone (SLTZ). During the period of 1.7-1.6 Ga, the Mazatzal Province, bounded the south of Yavapai Province, was added to Laurentia. The previous research mainly focuses on the southwestern Yavapai-Mazatzal boundary (Karlstrom et.al 2002, Magnani et.al 2004) but less in the northeastern area that we are interested in. The Granite-Rhyolite province is the product of the suturing event of juvenile arc crust reoccurring along the southeast margin of Laurentia between 1.55-1.35 Ga, which has been proved by the Nd model age (Whitmeyer et.al 2007). Here we will select the Mw>=5.5 teleseismic events with epicenter distance between 35 and 90 recorded by 300 available seismic stations in our study region. The receiver functions will be calculated by the water-level deconvolution in frequency domain (Langston 1979) and iterative deconvolution in time domain (Ligorria et.al 1999). The common conversion point (CCP) stacking method will then be applied to the receiver functions to create the 3-D image volume by imaging the conversion points in space from the time domain signals (Levander and Miller 2012). The preliminary results show that the accretion process of the tectonic provinces may have different models. The profiles of CCP image volume will inform us the seismic evidence to model the suturing process of juvenile Yavapai, Mozatzal and Granite-Rhyolite crust, hence providing great indication to interpret the growth of the North American plate. The updated results will be presented in the meeting.

Seismic Migration Imaging of the Mantle Transition Zone Beneath Continental US with Receiver Functions

NASA Astrophysics Data System (ADS)

Zhang, H.; Schmandt, B.

2017-12-01

The mantle transition zone has been widely studied by multiple sub-fields in geosciences including seismology, mineral physics and geodynamics. Due to the relatively high water storage capacity of olivine polymorphs (wadsleyite and ringwoodite) inside the transition zone, it is proposed to be a potential geochemical water reservoir that may contain one or more ocean masses of water. However, there is an ongoing debate about the hydration level of those minerals and how it varies from place to place. Considering that dehydration melting, which may happen during mantle flow across phase transitions between hydrated olivine polymorphs, may be seismically detectable, large-scale seismic imaging of heterogeneous scattering in the transition zone can contribute to the debate. To improve our understanding of the properties of the mantle transition zone and how they relate to mantle flow across its boundaries, it is important to gain an accurate image with large spatial coverage. The accuracy is primarily limited by the density of broadband seismic data and the imaging algorithms applied to the data, while the spatial coverage is limited by the availability of wide-aperture (>500 km) seismic arrays. Thus, the emergence of the USArray seismic data set (www.usarray.org) provides a nearly ideal data source for receiver side imaging of the mantle transition zone due to its large aperture ( 4000 km) with relatively small station spacing ( 70 km), which ensures that the transition zone beneath it is well sampled by teleseismic waves. In total, more than 200,000 P to S receiver functions will be used for imaging structures in depth range of 300 km to 800 km beneath the continental US with an improved 3D Kirchhoff pre-stacking migration method. The method uses 3-D wave fronts calculated for P and S tomography models to more accurately calculate point scattering coefficients and map receiver function lag times to 3-D position. The new images will help resolve any laterally sporadic or dipping interfaces that may be present at transition zone depths. The locations of sporadic velocity decreases will be compared with mantle flow models to evaluate the possibility of dehydration melting.

Detailed Configuration of the Underthrusting Indian Lithosphere Beneath Western Tibet Revealed by Receiver Function Images

NASA Astrophysics Data System (ADS)

Xu, Qiang; Zhao, Junmeng; Yuan, Xiaohui; Liu, Hongbing; Pei, Shunping

2017-10-01

We analyze the teleseismic waveform data recorded by 42 temporary stations from the Y2 and ANTILOPE-1 arrays using the P and S receiver function techniques to investigate the lithospheric structure beneath western Tibet. The Moho is reliably identified as a prominent feature at depths of 55-82 km in the stacked traces and in depth migrated images. It has a concave shape and reaches the deepest location at about 80 km north of the Indus-Yarlung suture (IYS). An intracrustal discontinuity is observed at 55 km depth below the southern Lhasa terrane, which could represent the upper border of the eclogitized underthrusting Indian lower crust. Underthrusting of the Indian crust has been widely observed beneath the Lhasa terrane and correlates well with the Bouguer gravity low, suggesting that the gravity anomalies in the Lhasa terrane are induced by topography of the Moho. At 20 km depth, a midcrustal low-velocity zone (LVZ) is observed beneath the Tethyan Himalaya and southern Lhasa terrane, suggesting a layer of partial melts that decouples the thrust/fold deformation of the upper crust from the shortening and underthrusting in the lower crust. The Sp conversions at the lithosphere-asthenosphere boundary (LAB) can be recognized at depths of 130-200 km, showing that the Indian lithospheric mantle is underthrusting with a ramp-flat shape beneath southern Tibet and probably is detached from the lower crust immediately under the IYS. Our observations reconstruct the configuration of the underthrusting Indian lithosphere and indicate significant along strike variations.

Mantle structure beneath eastern Africa: Evidence for a through going-mantle anomaly and its implications for the origin of Cenozoic tectonism in eastern Africa

NASA Astrophysics Data System (ADS)

Mulibo, G.; Tugume, F.; Julia, J.

2012-12-01

In this study, teleseismic earthquakes recorded on over 60 temporary AfricaArray seismic stations deployed in Uganda, Kenya, Tanzania and Zambia between 2007 and 2011 are used to invert P and S travel time residuals, together with travel time residuals from previous deployments, for a 3D image of mantle wave speeds and for examining relief on transition zone discontinuities using receiver function stacks. Tomographic images reveal a low wave speed anomaly (LWA) that dips to the SW beneath northern Zambia, extending to a depth of at least 900 km. The anomaly appears to be continuous across the transition zone, extending into the lower mantle. Receiver function stacks reveal an average transition zone thickness (TZT) across a wide region extending from central Zambia to the NE through Tanzania and into Kenya, which is ~30-40 km thinner than the global average. These results are not easily explained by models for the origin of the Cenozoic tectonism in eastern Africa that invoke a plume head or small scale convection either by edge flow or passive stretching of the lithosphere. However, the depth extent of the LWA coincident with a thin transition zone is consistent with a model invoking a through-going mantle anomaly beneath eastern Africa that links anomalous upper mantle to the African Superplume anomaly in the lower mantle beneath southern Africa. This finding indicates that geodynamic processes deep in the lower mantle are influencing surface dynamics across the Afro-Arabian rift system.

Crustal thickness and Vp/Vs beneath the southeastern United States: Constraints from receiver function stacking

NASA Astrophysics Data System (ADS)

Yang, Q.; Gao, S. S.; Liu, K. H.

2017-12-01

To provide new constraints on crustal structure and evolution models beneath a collage of tectonic provinces in the southeastern United States, a total of 10,753 teleseismic receiver functions recorded by 125 USArray and other seismic stations are used to compute crustal thickness and Vp/Vs values. The resulting crustal thicknesses range from 25 km at the coast to 51 km beneath the peak of the southern Appalachians with an average of 36.2 km ± 5.5 km. The resulting crustal thicknesses correlate well with surface elevation and Bouguer gravity anomalies. Beneath the Atlantic Coastal Plain, the crustal thicknesses show a clear eastward thinning with a magnitude of 10 km, from about 40 km beneath the western margin to 30 km beneath the coast. The Vp/Vs values for the entire study area range from 1.71 to 1.90 with a mean value of 1.80 ± 0.04. The mean Vp/Vs value is 1.82±0.035 in the southern Appalachian Mountain. The slightly larger than normal crustal Vp/Vs for this area might be the result of significant erosion of the felsic upper crust over the past 300 million years. Alternatively, it could also suggest the existence of pervasive magmatic intrusion into the Appalachian crust. The Vp/Vs measurements in the Atlantic Coastal Plain increase toward the east, ranging from 1.75 to 1.82, probably indicating a gradual increase of mafic magmatic intrusion into thinner crust during the development of the passive continental margin.

Virtual source reflection imaging of the Socorro Magma Body, New Mexico, using a dense seismic array

NASA Astrophysics Data System (ADS)

Finlay, T. S.; Worthington, L. L.; Schmandt, B.; Hansen, S. M.; Bilek, S. L.; Aster, R. C.; Ranasinghe, N. R.

2017-12-01

The Socorro Magma Body (SMB) is one of the largest known actively inflating continental magmatic intrusions. Previous studies have relied on sparse instrument coverage to determine its spatial extent, depth, and seismic signature, which characterized the body as a thin sill with a surface at 19 km below the Earth's surface. However, over the last two decades, InSAR and magneto-telluric (MT) studies have shed new light on the SMB and invigorated the scientific debate of the spatial distribution and uplift rate of the SMB. We return to seismic imaging of the SMB with the Sevilleta Array, a 12-day deployment of approximately 800 vertical component, 10-Hz geophones north of Socorro, New Mexico above and around the estimated northern half of the SMB. Teleseismic virtual source reflection profiling (TVR) employs the free surface reflection off of a teleseismic P as a virtual source in dense arrays, and has been used successfully to image basin structure and the Moho in multiple tectonic environments. The Sevilleta Array recorded 62 teleseismic events greater than M5. Applying TVR to the data collected by the Sevilleta Array, we present stacks from four events that produced the with high signal-to-noise ratios and simple source-time functions: the February 11, 2015 M6.7 in northern Argentina, the February 19, 2015 M5.4 in Kamchatka, Russia, and the February 21, 2015 M5.1 and February 22, 2015 M5.5 in western Colombia. Preliminary results suggest eastward-dipping reflectors at approximately 5 km depth near the Sierra Ladrones range in the northwestern corner of the array. Further analysis will focus on creating profiles across the area of maximum SMB uplift and constraining basin geometry.

Compilation of Moho boundary map for northern Fennoscandian shield

NASA Astrophysics Data System (ADS)

Silvennoinen, H.; Kozlovskaya, E.; Kissling, E.; Kosarev, G.

2012-04-01

POLENET/LAPNET project is a passive seismic array experiment in north-western Fennoscandian Shield. It is centered in northern Finland with some stations also in Sweden, Norway and Russia. The experiment was a part of International Polar Year (IPY) 2007-2009. One of the main targets of the project is to obtain a seismic model of the upper mantle using tomographic inversion of teleseismic travel times. To correct teleseismic travel times for crustal effect we present a new Moho map of POLENET/LAPNET study area located between 64˚ - 70˚ N and 18˚ - 34˚ E. The new Moho map is based on published models of previous 2-D controlled source seismic experiments and previous and new receiver function inversion results. There are four major seismic profiles crossing our research area. In addition there are some shorter profiles, but also quite large areas with no previous controlled source seismic information at all. These regions of data gaps were filled by a new analysis of receiver functions obtained from POLENET/LAPNET data. CRUST3D program was used for compiling these different types of 1-D and 2-D data to a 3-D crustal model. The program is designed to take advantage of the different methodological strengths and to compile a 3-D crustal model that fits all available data within its appropriate individual and methodological uncertainty limits. The original data leading to the published models were carefully analyzed to ascertain only information is used from locations where Moho reflectors/refractors were actually observed. The Moho interface is obtained by application of the principle of simplicity: the aim is to find a smoothest Moho interface that satisfies all reflector data within their a priori estimated error bars. On our Moho map we can see three main units in Moho, which suggests presence of three major crustal terrains. In south-western part of our study area the Moho is quite flat and shallow with average Moho depth of 44 km, which suggests that the crust there was formed in Achaean. In northern part of our study area the Moho is also quite flat and the average Moho depth is about 47 km, suggesting that the crust there has been reworked in Proterozoic. Two deepest Moho depressions were found in the north-eastern and south-eastern parts of study area, where it is reaching the depths of almost 60 km. The latter Moho depression appears to be limitation of the thin crust of the Karelian Craton not only from the West, but also from the East and is spatially coincident with the cluster of mineral deposits.

Intercorrelation of P and Pn Recordings for the North Korean Nuclear Tests

NASA Astrophysics Data System (ADS)

Lay, T.; Voytan, D.; Ohman, J.

2017-12-01

The relative waveform analysis procedure called Intercorrelation is applied to Pn and P waveforms at regional and teleseismic distances, respectively, for the 5 underground nuclear tests at the North Korean nuclear test site. Intercorrelation is a waveform equalization procedure that parameterizes the effective source function for a given explosion, including the reduced velocity potential convolved with a simplified Green's function that accounts for the free surface reflections (pPn and pP), and possibly additional arrivals such as spall. The source function for one event is convolved with the signal at a given station for a second event, and the recording at the same station for the first event is convolved with the source function for the second event. This procedure eliminates the need to predict the complex receiver function effects at the station, which are typically not well-known for short-period response. The parameters of the source function representation are yield and burial depth, and an explosion source model is required. Here we use the Mueller-Murphy representation of the explosion reduced velocity potential, which explicitly depends on yield and burial depth. We then search over yield and burial depth ranges for both events, constrained by a priori information about reasonable ranges of parameters, to optimize the simultaneous match of multiple station signals for the two events. This procedure, applied to the apparently overburied North Korean nuclear tests (no indications of spall complexity), assuming simple free surface interactions (elastic reflection from a flat surface), provides excellent waveform equalization for all combinations of 5 nuclear tests.

Teleseismic tomography for imaging Earth's upper mantle

NASA Astrophysics Data System (ADS)

Aktas, Kadircan

Teleseismic tomography is an important imaging tool in earthquake seismology, used to characterize lithospheric structure beneath a region of interest. In this study I investigate three different tomographic techniques applied to real and synthetic teleseismic data, with the aim of imaging the velocity structure of the upper mantle. First, by applying well established traveltime tomographic techniques to teleseismic data from southern Ontario, I obtained high-resolution images of the upper mantle beneath the lower Great Lakes. Two salient features of the 3D models are: (1) a patchy, NNW-trending low-velocity region, and (2) a linear, NE-striking high-velocity anomaly. I interpret the high-velocity anomaly as a possible relict slab associated with ca. 1.25 Ga subduction, whereas the low-velocity anomaly is interpreted as a zone of alteration and metasomatism associated with the ascent of magmas that produced the Late Cretaceous Monteregian plutons. The next part of the thesis is concerned with adaptation of existing full-waveform tomographic techniques for application to teleseismic body-wave observations. The method used here is intended to be complementary to traveltime tomography, and to take advantage of efficient frequency-domain methodologies that have been developed for inverting large controlled-source datasets. Existing full-waveform acoustic modelling and inversion codes have been modified to handle plane waves impinging from the base of the lithospheric model at a known incidence angle. A processing protocol has been developed to prepare teleseismic observations for the inversion algorithm. To assess the validity of the acoustic approximation, the processing procedure and modelling-inversion algorithm were tested using synthetic seismograms computed using an elastic Kirchhoff integral method. These tests were performed to evaluate the ability of the frequency-domain full-waveform inversion algorithm to recover topographic variations of the Moho under a variety of realistic scenarios. Results show that frequency-domain full-waveform tomography is generally successful in recovering both sharp and discontinuous features. Thirdly, I developed a new method for creating an initial background velocity model for the inversion algorithm, which is sufficiently close to the true model so that convergence is likely to be achieved. I adapted a method named Deformable Layer Tomography (DLT), which adjusts interfaces between layers rather than velocities within cells. I applied this method to a simple model comprising a single uniform crustal layer and a constant-velocity mantle, separated by an irregular Moho interface. A series of tests was performed to evaluate the sensitivity of the DLT algorithm; the results show that my algorithm produces useful results within a realistic range of incident-wave obliquity, incidence angle and signal-to-noise level. Keywords. Teleseismic tomography, full waveform tomography, deformable layer tomography, lower Great Lakes, crust and upper mantle.

Fully probabilistic earthquake source inversion on teleseismic scales

NASA Astrophysics Data System (ADS)

Stähler, Simon; Sigloch, Karin

2017-04-01

Seismic source inversion is a non-linear problem in seismology where not just the earthquake parameters but also estimates of their uncertainties are of great practical importance. We have developed a method of fully Bayesian inference for source parameters, based on measurements of waveform cross-correlation between broadband, teleseismic body-wave observations and their modelled counterparts. This approach yields not only depth and moment tensor estimates but also source time functions. These unknowns are parameterised efficiently by harnessing as prior knowledge solutions from a large number of non-Bayesian inversions. The source time function is expressed as a weighted sum of a small number of empirical orthogonal functions, which were derived from a catalogue of >1000 source time functions (STFs) by a principal component analysis. We use a likelihood model based on the cross-correlation misfit between observed and predicted waveforms. The resulting ensemble of solutions provides full uncertainty and covariance information for the source parameters, and permits propagating these source uncertainties into travel time estimates used for seismic tomography. The computational effort is such that routine, global estimation of earthquake mechanisms and source time functions from teleseismic broadband waveforms is feasible. A prerequisite for Bayesian inference is the proper characterisation of the noise afflicting the measurements. We show that, for realistic broadband body-wave seismograms, the systematic error due to an incomplete physical model affects waveform misfits more strongly than random, ambient background noise. In this situation, the waveform cross-correlation coefficient CC, or rather its decorrelation D = 1 - CC, performs more robustly as a misfit criterion than ℓp norms, more commonly used as sample-by-sample measures of misfit based on distances between individual time samples. From a set of over 900 user-supervised, deterministic earthquake source solutions treated as a quality-controlled reference, we derive the noise distribution on signal decorrelation D of the broadband seismogram fits between observed and modelled waveforms. The noise on D is found to approximately follow a log-normal distribution, a fortunate fact that readily accommodates the formulation of an empirical likelihood function for D for our multivariate problem. The first and second moments of this multivariate distribution are shown to depend mostly on the signal-to-noise ratio (SNR) of the CC measurements and on the back-azimuthal distances of seismic stations. References: Stähler, S. C. and Sigloch, K.: Fully probabilistic seismic source inversion - Part 1: Efficient parameterisation, Solid Earth, 5, 1055-1069, doi:10.5194/se-5-1055-2014, 2014. Stähler, S. C. and Sigloch, K.: Fully probabilistic seismic source inversion - Part 2: Modelling errors and station covariances, Solid Earth, 7, 1521-1536, doi:10.5194/se-7-1521-2016, 2016.

Crustal structure of northern Egypt from joint inversion of receiver functions and surface wave dispersion velocities

NASA Astrophysics Data System (ADS)

Badawy, Ahmed; Hegazi, Mona; Gaber, Hanan; Korrat, Ibrahim

2018-05-01

In this study, we used a combined inversion of body wave receiver functions and surface wave dispersion measurements to provide constraints on the crustal structure of northern Egypt. The two techniques are complementary to each other: receiver functions (RFs) are sensitive to shear-wave velocity contrasts, while surface wave dispersion (SWD) measurements are sensitive to finite variations of shear-wave velocity with depth. A database of 122 teleseismic events digitally recorded by the Egyptian National Seismological Network (ENSN) stations has been used as well. To enhance the resulting RFs at each ENSN station, the H-k stacking method was applied. A joint inversion process between the resulting receiver functions and the surface wave dispersion curves was applied as well. We have produced three averaged velocity structure models for distinct geographic and tectonic provinces namely Sinai, eastern desert, and western desert from east to the west respectively. These models will deeply help in estimation the epicenter distance of earthquake, focal mechanism solutions, and earthquake hazard analysis in northern Egypt. An obvious image of the subsurface structure has been determined which shows that generally the crustal structure of northern Egypt consists of three layers covered with a sequence of sediments that differs in thickness from across the region except in the Sharm area where the sedimentary cover is absent. The obtained results indicate that crustal thickness differs from east to west and reaches its maximum value of about 36 km at Siwa station (SWA) in the western desert and its minimum value of about 28 km at Sharm station (SHR) of the southern tip of the Sinai Peninsula. The Vp/Vs ratio varies between 1.71 and 2.07 in northern Egypt. Generally, the high values (1.93) of (Vp/Vs) at SWA station may reflect the well-known rich aquifer with fully saturated sediments of the Swia Oasis in the Western Desert. Moreover, the highest value (2.07) of (Vp/Vs) at BNS station may be attributed to the widespread recently discovered hydrocarbon fields at the Beni-Suef Basin along the Eastern Desert. Finally, an integrated geophysical and hydrological study of the dimensions and physical properties of the aquifer and hydrocarbon fields at SWA and BNS stations to confirm if they are sufficient to produce the elevated Vp/Vs ratios or not become essential and highly recommended.

Crustal structure of northern Egypt from joint inversion of receiver functions and surface wave dispersion velocities

NASA Astrophysics Data System (ADS)

Badawy, Ahmed; Hegazi, Mona; Gaber, Hanan; Korrat, Ibrahim

2018-01-01

In this study, we used a combined inversion of body wave receiver functions and surface wave dispersion measurements to provide constraints on the crustal structure of northern Egypt. The two techniques are complementary to each other: receiver functions (RFs) are sensitive to shear-wave velocity contrasts, while surface wave dispersion (SWD) measurements are sensitive to finite variations of shear-wave velocity with depth. A database of 122 teleseismic events digitally recorded by the Egyptian National Seismological Network (ENSN) stations has been used as well. To enhance the resulting RFs at each ENSN station, the H-k stacking method was applied. A joint inversion process between the resulting receiver functions and the surface wave dispersion curves was applied as well. We have produced three averaged velocity structure models for distinct geographic and tectonic provinces namely Sinai, eastern desert, and western desert from east to the west respectively. These models will deeply help in estimation the epicenter distance of earthquake, focal mechanism solutions, and earthquake hazard analysis in northern Egypt. An obvious image of the subsurface structure has been determined which shows that generally the crustal structure of northern Egypt consists of three layers covered with a sequence of sediments that differs in thickness from across the region except in the Sharm area where the sedimentary cover is absent. The obtained results indicate that crustal thickness differs from east to west and reaches its maximum value of about 36 km at Siwa station (SWA) in the western desert and its minimum value of about 28 km at Sharm station (SHR) of the southern tip of the Sinai Peninsula. The Vp/Vs ratio varies between 1.71 and 2.07 in northern Egypt. Generally, the high values (1.93) of (Vp/Vs) at SWA station may reflect the well-known rich aquifer with fully saturated sediments of the Swia Oasis in the Western Desert. Moreover, the highest value (2.07) of (Vp/Vs) at BNS station may be attributed to the widespread recently discovered hydrocarbon fields at the Beni-Suef Basin along the Eastern Desert. Finally, an integrated geophysical and hydrological study of the dimensions and physical properties of the aquifer and hydrocarbon fields at SWA and BNS stations to confirm if they are sufficient to produce the elevated Vp/Vs ratios or not become essential and highly recommended.

Crustal Structure and Deformation of the Sichuan-Yunnan Region Revealed by receiver Function Data

NASA Astrophysics Data System (ADS)

Zeng, S.; Zheng, Y.

2017-12-01

Sichuan-Yunnan and its surrounding areas locates in the southeast side to the Tibetan Plateau, due to the intrusion of the Indian Plate under the Tibetan Plateau, materials escape from the Tibetan Plateau and flow southward to southeastward. Because of such tectonic environment, the Sichuan-Yunnan region is experiencing high tectonic movement, and is capable of highly diffused seismicity. Based on dynamic simulation and field survey investigations, tectonic and geological studies proposed a decoupling model in this region and lower crustal flow may inflate in the crust. However, this idea needs more evidences, especially anisotropic structures to support it, since the anisotropic structures are usually directly related to the movement of materials, or to the tectonic distributions. In the past several years, a number of works have been done on the anisotropic structures in the Tibetan Plateau and its surroundings. In usually, previous studies were mainly carried out by two kinds of methods. First, the shear wave splitting of SKS, which mainly reflects the accumulation effect of the anisotropy of the crust to the mantle; the other way is use surface wave to investigate the anisotropic features at different azimuths and depths. In the recent years, receiver function is used to determine the inclination and anisotropy of the subsurface structure, comparing with the other two methods, receiver functions can provide higher resolution and reliable anisotropic features in the crust. Following the method of Liu and Niu(2012), we collected teleseismic data from the Himalayan first term network, and picked out high quality data based on the waveform SNR ratio, as well as the azimuthal distributions. Comparing with previous work (e.g., Sun et al.,2012), our work can provide more receiver functions results with higher reliability. We find that the crust beneath the Sichuan-Yunnan region has a thickness of 30-60 km and Vp/Vs ratio of 1.70-1.80. The Moho depth from northwest to southeast showed a trend of gradual thinning. We also find that the crust beneath this area is highly anisotropic, and align with the main fault. Crustal anisotropy is present at most stations with a fast axis trending N-S to NW-SE from west side to east side of this region.

Studying the Effects of Transparent vs. Opaque Shallow Thrust Faults Using Synthetic P and SH Seismograms

NASA Astrophysics Data System (ADS)

Smith, D. E.; Aagaard, B. T.; Heaton, T. H.

2001-12-01

It has been hypothesized (Brune, 1996) that teleseismic inversions may underestimate the moment of shallow thrust fault earthquakes if energy becomes trapped in the hanging wall of the fault, i.e. if the fault boundary becomes opaque. We address this by creating and analyzing synthetic P and SH seismograms for a variety of friction models. There are a total of five models: (1) crack model (slip weakening) with instantaneous healing (2) crack model without healing (3) crack model with zero sliding friction (4) pulse model (slip and rate weakening) (5) prescribed model (Haskell-like rupture with the same final slip and peak slip-rate as model 4). Models 1-4 are all dynamic models where fault friction laws determine the rupture history. This allows feedback between the ongoing rupture and waves from the beginning of the rupture that hit the surface and reflect downwards. Hence, models 1-4 can exhibit opaque fault characteristics. Model 5, a prescribed rupture, allows for no interaction between the rupture and reflected waves, therefore, it is a transparent fault. We first produce source time functions for the different friction models by rupturing shallow thrust faults in 3-D dynamic finite-element simulations. The source time functions are used as point dislocations in a teleseismic body-wave code. We examine the P and SH waves for different azimuths and epicentral distances. The peak P and S first arrival displacement amplitudes for the crack, crack with healing and pulse models are all very similar. These dynamic models with opaque faults produce smaller peak P and S first arrivals than the prescribed, transparent fault. For example, a fault with strike = 90 degrees, azimuth = 45 degrees has P arrivals smaller by about 30% and S arrivals smaller by about 15%. The only dynamic model that doesn't fit this pattern is the crack model with zero sliding friction. It oscillates around its equilibrium position; therefore, it overshoots and yields an excessively large peak first arrival. In general, it appears that the dynamic, opaque faults have smaller peak teleseismic displacements that would lead to lower moment estimates by a modest amount.

Developments of Finite-Frequency Seismic Theory and Applications to Regional Tomographic Imaging

DTIC Science & Technology

2009-01-31

banana -doughnut” sensitivity kernels of teleseismic body waves to image the crust and mantle beneath eastern Eurasia. We have collected and processed...In this project, we use the “ banana -doughnut” sensitivity kernels of teleseismic body waves to image the crust and mantle beneath eastern Eurasia...replaced body-wave ray paths with “ banana -doughnut” sensitivity kernels calculated in 1D (Dahlen et al., 2000; Hung et al., 2000; Zhao et al., 2000

A Unified Approach to Joint Regional/Teleseismic Calibration and Event Location with a 3D Earth Model

DTIC Science & Technology

2010-09-01

raytracing and travel-time calculation in 3D Earth models, such as the finite-difference eikonal method (e.g., Podvin and Lecomte, 1991), fast...by Reiter and Rodi (2009) in constructing JWM. Two teleseismic data sets were considered, both extracted from the EHB database (Engdahl et al...extracted from the updated EHB database distributed by the International Seismological Centre (http://www.isc.ac.uk/EHB/index.html). The new database

Imaging Basin Structure with Teleseismic Virtual Source Reflection Profiles

NASA Astrophysics Data System (ADS)

Yang, Z.; Sheehan, A. F.; Yeck, W. L.; Miller, K. C.; Worthington, L. L.; Erslev, E.; Harder, S. H.; Anderson, M. L.; Siddoway, C. S.

2011-12-01

We demonstrate a case of using teleseisms recorded on single channel high frequency geophones to image upper crustal structure across the Bighorn Arch in north-central Wyoming. The dataset was obtained through the EarthScope FlexArray Bighorn Arch Seismic Experiment (BASE). In addition to traditional active and passive source seismic data acquisition, BASE included a 12 day continuous (passive source) deployment of 850 geophones with 'Texan' dataloggers. The geophones were deployed in three E-W lines in north-central Wyoming extending from the Powder River Basin across the Bighorn Mountains and across the Bighorn Basin, and two N-S lines on east and west flanks of the Bighorn Mountains. The station interval is roughly 1.5-2 km, good for imaging coherent shallow structures. The approach used in this study uses the surface reflection as virtual seismic source and reverberated teleseismic P-wave phase (PpPdp) (teleseismic P-wave reflected at receiver side free surface and then reflected off crustal seismic interface) to construct seismic profiles. These profiles are equivalent to conventional active source seismic reflection profiles except that high-frequency (up to 2.4 Hz) transmitted wave fields from distant earthquakes are used as sources. On the constructed seismic profiles, the coherent PpPdp phases beneath Powder River and Bighorn Basins are distinct after the source wavelet is removed from the seismograms by deconvolution. Under the Bighorn Arch, no clear coherent signals are observed. We combine phases PpPdp and Ps to constrain the averaged Vp/Vs: 2.05-2.15 for the Powder River Basin and 1.9-2.0 for the Bighorn Basin. These high Vp/Vs ratios suggest that the layers within which P-wave reverberates are sedimentary. Assuming Vp as 4 km/s under the Powder River Basin, the estimated thickness of sedimentary layer above reflection below the profile is 3-4.5 km, consistent with the depth of the top of the Tensleep Fm. Therefore we interpret the coherent PpPdp phases about 1-3 s after direct P-wave arrival as the reflections off the interface between the Paleozoic carbonates/sandstones and Mesozoic shales.

Assessing the short-term clock drift of early broadband stations with burst events of the 26 s persistent and localized microseism

NASA Astrophysics Data System (ADS)

Xie, Jun; Ni, Sidao; Chu, Risheng; Xia, Yingjie

2018-01-01

Accurate seismometer clock plays an important role in seismological studies including earthquake location and tomography. However, some seismic stations may have clock drift larger than 1 s (e.g. GSC in 1992), especially in early days of global seismic networks. The 26 s Persistent Localized (PL) microseism event in the Gulf of Guinea sometime excites strong and coherent signals, and can be used as repeating source for assessing stability of seismometer clocks. Taking station GSC, PAS and PFO in the TERRAscope network as an example, the 26 s PL signal can be easily observed in the ambient noise cross-correlation function between these stations and a remote station OBN with interstation distance about 9700 km. The travel-time variation of this 26 s signal in the ambient noise cross-correlation function is used to infer clock error. A drastic clock error is detected during June 1992 for station GSC, but not found for station PAS and PFO. This short-term clock error is confirmed by both teleseismic and local earthquake records with a magnitude of 25 s. Averaged over the three stations, the accuracy of the ambient noise cross-correlation function method with the 26 s source is about 0.3-0.5 s. Using this PL source, the clock can be validated for historical records of sparsely distributed stations, where the usual ambient noise cross-correlation function of short-period (<20 s) ambient noise might be less effective due to its attenuation over long interstation distances. However, this method suffers from cycling problem, and should be verified by teleseismic/local P waves. Further studies are also needed to investigate whether the 26 s source moves spatially and its effects on clock drift detection.

Love and Rayleigh wave dispersion from regional Ambient Noise Tomography in the Eastern Alps of Europe

NASA Astrophysics Data System (ADS)

Behm, Michael; Nakata, Nori; Bianchi, Irene; Bokelmann, Götz

2014-05-01

ALPASS is an international passive seismic monitoring experiment aimed at understanding the upper mantle structure in the in the European Eastern Alps. Data were collected from May 2005 to June 2006 at about 50 locations with an average spacing of 20 km, and have been used for teleseismic travel time tomography and receiver function analysis in previous studies. We combine the ALPASS data from 23 broadband stations with additional data from the temporary CBP (Carpathian Basin Project) network (15 stations), and present results from ambient noise tomography applied to the region covering the easternmost part of the Alps and its transition to the adjacent tectonic provinces (Vienna Basin, Bohemian Massif, Dinarides). By turning each station into a virtual source, we are able to recover surface waves in the frequency range of 0.05 - 0.5 Hz, which are sensitive to depths of approximately 2 - 15 kilometers. The three-component recordings allow distinguishing between Rayleigh waves on the vertical/radial components and Love waves on the transverse component. On average, the Love waves have higher apparent velocity by about 15%. Owing to dense receiver spacing and high S/N ratio of the obtained interferograms, we are able to derive a large set of dispersion curves. The complicated 3D structure of the investigated region calls for a tomographic approach to transform these dispersion curves to be representative of local 1D structures. The results correlate well with surface geology and provide the input to inversion for the vertical shear-wave velocity distribution. Compared to data from active source experiments, we derive lower average shear wave velocities. This observation is comparable to receiver functions analysis which show a high Vp/Vs ratio for the area of the Molasse basin, where the shear wave velocities retrieved from the surface wave inversion are in particular low.

Calculation of Source and Strucutral Parameters at Regional and Teleseismic Distances

DTIC Science & Technology

1987-02-28

Explosions, Elsevier, Amsterdam-Oxford-New York, 440 pages, K 1977. h .- 34 Denham.D., L.G. Alexander, and G . Worotnicki, The stress field near the ...km. Support «.r.i; this conclusion is the existence of short-period siiz-face wave», ^ g waves in events 5, 6, 9, and 13 and Love waves in events 1...for Engineering Applications, New York- Wiley 126 Fig.l Normalized field amplitude as a function of G , the angular displacement from the y-axis. in

Determination of stress glut moments of total degree 2 from teleseismic surface wave amplitude spectra

NASA Astrophysics Data System (ADS)

Bukchin, B. G.

1995-08-01

A special case of the seismic source, where the stress glut tensor can be expressed as a product of a uniform moment tensor and a scalar function of spatial coordinates and time, is considered. For such a source, a technique of determining stress glut moments of total degree 2 from surface wave amplitude spectra is described. The results of application of this technique for the estimation of spatio-temporal characteristics of the Georgian earthquake, 29.04.91 are presented.

It's Still Downhill From Tonopah to Las Vegas, but the Crust Doesn't Ride for Free

NASA Astrophysics Data System (ADS)

Pettit, M. M.; Schulte-Pelkum, V.; Sheehan, A.

2008-12-01

We investigate the crustal thickness in the central Basin and Range province of the western US. There is a gravity anomaly at 37 degrees N latitude at which the gravity increases ~100mgal from North to South over a distance of ~100 km. The majority of recent publications ascribe the gravity signal to a mantle influence based on observations of near constant crustal thickness in the area. However, Moho depth estimates are sparse in the area, and therefore higher gravity due to a thinner crust in the south is still a possible explanation to date. In order to determine Moho depths, we examined teleseismic receiver functions from broadband and short-period stations from 1993 to 2008 located within the region, including stations from the recent Earthscope Transportable Array deployment. We used a total of 11,751 high-quality receiver functions at 80 stations and picked arrival times of the Moho converted phase from backazimuthal and moveout stacks. Moho depths were determined from these arrival times using a fixed velocity model, as well as from forward modeling of moveout curves of the direct conversion as well as multiples. Our results confirm the presence of thinner crust south of 37N latitude. Assuming an average crustal velocity of 6.3 km/s and a Vp/Vs ratio of 1.732, we found an average crustal thickness between 33 and 34 km north of 37N, and roughly 27 km south of 37N. We also found an interesting pattern of thin crust trending NE from the southern tip of Nevada to approximately 38N, 245E. The findings indicate that a least part of the gravity signal is of crustal origin.

Crustal geometry of the northeastern Gulf of Aden passive margin: localization of the deformation inferred from receiver function analysis

NASA Astrophysics Data System (ADS)

Tiberi, C.; Leroy, S.; d'Acremont, E.; Bellahsen, N.; Ebinger, C.; Al-Lazki, A.; Pointu, A.

2007-03-01

Here we use receiver function analysis to retrieve crustal thickness and crustal composition along the 35-My-old passive margin of the eastern Gulf of Aden. Our aims are to use results from the 3-D seismic array to map crustal stretching across and along the Aden margin in southern Oman. The array recorded local and teleseismic events between 2003 March and 2004 March. Seventy-eight events were used in our joint inversions for Vp/Vs ratio and depth. The major results are: (1) Crustal thickness decreases from the uplifted rift flank of the margin towards the Sheba mid-ocean ridge. We found a crustal thickness of about 35 km beneath the northern rift flank. This value decreases sharply to 26 km beneath the post-rift subsidence zone on the Salalah coastal plain. This 10 km of crustal thinning occurs across a horizontal distance of less than 30 km showing a localization of the crustal thinning below the first known rifted block of the margin. (2) A second rift margin transect located about 50 km to the east shows no thinning from the coast to 50 km onshore. The lack of crustal thickness variation indicates that the maximum crustal stretching could be restricted to offshore regions. (3) The along-strike variations in crustal structure demonstrate the scale and longevity of the regular along-axis rift segmentation. (4) Extension is still observed north of the rifted domain, 70 km onshore from the coast, making the width of margin larger than first expected from geology. (5) The crust has a felsic to normal composition with a probably strong effect of the sedimentary layer on the Vp/Vs ratio (comprised between 1.67 and 1.91).

Mapping the mantle transition zone beneath the central Mid-Atlantic Ridge using Ps receiver functions.

NASA Astrophysics Data System (ADS)

Agius, M. R.; Rychert, C.; Harmon, N.; Kendall, J. M.

2017-12-01

Determining the mechanisms taking place beneath ridges is important in order to understand how tectonic plates form and interact. Of particular interest is establishing the depth at which these processes originate. Anomalies such as higher temperature within the mantle transition zone may be inferred seismically if present. However, most ridges are found in remote locations beneath the oceans restricting seismologists to use far away land-based seismometers, which in turn limits the imaging resolution. In 2016, 39 broadband ocean-bottom seismometers were deployed across the Mid-Atlantic Ridge, along the Romanche and Chain fracture zones as part of the PI-LAB research project (Passive Imaging of the Lithosphere and Asthenosphere Boundary). The one-year long seismic data is now retrieved and analysed to image the mantle transition zone beneath the ridge. We determine P-to-s (Ps) receiver functions to illuminate the 410- and 660-km depth mantle discontinuities using the extended multitaper deconvolution. The data from ocean-bottom seismometers have tilt and compliance noise corrections and is filtered between 0.05-0.2 Hz to enhance the signal. 51 teleseismic earthquakes generated hundreds of good quality waveforms, which are then migrated to depth in 3-D. The topography at the d410 deepens towards the west of the Romanche and Chain fracture zone by 15 km, whereas the topography of d660 shallows beneath the ridge between the two zones. Transition zone thickness thins from 5 to 20 km. Thermal anomalies determined from temperature relationships with transition zone thickness and depth variations of the d410 and d660 suggests hotter temperatures of about 200 K. Overall, the result suggests mid-ocean ridges may have associated thermal signatures as deep as the transition zone.

Crustal structure beneath northeast India inferred from receiver function modeling

NASA Astrophysics Data System (ADS)

Borah, Kajaljyoti; Bora, Dipok K.; Goyal, Ayush; Kumar, Raju

2016-09-01

We estimated crustal shear velocity structure beneath ten broadband seismic stations of northeast India, by using H-Vp/Vs stacking method and a non-linear direct search approach, Neighbourhood Algorithm (NA) technique followed by joint inversion of Rayleigh wave group velocity and receiver function, calculated from teleseismic earthquakes data. Results show significant variations of thickness, shear velocities (Vs) and Vp/Vs ratio in the crust of the study region. The inverted shear wave velocity models show crustal thickness variations of 32-36 km in Shillong Plateau (North), 36-40 in Assam Valley and ∼44 km in Lesser Himalaya (South). Average Vp/Vs ratio in Shillong Plateau is less (1.73-1.77) compared to Assam Valley and Lesser Himalaya (∼1.80). Average crustal shear velocity beneath the study region varies from 3.4 to 3.5 km/s. Sediment structure beneath Shillong Plateau and Assam Valley shows 1-2 km thick sediment layer with low Vs (2.5-2.9 km/s) and high Vp/Vs ratio (1.8-2.1), while it is observed to be of greater thickness (4 km) with similar Vs and high Vp/Vs (∼2.5) in RUP (Lesser Himalaya). Both Shillong Plateau and Assam Valley show thick upper and middle crust (10-20 km), and thin (4-9 km) lower crust. Average Vp/Vs ratio in Assam Valley and Shillong Plateau suggest that the crust is felsic-to-intermediate and intermediate-to-mafic beneath Shillong Plateau and Assam Valley, respectively. Results show that lower crust rocks beneath the Shillong Plateau and Assam Valley lies between mafic granulite and mafic garnet granulite.

Seismic structure beneath the Tengchong volcanic area (southwest China) from receiver function analysis

NASA Astrophysics Data System (ADS)

Xu, Yi; Li, Xuelei; Wang, Sheng

2018-05-01

Tengchong is a young volcanic area on the collision boundary between the Indian and Euro-Asian plates of the southeastern Tibetan margin. Holocene volcanoes are concentrated in the Tengchong basin, where they align an N-S trending string-like cluster. To study the magma activity and its relation with the volcanoes, we deployed a passive seismic observation across the volcanic area in northern Tengchong. Using tele-seismic data and receiver function technique, we determined the S-wave velocity structure beneath nine temporary stations. Results show that the Tengchong basin is underlain by prominent low-velocity zones that are associated with the magma chambers of the volcanoes. In the north, a small and less pronounced magma chamber lies beneath two crater lakes, with a depth range of 9-16 km and a lateral width of <8 km. To the south, an interconnected magma chamber is found between the Dayingshan (DYS) volcano and the Dakongshan (DKS) volcanic cluster, with a depth range of 6-15 km and a lateral width of <12 km. In the south, the Laoguipo (LGP) volcano is characterized by anomalous low velocities throughout the upper-mid crust. Combined with other studies, we infer that the DYS volcano shares the same magma chamber with the DKS volcanic cluster, whereas the heat flow beneath the LGP volcano belongs to another thermal system, probably relating to the magma activity beneath the Rehai geothermal field in the south or affected by the intersection between the Tengchong volcanic fault zone and the Dayingjiang fault zone. In addition, mantle intrusion has resulted in the Moho elevation beneath the DKS volcanic cluster, and the thick transition zones on the crust-mantle boundary imply a possible penetration of the heat flow from the uppermost mantle into the lower crust.

eqMAXEL: A new automatic earthquake location algorithm implementation for Earthworm

NASA Astrophysics Data System (ADS)

Lisowski, S.; Friberg, P. A.; Sheen, D. H.

2017-12-01

A common problem with automated earthquake location systems for a local to regional scale seismic network is false triggering and false locations inside the network caused by larger regional to teleseismic distance earthquakes. This false location issue also presents a problem for earthquake early warning systems where societal impacts of false alarms can be very expensive. Towards solving this issue, Sheen et al. (2016) implemented a robust maximum-likelihood earthquake location algorithm known as MAXEL. It was shown with both synthetics and real-data for a small number of arrivals, that large regional events were easily identifiable through metrics in the MAXEL algorithm. In the summer of 2017, we collaboratively implemented the MAXEL algorithm into a fully functional Earthworm module and tested it in regions of the USA where false detections and alarming are observed. We show robust improvement in the ability of the Earthworm system to filter out regional and teleseismic events that would have falsely located inside the network using the traditional Earthworm hypoinverse solution. We also explore using different grid sizes in the implementation of the MAXEL algorithm, which was originally designed with South Korea as the target network size.

Along-strike slab segmentation under Greece from a 500 km long teleseismic receiver-function swath profile : control on large earthquakes, upper plate motion, and surface morphology

NASA Astrophysics Data System (ADS)

Sachpazi, M.; Laigle, M.; Diaz, J.; Gesret, A.; Charalampakis, M.; Kissling, E. H.; Hirn, A.

2010-12-01

Observations from teleseismic converted waves recorded at 100 sites in Greece from Crete to North Aegean in a 500 km swath along the slab strike during the EU project “Thales was right” allow imaging its top in 3D. Multiscale analysis brings high-resolution to interface imaging at depth which resolved for the first time a thin, oceanic, crust for the slab under southern Greece. This first indication of its large negative buoyancy suggests its roll-back and is consistent with the upper plate trenchward motion with the highest velocities there, as shown by GPS. With respect to up to now subduction zone surveys with receivers deployed along the presumed dip to get a cross-section of the downgoing slab, our swath was instead perpendicular, that is along strike. This was in order to track down lateral changes in slab attitude along the subduction zone, that is a possible segmentation. The expected subduction strike at shallow depth, as approximated by a line from SW of Crete to W of the Ionian Islands is about N 135°E. Instead, the slab top is found along an almost N-S line at several places, at 60-70 km depth. However the slab depth contours deviate from it in-between. Their broad correspondance with the Aegean coastline or extensional domain suggests a possible control on surface morphology, and on upper plate deformation as mirrored in the topography of its crust-mantle boundary. Indeed, this first image recovered with such a high lateral resolution reveals that several slab segments can be defined dipping N 60°E, that is with a N 160 °E strike, and that these are juxtaposed through domains of strong localized variations along-strike that suggest warping or tearing of the slab. Apart their strong bearing on geodynamic reconstructions, and the continental/oceanic nature of the slab fragments, these 3D images reach the high-resolution for their discussion with respect to major earthquakes. The attitude of the slab, the dip of its upper part and its buoyancy force enter the balance controlling the degree of seismic coupling, of the seismogenic interplate fault, as well as its along dip extent as discussed earlier for the Ionian Islands. The segmented nature revealed at depth suggests a possible segmentation of the shallower interplate seismogenic zone. The precise location of the stronger intermediate-depth earthquakes occurred during the deployment appears also related to this deep structural and tectonic control.

Imaging the crustal and lithospheric structures beneath the Alboran Domain and its surrounding area

NASA Astrophysics Data System (ADS)

Dündar, Süleyman; Kind, Rainer; Yuan, Xiaohui

2010-05-01

The knowledge of the crustal and lithospheric structures plays an important role in understanding the geodynamic evolution of the Earth's interiors within the framework of plate tectonics. The receiver function method is used to resolve the seismic discontinuity structure of the crust and upper mantle beneath a recording station and to infer possible geodynamic processes within the Earth. The methodology is developed based on the conversion of elastic body waves (P and S) at an interface which represents a boundary between different elastic properties. In this study, we analyze the P- and S-wave receiver functions in order to investigate seismic deep structures beneath the Alboran Domain which is still in debate despite a large amount of research effort conducted along the region of interest. The Alboran Domain is located at the western end of the Mediterranean and Betic-Rif orogenic system. The study area is on the edge of a prominent plate boundary, which is dominated by the tectonic interaction between the Africa and Iberian plates. Thus, it represents a complex tectonic process consisting of composite compressional and extensional regimes. The teleseismic recordings are extracted from the database of IRIS and GEOFON data centers according to the earthquake catalog obtained from U.S. Geological Survey. We analyzed totally 4976 P- and 12673 S- receiver functions.To achieve the sufficient energy in waveforms, we analyze events greater than M5.7, located at epicentral distance ranging from 35° to 90°, from 60° to 85° and from 85° to 120° for P-, S- and SKS phases, respectively. The data quality is manually checked to restrict the event database to the clear P-, S and SKS- onsets. The seismograms are rotated into P-, SH- and SV components of local ray coordinate system in order to get the highest energy of converted phases. We perform a time-domain deconvolution approach to derive the receiver functions in order to eliminate the source and path effects. Move-out correction is applied prior to stacking the individual traces in order to compare and then to better identify the coherent phases. We alternatively use piercing-point approach for stacking process subdividing the region into the grids with a size of 1°x1° and stack the individual traces based on their corresponding grids (piercing-points). The S-receiver function is used to avoid complications due to the crustal-reverberations and thus to better resolve the variation of lithosphere-asthenosphere boundary (LAB). The variation of crustal thickness derived from P-wave receiver functions is well-correlated with the pattern obtained from S-wave receiver functions.The results suggest that the thickness of the crust as well as the depth of LAB systematically decreases towards the east. The greatest crustal thickness is observed along the Betic and Rift mountains. The relatively shallow Moho as well as the shallow LAB beneath the Alboran Sea are consistent with the extensional nature of the boundary between Iberian and African plates.

Receiver Functions Imaging of the Moho and LAB in the Southern Caribbean plate boundary and Venezuela

NASA Astrophysics Data System (ADS)

Masy, J.; Levander, A.; Niu, F.

2011-12-01

We have made teleseismic Ps and Sp receiver functions from data recorded from 2003 to 2009 by the permanent national seismic network of Venezuela, the BOLIVAR (Broadband Onshore-offshore Lithospheric Investigation of Venezuela and the Antilles arc Region) and WAVE (Western Array for Venezuela) experiments. The receiver functions show rapid variations in Moho and lithosphere-asthenosphere boundary (LAB) depths both across and along the southern Caribbean plate boundary region. We used a total of 69 events with Mw > 6 occurring at epicentral distances from 30° to 90° for the Ps receiver functions, and 43 events with Mw > 5.7 from 55° to 85° to make Sp receiver functions. For CCP stacking we constructed a 3D velocity model from numerous active source profiles (Schmitz et al., 2001; Bezada et al., 2007; Clark et al., 2008; Guedez, 2008; Magnani et al., 2009), from finite-frequency P wave upper mantle tomography model of Bezada et al., (2010) and the Rayleigh wave tomography model of Miller et al., (2009). The Moho ranges in depth from ~25 km beneath the Caribbean Large Igneous Provinces to ~55 km beneath the Mérida Andes in western Venezuela. These results are consistent with previous receiver functions studies (Niu et al., 2007) and the available active source profiles. Beneath the Maracaibo Block in northwestern Venezuela, we observe a strong positive signal at 40 to 60 km depth dipping ~6° towards the continent. We interpret this as the Moho of the Caribbean slab subducting beneath northernmost South America from the west. Beneath northern Colombia and northwestern Venezuela the top of this slab has been previously inferred from intermediate depth seismicity (Malavé and Suarez, 1995), which indicates a slab dipping between 20° - 30° beneath Lake Maracaibo. Our results could indicate that the slab is tearing beneath Lake Maracaibo as suggested previously by Masy et al. (2011). The deeper (> 100 km depth) part of the slab has been imaged using P-wave tomography (Bezada et al, 2010). Like others we attribute the uplift of the Mérida Andes to flat Caribbean slab subduction (for example Kellogg and Bonini, 1982). In central Venezuela beneath the Cordillera de la Costa we observe a positive signal shallower than the Moho at <30 km depth beneath the entire range. We interpret this as a detachment surface beneath Caribbean & arc terranes thrust onto the SA margin (Bezada et al., 2010). The lithosphere-asthenosphere boundary (LAB) beneath the Mérida Andes is shallow, ~65km depth, and parallels the range. In the plate boundary region under the Cordillera de la Costa the lithosphere is also thin, ~65km, beneath the Cariaco basin the lithosphere thickens to 85 km. In the far east under Serranía del Interior the lithosphere is ~75 km. Cratonic lithosphere thickness varies from 85 to 100 km.

Crustal structure of the rifted volcanic margins and uplifted plateau of Western Yemen from receiver function analysis

NASA Astrophysics Data System (ADS)

Ahmed, Abdulhakim; Tiberi, Christel; Leroy, Sylvie; Stuart, Graham W.; Keir, Derek; Sholan, Jamal; Khanbari, Khaled; Al-Ganad, Ismael; Basuyau, Clémence

2013-06-01

We analyse P-wave receiver functions across the western Gulf of Aden and southern Red Sea continental margins in Western Yemen to constrain crustal thickness, internal crustal structure and the bulk seismic velocity characteristics in order to address the role of magmatism, faulting and mechanical crustal thinning during continental breakup. We analyse teleseismic data from 21 stations forming the temporary Young Conjugate Margins Laboratory (YOCMAL) network together with GFZ and Yemeni permanent stations. Analysis of computed receiver functions shows that (1) the thickness of unextended crust on the Yemen plateau is ˜35 km; (2) this thins to ˜22 km in coastal areas and reaches less than 14 km on the Red Sea coast, where presence of a high-velocity lower crust is evident. The average Vp/Vs ratio for the western Yemen Plateau is 1.79, increasing to ˜1.92 near the Red Sea coast and decreasing to 1.68 for those stations located on or near the granitic rocks. Thinning of the crust, and by inference extension, occurs over a ˜130-km-wide transition zone from the Red Sea and Gulf of Aden coasts to the edges of the Yemen plateau. Thinning of continental crust is particularly localized in a <30-km-wide zone near the coastline, spatially co-incident with addition of magmatic underplate to the lower crust, above which on the surface we observe the presence of seaward dipping reflectors (SDRs) and thickened Oligo-Miocene syn-rift basaltic flows. Our results strongly suggest the presence of high-velocity mafic intrusions in the lower crust, which are likely either synrift magmatic intrusion into continental lower crust or alternatively depleted upper mantle underplated to the base of the crust during the eruption of the SDRs. Our results also point towards a regional breakup history in which the onset of rifting was synchronous along the western Gulf of Aden and southern Red Sea volcanic margins followed by a second phase of extension along the Red Sea margin.

Crustal Seismic Structure of Central Alberta from Receiver Function Inversions

NASA Astrophysics Data System (ADS)

Chen, Y.; Dokht, R.; Gu, Y. J.; Sacchi, M. D.

2013-12-01

It is widely believed that the basement structure of central Alberta represents the tectonic assembly and evolution of several Archean lithospheric fragments. These fragments underwent episodes of rifting, collision, subduction and melting during the Proterozoic, giving rise to a complex network of tectonic domains with considerable differences in the crustal magnetic and seismic signatures. Observational support for these episodes, e.g., the coeval subduction around the Hearne province, has been limited due to the lack of exposed geology and insufficient teleseismic data prior to 2006. Since mid 2007, the establishment of the Canadian Rockies and Alberta Network (CRANE) has greatly improved the broadband seismic data coverage in central and southern Alberta. Based on 5+ years of CRANE data, we systematically analyze crust and shallow mantle shear velocities through simultaneous inversions of low and high frequency receiver functions. P-to-S converted waves from several stations in central Alberta suggest a significant mid crustal low velocity zone (LVZ), where shear velocity could vary by as much as 35 percent in a depth range of 15-35 km. This structure is not required by the receiver functions from stations along the foothills of the Rocky Mountains. While LVZ of notable magnitudes have been suggested previously in an overlapped part of the study region and attributed to the presence of serpentine or intrusive sill, the spatial dimensions of the anomaly reported in the current study is significantly larger: this anomaly spans hundreds of kilometers horizontally and is generally thicker than 10 km. The presence of mid/lower crustal LVZ in central Alberta is supported by recent group velocity maps based on noise correlation tomography, and the southeastward orientation of this LVZ is consistent with the proposed direction of the subducted oceanic microplate beneath the northwestern Hearne province during the Proterozoic. Still, the cause of the LVZ remains inconclusive, as the broad dimension of the anomaly would require major, potentially multiple, episodes of serpentinization and/or rework after the Paleoproterzoic. This process may span multiple domains and the correlation of present-day seismic structure with the domain boundaries (hence aeromagnetic signatures) remain questionable on the regional scale.

Investigating Transition Zone Thickness Variation under the Arabian Plate: Evidence Lacking for Deep Mantle Upwellings

NASA Astrophysics Data System (ADS)

Juliá, J.; Tang, Z.; Mai, P. M.; Zahran, H.

2014-12-01

Cenozoic volcanic outcrops in Arabia - locally known as harrats - span more than 2000 km along the western half of the Arabian plate, from eastern Yemen to southern Syria. The magmatism is bimodal in character, with older volcanics (30 to 20 My) being tholeiitic-to-transitional and paralleling the Red Sea margin, and younger volcanics (12 Ma to Recent) being transitional-to-strongly-alkalic and aligning in a more north-south direction. The bimodal character has been attributed to a two-stage rifting process along the Red Sea, where the old volcanics would have produced from shallow sources related to an initial passive rifting stage, and young volcanics would have originated from one or more deep-seated mantle plumes driving present active rifting. Early models suggested the harrats would have resulted from either lateral flow from the Afar plume in Ethiopia, or more locally from a separate mantle plume directly located under the shield. Most recently, tomographic images of the Arabian mantle have suggested the northern harrats could be resulting from flow originating at a deep plume under Jordan. In this work, we investigate the location of deep mantle plumes under the Arabian plate by mapping transition zone thickness with teleseismic receiver functions. The transition zone is bounded by seismic discontinuities, nominally at 410 and 660 km depth, originating from phase transitions in the olivine-normative component of the mantle. The precise depth of the discontinuities is strongly dependent on temperature and, due to the opposing signs of the corresponding Clapeyron slopes, positive temperature anomalies are expected to result in thinning of the transition zone. Our dataset consists of ~5000 low-frequency (fc < 0.25 Hz) receiver function waveforms obtained at ~110 broadband stations belonging to a number of permanent and temporary seismic networks in the region. The receiver functions were migrated to depth and stacked along a ~2000 km long record section displaying P-to-S conversions at seismic discontinuities under Western Arabia. Our results display a normal to thicker-than-average transition zone under the study area, suggesting thermal perturbations of the transition zone due to deep mantle upwellings under the western shield and/or Jordan are unlikely.

Lithospheric structure beneath the central and western North China Craton and adjacent regions from S-receiver function imaging

NASA Astrophysics Data System (ADS)

Yinshuang, A.; Zhang, Y.; Chen, L.

2016-12-01

The central and western NCC(CWNCC) only experienced localized lithospheric modification and has remained relatively stable since the Pre-Cambrian in contrast to the fundamental destruction in the east. For better unraveling the tectonic evolution and dynamics of CWNCC, detailed knowledge of lithospheric structure is thus important. However, most of the available seismological observations are dominated by regional seismic tomography and the resolutions are rather low due to the limited data coverage or intrinsic limitation of the methods. S receiver function(RF) contains information from deep velocity discontinuities and is free from the interference of crustal multiples, so it is widely used in subcontinental lithospheric structural studies. We collected teleseismic data from 340 broadband stations in CWNCC, and adopted 2-D wave equation-based poststack migration method to do S-receiver function CCP imaging. Finally, we get 8 migrated profile images in CWNCC and adjacent areas and integrate them for an overview. The most prominent feature of the LAB beneath central NCC is an sudden subsidence to 160km in the southern portion, and the dimension and extension of this deep anomaly is correlated to the lithosphere in Ordos, so we interpret it as a remnant cratonic mantle root. The LAB beneath western NCC can extend to the depth of 150-180 km but appears laterally variable. Western Ordos becomes shallower than its eastern counterpart and there are two obvious deep anomalies beneath the eastern Ordos, divided by a geological boundary at 37°N, which reflects that the lithosphere of Ordos is not so homogeneous or rigid as people thought before. Furthermore, a negative velocity discontinuity is widely identified at the depth of 80- 110 km within the thick lithosphere of CWNCC, and the location is spatially coincide with the modified LAB in ENCC. Although the cause of this mid-lithospheric discontinuity(MLD) is still controversial, mechanically, it may indicate an ancient, weak layer within the overall strong cratonic lithosphere. Our result is broadly consistent with the previous tomography studies, but shows more detailed information of lithospheric variations,. Moreover, it corroborates the existence of the similar discontinuities at 100 km depth under stable continental regions worldwide.

Crustal structure across the lateral edge of the Southern Tyrrhenian slab

NASA Astrophysics Data System (ADS)

Pio Lucente, Francesco; Piana Agostinetti, Nicola; Di Bona, Massimo; Govoni, Aladino; Bianchi, Irene

2015-04-01

In the southeastern corner of the Tyrrhenian basin, in the central Mediterranean Sea, a tight alignment of earthquakes along a well-defined Benioff zone reveals the presence of one of the narrowest active trenches worldwide, where one of the last fragments of the former Tethys ocean is consumed. Seismic tomography furnishes snapshot images of the present-day position and shape of this slab. Through receiver function analysis we investigate the layered structures overlying the slab. We compute receiver functions from the P-coda of teleseismic events at 13 temporary station deployed during the "Messina 1908-2008" research project (Margheriti, 2008), and operating for an average period of 15 months each. The crustal and uppermost mantle structure has been investigated using a trans-dimensional McMC algorithm developed by Piana Agostinetti and Malinverno (2010), obtaining a 1D S-wave velocity profile for each station. At three of the stations, operating for a longer period of time, the number and the azimuthal distribution of teleseisms allowed us to stack the RF data-set with back azimuth and to compute the harmonic expansion. The analysis of the back-azimuthal harmonics gave us insight on the presence of dipping interfaces and anisotropic layers at depth. The strike and the dip of interfaces and the anisotropic parameters have been quantified using the Neighbourhood Algorithm (Sambridge, 1999). Preliminary results highlight: (1) a neat differentiation of the isotropic S-wave velocity structure passing through the slab edge, from the tip of the Calabrian arc to the Peloritani Range, and (2) the presence of crustal complexities, such as dipping interfaces and anisotropic layers, both in the upper and lower crust. Margheriti, L. (2008), Understanding Crust Dynamics and Subduction in Southern Italy, Eos Trans. AGU, 89(25), 225-226, doi:10.1029/2008EO250002. Piana Agostinetti, N. and A. Malinverno (2010) Receiver Function inversion by trans-dimensional Monte Carlo sampling, Geophys. J. Int., 181(2) 858-872, doi: 10.1111/j.1365-246X.2010.04530.x Sambridge, M. (1999), Geophysical inversion with a neighbourhood algorithm-I. Searching a parameter space, Geophys. J. Int., 138, 479-494, doi:10.1046/j.1365-246X.1999.00876.x.

Moho Depth Variations in the Northeastern North China Craton Revealed by Receiver Function Imaging

NASA Astrophysics Data System (ADS)

Zhang, P.; Chen, L.; Yao, H.; Fang, L.

2016-12-01

The North China Craton (NCC), one of the oldest cratons in the world, has attracted wide attention in Earth Science for decades because of the unusual Mesozoic destruction of its cratonic lithosphere. Understanding the deep processes and mechanism of this craton destruction demands detailed knowledge about the deep structure of the region. In this study, we used two-year teleseismic receiver function data from the North China Seismic Array consisting of 200 broadband stations deployed in the northeastern NCC to image the Moho undulation of the region. A 2-D wave equation-based poststack depth migration method was employed to construct the structural images along 19 profiles, and a pseudo 3D crustal velocity model of the region based on previous ambient noise tomography and receiver function study was adopted in the migration. We considered both the Ps and PpPs phases, but in some cases we also conducted PpSs+PsPs migration using different back azimuth ranges of the data, and calculated the travel times of all the considered phases to constrain the Moho depths. By combining the structure images along the 19 profiles, we got a high-resolution Moho depth map beneath the northeastern NCC. Our results broadly consist with the results of previous active source studies [http://www.craton.cn/data], and show a good correlation of the Moho depths with geological and tectonic features. Generally, the Moho depths are distinctly different on the opposite sides of the North-South Gravity Lineament. The Moho in the west are deeper than 40 km and shows a rapid uplift from 40 km to 30 km beneath the Taihang Mountain Range in the middle. To the east in the Bohai Bay Basin, the Moho further shallows to 30-26 km depth and undulates by 3 km, coinciding well with the depressions and uplifts inside the basin. The Moho depth beneath the Yin-Yan Mountains in the north gradually decreases from 42 km in the west to 25 km in the east, varying much smoother than that to the south.

Deep Sources: New constraints on the tectonic origin of the Klyuchevskoy Group upper mantle anomaly

NASA Astrophysics Data System (ADS)

Bourke, J. R.; Nikulin, A.; Levin, V. L.

2017-12-01

Volcanoes of the Klyuchevskoy Group (KG) form one of the most active volcanic clusters on the planet, yet its position relative to the subducting Pacific Plate seems to be in violation of the understood principles of the flux-induced arc volcanism. Positioned at 170km above the accepted subduction contact, the KG is seemingly outside the maximum fluid flux release zone of 100km, as observed across global subduction zone environments. Past geophysical studies indicate presence of a planar seismic anomaly 110km below the KG, and it has been noted that the KG lavas exhibit anomalous geochemical signatures, possibly associated with two separate melt generation regions. This interpretation was largely based on receiver function analysis of seismic data recorded by 3 stations of the Partnership in International Research and Education (PIRE) network, done prior to this data becoming publically available. We present results of receiver function and a teleseismic, regional, and local source shear wave splitting study, focused on datasets obtained by the full PIRE network of 12 stations, as well as a hybrid summation of all stations. We present our findings in the form of depth migrated receiver function images convolved with a three-dimensional model of the subduction zone and shear-wave splitting measurements. Our results vastly increase the resolution of the previously identified upper mantle anomaly, further constraining its geometry both vertically and laterally. We complement our observations with a forward modeling effort aimed at assessing the geological nature of the anomaly. Specifically, we test three scenarios that were previously invoked to explain the presence of the low-velocity anomaly in the upper mantle below the KG: a 3D flow of mantle material around the corner of the subducting Pacific Plate, a sinking paleoslab left behind as a result of subduction rollback, and a plume of sediments from the subducting plate. We show that presence of remnant paleoslab material remains a likely geodynamic scenario that explains both the observed geophysical anomaly and its impact on volcanic activity of the KG.

Probing the Cypriot Lithosphere: Insights from Broadband Seismology

NASA Astrophysics Data System (ADS)

Ogden, C. S.; Bastow, I. D.; Pilidou, S.; Dimitriadis, I.; Iosif, P.; Constantinou, C.; Kounoudis, R.

2017-12-01

Cyprus, an island in the eastern Mediterranean Sea, is an ideal study locale for understanding both the final stages of subduction, and the internal structure of so-called `ophiolites' - rare, on-land exposures of oceanic crust. The Troodos ophiolite offers an excellent opportunity to interrogate a complete ophiolite sequence from mantle rocks to pillow lavas. However, determining its internal architecture, and that of the subducting African plate deep below it, cannot be easily achieved using traditional field geology. To address this issue, we have built a new network of five broadband seismograph stations across the island. These, along with existing permanent stations, record both local and teleseismic earthquakes that we are now using to image Cyprus' crust and mantle seismic structure. Receiver functions are time series, computed from three-component seismograms, which contain information about lithospheric seismic discontinuities. When a P-wave strikes a velocity discontinuity such as the Moho, energy is converted to S-waves (direct Ps phase). The widely-used H-K Stacking technique utilises this arrival, and subsequent crustal reverberations (PpPs and PsPs+PpSs), to calculate crustal thickness (H) and bulk-crustal Vp/Vs ratio (K). Central to the method is the assumption that the Moho produces the largest amplitude conversions, after the direct P-arrival, which is valid where the Moho is sharp. Where the Moho is gradational or upper crustal discontinuities are present, the Moho signals are weakened and masked by shallow crustal conversions, potentially rendering the H-K stacking method unreliable. Using a combination of synthetic and observed seismograms, we explore Cyprus' crustal structure and, specifically, the reliability of the H-K method in constraining it. Data quality is excellent across the island, but the receiver function Ps phase amplitude is low, and crustal reverberations are almost non-existent. Therefore, a simple, abrupt wavespeed jump at the Moho is lacking (perhaps due to the subducting African plate), and/or evidence for it is obscured by complex structure associated with the Troodos ophiolite. On-going analyses also include joint inversion of receiver functions and surface wave data, which together, are capable of resolving complex lithospheric seismic structure.

Lithospheric Structure Beneath Taiwan From Sp Converted Waves

NASA Astrophysics Data System (ADS)

Glasgow, D.; McGlashan, N.; Brown, L.

2006-12-01

Taiwan is the product of three dimensionally complex interaction between the Eurasian Plate (EP) and the Philippine Sea plate (PSP), with the EP subducting eastward beneath the PSP in southern Taiwan while the PSP subducts northward beneath the EP in northern Taiwan. The structural emplacement of Philippine Arc lithosphere onto Chinese passive margin lithosphere is an exemplar of continental amalgamation, yet there are relatively few contraints on the geometry of lithosphere involved at depth. We have used teleseismic data recorded by the Broadband Array for Taiwan Seismology (BATS) to compute S-to-p wave receiver functions for the Taiwan region to provide new constraints on deep geometries. Moho conversions provide independent new estimates of crustal thickness, which vary from 35 to 55 km across the island in agreement with previous P to S conversion studies and local tomography. More significantly, our results suggest that the lithosphere- asthenosphere boundary (LAB) varies in depth from ca 140 km beneath northeastern Taiwan to ca 120 km beneath central Taiwan to perhaps less than 80 km beneath southern Taiwan. We attribute this along strike variation to the depression and decapitation of the Eurasian plate in the transition to northward subduction of the PSP.

SCANLIPS - A Study of Epirogenic Uplift of Scandinavia

NASA Astrophysics Data System (ADS)

England, R. W.; Ebbing, J.

2007-12-01

Thermochronology data and geomorphological interpretation indicate that parts of the Scandinavian mountains have risen by over 1 km since the Miocene. This permanent uplift, the cause of which is still disputed, varies across Norway, being greatest in southern and northern areas and least in the central region. To investigate this the SCANLIPS project employs passive seismology, coupled with modelling of potential field data to determine variations in crustal properties and structure across Norway and Sweden. Initially we intend to test whether lateral variations in crustal structure and properties are correlated with the uplift pattern. This would suggest that the cause of the differential uplift lies in a modification of the crust. If the test of this hypothesis is null we will use the data to investigate the present day upper mantle structure for the cause. Between April and October 2006 28 seismometers were deployed at sites along a c. 600 km long profile between Trondheim in Norway and Harnosand in Sweden to record teleseismic arrivals. Receiver Functions have been calculated for teleseismic events recorded at these stations and then modelled to determine crustal velocity structure, estimate Vp/Vs and depth to Moho. Preliminary results suggest that crustal thickness increases eastward beneath Norway and then remains deep beneath the lower topography of central Sweden. Along the profile a gradual eastward increase in seismic velocity, including a very high velocity lower crust beneath Sweden explains the compensation of shallow topography by thick crust. Forward density and isostatic modelling shows that the introduction of the high-density lower crust adjusts both the gravity field and the isostatic compensation. Beneath Norway the crust thins rapidly toward the continental margin at a rate that is faster than the topography decreases. This suggests that at least part of the topography is supported by the flexural strength of the crust in the footwall of the More-Trondelag fault zone. Recently published results of Svenningsen et al. (2007) show a similar thickening below the high topography of southern Norway, indicating Airy type compensation. Further work is required before a direct comparison can be made of the crustal properties between the two regions and a possible cause for the differential uplift of Scandinavia determined.

The crust and upper mantle of central East Greenland - implications for continental accretion and rift evolution

NASA Astrophysics Data System (ADS)

Schiffer, Christian; Balling, Niels; Ebbing, Jörg; Holm Jacobsen, Bo; Bom Nielsen, Søren

2016-04-01

The geological evolution of the North Atlantic Realm during the past 450 Myr, which has shaped the present-day topographic, crustal and upper mantle features, was dominated by the Caledonian orogeny and the formation of the North Atlantic and associated igneous activity. The distinct high altitude-low relief landscapes that accompany the North Atlantic rifted passive margins are the focus of a discussion of whether they are remnant and modified Caledonian features or, alternatively, recently uplifted peneplains. Teleseismic receiver function analysis of 11 broadband seismometers in the Central Fjord Region in East Greenland indicates the presence of a fossil subduction complex, including a slab of eclogitised mafic crust and an overlying wedge of hydrated mantle peridotite. This model is generally consistent with gravity and topography. It is shown that the entire structure including crustal thickness variations and sub-Moho heterogeneity gives a superior gravity and isostatic topographic fit compared to a model with a homogeneous lithospheric layer (1). The high topography of >1000 m in the western part of the area is supported by the c. 40 km thick crust. The eastern part requires buoyancy from the low velocity/low density mantle wedge. The geometry, velocities and densities are consistent with structures associated with a fossil subduction zone. The spatial relations with Caledonian structures suggest a Caledonian origin. The results indicate that topography is isostatically compensated by density variations within the lithosphere and that significant present-day dynamic topography seems not to be required. Further, this structure is suggested to be geophysically very similar to the Flannan reflector imaged north of Scotland, and that these are the remnants of the same fossil subduction zone, broken apart and separated during the formation of the North Atlantic in the early Cenozoic (2). 1) Schiffer, C., Jacobsen, B.H., Balling, N., Ebbing, J. and Nielsen, S.B., 2015. The East Greenland Caledonides - teleseismic signature, gravity and isostasy. Geophysical Journal International, 203, 1400-1418. 2) Schiffer, C., Stephenson, R.A., Petersen, K.D., Nielsen, S.B., Jacobsen, B.H., Balling, N. and Macdonald, D.I.M., 2015. A sub-crustal piercing point for North Atlantic reconstructions and tectonic implications. Geology, 43, 1087-1090.

Shear wave velocities in the upper mantle of the Western Alps: new constraints using array analysis of seismic surface waves

NASA Astrophysics Data System (ADS)

Lyu, Chao; Pedersen, Helle A.; Paul, Anne; Zhao, Liang; Solarino, Stefano

2017-07-01

It remains challenging to obtain absolute shear wave velocities of heterogeneities of small lateral extension in the uppermost mantle. This study presents a cross-section of Vs across the strongly heterogeneous 3-D structure of the western European Alps, based on array analysis of data from 92 broad-band seismic stations from the CIFALPS experiment and from permanent networks in France and Italy. Half of the stations were located along a dense sublinear array. Using a combination of these stations and off-profile stations, fundamental-mode Rayleigh wave dispersion curves were calculated using a combined frequency-time beamforming approach. We calculated dispersion curves for seven arrays of approximately 100 km aperture and 14 arrays of approximately 50 km aperture, the latter with the aim of obtaining a 2-D vertical cross-section of Vs beneath the western Alps. The dispersion curves were inverted for Vs(z), with crustal interfaces imposed from a previous receiver function study. The array approach proved feasible, as Vs(z) from independent arrays vary smoothly across the profile length. Results from the seven large arrays show that the shear velocity of the upper mantle beneath the European plate is overall low compared to AK135 with the lowest velocities in the internal part of the western Alps, and higher velocities east of the Alps beneath the Po plain. The 2-D Vs model is coherent with (i) a ∼100 km thick eastward-dipping European lithosphere west of the Alps, (ii) very high velocities beneath the Po plain, coherent with the presence of the Alpine (European) slab and (iii) a narrow low-velocity anomaly beneath the core of the western Alps (from the Briançonnais to the Dora Maira massif), and approximately colocated with a similar anomaly observed in a recent teleseismic P-wave tomography. This intriguing anomaly is also supported by traveltime variations of subvertically propagating body waves from two teleseismic events that are approximately located on the profile great circle.

A reference model for crust and uppermost mantle beneath Antarctica

NASA Astrophysics Data System (ADS)

Shen, W.; Wiens, D.; Gerstoft, P.; Bromirski, P. D.; Stephen, R. A.; Aster, R. C.; Nyblade, A.; Winberry, J. P.; Huerta, A. D.; Anandakrishnan, S.; Hansen, S. E.; Wilson, T. J.; Heeszel, D.

2016-12-01

Since the last decade of the 20th Century, over 300 broad-band seismic stations have been deployed across the continent of Antarctica (e.g., temporary networks such as TAMSEIS, AGAP/GAMSEIS, POLENET/ANET, TAMNNET and RIS/DRIS by US geoscientists, as well as stations deployed by other countries). In this presentation, we discuss our recent effort that builds a reference crustal and uppermost mantle shear velocity (Vs) model for continental Antarctica based on those seismic arrays. The data analysis for this effort consists of four steps. First, we compute ambient noise cross-correlations between all possible station pairs and use them to construct Rayleigh wave phase and group velocity maps at a continental scale. Coherence of the new maps with maps generated from teleseismic earthquake data from an earlier study (Heeszel et al., 2016) confirms the high quality of both maps and the minor difference helps quantify the map uncertainties. Second, we compute P receiver function waveforms for each station in Antarctica. Third, we collect Rayleigh waves generated by teleseismic earthquakes and measure their horizontal to vertical (H/V) ratio at each station. Fourth and finally, by combing all seismic measurements from the first three steps together with the phase velocity maps by Heeszel et al.(2016) using a non-linear Monte Carlo (MC) inversion algorithm, we built a 3-D model for the crust and uppermost mantle beneath continental Antarctica and its periphery to a depth of 150 km. This high resolution model, together with associated uncertainty estimates from the MC inversion, serve as a starting point for further improvement and geological interpretation. A variety of tectonic features, including a slower but highly heterogeneous West Antarctica and a much faster East Antarctica, are present in the 3D model. A better image of these features from the 3D model helps further investigation of the thermal and dynamic state of Antarctica's lithosphere and underlying asthenosphere and provides key constraints on the interaction of the solid earth with the West Antarctic Ice Sheet.

Detailed Northern Anatolian Fault Zone crustal structure from receiver functions

NASA Astrophysics Data System (ADS)

Cornwell, D. G.; Kahraman, M.; Thompson, D. A.; Houseman, G. A.; Rost, S.; Turkelli, N.; Teoman, U.; Altuncu Poyraz, S.; Gülen, L.; Utkucu, M.

2013-12-01

We present high resolution images derived from receiver functions of the continental crust in Northern Turkey that is dissected by two fault strands of the Northern Anatolian Fault Zone (NAFZ). The NAFZ is a major continental strike-slip fault system that is comparable in length and slip rate to the San Andreas Fault Zone. Recent large earthquakes occurred towards the western end of the NAFZ in 1999 at Izmit (M7.5) and Düzce (M7.2). As part of the multi-disciplinary Faultlab project, we aim to develop a model of NAFZ crustal structure and locate deformation by constraining variations in seismic properties and anisotropy in the upper and lower crust. The crustal model will be an input to test deformation scenarios in order to match geodetic observations from different phases of the earthquake loading cycle. We calculated receiver functions from teleseismic earthquakes recorded by a rectangular seismometer array spanning the NAFZ with 66 stations at a nominal inter-station spacing of 7 km and 7 additional stations further afield. This Dense Array for North Anatolia (DANA) was deployed from May 2012 until September 2013 and we selected large events (Mw>5.5) from the high quality seismological dataset to analyze further. Receiver functions were calculated for different frequency bands then collected into regional stacks before being inverted for crustal S-wave velocity structure beneath the entire DANA array footprint. In addition, we applied common conversion point (CCP) migration using a regional velocity model to construct a migrated 3D volume of P-to-S converted and multiple energy in order to identify the major crustal features and layer boundaries. We also performed the CCP migration with transverse receiver functions in order to identify regions of anisotropy within the crustal layers. Our preliminary results show a heterogeneous crust above a flat Moho that is typically at a depth of 33 km. We do not observe a prominent step in the Moho beneath the surface locations at either of the NAFZ fault branches. We observe first-order differences in crustal structure between the crustal blocks that are separated by the faults. Each crustal block also contains regions of strong anisotropy at various depths that will be analyzed further with the full seismological dataset and compared to petrofabric analyses of exhumed fault segments. We will compare our results with other seismological imaging techniques to attempt to resolve the distribution of fault zone deformation with respect to depth. This information will be useful to other complementary Faultlab techniques that will add a detailed insight into the fault structure and dynamics of the NAFZ and contribute more broadly into ongoing research into major strike-slip fault zones.

Earthquake source imaging by high-resolution array analysis at regional distances: the 2010 M7 Haiti earthquake as seen by the Venezuela National Seismic Network

NASA Astrophysics Data System (ADS)

Meng, L.; Ampuero, J. P.; Rendon, H.

2010-12-01

Back projection of teleseismic waves based on array processing has become a popular technique for earthquake source imaging,in particular to track the areas of the source that generate the strongest high frequency radiation. The technique has been previously applied to study the rupture process of the Sumatra earthquake and the supershear rupture of the Kunlun earthquakes. Here we attempt to image the Haiti earthquake using the data recorded by Venezuela National Seismic Network (VNSN). The network is composed of 22 broad-band stations with an East-West oriented geometry, and is located approximately 10 degrees away from Haiti in the perpendicular direction to the Enriquillo fault strike. This is the first opportunity to exploit the privileged position of the VNSN to study large earthquake ruptures in the Caribbean region. This is also a great opportunity to explore the back projection scheme of the crustal Pn phase at regional distances,which provides unique complementary insights to the teleseismic source inversions. The challenge in the analysis of the 2010 M7.0 Haiti earthquake is its very compact source region, possibly shorter than 30km, which is below the resolution limit of standard back projection techniques based on beamforming. Results of back projection analysis using the teleseismic USarray data reveal little details of the rupture process. To overcome the classical resolution limit we explored the Multiple Signal Classification method (MUSIC), a high-resolution array processing technique based on the signal-noise orthognality in the eigen space of the data covariance, which achieves both enhanced resolution and better ability to resolve closely spaced sources. We experiment with various synthetic earthquake scenarios to test the resolution. We find that MUSIC provides at least 3 times higher resolution than beamforming. We also study the inherent bias due to the interferences of coherent Green’s functions, which leads to a potential quantification of biased uncertainty of the back projection. Preliminary results from the Venezuela data set shows an East to West rupture propagation along the fault with sub-Rayleigh rupture speed, consistent with a compact source with two significant asperities which are confirmed by source time function obtained from Green’s function deconvolution and other source inversion results. These efforts could lead the Venezuela National Seismic Network to play a prominent role in the timely characterization of the rupture process of large earthquakes in the Caribbean, including the future ruptures along the yet unbroken segments of the Enriquillo fault system.

Seismic anisotropy in central North Anatolian Fault Zone and its implications on crustal deformation

NASA Astrophysics Data System (ADS)

Licciardi, A.; Eken, T.; Taymaz, T.; Piana Agostinetti, N.; Yolsal-Çevikbilen, S.

2018-04-01

We investigate the crustal seismic structure and anisotropy around the central portion of the North Anatolian Fault Zone, a major plate boundary, using receiver function analysis. The characterization of crustal seismic anisotropy plays a key role in our understanding of present and past deformation processes at plate boundaries. The development of seismic anisotropy in the crust arises from the response of the rocks to complicated deformation regimes induced by plate interaction. Through the analysis of azimuthally-varying signals of teleseismic receiver functions, we map the anisotropic properties of the crust as a function of depth, by employing the harmonic decomposition technique. Although the Moho is located at a depth of about 40 km, with no major offset across the area, our results show a clear asymmetric distribution of crustal properties between the northern and southern blocks, divided by the North Anatolian Fault Zone. Heterogeneous and strongly anisotropic crust is present in the southern block, where complex intra-crustal signals are the results of strong deformation. In the north, a simpler and weakly anisotropic crust is typically observed. The strongest anisotropic signal is located in the first 15 km of the crust and is widespread in the southern block. Stations located on top of the main active faults in the area indicate the highest amplitudes, together with fault-parallel strikes of the fast plane of anisotropy. We interpret the origin of this signal as due to structure-induced anisotropy, and roughly determine its depth extent up to 15-20 km for these stations. Away from the faults, we suggest the contribution of previously documented uplifted basement blocks to explain the observed anisotropy at upper and middle crustal depths. Finally, we interpret coherent NE-SW orientations below the Moho as a result of frozen-in anisotropy in the upper mantle, as suggested by previous studies.

Resolving mantle structure beneath the Pacific Northwest

NASA Astrophysics Data System (ADS)

Darold, A. P.; Humphreys, E.; Schmandt, B.; Gao, H.

2011-12-01

Cenozoic tectonics of the Pacific Northwest (PNW) and the associated mantle structures are remarkable, the latter revealed only recently by EarthScope seismic data. Over the last ~66 Ma this region experienced a wide range of tectonic and magmatic conditions: Laramide compression, ~75-53 Ma, involving Farallon flat-slab subduction, regional uplift, and magmatic quiescence. With the ~53 Ma accretion of Siletzia ocean lithosphere within the Columbia Embayment, westward migration of subduction beginning Cascadia, along with initiation of the Cascade volcanic arc. Within the continental interior the Laramide orogeny was quickly followed by a period of extension involving metamorphic core complexes and the associated initial ignimbrite flare-up (both in northern Washington, Idaho, and western Montana); interior magmo-tectonic activity is attributed to flat-slab removal and (to the south) slab rollback. Rotation of Siletzia created new crust on SE Oregon and, at ~16 Ma, the Columbia River Flood Basalt (CRB) eruptions renewed vigorous magmatism. We have united several EarthScope studies in the Pacific Northwest and have focused on better resolving the major mantle structures that have been discovered. We have tomographically modeled the body waves with teleseismic, finite-frequency code under the constraints of ambient noise tomography and teleseismic receiver function models of Gao et al. (2011), and teleseismic anisotropy models of Long et al. (2009) in order to resolve structures continuously from the surface to the base of the upper mantle. We now have clear imaging of two episodes of subduction: Juan De Fuca slab deeper than ~250 km is absent across much of the PNW, and it has an E-W tear located beneath northern Oregon; Farallon slab (the "Siletzia curtain") is still present, hanging vertically just inboard of the core complexes, and with a basal tear causing the structure to extend deeper (~600 km) beneath north-central Idaho than beneath south-central Idaho and northern Washington (~300 km). Lying just west of the Siletzia curtain, beneath NE Oregon, is a prominent high-velocity body centered on 250 km depth. Its nearly circular plan view corresponds with the area of intense Columbia River Basalt eruptions and with the circular topographic bull's eye centered on the recently uplifted (post CRB) Wallowa Mountains. Finally, we are investigating a very low-velocity volume of mantle present between the E-W Juan de Fuca tear and the high-velocity body beneath the Wallowa Mountains. At 250 km depth this is the strongest low-velocity anomaly beneath the western U.S. Presently we are completing resolution testing on the structures revealed through our imaging in order to resolve their structural details. These synthetic resolution tests along with the high resolution imaging of the crust and upper mantle will clarify several previously cited structures as well as strengthen our conclusions on the tectonic history and geodynamical evolution of the mantle while aiding in putting together a comprehensive story for the area.

Preliminary Seismic Velocity Structure Results from Ambient Noise and Teleseismic Tomography: Laguna del Maule Volcanic Field, Chile

NASA Astrophysics Data System (ADS)

Wespestad, C.; Thurber, C. H.; Zeng, X.; Bennington, N. L.; Cardona, C.; Singer, B. S.

2016-12-01

Laguna del Maule Volcanic Field is a large, restless, rhyolitic system in the Southern Andes that is being heavily studied through several methods, including seismology, by a collaborative team of research institutions. A temporary array of 52 seismometers from OVDAS (the Southern Andean Volcano Observatory), PASSCAL (Portable Array Seismic Studies of the Continental Lithosphere), and the University of Wisconsin-Madison was used to collect the 1.3 years worth of data for this preliminary study. Ambient noise tomography uses surface wave dispersion data obtained from noise correlation functions (NCFs) between pairs of seismic stations, with one of each pair acting as a virtual source, in order to image the velocity structure in 3-D. NCFs were computed for hour-long time windows, and the final NCFs were obtained with phase-weighted stacking. The Frequency-Time Analysis technique was then utilized to measure group velocity between station pairs. NCFs were also analyzed to detect temporal changes in seismic velocity related to magmatic activity at the volcano. With the surface wave data from ambient noise, our small array aperture limits our modeling to the upper crust, so we employed teleseismic tomography to study deeper structures. For picking teleseismic arrivals, we tested two different correlation and stacking programs, which utilize adaptive stacking and multi-channel cross-correlation, to get relative arrival time data for a set of high quality events. Selected earthquakes were larger than magnitude 5 and between 30 and 95 degrees away from the center of the array. Stations that consistently show late arrivals may have a low velocity body beneath them, more clearly visualized via a 3-D tomographic model. Initial results from the two tomography methods indicate the presence of low-velocity zones at several depths. Better resolved velocity models will be developed as more data are acquired.

Joint inversion of GNSS and teleseismic data for the rupture process of the 2017 M w6.5 Jiuzhaigou, China, earthquake

NASA Astrophysics Data System (ADS)

Li, Qi; Tan, Kai; Wang, Dong Zhen; Zhao, Bin; Zhang, Rui; Li, Yu; Qi, Yu Jie

2018-02-01

The spatio-temporal slip distribution of the earthquake that occurred on 8 August 2017 in Jiuzhaigou, China, was estimated from the teleseismic body wave and near-field Global Navigation Satellite System (GNSS) data (coseismic displacements and high-rate GPS data) based on a finite fault model. Compared with the inversion results from the teleseismic body waves, the near-field GNSS data can better restrain the rupture area, the maximum slip, the source time function, and the surface rupture. The results show that the maximum slip of the earthquake approaches 1.4 m, the scalar seismic moment is 8.0 × 1018 N·m (M w ≈ 6.5), and the centroid depth is 15 km. The slip is mainly driven by the left-lateral strike-slip and it is initially inferred that the seismogenic fault occurs in the south branch of the Tazang fault or an undetectable fault, a NW-trending left-lateral strike-slip fault, and belongs to one of the tail structures at the easternmost end of the eastern Kunlun fault zone. The earthquake rupture is mainly concentrated at depths of 5-15 km, which results in the complete rupture of the seismic gap left by the previous four earthquakes with magnitudes > 6.0 in 1973 and 1976. Therefore, the possibility of a strong aftershock on the Huya fault is low. The source duration is 30 s and there are two major ruptures. The main rupture occurs in the first 10 s, 4 s after the earthquake; the second rupture peak arrives in 17 s. In addition, the Coulomb stress study shows that the epicenter of the earthquake is located in the area where the static Coulomb stress change increased because of the 12 May 2017 M w7.9 Wenchuan, China, earthquake. Therefore, the Wenchuan earthquake promoted the occurrence of the 8 August 2017 Jiuzhaigou earthquake.

Joint inversion of GNSS and teleseismic data for the rupture process of the 2017 M w6.5 Jiuzhaigou, China, earthquake

NASA Astrophysics Data System (ADS)

Li, Qi; Tan, Kai; Wang, Dong Zhen; Zhao, Bin; Zhang, Rui; Li, Yu; Qi, Yu Jie

2018-05-01

The spatio-temporal slip distribution of the earthquake that occurred on 8 August 2017 in Jiuzhaigou, China, was estimated from the teleseismic body wave and near-field Global Navigation Satellite System (GNSS) data (coseismic displacements and high-rate GPS data) based on a finite fault model. Compared with the inversion results from the teleseismic body waves, the near-field GNSS data can better restrain the rupture area, the maximum slip, the source time function, and the surface rupture. The results show that the maximum slip of the earthquake approaches 1.4 m, the scalar seismic moment is 8.0 × 1018 N·m ( M w ≈ 6.5), and the centroid depth is 15 km. The slip is mainly driven by the left-lateral strike-slip and it is initially inferred that the seismogenic fault occurs in the south branch of the Tazang fault or an undetectable fault, a NW-trending left-lateral strike-slip fault, and belongs to one of the tail structures at the easternmost end of the eastern Kunlun fault zone. The earthquake rupture is mainly concentrated at depths of 5-15 km, which results in the complete rupture of the seismic gap left by the previous four earthquakes with magnitudes > 6.0 in 1973 and 1976. Therefore, the possibility of a strong aftershock on the Huya fault is low. The source duration is 30 s and there are two major ruptures. The main rupture occurs in the first 10 s, 4 s after the earthquake; the second rupture peak arrives in 17 s. In addition, the Coulomb stress study shows that the epicenter of the earthquake is located in the area where the static Coulomb stress change increased because of the 12 May 2017 M w7.9 Wenchuan, China, earthquake. Therefore, the Wenchuan earthquake promoted the occurrence of the 8 August 2017 Jiuzhaigou earthquake.

Structure of the Sumatra-Andaman subduction zone

NASA Astrophysics Data System (ADS)

Pesicek, Jeremy Dale

We have conducted studies of the Sumatra-Andaman subduction zone using newly available teleseismic data resulting from the aftershock sequences of the 2004, 2005, and 2007 great earthquakes that occurred offshore of the island of Sumatra. In order to better exploit the new data, existing methodologies have been adapted and advanced in several ways to obtain results at a level of precision not previously possible from teleseismic data. Seismic tomography studies of the mantle were conducted using an improved iterative technique that accounts for fine-scale three-dimensional (3-D) velocity variations inside the study region and coarser global velocity variations outside the region. More precise earthquake locations were determined using a double-difference technique that has been extended to teleseismic distances using spherical ray tracing through the nested 3-D regional-global velocity models. Earthquake relocation was included in the iterative tomography scheme and was found to significantly enhance the recovery of slab velocity anomalies. Finally, because crustal structure is poorly constrained by the teleseismic data, 3-D density modeling of the crust was conducted using newly available satellite gravity data and a spherical prism gravity algorithm. The results of these studies better constrain the structure of the Sumatra-Andaman subduction zone, including the geometry of the mantle slab, position of the megathrust, and structural features of the downgoing plate. Tomography results reveal continuous upper mantle slab anomalies with significant variations in dip throughout the region. Broad curvature of the fast anomalies beneath northern Sumatra, similar to curvature of the trench and volcanic arc at the surface, is interpreted as folding of the upper mantle slab. Earthquake relocations show systematic shifts of the hypocenters to the northeast and to shallower depths, each with average changes of 5 km. Reduced scatter in the relocations better constrain the megathrust plate boundary and the regions of coseismic slip during the 2004 and 2005 great earthquakes. In addition, the relocations reveal discrete seismic features on the downgoing plate not previously visible in teleseismic catalogs. The new velocity model and earthquake locations provide the most comprehensive view of the deep structure of the Sumatra-Andaman subduction zone yet available.

Joint inversion of teleseismic body-waves and geodetic data for the Mw6.8 aftershock of the Balochistan earthquake with refined epicenter location

NASA Astrophysics Data System (ADS)

Wei, S.; Wang, T.; Jonsson, S.; Avouac, J. P.; Helmberger, D. V.

2014-12-01

Aftershocks of the 2013 Balochistan earthquake are mainly concentrated along the northeastern end of the mainshock rupture despite of much larger coseismic slip to the southwest. The largest event among them is an Mw6.8 earthquake which occurred three days after the mainshock. A kinematic slip model of the mainshock was obtained by joint inversion of the teleseismic body-waves and horizontal static deformation field derived from remote sensing optical and SAR data, which is composed of seven fault segments with gradually changing strikes and dips [Avouac et al., 2014]. The remote sensing data provide well constraints on the fault geometry and spatial distribution of slip but no timing information. Meanwhile, the initiation of the teleseismic waveform is very sensitive to fault geometry of the epicenter segment (strike and dip) and spatial slip distribution but much less sensitive to the absolute location of the epicenter. The combination of the two data sets allows a much better determination of the absolute epicenter location, which is about 25km to the southwest of the NEIC epicenter location. The well located mainshock epicenter is used to establish path calibrations for teleseismic P-waves, which are essential for relocating the Mw6.8 aftershock. Our grid search shows that the refined epicenter is located right at the northeastern end of the mainshock rupture. This is confirmed by the SAR offsets calculated from images acquired after the mainshock. The azimuth and range offsets display a discontinuity across the rupture trace of the mainshock. Teleseismic only and static only, as well as joint inversions all indicate that the aftershock ruptured an asperity with 25km along strike and range from 8km to 20km in depth. The earthquake was originated in a positive Coulomb stress change regime due to the mainshock and has complementary slip distribution to the mainshock rupture at the northeastern end, suggesting that the entire seismic generic zone in the crust was ruptured during the earthquake sequence.

Joint Inversion of Earthquake Source Parameters with local and teleseismic body waves

NASA Astrophysics Data System (ADS)

Chen, W.; Ni, S.; Wang, Z.

2011-12-01

In the classical source parameter inversion algorithm of CAP (Cut and Paste method, by Zhao and Helmberger), waveform data at near distances (typically less than 500km) are partitioned into Pnl and surface waves to account for uncertainties in the crustal models and different amplitude weight of body and surface waves. The classical CAP algorithms have proven effective for resolving source parameters (focal mechanisms, depth and moment) for earthquakes well recorded on relatively dense seismic network. However for regions covered with sparse stations, it is challenging to achieve precise source parameters . In this case, a moderate earthquake of ~M6 is usually recorded on only one or two local stations with epicentral distances less than 500 km. Fortunately, an earthquake of ~M6 can be well recorded on global seismic networks. Since the ray paths for teleseismic and local body waves sample different portions of the focal sphere, combination of teleseismic and local body wave data helps constrain source parameters better. Here we present a new CAP mothod (CAPjoint), which emploits both teleseismic body waveforms (P and SH waves) and local waveforms (Pnl, Rayleigh and Love waves) to determine source parameters. For an earthquake in Nevada that is well recorded with dense local network (USArray stations), we compare the results from CAPjoint with those from the traditional CAP method involving only of local waveforms , and explore the efficiency with bootstraping statistics to prove the results derived by CAPjoint are stable and reliable. Even with one local station included in joint inversion, accuracy of source parameters such as moment and strike can be much better improved.

Three-Dimensional Velocity Structure in Southern California from Teleseismic Surface Waves and Body Waves.

NASA Astrophysics Data System (ADS)

Prindle-Sheldrake, K. L.; Tanimoto, T.

2003-12-01

Analysis of teleseismic waves generated by large earthquakes worldwide across the Southern California TriNet Seismic Broadband Array has yielded high quality measurements of both surface waves and body waves. Rayleigh waves and Love waves were previously analyzed using a spectral fitting technique (Tanimoto. and Prindle-Sheldrake, GRL 2002; Prindle-Sheldrake and Tanimoto, submitted to JGR), producing a three-dimensional S-wave velocity structure. Features in our velocity structure show some regional contrasts with respect to the starting model (SCEC 2.2), which has detailed crustal structure, but laterally homogeneous upper mantle structure. The most prominent of which is a postulated fast velocity anomaly located west of the Western Transverse Ranges that could be related to a rotated remnant plate from Farallon subduction. Analysis indicates that, while Rayleigh wave data are mostly sensitive to mantle structure, Love wave data require some modifications of crustal structure from SCEC 2.2 model. Recent advances in our velocity structure focus on accommodation of finite frequency effect, and the addition of body waves to the data. Thus far, 118 events have been analyzed for body waves. A simple geometrical approach is used to represent the finite frequency effect in phase velocity maps. Due to concerns that, for seismic phases between 10-100 seconds, structure away from the ray theoretical is also sampled by a propagating surface wave, we have adopted a technique which examines a normal mode formula in its asymptotic limit (Tanimoto, GRL 2003 in press). An ellipse, based on both distance from source to receiver and wavelength, can be used to approximate the effect on the structure along the ray path and adjacent structure. Three models were tested in order to select the appropriate distribution within the ellipse; the first case gives equal weight to all blocks within the ellipse; case 2 incorporates a Gaussian function which falls off perpendicular to the ray path, allowing the amplitude to peak at the receiver; case 3 is the same as case 2, yet removes the effect of the peak at the receiver. A major improvement is that the locale under consideration has expanded due to the effect of ray paths spreading over a larger area than the ray theoretical. Comparison of the three techniques yields very similar results, and all techniques show an exceptional correlation to the ray theoretical phase velocity maps. After analyzing our data in terms of the finite frequency effect, we find that little change has occurred as a result of employing this technique other than expanding our region of study. P-wave measurements were obtained from the data set for 118 events. Preliminary results show systematic patterns. We have successfully measured 30 S-wave events which we plan to incorporate into our velocity structure. Our goal is to proceed with a joint inversion of P-waves, S-waves and Surface waves for a collective Southern California velocity structure.

Application of an iterative least-squares waveform inversion of strong-motion and teleseismic records to the 1978 Tabas, Iran, earthquake

USGS Publications Warehouse

Hartzell, S.; Mendoza, C.

1991-01-01

An iterative least-squares technique is used to simultaneously invert the strong-motion records and teleseismic P waveforms for the 1978 Tabas, Iran, earthquake to deduce the rupture history. The effects of using different data sets and different parametrizations of the problem (linear versus nonlinear) are considered. A consensus of all the inversion runs indicates a complex, multiple source for the Tabas earthquake, with four main source regions over a fault length of 90 km and an average rupture velocity of 2.5 km/sec. -from Authors

Interferometric imaging of crustal structure from wide-angle multicomponent OBS-airgun data

NASA Astrophysics Data System (ADS)

Shiraishi, K.; Fujie, G.; Sato, T.; Abe, S.; Asakawa, E.; Kodaira, S.

2015-12-01

In wide-angle seismic surveys with ocean bottom seismograph (OBS) and airgun, surface-related multiple reflections and upgoing P-to-S conversions are frequently observed. We applied two interferometric imaging methods to the multicomponent OBS data in order to highly utilize seismic signals for subsurface imaging.First, seismic interferometry (SI) is applied to vertical component in order to obtain reflection profile with multiple reflections. By correlating seismic traces on common receiver records, pseudo seismic data are generated with virtual sources and receivers located on all original shot positions. We adopt the deconvolution SI because source and receiver spectra can be canceled by spectral division. Consequently, gapless reflection images from just below the seafloor to the deeper are obtained.Second, receiver function (RF) imaging is applied to multicomponent OBS data in order to image P-to-S conversion boundary. Though RF is commonly applied to teleseismic data, our purpose is to extract upgoing PS converted waves from wide-angle OBS data. The RF traces are synthesized by deconvolution of radial and vertical components at same OBS location for each shot. Final section obtained by stacking RF traces shows the PS conversion boundaries beneath OBSs. Then, Vp/Vs ratio can be estimated by comparing one-way traveltime delay with two-way traveltime of P wave reflections.We applied these methods to field data sets; (a) 175 km survey in Nankai trough subduction zone using 71 OBSs with from 1 km to 10 km intervals and 878 shots with 200 m interval, and (b) 237 km survey in northwest pacific ocean with almost flat layers before subduction using 25 OBSs with 6km interval and 1188 shots with 200 m interval. In our study, SI imaging with multiple reflections is highly applicable to OBS data even in a complex geological setting, and PS conversion boundary is well imaged by RF imaging and Vp/Vs ratio distribution in sediment is estimated in case of simple structure.

A feasibility study for the application of seismic interferometry by multidimensional deconvolution for lithospheric-scale imaging

NASA Astrophysics Data System (ADS)

Ruigrok, Elmer; van der Neut, Joost; Djikpesse, Hugues; Chen, Chin-Wu; Wapenaar, Kees

2010-05-01

Active-source surveys are widely used for the delineation of hydrocarbon accumulations. Most source and receiver configurations are designed to illuminate the first 5 km of the earth. For a deep understanding of the evolution of the crust, much larger depths need to be illuminated. The use of large-scale active surveys is feasible, but rather costly. As an alternative, we use passive acquisition configurations, aiming at detecting responses from distant earthquakes, in combination with seismic interferometry (SI). SI refers to the principle of generating new seismic responses by combining seismic observations at different receiver locations. We apply SI to the earthquake responses to obtain responses as if there was a source at each receiver position in the receiver array. These responses are subsequently migrated to obtain an image of the lithosphere. Conventionally, SI is applied by a crosscorrelation of responses. Recently, an alternative implementation was proposed as SI by multidimensional deconvolution (MDD) (Wapenaar et al. 2008). SI by MDD compensates both for the source-sampling and the source wavelet irregularities. Another advantage is that the MDD relation also holds for media with severe anelastic losses. A severe restriction though for the implementation of MDD was the need to estimate responses without free-surface interaction, from the earthquake responses. To mitigate this restriction, Groenestijn en Verschuur (2009) proposed to introduce the incident wavefield as an additional unknown in the inversion process. As an alternative solution, van der Neut et al. (2010) showed that the required wavefield separation may be implemented after a crosscorrelation step. These last two approaches facilitate the application of MDD for lithospheric-scale imaging. In this work, we study the feasibility for the implementation of MDD when considering teleseismic wavefields. We address specific problems for teleseismic wavefields, such as long and complicated source wavelets, source-side reverberations and illumination gaps. We exemplify the feasibility of SI by MDD on synthetic data, based on field data from the Laramie and the POLARIS-MIT array. van Groenestijn, G.J.A. & Verschuur, D.J., 2009. Estimation of primaries by sparse inversion from passive seismic data, Expanded abstracts, 1597-1601, SEG. van der Neut, J.R, Ruigrok, E.N., Draganov, D.S., & Wapenaar, K., 2010. Retrieving the earth's reflection response by multi-dimensional deconvolution of ambient seismic noise, Extended abstracts, submitted, EAGE. Wapenaar, K., van der Neut, J., & Ruigrok, E.N., 2008. Passive seismic interferometry by multidimensional deconvolution, Geophysics, 75, A51-A56.

Kinematic Rupture Process of the 2015 Gorkha (Nepal) Earthquake Sequence from Joint Inversion of Teleseismic, hr-GPS, Strong-Ground Motion, InSAR interferograms and pixel offsets

NASA Astrophysics Data System (ADS)

Yue, H.; Simons, M.; Jiang, J.; Fielding, E. J.; Owen, S. E.; Moore, A. W.; Riel, B. V.; Polet, J.; Duputel, Z.; Samsonov, S. V.; Avouac, J. P.

2015-12-01

The April 2015 Gorkha, Nepal (Mw 7.8) earthquake ruptured the front of Himalaya thrust belt, causing more than 9,000 fatalities. 17 days after the main event, a large aftershock (Mw 7.2) ruptured to down-dip and east of the main rupture area. To investigate the kinematic rupture process of this earthquake sequence, we explored linear and non-linear inversion techniques using a variety of datasets including teleseismic, high rate and conventional GPS, InSAR interferograms and pixel-offsets. InSAR interferograms from ALOS-2, RADARSAT-2 and Sentinel-1a satellites are used in the joint inversion. The main event is characterized by unilateral rupture extending along strike approximately 70 km to the southeast and 40 km along dip direction. The rupture velocity is well resolved to be lie between 2.8 and 3.0 km/s, which is consistent with back-projection results. An emergent initial phase is observed in teleseismic body wave records, which is consistent with a narrow area of rupture initiation near the hypocenter. The rupture mode of the main event is pulse like. The aftershock ruptured down-dip to the northeast of the main event rupture area. The aftershock rupture area is compact and contained within 40 km of its hypocenter. In contrast to the main event, teleseismic body wave records of the aftershock suggest an abrupt initial phase, which is consistent with a crack like rupture mode. The locations of most of the aftershocks (small and large) surround the rupture area of the main shock with little, if any, spatial overlap.

Finite-fault source inversion using teleseismic P waves: Simple parameterization and rapid analysis

USGS Publications Warehouse

Mendoza, C.; Hartzell, S.

2013-01-01

We examine the ability of teleseismic P waves to provide a timely image of the rupture history for large earthquakes using a simple, 2D finite‐fault source parameterization. We analyze the broadband displacement waveforms recorded for the 2010 Mw∼7 Darfield (New Zealand) and El Mayor‐Cucapah (Baja California) earthquakes using a single planar fault with a fixed rake. Both of these earthquakes were observed to have complicated fault geometries following detailed source studies conducted by other investigators using various data types. Our kinematic, finite‐fault analysis of the events yields rupture models that similarly identify the principal areas of large coseismic slip along the fault. The results also indicate that the amount of stabilization required to spatially smooth the slip across the fault and minimize the seismic moment is related to the amplitudes of the observed P waveforms and can be estimated from the absolute values of the elements of the coefficient matrix. This empirical relationship persists for earthquakes of different magnitudes and is consistent with the stabilization constraint obtained from the L‐curve in Tikhonov regularization. We use the relation to estimate the smoothing parameters for the 2011 Mw 7.1 East Turkey, 2012 Mw 8.6 Northern Sumatra, and 2011 Mw 9.0 Tohoku, Japan, earthquakes and invert the teleseismic P waves in a single step to recover timely, preliminary slip models that identify the principal source features observed in finite‐fault solutions obtained by the U.S. Geological Survey National Earthquake Information Center (USGS/NEIC) from the analysis of body‐ and surface‐wave data. These results indicate that smoothing constraints can be estimated a priori to derive a preliminary, first‐order image of the coseismic slip using teleseismic records.

Kinematic source inversions of teleseismic data based on the QUESO library for uncertainty quantification and prediction

NASA Astrophysics Data System (ADS)

Zielke, O.; McDougall, D.; Mai, P. M.; Babuska, I.

2014-12-01

One fundamental aspect of seismic hazard mitigation is gaining a better understanding of the rupture process. Because direct observation of the relevant parameters and properties is not possible, other means such as kinematic source inversions are used instead. By constraining the spatial and temporal evolution of fault slip during an earthquake, those inversion approaches may enable valuable insights in the physics of the rupture process. However, due to the underdetermined nature of this inversion problem (i.e., inverting a kinematic source model for an extended fault based on seismic data), the provided solutions are generally non-unique. Here we present a statistical (Bayesian) inversion approach based on an open-source library for uncertainty quantification (UQ) called QUESO that was developed at ICES (UT Austin). The approach has advantages with respect to deterministic inversion approaches as it provides not only a single (non-unique) solution but also provides uncertainty bounds with it. Those uncertainty bounds help to qualitatively and quantitatively judge how well constrained an inversion solution is and how much rupture complexity the data reliably resolve. The presented inversion scheme uses only tele-seismically recorded body waves but future developments may lead us towards joint inversion schemes. After giving an insight in the inversion scheme ifself (based on delayed rejection adaptive metropolis, DRAM) we explore the method's resolution potential. For that, we synthetically generate tele-seismic data, add for example different levels of noise and/or change fault plane parameterization and then apply our inversion scheme in the attempt to extract the (known) kinematic rupture model. We conclude with exemplary inverting real tele-seismic data of a recent large earthquake and compare those results with deterministically derived kinematic source models provided by other research groups.

TrigDB for improving the reliability of the epicenter locations by considering the neighborhood station's trigger and cutting out of outliers in operation of Earthquake Early Warning System.

NASA Astrophysics Data System (ADS)

Chi, H. C.; Park, J. H.; Lim, I. S.; Seong, Y. J.

2016-12-01

TrigDB is initially developed for the discrimination of teleseismic-origin false alarm in the case with unreasonably associated triggers producing mis-located epicenters. We have applied TrigDB to the current EEWS(Earthquake Early Warning System) from 2014. During the early stage of testing EEWS from 2011, we adapted ElarmS from US Berkeley BSL to Korean seismic network and applied more than 5 years. We found out that the real-time testing results of EEWS in Korea showed that all events inside of seismic network with bigger than magnitude 3.0 were well detected. However, two events located at sea area gave false location results with magnitude over 4.0 due to the long period and relatively high amplitude signals related to the teleseismic waves or regional deep sources. These teleseismic-relevant false events were caused by logical co-relation during association procedure and the corresponding geometric distribution of associated stations is crescent-shaped. Seismic stations are not deployed uniformly, so the expected bias ratio varies with evaluated epicentral location. This ratio is calculated in advance and stored into database, called as TrigDB, for the discrimination of teleseismic-origin false alarm. We upgraded this method, so called `TrigDB back filling', updating location with supplementary association of stations comparing triggered times between sandwiched stations which was not associated previously based on predefined criteria such as travel-time. And we have tested a module to reject outlier trigger times by setting a criteria comparing statistical values(Sigma) to the triggered times. The criteria of cutting off the outlier is slightly slow to work until the number of stations more than 8, however, the result of location is very much improved.

Rayleigh Wave Tomography of Mid-Continent Rift (MCR) using Earthquake and Ambient Noise Data

NASA Astrophysics Data System (ADS)

Aleqabi, G. I.; Wiens, D.; Wysession, M. E.; Shen, W.; van der Lee, S.; Revenaugh, J.; Frederiksen, A. W.; Darbyshire, F. A.; Stein, S. A.; Jurdy, D. M.; Wolin, E.; Bollmann, T. A.

2015-12-01

The structure of the North American Mid-Continent Rift Zone (MCRZ) is examined using Rayleigh waves from teleseismic earthquakes and ambient seismic noise recorded by the Superior Province Rifting EarthScope Experiment (SPREE). Eighty-four broadband seismometers were deployed during 2011-2013 in Minnesota and Wisconsin, USA, and Ontario, CA, along three lines; two across the rift axis and the third along the rift axis. These stations, together with the EarthScope Transportable Array, provided excellent coverage of the MCRZ. The 1.1 Ga Mesoproterozoic failed rift consists of two arms, buried under post-rifting sedimentary formations that meet at Lake Superior. We compare two array-based tomography methods using teleseismic fundamental mode Rayleigh waves phase and amplitude measurements: the two-plane wave method (TPWM, Forsyth, 1998) and the automated surface wave phase velocity measuring system (ASWMS, Jin and Gaherty, 2015). Both array methods and the ambient noise method give relatively similar results showing low velocity zones extending along the MCRZ arms. The teleseismic Rayleigh wave results from 18 - 180 s period are combined with short period phase velocity results (period 8-30 s) obtained from ambient noise by cross correlation. Phase velocities from the methods are very similar at periods of 18-30 where results overlap; in this period range we use the average of the noise and teleseismic results. Finally the combined phase velocity curve is inverted using a Monte-Carlo inversion method at each geographic point in the model. The results show low velocities at shallow depths (5-10 km) that are the result of very deep sedimentary fill within the MCRZ. Deeper-seated low velocity regions may correspond to mafic underplating of the rift zone.

HFT events - Shallow moonquakes. [High-Frequency Teleseismic

NASA Technical Reports Server (NTRS)

Nakamura, Y.

1977-01-01

A few large distant seismic events of distinctly high signal frequency, designated HFT (high-frequency teleseismic) events, are observed yearly by the Apollo lunar seismic network. Their sources are located on or near the surface of the moon, leaving a large gap in seismic activity between the zones of HFT sources and deep moonquakes. No strong regularities are found in either their spatial or temporal distributions. Several working hypotheses for the identity of these sources have advanced, but many characteristics of the events seem to favor a hypothesis that they are shallow moonquakes. Simultaneous observations of other lunar phenomena may eventually enable the determination of their true identity.

A geometricla error in some Computer Programs based on the Aki-Christofferson-Husebye (ACH) Method of Teleseismic Tomography

USGS Publications Warehouse

Julian, B.R.; Evans, J.R.; Pritchard, M.J.; Foulger, G.R.

2000-01-01

Some computer programs based on the Aki-Christofferson-Husebye (ACH) method of teleseismic tomography contain an error caused by identifying local grid directions with azimuths on the spherical Earth. This error, which is most severe in high latitudes, introduces systematic errors into computed ray paths and distorts inferred Earth models. It is best dealt with by explicity correcting for the difference between true and grid directions. Methods for computing these directions are presented in this article and are likely to be useful in many other kinds of regional geophysical studies that use Cartesian coordinates and flat-earth approximations.

The detectability of high frequency energy at teleseismic and regional distances, 1. Studies of radiation from high-explosive and nuclear cratering events, 2

NASA Astrophysics Data System (ADS)

der, Z. A.; Blandford, R. R.

1981-03-01

A survey of the literature on short period seismic studies showed that seismic waves of high frequency in the 3-10 Hz range can be observed regularly at both regional and teleseismic distances. These observations show that the low Q values proposed for the long period seismic waves cannot be valid in the short period band. The data indicate that, in the mantle, Q increases with frequency and may be as much as five times higher at 5-10 Hz than in the long period band. Even with the most conservative assumptions, the level of high frequency amplitudes in the teleseismic P waves exceeds that predicted with a constant t*p = 1 sec by a factor of at least 100,000 at and beyond 5 Hz. The apparent Q beta of the lithosphere, which may be largely due to scattering by the small scale inhomogeneities in the crust, also shows an increase with frequency by as much as a factor of four within the 1-10 Hz band. This parameter controls the attenuation and the detectability of seismic waves at regional distances such as Pn, Pg, Su and Lg.

Preliminary Results of P & S-wave Teleseismic Tomography of the Superior Region

NASA Astrophysics Data System (ADS)

Bollmann, T. A.; van der Lee, S.; Frederiksen, A. W.

2013-12-01

In continental North America, the Midcontinent Rift System (MRS) is the most prominent feature in gravity and magnetic anomaly maps. These anomalies are associated with large amount of igneous material deposited there around 1.1 Ga. Preliminary evidence from ambient seismic noise analysis of the area has found that the MRS crustal structure has a low velocity along its axis. A major question remains as to whether any structural evidence for the MRS' rifting episodes or its failure were retained in the lithospheric mantle beneath it. To this end we measured teleseismic P and S travel times at Earthscope seismic stations from the Flexible Array SPREE, the Transportable Array, and several US ANSS Backbone stations. These measurements constitute a major resource for upgrading an existing teleseismic, pre-SPREE tomography model for the region, (Frederiksen et al., 2013) as well as longer wavelength regional models, such as NA07 (Bedle and Van der Lee, 2007). We measured the delay times of about 25 thousand teleseismic P arrivals from over a hundred events with magnitudes of 5.5 and greater, and about half as many for S arrivals. Nearly half of these teleseismic events are to the NNW (Alaska-Japan) and about one third are from Central and South America to the SSE. We inverted the P delays for common station-side delays and common event-side delays. Station-side P delays vary by about 1.5 s over the region, with the Archean Superior Craton recording earlier arrivals than Proterozoic terrains in Wisconsin. SPREE stations show later arrivals closer to the rift axis compare to earlier onesfurther away from the rift, but a correlation with the large rift-related gravity anomaly is not obvious. To examine whether the mantle has any rift-related structures, for example from meltdepletion, we are measuring delay times from additional events recorded and recovered during the spring SPREE service run, applying corrections to the delay times for topography and crustal structure, and will invert the corrected delay times for 3D mantle structure. The average P-wave delay time from all events for each station fit to a surface. An interpolated surface of these values is shown in the background. The gray lines are accretionary provinces from Whitmeyer & Karlstrom (2007).

Mantle plumes and associated flow beneath Arabia and East Africa

NASA Astrophysics Data System (ADS)

Chang, Sung-Joon; Van der Lee, Suzan

2011-02-01

We investigate mantle plumes and associated flow beneath the lithosphere by imaging the three-dimensional S-velocity structure beneath Arabia and East Africa. This image shows elongated vertical and horizontal low-velocity anomalies down to at least mid mantle depths. This three-dimensional S-velocity model is obtained through the joint inversion of teleseismic S- and SKS-arrival times, regional S- and Rayleigh waveform fits, fundamental-mode Rayleigh-wave group velocities, and independent Moho constraints from receiver functions, reflection/refraction profiles, and gravity measurements. In the resolved parts of our S-velocity model we find that the Afar plume is distinctly separate from the Kenya plume, showing the Afar plume's origin in the lower mantle beneath southwestern Arabia. We identify another quasi-vertical low-velocity anomaly beneath Jordan and northern Arabia which extends into the lower mantle and may be related to volcanism in Jordan, northern Arabia, and possibly southern Turkey. Comparing locations of mantle plumes from the joint inversion with fast axes of shear-wave splitting, we confirm horizontal mantle flow radially away from Afar. Low-velocity channels in our model support southwestward flow beneath Ethiopia, eastward flow beneath the Gulf of Aden, but not northwestwards beneath the entire Red Sea. Instead, northward mantle flow from Afar appears to be channeled beneath Arabia.

Seismological evidence of the Hales discontinuity in northeast India

NASA Astrophysics Data System (ADS)

Anand, Aakash; Bora, Dipok K.; Borah, Kajaljyoti; Madhab Borgohain, Jayanta

2018-04-01

The crust and upper mantle shear wave velocity structure beneath the northeast India is estimated by joint inversion of Rayleigh wave group velocity and receiver function, calculated from teleseismic earthquakes data recorded at nine broadband seismic stations. The Assam valley and the Shillong-Mikir plateau are the two important tectonic blocks in the northeast India, which are surrounded by the Himalayan collision zone in the north, Indo-Burma subduction zone in the east and by the Bengal basin in the south. The joint inversion followed by forward modeling reveal crustal thicknesses of 30-34 km beneath the Shillong plateau, 36 km beneath the Mikir hills and 38-40 km beneath the Assam valley with an average shear wave velocity (Vs) of 3.4-3.5 km/s. The estimated low upper mantle shear wave velocity (Vsn) 4.2-4.3 km/s may be due to the rock composition or grain size or increased temperature and partial melt (<1%) in the upper mantle, or an effect of all. Also, we report for the first time, the existence of the Hales discontinuity at depths 56-74 km with Vs ∼4.4-4.6 km/s. Variable depth of the Hales discontinuity may be explained by the geotherm and/or addition of Cr3+ and Fe2+ in the spinel-garnet system.

Tomographic Imaging of the Peru Subduction Zone beneath the Altiplano and Implications for Andean Tectonics

NASA Astrophysics Data System (ADS)

Davis, P. M.; Foote, E. J.; Stubailo, I.; Phillips, K. E.; Clayton, R. W.; Skinner, S.; Audin, L.; Tavera, H.; Dominguez Ramirez, L. A.; Lukac, M. L.

2010-12-01

This work describes preliminary tomography results from the Peru Seismic Experiment (PERUSE) a 100 station broadband seismic network installed in Peru. The network consists a linear array of broadband seismic stations that was installed mid-2008 that runs from the Peruvian coast near Mollendo to Lake Titicaca. A second line was added in late 2009 between Lake Titicaca and Cusco. Teleseismic and local earthquake travel time residuals are being combined in the tomographic inversions. The crust under the Andes is found to be 70-80 km thick decreasing to 30 km near the coast. The morphology of the Moho is consistent with the receiver function images (Phillips et al., 2010; this meeting) and also gravity. Ray tracing through the heterogeneous structure is used to locate earthquakes. However the rapid spatial variation in crustal thickness, possibly some of the most rapid in the world, generates shadow zones when using conventional ray tracing for the tomography. We use asymptotic ray theory that approximates effects from finite frequency kernels to model diffracted waves in these regions. The observation of thickened crust suggests that models that attribute the recent acceleration of the Altiplano uplift to crustal delamination are less likely than those that attribute it to crustal compression.

Seismic investigation of the southern Rio Grande Rift

NASA Astrophysics Data System (ADS)

Thompson, Lennox E.

Competing models exist to explain what caused the Earth's crust to spread apart 29 million years ago to create a region known today as the Rio Grande Rift (RGR). The RGR extends from central Colorado through New Mexico to northern Mexico, near El Paso. The RGR has different geologic features that distinguish it from most other valleys (e.g., the RGR was not cut by a river nor does a river branch upstream). A growing body of evidence shows that geologic activity still occurs in the RGR, with a continuation of faulting, seismicity and widening at a small rate of about 0.3 mm/yr (Woodward , 1977). We map of the seismic velocity structure and crustal thickness using data from the Rio Grande Rift Seismic TRAnsect (RISTRA) experiment and the EarthScope Transportable Array (USArray) dataset. In addition to the data we collected from the RISTRA experiment and USArray dataset, we also acquired receiver functions from the EarthScope Automatic Receiver Survey (EARS) website (http://www.earthscope.org/data) and waveform data from the Incorporated Research Institutes for Seismology (IRIS) Data Management Center (DMC). We requested seismograms from the IRIS DMC database where we acquired teleseismic events from Jan 2000 to Dec 2009. This includes 7,259 seismic events with a minimum magnitude of 5.5 and 106,389 continuous waveforms. This data was preprocessed (merged, rotated) using a program called Standing Order of Data (SOD). The RISTRA experiment and the USArray were designed to image crust and mantle structures by computing receiver functions for all data in the Southern Rio Grande Rift (SRGR). We map the crustal thickness, seismic velocity, and mantle structure for the sole purpose to better determine the nature of tectonic activity that is presently taking place and further investigate the regional extension of the Southern Rio Grande Rift (SRGR). Here we present preliminary results of the crustal and velocity structure using the kriging interpolation scheme seem stable and we are now able to clearly observe certain patterns we can use to interpret the southern RGR deformation and extension.

Crustal anisotropy along the North Anatolian Fault Zone from receiver functions

NASA Astrophysics Data System (ADS)

Licciardi, Andrea; Eken, Tuna; Taymaz, Tuncay; Piana Agostinetti, Nicola; Yolsal-Çevikbilen, Seda; Tilmann, Frederik

2016-04-01

The North Anatolian Fault Zone (NAFZ) that is considered to be one of the largest plate-bounding transform faults separates the Anatolian Plate to the south from the Eurasian Plate to the north. A proper estimation of the crustal anisotropy in the area is a key point to understand the present and past tectonic processes associated with the plate boundary as well as for assessing its strength and stability. In this work we used data from the North Anatolian Fault (NAF) passive seismic experiment in order to retrieve the anisotropic properties of the crust by means of the receiver function (RF) method. This approach provides robust constraints on the location at depth of anisotropic bodies compared to other seismological tools like S-waves splitting observations where anisotropic parameters are obtained through a path-integrated measurement process over depth. We computed RFs from teleseismic events, for 39 stations with a recording period of nearly 2 years, providing an excellent azimuthal coverage. The observed azimuthal variations in amplitudes and delay times on the Radial and Transverse RF indicate the presence of anisotropy in the crust. Isotropic and anisotropic effects on the RFs are analyzed separately after harmonic decomposition of the RF dataset (Bianchi et al. 2010). Pseudo 2D profiles are built to observe both the seismic isotropic structure and the depth-dependent lateral variations of crustal anisotropy in the area, including orientation of the symmetry axis. Preliminary results show that the isotropic structure is characterized by a complex crustal setting above a nearly flat Moho at a depth of ~40 km in the central portion of the studied area. Strong anisotropy is present in the upper crust along some portions of the NAFZ and the Ezinepazari-Sungurlu Fault (ESF), with a strong correlation between the orientation of the symmetry axis of anisotropy and the strike of the main geological structures. More complex patterns of anisotropy are present in the middle and lower crust as well as in the upper mantle. Bianchi, I., J. Park, N. Piana Agostinetti, and V. Levin (2010), Mapping seismic anisotropy using harmonic decomposition of receiver functions: An application to Northern Apennines, Italy, J. Geophys. Res., 115, B12317, doi:10.1029/2009JB007061.

Structure of the Upper Mantle and Mantle Transition Zone in Central Mongolia

NASA Astrophysics Data System (ADS)

Cui, Z.; Meltzer, A.; Stachnik, J.; Fischer, K. M.; Russo, R. M.; Munkhuu, U.; Baasanbat, T.

2016-12-01

Located between two major Archean cratons, the Siberian Craton to the north and the Tarim and Sino-Korean Cratons to the south, the lithosphere of Central Mongolia was constructed over an extended period of orogenesis associated with the Central Asian Orogenic Belt. Archean to Early Proterozoic basement was modified by accreted subduction complexes during the Paleozoic and early Mesozoic and basalt magmatism in the Cenozoic. Central and western Mongolia constitute a significant portion of the greater Mongolian plateau, an approximately 2.6 million km2area of Central Asia with an average elevation of 1500 meters. The high topography of the Mongolian Plateau has been attributed to far-field effects of India-Asia convergence, Pacific plate subduction, mantle plume activity, convective mantle flow, and magmatic underplating. The origin and persistence of continental plateaus through time provides insight into the evolution of continents and interactions between mantle dynamics and surface processes. As part of a larger interdisciplinary project to understand the origin of high topography in continental interiors we deployed 112 seismic broadband stations in central Mongolia as three separate subarrays in two separate mobilizations over a four year period (2012-2016). The stations extend from the Hovsgol rift in northern Mongolia, through the Hangay Dome, and into the Gobi Altai in southern Mongolia. We use S wave Receiver functions (SRF) to examine the lithosphere asthenosphere boundary and P wave Receiver functions (PRF) to investigate the mantle transition zone (MTZ). Preliminary SRF results from the subarray in the Hangay show lithospheric thinning and E-W variation. The LAB beneath the Hangay is 100km. It gradually thins to 90 km at the western end of the central Hangay and thins more abruptly to 80km at the eastern end of the central Hangay. These results are in agreement with results from joint inversion of receiver functions and surface waves and teleseismic tomography. PRF exhibit a clear Ps arrival from the 410-km discontinuity but a weak Ps phase converted from 660-km discontinuity in both station and CCP stacks. The 410-km Ps arrival is gradually delayed from south to north corresponding to a depth change of 20km. Several stations with visible 660-km Ps suggest a 10-30 km thinning within the MTZ.

Crustal structure of Tolfa domes complex (northern Latium - Italy) inferred from receiver functions analysis: an interplay between tectonics and magmatism

NASA Astrophysics Data System (ADS)

Buttinelli, M.; Bianchi, I.; Anselmi, M.; Chiarabba, C.; de Rita, D.; Quattrocchi, F.

2010-12-01

The Tolfa-Cerite volcanic district developed along the Tyrrhenian passive margin of central Italy, as part of magmatic processes started during the middle Pliocene. In this area the uncertainties on the deep crustal structures and the definition of the intrusive bodies geometry are focal issues that still need to be addressed. After the onset of the spreading of the Tyrrhenian sea during the Late Miocene, the emplacement of the intrusive bodies of the Tolfa complex (TDC), in a general back-arc geodynamical regime, generally occurred in a low stretching rate, in correspondence of the junctions between major lithospheric discontinuities. Normal faults, located at the edge of Mio-Pliocene basins, were used as preferential pathways for the rising of magmatic masses from the mantle to the surface. We used teleseismic recordings at the TOLF and MAON broad band station of the INGV seismic network (located between the Argentario promontory and Tolfa-Ceriti dome complexes -TDC-) to image the principal seismic velocity discontinuities by receiver function analysis (RF's). Together with RF’s velocity models of the area computed using the teleseismic events recorded by a temporary network of eight stations deployed around the TDC, we achieve a general crustal model of this area. The geometry of the seismic network has been defined to focus on the crustal structure beneath the TDC, trying to define the main velocity changes attributable to the intrusive bodies, the calcareous basal complex, the deep metamorphic basement, the lower crust and the Moho. The analysis of these data show the Moho at a depth of 23 km in the TDC area and 20 km in the Argentario area. Crustal models also show an unexpected velocity decrease between 12 and 18 km, consistent with a slight dropdown of the Vp/Vs ratio, imputable to a regional mid-crustal shear zone inherited from the previous alpine orogenesis, re-activated in extensional tectonic by the early opening phases of the Tyrrhenian sea. Above this low Vs layer, we find some interesting features corresponding to: - a low Vs shallow and 2 km thick layer of Liguride and Plio-Pleistocene units (z = 0-2 km of depth) - a high Vs 4-5 km thick anisotropic layer of limestones (z = 2-7 km of depth) - a very high Vs (3.8 km/s) 4 km thick layer probably corresponding to the metamorphic basement. The analysis of the geometry of the velocity changes between these layers (from the surface to the bottom of metamorphic basement), also yield evidence of crustal block tilting, due to the development of the eastern continental passive margin of the Tyrrhenian sea. The general crustal setting observed between the TDC and the Argentario areas is also consistent with the simple shear models suggested for back-arc basins opening. Comparison of RF’s TDC models with MAON station data also led to important considerations confirming the initial evolutive phase of the Tyrrhenian sea opening, in association with the first occurrences of intrusive magmatism in these areas.

Crustal thickness and composition beneath the High Lava Plains of Eastern Oregon from teleseismic receiver functions

NASA Astrophysics Data System (ADS)

Eagar, K. C.; Fouch, M. J.; James, D. E.; Carlson, R. W.

2009-12-01

The nature of the crust beneath the High Lava Plains of eastern Oregon is fundamental for understanding the origins of widespread Cenozoic volcanism in the region. Eruptions of flood basalts in the southern Cascadian back arc peaked ~17-15 Ma, and were followed by distributed bimodal volcanism along two perpendicular migrating tracks; the Snake River Plain and the High Lava Plains. The orientations of eruptive centers have led to several competing hypotheses about their cause, including a deep mantle plume, slab retreat and asthenospheric inflow, lithospheric delamination, and lithospheric extension. The goal of this project is to constrain the nature, geometry, and depth of the Moho across the High Lava Plains, which will shed light on questions regarding crustal influence on melt generation and differentiation and the degree of magmatic underplating. In this study, we analyze teleseismic receiver functions from 118 stations of the High Lava Plains temporary broadband array, 34 nearby EarthScope/USArray stations, and 5 other regional broadband stations to determine bulk crustal features of thickness (H) and Vp/Vs ratio (κ). Applying the H-κ stacking method, we search for the best-fitting solution of timing predictions for direct and multiple P-to-S conversions from the Moho interface. Converting Vp/Vs to Poisson ratio, which is dependent primarily upon rock composition, allows for comparison with other direct geological observations. Preliminary results show that the crust of the High Lava Plains is relatively thin (~31 km) with a very sharp gradient to thicker crust (~42 km) at the western edge of the Owyhee Plateau in southwestern Idaho. This gradient is co-located with the western margin of Precambrian North America and is in the vicinity of the Jordan Craters volcanic center. The sharp topography of the Moho might have been a factor in melt migration beneath this area. West of the High Lava Plains, the crust thickens to ~40 km into the Cascade volcanic arc. We note that these results are consistent with preliminary crustal images from the complementary active source imaging effort of the High Lava Plains project. In contrast to crustal thickening to the east and west, there appears to be no change into the Blue Mountains to the north or the northern Basin and Range to the south. Although not as sharp and well defined as the Moho relief, there is variation in bulk crustal composition between the Owyhee Plateau and the High Lava Plains as defined by Vp/Vs. In contrast to the crust of Precambrian North America and the northern Basin and Range, whose Poisson ratio (~0.26) is comparable to average continental crust (0.27), the High Lava Plains exhibits high values (~0.30) typical of more mafic bulk crustal composition. This result suggests that the crust of the High Lava Plains evolved in a fundamentally different fashion in response to widespread magmatism relative to surrounding terranes. High Poisson ratios support the suggestion of mafic underplating and high, near crustal melting temperatures, possibly explaining the occurrence of rhyolitic volcanism across the High Lava Plains.

Revision of earthquake hypocenter locations in GEOFON bulletin data using global source-specific station terms technique

NASA Astrophysics Data System (ADS)

Nooshiri, N.; Saul, J.; Heimann, S.; Tilmann, F. J.; Dahm, T.

2015-12-01

The use of a 1D velocity model for seismic event location is often associated with significant travel-time residuals. Particularly for regional stations in subduction zones, where the velocity structure strongly deviates from the assumed 1D model, residuals of up to ±10 seconds are observed even for clear arrivals, which leads to strongly biased locations. In fact, due to mostly regional travel-time anomalies, arrival times at regional stations do not match the location obtained with teleseismic picks, and vice versa. If the earthquake is weak and only recorded regionally, or if fast locations based on regional stations are needed, the location may be far off the corresponding teleseismic location. In this case, implementation of travel-time corrections may leads to a reduction of the travel-time residuals at regional stations and, in consequence, significantly improve the relative location accuracy. Here, we have extended the source-specific station terms (SSST) technique to regional and teleseismic distances and adopted the algorithm for probabilistic, non-linear, global-search earthquake location. The method has been applied to specific test regions using P and pP phases from the GEOFON bulletin data for all available station networks. By using this method, a set of timing corrections has been calculated for each station varying as a function of source position. In this way, an attempt is made to correct for the systematic errors, introduced by limitations and inaccuracies in the assumed velocity structure, without solving for a new earth model itself. In this presentation, we draw on examples of the application of this global SSST technique to relocate earthquakes from the Tonga-Fiji subduction zone and from the Chilean margin. Our results have been showing a considerable decrease of the root-mean-square (RMS) residual in earthquake location final catalogs, a major reduction of the median absolute deviation (MAD) of the travel-time residuals at regional stations and sharper images of the seismicity compared to the initial locations.

Seismic Attenuation of Teleseismic Body Waves in Cascadia, Measured on the Amphibious Array

NASA Astrophysics Data System (ADS)

Eilon, Z.; Abers, G. A.

2015-12-01

Fundamental questions remain about the nature of the asthenosphere, including its dynamical relationship to overlying lithosphere, melt content, and entrainment in subduction zones. We examine the evolution of this low-velocity, highly attenuating layer using data from the Cascadia Initiative's Amphibious Array, which provides unprecedented coverage of an oceanic plate from ridge crest to trench to sub-arc. Our study extends the suite of measurements achievable with OBS data, augmenting traditional travel time analysis with integrated attenuation data that are a powerful tool for imaging melt/fluids and the variation of asthenospheric character with age. Cooling models, coupled with experimentally-derived anelastic scaling relationships, indicate that thermal gradients should cause appreciable decrease in attenuation of teleseismic body waves with increasing age. This long-wavelength cooling trend may be perturbed by highly attenuating melt or volatiles concentrated at the ridge axis or beneath the Cascades arc, depending on melt fraction and pore geometry. Attenuation beyond the trench should be a strong function of the fate of asthenospheric entrainment beneath subducted plates, with implications for mass transfer to the deep mantle as well as recent models of sub-slab anisotropy. The Amphibious Array, with <70 km spacing of OBS and on-land broadband seismometers deployed between 2011 and 2015, provides a dataset of ~1 x 105 arrivals from ~700 Mw>6.0 teleseismic earthquakes. We use a spectral ratio method to compute differential attenuation (Δt*) from body wave teleseisms recorded at OBS and land stations, allowing us to estimate path-integrated quality factor in the upper mantle. Preliminary results reveal variations of ~3 s in differential travel time and >0.5 s in ΔtS* across the 0-10 Ma oceanic plate, demonstrating the strong thermal control on anelasticity. Large values of Δt* observed east of the trench may indicate entrainment of highly attenuating asthenosphere during subduction, although more work is required to categorize and remove the signal of the overriding plate. This work complements previous studies using surface waves and contributes to our developing understanding of anelastic controls on seismic parameters by probing the Earth in a different frequency range.

Rupture imaging of the Mw 7.9 12 May 2008 Wenchuan earthquake from back projection of teleseismic P waves

USGS Publications Warehouse

Xu, Y.; Koper, K.D.; Sufri, O.; Zhu, L.; Hutko, Alexander R.

2009-01-01

[1] The Mw 7.9 Wenchuan earthquake of 12 May 2008 was the most destructive Chinese earthquake since the 1976 Tangshan event. Tens of thousands of people were killed, hundreds of thousands were injured, and millions were left homeless. Here we infer the detailed rupture process of the Wenchuan earthquake by back-projecting teleseismic P energy from several arrays of seismometers. This technique has only recently become feasible and is potentially faster than traditional finite-fault inversion of teleseismic body waves; therefore, it may reduce the notification time to emergency response agencies. Using the IRIS DMC, we collected 255 vertical component broadband P waves at 30-95?? from the epicenter. We found that at periods of 5 s and greater, nearly all of these P waves were coherent enough to be used in a global array. We applied a simple down-sampling heuristic to define a global subarray of 70 stations that reduced the asymmetry and sidelobes of the array response function (ARF). We also considered three regional subarrays of seismometers in Alaska, Australia, and Europe that had apertures less than 30?? and P waves that were coherent to periods as short as 1 s. Individual ARFs for these subarrays were skewed toward the subarrays; however, the linear sum of the regional subarray beams at 1 s produced a symmetric ARF, similar to that of the groomed global subarray at 5 s. For both configurations we obtained the same rupture direction, rupture length, and rupture time. We found that the Wenchuan earthquake had three distinct pulses of high beam power at 0, 23, and 57 s after the origin time, with the pulse at 23 s being highest, and that it ruptured unilaterally to the northeast for about 300 km and 110 s, with an average speed of 2.8 km/s. It is possible that similar results can be determined for future large dip-slip earthquakes within 20-30 min of the origin time using relatively sparse global networks of seismometers such as those the USGS uses to locate earthquakes in near-real time. Copyright 2009 by the American Geophysical Union.

Focal Depth of the WenChuan Earthquake Aftershocks from modeling of Seismic Depth Phases

NASA Astrophysics Data System (ADS)

Luo, Y.; Zeng, X.; Chong, J.; Ni, S.; Chen, Y.

2008-12-01

After the 05/12/2008 great WenChuan earthquake in Sichuan Province of China, tens of thousands earthquakes occurred with hundreds of them stronger than M4. Those aftershocks provide valuable information about seismotectonics and rupture processes for the mainshock, particularly accurate spatial distribution of aftershocks is very informational for determining rupture fault planes. However focal depth can not be well resolved just with first arrivals recorded by relatively sparse network in Sichuan Province, therefore 3D seismicity distribution is difficult to obtain though horizontal location can be located with accuracy of 5km. Instead local/regional depth phases such as sPmP, sPn, sPL and teleseismic pP,sP are very sensitive to depth, and be readily modeled to determine depth with accuracy of 2km. With reference 1D velocity structure resolved from receiver functions and seismic refraction studies, local/regional depth phases such as sPmP, sPn and sPL are identified by comparing observed waveform with synthetic seismograms by generalized ray theory and reflectivity methods. For teleseismic depth phases well observed for M5.5 and stronger events, we developed an algorithm in inverting both depth and focal mechanism from P and SH waveforms. Also we employed the Cut and Paste (CAP) method developed by Zhao and Helmberger in modeling mechanism and depth with local waveforms, which constrains depth by fitting Pnl waveforms and the relative weight between surface wave and Pnl. After modeling all the depth phases for hundreds of events , we find that most of the M4 earthquakes occur between 2-18km depth, with aftershocks depth ranging 4-12km in the southern half of Longmenshan fault while aftershocks in the northern half featuring large depth range up to 18km. Therefore seismogenic zone in the northern segment is deeper as compared to the southern segment. All the aftershocks occur in upper crust, given that the Moho is deeper than 40km, or even 60km west of the Longmenshan fault. Absence of mid-lower crustal shocks supports the model of lower crustal flow beneath eastern Tibetan plateau, which is probably responsible for Longmenshan uplifting and hence the Wenchuan earthquake.

Towards a new technique to construct a 3D shear-wave velocity model based on converted waves

NASA Astrophysics Data System (ADS)

Hetényi, G.; Colavitti, L.

2017-12-01

A 3D model is essential in all branches of solid Earth sciences because geological structures can be heterogeneous and change significantly in their lateral dimension. The main target of this research is to build a crustal S-wave velocity structure in 3D. The currently popular methodologies to construct 3D shear-wave velocity models are Ambient Noise Tomography (ANT) and Local Earthquake Tomography (LET). Here we propose a new technique to map Earth discontinuities and velocities at depth based on the analysis of receiver functions. The 3D model is obtained by simultaneously inverting P-to-S converted waveforms recorded at a dense array. The individual velocity models corresponding to each trace are extracted from the 3D initial model along ray paths that are calculated using the shooting method, and the velocity model is updated during the inversion. We consider a spherical approximation of ray propagation using a global velocity model (iasp91, Kennett and Engdahl, 1991) for the teleseismic part, while we adopt Cartesian coordinates and a local velocity model for the crust. During the inversion process we work with a multi-layer crustal model for shear-wave velocity, with a flexible mesh for the depth of the interfaces. The RFs inversion represents a complex problem because the amplitude and the arrival time of different phases depend in a non-linear way on the depth of interfaces and the characteristics of the velocity structure. The solution we envisage to manage the inversion problem is the stochastic Neighbourhood Algorithm (NA, Sambridge, 1999), whose goal is to find an ensemble of models that sample the good data-fitting regions of a multidimensional parameter space. Depending on the studied area, this method can accommodate possible independent and complementary geophysical data (gravity, active seismics, LET, ANT, etc.), helping to reduce the non-linearity of the inversion. Our first focus of application is the Central Alps, where a 20-year long dataset of high-quality teleseismic events recorded at 81 stations is available, and we have high-resolution P-wave velocity model available (Diehl et al., 2009). We plan to extend the 3D shear-wave velocity inversion method to the entire Alpine domain in frame of the AlpArray project, and apply it to other areas with a dense network of broadband seismometers.

Imaging megathrust zone and Yakutat/Pacific plate interface in Alaska subduction zone

NASA Astrophysics Data System (ADS)

Kim, Y.; Abers, G. A.; Li, J.; Christensen, D. H.; Calkins, J. A.

2013-05-01

We image the subducted slab underneath a 450 km long transect of the Alaska subduction zone. Dense stations in southern Alaska are set up to investigate (1) the geometry and velocity structure of the downgoing plate and their relation to slab seismicity, and (2) the interplate coupled zone where the great 1964 (magnitude 9.3) had greatest rupture. The joint teleseismic migration of two array datasets (MOOS, Multidisciplinary Observations of Onshore Subduction, and BEAAR, Broadband Experiment Across the Alaska Range) based on teleseismic receiver functions (RFs) using the MOOS data reveal a shallow-dipping prominent low-velocity layer at ~25-30 km depth in southern Alaska. Modeling of these RF amplitudes shows a thin (<6.5 km) low-velocity layer (shear wave velocity of ~3 km/s), which is ~20-30% slower than normal oceanic crustal velocities, between the subducted slab and the overriding North American plate. The observed low-velocity megathrust layer (with P-to-S velocity ratio (Vp/Vs) exceeding 2.0) may be due to a thick sediment input from the trench in combination of elevated pore fluid pressure in the channel. The subducted crust below the low-velocity channel has gabbroic velocities with a thickness of 11-12 km. Both velocities and thickness of the low-velocity channel abruptly increase as the slab bends in central Alaska, which agrees with previously published RF results. Our image also includes an unusually thick low-velocity crust subducting with a ~20 degree dip down to 130 km depth at approximately 200 km inland beneath central Alaska. The unusual nature of this subducted segment has been suggested to be due to the subduction of the Yakutat terrane. We also show a clear image of the Yakutat and Pacific plate subduction beneath the Kenai Peninsula, and the along-strike boundary between them at megathrust depths. Our imaged western edge of the Yakutat terrane, at 25-30 km depth in the central Kenai along the megathrust, aligns with the western end of the geodetically locked patch with high slip deficit, and coincides with the boundary of aftershock events from the 1964 earthquake. It seems plausible that this sharp change in the nature of the downgoing plate controls the slip distribution of great earthquakes on this plate interface.

Imaging megathrust zone and Yakutat/Pacific plate interface in Alaska subduction zone

NASA Astrophysics Data System (ADS)

Kim, Y.; Abers, G. A.; Li, J.; Christensen, D. H.; Calkins, J. A.

2012-12-01

We image the subducted slab underneath a 450 km long transect of the Alaska subduction zone. Dense stations in southern Alaska are set up to investigate (1) the geometry and velocity structure of the downgoing plate and their relation to slab seismicity, and (2) the interplate coupled zone where the great 1964 (magnitude 9.3) had greatest rupture. The joint teleseismic migration of two array datasets (MOOS, Multidisciplinary Observations of Onshore Subduction, and BEAAR, Broadband Experiment Across the Alaska Range) based on teleseismic receiver functions (RFs) using the MOOS data reveal a shallow-dipping prominent low-velocity layer at ~25-30 km depth in southern Alaska. Modeling of these RF amplitudes shows a thin (3-6.5 km) low-velocity layer (shear wave velocity less than 3 km/s), which is ~20-30% slower than normal oceanic crustal velocities, between the subducted slab and the overriding North America plate. The observed low-velocity megathrust layer (with Vp/Vs ratio exceeding 2.0) may be due to a thick sediment input from the trench in combination of elevated pore fluid pressure in the channel. The subducted crust below the low-velocity channel has gabbroic velocities with a thickness of 11-15 km. Both velocities and thickness of the low-velocity channel abruptly increase as the slab bends in central Alaska, which agrees with previously published RF results. Our image also includes an unusually thick low-velocity crust subducting with a ~20 degree dip down to 130 km depth at approximately 200 km inland beneath central Alaska. The unusual nature of this subducted segment has been suggested to be due to the subduction of the Yakutat terrane. Subduction of this buoyant crust could explain the shallow dip of the thrust zone beneath southern Alaska. We also show a clear image of the Yakutat and Pacific plate subduction beneath the Kenai Peninsula, and the along-strike boundary between them at megathrust depths. Our imaged western edge of the Yakutat terrane, at ~30-42 km depth in the central Kenai along the megathrust, aligns with the western end of the geodetically locked patch with high slip deficit, and coincides with the boundary of aftershock events from the 1964 earthquake. It seems plausible that this sharp change in the nature of the downgoing plate controls the slip distribution of great earthquakes on this plate interface.

FaultLab: Results on the crustal structure of the North Anatolian Fault from a dense seismic network

NASA Astrophysics Data System (ADS)

Thompson, David; Rost, Sebastian; Houseman, Greg; Cornwell, David; Türkelli, Niyazi; Uǧur, Teoman, Kahraman, Metin; Altuncu Poyraz, Selda; Gülen, Levent; Utkucu, Murat; Frederiksen, Andrew

2013-04-01

The North Anatolian Fault Zone (NAFZ) is a major continental strike-slip fault system, similar in size and scale to the San Andreas system, that extends ~1200 km across Turkey from the Aegean coast on the west to the Lake Van region in the east. FaultLab is a multidisciplinary project that aims to better understand deformation throughout the entire crust in the NAFZ, in particular the expected transition from narrow zones of brittle deformation in the upper crust to broad shear zones in the lower crust/upper mantle and how these features contribute to the earthquake loading cycle. The project incorporates broadband seismology, satellite geodesy, structural geology and numerical modelling in order to give an unprecedented view of the dynamic state of the NAFZ in the vicinity of the devastating 1999 Izmit and Düzce earthquakes. This contribution will discuss the first results from the seismic component of the project, a 73 station network encompassing the northern and southern branches of the NAFZ in the Sakarya region. Deployed in May 2012, the Dense Array for North Anatolia (DANA) is arranged as a 6×11 grid with a nominal station spacing of 7 km, with a further 7 stations located outside of the grid. Receiver function analysis will provide estimates of bulk crustal properties, along with information regarding heterogeneity at depth (dipping interfaces/anisotropy). With the excellent resolution afforded by the DANA network, we will present results using the technique of teleseismic scattering tomography. The method uses a full waveform inversion of teleseismic signals coupled with array processing techniques to infer the properties and location of small-scale heterogeneities (with scales on the order of the seismic wavelength) within the crust. Images obtained using these methods will provide evidence for how the deformation is distributed within the fault zone at depth, providing constraints that can be used in conjunction with structural analyses of exhumed fault segments elsewhere, and models of geodetic strain-rate across the fault system. By linking together results from the complementary techniques being employed in the FaultLab project, we aim to produce a comprehensive picture of fault structure and dynamics throughout the crust and shallow upper mantle of this major active fault zone.

Constraining the crustal root geometry beneath Northern Morocco

NASA Astrophysics Data System (ADS)

Díaz, J.; Gil, A.; Carbonell, R.; Gallart, J.; Harnafi, M.

2016-10-01

Consistent constraints of an over-thickened crust beneath the Rif Cordillera (N. Morocco) are inferred from analyses of recently acquired seismic datasets including controlled source wide-angle reflections and receiver functions from teleseismic events. Offline arrivals of Moho-reflected phases recorded in RIFSIS project provide estimations of the crustal thicknesses in 3D. Additional constraints on the onshore-offshore transition are inferred from shots in a coeval experiment in the Alboran Sea recorded at land stations in northern Morocco. A regional crustal thickness map is computed from all these results. In parallel, we use natural seismicity data collected throughout TopoIberia and PICASSO experiments, and from a new RIFSIS deployment, to obtain receiver functions and explore the crustal thickness variations with a H-κ grid-search approach. This larger dataset provides better resolution constraints and reveals a number of abrupt crustal changes. A gridded surface is built up by interpolating the Moho depths inferred for each seismic station, then compared with the map from controlled source experiments. A remarkably consistent image is observed in both maps, derived from completely independent data and methods. Both approaches document a large crustal root, exceeding 50 km depth in the central part of the Rif, in contrast with the rather small topographic elevations. This large crustal thickness, consistent with the available Bouguer anomaly data, favors models proposing that the high velocity slab imaged by seismic tomography beneath the Alboran Sea is still attached to the lithosphere beneath the Rif, hence pulling down the lithosphere and thickening the crust. The thickened area corresponds to a quiet seismic zone located between the western Morocco arcuate seismic zone, the deep seismicity area beneath western Alboran Sea and the superficial seismicity in Alhoceima area. Therefore, the presence of a crustal root seems to play also a major role in the seismicity distribution in northern Morocco.

Receiver Function Imaging of Crust and Uppermost Mantle Structure beneath the Japan Islands -Inclusion of Hi-net Data-

NASA Astrophysics Data System (ADS)

Yamauchi, M.; Hirahara, K.; Shibutani, T.

2001-12-01

We are examining a large number of teleseismic waveforms observed at stations closely distributed over the Japan Islands to construct body-wave waveform tomography data for determining 3-D crust and upper mantle structure including velocity discontinuities. As one of preparatory studies toward this final goal, we are executing array analyses of Receiver Functions (RF). RF analyses of J-array data ( 32 broad band stations and 269 short period stations ) and Freesia data ( 15 broad band stations ), whose stations are closely distributed, have provided us with new information on the structure including velocity discontinuities beneath the Japan Islands (Tada et al, 2001). In their study, for crustal imaging, RFs transformed from time to depth domain after SVD filtering ( Chevrot and Giardin, 2000 ) are projected onto 2-D profiles, which show average values for cells within +/- 50km from each cross section. However, this cell size does not satisfy our demand to draw the detailed image beneath the Japan Islands. In addition, J-array short period RFs available for the analyses are limited because of high frequency noises. In this research, Hi-net data (short period), whose stations are far more closely distributed, are newly included into our data. We make RF image with α =3 of short period J-array data and Hi-net data for events observed during a period from September, 2000 to July, 2001 with the magnitudes larger than 5.5. The total number of the stations with their average spacing of 10km is about 800 (J-array; 270, Hi-net; 500), which enables to reduce the cell size to +/- 20km at most. We show a new 3-D RF image of the crust and the uppermost mantle, whose best spatial resolution is reaching less than 5km. Therefore we can obtain much more detailed 3-D RF image beneath the whole Japan Islands.

From underplating to delamination-retreat in the northern Apennines

NASA Astrophysics Data System (ADS)

Chiarabba, C.; Giacomuzzi, G.; Bianchi, I.; Agostinetti, N. P.; Park, J.

2014-10-01

Recordings of teleseismic earthquakes from a dense set of temporary and permanent broadband seismic stations reveal the lithospheric structure of the northern Apennines and support the scenario of a retreating detachment within the mid-crust. Lithospheric delamination appears crucial to the formation and evolution of the Apennines orogen. Receiver-function (RF) stacks outline a continuous west-dipping Ps converted phase from a positive velocity jump that we interpret as the top of the lower crust and mantle of the Adria continental lithosphere, which is descending into the shallow mantle. The correlation of seismicity with two RF profiles across the northern Apennines suggests distinct stages of lithospheric delamination. Active penetration of the detachment into the Adria lithosphere seems evident in the south/east, with induced shallow-mantle flow facilitated by slab dehydration. Penetration of the detachment in the north/west seems to have arrested, and is possibly marked by crustal underplating. This layer atop the Apennines slab is visible only down to 80 km depth and suspends above an oppositely-dipping paired positive/negative Ps converted phase in stacked receiver functions. The break in the west-dipping Adria lithosphere conflicts with a westward-subduction scenario continuous from the Oligocene. Lateral changes of deep structure and seismicity along the northern Apennines suggest that underplating of crustal material and delamination-retreat are distinct mechanisms active today in the western and eastern sectors, respectively, of the northern Apennines. Negative Ps-pulses at 100-120 km depth help to define a seismic lithosphere-asthenosphere boundary (LAB), but cross-cut a volume of high-velocity mantle rock, as inferred from tomographic models. We hypothesize that this seismic LAB is a rheological discontinuity that affects the frequency band of seismic body waves, but not the long-term viscous response that governs the evolution and eventual detachment of the continental slab.

Lithospheric Structure of the Yamato Basin Inferred from Trans-dimensional Inversion of Receiver Functions

NASA Astrophysics Data System (ADS)

Akuhara, T.; Nakahigashi, K.; Shinohara, M.; Yamada, T.; Yamashita, Y.; Shiobara, H.; Mochizuki, K.

2017-12-01

The Yamato Basin, located at the southeast of the Japan Sea, has been formed by the back-arc opening of the Japan Sea. Wide-angle reflection surveys have revealed that the basin has anomalously thickened crust compared with a normal oceanic crust [e.g., Nakahigashi et al., 2013] while deeper lithospheric structure has not known so far. Revealing the lithospheric structure of the Yamato Basin will lead to better understanding of the formation process of the Japan Sea and thus the Japanese island. In this study, as a first step toward understanding the lithospheric structure, we aim to detect the lithosphere-asthenosphere boundary (LAB) using receiver functions (RFs). We use teleseismic P waveforms recorded by broad-band ocean-bottom seismometers (BBOBS) deployed at the Yamato Basin. We calculated radial-component RFs using the data with the removal of water reverberations from the vertical-component records [Akuhara et al., 2016]. The resultant RFs are more complicated than those calculated at an on-land station, most likely due to sediment-related reverberations. This complexity does not allow either direct detection of a Ps conversion from the LAB or forward modeling by a simple structure composed of a handful number of layers. To overcome this difficulty, we conducted trans-dimensional Markov Chain Monte Carlo inversion of RFs, where we do not need to assume the number of layers in advance [e.g., Bodin et al., 2012; Sambridge et al., 2014]. Our preliminary results show abrupt velocity reduction at 70 km depth, far greater depth than the expected LAB depth from the age of the lithosphere ( 20 Ma, although still debated). If this low-velocity jump truly reflects the LAB, the anomalously thickened lithosphere will provide a new constraint on the complex formation history of the Japan Sea. Further study, however, is required to deny the possibility that the obtained velocity jump is an artificial brought by the overfitting of noisy data.

Crustal Structure and Subsidence of the Williston Basin: Evidence from Receiver Function Stacking and Gravity Modeling

NASA Astrophysics Data System (ADS)

Song, J.; Liu, K. H.; Yu, Y.; Mickus, K. L.; Gao, S. S.

2017-12-01

The Williston Basin of the northcentral United States and southern Canada is a typical intracratonic sag basin, with nearly continuous subsidence from the Cambrian to the Jurassic. A number of contrasting models on the subsidence mechanism of this approximately circular basin have been proposed. While in principle 3D variations of crustal thickness, layering, and Poisson's ratio can provide essential constraints on the models, thick layers of Phanerozoic sediment with up to 4.5 km thickness prevented reliable determinations of those crustal properties using active or passive source seismic techniques. Specifically, the strong reverberations of teleseismic P-to-S converted waves (a.k.a. receiver functions or RFs) from the Moho and intracrustal interfaces in the loose sedimentary layer can severely contaminate the RFs. Here we use RFs recorded by about 200 USArray and other stations in the Williston Basin and adjacent areas to obtain spatial distributions of the crustal properties. We have found that virtually all of the RFs recorded by stations in the Basin contain strong reverberations, which are effectively removed using a recently developed deconvolution-based filter (Yu et al., 2015, DOI: 10.1002/2014JB011610). A "double Moho" structure is clearly imaged beneath the Basin. The top interface has a depth of about 40 km beneath the Basin, and shallows gradually toward the east from the depocenter. It joins with the Moho beneath the western margin of the Superior Craton, where the crust is about 30 km thick. The bottom interface has a depth of 55 km beneath the Wyoming Craton, and deepens to about 70 km beneath the depocenter. Based on preliminary results of H-k stacking and gravity modeling, we interpret the layer between the two interfaces as a high density, probably eclogized layer. Continuous eclogitization from the Cambrian to the Jurassic resulted in the previously observed rates of subsidence being nearly linear rather than exponential.

S-wave velocities of the lithosphere-asthenosphere system in the Lesser Antilles from the joint inversion of surface wave dispersion and receiver function analysis

NASA Astrophysics Data System (ADS)

González, O'Leary; Clouard, Valerie; Tait, Stephen; Panza, Giuliano F.

2018-06-01

We present an overview of S-wave velocities (Vs) within the crust and upper mantle of the Lesser Antilles as determined with 19 seismic broadband stations. Receiver functions (RF) have been computed from teleseismic recordings of earthquakes, and Rayleigh wave group velocity dispersion relations have been taken from earlier surface wave tomographic studies in the Caribbean area. Local smoothness optimization (LSO) procedure has been applied, combined with an H-K stacking method, the spatial distribution of hypocenters of local earthquakes and of the energy they released, in order to identify an optimum 1D model of Vs below each station. Several features of the Caribbean plate and its interaction with the Atlantic subducting slab are visible in the resulting models: (a) relatively thick oceanic crust below these stations ranges from 21 km to 33 km, being slight thinner in the middle of the island arc; (b) crustal low velocity zones are present below stations SABA, SEUS, SKI, SMRT, CBE, DSD, GCMP and TDBA; (c) lithospheric thickness range from 40 km to 105 km but lithosphere-asthenosphere boundary was not straightforward to correlate between stations; (d) the aseismic mantle wedge between the Caribbean seismic lithosphere and the subducted slab varies in thickness as well as Vs values which are, in general, lower below the West of Martinique than below the West of Guadeloupe; (e) the depth of the subducted slab beneath the volcanic arc, appears to be greater to the North, and relatively shallower below some stations (e.g. DLPL, SAM, BIM and FDF) than was estimated in previous studies based on the depth-distribution of seismicity; f) the WBZ is >10-15 km deeper than the top of the slab below the Central Lesser Antilles (Martinique and Dominica) where the presence of partial melt in the mantle wedge seems also to be more evident.

Joint inversion of regional and teleseismic earthquake waveforms

NASA Astrophysics Data System (ADS)

Baker, Mark R.; Doser, Diane I.

1988-03-01

A least squares joint inversion technique for regional and teleseismic waveforms is presented. The mean square error between seismograms and synthetics is minimized using true amplitudes. Matching true amplitudes in modeling requires meaningful estimates of modeling uncertainties and of seismogram signal-to-noise ratios. This also permits calculating linearized uncertainties on the solution based on accuracy and resolution. We use a priori estimates of earthquake parameters to stabilize unresolved parameters, and for comparison with a posteriori uncertainties. We verify the technique on synthetic data, and on the 1983 Borah Peak, Idaho (M = 7.3), earthquake. We demonstrate the inversion on the August 1954 Rainbow Mountain, Nevada (M = 6.8), earthquake and find parameters consistent with previous studies.

The September 25, 2003 Tokachi-Oki Mw 8.3 Earthquake: Rupture Process From Joint Inversion of Tsunami Waveform, GPS, and Pressure Gages Data

NASA Astrophysics Data System (ADS)

Romano, F.; Lorito, S.; Piatanesi, A.; Antonioli, A.; George, D. L.; Hirata, K.

2008-12-01

We infer the slip distribution along the rupture zone of the September 25, 2003 Hokkaido Region (Japan) from tide-gages records of the tsunami, pressure gages, and GPS measured static coseismic displacements. According to USGS, this one has been the largest earthquake in 2003. We select waveforms from 16 stations, distributed along the east coast of the Hokkaido Region and the north-east coast of the Tohoku Region. Furthermore we select more than 100 GPS stations positioned on these regions and 2 high-precision pressure gages positioned in open sea near the epicenter; indeed the seafloor measurement of the water pressure is an innovative geodetic observation because the displacement of the seafloor is directly proportional to water pressure increase. We assume the fault plane to be consistent with the geometry of the subducting plate and the slip direction with the focal mechanism solutions and previous inversions of teleseismic body waves. We subdivide the fault plane into several subfaults (both along strike and down dip) and we compute the corresponding Green's function for the coseismic displacement considering a 3D Earth's model implemented in a Finite-Element code. As for the tsunami Green's function we use the shallow water equations and a bathymetric dataset with 10 arcsec of spatial resolution. The slip distribution is determined by means of a simulated annealing technique. Synthetic checkerboard tests, using the station coverage of the available data, indicate that the main features of the rupture process may be robustly inverted with a minimum subfault area of 30x30 km. We compare our results with those obtained by previous inversions of teleseismic, GPS and tsunami data.

Body wave tomography of Iranian Plateau

NASA Astrophysics Data System (ADS)

Alinaghi, A.; Koulakov, I.; Thybo, H.

2004-12-01

The inverse teleseismic tomography approach has been adopted to study the P and S velocity structure of the crust and upper mantle across the Iranian Plateau. The method uses phase readings from earthquakes in a study area as reported by stations at teleseismic and regional distances to compute the velocity anomalies in the area. This use of source-receiver reciprocity allows tomographic studies of regions with sparse distribution of seismic stations, if only the region has sufficient seismicity. The input data for the algorithm are the arrival times of events located in Iran which were taken from the ISC catalogue (1964-1996). All the sources were located anew using a 1D spherical Earth model taking into account variable Moho depth and topography. The inversion provides relocation of events which is done simultaneously with calculation of velocity perturbations. With a series of synthetic tests we demonstrate the power of the algorithm to resolve both fancy and realistic anomalies using available earthquake sources and introducing measurement errors and outliers. The velocity anomalies show that the crust and upper mantle below the Iranian Plateau comprises a low velocity domain between the Arabian Plate and the Caspian Block, in agreement with models of the active Iranian plate trapped between the stable Turan plate in the north and the Arabian shield in the south. Our results show clear evidence of subduction at Makran in the southeastern corner of Iran where the oceanic crust of the Oman Sea subducts underneath the Iranian Plateau, a movement which is mainly aseismic. On the other hand, the subduction and collision of the two plates along the Zagros suture zone is highly seismic and in our images appear less consistent than the Makran region.

Two-receiver measurements of phase velocity: cross-validation of ambient-noise and earthquake-based observations

NASA Astrophysics Data System (ADS)

Kästle, Emanuel D.; Soomro, Riaz; Weemstra, Cornelis; Boschi, Lapo; Meier, Thomas

2016-12-01

Phase velocities derived from ambient-noise cross-correlation are compared with phase velocities calculated from cross-correlations of waveform recordings of teleseismic earthquakes whose epicentres are approximately on the station-station great circle. The comparison is conducted both for Rayleigh and Love waves using over 1000 station pairs in central Europe. We describe in detail our signal-processing method which allows for automated processing of large amounts of data. Ambient-noise data are collected in the 5-80 s period range, whereas teleseismic data are available between about 8 and 250 s, resulting in a broad common period range between 8 and 80 s. At intermediate periods around 30 s and for shorter interstation distances, phase velocities measured from ambient noise are on average between 0.5 per cent and 1.5 per cent lower than those observed via the earthquake-based method. This discrepancy is small compared to typical phase-velocity heterogeneities (10 per cent peak-to-peak or more) observed in this period range.We nevertheless conduct a suite of synthetic tests to evaluate whether known biases in ambient-noise cross-correlation measurements could account for this discrepancy; we specifically evaluate the effects of heterogeneities in source distribution, of azimuthal anisotropy in surface-wave velocity and of the presence of near-field, rather than far-field only, sources of seismic noise. We find that these effects can be quite important comparing individual station pairs. The systematic discrepancy is presumably due to a combination of factors, related to differences in sensitivity of earthquake versus noise data to lateral heterogeneity. The data sets from both methods are used to create some preliminary tomographic maps that are characterized by velocity heterogeneities of similar amplitude and pattern, confirming the overall agreement between the two measurement methods.

Preliminary results of receiver function analysis of seismic data recorded from a broadband deployment across the Gulf Coast Plain

NASA Astrophysics Data System (ADS)

Gurrola, H.; Pratt, K. W.; Pulliam, J.; Dunbar, J. A.

2011-12-01

In summer of 2010, 21 broadband seismographs were installed at 16-18 km spacing along a transect running from Johnson City, TX, (on the Edwards Plateau), to Matagorda Island to study the current structure of this rifted passive margin. The large magnetic anomaly that parallels the coast throughout the Gulf region moves on-shore beneath our transect such that we will be able to investigate the source of this anomaly. A second important target that will be imaged in this Balcones fault which is associated with the Ouachita front. This project is funded by a grant from the Norman Hackerman Advanced Research Program (NHARP), a biannual competition among Texas Universities to support research, and makes use of Texas Tech, Baylor, and UT Austin equipment. As a result, the deployment includes a less uniform array of seismic equipment, (10 Trillium compact seismometers and 10 Guralps; including 40Ts, 3Ts and 3ESPs), than projects supported by the IRIS PASSCAL center. Our vault construction was similar to Flexible array vaults, but Gulf Coast provides a more challenging environment for deployment than most encountered in the western US. The shallow water table and loose sediment can become almost fluid when storms deluge the area with rain. In dry periods, mud cracks near the vaults cause the vaults to tilt. As a result, even high quality, shallow seismic vaults can "float" or shift sufficiently to cause one or two components of the seismic stations to drift against their stops in days or weeks. As a result, the only data consistently available from all our stations, are vertical components. Horizontal component data from the summer of 2010 can be hit and miss due to the tilting of the vaults. These issues have been reduced in the summer of 2011 due to the drought. To address the data's shortcomings, we will average the vertical components from our stations and nearby EarthScope TA stations, (up 300 km away), to isolate the cleanest representation of the incoming P-wave, (with local PPp reverberations averaged out). This is essentially beam forming for the optimal teleseismic ray path. The clean P-wave will then be deconvolved from the vertical components at each station to produce a vertical component receiver function that will enable us to model and stack local P-wave reverberations to produce a 2-D image of lithospheric structure. To produce traditional receiver functions from time periods where one component is lost from several stations, we will treat neighboring stations as arrays and recover an "array averaged three-component seismogram" for each loacation. These "beamed" seismograms will allow imaging of the crust, lithospheric mantle, and transition zone beneath the broadband array using traditional receiver function stacking or migration.

Determination of source process and the tsunami simulation of the 2013 Santa Cruz earthquake

NASA Astrophysics Data System (ADS)

Park, S. C.; Lee, J. W.; Park, E.; Kim, S.

2014-12-01

In order to understand the characteristics of large tsunamigenic earthquakes, we analyzed the earthquake source process of the 2013 Santa Cruz earthquake and simulated the following tsunami. We first estimated the fault length of about 200 km using 3-day aftershock distribution and the source duration of about 110 seconds using the duration of high-frequency energy radiation (Hara, 2007). Moment magnitude was estimated to be 8.0 using the formula of Hara (2007). From the results of 200 km of fault length and 110 seconds of source duration, we used the initial value of rupture velocity as 1.8 km/s for teleseismic waveform inversions. Teleseismic body wave inversion was carried out using the inversion package by Kikuchi and Kanamori (1991). Teleseismic P waveform data from 14 stations were used and band-pass filter of 0.005 ~ 1 Hz was applied. Our best-fit solution indicated that the earthquake occurred on the northwesterly striking (strike = 305) and shallowly dipping (dip = 13) fault plane. Focal depth was determined to be 23 km indicating shallow event. Moment magnitude of 7.8 was obtained showing somewhat smaller than the result obtained above and that of previous study (Lay et al., 2013). Large slip area was seen around the hypocenter. Using the slip distribution obtained by teleseismic waveform inversion, we calculated the surface deformations using formulas of Okada (1985) assuming as the initial change of sea water by tsunami. Then tsunami simulation was carred out using Conell Multi-grid Coupled Tsunami Model (COMCOT) code and 1 min-grid topographic data for water depth from the General Bathymetric Chart of the Ocenas (GEBCO). According to the tsunami simulation, most of tsunami waves propagated to the directions of southwest and northeast which are perpendicular to the fault strike. DART buoy data were used to verify our simulation. In the presentation, we will discuss more details on the results of source process and tsunami simulation and compare them with the previous study.

Dynamic Triggering of Seismic Events and Their Relation to Slow Slip in Interior Alaska

NASA Astrophysics Data System (ADS)

Sims, N. E.; Holtkamp, S. G.

2017-12-01

We conduct a search for dynamically triggered events in the Minto Flats Fault Zone (MFFZ), a left-lateral strike-slip zone expressed as multiple, partially overlapping faults, in central Alaska. We focus on the MFFZ because we have observed slow slip processes (earthquake swarms and Very Low Frequency Earthquakes) and interaction between earthquake swarms and larger main-shock (MS) events in this area before. We utilize the Alaska Earthquake Center catalog to identify potential earthquake swarms and dynamically triggered foreshock and mainshock events along the fault zone. We find 30 swarms occurring in the last two decades, five of which we classify as foreshock (FS) swarms due to their close proximity in both time and space to MS events. Many of the earthquake swarms cluster around 15-20 km depth, which is near the seismic-aseismic transition along this fault zone. Additionally, we observe instances of large teleseismic events such as the M8.6 2012 Sumatra earthquake and M7.4 2012 Guatemala earthquake triggering seismic events within the MFFZ, with the Sumatra earthquake triggering a mainshock event that was preceded by an ongoing earthquake swarm and the Guatemala event triggering earthquake swarms that subsequently transition into a larger mainshock event. In both cases an earthquake swarm transitioned into a mainshock-aftershock event and activity continued for several days after the teleseismic waves had passed, lending some evidence to delayed dynamic triggering of seismic events. We hypothesize that large dynamic transient strain associated with the passage of teleseismic surface waves is triggering slow slip processes near the base of the seismogenic zone. These triggered aseismic transient events result in earthquake swarms, which sometimes lead to the nucleation of larger earthquakes. We utilize network matched filtering to build more robust catalogs of swarm earthquake families in this region to search for additional swarm-like or triggered activity in response to teleseismic surface waves, and to test dynamic triggering hypotheses.

Orientations and Relative Shear-strain Response Coefficients for PBO Gladwin Tensor Strainmeters from Teleseismic Love Waves

NASA Astrophysics Data System (ADS)

Roeloffs, E. A.

2016-12-01

A Gladwin Tensor Strainmeter (GTSM) is designed to measure changes of the horizontal strain tensor, derived as linear combinations of radial elongations or contractions of the strainmeter's cylindrical housing measured at four azimuths. Each radial measurement responds to changes in the areal, horizontal shear and vertical components of the strain tensor in the surrounding formation. The elastic response coefficients to these components depend on the relative elastic moduli of the housing, formation, and cement. These coefficients must be inferred for each strainmeter after it is cemented into its borehole by analyzing the instrument response to well-characterized strain signals such as earth tides. For some GTSMs of the Earthscope Plate Boundary Observatory (PBO), however, reconciling observed earth-tide signals with modeled tidal strains requires response coefficients that differ substantially between the instrument's four gauges, and/or orientation corrections of tens of degrees. GTSM response coefficients can also be estimated from high-resolution records of teleseismic Love waves from great earthquakes around the world. Such records can be used in conjunction with apparent propagation azimuths from nearby broadband seismic stations to determine the GTSM's orientation. Knowing the orientation allows the ratios between the shear strain response coefficients of a GTSM's four gauges to be estimated. Applying this analysis to 14 PBO GTSMs confirms that orientations of some instruments differ significantly from orientations measured during installation. Orientations inferred from earth-tide response tend to agree with those inferred from Love waves for GTSMs far from tidal water bodies, but to differ for GTSMs closer to coastlines. Orientations derived from teleseismic Love waves agree with those estimated by Grant and Langston (2010) using strains from a broadband seismic array near Anza, California. PBO GTSM recordings of teleseismic Love waves show differences of more than 20% among the shear-strain response coefficients of the four gauges. Love-wave derived orientations and relative shear-strain response coefficients can reduce uncertainties in shear strains derived from PBO GTSM data.

Imaging the North Anatolian Fault using the scattered teleseismic wavefield

NASA Astrophysics Data System (ADS)

Thompson, D. A.; Rost, S.; Houseman, G. A.; Cornwell, D. G.; Turkelli, N.; Teoman, U.; Kahraman, M.; Altuncu Poyraz, S.; Gülen, L.; Utkucu, M.; Frederiksen, A. W.; Rondenay, S.

2013-12-01

The North Anatolian Fault Zone (NAFZ) is a major continental strike-slip fault system, similar in size and scale to the San Andreas system, that extends ˜1200 km across Turkey. In 2012, a new multidisciplinary project (FaultLab) was instigated to better understand deformation throughout the entire crust in the NAFZ, in particular the expected transition from narrow zones of brittle deformation in the upper crust to possibly broader shear zones in the lower crust/upper mantle and how these features contribute to the earthquake loading cycle. This contribution will discuss the first results from the seismic component of the project, a 73 station network encompassing the northern and southern branches of the NAFZ in the Sakarya region. The Dense Array for North Anatolia (DANA) is arranged as a 6×11 grid with a nominal station spacing of 7 km, with a further 7 stations located outside of the main grid. With the excellent resolution afforded by the DANA network, we will present images of crustal structure using the technique of teleseismic scattering tomography. The method uses a full waveform inversion of the teleseismic scattered wavefield coupled with array processing techniques to infer the properties and location of small-scale heterogeneities (with scales on the order of the seismic wavelength) within the crust. We will also present preliminary results of teleseismic scattering migration, another powerful method that benefits from the dense data coverage of the deployed seismic network. Images obtained using these methods together with other conventional imaging techniques will provide evidence for how the deformation is distributed within the fault zone at depth, providing constraints that can be used in conjunction with structural analyses of exhumed fault segments and models of geodetic strain-rate across the fault system. By linking together results from the complementary techniques being employed in the FaultLab project, we aim to produce a comprehensive picture of fault structure and dynamics throughout the crust and shallow upper mantle of this major active fault zone.

Genetic algorithms and their use in Geophysical Problems

DOE Office of Scientific and Technical Information (OSTI.GOV)

Parker, Paul B.

1999-04-01

Genetic algorithms (GAs), global optimization methods that mimic Darwinian evolution are well suited to the nonlinear inverse problems of geophysics. A standard genetic algorithm selects the best or ''fittest'' models from a ''population'' and then applies operators such as crossover and mutation in order to combine the most successful characteristics of each model and produce fitter models. More sophisticated operators have been developed, but the standard GA usually provides a robust and efficient search. Although the choice of parameter settings such as crossover and mutation rate may depend largely on the type of problem being solved, numerous results show thatmore » certain parameter settings produce optimal performance for a wide range of problems and difficulties. In particular, a low (about half of the inverse of the population size) mutation rate is crucial for optimal results, but the choice of crossover method and rate do not seem to affect performance appreciably. Optimal efficiency is usually achieved with smaller (< 50) populations. Lastly, tournament selection appears to be the best choice of selection methods due to its simplicity and its autoscaling properties. However, if a proportional selection method is used such as roulette wheel selection, fitness scaling is a necessity, and a high scaling factor (> 2.0) should be used for the best performance. Three case studies are presented in which genetic algorithms are used to invert for crustal parameters. The first is an inversion for basement depth at Yucca mountain using gravity data, the second an inversion for velocity structure in the crust of the south island of New Zealand using receiver functions derived from teleseismic events, and the third is a similar receiver function inversion for crustal velocities beneath the Mendocino Triple Junction region of Northern California. The inversions demonstrate that genetic algorithms are effective in solving problems with reasonably large numbers of free parameters and with computationally expensive objective function calculations. More sophisticated techniques are presented for special problems. Niching and island model algorithms are introduced as methods to find multiple, distinct solutions to the nonunique problems that are typically seen in geophysics. Finally, hybrid algorithms are investigated as a way to improve the efficiency of the standard genetic algorithm.« less

A semi-automated method for the detection of seismic anisotropy at depth via receiver function analysis

NASA Astrophysics Data System (ADS)

Licciardi, A.; Piana Agostinetti, N.

2016-06-01

Information about seismic anisotropy is embedded in the variation of the amplitude of the Ps pulses as a function of the azimuth, on both the Radial and the Transverse components of teleseismic receiver functions (RF). We develop a semi-automatic method to constrain the presence and the depth of anisotropic layers beneath a single seismic broad-band station. An algorithm is specifically designed to avoid trial and error methods and subjective crustal parametrizations in RF inversions, providing a suitable tool for large-size data set analysis. The algorithm couples together information extracted from a 1-D VS profile and from a harmonic decomposition analysis of the RF data set. This information is used to determine the number of anisotropic layers and their approximate position at depth, which, in turn, can be used to, for example, narrow the search boundaries for layer thickness and S-wave velocity in a subsequent parameter space search. Here, the output of the algorithm is used to invert an RF data set by means of the Neighbourhood Algorithm (NA). To test our methodology, we apply the algorithm to both synthetic and observed data. We make use of synthetic RF with correlated Gaussian noise to investigate the resolution power for multiple and thin (1-3 km) anisotropic layers in the crust. The algorithm successfully identifies the number and position of anisotropic layers at depth prior the NA inversion step. In the NA inversion, strength of anisotropy and orientation of the symmetry axis are correctly retrieved. Then, the method is applied to field measurement from station BUDO in the Tibetan Plateau. Two consecutive layers of anisotropy are automatically identified with our method in the first 25-30 km of the crust. The data are then inverted with the retrieved parametrization. The direction of the anisotropic axis in the uppermost layer correlates well with the orientation of the major planar structure in the area. The deeper anisotropic layer is associated with an older phase of crustal deformation. Our results are compared with previous anisotropic RF studies at the same station, showing strong similarities.

Genetic algorithms and their use in geophysical problems

NASA Astrophysics Data System (ADS)

Parker, Paul Bradley

Genetic algorithms (GAs), global optimization methods that mimic Darwinian evolution are well suited to the nonlinear inverse problems of geophysics. A standard genetic algorithm selects the best or "fittest" models from a "population" and then applies operators such as crossover and mutation in order to combine the most successful characteristics of each model and produce fitter models. More sophisticated operators have been developed, but the standard GA usually provides a robust and efficient search. Although the choice of parameter settings such as crossover and mutation rate may depend largely on the type of problem being solved, numerous results show that certain parameter settings produce optimal performance for a wide range of problems and difficulties. In particular, a low (about half of the inverse of the population size) mutation rate is crucial for optimal results, but the choice of crossover method and rate do not seem to affect performance appreciably. Also, optimal efficiency is usually achieved with smaller (<50) populations. Lastly, tournament selection appears to be the best choice of selection methods due to its simplicity and its autoscaling properties. However, if a proportional selection method is used such as roulette wheel selection, fitness scaling is a necessity, and a high scaling factor (>2.0) should be used for the best performance. Three case studies are presented in which genetic algorithms are used to invert for crustal parameters. The first is an inversion for basement depth at Yucca mountain using gravity data, the second an inversion for velocity structure in the crust of the south island of New Zealand using receiver functions derived from teleseismic events, and the third is a similar receiver function inversion for crustal velocities beneath the Mendocino Triple Junction region of Northern California. The inversions demonstrate that genetic algorithms are effective in solving problems with reasonably large numbers of free parameters and with computationally expensive objective function calculations. More sophisticated techniques are presented for special problems. Niching and island model algorithms are introduced as methods to find multiple, distinct solutions to the nonunique problems that are typically seen in geophysics. Finally, hybrid algorithms are investigated as a way to improve the efficiency of the standard genetic algorithm.

Expression of Lithospheric Shear Zones in Rock Elasticity Tensors and in Anisotropic Receiver Functions and Inferences on the Roots of Faults and Lower Crustal Deformation

NASA Astrophysics Data System (ADS)

Schulte-Pelkum, V.; Condit, C.; Brownlee, S. J.; Mahan, K. H.; Raju, A.

2016-12-01

We investigate shear zone-related deformation fabric from field samples, its dependence on conditions during fabric formation, and its detection in situ using seismic data. We present a compilation of published rock elasticity tensors measured in the lab or calculated from middle and deep crustal samples and compare the strength and symmetry of seismic anisotropy as a function of location within a shear zone, pressure-temperature conditions during formation, and composition. Common strengths of seismic anisotropy range from a few to 10 percent. Apart from the typically considered fabric in mica, amphibole and quartz also display fabrics that induce seismic anisotropy, although the interaction between different minerals can result in destructive interference in the total measured anisotropy. The availability of full elasticity tensors enables us to predict the seismic signal from rock fabric at depth. A method particularly sensitive to anisotropy of a few percent in localized zones of strain at depth is the analysis of azimuthally dependent amplitude and polarity variations in teleseismic receiver functions. We present seismic results from California and Colorado. In California, strikes of seismically detected fabric show a strong alignment with current strike-slip motion between the Pacific and North American plates, with high signal strength near faults and from depths below the brittle-ductile transition. These results suggest that the faults have roots in the ductile crust; determining the degree of localization, i.e., the width of the fault-associated shear zones, would require an analysis with denser station coverage, which now exists in some areas. In Colorado, strikes of seismically detected fabric show a broad NW-SE to NNW-SSE alignment that may be related to Proterozoic fabric developed at high temperatures, but locally may also show isotropic dipping contrasts associated with Laramide faulting. The broad trend is punctuated with NE-SW-trending strikes parallel to exhumed and highly localized structures such as the Idaho Springs-Ralston and Black Canyon shear zones. In either case, denser seismic studies should elucidate the width of the deep seismic expression of the shear zones.

Teleseismic and regional data analysis for estimating depth, mechanism and rupture process of the 3 April 2017 MW 6.5 Botswana earthquake and its aftershock (5 April 2017, MW 4.5)

NASA Astrophysics Data System (ADS)

Letort, J.; Guilhem Trilla, A.; Ford, S. R.; Sèbe, O.; Causse, M.; Cotton, F.; Campillo, M.; Letort, G.

2017-12-01

We constrain the source, depth, and rupture process of the Botswana earthquake of April 3, 2017, as well as its largest aftershock (5 April 2017, Mw 4.5). This earthquake is the largest recorded event (Mw 6.5) in the East African rift system since 1970, making one important case study to better understand source processes in stable continental regions. For the two events an automatic cepstrum analysis (Letort et al., 2015) is first applied on respectively 215 and 219 teleseismic records, in order to detect depth phase arrivals (pP, sP) in the P-coda. Coherent detections of depth phases for different azimuths allow us to estimate the hypocentral depths respectively at 28 and 23 km, suggesting that the events are located in the lower crust. A same cepstrum analysis is conducted on five other earthquakes with mb>4 in this area (from 2002 to 2017), and confirms a deep crustal seismicity cluster (around 20-30 km). The source mechanisms are then characterized using a joint inversion method by fitting both regional long-period surface-waves and teleseismic high-frequency body-waves. Combining regional and teleseismic data (as well as systematic comparisons between theoretical and observed regional surface-waves dispersion curves prior to the inversion) allows us to decrease epistemic uncertainties due to lack of regional data and poor knowledge about the local velocity structure. Focal mechanisms are both constrained as normal faulting with a northwest trending, and hypocentral depths are confirmed at 28 and 24 km. Finally, in order to study the mainshock rupture process, we originally apply a kymograph analysis method (an image processing method, commonly used in the field of cell biology for identifying motions of molecular motors, e.g. Mangeol et al., 2016). Here, the kymograph allows us to better identify high-frequency teleseismic P-arrivals inside the P-coda by tracking both reflected depth phase and direct P-wave arrivals radiated from secondary sources during the faulting process. Secondary P-arrivals are thus identified with a significant azimuthal variation of their arrival times (until 4s), allowing the localization of the source that generated these secondary waves. This analysis shows that the mainshock is probably a mix of at least two events, the second being 20-30 km further northwest along the fault.

Quantification of Explosion Source Characteristics from Near Source, Regional and Teleseismic Distances

DTIC Science & Technology

1989-07-31

may be expressed as a combination of geometrical spreading and anelastic attenuation as, A = e -a I rn where: A = attenuation as a function of...of the source parameter estimates. Similar consistent results are obtained for the vertical component records. 26 . E .- .0 ~ Ub C>2 a) o 0~ M- PC oa -0...U - V9suu) apl)dr F.p LO. ______ ___2 - a. 4) 440 44~ 0 A* 0 4-4 0 uu 14 E 0. 0 w a- .6aa w ad 0 IV E E 28 UCZ 40 F.) o .*L 101 V- z Z. Ica 29 0 Long

Using a Genetic Algorithm to Model Broadband Regional Waveforms for Crustal Structure in the Western United States

NASA Technical Reports Server (NTRS)

Bhattacharyya, Joydeep; Sheehan, Anne F.; Tiampo, Kristy; Rundle, John

1999-01-01

In this study, we analyze regional seismograms to obtain the crustal structure in the eastern Great Basin and western Colorado plateau. Adopting a for- ward-modeling approach, we develop a genetic algorithm (GA) based parameter search technique to constrain the one-dimensional crustal structure in these regions. The data are broadband three-component seismograms recorded at the 1994-95 IRIS PASSCAL Colorado Plateau to Great Basin experiment (CPGB) stations and supplemented by data from U.S. National Seismic Network (USNSN) stations in Utah and Nevada. We use the southwestern Wyoming mine collapse event (M(sub b) = 5.2) that occurred on 3 February 1995 as the seismic source. We model the regional seismograms using a four-layer crustal model with constant layer parameters. Timing of teleseismic receiver functions at CPGB stations are added as an additional constraint in the modeling. GA allows us to efficiently search the model space. A carefully chosen fitness function and a windowing scheme are added to the algorithm to prevent search stagnation. The technique is tested with synthetic data, both with and without random Gaussian noise added to it. Several separate model searches are carried out to estimate the variability of the model parameters. The average Colorado plateau crustal structure is characterized by a 40-km-thick crust with velocity increases at depths of about 10 and 25 km and a fast lower crust while the Great Basin has approximately 35- km-thick crust and a 2.9-km-thick sedimentary layer.

Mapping the influence of the deep Nazca slab on the geometry of the 660-km discontinuity beneath stable South America

NASA Astrophysics Data System (ADS)

Bianchi, M. B. D.; Assumpcao, M.; Julià, J.

2017-12-01

The fate of the deep Nazca subducted plate is poorly mapped under stable South America. Transition zone thickness and position is greatly dependent on mantle temperature and so is influenced by the colder Nazca plate position. We use a database of 35,000 LQT deconvolved receiver function traces to image the mantle transition zone and other upper mantle discontinuities under different terranes of stable South American continent. Data from the entire Brazilian Seismographic Network database, consisting of more than 80 broadband stations supplemented by 35 temporary stations deployed in west Brazil, Argentina, Paraguay, Bolivia and Uruguay were processed. Our results indicates that upper mantle velocities are faster than average under stable cratons and that most of the discontinuities are positioned with small variations in respect to nominal depths, except in places were the Nazca plate interacts with the transition zone. Under the Chaco-Pantanal basin the Nazca plate appears to be trapped in the transition zone for more than 1000 km with variations of up to 30 km in 660 km discontinuity topography under this region consistent with global tomographic models. Additional results obtained from SS precursor analysis of South Sandwich Islands teleseismic events recorded at USArray stations indicates that variations of transition zones thickness occur where the Nazca plate interacts with the upper mantle discontinuities in the northern part of Stable South American continent.

The North Tanzania Rift seen from multi geophysical tools: link between seismicity and resistivity

NASA Astrophysics Data System (ADS)

Gautier, S.; Plasman, M.; Tarits, P.; Hautot, S.; Tiberi, C.; Albaric, J.; Le Gall, B.; Deverchere, J.; Ebinger, C. J.; Roecker, S. W.; Ferdinand, R.; Muzuka, A.; Msabi, M.; Khalfan, M.; Gama, R.; Mulibo, G. D.

2016-12-01

The North Tanzania part of the East African Rift is the place of an incipient break up of the lithosphere. In this region, seismicity and volcanism seem strongly linked to the inherited structures, magmatic intrusion, and tectonic. Natron Lake is characterized by a shallow seismicity and present volcanic activity, whereas Manyara area is the location of a deeper seismicity and sparse volcanism. It is thus of prime interest to image the structure of this area to fully understand the role of each factor on the localisation of the current deformation at the surface. Since 2007 different multidisciplinary projects have taken place in this area to address this question. We present here a work based on a collaborative work between French, American and Tanzanian institutes that started in 2013. We have analysed more than a hundred teleseismic events and local seismicity to compute receiver function and local tomography. We combine this information with two MT profiles in order to image crustal and upper mantle structures. The resistivity deduced from the MT observations confirms the seismic results with a great difference within the crust and upper mantle between Natron and Manyara. The MT profiles evidence crustal structures such as major volcanic edifices, main tectonic units and interfaces. We discuss our combined images in terms of rift-craton interaction and magmatic intrusions.

Crustal structure in Tengchong Volcano-Geothermal Area, western Yunnan, China

NASA Astrophysics Data System (ADS)

Wang, Chun-Yong; Huangfu, Gang

2004-02-01

Based upon the deep seismic sounding profiles carried out in the Tengchong Volcano-Geothermal Area (TVGA), western Yunnan Province of China, a 2-D crustal P velocity structure is obtained by use of finite-difference inversion and forward travel-time fitting method. The crustal model shows that a low-velocity anomaly zone exists in the upper crust, which is related to geothermal activity. Two faults, the Longling-Ruili Fault and Tengchong Fault, on the profile extend from surface to the lower crust and the Tengchong Fault likely penetrates the Moho. Moreover, based on teleseismic receiver functions on a temporary seismic network, S-wave velocity structures beneath the geothermal field show low S-wave velocity in the upper crust. From results of geophysical survey, the crust of TVGA is characterized by low P-wave and S-wave velocities, low resistivity, high heat-flow value and low Q. The upper mantle P-wave velocity is also low. This suggests presence of magma in the crust derived from the upper mantle. The low-velocity anomaly in upper crust may be related to the magma differentiation. The Tengchong volcanic area is located on the northeast edge of the Indian-Eurasian plate collision zone, away from the eastern boundary of the Indian plate by about 450 km. Based on the results of this paper and related studies, the Tengchong volcanoes can be classified as plate boundary volcanoes.

Lithosphere-asthenosphere interaction beneath Ireland from joint inversion of teleseismic P-wave delay times and GRACE gravity

NASA Astrophysics Data System (ADS)

O'Donnell, J. P.; Daly, E.; Tiberi, C.; Bastow, I. D.; O'Reilly, B. M.; Readman, P. W.; Hauser, F.

2011-03-01

The nature and extent of the regional lithosphere-asthenosphere interaction beneath Ireland and Britain remains unclear. Although it has been established that ancient Caledonian signatures pervade the lithosphere, tertiary structure related to the Iceland plume has been inferred to dominate the asthenosphere. To address this apparent contradiction in the literature, we image the 3-D lithospheric and deeper upper-mantle structure beneath Ireland via non-linear, iterative joint teleseismic-gravity inversion using data from the ISLE (Irish Seismic Lithospheric Experiment), ISUME (Irish Seismic Upper Mantle Experiment) and GRACE (Gravity Recovery and Climate Experiment) experiments. The inversion combines teleseismic relative arrival time residuals with the GRACE long wavelength satellite derived gravity anomaly by assuming a depth-dependent quasilinear velocity-density relationship. We argue that anomalies imaged at lithospheric depths probably reflect compositional contrasts, either due to terrane accretion associated with Iapetus Ocean closure, frozen decompressional melt that was generated by plate stretching during the opening of the north Atlantic Ocean, frozen Iceland plume related magmatic intrusions, or a combination thereof. The continuation of the anomalous structure across the lithosphere-asthenosphere boundary is interpreted as possibly reflecting sub-lithospheric small-scale convection initiated by the lithospheric compositional contrasts. Our hypothesis thus reconciles the disparity which exists between lithospheric and asthenospheric structure beneath this region of the north Atlantic rifted margin.

Finite‐fault Bayesian inversion of teleseismic body waves

USGS Publications Warehouse

Clayton, Brandon; Hartzell, Stephen; Moschetti, Morgan P.; Minson, Sarah E.

2017-01-01

Inverting geophysical data has provided fundamental information about the behavior of earthquake rupture. However, inferring kinematic source model parameters for finite‐fault ruptures is an intrinsically underdetermined problem (the problem of nonuniqueness), because we are restricted to finite noisy observations. Although many studies use least‐squares techniques to make the finite‐fault problem tractable, these methods generally lack the ability to apply non‐Gaussian error analysis and the imposition of nonlinear constraints. However, the Bayesian approach can be employed to find a Gaussian or non‐Gaussian distribution of all probable model parameters, while utilizing nonlinear constraints. We present case studies to quantify the resolving power and associated uncertainties using only teleseismic body waves in a Bayesian framework to infer the slip history for a synthetic case and two earthquakes: the 2011 Mw 7.1 Van, east Turkey, earthquake and the 2010 Mw 7.2 El Mayor–Cucapah, Baja California, earthquake. In implementing the Bayesian method, we further present two distinct solutions to investigate the uncertainties by performing the inversion with and without velocity structure perturbations. We find that the posterior ensemble becomes broader when including velocity structure variability and introduces a spatial smearing of slip. Using the Bayesian framework solely on teleseismic body waves, we find rake is poorly constrained by the observations and rise time is poorly resolved when slip amplitude is low.

Mantle transition zone, stagnant slab and intraplate volcanism in Northeast Asia

NASA Astrophysics Data System (ADS)

Chen, Chuanxu; Zhao, Dapeng; Tian, You; Wu, Shiguo; Hasegawa, Akira; Lei, Jianshe; Park, Jung-Ho; Kang, Ik-Bum

2017-04-01

3-D P- and S-wave velocity structures of the mantle down to a depth of 800 km beneath NE Asia are investigated using ∼981 000 high-quality arrival-time data of local earthquakes and teleseismic events recorded at 2388 stations of permanent and portable seismic networks deployed in NE China, Japan and South Korea. Our results do not support the existence of a gap (or a hole) in the stagnant slab under the Changbai volcano, which was proposed by a previous study of teleseismic tomography. In this work we conducted joint inversions of both local-earthquake arrival times and teleseismic relative traveltime residuals, leading to a robust tomography of the upper mantle and the mantle transition zone (MTZ) beneath NE Asia. Our joint inversion results reveal clearly the subducting Pacific slab beneath the Japan Islands and the Japan Sea, as well as the stagnant slab in the MTZ beneath the Korean Peninsula and NE China. A big mantle wedge (BMW) has formed in the upper mantle and the upper part of the MTZ above the stagnant slab. Localized low-velocity anomalies are revealed clearly in the crust and the BMW directly beneath the active Changbai and Ulleung volcanoes, indicating that the intraplate volcanism is caused by hot and wet upwelling in the BMW associated with corner flows in the BMW and deep slab dehydration as well.

Assessing the short-term clock drift of early broadband stations with burst events of the 26 s persistent and localized microseism

NASA Astrophysics Data System (ADS)

Xie, J.; Ni, S.; Chu, R.; Xia, Y.

2017-12-01

Accurate seismometer clock plays an important role in seismological studies including earthquake location and tomography. However, some seismic stations may have clock drift larger than 1 second, especially in early days of global seismic network. The 26 s Persistent Localized (PL) microseism event in the Gulf of Guinea sometime excites strong and coherent signals, and can be used as repeating source for assessing stability of seismometer clocks. Taking station GSC/TS in southern California, USA as an example, the 26 s PL signal can be easily observed in the ambient Noise Cross-correlation Function (NCF) between GSC/TS and a remote station. The variation of travel-time of this 26 s signal in the NCF is used to infer clock error. A drastic clock error is detected during June, 1992. This short-term clock error is confirmed by both teleseismic and local earthquake records with a magnitude of ±25 s. Using 26 s PL source, the clock can be validated for historical records of sparsely distributed stations, where usual NCF of short period microseism (<20 s) might be less effective due to its attenuation over long interstation distances. However, this method suffers from cycling problem, and should be verified by teleseismic/local P waves. The location change of the 26 s PL source may influence the measured clock drift, using regional stations with stable clock, we estimate the possible location change of the source.

Chacterization of Teleseismic Earthquakes Observed on an Ice Shelf

NASA Astrophysics Data System (ADS)

Baker, M. G.; Aster, R. C.; Anthony, R. E.; Wiens, D.; Nyblade, A.; Bromirski, P. D.; Stephen, R. A.; Gerstoft, P.

2016-12-01

Broadband seismographs deployed atop large tabular icebergs and ice shelves record a rich superposition of atmospheric, oceanic, and solid earth signals. We characterize these signals, including body and surface wave arrivals from approximately 200 global earthquakes, using a 34-station broadband array spanning the Ross Ice Shelf, Antarctica. Teleseismic earthquake arrivals are essential for constructing models of crustal and upper mantle structure, and observations on the ice shelf are key to resolving the structure of the underlying West Antarctic Rift System. To test the plausibility of passive imaging in this unique environment, we examine seasonal and spatial dependence of signal-to-noise ratios of body wave arrivals and the impact of ice shelf dynamics on surface wave dispersion. We also note unusual phase mechanics arising from the floating platform geometry.

Anisotropic feature inferred from receiver functions and S-wave splitting in and around the high strain rate zone, central Japan

NASA Astrophysics Data System (ADS)

Shiomi, K.; Takeda, T.; Sekiguchi, S.

2012-12-01

By the recent dense GPS observation, the high strain rate zone (HSRZ) crossing the central Japan was discovered. In the HSRZ, E-W compressive stress field is observed, and large earthquakes with M>6 are frequently occurred. In this study, we try to reveal depth-dependent anisotropic feature in this region by using teleseismic receiver functions (RFs) and S-wave splitting information. As a target, we select NIED Hi-net stations N.TGWH and N.TSTH, which are located inside and outside of the HSRZ respectively. For RF analysis, we choose M>5.5 teleseismic events from October 2000 to November 2011. Low-pass filters with fc = 1 and 2 Hz are applied to estimate RFs. In the radial RFs, we find clear positive phase arrivals at 4 to 4.5 s in delay time for both stations. Since this time delay corresponds to 35 km-depth velocity discontinuity existence, these phases may be the converted phases generated at the Moho discontinuity. Seeing the back-azimuth paste-ups of the transverse RFs, we can find polarity changes of later phases at 4 to 4.5 s in delay time at the N.TSTH station. This polarity change occurs for direction of N0E (north), N180E (south), and N270E (west). Although we have no data in N90E (east) direction, this feature implies that anisotropic rocks may exist around the Moho. In order to check this feature, we consider 6-layered subsurface model and compare synthetic RFs with the observation. The first three layers are for thick sediments and upper crust including a dipping velocity interface. The fourth, fifth and sixth layer corresponds to the mid crust, lower crust and uppermost mantle, respectively. The best model infers that the mid- and lower-crust beneath the N.TSTH station should have strong anisotropy whose fast axis directs to the N-S, though the fast axis in the uppermost mantle seems to show E-W direction. Moreover, to explain the observation, the symmetric axes in the lower crust and the uppermost mantle should be dipping about 20 degrees. To check anisotropic feature of this station, we also apply S-wave splitting analysis to the local events. In order to avoid contaminations of scattered phases, we select seismic waveforms with incident angle less than 35 degrees. We select good S/N records and apply 2-8 Hz butter-worth type band-pass filter to the waveforms. Then, we estimate the leading S wave polarization direction (LSPD) and delay time of each event. At the N.TSTH station, we can select crustal earthquakes (< 30 km in depth) and the deep earthquakes (> 90 km) which occur along the subducting Pacific slab. For deeper events, LSPD shows two possibilities: ENE-WSW or NNW-SSE. On the other hand, only for shallow events, LSPD indicates NS. This result is consistent with the feature of RFs. We can conclude that the crustal anisotropic feature beneath the station N.TSTH corresponds to the lineament on the ground surface, not to the E-W compressive stress field. The LSPD in the uppermost mantle reflects to the lattice-preferred orientation of anisotropic minerals beneath this station.

Earthquake location in island arcs

USGS Publications Warehouse

Engdahl, E.R.; Dewey, J.W.; Fujita, K.

1982-01-01

A comprehensive data set of selected teleseismic P-wave arrivals and local-network P- and S-wave arrivals from large earthquakes occurring at all depths within a small section of the central Aleutians is used to examine the general problem of earthquake location in island arcs. Reference hypocenters for this special data set are determined for shallow earthquakes from local-network data and for deep earthquakes from combined local and teleseismic data by joint inversion for structure and location. The high-velocity lithospheric slab beneath the central Aleutians may displace hypocenters that are located using spherically symmetric Earth models; the amount of displacement depends on the position of the earthquakes with respect to the slab and on whether local or teleseismic data are used to locate the earthquakes. Hypocenters for trench and intermediate-depth events appear to be minimally biased by the effects of slab structure on rays to teleseismic stations. However, locations of intermediate-depth events based on only local data are systematically displaced southwards, the magnitude of the displacement being proportional to depth. Shallow-focus events along the main thrust zone, although well located using only local-network data, are severely shifted northwards and deeper, with displacements as large as 50 km, by slab effects on teleseismic travel times. Hypocenters determined by a method that utilizes seismic ray tracing through a three-dimensional velocity model of the subduction zone, derived by thermal modeling, are compared to results obtained by the method of joint hypocenter determination (JHD) that formally assumes a laterally homogeneous velocity model over the source region and treats all raypath anomalies as constant station corrections to the travel-time curve. The ray-tracing method has the theoretical advantage that it accounts for variations in travel-time anomalies within a group of events distributed over a sizable region of a dipping, high-velocity lithospheric slab. In application, JHD has the practical advantage that it does not require the specification of a theoretical velocity model for the slab. Considering earthquakes within a 260 km long by 60 km wide section of the Aleutian main thrust zone, our results suggest that the theoretical velocity structure of the slab is presently not sufficiently well known that accurate locations can be obtained independently of locally recorded data. Using a locally recorded earthquake as a calibration event, JHD gave excellent results over the entire section of the main thrust zone here studied, without showing a strong effect that might be attributed to spatially varying source-station anomalies. We also calibrated the ray-tracing method using locally recorded data and obtained results generally similar to those obtained by JHD. ?? 1982.

­Tectonic and geomorphic setting of the Pamir Plateau: Insights from InSAR and teleseismic analysis of the 2015 Lake Saurez and 2016 Muji fault earthquake sequence

NASA Astrophysics Data System (ADS)

Nanjundiah, P.; Barbot, S.; Wei, S.; Tapponnier, P.; Feng, W.; Wang, T.

2017-12-01

The Pamir Plateau lies on the western edge of the India- Eurasia collision zone and has been the sight of complex subduction regime in the past 50 Ma. In our study, we focus on two earthquakes and their aftershocks that occurred between December 2015 and December 2016. The first earthquake (Mw7.2), on 7 December 2015 between the Karakoram and Darwas fault systems, was sinstral strike slip in nature. The earthquake on 25 November (Mw6.6) occurred on the western end of Muji Fault, a dextral strike slip fault with an avg slip rate of 4mm/yr. We aim to better understand the structure, stress and deformation patterns in the northern and central Pamir plateau by analyzing InSAR, teleseismic, and optical data for these events and their aftershocks. We aim to better understand the structure, stress and deformation patterns in the northern and central Pamir plateau by analysing InSAR, teleseismic, and optical data for these events and their aftershocks. We constrain the fault geometry by precisely relocating aftershocks using the double difference technique implemented in HypoDD (Waldhauser & Ellsworth 2000). We used the Green's functions of Okada (1992) to invert for slip on the fault with rectangular dislocation and edgreen to numerically invert for the slip in a layered medium (Wang et al. 2003). The surface rupture of the December 2015 Lake Saurez earthquake shows evidence of multiple segments and step-overs. The combination of data sets used in this study highlights the existence of a seismic gap south of Lake Karakul as well as coupling between the Muji and Darwas-Karakoram fault systems. Mapping of past ruptures shows that the Sarez fault continues along the eastern coast of Lake Karakul almost until the Muji fault. With near field geodetic data in the form of InSAR, we can get a better insight into complex fault structures as well as post seismic slip and strain along the faults and its surroundings. We emphasize the role of smaller faults and their interactions in accommodating the overall strain in the Pamir region and their effect on estimating local seismic hazard.

Ambient Tremor, But No Triggered Tremor at the Northern Costa Rica Subduction Zone

NASA Astrophysics Data System (ADS)

Swiecki, Z.; Schwartz, S. Y.

2010-12-01

Non-volcanic tremor (NVT) has been found to be triggered during the passage of surface waves from various teleseismic events in locations around the world including Cascadia, Southwest Japan, Taiwan, and California. In this study we examine the northern Costa Rica subduction zone for evidence of triggered tremor. The Nicoya Peninsula segment of the northern Costa Rica margin experiences both slow-slip and tremor and is thus a prime candidate for triggered tremor observations. Eleven teleseismic events with magnitudes (Mw) greater than 8 occurring between 2006 and 2010 were examined using data from both broadband and short period sensors deployed on the Nicoya Peninsula, Costa Rica. Waveforms from several large regional events were also considered. The largest teleseismic and regional events (27 February 2010 Chile, Mw 8.8 and 28 May 2009 Honduras, Mw 7.3) induced peak ground velocities (PGV) at the NIcoya stations of ~2 and 6 mm/s, respectively; larger than PGVs in other locations that have triggered tremor. Many of the earthquakes examined occurred during small episodes of background ambient tremor. In spite of this, no triggered tremor was observed during the passage of seismic waves from any event. This is significant because other studies have demonstrated that NVT is not triggered everywhere by all events above some threshold magnitude, indicating that unique conditions are required for its occurrence. The lack of triggered tremor at the Costa Rica margin can help to better quantify the requisite conditions and triggering mechanisms. An inherent difference between the Costa Rica margin and the other subduction zones where triggered tremor exists is its erosional rather than accretionary nature. Its relatively low sediment supply likely results in a drier, lower pore fluid pressure, stronger and less compliant thrust interface that is less receptive to triggering tremor from external stresses generated by teleseismic or strong local earthquakes. Another important factor is Costa Rica’s relatively cool subduction zone structure where temperatures required for the fluid generating basalt/ecloginte reaction are not reached until far below tremor producing depths.

Teleseismic tomography beneath the mid-lower Yangtze region in China

NASA Astrophysics Data System (ADS)

Jiang, G.; Zhang, G.; Lu, Q.; Shi, D.

2010-12-01

We have applied the teleseismic tomography method proposed by Professor Zhao in 1994 to determine a 3-D P-wave velocity structure of the mantle down to 400 km depth beneath the mid-lower Yangtze region, by using 1508 P-wave arrival times collected from 46 teleseismic events recorded by 38 portable seismic stations in this region (Fig. 1). All the seismograms were recorded only for 5 months from November 2009 to March 2010. Most events located to the south and southeast of stations, and the stations almost align in a line, which causes the rays crossing not well and decreases the resolution in the depth, especially beneath the region in the north-west of the stations. All the relative arrival times were obtained by using the multi-channel cross-correlation method with higher precisions than by hand-picking. Our tomographic result along the profile line of OO’ shows that the lithosphere with high velocity anomalies is shorten gradually from the northwest to the southeast direction, and the thickness of lithosphere is about 150~200 km beneath the northwest of mid-lower Yangtze region (Fig. 1). However, the asthenosphere with low velocity anomalies characterized reversely to the lithosphere, which is consistent with other previous results. Therefore, we can propose that the upwelling of mantle material plays a primary role for the thinning of lithosphere beneath the southeast of mid-lower Yangtze region. Acknowledgment: We thank the financial support of SinoProbe by the Ministry of Finance and Ministry of Land and Resources, P. R. China, under Grant sinoprobe-03, the financial support by National Natural Science Foundation of China under Grants 40930418 and 40904021, and the financial supported by “the Fundamental Research Funds for the Central Universities” under Grant 2010ZD09. Fig. 1 (A) Epicentral locations of the 46 teleseismic events (blue squares) used in this study. The triangle denotes the center of seismic stations; (B) Locations of 38 portable seismic stations (blue squares) and the profile line of OO’ (red line); (C) Vertical cross section of P-wave velocity image by this study along the profile line of OO’. Red and blue colors represent slow and fast velocities, respectively.

Box Tomography: first application to the imaging of upper-mantle shear velocity and radial anisotropy structure beneath the North American continent

NASA Astrophysics Data System (ADS)

Clouzet, P.; Masson, Y.; Romanowicz, B.

2018-06-01

The EarthScope Transpotable Array (TA) deployment provides dense array coverage throughout the continental United States and with it, the opportunity for high-resolution 3-D seismic velocity imaging of the stable part of the North American (NA) upper mantle. Building upon our previous long-period waveform tomographic modeling, we present a higher resolution 3-D isotropic and radially anisotropic shear wave velocity model of the NA lithosphere and asthenosphere. The model is constructed using a combination of teleseismic and regional waveforms down to 40 s period and wavefield computations are performed using the spectral element method both for regional and teleseismic data. Our study is the first tomographic application of `Box Tomography', which allows us to include teleseismic events in our inversion, while computing the teleseismic wavefield only once, thus significantly reducing the numerical computational cost of several iterations of the regional inversion. We confirm the presence of high-velocity roots beneath the Archean part of the continent, reaching 200-250 km in some areas, however the thickness of these roots is not everywhere correlated to the crustal age of the corresponding cratonic province. In particular, the lithosphere is thick (˜250 km) in the western part of the Superior craton, while it is much thinner (˜150 km) in its eastern part. This may be related to a thermomechanical erosion of the cratonic root due to the passage of the NA plate over the Great Meteor hotspot during the opening of the Atlantic ocean 200-110 Ma. Below the lithosphere, an upper-mantle low-velocity zone (LVZ) is present everywhere under the NA continent, even under the thickest parts of the craton, although it is less developed there. The depth of the minimum in shear velocity has strong lateral variations, whereas the bottom of the LVZ is everywhere relatively flat around 270-300 km depth, with minor undulations of maximum 30 km that show upwarping under the thickest lithosphere and downwarping under tectonic regions, likely reflecting residual temperature anomalies. The radial anisotropy structure is less well resolved, but shows distinct signatures in highly deformed regions of the lithosphere.

Seismic velocity structure of the forearc in northern Cascadia from Bayesian inversion of teleseismic data

NASA Astrophysics Data System (ADS)

Gosselin, J.; Audet, P.; Schaeffer, A. J.

2017-12-01

The seismic velocity structure in the forearc of subduction zones provides important constraints on material properties, with implications for seismogenesis. In Cascadia, previous studies have imaged a downgoing low-velocity zone (LVZ) characterized by an elevated P-to-S velocity ratio (Vp/Vs) down to 45 km depth, near the intersection with the mantle wedge corner, beyond which the signature of the LVZ disappears. These results, combined with the absence of a "normal" continental Moho, indicate that the down-going oceanic crust likely carries large amounts of overpressured free fluids that are released downdip at the onset of crustal eclogitization, and are further stored in the mantle wedge as serpentinite. These overpressured free fluids affect the stability of the plate interface and facilitate slow slip. These results are based on the inversion and migration of scattered teleseismic data for individual layer properties; a methodology which suffers from regularization and smoothing, non-uniqueness, and does not consider model uncertainty. This study instead applies trans-dimensional Bayesian inversion of teleseismic data collected in the forearc of northern Cascadia (the CAFÉ experiment in northern Washington) to provide rigorous, quantitative estimates of local velocity structure, and associated uncertainties (particularly Vp/Vs structure and depth to the plate interface). Trans-dimensional inversion is a generalization of fixed-dimensional inversion that includes the number (and type) of parameters required to describe the velocity model (or data error model) as unknown in the problem. This allows model complexity to be inherently determined by data information content, not by subjective regularization. The inversion is implemented here using the reversible-jump Markov chain Monte Carlo algorithm. The result is an ensemble set of candidate velocity-structure models which approximate the posterior probability density (PPD) of the model parameters. The solution to the inverse problem, and associated uncertainties, are described by properties of the PPD. The results obtained here will eventually be integrated with teleseismic data from OBS stations from the Cascadia Initiative to provide constraints across the entire seismogenic portion of the plate interface.

Source mechanism of the 2006 M5.1 Wen'an Earthquake determined from a joint inversion of local and teleseismic broadband waveform data

NASA Astrophysics Data System (ADS)

Huang, J.; Ni, S.; Niu, F.; Fu, R.

2007-12-01

On July 4th, 2006, a magnitude 5.1 earthquake occurred at Wen'an, {~}100 km south of Beijing, which was felt at Beijing metropolitan area. To better understand the regional tectonics, we have inverted local and teleseismic broadband waveform data to determine the focal mechanism of this earthquake. We selected waveform data of 9 stations from the recently installed Beijing metropolitan digital Seismic Network (BSN). These stations are located within 600 km and cover a good azimuthal range to the earthquake. To better fit the lower amplitude P waveform, we employed two different weights for the P wave and surface wave arrivals, respectively. A grid search method was employed to find the strike, dip and slip of the earthquake that best fits the P and surface waveforms recorded at all the three components (the tangential component of the P-wave arrivals was not used). Synthetic waveforms were computed with an F-K method. Two crustal velocity models were used in the synthetic calculation to reflect a rapid east-west transition in crustal structure observed by seismic and geological studies in the study area. The 3D grid search results in reasonable constraints on the fault geometry and the slip vector with a less well determined focal depth. As such we combined teleseismic waveform data from 8 stations of the Global Seismic Network in a joint inversion. Clearly identifiable depth phases (pP, sP) recorded in the teleseismic stations obviously provided a better constraint on the resulting source depth. Results from the joint inversion indicate that the Wen'an earthquake is mainly a right-lateral strike slip event (-150°) which occurred at a near vertical (dip, 80° ) NNE trend (210°º) fault. The estimated focal depth is {~}14- 15km, and the moment magnitude is 5.1. The estimated fault geometry here agrees well with aftershock distribution and is consistent with the major fault systems in the area which were developed under a NNE-SSW oriented compressional stress field. Key word: waveform modeling method, source mechanism, grid search method, cut and paste method, aftershocks distribution

Three-dimensional modeling of the Nevada Test Site and vicinity from teleseismic P-wave residuals

USGS Publications Warehouse

Monfort, Mary E.; Evans, John R.

1982-01-01

A teleseismic P-wave travel-time residual study is described which reveals the regional compressional-velocity structure of southern Nevada and neighboring parts of California to a depth of 280 km. During 1980, 98 teleseismic events were recorded at as many as 53 sites in this area. P-wave residuals were calculated relative to a network-wide average residual for each event and are displayed on maps of the stations for each of four event-azimuth quadrants. Fluctuations in these map-patterns of residuals with approach azimuth combined with results of linear, three-dimensional inversions of some 2887 residuals indicate the following characteristics of the velocity structure of the southern Nevada region: 1) a low-velocity body exists in the upper crust 50 km northeast of Beatty, Nevada, near the Miocene Timber Mountain-Silent Canyon caldera complex. Another highly-localized low-velocity anomaly occurs near the southwest corner of the Nevada Test Site (NTS). These two anomalies seem to be part of a low-velocity trough extending from Death Valley, California, to about 50 km north of NTS. 2) There is a high-velocity body in the mantle between 81 and 131 km deep centered about i0 km north of the edge of the Timber Mountain caldera, 3) a broad low-velocity body is delineated between 81 and 131 km deep centered about 30 km north of Las Vegas, 4) there is a monotonic increase in travel-time delays from west to east across the region, probably indicating an eastward decrease in velocity, and lower than average velocities in southeastern Nevada below 31 km, and 5) considerable complexity in three-dimensional velocity structure exists in this part of the southern Great Basin. Inversions of teleseismic P-wave travel-time residuals were also performed on data from 12 seismometers in the immediate vicinity of the Nevada Test Site to make good use of the closer station spacing i in that area. Results of these inversions show more details of the velocity structure but generally the same features as those found in the regional study.

Large-scale trench-perpendicular mantle flow beneath northern Chile

NASA Astrophysics Data System (ADS)

Reiss, M. C.; Rumpker, G.; Woelbern, I.

2017-12-01

We investigate the anisotropic properties of the forearc region of the central Andean margin by analyzing shear-wave splitting from teleseismic and local earthquakes from the Nazca slab. The data stems from the Integrated Plate boundary Observatory Chile (IPOC) located in northern Chile, covering an approximately 120 km wide coastal strip between 17°-25° S with an average station spacing of 60 km. With partly over ten years of data, this data set is uniquely suited to address the long-standing debate about the mantle flow field at the South American margin and in particular whether the flow field beneath the slab is parallel or perpendicular to the trench. Our measurements yield two distinct anisotropic layers. The teleseismic measurements show a change of fast polarizations directions from North to South along the trench ranging from parallel to subparallel to the absolute plate motion and, given the geometry of absolute plate motion and strike of the trench, mostly perpendicular to the trench. Shear-wave splitting from local earthquakes shows fast polarizations roughly aligned trench-parallel but exhibit short-scale variations which are indicative of a relatively shallow source. Comparisons between fast polarization directions and the strike of the local fault systems yield a good agreement. We use forward modelling to test the influence of the upper layer on the teleseismic measurements. We show that the observed variations of teleseismic measurements along the trench are caused by the anisotropy in the upper layer. Accordingly, the mantle layer is best characterized by an anisotropic fast axes parallel to the absolute plate motion which is roughly trench-perpendicular. This anisotropy is likely caused by a combination of crystallographic preferred orientation of the mantle mineral olivine as fossilized anisotropy in the slab and entrained flow beneath the slab. We interpret the upper anisotropic layer to be confined to the crust of the overriding continental plate. This is explained by the shape-preferred orientation of micro-cracks in relation to local fault zones which are oriented parallel the overall strike of the Andean range. Our results do not provide any evidence for a significant contribution of trench-parallel mantle flow beneath the subducting slab to the measurements.

Lithospheric structure beneath the Caribbean- South American plate boundary from S receiver functions

NASA Astrophysics Data System (ADS)

Masy, J.; Levander, A.; Niu, F.

2010-12-01

We have analyzed teleseismic S-wave data recorded by the permanent national seismic network of Venezuela and the BOLIVAR broadband array (Broadband Onshore-offshore Lithospheric Investigation of Venezuela and the Antilles arc Region) deployed from 2003 to 2005. A total of 28 events with Mw > 5.7 occurring at epicentral distances from 55° to 85° were used. We made Sp receiver functions to estimate the rapid variations of lithospheric structure in the southern Caribbean plate boundary region to try to better understand the complicated tectonic history of the region. Estimated Moho depth ranges from ~20 km beneath the Caribbean Large Igneous Provinces to ~50 km beneath the Mérida Andes in western Venezuela and the Sierra del Interior in northeastern Venezuela. These results are consistent with previous receiver functions studies (Niu et al., 2007) and active source profiles (Schmitz et al., 2001; Bezada et al., 2007; Clark et al., 2008; Guedez, 2008; Magnani et al., 2009). Beneath the Maracaibo Block we observe a signal at a depth of 100 km dipping ~24° towards the continent, which we interpret as the top of the oceanic Caribbean slab that is subducting beneath South America from the west. The deeper part of the slab was previously imaged using P-wave tomography (Bezada et al, 2010), and the upper part inferred from intermediate depth seismicity (Malavé and Suarez, 1995). These studies indicate flat slab subduction beneath northern Colombia and northwestern Venezuela with the slab dipping between 20° - 30° beneath Lake Maracaibo. Like others we attribute the flat slab subduction to the uplift of the Mérida Andes (for example Kellogg and Bonini, 1982). In eastern Venezuela beneath the Sierra del Interior we also observe a deep signal that we interpret as deep South American lithosphere that is detaching from the overriding plate as the Atlantic subducts and tears away from SA (Bezada et al., 2010; Clark et al, 2008). The lithosphere-asthenosphere boundary (LAB) is not a continuous feature under the entire region, instead it is seen beneath the Cordillera de la Costa in central Venezuela at ~130 km, also under the Perijá Range and the Sierra del Interior. Under the Guayana Shield we observe two distinct regions with LAB depths at ~150 km depth. We also see the LAB at this depth in places north of the Orinoco River, suggesting the presence of cratonic structures north of the river. These results are in good agreement with the structures observed by Miller et al. (2009) in Rayleigh wave tomography images.

Crustal seismic anisotropy and structure from textural and seismic investigations in the Cycladic region, Greece

NASA Astrophysics Data System (ADS)

Cossette, Élise; Schneider, David; Audet, Pascal; Grasemann, Bernhard

2016-04-01

Seismic anisotropy data are often used to resolve rock structures and deformation styles in the crust based on compilations of rock properties that may not be representative of the exposed geology. We use teleseismic receiver functions jointly with in situ rock property data to constrain the seismic structure and anisotropy of the crust in the Cyclades, Greece, located in the back arc region of the Hellenic subduction zone. Crystallographic preferred orientations (CPOs) via electron backscatter diffraction (EBSD) analyses were measured on a suite of samples representative of different structural depths along the West Cycladic Detachment System; average seismic properties of the rocks were calculated with the Voigt-Reuss-Hill average of the single minerals' elastic stiffness tensor. The calcitic and quartzitic rocks have P- and S-wave velocity anisotropies (AVp, AVs) averaging 8.1% and 7.1%, respectively. The anisotropy increases with depth represented by blueschist assemblages, with AVp averaging 20.3% and AVs averaging 14.5% due to the content of aligned glaucophane and mica, which strongly control the seismic properties of the rocks. Localized anisotropies of very high magnitude are caused by the presence of mica schists as they possess the strongest anisotropies, with values of ~25% for AVp and AVs. The direction of the fast and slow P-wave velocities occur parallel and perpendicular to the foliation, respectively, for most samples. The fast propagation has the same NE-SW orientation as the lithospheric stretching direction present in the Cyclades since the Late Oligocene. The maximum shear wave anisotropy is subhorizontal, similarly concordant with mineral alignment that developed during back-arc extension. Our results strongly favor radial anisotropy in the Aegean mid-crust over azimuthal anisotropy. The receiver function data indicate that the Moho is relatively flat at 25 km depth in the south and deepens to 33 km in the north, consistent with previous studies, and reveal an intra-crustal discontinuity at depth varying from 3 to 11 km, mostly observed in the south-central Aegean. Harmonic decomposition of the receiver functions further indicates layering of both shallow and deep crustal anisotropy related to crustal structures. We model synthetic receiver functions based on constraints from the in situ rock properties that we measured using the EBSD technique. Our results indicate that the shallow upper crustal layer is characterized by metapelites with ~5% anisotropy, underlain by a 20 km thick and anisotropic layer of possible high-pressure rocks comprising blueschist and eclogite and/or restitic crust as a consequence of Miocene magmatism. Seismic anisotropy models require a sub-vertical axis of hexagonal symmetry in the upper crust (i.e. radial anisotropy), consistent with in situ rock data. Finally, a thinned crust is likely caused by back-arc extension associated with elevated sub-crustal temperatures, in agreement with thermal isostasy models of back arcs. This study demonstrates the importance of integrating rock textural data with seismic velocity profiles in the interpretation of crustal architecture.

Tremor evidence for dynamically triggered creep events on the deep San Andreas Fault

NASA Astrophysics Data System (ADS)

Peng, Z.; Shelly, D. R.; Hill, D. P.; Aiken, C.

2010-12-01

Deep tectonic tremor has been observed along major subduction zones and the San Andreas fault (SAF) in central and southern California. It appears to reflect deep fault slip, and it is often seen to be triggered by small stresses, including passing seismic waves from large regional and teleseismic earthquakes. Here we examine tremor activity along the Parkfield-Cholame section of the SAF from mid-2001 to early 2010, scrutinizing its relationship with regional and teleseismic earthquakes. Based on similarities in the shape and timing of seismic waveforms, we conclude that triggered and ambient tremor share common sources and a common physical mechanism. Utilizing this similarity in waveforms, we detect tremor triggered by numerous large events, including previously unreported triggering from the recent 2009 Mw7.3 Honduras, 2009 Mw8.1 Samoa, and 2010 Mw8.8 Chile earthquakes at teleseismic distances, and the relatively small 2007 Mw5.4 Alum Rock and 2008 Mw5.4 Chino Hills earthquakes at regional distances. We also find multiple examples of systematic migration in triggered tremor, similar to ambient tremor migration episodes observed at other times. Because these episodes propagate much more slowly than the triggering waves, the migration likely reflects a small, triggered creep event. As with ambient tremor bursts, triggered tremor at times persists for multiple days, probably indicating a somewhat larger creep event. This activity provides a clear example of delayed dynamic triggering, with a mechanism perhaps also relevant for triggering of regular earthquakes.

Shallow earthquake swarms in southern Ryukyu area: manifestation of dynamics of fluid and/or magma plumbing system revealed by teleseismic and regional datasets

NASA Astrophysics Data System (ADS)

Špičák, Aleš; Vaněk, Jiří

2017-04-01

Earthquake swarm occurrence beneath volcanic domains is one of the indicators of current magmatic activity in the Earth's crust. Repeated occurrence of teleseismically recorded earthquake swarms has been observed in the lithospheric wedge of the southern Ryukyu area above the subducting slab of the Philippine Sea Plate. The swarms were analyzed using the EHB, ISC and JMA catalogs of hypocenter parameters. The swarm earthquakes are shallow (1-60 km), in the body-wave magnitude range up to 5.8. The swarms are distributed beneath the seafloor, parallel to the Ryukyu Trench along a belt connecting active subaerial volcanoes Io-Torishima north-east and Kueishantao west of the investigated area. Epicentral zones of the swarms often coincide with distinct elevations at the seafloor—seamounts and seamount ranges. The top of the subducting slab reaches a depth of about 100 km beneath the zones of earthquake swarm occurrence, which is an average depth of a slab beneath volcanoes in general. The repeated occurrence of relatively strong, teleseismically recorded earthquake swarms thus probably reflects fluid and/or magma migration in the plumbing system of the volcanic arc and points to brittle character of the lithospheric wedge at respective depths. In addition to the factual results, this study documents the high accuracy of hypocenter parameter determinations published by the International Seismological Centre and the usefulness of the EHB relocation procedure.

Crustal and upper-mantle structure of South China from Rayleigh wave tomography

NASA Astrophysics Data System (ADS)

Shan, B.; Xiong, X.; Zhao, K. F.; Xie, Z. J.; Zheng, Y.; Zhou, L.

2017-03-01

In this study, we image the crust and upper-mantle seismic velocity structures in South China using teleseismic Rayleigh waves recorded at 354 stations from the Chinese provincial networks (CEArray). We process Rayleigh wave data from 1087 teleseismic events and construct phase velocity maps at periods of 40-150 s. By combining dispersion curves at 6-70 s from Zhou et al. and at 40-150 s from the teleseismic surface wave tomography of this study, we construct a 3-D shear velocity model of the crust and upper mantle of South China. Distinct seismic structures are revealed from the eastern part of South China (including the South China Fold System and the eastern Yangtze Craton) to the western Yangtze Craton. The South China Fold System and eastern Yangtze Craton are characterized by lower velocities and shallow lithosphere-asthenosphere boundary (∼90 km), which are similar to the lithospheric thermal and seismic velocity structures of the North China basin. These observations may imply that the lithospheric destruction and thinning occurred not only beneath the North China Craton, but also beneath the eastern part of South China. The western Yangtze Craton, including the Sichuan Basin and Jiangnan Orogen, is underlain by a thicker and colder lithosphere with high velocities. The contrast of the lithosphere structure between the western Yangtze Craton and other parts of South China indicates that the lithospheric destruction and thinning of the east and southeast parts of South China may terminate at the boundary of the Jiangnan Orogen.

Crustal and mantle structure and anisotropy beneath the incipient segments of the East African Rift System: Preliminary results from the ongoing SAFARI

NASA Astrophysics Data System (ADS)

Yu, Y.; Reed, C. A.; Gao, S. S.; Liu, K. H.; Massinque, B.; Mdala, H. S.; moidaki, M.; Mutamina, D. M.; Atekwana, E. A.; Ingate, S. F.; Reusch, A.; Barstow, N.

2013-12-01

Despite the vast wealth of research conducted toward understanding processes associated with continental rifting, the extent of our knowledge is derived primarily from studies focused on mature rift systems, such as the well-developed portions of the East African Rift System (EARS) north of Lake Malawi. To explore the dynamics of early rift evolution, the SAFARI (Seismic Arrays for African Rift Initiation) team deployed 50 PASSCAL broadband seismic stations across the Malawi, Luangwa, and Okavango rifts of the EARS during the summer of 2012. The cumulative length of the profiles is about 2500 km and the planned recording duration is 2 years. Here we present the preliminary results of systematic analyses of data obtained from the first year of acquisition for all 50 stations. A total of 446 high-quality shear-wave splitting measurements using PKS, SKKS, and SKS phases from 84 teleseismic events were used to constrain fast polarization directions and splitting times throughout the region. The Malawi and Okavango rifts are characterized by mostly NE trending fast directions with a mean splitting time of about 1 s. The fast directions on the west side of the Luangwa Rift Zone are parallel to the rift valley, and those on the east side are more N-S oriented. Stacking of approximately 1900 radial receiver functions reveals significant spatial variations of both crustal thickness and the ratio of crustal P and S wave velocities, as well as the thickness of the mantle transition zone. Stations situated within the Malawi rift demonstrate a southward increase in observed crustal thickness, which is consistent with the hypothesis that the Malawi rift originated at the northern end of the rift system and propagated southward. Both the Okavango and Luangwa rifts are associated with thinned crust and increased Vp/Vs, although additional data is required at some stations to enhance the reliability of the observations. Teleseismic P-wave travel-time residuals show a delay of about 1 s at stations in the Okavango rift relative to the Limpopo belt. The study region is characterized by a relatively average mantle transition zone thickness of 250 km except for stations located within and to the immediate NW of the Okavango rift, where it is probably abnormally thin. Additional seismological techniques will be applied to the data set, and the preliminary results from the above initial analyses will be confirmed or modified by data from the SAFARI stations in the second year.

Rigorous Approach in Investigation of Seismic Structure and Source Characteristicsin Northeast Asia: Hierarchical and Trans-dimensional Bayesian Inversion

NASA Astrophysics Data System (ADS)

Mustac, M.; Kim, S.; Tkalcic, H.; Rhie, J.; Chen, Y.; Ford, S. R.; Sebastian, N.

2015-12-01

Conventional approaches to inverse problems suffer from non-linearity and non-uniqueness in estimations of seismic structures and source properties. Estimated results and associated uncertainties are often biased by applied regularizations and additional constraints, which are commonly introduced to solve such problems. Bayesian methods, however, provide statistically meaningful estimations of models and their uncertainties constrained by data information. In addition, hierarchical and trans-dimensional (trans-D) techniques are inherently implemented in the Bayesian framework to account for involved error statistics and model parameterizations, and, in turn, allow more rigorous estimations of the same. Here, we apply Bayesian methods throughout the entire inference process to estimate seismic structures and source properties in Northeast Asia including east China, the Korean peninsula, and the Japanese islands. Ambient noise analysis is first performed to obtain a base three-dimensional (3-D) heterogeneity model using continuous broadband waveforms from more than 300 stations. As for the tomography of surface wave group and phase velocities in the 5-70 s band, we adopt a hierarchical and trans-D Bayesian inversion method using Voronoi partition. The 3-D heterogeneity model is further improved by joint inversions of teleseismic receiver functions and dispersion data using a newly developed high-efficiency Bayesian technique. The obtained model is subsequently used to prepare 3-D structural Green's functions for the source characterization. A hierarchical Bayesian method for point source inversion using regional complete waveform data is applied to selected events from the region. The seismic structure and source characteristics with rigorously estimated uncertainties from the novel Bayesian methods provide enhanced monitoring and discrimination of seismic events in northeast Asia.

Source analysis using regional empirical Green's functions: The 2008 Wells, Nevada, earthquake

USGS Publications Warehouse

Mendoza, C.; Hartzell, S.

2009-01-01

We invert three-component, regional broadband waveforms recorded for the 21 February 2008 Wells, Nevada, earthquake using a finite-fault methodology that prescribes subfault responses using eight MW∼4 aftershocks as empirical Green's functions (EGFs) distributed within a 20-km by 21.6-km fault area. The inversion identifies a seismic moment of 6.2 x 1024 dyne-cm (5.8 MW) with slip concentrated in a compact 6.5-km by 4-km region updip from the hypocenter. The peak slip within this localized area is 88 cm and the stress drop is 72 bars, which is higher than expected for Basin and Range normal faults in the western United States. The EGF approach yields excellent fits to the complex regional waveforms, accounting for strong variations in wave propagation and site effects. This suggests that the procedure is useful for studying moderate-size earthquakes with limited teleseismic or strong-motion data and for examining uncertainties in slip models obtained using theoretical Green's functions.

Revision of earthquake hypocentre locations in global bulletin data sets using source-specific station terms

NASA Astrophysics Data System (ADS)

Nooshiri, Nima; Saul, Joachim; Heimann, Sebastian; Tilmann, Frederik; Dahm, Torsten

2017-02-01

Global earthquake locations are often associated with very large systematic travel-time residuals even for clear arrivals, especially for regional and near-regional stations in subduction zones because of their strongly heterogeneous velocity structure. Travel-time corrections can drastically reduce travel-time residuals at regional stations and, in consequence, improve the relative location accuracy. We have extended the shrinking-box source-specific station terms technique to regional and teleseismic distances and adopted the algorithm for probabilistic, nonlinear, global-search location. We evaluated the potential of the method to compute precise relative hypocentre locations on a global scale. The method has been applied to two specific test regions using existing P- and pP-phase picks. The first data set consists of 3103 events along the Chilean margin and the second one comprises 1680 earthquakes in the Tonga-Fiji subduction zone. Pick data were obtained from the GEOFON earthquake bulletin, produced using data from all available, global station networks. A set of timing corrections varying as a function of source position was calculated for each seismic station. In this way, we could correct the systematic errors introduced into the locations by the inaccuracies in the assumed velocity structure without explicitly solving for a velocity model. Residual statistics show that the median absolute deviation of the travel-time residuals is reduced by 40-60 per cent at regional distances, where the velocity anomalies are strong. Moreover, the spread of the travel-time residuals decreased by ˜20 per cent at teleseismic distances (>28°). Furthermore, strong variations in initial residuals as a function of recording distance are smoothed out in the final residuals. The relocated catalogues exhibit less scattered locations in depth and sharper images of the seismicity associated with the subducting slabs. Comparison with a high-resolution local catalogue reveals that our relocation process significantly improves the hypocentre locations compared to standard locations.

Complete synthetic seismograms based on a spherical self-gravitating Earth model with an atmosphere-ocean-mantle-core structure

NASA Astrophysics Data System (ADS)

Wang, Rongjiang; Heimann, Sebastian; Zhang, Yong; Wang, Hansheng; Dahm, Torsten

2017-04-01

A hybrid method is proposed to calculate complete synthetic seismograms based on a spherically symmetric and self-gravitating Earth with a multi-layered structure of atmosphere, ocean, mantle, liquid core and solid core. For large wavelengths, a numerical scheme is used to solve the geodynamic boundary-value problem without any approximation on the deformation and gravity coupling. With the decreasing wavelength, the gravity effect on the deformation becomes negligible and the analytical propagator scheme can be used. Many useful approaches are used to overcome the numerical problems that may arise in both analytical and numerical schemes. Some of these approaches have been established in the seismological community and the others are developed for the first time. Based on the stable and efficient hybrid algorithm, an all-in-one code QSSP is implemented to cover the complete spectrum of seismological interests. The performance of the code is demonstrated by various tests including the curvature effect on teleseismic body and surface waves, the appearance of multiple reflected, teleseismic core phases, the gravity effect on long period surface waves and free oscillations, the simulation of near-field displacement seismograms with the static offset, the coupling of tsunami and infrasound waves, and free oscillations of the solid Earth, the atmosphere and the ocean. QSSP is open source software that can be used as a stand-alone FORTRAN code or may be applied in combination with a Python toolbox to calculate and handle Green's function databases for efficient coding of source inversion problems.

Complete synthetic seismograms based on a spherical self-gravitating Earth model with an atmosphere-ocean-mantle-core structure

NASA Astrophysics Data System (ADS)

Wang, Rongjiang; Heimann, Sebastian; Zhang, Yong; Wang, Hansheng; Dahm, Torsten

2017-09-01

A hybrid method is proposed to calculate complete synthetic seismograms based on a spherically symmetric and self-gravitating Earth with a multilayered structure of atmosphere, ocean, mantle, liquid core and solid core. For large wavelengths, a numerical scheme is used to solve the geodynamic boundary-value problem without any approximation on the deformation and gravity coupling. With decreasing wavelength, the gravity effect on the deformation becomes negligible and the analytical propagator scheme can be used. Many useful approaches are used to overcome the numerical problems that may arise in both analytical and numerical schemes. Some of these approaches have been established in the seismological community and the others are developed for the first time. Based on the stable and efficient hybrid algorithm, an all-in-one code QSSP is implemented to cover the complete spectrum of seismological interests. The performance of the code is demonstrated by various tests including the curvature effect on teleseismic body and surface waves, the appearance of multiple reflected, teleseismic core phases, the gravity effect on long period surface waves and free oscillations, the simulation of near-field displacement seismograms with the static offset, the coupling of tsunami and infrasound waves, and free oscillations of the solid Earth, the atmosphere and the ocean. QSSP is open source software that can be used as a stand-alone FORTRAN code or may be applied in combination with a Python toolbox to calculate and handle Green's function databases for efficient coding of source inversion problems.

Rupture process of the 2016 Mw 7.8 Ecuador earthquake from joint inversion of InSAR data and teleseismic P waveforms

NASA Astrophysics Data System (ADS)

Yi, Lei; Xu, Caijun; Wen, Yangmao; Zhang, Xu; Jiang, Guoyan

2018-01-01

The 2016 Ecuador earthquake ruptured the Ecuador-Colombia subduction interface where several historic megathrust earthquakes had occurred. In order to determine a detailed rupture model, Interferometric Synthetic Aperture Radar (InSAR) images and teleseismic data sets were objectively weighted by using a modified Akaika's Bayesian Information Criterion (ABIC) method to jointly invert for the rupture process of the earthquake. In modeling the rupture process, a constrained waveform length method, unlike the traditional subjective selected waveform length method, was used since the lengths of inverted waveforms were strictly constrained by the rupture velocity and rise time (the slip duration time). The optimal rupture velocity and rise time of the earthquake were estimated from grid search, which were determined to be 2.0 km/s and 20 s, respectively. The inverted model shows that the event is dominated by thrust movement and the released moment is 5.75 × 1020 Nm (Mw 7.77). The slip distribution extends southward along the Ecuador coast line in an elongated stripe at a depth between 10 and 25 km. The slip model is composed of two asperities and slipped over 4 m. The source time function is approximate 80 s that separated into two segments corresponding to the two asperities. The small magnitude of the slip occurred in the updip section of the fault plane resulted in small tsunami waves that were verified by observations near the coast. We suggest a possible situation that the rupture zone of the 2016 earthquake is likely not overlapped with that of the 1942 earthquake.

Interaction between fault systems in a complex tectonic setting: Insights from InSAR and Teleseismic analysis of the 2015 Lake Saurez and 2016 Muji fault earthquake sequence

NASA Astrophysics Data System (ADS)

Nanjundiah, P.; Barbot, S.; Wei, S.; Tapponnier, P.; Feng, W.; Wang, T.

2017-12-01

The Pamir Plateau is a complex and important component of the India-Eurasia Collision zone. Despite being similar to the Tibetan plateau in elevation and collision processes, quite a bit is still unknown about the structure and the tectonic processes occurring in this region. We aim to better understand the structure, stress and deformation patterns in the northern and central Pamir plateau by analysing InSAR, teleseismic, and optical data for two large earthquakes that occurred in this region between December 2015 (Mw 7.2, Lake Saurez) and November 2016 (Mw 6.6 Muji Fault). We constrain the fault geometry by precisely relocating aftershocks using the double difference technique implemented in HypoDD (Waldhauser & Ellsworth 2000). We used Okada's (1992) Green Functions to invert for slip on the fault with a rectangular dislocation and edgreen to numerically invert for the slip in a layered medium (Wang et al. 2005). The combined datasets highlight the existence of an oblique fault between two major thrust fault systems i.e. the Darwas & the Karakoram faults. The December 2015 event highlights complexity in this fault system. The combination of data sets used in this study highlights the existence of a seismic gap south of Lake Karakul as well as coupling between the Muji and Darwas-Karakoram fault systems. We emphasise the role of smaller faults and their interactions in accommodating the overall strain and tectonics in the Pamir region and their effect on estimating local seismic hazard.

Near surface characterisation with passive seismic data - a case study from the La Barge basin (Wyoming)

NASA Astrophysics Data System (ADS)

Behm, M.; Snieder, R.; Tomic, J.

2012-12-01

In regions where active source seismic data are inadequate for imaging purposes due to energy penetration and recovery, cost and logistical concerns, or regulatory restrictions, analysis of natural source and ambient seismic data may provide an alternative. In this study, we investigate the feasibility of using locally-generated seismic noise and teleseismic events in the 2-10 Hz band to obtain a subsurface model. We apply different techniques to 3-component data recorded during the LaBarge Passive Seismic Experiment, a local deployment in southwestern Wyoming in a producing hydrocarbon basin. Fifty-five broadband instruments with an inter-station distance of 250 m recorded continuous seismic data between November 2008 and June 2009. The consistency and high quality of the data set make it an ideal test ground to determine the value of passive seismology techniques for exploration purposes. The near surface is targeted by interferometric analysis of ambient noise. Our results indicate that traffic noise from a state highway generates coherent Rayleigh and Love waves that can then be inverted for laterally varying velocities. The results correlate well with surface geology, and are thought to represent the average of the few upper hundred meters. The autocorrelation functions (ACF) of teleseismic body waves provide information on the uppermost part (1 to 5 km depth) of the crust. ACFs from P-waves correlate with the shallow structure as known from active source studies. The analysis of S-waves exhibits a pronounced azimuthal dependency, which might be used to gain insights on anisotropy.

Simultaneous Determination of Average Thickness and P-wave Speed of the Crust by Virtual Deep Seismic Sounding (VDSS)

NASA Astrophysics Data System (ADS)

Kang, D.; Yu, C.; Ning, J.; TAO, K.; Chen, W. P.

2014-12-01

Using teleseismic S-waves, VDSS treats the SV-to-P conversion under the free surface (on the station-side) as a virtual source to generate strong, post-critical reflection off the Moho (SsPmp phase). With just a single, good-quality earthquake, arrival-time difference between SsPmp and the direct S-phase (TSsPmp-Ss) can effectively determine the crustal thickness (H) near the receiver. However, there is a strong trade-off between H and P-wave speed (Vp) in the crust. Here we extend VDSS to constrain both H and Vp by taking advantage of the variation in ray-parameters, or incident angles, as a function of epicentral distance. Note that in conventional receiver functions, information contained in data of different ray-parameters is usually lost, because stacking over move-out corrected data is required to get a clear signal. At a given station, we collect data from many events, each with a different ray-parameter of the direct S-phase (ps­). For each event, we 1) estimate the source wavelet of the direct S-wave through particle motion analysis; 2) deconvolve this wavelet from the vertical- and radial-component seismograms (Yu et al., GJI, 2013); and then 3) determine TSsPmp-Ss through waveform modeling. Finally, we analyze data pairs (ps2, T2SsPmp-Ss) to find the best-fitting values of H and Vp. Synthetic tests verify the robustness of the method even with 15% of white noise. Moreover, we applied the method to public domain data from Forrest (FORT), located in the Eucla basin of western Australia. Based on 30 earthquakes from a narrow back-azimuth range (105±15°) but with ps changing from 0.1221 to 0.1349 s/km, we estimate that near FORT, H and Vp are about 44±2 km and 6.67±0.35 km/s, respectively. This crustal thickness is consistent with previous reports - a surprisingly high value for a region where the elevation is less than 200 m. Together with the high Vp, our results imply that the crust has a dense, mafic component.

Imaging the Juan de Fuca subduction plate using 3D Kirchoff Prestack Depth Migration

NASA Astrophysics Data System (ADS)

Cheng, C.; Bodin, T.; Allen, R. M.; Tauzin, B.

2014-12-01

We propose a new Receiver Function migration method to image the subducting plate in the western US that utilizes the US array and regional network data. While the well-developed CCP (common conversion point) poststack migration is commonly used for such imaging; our method applies a 3D prestack depth migration approach. The traditional CCP and post-stack depth mapping approaches implement the ray tracing and moveout correction for the incoming teleseismic plane wave based on a 1D earth reference model and the assumption of horizontal discontinuities. Although this works well in mapping the reflection position of relatively flat discontinuities (such as the Moho or the LAB), CCP is known to give poor results in the presence of lateral volumetric velocity variations and dipping layers. Instead of making the flat layer assumption and 1D moveout correction, seismic rays are traced in a 3D tomographic model with the Fast Marching Method. With travel time information stored, our Kirchoff migration is done where the amplitude of the receiver function at a given time is distributed over all possible conversion points (i.e. along a semi-elipse) on the output migrated depth section. The migrated reflectors will appear where the semicircles constructively interfere, whereas destructive interference will cancel out noise. Synthetic tests show that in the case of a horizontal discontinuity, the prestack Kirchoff migration gives similar results to CCP, but without spurious multiples as this energy is stacked destructively and cancels out. For 45 degree and 60 degree dipping discontinuities, it also performs better in terms of imaging at the right boundary and dip angle. This is especially useful in the Western US case, beneath which the Juan de Fuca plate subducted to ~450km with a dipping angle that may exceed 50 degree. While the traditional CCP method will underestimate the dipping angle, our proposed imaging method will provide an accurate 3D subducting plate image without heavy computation. This will provide further thoughts for geodynamic research on the evolution of western US.

Crustal thickness variations in the Zagros continental collision zone (Iran) from joint inversion of receiver functions and surface wave dispersion

NASA Astrophysics Data System (ADS)

Tatar, M.; Nasrabadi, A.

2013-10-01

Variations in crustal thickness in the Zagros determined by joint inversion of P wave receiver functions (RFs) and Rayleigh wave group and phase velocity dispersion. The time domain iterative deconvolution procedure was employed to compute RFs from teleseismic recordings at seven broadband stations of INSN network. Rayleigh wave phase velocity dispersion curves were estimated employing two-station method. Fundamental mode Rayleigh wave group velocities for each station is taken from a regional scale surface wave tomographic imaging. The main variations in crustal thickness that we observe are between stations located in the Zagros fold and thrust belt with those located in the Sanandaj-Sirjan zone (SSZ) and Urumieh-Dokhtar magmatic assemblage (UDMA). Our results indicate that the average crustal thickness beneath the Zagros Mountain Range varies from ˜46 km in Western and Central Zagros beneath SHGR and GHIR up to ˜50 km beneath BNDS located in easternmost of the Zagros. Toward NE, we observe an increase in Moho depth where it reaches ˜58 km beneath SNGE located in the SSZ. Average crustal thickness also varies beneath the UDMA from ˜50 km in western parts below ASAO to ˜58 in central parts below NASN. The observed variation along the SSZ and UDMA may be associated to ongoing slab steepening or break off in the NW Zagros, comparing under thrusting of the Arabian plate beneath Central Zagros. The results show that in Central Iran, the crustal thickness decrease again to ˜47 km below KRBR. There is not a significant crustal thickness difference along the Zagros fold and thrust belt. We found the same crystalline crust of ˜34 km thick beneath the different parts of the Zagros fold and thrust belt. The similarity of crustal structure suggests that the crust of the Zagros fold and thrust belt was uniform before subsidence and deposition of the sediments. Our results confirm that the shortening of the western and eastern parts of the Zagros basement is small and has only started recently.

Seismic signals from the slab surface within and downdip of the thrust zone: blind men and the elephant?

NASA Astrophysics Data System (ADS)

Abers, G. A.; Janiszewski, H. A.; Keranen, K. M.; Li, J.; Saffer, D. M.; Shillington, D. J.; Schindler, K.

2016-12-01

The subduction plate interface as been variably described as a narrow discontinuity, a decoupling layer, a viscous channel, or an intensely deformed mixing zone. The 1994 Subduction Conference depicted this ambiguity with the parable of the blind men and the elephant, as participants struggled to rationalize dissimilar observations with an integrated view of subduction. We illustrate here how different seismological tools reveal contradictory natures to the slab surface at 30-80 km depth, highlighting new examples from Alaska and Cascadia. At the km scale and 0.05-0.5 Hz, the teleseismic scattered wavefield that generates receiver functions shows strong consistent patterns. It indicates a uniformly layered plate interface structure with a low-velocity channel along the thrust zone where earthquakes and slow slip events occur. These channels appear homogeneous and 1-4 km thick over wide areas, with hints of strong anisotropy. By contrast, reflection seismology shows complex discontinuous reflectivity packages at the tens of m scale and 5-20 Hz that imply much greater heterogeneity, in both in normal-incidence and wide-angle reflections. To span the intervening frequency band we analyze P-S conversions from in-slab earthquakes recorded nearby, in southern Alaska. These 1-10 Hz signals arrive between P and S and have comparable amplitude, indicating sharp or complex boundaries near the slab surface at 30-50 km depth. However the signals are not uniformly observed and indicate significant heterogeneity in the causative structure. The conversion points lie within those sampled by receiver functions so the differences are not due to geographic variations, and can be analyzed jointly. Taken together, these observations suggest that the same boundary is continuous and relatively homogenous, or highly heterogeneous and laminated, depending on its sampling. They can be reconciled if the structure at km-scale is relatively simple, while it is complex and highly heterogeneous at shorter wavelengths. A complex but sharply bounded low-velocity shear zone best explains these observations, indicating that most deformation is highly localized rather than forming broadly-distributed ductile flow features even well into the aseismic region.

Lithospheric structure of an incipient rift basin: Results from receiver function analysis of Bransfield Strait, NW Antarctic Peninsula

NASA Astrophysics Data System (ADS)

Biryol, C. Berk; Lee, Stephen J.; Lees, Jonathan M.; Shore, Michael J.

2018-06-01

Bransfield Basin (BB), located northwest of the Antarctic Peninsula (AP) and southeast of the South Shetland Islands (SSI), is the most active section of the Antarctic continental margin. The region has long been (50 Ma) a convergent plate boundary where the Phoenix plate was subducting beneath the Antarctic Plate and is characterized by long-lived arc magmatism and accretion. However, the collision of the Antarctic-Phoenix spreading center with the subduction front near SSI (ca. 4 Ma) gave way to the opening of slab windows and dramatic decrease in the subduction rate of the Phoenix plate beneath AP and SSI. Consequently, the Phoenix slab began to rollback slowly along the South Shetland Trench (SST), giving way to slow extension in the back-arc region and rifting along the BB. Although there is consensus on the factors that control the current deformation and extension of the BB, the origin of the BB and the tectonic configuration of the basin are still unclear. Most of the controversy stems from uncertainties regarding the crustal thickness of the BB. Hence, we computed teleseismic receiver functions for 10 broadband stations in the region that belong to existing permanent and temporary deployments in order obtain robust constraints on the lithospheric structure and crustal thickness of the BB, as well as the AP and SSI. Our results indicate that the crust is thinning from 30 km to 26 km from the AP towards the South Shetland trench and Central BB showing the asymmetrical character of the rift basin. The crustal thickness and Vp/Vs variations are less pronounced along the AP but very significant across the SSB indicating the lithospheric scale segmentation of the South Shetland Block (SSB) and the incipient rift basin under the control of the opening of slab window and the roll-back of stalled Phoenix slab. High Vp/Vs ratios (∼1.9) beneath BB and SSI, agree well with the nascent rift character of BB, the presence of a steep Phoenix slab and consequently a wider mantle wedge characterized by the presence of underplating partial melts beneath SSI and BB.

The Crust and Upper Mantle Structure of the Iranian Plateau from Joint Waveform Tomography Imaging of Body and Surface Waves

NASA Astrophysics Data System (ADS)

Roecker, S. W.; Priestley, K. F.; Tatar, M.

2014-12-01

The Iranian Plateau forms a broad zone of deformation between the colliding Arabian and Eurasian plates. The convergence is accommodated in the Zagros Mountains of SW Iran, the Alborz Mountains of northern Iran, and the Kopeh Dagh Mountains of NE Iran. These deforming belts are separated by relatively aseismic depressions such as the Lut Block. It has been suggested that the Arabia-Eurasia collision is similar to the Indo-Eurasia collision but at a early point of development and therefore, it may provide clues to our understanding of the earlier stages of the continent-continent collision process. We present results of the analysis of seismic data collected along two NE-SW trending transects across the Iranian Plateau. The first profile extends from near Bushere on the Persian Gulf coast to near to the Iran-Turkmenistan border north of Mashad, and consists of seismic recordings along the SW portion of the line in 2000-2001 and recording along the NE portion of the line in 2003 and 2006-2008. The second profile extends from near the Iran-Iraq border near the Dezfel embayment to the south Caspian Sea coast north of Tehran. We apply the combined 2.5D finite element waveform tomography algorithm of Baker and Roecker [2014] to jointly invert teleseismic body and surface waves to determine the elastic wavespeed structures of these areas. The joint inversion of these different types of waves affords similar types of advantages that are common to combined surface wave dispersion/receiver function inversions in compensating for intrinsic weaknesses in horizontal and vertical resolution capabilities. We compare results recovered from a finite difference approach to document the effects of various assumptions related to their application, such as the inclusion of topography, on the models recovered. We also apply several different inverse methods, starting with simple gradient techniques to the more sophisticated pseudo-Hessian or L-BFGS approach, and find that the latter are generally more robust. Modeling of receiver functions and surface wave dispersion prior to the analysis is shown to be an efficacious way to generate starting models for this analysis.

Seismological observations at the Northern Andean region of Colombia: Evidence for a shallowly subducting Caribbean Slab and an extensional regime in the upper plate

NASA Astrophysics Data System (ADS)

Monsalve, G.; Cardona, A.; Yarce, J.; Alvira, D.; Poveda, E.

2013-05-01

A number of seismological observations, among which we can mention teleseismic travel time residuals, P to S receiver functions and Pn velocity quantification, suggest a clear distinction between the seismic structure of the crust and uppermost mantle between the plains on the Caribbean coast of Colombia and the mountains at the Northern Andean region. Absolute and relative travel time residuals indicate the presence of a seismically fast material in the upper mantle beneath northern Colombia; preliminary results of Pn studies show a region of relatively slow Pn velocities (between 7.8 and 7.9 km/s) underneath the Caribbean coast, contrasting with values greater than 8 km/s beneath the Central and Western cordilleras of Colombia, and the Pacific coast; receiver functions suggest a significantly thinner crust beneath the Caribbean coast, with a crustal thickness between 25 and 30 km, than beneath the Northern Andean zone at the cordilleras of Colombia, where it exceeds 40 km and reaches about 57 km at the location of Bogota. Besides the obviuos discrepancies that appear in response to different topography, we think that the seismological observations are a consequence of the presence of two very distinct slab segments beneath Colombia and contrasting behaviors of the upper plate, which correspond to Caribbean and Nazca subductions. Our seismic observations can be explained by a shallowly subducting Caribbean Plate, in the absence of an asthenospheric wedge, that steepens at about the location of the Bucaramanga nest, and a thinned continental crust that reflects an extensional component linked to oblique convergence of the Caribbean, which contrasts with the crustal thickening in the Andean Cordillera linked to crustal shortening and Nazca plate subuction. These new data are consistent with the idea of of a relatively warm Nazca slab of Neogene age which seems to have a relatively frontal convergence, and a colder, more buoyant Caribbean slab which represents an oceanic plateau of Cretaceous age that is characterized by an oblique convergence relation that has promoted extensional tectonics in the upper plate.

Coulomb Stress Change and Seismic Hazard of Rift Zones in Southern Tibet after the 2015 Mw7.8 Nepal Earthquake and Its Mw7.3 Aftershock

NASA Astrophysics Data System (ADS)

Dai, Z.; Zha, X.; Lu, Z.

2015-12-01

In southern Tibet (30~34N, 80~95E), many north-trending rifts, such as Yadong-Gulu and Lunggar rifts, are characterized by internally drained graben or half-graben basins bounded by active normal faults. Some developed rifts have become a portion of important transportation lines in Tibet, China. Since 1976, eighty-seven >Mw5.0 earthquakes have happened in the rift regions, and fifty-five events have normal faulting focal mechanisms according to the GCMT catalog. These rifts and normal faults are associated with both the EW-trending extension of the southern Tibet and the convergence between Indian and Tibet. The 2015 Mw7.8 Nepal great earthquake and its Mw7.3 aftershock occurred at the main Himalayan Thrust zone and caused tremendous damages in Kathmandu region. Those earthquakes will lead to significant viscoelastic deformation and stress changes in the southern Tibet in the future. To evaluate the seismic hazard in the active rift regions in southern Tibet, we modeled the slip distribution of the 2015 Nepal great earthquakes using the InSAR displacement field from the ALOS-2 satellite SAR data, and calculated the Coulomb failure stress (CFS) on these active normal faults in the rift zones. Because the estimated CFS depends on the geometrical parameters of receiver faults, it is necessary to get the accurate fault parameters in the rift zones. Some historical earthquakes have been studied using the field data, teleseismic data and InSAR observations, but results are in not agreement with each other. In this study, we revaluated the geometrical parameters of seismogenic faults occurred in the rift zones using some high-quality coseismic InSAR observations and teleseismic body-wave data. Finally, we will evaluate the seismic hazard in the rift zones according to the value of the estimated CFS and aftershock distribution.

Teleseismically recorded seismicity before and after the May 7, 1986, Andreanof Islands, Alaska, earthquake

USGS Publications Warehouse

Engdahl, E.R.; Billington, S.; Kisslinger, C.

1989-01-01

The Andreanof Islands earthquake (Mw 8.0) is the largest event to have occurred in that section of the Aleutian arc since the March 9, 1957, Aleutian Islands earthquake (Mw 8.6). Teleseismically well-recorded earthquakes in the region of the 1986 earthquake are relocated with a plate model and with careful attention to the focal depths. The data set is nearly complete for mb???4.7 between longitudes 172??W and 179??W for the period 1964 through April 1987 and provides a detailed description of the space-time history of moderate-size earthquakes in the region for that period. Additional insight is provided by source parameters which have been systematically determined for Mw???5 earthquakes that occurred in the region since 1977 and by a modeling study of the spatial distribution of moment release on the mainshock fault plane. -from Authors

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shiddiqi, Hasbi Ash, E-mail: h.a.shiddiqi@students.itb.ac.id, E-mail: h.a.shiddiqi@gmail.com; Widiyantoro, Sri; Nugraha, Andri Dian

We have relocated hypocenters of earthquakes occurring in the Molucca collision zone and surrounding region taken from the BMKG catalog using teleseismic double-difference relocation algorithm (teletomoDD). We used P-wave arrival times of local, regional, and teleseismic events recorded at 304 recording stations. Over 7,000 earthquakes were recorded by the BMKG seismographicnetworkin the study region from April, 2009 toJune, 2014. We used a 3D regional-global nested velocity modelresulting fromprevious global tomographystudy. In this study, the3D seismic velocity model was appliedto theIndonesian region, whilethe1D seismicvelocity model (ak135)wasused for regions outside of Indonesia. Our relocation results show a better improvement in travel-time RMSmore » residuals comparedto those of the BMKG catalog.Ourresultsalso show that relocation shifts were dominated intheeast-west direction, whichmaybeinfluenced by theexistingvelocity anomaly related to the reversed V-shaped slabbeneaththestudy region. Our eventrelocation results refine the geometry of slabs beneath the Halmahera and Sangihe arcs.« less

Amphibious Shear Velocity Structure of the Cascadia Subduction Zone

NASA Astrophysics Data System (ADS)

Janiszewski, H. A.; Gaherty, J. B.; Abers, G. A.; Gao, H.

2017-12-01

The amphibious Cascadia Initiative crosses the coastline of the Cascadia subduction zone (CSZ) deploying seismometers from the Juan de Fuca ridge offshore to beyond the volcanic arc onshore. This allows unprecedented seismic imaging of the CSZ, enabling examination of both the evolution of the Juan de Fuca plate prior to and during subduction as well as the along strike variability of the subduction system. Here we present new results from an amphibious shear velocity model for the crust and upper mantle across the Cascadia subduction zone. The primary data used in this inversion are surface-wave phase velocities derived from ambient-noise Rayleigh-wave data in the 10 - 20 s period band, and teleseismic earthquake Rayleigh wave phase velocities in the 20 - 160 s period band. Phase velocity maps from these data reflect major tectonic structures including the transition from oceanic to continental lithosphere, Juan de Fuca lithosphere that is faster than observations in the Pacific for oceanic crust of its age, slow velocities associated with the accretionary prism, the front of the fast subducting slab, and the Cascades volcanic arc which is associated with slower velocities in the south than in the north. Crustal structures are constrained by receiver functions in the offshore forearc and onshore regions, and by active source constraints on the Juan de Fuca plate prior to subduction. The shear-wave velocities are interpreted in their relationships to temperature, presence of melt or hydrous alteration, and compositional variation of the CSZ.

MOZART - A seismological investigation of Central Mozambique

NASA Astrophysics Data System (ADS)

Domingues, Ana; Chamussa, Jose; Helffrich, George; Fishwick, Stewart; Ferreira, Ana; Custodio, Susana; Silveira, Graca; Manhica, Vladimiro; Fonseca, Joao

2013-04-01

Project MOZART (MOZAmbique Rift Tomography) aims to investigate the geological structure and current tectonic activity of the Mozambique sector of the East African Rift System (EARS). Space geodesy has indicated in recent years that the border between Nubia and the Somalian plate at these latitudes (16°S to 24°S) encompasses the Rovuma microplate, but little is known about its geometry or seismotectonics. The M7 Machaze earthquake of 2006 highlighted the relevance of the associated deformation, and motivated the MOZART deployment. Besides the regional seismotectonics, other targets of the project are the illumination of the Mesoproterozoic structures of the Mozambique Belt, and the study of its role in the current incipient rifting. The seismic network is composed of 30 VBB seismographic stations on loan from NERC's SEIS-UK Pool (Guralp CMG-3T 120s sensors) covering Central Mozambique (Manica, Sofala, Gaza and Inhambane provinces) with average inter-station spaces of the order of 100 km. Four stations are across the border in South Africa (Kruger Park). Data acquisition started in March 2011, and decommissioning is foreseen for August 2013. Data processing is underway, and includes local seismicity analysis, receiver function estimation and the study of surface wave dispersion (both ambient noise and teleseismic). Once a preliminary velocity model is developed with these techniques, further refinements will be attempted through waveform tomography. For this purpose, SPECFEM waveform modelling with a 3D velocity model is currently being implemented. Preliminary results of the ongoing data processing and analysis will be presented.

Multi Plumes and Their Flows beneath Arabia and East Africa

NASA Astrophysics Data System (ADS)

Chang, S.; van der Lee, S.

2010-12-01

The three-dimensional S-velocity structure beneath Arabia and East Africa is estimated down to the lower mantle to investigate vertical and horizontal extension of low-velocity anomalies that bear out the presence of mantle plumes and their flows beneath lithosphere. We estimated this model through joint inversion of teleseismic S- and SKS-arrival times, regional S- and Rayleigh waveform fits, fundamental-mode Rayleigh-wave group velocities, and independent Moho constraints from receiver functions, reflection/refraction profiles, and gravity measurements. With the unprecedented resolution in our S-velocity model, we found different flow patterns of hot materials upwelling beneath Afar beneath the Red Sea and the Gulf of Aden. While the low-velocity anomaly from Afar is well confined beneath the Gulf of Aden, inferring mantle flow along the gulf, N-S channel of low velocity is found beneath Arabia, not along the Red Sea. The Afar plume is distinctively separate from the Kenya plume, showing its origin in the lower mantle beneath southwestern Arabia. We identified another low-velocity extension to the lower mantle beneath Jordan and northern Arabia, which is thought to have caused volcanism in Jordan, northern Arabia, and possibly southern Turkey. Comparing locations of mantle plumes from the joint inversion with fast axes of shear-wave splitting, we confirmed horizontal plume flow from Afar in NS direction beneath Arabia and in NE-SW direction beneath Ethiopia as a likely cause of the observed seismic anisotropy.

Two-stage magmatism during the evolution of the transitional Ethiopian rift

NASA Astrophysics Data System (ADS)

Cornwell, D. G.; England, R. W.; Maguire, P. K.; Kendall, M.; Stuart, G. W.

2008-12-01

The Ethiopian rift marks the transition between continental rifting and incipient seafloor spreading. The Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE) included a 400 km-long cross-rift profile with 97 broadband passive seismometers with the aim to investigate the change from mechanical to magmatic extension by defining the lithospheric structure and extent of magmatism beneath the rift. Complimentary studies of P-wave receiver functions, shear-wave splitting and teleseismic earthquake arrival times show that the lithospheric structure is inherently different beneath the north-western rift flank, rift valley and south- eastern rift flank, with contrasting crustal thickness and composition, upper mantle velocity and lithospheric anisotropy. Two stages of magmatic addition are interpreted: 1) a 6--18 km-thick underplate lens at the base of the crust, which probably formed synchronous with an Oligocene flood basalt event (and therefore pre-dates the adjacent rifting by ~20 Myr); and 2) a 20--30 km-wide zone of intense dyking and partial melt, which most likely pervades the entire crust beneath the rift valley and marks the locus of current rift extension. Furthermore, Precambrian collision-related lithospheric fabric is proposed to be the main source of the strong anisotropy that is observed along the entire cross-rift profile, which may be augmented by magmatism beneath the rift. An active, followed by a passive magma-assisted rifting model that is controlled by a combination of far-field plate stresses, the pre-existing lithospheric framework and magmatism is invoked to explain the rift evolution.

Control strategy to limit duty cycle impact of earthquakes on the LIGO gravitational-wave detectors

NASA Astrophysics Data System (ADS)

Biscans, S.; Warner, J.; Mittleman, R.; Buchanan, C.; Coughlin, M.; Evans, M.; Gabbard, H.; Harms, J.; Lantz, B.; Mukund, N.; Pele, A.; Pezerat, C.; Picart, P.; Radkins, H.; Shaffer, T.

2018-03-01

Advanced gravitational-wave detectors such as the laser interferometer gravitational-wave observatories (LIGO) require an unprecedented level of isolation from the ground. When in operation, they measure motion of less than 10‑19 m. Strong teleseismic events like earthquakes disrupt the proper functioning of the detectors, and result in a loss of data. An earthquake early-warning system, as well as a prediction model, have been developed to understand the impact of earthquakes on LIGO. This paper describes a control strategy to use this early-warning system to reduce the LIGO downtime by  ∼30%. It also presents a plan to implement this new earthquake configuration in the LIGO automation system.

Seismic interferometry of the Bighorn Mountains: Using virtual source gathers to increase fold in sparse-source, dense-receiver data

NASA Astrophysics Data System (ADS)

Plescia, S. M.; Sheehan, A. F.; Haines, S. S.; Cook, S. W.; Worthington, L. L.

2016-12-01

The Bighorn Arch Seismic Experiment (BASE) was a combined active- and passive-source seismic experiment designed to image deep structures including the Moho beneath a basement-involved foreland arch. In summer 2010, over 1800 Texan receivers, with 4.5 Hz vertical component geophones, were deployed at 100-m to 1-km spacing in a region spanning the Bighorn Arch and the adjacent Bighorn and Powder River Basins. Twenty explosive sources were used to create seismic energy during a two-week acquisition period. Teleseismic earthquakes and mine blasts were also recorded during this time period. We utilize both virtual source interferometry and traditional reflection processing to better understand the deep crustal features of the region and the Moho. The large number of receivers, compared to the limited, widely spaced (10 - 30 km) active-source shots, makes the data an ideal candidate for virtual source seismic interferometry to increase fold. Virtual source interferometry results in data representing a geometry where receiver locations act as if they were seismic source positions. A virtual source gather, the product of virtual source interferometry, is produced by the cross correlation of one receiver's recording, the reference trace, with the recordings of all other receivers in a given shot gather. The cross correlation is repeated for all shot gathers and the resulting traces are stacked. This process is repeated until a virtual source gather has been determined for every real receiver location. Virtual source gathers can be processed with a standard reflection seismic processing flow to yield a reflection section. Improper static corrections can be detrimental to effective stacking, and determination of proper statics is often difficult in areas of significant contrast such as between basin and mountain areas. As such, a natural synergy exists between virtual source interferometry and modern industry reflection seismic processing, with its emphasis on detailed static correction and dense acquisition geometries.

Seismic wave triggering of nonvolcanic tremor, episodic tremor and slip, and earthquakes on Vancouver Island

NASA Astrophysics Data System (ADS)

Rubinstein, Justin L.; Gomberg, Joan; Vidale, John E.; Wech, Aaron G.; Kao, Honn; Creager, Kenneth C.; Rogers, Garry

2009-02-01

We explore the physical conditions that enable triggering of nonvolcanic tremor and earthquakes by considering local seismic activity on Vancouver Island, British Columbia during and immediately after the arrival of large-amplitude seismic waves from 30 teleseismic and 17 regional or local earthquakes. We identify tremor triggered by four of the teleseismic earthquakes. The close temporal and spatial proximity of triggered tremor to ambient tremor and aseismic slip indicates that when a fault is close to or undergoing failure, it is particularly susceptible to triggering of further events. The amplitude of the triggering waves also influences the likelihood of triggering both tremor and earthquakes such that large amplitude waves triggered tremor in the absence of detectable aseismic slip or ambient tremor. Tremor and energy radiated from regional/local earthquakes share the same frequency passband so that tremor cannot be identified during these smaller, more frequent events. We confidently identify triggered local earthquakes following only one teleseism, that with the largest amplitude, and four regional or local events that generated vigorous aftershock sequences in their immediate vicinity. Earthquakes tend to be triggered in regions different from tremor and with high ambient seismicity rates. We also note an interesting possible correlation between large teleseismic events and episodic tremor and slip (ETS) episodes, whereby ETS events that are "late" and have built up more stress than normal are susceptible to triggering by the slight nudge of the shaking from a large, distant event, while ETS events that are "early" or "on time" are not.

AniTomo - New Anisotropic Teleseismic Body-Wave Tomography Code to Unravel Structure of the Upper Mantle: Impact of Inversion Settings on Inferences of the Output Model

NASA Astrophysics Data System (ADS)

Munzarova, H.; Plomerova, J.; Kissling, E. H.

2015-12-01

Consideration of only isotropic wave propagation and neglecting anisotropy in tomography studies is a simplification obviously incongruous with current understanding of mantle-lithosphere plate dynamics. Both fossil anisotropy in the mantle lithosphere and anisotropy due to the present-day flow in the asthenosphere may significantly influence propagation of seismic waves. We present a novel code for anisotropic teleseismic tomography (AniTomo) that allows to invert relative P-wave travel time residuals simultaneously for coupled isotropic-anisotropic P-wave velocity models of the upper mantle. We have modified frequently-used isotropic teleseismic tomography code Telinv by assuming weak hexagonal anisotropy with symmetry axis oriented generally in 3D to be, together with heterogeneities, a source of the observed P-wave travel-time residuals. Careful testing of the new code with synthetics, concentrating on strengths and limitations of the inversion method, is a necessary step before AniTomo is applied to real datasets. We examine various aspects of anisotropic tomography and particularly influence of ray coverage on resolvability of individual model parameters and of initial models on the result. Synthetic models are designed to schematically represent heterogeneous and anisotropic structures in the upper mantle. Several synthetic tests mimicking a real tectonic setting, e.g. the lithosphere subduction in the Northern Apennines in Italy (Munzarova et al., G-Cubed, 2013), allow us to make quantitative assessments of the well-known trade-off between effects of seismic anisotropy and heterogeneities. Our results clearly document that significant distortions of imaged velocity heterogeneities may result from neglecting anisotropy.

Rupture processes of the 2013-2014 Minab earthquake sequence, Iran

NASA Astrophysics Data System (ADS)

Kintner, Jonas A.; Ammon, Charles J.; Cleveland, K. Michael; Herman, Matthew

2018-06-01

We constrain epicentroid locations, magnitudes and depths of moderate-magnitude earthquakes in the 2013-2014 Minab sequence using surface-wave cross-correlations, surface-wave spectra and teleseismic body-wave modelling. We estimate precise relative locations of 54 Mw ≥ 3.8 earthquakes using 48 409 teleseismic, intermediate-period Rayleigh and Love-wave cross-correlation measurements. To reduce significant regional biases in our relative locations, we shift the relative locations to align the Mw 6.2 main-shock centroid to a location derived from an independent InSAR fault model. Our relocations suggest that the events lie along a roughly east-west trend that is consistent with the faulting geometry in the GCMT catalogue. The results support previous studies that suggest the sequence consists of left-lateral strain release, but better defines the main-shock fault length and shows that most of the Mw ≥ 5.0 aftershocks occurred on one or two similarly oriented structures. We also show that aftershock activity migrated westwards along strike, away from the main shock, suggesting that Coulomb stress transfer played a role in the fault failure. We estimate the magnitudes of the relocated events using surface-wave cross-correlation amplitudes and find good agreement with the GCMT moment magnitudes for the larger events and underestimation of small-event size by catalogue MS. In addition to clarifying details of the Minab sequence, the results demonstrate that even in tectonically complex regions, relative relocation using teleseismic surface waves greatly improves the precision of relative earthquake epicentroid locations and can facilitate detailed tectonic analyses of remote earthquake sequences.

A new, improved and fully automatic method for teleseismic depth estimation of moderate earthquakes (4.5 < M < 5.5): application to the Guerrero subduction zone (Mexico)

NASA Astrophysics Data System (ADS)

Letort, Jean; Guilbert, Jocelyn; Cotton, Fabrice; Bondár, István; Cano, Yoann; Vergoz, Julien

2015-06-01

The depth of an earthquake is difficult to estimate because of the trade-off between depth and origin time estimations, and because it can be biased by lateral Earth heterogeneities. To face this challenge, we have developed a new, blind and fully automatic teleseismic depth analysis. The results of this new method do not depend on epistemic uncertainties due to depth-phase picking and identification. The method consists of a modification of the cepstral analysis from Letort et al. and Bonner et al., which aims to detect surface reflected (pP, sP) waves in a signal at teleseismic distances (30°-90°) through the study of the spectral holes in the shape of the signal spectrum. The ability of our automatic method to improve depth estimations is shown by relocation of the recent moderate seismicity of the Guerrero subduction area (Mexico). We have therefore estimated the depth of 152 events using teleseismic data from the IRIS stations and arrays. One advantage of this method is that it can be applied for single stations (from IRIS) as well as for classical arrays. In the Guerrero area, our new cepstral analysis efficiently clusters event locations and provides an improved view of the geometry of the subduction. Moreover, we have also validated our method through relocation of the same events using the new International Seismological Centre (ISC)-locator algorithm, as well as comparing our cepstral depths with the available Harvard-Centroid Moment Tensor (CMT) solutions and the three available ground thrust (GT5) events (where lateral localization is assumed to be well constrained with uncertainty <5 km) for this area. These comparisons indicate an overestimation of focal depths in the ISC catalogue for deeper parts of the subduction, and they show a systematic bias between the estimated cepstral depths and the ISC-locator depths. Using information from the CMT catalogue relating to the predominant focal mechanism for this area, this bias can be explained as a misidentification of sP phases by pP phases, which shows the greater interest for the use of this new automatic cepstral analysis, as it is less sensitive to phase identification.

Radiated energy and the rupture process of the Denali fault earthquake sequence of 2002 from broadband teleseismic body waves

USGS Publications Warehouse

Choy, G.L.; Boatwright, J.

2004-01-01

Displacement, velocity, and velocity-squared records of P and SH body waves recorded at teleseismic distances are analyzed to determine the rupture characteristics of the Denali fault, Alaska, earthquake of 3 November 2002 (MW 7.9, Me 8.1). Three episodes of rupture can be identified from broadband (???0.1-5.0 Hz) waveforms. The Denali fault earthquake started as a MW 7.3 thrust event. Subsequent right-lateral strike-slip rupture events with centroid depths of 9 km occurred about 22 and 49 sec later. The teleseismic P waves are dominated by energy at intermediate frequencies (0.1-1 Hz) radiated by the thrust event, while the SH waves are dominated by energy at lower frequencies (0.05-0.2 Hz) radiated by the strike-slip events. The strike-slip events exhibit strong directivity in the teleseismic SH waves. Correcting the recorded P-wave acceleration spectra for the effect of the free surface yields an estimate of 2.8 ?? 1015 N m for the energy radiated by the thrust event. Correcting the recorded SH-wave acceleration spectra similarly yields an estimate of 3.3 ?? 10 16 N m for the energy radiated by the two strike-slip events. The average rupture velocity for the strike-slip rupture process is 1.1??-1.2??. The strike-slip events were located 90 and 188 km east of the epicenter. The rupture length over which significant or resolvable energy is radiated is, thus, far shorter than the 340-km fault length over which surface displacements were observed. However, the seismic moment released by these three events, 4 ?? 1020 N m, was approximately half the seismic moment determined from very low-frequency analyses of the earthquake. The difference in seismic moment can be reasonably attributed to slip on fault segments that did not radiate significant or coherent seismic energy. These results suggest that very large and great strike-slip earthquakes can generate stress pulses that rapidly produce substantial slip with negligible stress drop and little discernible radiated energy on fault segments distant from the initial point of nucleation. The existence of this energy-deficient rupture mode has important implications for the evaluation of the seismic hazard of very large strike-slip earthquakes.

Centroid-moment tensor inversions using high-rate GPS waveforms

NASA Astrophysics Data System (ADS)

O'Toole, Thomas B.; Valentine, Andrew P.; Woodhouse, John H.

2012-10-01

Displacement time-series recorded by Global Positioning System (GPS) receivers are a new type of near-field waveform observation of the seismic source. We have developed an inversion method which enables the recovery of an earthquake's mechanism and centroid coordinates from such data. Our approach is identical to that of the 'classical' Centroid-Moment Tensor (CMT) algorithm, except that we forward model the seismic wavefield using a method that is amenable to the efficient computation of synthetic GPS seismograms and their partial derivatives. We demonstrate the validity of our approach by calculating CMT solutions using 1 Hz GPS data for two recent earthquakes in Japan. These results are in good agreement with independently determined source models of these events. With wider availability of data, we envisage the CMT algorithm providing a tool for the systematic inversion of GPS waveforms, as is already the case for teleseismic data. Furthermore, this general inversion method could equally be applied to other near-field earthquake observations such as those made using accelerometers.

The Under-side of the Andes: Using Receiver Functions to Map the North Central Andean Subsurface

NASA Astrophysics Data System (ADS)

Ryan, J. C.; Beck, S. L.; Zandt, G.; Wagner, L. S.; Minaya, E.; Tavera, H.

2012-12-01

The Central Andean Uplift and Geodynamics of High Topography (CAUGHT) project is an interdisciplinary project to investigate connections between lithospheric removal, crustal shortening and surface uplift in the northern Bolivia and southern Peru region of the South American Andean orogen. The central Andes are defined by six major tectonomorphic provinces; the forearc, the volcanically active Western Cordillera (WC, ~6 km elevation), the internally drained Altiplano (~4 km elevation), an inactive fold and thrust belt in the Eastern Cordillera (EC, ~6 km elevation), a lower elevation active fold and thrust belt in the Subandean (SA) zone and the Beni, a foreland basin. Forty seismic stations installed for the CAUGHT project were deployed between 13° and 18° S latitude, covering the transition zone where the Altiplano region pinches out in southern Peru, in an effort to better constrain the changing character of the crust and mantle lithosphere. Geologic studies across the northern Bolivian portion of the eastern Andean margin (15-17° S) have documented a total of 275 km of upper crustal shortening (McQuarrie et al, Tectonics, v27, 2008), which may be associated with crustal thickening and/or the removal of lithospheric material as a thickened lithosphere root becomes unstable. For this receiver function (converted wave) study, we have little coverage in the forearc and foreland, ~75 km spacing in most of the array, and a relatively dense ~20 km spaced profile along the Charaña-La Paz-Yucumo transect, the eastern portion of which is nearly coincident with the balanced cross-section of McQuarrie et al. (2008). Using the first year of available data, more than 1200 receiver functions have been calculated using an iterative deconvolution method, and stacked using the common conversion point (CCP) method, along profiles parallel to and nearly coincident to those used for the geologic shortening estimates. We identified arrivals for the Moho and generated a 3D map of crustal thickness underneath the array that reveals distinct characteristics for the 4 major tectonomorphic provinces. The SA crust thickens westward from 40 km adjacent to the foreland to 50 km adjacent to the EC. The crust beneath the EC varies between 50-65 km, generally thickening westward. The crustal thickness beneath the Altiplano is variable along strike between 60-70 km, with a rapid change within the southern part of the eastern Altiplano with a northerly strike crossing into the EC east of Lake Titicaca. The crust under the WC is equally variable between 60-70 km, with numerous intracrustal interfaces. Comparisons with results from teleseismic tomography (Scire et al., this mtg.) and ambient noise tomography (Ward et al., this mtg.) show that this rapid Moho change is associated with changes in lower crustal and upper mantle velocities, suggesting it is an important deep-seated structure. We consider two possible interpretations of this structure; underthrusting of the Brazilian cratonic crust or a delamination structure. With a second year of data and enhanced processing we expect to improve the resolution of our receiver function study.

Global search of triggered non-volcanic tremor

NASA Astrophysics Data System (ADS)

Chao, Tzu-Kai Kevin

Deep non-volcanic tremor is a newly discovered seismic phenomenon with low amplitude, long duration, and no clear P- and S-waves as compared with regular earthquake. Tremor has been observed at many major plate-boundary faults, providing new information about fault slip behaviors below the seismogenic zone. While tremor mostly occurs spontaneously (ambient tremor) or during episodic slow-slip events (SSEs), sometimes tremor can also be triggered during teleseismic waves of distance earthquakes, which is known as "triggered tremor". The primary focus of my Ph.D. work is to understand the physical mechanisms and necessary conditions of triggered tremor by systematic investigations in different tectonic regions. In the first chapter of my dissertation, I conduct a systematic survey of triggered tremor beneath the Central Range (CR) in Taiwan for 45 teleseismic earthquakes from 1998 to 2009 with Mw ≥ 7.5. Triggered tremors are visually identified as bursts of high-frequency (2-8 Hz), non-impulsive, and long-duration seismic energy that are coherent among many seismic stations and modulated by the teleseismic surface waves. A total of 9 teleseismic earthquakes has triggered clear tremor in Taiwan. The peak ground velocity (PGV) of teleseismic surface waves is the most important factor in determining tremor triggering potential, with an apparent threshold of ˜0.1 cm/s, or 7-8 kPa. However, such threshold is partially controlled by the background noise level, preventing triggered tremor with weaker amplitude from being observed. In addition, I find a positive correlation between the PGV and the triggered tremor amplitude, which is consistent with the prediction of the 'clock-advance' model. This suggests that triggered tremor can be considered as a sped-up occurrence of ambient tremor under fast loading from the passing surface waves. Finally, the incident angles of surface waves also play an important rule in controlling the tremor triggering potential. The next chapter focuses on a systematic comparison of triggered tremor around the Calaveras Fault (CF) in northern California (NC), the Parkfield-Cholame section of the San Andreas Fault (SAF) in central California (CC), and the San Jacinto Fault (SJF) in southern California (SC). Out of 42 large (Mw ≥7.5) earthquakes between 2001 and 2010, only the 2002 Mw 7.9 Denali fault earthquake triggered clear tremor in NC and SC. In comparison, abundant triggered and ambient tremor has been observed in CC. Further analysis reveal that the lack of triggered tremor observations in SC and NC is not simply a consequence of their different background noise levels as compared to CC, but rather reflects different background tremor rates in these regions. In the final chapter, I systematically search for triggered tremor following the 2011 Mw9.0 Tohoku-Oki earthquake in the regions where ambient or triggered tremor has been found before. The main purpose is to check whether triggered tremor is observed in regions when certain conditions (e.g., surface wave amplitudes) are met. Triggered tremor is observed in southwest Japan, Taiwan, the Aleutian Arc, south-central Alaska, northern Vancouver Island, the Parkfield-Cholame section of the SAF in CC and the SJF in SC, and the North Island of New Zealand. Such a widespread triggering of tremor is not too surprising because of the large amplitude surface waves (minimum peak value of ˜0.1 cm/s) and the associated dynamic stresses (at least ˜7-8 kPa), which is one of the most important factors in controlling the triggering threshold. The triggered tremor in different region is located close to or nearby the ambient tremor active area. In addition, the amplitudes of triggered tremor have positive correlations with the amplitudes of teleseismic surface waves among many regions. Moreover, both Love and Rayleigh waves participate in triggering tremor in different regions, and their triggering potential is somewhat controlled by the incident angles. In summary, systematically surveys of triggered tremor in different tectonic regions reveal that triggered tremor shares similar physical mechanism (shear failure on the fault interface) as ambient tremor but with different loading conditions. The amplitude of the teleseismic surface wave is one of the most important factors in controlling the tremor triggering threshold. In addition, the frequency contents and incident angles of the triggering waves, and local fault geometry and ambient conditions also play certain roles in determining the triggering potential. On the other hand, the background noise level and seismic network coverage and station quality also could affect the apparent triggering threshold. (Abstract shortened by UMI.).

Optimal filter parameters for low SNR seismograms as a function of station and event location

NASA Astrophysics Data System (ADS)

Leach, Richard R.; Dowla, Farid U.; Schultz, Craig A.

1999-06-01

Global seismic monitoring requires deployment of seismic sensors worldwide, in many areas that have not been studied or have few useable recordings. Using events with lower signal-to-noise ratios (SNR) would increase the amount of data from these regions. Lower SNR events can add significant numbers to data sets, but recordings of these events must be carefully filtered. For a given region, conventional methods of filter selection can be quite subjective and may require intensive analysis of many events. To reduce this laborious process, we have developed an automated method to provide optimal filters for low SNR regional or teleseismic events. As seismic signals are often localized in frequency and time with distinct time-frequency characteristics, our method is based on the decomposition of a time series into a set of subsignals, each representing a band with f/Δ f constant (constant Q). The SNR is calculated on the pre-event noise and signal window. The band pass signals with high SNR are used to indicate the cutoff filter limits for the optimized filter. Results indicate a significant improvement in SNR, particularly for low SNR events. The method provides an optimum filter which can be immediately applied to unknown regions. The filtered signals are used to map the seismic frequency response of a region and may provide improvements in travel-time picking, azimuth estimation, regional characterization, and event detection. For example, when an event is detected and a preliminary location is determined, the computer could automatically select optimal filter bands for data from non-reporting stations. Results are shown for a set of low SNR events as well as 379 regional and teleseismic events recorded at stations ABKT, KIV, and ANTO in the Middle East.

A statistical kinematic source inversion approach based on the QUESO library for uncertainty quantification and prediction

NASA Astrophysics Data System (ADS)

Zielke, Olaf; McDougall, Damon; Mai, Martin; Babuska, Ivo

2014-05-01

Seismic, often augmented with geodetic data, are frequently used to invert for the spatio-temporal evolution of slip along a rupture plane. The resulting images of the slip evolution for a single event, inferred by different research teams, often vary distinctly, depending on the adopted inversion approach and rupture model parameterization. This observation raises the question, which of the provided kinematic source inversion solutions is most reliable and most robust, and — more generally — how accurate are fault parameterization and solution predictions? These issues are not included in "standard" source inversion approaches. Here, we present a statistical inversion approach to constrain kinematic rupture parameters from teleseismic body waves. The approach is based a) on a forward-modeling scheme that computes synthetic (body-)waves for a given kinematic rupture model, and b) on the QUESO (Quantification of Uncertainty for Estimation, Simulation, and Optimization) library that uses MCMC algorithms and Bayes theorem for sample selection. We present Bayesian inversions for rupture parameters in synthetic earthquakes (i.e. for which the exact rupture history is known) in an attempt to identify the cross-over at which further model discretization (spatial and temporal resolution of the parameter space) is no longer attributed to a decreasing misfit. Identification of this cross-over is of importance as it reveals the resolution power of the studied data set (i.e. teleseismic body waves), enabling one to constrain kinematic earthquake rupture histories of real earthquakes at a resolution that is supported by data. In addition, the Bayesian approach allows for mapping complete posterior probability density functions of the desired kinematic source parameters, thus enabling us to rigorously assess the uncertainties in earthquake source inversions.

Seismological analyses of the 2010 March 11, Pichilemu, Chile Mw 7.0 and Mw 6.9 coastal intraplate earthquakes

USGS Publications Warehouse

Ruiz, Javier A.; Hayes, Gavin P.; Carrizo, Daniel; Kanamori, Hiroo; Socquet, Anne; Comte, Diana

2014-01-01

On 2010 March 11, a sequence of large, shallow continental crust earthquakes shook central Chile. Two normal faulting events with magnitudes around Mw 7.0 and Mw 6.9 occurred just 15 min apart, located near the town of Pichilemu. These kinds of large intraplate, inland crustal earthquakes are rare above the Chilean subduction zone, and it is important to better understand their relationship with the 2010 February 27, Mw 8.8, Maule earthquake, which ruptured the adjacent megathrust plate boundary. We present a broad seismological analysis of these earthquakes by using both teleseismic and regional data. We compute seismic moment tensors for both events via a W-phase inversion, and test sensitivities to various inversion parameters in order to assess the stability of the solutions. The first event, at 14 hr 39 min GMT, is well constrained, displaying a fault plane with strike of N145°E, and a preferred dip angle of 55°SW, consistent with the trend of aftershock locations and other published results. Teleseismic finite-fault inversions for this event show a large slip zone along the southern part of the fault, correlating well with the reported spatial density of aftershocks. The second earthquake (14 hr 55 min GMT) appears to have ruptured a fault branching southward from the previous ruptured fault, within the hanging wall of the first event. Modelling seismograms at regional to teleseismic distances (Δ > 10°) is quite challenging because the observed seismic wave fields of both events overlap, increasing apparent complexity for the second earthquake. We perform both point- and extended-source inversions at regional and teleseismic distances, assessing model sensitivities resulting from variations in fault orientation, dimension, and hypocentre location. Results show that the focal mechanism for the second event features a steeper dip angle and a strike rotated slightly clockwise with respect to the previous event. This kind of geological fault configuration, with secondary rupture in the hanging wall of a large normal fault, is commonly observed in extensional geological regimes. We propose that both earthquakes form part of a typical normal fault diverging splay, where the secondary fault connects to the main fault at depth. To ascertain more information on the spatial and temporal details of slip for both events, we gathered near-fault seismological and geodetic data. Through forward modelling of near-fault synthetic seismograms we build a kinematic k−2 earthquake source model with spatially distributed slip on the fault that, to first-order, explains both coseismic static displacement GPS vectors and short-period seismometer observations at the closest sites. As expected, the results for the first event agree with the focal mechanism derived from teleseismic modelling, with a magnitude Mw 6.97. Similarly, near-fault modelling for the second event suggests rupture along a normal fault, Mw 6.90, characterized by a steeper dip angle (dip = 74°) and a strike clockwise rotated (strike = 155°) with respect to the previous event.

Nuclear Test Depth Determination with Synthetic Modelling: Global Analysis from PNEs to DPRK-2016

NASA Astrophysics Data System (ADS)

Rozhkov, Mikhail; Stachnik, Joshua; Baker, Ben; Epiphansky, Alexey; Bobrov, Dmitry

2016-04-01

Seismic event depth determination is critical for the event screening process at the International Data Center, CTBTO. A thorough determination of the event depth can be conducted mostly through additional special analysis because the IDC's Event Definition Criteria is based, in particular, on depth estimation uncertainties. This causes a large number of events in the Reviewed Event Bulletin to have depth constrained to the surface making the depth screening criterion not applicable. Further it may result in a heavier workload to manually distinguish between subsurface and deeper crustal events. Since the shape of the first few seconds of signal of very shallow events is very sensitive to the depth phases, cross correlation between observed and theoretic seismograms can provide a basis for the event depth estimation, and so an expansion to the screening process. We applied this approach mostly to events at teleseismic and partially regional distances. The approach was found efficient for the seismic event screening process, with certain caveats related mostly to poorly defined source and receiver crustal models which can shift the depth estimate. An adjustable teleseismic attenuation model (t*) for synthetics was used since this characteristic is not known for most of the rays we studied. We studied a wide set of historical records of nuclear explosions, including so called Peaceful Nuclear Explosions (PNE) with presumably known depths, and recent DPRK nuclear tests. The teleseismic synthetic approach is based on the stationary phase approximation with hudson96 program, and the regional modelling was done with the generalized ray technique by Vlastislav Cerveny modified to account for the complex source topography. The software prototype is designed to be used for the Expert Technical Analysis at the IDC. With this, the design effectively reuses the NDC-in-a-Box code and can be comfortably utilized by the NDC users. The package uses Geotool as a front-end for data retrieval and pre-processing. After the event database is compiled, the control is passed to the driver software, running the external processing and plotting toolboxes, which controls the final stage and produces the final result. The modules are mostly Python coded, C-coded (Raysynth3D complex topography regional synthetics) and FORTRAN coded synthetics from the CPS330 software package by Robert Herrmann of Saint Louis University. The extension of this single station depth determination method is under development and uses joint information from all stations participating in processing. It is based on simultaneous depth and moment tensor determination for both short and long period seismic phases. A novel approach recently developed for microseismic event location utilizing only phase waveform information was migrated to a global scale. It should provide faster computation as it does not require intensive synthetic modelling, and might benefit processing noisy signals. A consistent depth estimate for all recent nuclear tests was produced for the vast number of IMS stations (primary and auxiliary) used in processing.

Shallow and deep lithosphere slabs beneath the Dinarides from teleseismic tomography as the result of the Adriatic lithosphere downwelling

NASA Astrophysics Data System (ADS)

Šumanovac, Franjo; Markušić, Snježana; Engelsfeld, Tihomir; Jurković, Klaudia; Orešković, Jasna

2017-08-01

The study area covers the Dinarides and southwestern part of the Pannonian basin as the marginal zone between the Adriatic microplate (African plate) and the Pannonian tectonic segment (Eurasian plate). We created a three-dimensional seismic velocity model to 450 km depth using teleseismic tomography. Our travel-time dataset was collected by means of 40 seismic stations from the ORFEUS database and Croatian Seismological Survey database. A set of 90 teleseismic earthquakes were selected in the time range 2014-2015, and relative P-wave travel-time residuals were calculated. For the first time the seismic P-wave velocity model of a relatively high resolution on the entire Dinaridic mountain belt was obtained. Based on this model, a more reliable insight in the relations of the lithosphere plates has been achieved. We imaged a fast velocity anomaly extending underneath the entire Dinaridic mountain belt which indicates cold, rigid materials. The anomaly is steeply sloping towards the northeast and directly indicates the sinking of the Adriatic microplate underneath the Pannonian tectonic segment. In the Northern Dinarides the anomaly extends to the depth of 250 km, whereas in the Southern Dinarides it covers greater depths, up to 450 km. The shallow Adriatic slab extends along the External Dinarides, while the deep Adriatic slab extends beneath the Internal Dinarides and ophiolite zones in the area of central and southern Dinarides. Different slab depths are interpreted as the faster convergence of the plate in the southern Dinarides than in the northern, or the convergence of the plates had started in the southern part and systematically developed to the north.

Active backstop faults in the Mentawai region of Sumatra, Indonesia, revealed by teleseismic broadband waveform modeling

NASA Astrophysics Data System (ADS)

Wang, Xin; Bradley, Kyle Edward; Wei, Shengji; Wu, Wenbo

2018-02-01

Two earthquake sequences that affected the Mentawai islands offshore of central Sumatra in 2005 (Mw 6.9) and 2009 (Mw 6.7) have been highlighted as evidence for active backthrusting of the Sumatran accretionary wedge. However, the geometry of the activated fault planes is not well resolved due to large uncertainties in the locations of the mainshocks and aftershocks. We refine the locations and focal mechanisms of medium size events (Mw > 4.5) of these two earthquake sequences through broadband waveform modeling. In addition to modeling the depth-phases for accurate centroid depths, we use teleseismic surface wave cross-correlation to precisely relocate the relative horizontal locations of the earthquakes. The refined catalog shows that the 2005 and 2009 "backthrust" sequences in Mentawai region actually occurred on steeply (∼60 degrees) landward-dipping faults (Masilo Fault Zone) that intersect the Sunda megathrust beneath the deepest part of the forearc basin, contradicting previous studies that inferred slip on a shallowly seaward-dipping backthrust. Static slip inversion on the newly-proposed fault fits the coseismic GPS offsets for the 2009 mainshock equally well as previous studies, but with a slip distribution more consistent with the mainshock centroid depth (∼20 km) constrained from teleseismic waveform inversion. Rupture of such steeply dipping reverse faults within the forearc crust is rare along the Sumatra-Java margin. We interpret these earthquakes as 'unsticking' of the Sumatran accretionary wedge along a backstop fault separating imbricated material from the stronger Sunda lithosphere. Alternatively, the reverse faults may have originated as pre-Miocene normal faults of the extended continental crust of the western Sunda margin. Our waveform modeling approach can be used to further refine global earthquake catalogs in order to clarify the geometries of active faults.

The KRISP 90 seismic experiment-a technical review

USGS Publications Warehouse

Prodehl, C.; Mechie, J.; Achauer, U.; Keller, Gordon R.; Khan, M.A.; Mooney, W.D.; Gaciri, S.J.; Obel, J.D.

1994-01-01

On the basis of a preliminary experiment in 1985 (KRISP 85), a seismic refraction/wide-angle reflection survey and a teleseismic tomography experiment were jointly undertaken to study the lithospheric structure of the Kenya rift down to depths of greater than 200 km. This report serves as an introduction to a series of subsequent papers and will focus on the technical description of the seismic surveys of the main KRISP 90 effort. The seismic refraction/wide-angle reflection survey was carried out in a 4-week period in January and February 1990. It consisted of three profiles: one extending along the rift valley from Lake Turkana to Lake Magadi, one crossing the rift at Lake Baringo, and one located on the eastern flank of the rift proper. A total of 206 mobile vertical-component seismographs, with an average station interval of about 2 km, recorded the energy of underwater and borehole explosions to distances of up to about 550 km. During the teleseismic survey an array of 65 seismographs was deployed to record teleseismic, regional and local events for a period of about 7 months from October 1989 to April 1990. The elliptical array spanned the central portion of the rift, with Nakuru at its center, and covered an area about 300 ?? 200 km, with an average station spacing of 10-30 km. Major scientific goals of the project were to reveal the detailed crustal and upper-mantle structure under the Kenya rift, to study the relationship between deep crustal and mantle structure and the development of sedimentary basins and volcanic features within the rift, to understand the role of the Kenya rift within the Afro-Arabian rift system, and to answer fundamental questions such as the mode and mechanism of continental rifting. ?? 1994.

Teleseismic depth estimation of the 2015 Gorkha-Nepal aftershocks

NASA Astrophysics Data System (ADS)

Letort, Jean; Bollinger, Laurent; Lyon-Caen, Helene; Guilhem, Aurélie; Cano, Yoann; Baillard, Christian; Adhikari, Lok Bijaya

2016-12-01

The depth of 61 aftershocks of the 2015 April 25 Gorkha, Nepal earthquake, that occurred within the first 20 d following the main shock, is constrained using time delays between teleseismic P phases and depth phases (pP and sP). The detection and identification of these phases are automatically processed using the cepstral method developed by Letort et al., and are validated with computed radiation patterns from the most probable focal mechanisms. The events are found to be relatively shallow (13.1 ± 3.9 km). Because depth estimations could potentially be biased by the method, velocity model or selected data, we also evaluate the depth resolution of the events from local catalogues by extracting 138 events with assumed well-constrained depth estimations. Comparison between the teleseismic depths and the depths from local and regional catalogues helps decrease epistemic uncertainties, and shows that the seismicity is clustered in a narrow band between 10 and 15 km depth. Given the geometry and depth of the major tectonic structures, most aftershocks are probably located in the immediate vicinity of the Main Himalayan Thrust (MHT) shear zone. The mid-crustal ramp of the flat/ramp MHT system is not resolved indicating that its height is moderate (less than 5-10 km) in the trace of the sections that ruptured on April 25. However, the seismicity depth range widens and deepens through an adjacent section to the east, a region that failed on 2015 May 12 during an Mw 7.3 earthquake. This deeper seismicity could reflect a step-down of the basal detachment of the MHT, a lateral structural variation which probably acted as a barrier to the dynamic rupture propagation.

VP and VS structure of the Yellowstone hot spot from teleseismic tomography: Evidence for an upper mantle plume

USGS Publications Warehouse

Waite, Gregory P.; Smith, Robert B.; Allen, Richard M.

2006-01-01

The movement of the lithosphere over a stationary mantle magmatic source, often thought to be a mantle plume, explains key features of the 16 Ma Yellowstone–Snake River Plain volcanic system. However, the seismic signature of a Yellowstone plume has remained elusive because of the lack of adequate data. We employ new teleseismic P and S wave traveltime data to develop tomographic images of the Yellowstone hot spot upper mantle. The teleseismic data were recorded with two temporary seismograph arrays deployed in a 500 km by 600 km area centered on Yellowstone. Additional data from nearby regional seismic networks were incorporated into the data set. The VP and VS models reveal a strong low-velocity anomaly from ∼50 to 200 km directly beneath the Yellowstone caldera and eastern Snake River Plain, as has been imaged in previous studies. Peak anomalies are −2.3% for VP and −5.5% for VS. A weaker, anomaly with a velocity perturbation of up to −1.0% VP and −2.5% VS continues to at least 400 km depth. This anomaly dips 30° from vertical, west-northwest to a location beneath the northern Rocky Mountains. We interpret the low-velocity body as a plume of upwelling hot, and possibly wet rock, from the mantle transition zone that promotes small-scale convection in the upper ∼200 km of the mantle and long-lived volcanism. A high-velocity anomaly, 1.2%VP and 1.9% VS, is located at ∼100 to 250 km depth southeast of Yellowstone and may represent a downwelling of colder, denser mantle material.

Teleseismic traveltime tomography of Jeju Island, South Korea

NASA Astrophysics Data System (ADS)

Song, J.; Rhie, J.; Kim, S.; Lee, S. H.

2017-12-01

Jeju Island is the largest volcanic island in South Korea, which lies off the south coast of the Korean Peninsula. It is well known that the volcanism started in the Early Pleistocene (c. 1.7 Ma) and subsequent eruptions during Late Pleistocene to Holocene formed the bulk of the island with a number of small cones. However, the origin of magma and detailed mechanism of eruptions have not been fully understood yet. To address these issues, we applied teleseismic travel time tomography to image the underlying crust and upper mantle of the island. We carefully analyzed 185 teleseismic earthquakes (5.5 < Mw < 7.9) that occurred between Oct. 2013 and Nov. 2015. Broadband waveforms recorded by 23 seismic stations covering the whole island were used to measure travel time residuals of P and S waves using semi-automated adaptive stacking technique. The residuals are mapped as three-dimensional perturbations of velocity using iterative non-linear tomographic process with a subspace inversion technique and the fast marching method for grid based eikonal solver. We used AK135 global reference model as a starting velocity model for tomography inversion. The resulting P wave tomographic images exhibit relatively low velocity anomaly in the upper mantle, which extends to depths of nearly 60 km under the summit of the island, Mt. Halla. The anomaly is likely related to a relatively high-temperature magmatic body, which might be associated to the volcanism lasted until late Cenozoic. To better constrain possible compositions of the anomalies and the existence of melt fractions, we will continue to examine perturbation of Vp/Vs ratios and discuss the evolution of the volcanic island.

Pipeline Processing with an Iterative, Context-based Detection Model

DTIC Science & Technology

2014-04-19

stripping the incoming data stream of repeating and irrelevant signals prior to running primary detectors , adaptive beamforming and matched field processing...framework, pattern detectors , correlation detectors , subspace detectors , matched field detectors , nuclear explosion monitoring 16. SECURITY CLASSIFICATION...10 5. Teleseismic paths from earthquakes in

Constructing a Teleseismic Tomographic Image of Taiwan using BATS Recordings

NASA Astrophysics Data System (ADS)

Krajewski, J.; Roecker, S.

2005-12-01

Taiwan is an evolving arc-continent collision located at a complicated part of the plate boundary between the Eurasian and Philippine Sea plates. To better understand the role of the upper mantle in the dynamics of this collision, we reviewed 4 years of data from the Broadband Array in Taiwan for Seismology (BATS) in Taiwan to construct a teleseismic dataset for tomographic imaging of the subsurface of the island. From an initial selection of approximately 300 events, we used waveform correlation to generate a dataset of 4500 relative arrival times. To calculate accurate travel times in three dimensional wavespeed models over the large lateral distances in our model (~800 km), we solve the eikonal equation directly in a spherical coordinate system. We reduce the influence of smearing of crustal heterogeneity into the deeper mantle, we fix the upper 30 km to a previously determined P wavespeed model for the region. Initial resolution tests suggest a spatial limit on the order of 40 km.

Inversion for slip distribution using teleseismic P waveforms: North Palm Springs, Borah Peak, and Michoacan earthquakes

USGS Publications Warehouse

Mendoza, C.; Hartzell, S.H.

1988-01-01

We have inverted the teleseismic P waveforms recorded by stations of the Global Digital Seismograph Network for the 8 July 1986 North Palm Springs, California, the 28 October 1983 Borah Peak, Idaho, and the 19 September 1985 Michoacan, Mexico, earthquakes to recover the distribution of slip on each of the faults using a point-by-point inversion method with smoothing and positivity constraints. Results of the inversion indicate that the Global digital Seismograph Network data are useful for deriving fault dislocation models for moderate to large events. However, a wide range of frequencies is necessary to infer the distribution of slip on the earthquake fault. Although the long-period waveforms define the size (dimensions and seismic moment) of the earthquake, data at shorter period provide additional constraints on the variation of slip on the fault. Dislocation models obtained for all three earthquakes are consistent with a heterogeneous rupture process where failure is controlled largely by the size and location of high-strength asperity regions. -from Authors

Upper mantle anisotropy from teleseismic SKS splitting beneath Lützow-Holm Bay Region, East Antarctica

USGS Publications Warehouse

Usui, Y.; Kanao, M.; Kubo, A.; Hiramatsu, Y.; Negishi, H.

2007-01-01

Investigations of SKS wave splitting of teleseismic events from digital seismographs recorded at eight stations around the Lützow-Holm Bay Region have lead to understanding the evolution of the Antarctic Plate. The observed delay times of SKS splitting are up to 1.3 s, which are generally equal to the global average. A two-layer model reveals that the lower layer anisotropy is caused by the recent asthenospheric flow, as compared with the Absolute Plate Motion by the HS3-NUVEL1 model. The upper layer anisotropy corresponds well to polarization of NE–SW convergence direction between East and West Gondwana in Pan-African age. We suggest that the upper layer anisotropy was formed during Pan-African orogeny and was possibly influenced by the preexisting structure during Gondwana break-up. The origin of anisotropy is the Lattice Preferred Orientation of olivine which was caused by both paleo-tectonic events and the recent asthenospheric flow.

Imaging the 2016 Mw 7.8 Kaikoura, New Zealand, earthquake with teleseismic P waves: A cascading rupture across multiple faults

NASA Astrophysics Data System (ADS)

Zhang, Hao; Koper, Keith D.; Pankow, Kristine; Ge, Zengxi

2017-05-01

The 13 November 2016 Mw 7.8 Kaikoura, New Zealand, earthquake was investigated using teleseismic P waves. Backprojection of high-frequency P waves from two regional arrays shows unilateral rupture of at least two southwest-northeast striking faults with an average rupture speed of 1.4-1.6 km/s and total duration of 100 s. Guided by these backprojection results, 33 globally distributed low-frequency P waves were inverted for a finite fault model (FFM) of slip. The FFM showed evidence of several subevents; however, it lacked significant moment release near the epicenter, where a large burst of high-frequency energy was observed. A local strong-motion network recorded strong shaking near the epicenter; hence, for this earthquake the distribution of backprojection energy is superior to the FFM as a guide of strong shaking. For future large earthquakes that occur in regions without strong-motion networks, initial shaking estimates could benefit from backprojection constraints.

Slip history of the 2003 San Simeon earthquake constrained by combining 1-Hz GPS, strong motion, and teleseismic data

USGS Publications Warehouse

Ji, C.; Larson, K.M.; Tan, Y.; Hudnut, K.W.; Choi, K.

2004-01-01

The slip history of the 2003 San Simeon earthquake is constrained by combining strong motion and teleseismic data, along with GPS static offsets and 1-Hz GPS observations. Comparisons of a 1-Hz GPS time series and a co-located strong motion data are in very good agreement, demonstrating a new application of GPS. The inversion results for this event indicate that the rupture initiated at a depth of 8.5 km and propagated southeastwards with a speed ???3.0 km/sec, with rake vectors forming a fan structure around the hypocenter. We obtained a peak slip of 2.8 m and total seismic moment of 6.2 ?? 1018 Nm. We interpret the slip distribution as indicating that the hanging wall rotates relative to the footwall around the hypocenter, in a sense that appears consistent with the shape of the mapped fault trace. Copyright 2004 by the American Geophysical Union.

Earthquake swarms on the Mid-Atlantic Ridge - Products of magmatism or extensional tectonics?

NASA Technical Reports Server (NTRS)

Bergman, Eric A.; Solomon, Sean C.

1990-01-01

The spatial and temporal patterns and other characteristics of earthquakes in 34 earthquake swarms on the Mid-Atlantic Ridge were compared with those of well-studied earthquake swarms which accompany terrestrial volcanic eruptions, to test the assumption that the teleseismically observed earthquake swarms along mid-ocean ridges are indicators of volcanism. Improved resolution of these patterns for the mid-ocean ridge events was achieved by a multiple-event relocation technique. It was found that the teleseismically located earthquake swarms on the mid-ocean ridge system have few features in common with swarms directly associated with active magmatism in terrestrial volcanic rift zones such as Hawaii and Iceland. While the possibility that some of the mid-ocean earthquake swarms might be directly associated with a current episode of eruptive activity on the Mid-Atlantic Ridge cannot be excluded, none of the 34 swarms studied in this work was found to be a conspicuously attractive candidate for such a role.

A teleseismic study of the 2002 Denali fault, Alaska, earthquake and implications for rapid strong-motion estimation

USGS Publications Warehouse

Ji, C.; Helmberger, D.V.; Wald, D.J.

2004-01-01

Slip histories for the 2002 M7.9 Denali fault, Alaska, earthquake are derived rapidly from global teleseismic waveform data. In phases, three models improve matching waveform data and recovery of rupture details. In the first model (Phase I), analogous to an automated solution, a simple fault plane is fixed based on the preliminary Harvard Centroid Moment Tensor mechanism and the epicenter provided by the Preliminary Determination of Epicenters. This model is then updated (Phase II) by implementing a more realistic fault geometry inferred from Digital Elevation Model topography and further (Phase III) by using the calibrated P-wave and SH-wave arrival times derived from modeling of the nearby 2002 M6.7 Nenana Mountain earthquake. These models are used to predict the peak ground velocity and the shaking intensity field in the fault vicinity. The procedure to estimate local strong motion could be automated and used for global real-time earthquake shaking and damage assessment. ?? 2004, Earthquake Engineering Research Institute.

The 2017 North Korea M6 seismic sequence: moment tensor, source time function, and aftershocks

NASA Astrophysics Data System (ADS)

Ni, S.; Zhan, Z.; Chu, R.; He, X.

2017-12-01

On September 3rd, 2017, an M6 seismic event occurred in North Korea, with location near previous nuclear test sites. The event features strong P waves and short period Rayleigh waves are observed in contrast to weak S waves, suggesting mostly explosion mechanism. We performed joint inversion for moment tensor and depth with both local and teleseismic waveforms, and find that the event is shallow with mostly isotropic yet substantial non-isotropic components. Deconvolution of seismic waveforms of this event with respect to previous nuclear test events shows clues of complexity in source time function. The event is followed by smaller earthquakes, as early as 8.5 minutes and lasted at least to October. The later events occurred in a compact region, and show clear S waves, suggesting double couple focal mechanism. Via analyzing Rayleigh wave spectrum, these smaller events are found to be shallow. Relative locations, difference in waveforms of the events are used to infer their possible links and generation mechanism.

Preliminary analysis of seismic anisotropy in the uppermost mantle beneath NW Pacific reveled by the Normal Oceanic Mantle project

NASA Astrophysics Data System (ADS)

Takeo, A.; Isse, T.; Nishida, K.; Kawakatsu, H.; Shiobara, H.; Sugioka, H.; Ito, A.; Utada, H.

2013-12-01

Seismic structure including anisotropy in the oceanic uppermost mantle is essential for understanding deformation related to plate tectonics. Recent reports of a sharp discontinuity between the high velocity LID and the low velocity zone (LVZ) especially emphasize the importance of observation in oceanic basins apart from ridges and hotspots for determining the structure including LID and LVZ. In this study, we analyzed records of four broadband ocean bottom seismometers (BBOBSs) deployed in the northwest of Shatsky Rise by the pilot observation of the Normal Oceanic Mantle (NOMan) project in 2010-2011. We first measured average phase velocities of surface waves at periods of 5-30 s by the ambient-noise cross correlation method. Based on the method of Takeo et al. (in prep. GJI), we analyzed fundamental- and first higher- mode Rayleigh waves and fundamental-mode Love wave simultaneously by waveform fitting after the correction of clock delay. At periods of 25-100 s, we measured phase velocities of fundamental-mode surface waves by the array analysis of teleseismic waveforms. We then determined one-dimensional radially anisotropic structure beneath the array by the method of Takeo et al. (2013, JGR). The obtained structure shows transition from LID to LVZ at depths of 50-80km, which is marginally consistent with the depth of ~80 km estimated by a receiver function analysis at WP2 station situated at east of the studies area (Kawakatsu et al., 2009). The velocity gradient in the LID is almost zero and inconsistent with the simple cooling model of homogeneous oceanic plate. The average intensity of S-wave radial anisotropy at depths of ~10-220 km is ~3% (VSH>VSV). We further estimated S-wave azimuthal anisotropy at depths of ~30-100 km by analyzing teleseismic fundamental-mode Rayleigh waves at periods of 25-50 s. The intensity of anisotropy is 2-3%. The fastest direction is about N35W, close to that of Sn-wave velocity around WP2 station obtained by a refraction survey (Shinohara et al., 2008), and indicates the presence of past mantle flow almost perpendicular to the ancient mid ocean ridge or the presence of current mantle flow parallel to the plate motion at depths of 30-100 km. We will further analyze new records after the recovery of 13 BBOBSs in August 2013 and will present more detailed structure around Shatsky Rise. BBOBS stations of pilot observation of NOMan project (white crosses), WP2 station (circle), isochrons (white lines). Black bars show the fastest directions of Rayleigh wave at periods of 25-50 s and the fastest direction of Sn-wave velocity (Shinohara et al. 2008).

The TIPTEQ seismological network in Southern Chile - Studying the Seismogenic Coupling Zone

NASA Astrophysics Data System (ADS)

Haberland, C.; Rietbrock, A.; Lange, D.; Bataille, K.; Hofmann, S.; Dahm, T.; Scherbaum, F.; Tilman, F.; Hermosilla, G.; Group, T. S.

2005-12-01

Subduction zones generate the world's largest and most destructive earthquakes. Understanding the factors leading to these earthquakes in the coupling zone of convergent margins and their interrelation with surface deformation are the main aims of the international and interdisciplinary research initiative TIPTEQ (From The Incoming Plate To megaThrust EarthQuake Processes) which is financed by the German Ministry for Education and Research (BMBF). These aims shall be achieved by obtaining high resolution images of the seismogenic zone and the forearc structure, which will form the base for identifying the processes involved. Our studies focus spatially on the nucleation zone of the Mw=9.5 1960 Chile earthquake, the worldwide largest instrumentally ever recorded earthquake. Within this project a large temporary seismological network is installed in southern Chile since Nov. 2004, covering the forearc between 37° and 39°S. It consists of 120 digitally recording and continuously running seismic stations equipped with short period sensors. The network covers the forearc between 37° and 39°S. The onshore network is complemented by 10 ocean bottom seismometers/hydrophones (OBS/OBH), and the stations (except for 20 stations which will operate until October 2005) were in operation until July 2005. The network is characterized by very short station spacings in the centre which will assure an increased quantity of P and S phase onset times and which will achieve the observation of the whole wavefield (coherent waveforms). A second network of 20 onshore and 20 offshore stations is installed at and around Chiloe Island for a one year period. Until now we collected about 1.2 TByte of data. First steps of the data processing are the event detection, the onset time picking, and the localisation of the (local) earthquakes (catalog). Later steps include the determination of the velocity and attenuation structure (tomography), the analysis of the stress field by moment tensor inversion, the analysis of later phases such as guided waves and scattered/converted/reflected arrivals, the analysis of teleseismic recordings (receiver functions, anisotropy), and many more. The results will be jointly interpreted with the findings of controlled source seismic studies, magnetotelluric measurements, and surface geological studies. Each day 2 to 3 local earthquakes and several teleseismic events were recorded. We present first results including data examples, seismicity distribution, and first 1D velocity models of this ongoing research project. Most of the crustal seismicity in the northern network is concentrated in two clusters close to the coast line; Benioff seismicity can be found down to 100 km depth. Between 41.5° and 43.5° S (at and around Chiloe Island) we also found several earthquakes in the continental (forearc) crust and in the downgoing slab.

P-wave and S-wave traveltime residuals in Caledonian and adjacent units of Northern Europe and Greenland

NASA Astrophysics Data System (ADS)

Hejrani, Babak; Balling, Niels; Holm Jacobsen, Bo; Kind, Rainer; Tilmann, Frederik; England, Richard; Bom Nielsen, Søren

2014-05-01

This work combines P-wave and S-wave travel time residuals from in total 477 temporary and 56 permanent stations deployed across Caledonian and adjacent units in Northern Europe and Greenland (Tor, Gregersen et al. 2002; SVEKALAPKO, Sandoval et al., 2003; CALAS, Medhus et al, 2012a; MAGNUS, Weidle et al. 2010; SCANLIPS south, England & Ebbing 2012; SCANLIPS north, Hejrani et al. 2012; JULS Hejrani et al. 2013; plus permanent stations in the region). We picked data from 2002 to 2012 (1221 events) using a cross correlation technique on all waveforms recorded for each event. In this way we achieve maximum consistency of relative residuals over the whole region (Medhus et al. 2012b). On the European side 18362 P-wave travel time residuals was delivered. In East Greenland 1735 P-wave residuals were recovered at the Central Fjord array (13 stations) and 2294 residuals from the sparse GLISN-array (23 stations). Likewise, we picked a total of 6034 residuals of the SV phase (For the Tor and SVEKALAPKO projects we used data from Amaru et al. 2008). Relative residuals within the region are mainly due to sub-crustal uppermost mantle velocity anomalies. A dominant subvertical boundary was detected by Medhus et al. (2012), running along the Tornquist zone, east of the Oslo Graben and crossing under high topography of the southern Scandes. We delineated this boundary in more detail, tracking it towards the Atlantic margin north of Trondheim. Further north (Scanlips north), a similar subvertical upper mantle boundary seems to be present close to the coast, coinciding with the edge of the stretched crust. The North German Caledonides were probed by the new JULS (JUtland Lower Saxony) profile which closes the gap between Tor and CALAS arrays. Mantle structure found by the Tor project was confirmed, and modelling was extended to the eastern edge of the North Sea. References: Amaru, M. L., Spakman, W., Villaseñor, A., Sandoval, S., Kissling, E., 2008, A new absolute arrival time data set for Europe.Geophysical Journal International, 173, 465-472. England, R. W.; Ebbing, J., 2012, Crustal structure of central Norway and Sweden from integrated modelling of teleseismic receiver functions and the gravity anomaly.GEOPHYSICAL JOURNAL INTERNATIONAL, 191, 1-11. Gregersen S., Voss P., TOR Working Group, 2002. Summary of project TOR: delineation of a stepwise, sharp, deep lithosphere transition across Germany-Denmark-Sweden, Tectonophysics, 360, 61-73. Hejrani, B., Jacobsen, B. H., Balling,N. and England, R. W.. 2012, A seismic tomography study of lithospheric structure under the Norwegian Caledonides.Geophysical Research Abstracts, 14, 4334. Hejrani, B.; Jacobsen, B.H.; Balling, N.;Tilmann, F.; Kind, R., 2013, Upper-mantle velocity structure beneathJutland, Denmark and northern Germany:Preliminary results. Joint Assembly Gothenburg Abstract S401S2.01, Medhus, A. B., Balling, N., Jacobsen, B. H., Weidle, C., England, R. W., Kind, R., Thybo, H., Voss, P. (2012a): Upper-mantle structure beneath the Southern Scandes Mountains and the Northern Tornquist Zone revealed by P-wave traveltime tomography. Geophysical Journal International, 189, 3, 1315-1334. Medhus, Jacobsen, B. H.,A. B., Balling, N., 2012b, Bias Problems in Existing Teleseismic Travel Time Databases: Ignore or Repair? Seismological Research Letters, 83, 1030-1037. Sandoval, S., Kissling, E. &Ansorge, J., 2003.High-resolution body wave tomography beneath the SVEKALAPKO array: I. A priori three-dimensional crustal model and associated traveltime effects on teleseismic wave fronts, Geophys. J. Int., 153, 75-87. Weidle, C., Maupin, V., Ritter, J.,Kværna, T., Schweitzer J., Balling, N.,Thybo, H.,Faleide, J. I.,and,Wenzel, F., 2010, MAGNUS-A Seismological Broadband Experiment to Resolve Crustal and Upper Mantle Structure beneath the Southern Scandes Mountains in Norway. SEISMOLOGICAL RESEARCH LETTERS, 81, 76-84.

Seismological Structure of the 1.8Ga Trans-Hudson Orogen of North America and its affinity to present-day Tibet

NASA Astrophysics Data System (ADS)

Gilligan, A.; Bastow, I. D.; Darbyshire, F. A.

2015-12-01

How tectonic processes operated and changed through the Precambrian is debated: what was the nature and scale of orogenic events and were they different on the younger, hotter, more ductile Earth? The geology of northern Hudson Bay records the Paleoproterozoic collision between the Western Churchill and Superior plates: the 1.8Ga Trans-Hudson Orogeny (THO) and is thus an ideal study locale to address this issue. It has been suggested, primarily on the strength of traditional field geology, that the THO was comparable in scale and style to the present-day Himalayan-Karakoram-Tibet Orogen (HKTO). However, understanding of the deep crustal architecture of the THO, and how it compares to the evolving HKTO is presently lacking. Through joint inversion of teleseismic receiver functions and surface wave data, we obtain new Moho depth estimates and shear velocity models for the crust and upper mantle. Archean crust in the Rae, Hearne and Churchill domains is thin and structurally simple, with a sharp Moho; upper crustal wavespeed variations are readily attributed to post-formation events. However, the Paleoproterozoic Quebec-Baffin segment of the THO has a deeper Moho and more complex crustal structure. Our observations are strikingly similar to recent models, computed using the same methods, of the HKTO lithosphere, where deformation also extends >400km beyond the collision front. On the strength of Moho character, present-day crustal thickness, and metamorphic grade, we thus propose that southern Baffin experienced uplift of a similar magnitude and spatial extent to the Himalayas during the Paleoproterozoic Trans-Hudson Orogeny.

Structure of the Cascadia Subduction Zone Imaged Using Surface Wave Tomography

NASA Astrophysics Data System (ADS)

Schaeffer, A. J.; Audet, P.

2017-12-01

Studies of the complete structure of the Cascadia subduction zone from the ridge to the arc have historically been limited by the lack of offshore ocean bottom seismograph (OBS) infrastructure. On land, numerous dense seismic deployments have illuminated detailed structures and dynamics associated with the interaction between the subducting oceanic plate and the overriding continental plate, including cycling of fluids, serpentinization of the overlying forearc mantle wedge, and the location of the upper surface of the Juan de Fuca plate as it subducts beneath the Pacific Northwest. In the last half-decade, the Cascadia Initiative (CI), along with Neptune (ONC) and several other OBS initiatives, have instrumented both the continental shelf and abyssal plains off shore of the Cascadia subduction zone, facilitating the construction of a complete picture of the subduction zone from ridge to trench and volcanic arc. In this study, we present a preliminary azimuthally anisotropic surface-wave phase-velocity based model of the complete system, capturing both the young, unaltered Juan de Fuca plate from the ridge, to its alteration as it enters the subduction zone, in addition to the overlying continent. This model is constructed from a combination of ambient noise cross-correlations and teleseismic two station interferometry, and combines together concurrently running offshore OBS and onshore stations. We furthermore perform a number of representative 1D depth inversions for shear velocity to categorize the pristine oceanic, subducted oceanic, and continental crust and lithospheric structure. In the future the dispersion dataset will be jointly inverted with receiver functions to constrain a 3D shear-velocity model of the complete region.

Thin Crust and High Crustal Vp/Vs beneath the Central Armenia Plateau of the Lesser Caucasus

NASA Astrophysics Data System (ADS)

Tseng, T. L.; Lin, C. M.; Huang, B. S.; Karakhanyan, A.

2017-12-01

Armenia volcanic highland is part of the Lesser Caucasus directly connected with the East Anatolian Plateau to the west and Iranian Plateau to the east. Abundant Quaternary volcanoes in Armenia are the youngest among those associated with post-collision of Arabia-Eurasian since Miocene ( 11 Ma). In this study, teleseismic receiver functions were analyzed from a temporary array to constrain the crustal structures under Armenia and the vicinity. The results show that the Moho depth is shallowest beneath central Armenia where the estimated crustal thickness is 32 km with high averaged crustal Vp/Vs of 1.8-2.0 using H-κ technique. The high crustal Vp/Vs is distributed in a wider area but thin crust is confined more locally around stratovolcano Aragats, whose last eruption was about 0.5 Ma. High crustal Vp/Vs value approaching to 2.1 is found near East of volcano Ghegam complex and NW of volcano Ararat with last dated ages of 0.5 and <0.1 Ma, respectively. Such high Vp/Vs (2.0) cannot be explained without high mafic content and the presence of partial melt in the crust. The 1-D velocity models inverted demonstrate that the partial melt is more likely in the low-velocity layer of the lower crust. To support the unusually thin crust in central Armenia, it requires additional thermal buoyancy in the uppermost mantle which is consistent with regionally low Pn velocity found in previous studies. We propose that the volcanism here is facilitated by the stretches of lithosphere.

The January 2001, El Salvador Earthquake: A Multi-data Analysis

NASA Astrophysics Data System (ADS)

Vallee, M.; Bouchon, M.; Schwartz, S. Y.

On January 13, 2001, a large normal intermediate depth event (Mw=7.7) occured 40 km away from the Salvadorian coast (Central America). We analysed this earthquake with different sets of seismic data. Because teleseismic waves are the only data which offer a good azimuthal coverage, we first built a kinematic source model with P, SH and surface waves provided by the IRIS,GEOSCOPE and NCEDC networks. P and SH waves were used through a theoretical Green function approach whereas surface waves were used through an Empirical Green Function (EGF) approach. The ambigu- ity between the 30-dipping plane (plunging toward Pacific Ocean) and the 60-degree dipping plane (plunging toward Central America) lead us to do a parallel analysis of the two possible planes. After having relocated the hypocentral depth to 54 km, we tried to retrieve the kinematic features of the rupture. We allowed variable rupture ve- locity (through a finite difference scheme) and variable slip and solved this inverse problem with a combination of the Neighborhood algorithm of Sambridge (1999) and the Simplex method. We found for both planes an updip and northwest rupture prop- agation yielding a centroid depth around 48km. The teleseismic data give a slight preferrence for the 60-dipping plane. In the second part of the study, we tested the two possible fault models with other seismological data, that are (1) regional broad- band data and (2) near-field accelerometers provided by Universidad Centroameri- cana (UCA). Regional data do not allow to discriminate between the two models but near-field data confirm that the fault plane is the steeper one plunging toward Central America. This event initiated at a depth of about 54km on the 60-dipping plane, and rupture propagated mostly updip and to the northwest, breaking a surface of approx- imately 30km*50km with an average slip of about 3.5 m. The large amount of slip occurs updip from the hypocenter near the plate interface. This is better explained by the bending of the subducting plate rather than by a complete decoupling of the slab.

Comparison of Orthogonal Transforms for Teleseismic Data

DTIC Science & Technology

1974-10-31

inverse transform Because the computations are done in-plaee( Y is both input and output arrays; X is a complex buffer array. The program generates...the forward transform the FFT is done first, then the array is phase-shifted; for the inverse transform the reverse procedure is followed. Each

Relationship between Near-Field and Teleseismic Observations of Seismic Source Parameters

DTIC Science & Technology

1978-07-01

For example, foreshocks and aftershocks of the Utah-Idaho border and Oroville, California sequences of 1975, as recorded at the Albuquerque SRO station...have been analyzed and compared; in both cases the principal foreshock exhibited the same mechanisms as the main shock, while the aftershocks are

ANZA Seismic Network- From Monitoring to Science

NASA Astrophysics Data System (ADS)

Vernon, F.; Eakin, J.; Martynov, V.; Newman, R.; Offield, G.; Hindley, A.; Astiz, L.

2007-05-01

The ANZA Seismic Network (http:eqinfo.ucsd.edu) utilizes broadband and strong motion sensors with 24-bit dataloggers combined with real-time telemetry to monitor local and regional seismicity in southernmost California. The ANZA network provides real-time data to the IRIS DMC, California Integrated Seismic Network (CISN), other regional networks, and the Advanced National Seismic System (ANSS), in addition to providing near real-time information and monitoring to the greater San Diego community. Twelve high dynamic range broadband and strong motion sensors adjacent to the San Jacinto Fault zone contribute data for earthquake source studies and continue the monitoring of the seismic activity of the San Jacinto fault initiated 24 years ago. Five additional stations are located in the San Diego region with one more station on San Clemente Island. The ANZA network uses the advance wireless networking capabilities of the NSF High Performance Wireless Research and Education Network (http:hpwren.ucsd.edu) to provide the communication infrastructure for the real-time telemetry of Anza seismic stations. The ANZA network uses the Antelope data acquisition software. The combination of high quality hardware, communications, and software allow for an annual network uptime in excess of 99.5% with a median annual station real-time data return rate of 99.3%. Approximately 90,000 events, dominantly local sources but including regional and teleseismic events, comprise the ANZA network waveform database. All waveform data and event data are managed using the Datascope relational database. The ANZA network data has been used in a variety of scientific research including detailed structure of the San Jacinto Fault Zone, earthquake source physics, spatial and temporal studies of aftershocks, array studies of teleseismic body waves, and array studies on the source of microseisms. To augment the location, detection, and high frequency observations of the seismic source spectrum from local earthquakes, the ANZA network is receiving real-time data from borehole arrays located at the UCSD Thornton Hospital, and from UCSB's Borrego Valley and Garner Valley Downhole Arrays. Finally the ANZA network is acquiring data from seven PBO sites each with 300 meter deep MEMs accelerometers, passive seismometers, and a borehole strainmeter.

Optimal observables for multiparameter seismic tomography

NASA Astrophysics Data System (ADS)

Bernauer, Moritz; Fichtner, Andreas; Igel, Heiner

2014-08-01

We propose a method for the design of seismic observables with maximum sensitivity to a target model parameter class, and minimum sensitivity to all remaining parameter classes. The resulting optimal observables thereby minimize interparameter trade-offs in multiparameter inverse problems. Our method is based on the linear combination of fundamental observables that can be any scalar measurement extracted from seismic waveforms. Optimal weights of the fundamental observables are determined with an efficient global search algorithm. While most optimal design methods assume variable source and/or receiver positions, our method has the flexibility to operate with a fixed source-receiver geometry, making it particularly attractive in studies where the mobility of sources and receivers is limited. In a series of examples we illustrate the construction of optimal observables, and assess the potentials and limitations of the method. The combination of Rayleigh-wave traveltimes in four frequency bands yields an observable with strongly enhanced sensitivity to 3-D density structure. Simultaneously, sensitivity to S velocity is reduced, and sensitivity to P velocity is eliminated. The original three-parameter problem thereby collapses into a simpler two-parameter problem with one dominant parameter. By defining parameter classes to equal earth model properties within specific regions, our approach mimics the Backus-Gilbert method where data are combined to focus sensitivity in a target region. This concept is illustrated using rotational ground motion measurements as fundamental observables. Forcing dominant sensitivity in the near-receiver region produces an observable that is insensitive to the Earth structure at more than a few wavelengths' distance from the receiver. This observable may be used for local tomography with teleseismic data. While our test examples use a small number of well-understood fundamental observables, few parameter classes and a radially symmetric earth model, the method itself does not impose such restrictions. It can easily be applied to large numbers of fundamental observables and parameters classes, as well as to 3-D heterogeneous earth models.

The Two Subduction Zones of the Southern Caribbean: Lithosphere Tearing and Continental Margin Recycling in the East, Flat Slab Subduction and Laramide-Style Uplifts in the West

NASA Astrophysics Data System (ADS)

Levander, A.; Bezada, M. J.; Niu, F.; Schmitz, M.

2015-12-01

The southern Caribbean plate boundary is a complex strike-slip fault system bounded by oppositely vergent subduction zones, the Antilles subduction zone in the east, and a currently locked Caribbean-South American subduction zone in the west (Bilham and Mencin, 2013). Finite-frequency teleseismic P-wave tomography images both the Atlanic (ATL) and the Caribbean (CAR) plates subducting steeply in opposite directions to transition zone depths under northern South America. Ps receiver functions show a depressed 660 discontinuity and thickened transition zone associated with each subducting plate. In the east the oceanic (ATL) part of the South American (SA) plate subducts westward beneath the CAR, initiating the El Pilar-San Sebastian strike slip system, a subduction-transform edge propagator (STEP) fault (Govers and Wortel, 2005). The point at which the ATL tears away from SA as it descends into the mantle is evidenced by the Paria cluster seismicity at depths of 60-110 km (Russo et al, 1993). Body wave tomography and lithosphere-asthenosphere boundary (LAB) thickness determined from Sp and Ps receiver functions and Rayleigh waves suggest that the descending ATL also viscously removes the bottom third to half of the SA continental margin lithospheric mantle as it descends. This has left thinned continental lithosphere under northern SA in the wake of the eastward migrating Antilles subduction zone. The thinned lithosphere occupies ~70% of the length of the El Pilar-San Sebastian fault system, from ~64oW to ~69oW, and extends inland several hundred kilometers. In northwestern SA the CAR subducts east-southeast at low angle under northern Colombia and western Venezuela. The subducting CAR is at least 200 km wide, extending from northernmost Colombia as far south as the Bucaramanga nest seismicity. The CAR descends steeply under Lake Maracaibo and the Merida Andes. This flat slab is associated with three Neogene basement cored, Laramide-style uplifts: the Santa Marta block, the Perija Range, and the Merida Andes (Kellogg and Bonini, 1982). The steep descent of the CAR under Maracaibo implies that the CAR plate is torn somewhere between the Merida Andes and the Caribbean Sea, where it forms the ocean floor. An upcoming broadband seismic experiment will examine the CAR flat slab and the suspected slab tear in detail.

The contribution of the seismic component of Topo-Iberia to the imaging of the deep structure of the Iberian Peninsula and North Morocco

NASA Astrophysics Data System (ADS)

Diaz, Jordi; Gallart, Josep; de Lis Mancilla, Flor; Villaseñor, Antonio; Bonatto, Luciana; Schimmel, Martin; Harnafi, Mimoun; El Moudnib, Lahcen

2015-04-01

Topo-Iberia has been a large-scale Spanish project running from 2007 to 2013 that integrated more than 150 researchers on Earth Sciences. One of its key assets was the management of an observatory platform, named IberArray, aimed to provide new geophysical datasets (seismic, GPS, MT) to constrain the structure of Iberia with unprecedented resolution. The IberArray seismic pool was composed by 70+ BB stations, covering the study area in 3 deployments with a site-density of 60km x 60km. The data base holds ~300 sites, including the permanent networks in the area. Hence it forms a unique seismic database in Europe that allows for multiple analyses to constrain the complex geodinamics of the Western Mediterranean. A summary of new results coming from different techniques is presented here. The SKS splitting analysis has provided a spectacular image of the rotation of the fast velocity direction along the Gibraltar Arc. In central and northern Iberia, the fast polarization directions are close to EW, consistently with global mantle flow models considering contributions of surface plate motion, density variations and net lithosphere rotation. Those results suggest an asthenospheric origin of the observed anisotropy related to present-day mantle flow. Receiver functions have revealed the crustal thickness variations beneath the Atlas, Rif and southern Iberia, evidencing a relevant crustal root beneath the Rif, in agreement with recent, high-density active seismic experiments. The Variscan Iberian massif shows a flat Moho discontinuity, while the areas reworked in the Alpine orogeny show a slightly thicker crust. Beneath N Iberia, the imbrication of the Iberian and Eurasian crusts results in complex receiver functions. Depths exceeding 45 km are observed along the Pyrenean range, while the crust thins to values of 26-28 km close to the Atlantic coasts. The geometry of the 410-km and 660-km discontinuities has been investigated using novel cross-correlation/stacking techniques. Ambient noise tomography allows to identify the main sedimentary basins and to discriminate between the Variscan and the Alpine reworked areas. Local body-wave tomography in North Morocco has improved the location of the small magnitude events on the area and the details of the crustal structure. Teleseismic tomography has confirmed, using an independent data set, the presence of a high-velocity slab beneath the Gibraltar Arc.

The IberArray BB seismic network of Topo-Iberia: new constraints revealing the deep structure of the Iberian Peninsula and North Morocco

NASA Astrophysics Data System (ADS)

Gallart, J.; Diaz Cusi, J.; Villasenor, A.; Mancilla, F. D. L.; Bonatto, L.; Schimmel, M.; El moudnib, L.

2014-12-01

Topo-Iberia has been a large-scale Spanish project running from 2007 to 2013 that integrated more than 150 researchers on Earth Sciences. One of its key assets was the management of an observatory platform, named IberArray, aimed to provide new geophysical datasets (seismic, GPS, MT) to constrain the structure of Iberia with unprecedented resolution. The IberArray seismic pool was composed by 70+ BB stations, covering the study area in 3 deployments with a site-density of 60km x 60km. The data base holds ~300 sites, including the permanent networks in the area. Hence it forms a unique seismic database in Europe that allow for multiple analyses to constrain the complex geodinamics of the Western Mediterranean. A summary of new results coming from different techniques is presented here. The SKS splitting analysis has provided a spectacular image of the rotation of the fast velocity direction along the Gibraltar Arc. In central and northern Iberia, the fast polarization directions are close to EW, consistently with global mantle flow models considering contributions of surface plate motion, density variations and net lithosphere rotation. Those results suggest an asthenospheric origin of the observed anisotropy related to present-day mantle flow. Receiver functions have revealed the crustal thickness variations beneath the Rif and southern Iberia, including a crustal root beneath the Rif. The Variscan Iberian massif shows a flat Moho discontinuity, while the areas reworked in the Alpine orogeny show a slightly thicker crust. Beneath N Iberia, the imbrication of the Iberian and Eurasian crusts results in complex receiver functions. Depths exceeding 45 km are observed along the Pyrenean range, while the crust thins to values of 26-28 km close to the Atlantic coasts. The geometry of the 410-km and 660-km discontinuities has been investigated using novel cross-correlation/stacking techniques. Ambient noise tomography allows to identify the main sedimentary basins and to discriminate between the Variscan and the Alpine reworked areas. Local body-wave tomography in North Morocco has improved the location of the small magnitude events on the area and the details of the crustal structure. Teleseismic tomography has confirmed, using an independent data set, the presence of a high-velocity slab beneath the Gibraltar Arc.

Seismically imaging the structural legacy of rifting and collision events in the central and eastern U.S. crust

NASA Astrophysics Data System (ADS)

Schmandt, B.; Lin, F. C.; Karlstrom, K. E.

2015-12-01

EarthScope's USArray now provides broadband seismic data across the contiguous U.S. and southeastern Canada. We used teleseismic receiver functions and surface wave tomography to map crustal structure beneath the entire array. Crust thickness was estimated with multi-mode Ps receiver function images using <0.5 Hz Ps and <0.25 Hz 2p1s and 2s1p reverberations between the free-surface and Moho. In areas of sedimentary basins or large impedance contrasts in the middle crust the reverberations alone often provide clearer images of the Moho than the Ps mode, because of interference from conversions at shallow interfaces is reduced at greater lag times. The new results enable large-scale comparison of the structural legacy of multiple rifting and collision events in eastern North America. Some Proterozoic rift segments defined by Bouguer gravity and surface geology maintain locally thin crust while others lack correlated Moho topography or are areas of locally thicker crust. Locally thin crust is found at southern end of the mid-continent rift (MCR) in northern Kansas and southern Nebraska, along the Reelfoot rift, and beneath inferred rifts in Michigan, Indiana, and Ohio. The Oklahoma aulacogen is not associated with a coherent change in crust thickness along its length, at least at a scale resolvable by USArray data and our imaging approach. The MCR extending northeast from Nebraska to Lake Superior has locally thicker crust, consistent with other recent results. We suggest that magmatic additions to the lower crust overwhelmed extension in the northern mid-continent rift, but not the rift segments further south and east. Collision events of the Grenville orogeny and Paleozoic orogens that created the Appalachian Mountains are still associated with ~45-55 km thick crust extending from the Grenville front eastward across the Appalachian Mountains to the fall line that marks the abrupt geomorphic transition to the coastal plains. Despite the ~45-55 km crust thickness long-wavelength elevations (>50 km) across this area rarely exceed 1 km. Along the fall line we find ~15-20 km of seaward thinning that is coherent from Alabama to Pennsylvania, with a transition width similar to or less than the ~70 km.

Geophysical character of the intraplate Wabash Fault System from the Wabash EarthScope FlexArray

NASA Astrophysics Data System (ADS)

Conder, J. A.; Zhu, L.; Wood, J. D.

2017-12-01

The Wabash Seismic Array was an EarthScope funded FlexArray deployment across the Wabash Fault System. The Wabash system is long known for oil and gas production. The fault system is often characterized as an intraplate seismic zone as it has produced several earthquakes above M4 in the last 50 years and potentially several above M7 in the Holocene. While earthquakes are far less numerous in the Wabash system than in the nearby New Madrid seismic zone, the seismic moment is nearly twice that of New Madrid over the past 50 years. The array consisted of 45 broadband instruments deployed across the axis to study the larger structure and 3 smaller phased arrays of 9 short-period instruments each to get a better sense of the local seismic output of smaller events. First results from the northern phased array indicate that seismicity in the Wabash behaves markedly differently than in New Madrid, with a low b-value around 0.7. Receiver functions show a 50 km thick crust beneath the system, thickening somewhat to the west. A variable-depth, positive-amplitude conversion in the deep crust gives evidence for a rift pillow at the base of the system within a dense lowermost crustal layer. Low Vs and a moderate negative amplitude conversion in the mid crust suggest a possible weak zone that could localize deformation. Shear wave splitting shows fast directions consistent with absolute plate motion across the system. Split times drop in magnitude to 0.5-0.7 seconds within the valley while in the 1-1.5 second range outside the valley. This magnitude decrease suggests a change in mantle signature beneath the fault system, possibly resulting from a small degree of local flow in the asthenosphere either along axis (as may occur with a thinned lithosphere) or by vertical flow (e.g., from delamination or dripping). We are building a 2D tomographic model across the region, relying primarily on teleseismic body waves. The tomography will undoubtedly show variations in crustal structure that will give additional context to the receiver function results. Possibly more importantly, the lithospheric structure will discriminate between hypotheses of mantle flow required to give the observed shear wave splitting signature.

The shallow seismic structure of the Larderello geothermal field (Italy) as seen from Receiver Function analysis

NASA Astrophysics Data System (ADS)

Piana Agostinetti, Nicola; Licciardi, Andrea; Piccinini, Davide; Mazzarini, Francesco; Musumeci, Giovanni; Saccorotti, Gilberto

2017-04-01

The Larderello field (Tuscany, Italy) is the oldest example in the world of geothermal energy exploitation for industrial purposes. Despite its century long history of exploration and exploitation, the deep structure (4-8km depth) of the Larderello field is still poorly known, due to (a) the lack of resolution of the applied exploration techniques and (b) the lack of interest in the investigation of deep geothermal reservoirs, given the abundant amount of energy extracted from the shallow reservoirs. Recently, the increasing demand of green-energy promoted a renewed interest in the geothermal industrial sector, which translated into new exploration efforts, especially to obtain a detailed characterization of deep geothermal sources. We investigate the seismic structure of the Larderello geothermal field using Receiver Function (RF) analysis. Crustal seismic structures are routinely investigated using the RF methodology, where teleseismic P-wave are analysed to extract P-to-S converted phases that can be related to the propagation of the P-wave across a seismic discontinuity. We compute RF from 26 seismic stations, belonging to both temporary and permanent networks: the GAPSS and RETREAT experiments and the Italian Seismic Network. The RF data-set is migrated at depth and decomposed into azimuthal harmonics. Computing the first, k=0, and the second, k=1, harmonics allows to separate the "isotropic" contribution, due to the change of the isotropic properties of the sampled materials (recorded on the k=0 harmonics), from the "anisotropic" contribution, where the energy is related to the propagation of the P-wave through anisotropic materials (recorded on the k=1 harmonics). Preliminary results allow us: (1) to infer the position of the main S-wave velocity discontinuities in the study area, mainly a shallow Tyrrhenian Moho and a very-low S-wave velocity body in the center of the Larderello dome, at about 5-15km depth; and (2) to map the presence of anisotropic materials at depth beneath the central part of the geothermal field. Our finding are discussed in relation to the distribution of local microseismicity recorded during the GAPSS experiment and to the geometry of the main seismic interfaces inferred from the analysis of active seismic data.

An automated method for depth-dependent crustal anisotropy detection with receiver function

NASA Astrophysics Data System (ADS)

Licciardi, Andrea; Piana Agostinetti, Nicola

2015-04-01

Crustal seismic anisotropy can be generated by a variety of geological factors (e.g. alignment of minerals/cracks, presence of fluids etc...). In the case of transversely isotropic media approximation, information about strength and orientation of the anisotropic symmetry axis (including dip) can be extracted from the analysis of P-to-S conversions by means of teleseismic receiver functions (RF). Classically this has been achieved through probabilistic inversion encoding a forward solver for anisotropic media. This approach strongly relies on apriori choices regarding Earth's crust parameterization and velocity structure, requires an extensive knowledge of the RF method and involves time consuming trial and error steps. We present an automated method for reducing the non-uniqueness in this kind of inversions and for retrieving depth-dependent seismic anisotropy parameters in the crust with a resolution of some hundreds of meters. The method involves a multi-frequency approach (for better absolute Vs determination) and the decomposition of the RF data-set in its azimuthal harmonics (to separate the effects of isotropic and anisotropic component). A first inversion of the isotropic component (Zero-order harmonics) by means of a Reversible jump Markov Chain Monte Carlo (RjMCMC) provides the posterior probability distribution for the position of the velocity jumps at depth, from which information on the number of layers and the S-wave velocity structure below a broadband seismic station can be extracted. This information together with that encoded in the first order harmonic is jointly used in an automated way to: (1) determine the number of anisotropic layers and their approximate position at depth, and (2) narrow the search boundaries for layer thickness and S-wave velocity. Finaly, an inversion is carried out with a Neighbourhood Algorithm (NA), where the free parameters are represented by the anisotropic structure beneath the seismic station. We tested the method against synthetic RF with correlated Gaussian noise to investigate the resolution power for multiple and thin (1-5 km) anisotropic layers in the crust. The algorithm correctly retrieves the true models for the number and the position of the anisotropic layers, their strength and orientation of the anisotropic symmetry axis, although the trend direction is better constrained than the dip angle. The method is then applied to a real data-set and the results compared with previous RF studies.

Preliminary results from receiver function analysis in a seismological network across the Pamir

NASA Astrophysics Data System (ADS)

Schneider, Felix M.; Yuan, Xiaohui; Sippl, Christan; Schurr, Bernd; Mechie, James; Minaev, Vlad; Oimahmadov, Ilhomjon; Gadoev, Mustafo; Abdybachaev, Ulan A.

2010-05-01

The multi-disciplinary TIen Shan-PAmir GEodynamic (TIPAGE) program aims to investigate the dynamics of the orogeny of the Tien Shan and Pamir mountains, which are situated in south Kyrgyzstan and east Tajikistan in Central Asia. Deformation and uplift accompanied by crustal thickening is mainly induced by the collision between the Indian and Eurasian continental plates. As a local feature this collision provides the world's largest active intra-continental subduction zone. Within the framework of the TIPAGE program we operate a temporary seismic array consisting of 32 broadband and 8 short period seismic stations for a period of two years (from 2008 to 2010) covering an area of 300 x 300 km over the main part of the central Pamir plateau and the Alai-range of the southern Tien Shan. In the first year 24 broadband stations were set up in a 350-km long north-south profile geometry from Osh in southern Kyrgyzstan to Zorkul in south-eastern Tajikistan with approximately 15 km station spacing. We perform a receiver function (RF) analysis of converted P and S waves from teleseismic earthquakes at epicentral distances of 35-95 degrees with a minimum magnitude of 5.5. Therefore we decompose their wavefields by rotating the coordinate systems of the recorded seismograms from a N,E,Z into a SH,SV,P system. RFs are isolated by deconvolution of the P-component from the SH- and SV-component. They provide a robust tool to locate discontinuities in wave velocity like the Moho and thus represent the method of choice to determine crustal thickness. First results show a crustal thickness of 70-80km. Xenolith findings from depths of 100km reported by Hacker et al. (2005) give indication for even higher values. The N-S profile geometry will produce a high resolution RF image to map the gross crustal and lithospheric structure. In addition a 2D network with additional 16 stations will enable an investigation of lateral structure variation. We give an introduction to the project and methodology as well as a presentation of our preliminary most recent results on crustal thickness and Poisson's ratio along the profile from the data of the first year.

Geophysical investigations of the Anna, Ohio earthquake zone. Annual progess report July 1979-June 1980

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mauk, F.J.; Henry, S.G.; Christensen, D.H.

The Anna, Ohio seismic array, converted to solar recharge power systems, has been in continuous operation. No local earthquakes above m sub b 1.5 have occurred. Near regional earthquakes from 1977 through 1980 supplemented with quarry blast recordings have been used to determine the regional travel time curves. Theoretical estimates of earthquake detection and location capabilities for m sub b 2.5, 2.0, and 1.5 earthquakes in the Anna, Ohio region are included to demonstrate the coverage effectiveness of the network. Teleseismic P-wave residuals as a function of azimuth are included to demonstrate the lower crustal velocity variation for the region.more » Finally, an exhaustive catalog of water and gas well data is included from which a regional depth to bedrock map has been produced.« less

Extended fault inversion with random slipmaps: A resolution test for the 2012 Mw 7.6 Nicoya, Costa Rica earthquake from a Popperian inversion strategy.

NASA Astrophysics Data System (ADS)

Ángel López Comino, José; Stich, Daniel; Ferreira, Ana M. G.; Morales Soto, José

2015-04-01

The inversion of seismic data for extended fault slip distributions provides us detailed models of earthquake sources. The validity of the solutions depends on the fit between observed and synthetic seismograms generated with the source model. However, there may exist more than one model that fit the data in a similar way, leading to a multiplicity of solutions. This underdetermined problem has been analyzed and studied by several authors, who agree that inverting for a single best model may become overly dependent on the details of the procedure. We have addressed this resolution problem by using a global search that scans the solutions domain using random slipmaps, applying a Popperian inversion strategy that involves the generation of a representative set of slip distributions. The proposed technique solves the forward problem for a large set of models calculating their corresponding synthetic seismograms. Then, we propose to perform extended fault inversion through falsification, that is, falsify inappropriate trial models that do not reproduce the data within a reasonable level of mismodelling. The remainder of surviving trial models forms our set of coequal solutions. Thereby the ambiguities that might exist can be detected by taking a look at the solutions, allowing for an efficient assessment of the resolution. The solution set may contain only members with similar slip distributions, or else uncover some fundamental ambiguity like, for example, different patterns of main slip patches or different patterns of rupture propagation. For a feasibility study, the proposed resolution test has been evaluated using teleseismic body wave recordings from the September 5th 2012 Nicoya, Costa Rica earthquake. Note that the inversion strategy can be applied to any type of seismic, geodetic or tsunami data for which we can handle the forward problem. A 2D von Karman distribution is used to describe the spectrum of heterogeneity in slipmaps, and we generate possible models by spectral synthesis for random phase, keeping the rake angle, rupture velocity and slip velocity function fixed. The 2012 Nicoya earthquake turns out to be relatively well constrained from 50 teleseismic waveforms. The solution set contains 252 out of 10.000 trial models with normalized L1-fit within 5 percent from the global minimum. The set includes only similar solutions -a single centred slip patch- with minor differences. Uncertainties are related to the details of the slip maximum, including the amount of peak slip (2m to 3.5m), as well as the characteristics of peripheral slip below 1 m. Synthetic tests suggest that slip patterns like Nicoya may be a fortunate case, while it may be more difficult to unambiguously reconstruct more distributed slip from teleseismic data.

An efficient Matlab script to calculate heterogeneous anisotropically elastic wave propagation in three dimensions

USGS Publications Warehouse

Boyd, O.S.

2006-01-01

We have created a second-order finite-difference solution to the anisotropic elastic wave equation in three dimensions and implemented the solution as an efficient Matlab script. This program allows the user to generate synthetic seismograms for three-dimensional anisotropic earth structure. The code was written for teleseismic wave propagation in the 1-0.1 Hz frequency range but is of general utility and can be used at all scales of space and time. This program was created to help distinguish among various types of lithospheric structure given the uneven distribution of sources and receivers commonly utilized in passive source seismology. Several successful implementations have resulted in a better appreciation for subduction zone structure, the fate of a transform fault with depth, lithospheric delamination, and the effects of wavefield focusing and defocusing on attenuation. Companion scripts are provided which help the user prepare input to the finite-difference solution. Boundary conditions including specification of the initial wavefield, absorption and two types of reflection are available. ?? 2005 Elsevier Ltd. All rights reserved.

Teleseismic array analysis of upper mantle compressional velocity structure. Ph.D. Thesis

NASA Technical Reports Server (NTRS)

Walck, M. C.

1984-01-01

Relative array analysis of upper mantle lateral velocity variations in southern California, analysis techniques for dense data profiles, the P-wave upper mantle structure beneath an active spreading center: the Gulf of California, and the upper mantle under the Cascade ranges: a comparison with the Gulf of California are presented.

Global-Scale P-Wave Tomography Designed for Accurate Prediction of Regional and Teleseismic Travel Times for Middle East Events

DTIC Science & Technology

2010-09-01

latitude-longitude grid. Building a spherical tessellation mesh involves recursive subdivision of triangular facets of an initial polyhedron (in our...of our local database at Lawrence Livermore National Laboratory (Ruppert et al. 2005) and the publicly available Engdahl-van der Hilst-Buland (EHB

Proceedings of the 11th Annual DARPA/AFGL Seismic Research symposium

NASA Astrophysics Data System (ADS)

Lewkowicz, James F.; McPhetres, Jeanne M.

1990-11-01

The following subjects are covered: near source observations of quarry explosions; small explosion discrimination and yield estimation; Rg as a depth discriminant for earthquakes and explosions: a case study in New England; a comparative study of high frequency seismic noise at selected sites in the USSR and USA; chemical explosions and the discrimination problem; application of simulated annealing to joint hypocenter determination; frequency dependence of Q(sub Lg) and Q in the continental crust; statistical approaches to testing for compliance with a threshold test ban treaty; broad-band studies of seismic sources at regional and teleseismic distances using advanced time series analysis methods; effects of depth of burial and tectonic release on regional and teleseismic explosion waveforms; finite difference simulations of seismic wave excitation at Soviet test sites with deterministic structures; stochastic geologic effects on near-field ground motions; the damage mechanics of porous rock; nonlinear attenuation mechanism in salt at moderate strain; compressional- and shear-wave polarizations at the Anza seismic array; and a generalized beamforming approach to real time network detection and phase association.

Coseismic slip of two large Mexican earthquakes from teleseismic body waveforms - Implications for asperity interaction in the Michoacan plate boundary segment

NASA Astrophysics Data System (ADS)

Mendoza, Carlos

1993-05-01

The distributions and depths of coseismic slip are derived for the October 25, 1981 Playa Azul and September 21, 1985 Zihuatanejo earthquakes in western Mexico by inverting the recorded teleseismic body waves. Rupture during the Playa Azul earthquake appears to have occurred in two separate zones both updip and downdip of the point of initial nucleation, with most of the slip concentrated in a circular region of 15-km radius downdip from the hypocenter. Coseismic slip occurred entirely within the area of reduced slip between the two primary shallow sources of the Michoacan earthquake that occurred on September 19, 1985, almost 4 years later. The slip of the Zihuatanejo earthquake was concentrated in an area adjacent to one of the main sources of the Michoacan earthquake and appears to be the southeastern continuation of rupture along the Cocos-North America plate boundary. The zones of maximum slip for the Playa Azul, Zihuatanejo, and Michoacan earthquakes may be considered asperity regions that control the occurrence of large earthquakes along the Michoacan segment of the plate boundary.

Rupture process of a multiple main shock sequence: analysis of teleseismic, local and field observations of the Tennant Creek, Australia, earthquakes of January 22, 1988

USGS Publications Warehouse

Choy, G.L.; Bowman, J.R.

1990-01-01

On January 22, 1988, three large intraplate earthquakes (with MS 6.3, 6.4 and 6.7) occurred within a 12-hour period near Tennant Creek, Australia. Broadband displacement and velocity records of body waves from teleseismically recorded data are analyzed to determine source mechanisms, depths, and complexity of rupture of each of the three main shocks. Hypocenters of an additional 150 foreshocks and aftershocks constrained by local arrival time data and field observations of surface rupture are used to complement the source characteristics of the main shocks. The interpretation of the combined data sets suggests that the overall rupture process involved unusually complicated stress release. Rupture characteristics suggest that substantial slow slip occurred on each of the three fault interfaces that was not accompanied by major energy release. Variation of focal depth and the strong increase of moment and radiated energy with each main shock imply that lateral variations of strength were more important than vertical gradients of shear stress in controlling the progression of rupture. -from Authors

Rupture history of the 1997 Cariaco, Venezuela, earthquake from teleseismic P waves

USGS Publications Warehouse

Mendoza, C.

2000-01-01

A two-step finite-fault waveform inversion scheme is applied to the broadband teleseismic P waves recorded for the strike-slip, Cariaco, Venezuela, earthquake of 9 July 1997 to recover the distribution of mainshock slip. The earthquake is first analyzed using a long narrow fault with a maximum rise time of 20 sec. This line-source analysis indicates that slip propagated to the west with a constant rupture velocity and a relatively short rise time. The results are then used to constrain a second inversion of the P waveforms using a 60-km by 20-km two-dimensional fault. The rupture shows a zone of large slip (1.3-m peak) near the hypocenter and a second, broader source extending updip and to the west at depths shallower than 5 km. The second source has a peak slip of 2.1 meters and accounts for most of the moment of 1.1 × 1026 dyne-cm (6.6 Mww) estimated from the P waves. The inferred rupture pattern is consistent with macroseismic effects observed in the epicentral area.

Robust automatic P-phase picking: an on-line implementation in the analysis of broadband seismogram recordings

NASA Astrophysics Data System (ADS)

Sleeman, Reinoud; van Eck, Torild

1999-06-01

The onset of a seismic signal is determined through joint AR modeling of the noise and the seismic signal, and the application of the Akaike Information Criterion (AIC) using the onset time as parameter. This so-called AR-AIC phase picker has been tested successfully and implemented on the Z-component of the broadband station HGN to provide automatic P-phase picks for a rapid warning system. The AR-AIC picker is shown to provide accurate and robust automatic picks on a large experimental database. Out of 1109 P-phase onsets with signal-to-noise ratio (SNR) above 1 from local, regional and teleseismic earthquakes, our implementation detects 71% and gives a mean difference with manual picks of 0.1 s. An optimal version of the well-established picker of Baer and Kradolfer [Baer, M., Kradolfer, U., An automatic phase picker for local and teleseismic events, Bull. Seism. Soc. Am. 77 (1987) 1437-1445] detects less than 41% and gives a mean difference with manual picks of 0.3 s using the same dataset.

A teleseismic analysis of the New Brunswick earthquake of January 9, 1982.

USGS Publications Warehouse

Choy, G.L.; Boatwright, J.; Dewey, J.W.; Sipkin, S.A.

1983-01-01

The analysis of the New Brunswick earthquake of January 9, 1982, has important implications for the evaluation of seismic hazards in eastern North America. Although moderate in size (mb, 5.7), it was well-recorded teleseismically. Source characteristics of this earthquake have been determined from analysis of data that were digitally recorded by the Global Digital Seismography Network. From broadband displacement and velocity records of P waves, we have obtained a dynamic description of the rupture process as well as conventional static properties of the source. The depth of the hypocenter is estimated to be 9km from depth phases. The focal mechanism determined from the broadband data corresponds to predominantly thrust faulting. From the variation in the waveforms the direction of slip is inferred to be updip on a west dipping NNE striking fault plane. The steep dip of the inferred fault plane suggests that the earthquake occurred on a preexisting fault that was at one time a normal fault. From an inversion of body wave pulse durations, the estimated rupture length is 5.5km.-from Authors

DOE Office of Scientific and Technical Information (OSTI.GOV)

Murphy, J.R.; Marshall, M.E.; Barker, B.W.

In situations where cavity decoupling of underground nuclear explosions is a plausible evasion scenario, comprehensive seismic monitoring of any eventual CTBT will require the routine identification of many small seismic events with magnitudes in the range 2.0 < m sub b < 3.5. However, since such events are not expected to be detected teleseismically, their magnitudes will have to be estimated from regional recordings using seismic phases and frequency bands which are different from those employed in the teleseismic m sub b scale which is generally used to specify monitoring capability. Therefore, it is necessary to establish the m submore » b equivalences of any selected regional magnitude measures in order to estimate the expected detection statistics and thresholds of proposed CTBT seismic monitoring networks. In the investigations summarized in this report, this has been accomplished through analyses of synthetic data obtained by theoretically scaling observed regional seismic data recorded in Scandinavia and Central Asia from various tamped nuclear tests to obtain estimates of the corresponding seismic signals to be expected from small cavity decoupled nuclear tests at those same source locations.« less

Some possible causes of and corrections for STS-1 response changes in the Global Seismographic Network

USGS Publications Warehouse

Hutt, C.R.; Ringler, A.T.

2011-01-01

The Global Seismographic Network (GSN) (Figure 1) plays a key role in providing seismic data for global earthquake monitoring (e.g., Benz et al. 2005), earthquake science (e.g., Tsai et al. 2005), and studies of Earth structure (e.g., Dalton et al. 2008). One of the key GSN design goals is to "provide high fidelity digital recordings of all teleseismic ground motions (adequate to resolve at or near ambient noise up to the largest teleseismic signals over the bandwidth from free oscillations (10-4 Hz) to teleseismic body waves (up to approximately 15 Hz))" (GSN ad hoc Design Goals Subcommittee 2002). To help meet this goal, Streckeisen STS-1 seismometers were deployed at 80 GSN stations. Some of the GSN sensors have been deployed for more than 25 years. Several recent studies (Davis et al. 2005; Ekström et al. 2006; Davis and Berger 2007) have examined the question of overall calibration of the GSN. Ekström et al. (2006) indicated that a number of sites showed anomalous responses and suggested a gradual decay in the sensitivity. We have investigated the anomalous responses at several GSN sites. At least some of the problems observed by Ekström et al. (2006) may be attributed to humid air leaking into the feedback electronics of the STS-1 seismometers, which produces lower than normal sensitivities near the long-period corner of the instrument (360 seconds period). It appears that even though the feedback electronics boxes are designed to be sealed, water vapor can penetrate their interior after they have been exposed to highly humid seismometer vault air for extended periods. Highly humid air was also found to be present inside some STS-1 bell-jars (especially horizontal instruments) after loss of vacuum, resulting in corrosion and leakage between electrical conductors in connectors.

TSOS and TSOS-FK hybrid methods for modelling the propagation of seismic waves

NASA Astrophysics Data System (ADS)

Ma, Jian; Yang, Dinghui; Tong, Ping; Ma, Xiao

2018-05-01

We develop a new time-space optimized symplectic (TSOS) method for numerically solving elastic wave equations in heterogeneous isotropic media. We use the phase-preserving symplectic partitioned Runge-Kutta method to evaluate the time derivatives and optimized explicit finite-difference (FD) schemes to discretize the space derivatives. We introduce the averaged medium scheme into the TSOS method to further increase its capability of dealing with heterogeneous media and match the boundary-modified scheme for implementing free-surface boundary conditions and the auxiliary differential equation complex frequency-shifted perfectly matched layer (ADE CFS-PML) non-reflecting boundaries with the TSOS method. A comparison of the TSOS method with analytical solutions and standard FD schemes indicates that the waveform generated by the TSOS method is more similar to the analytic solution and has a smaller error than other FD methods, which illustrates the efficiency and accuracy of the TSOS method. Subsequently, we focus on the calculation of synthetic seismograms for teleseismic P- or S-waves entering and propagating in the local heterogeneous region of interest. To improve the computational efficiency, we successfully combine the TSOS method with the frequency-wavenumber (FK) method and apply the ADE CFS-PML to absorb the scattered waves caused by the regional heterogeneity. The TSOS-FK hybrid method is benchmarked against semi-analytical solutions provided by the FK method for a 1-D layered model. Several numerical experiments, including a vertical cross-section of the Chinese capital area crustal model, illustrate that the TSOS-FK hybrid method works well for modelling waves propagating in complex heterogeneous media and remains stable for long-time computation. These numerical examples also show that the TSOS-FK method can tackle the converted and scattered waves of the teleseismic plane waves caused by local heterogeneity. Thus, the TSOS and TSOS-FK methods proposed in this study present an essential tool for the joint inversion of local, regional, and teleseismic waveform data.

Grid-search Moment Tensor Estimation: Implementation and CTBT-related Application

NASA Astrophysics Data System (ADS)

Stachnik, J. C.; Baker, B. I.; Rozhkov, M.; Friberg, P. A.; Leifer, J. M.

2017-12-01

This abstract presents a review work related to moment tensor estimation for Expert Technical Analysis at the Comprehensive Test Ban Treaty Organization. In this context of event characterization, estimation of key source parameters provide important insights into the nature of failure in the earth. For example, if the recovered source parameters are indicative of a shallow source with large isotropic component then one conclusion is that it is a human-triggered explosive event. However, an important follow-up question in this application is - does an alternative hypothesis like a deeper source with a large double couple component explain the data approximately as well as the best solution? Here we address the issue of both finding a most likely source and assessing its uncertainty. Using the uniform moment tensor discretization of Tape and Tape (2015) we exhaustively interrogate and tabulate the source eigenvalue distribution (i.e., the source characterization), tensor orientation, magnitude, and source depth. The benefit of the grid-search is that we can quantitatively assess the extent to which model parameters are resolved. This provides a valuable opportunity during the assessment phase to focus interpretation on source parameters that are well-resolved. Another benefit of the grid-search is that it proves to be a flexible framework where different pieces of information can be easily incorporated. To this end, this work is particularly interested in fitting teleseismic body waves and regional surface waves as well as incorporating teleseismic first motions when available. Being that the moment tensor search methodology is well-established we primarily focus on the implementation and application. We present a highly scalable strategy for systematically inspecting the entire model parameter space. We then focus on application to regional and teleseismic data recorded during a handful of natural and anthropogenic events, report on the grid-search optimum, and discuss the resolution of interesting and/or important recovered source properties.

SplitRacer - a new Semi-Automatic Tool to Quantify And Interpret Teleseismic Shear-Wave Splitting

NASA Astrophysics Data System (ADS)

Reiss, M. C.; Rumpker, G.

2017-12-01

We have developed a semi-automatic, MATLAB-based GUI to combine standard seismological tasks such as the analysis and interpretation of teleseismic shear-wave splitting. Shear-wave splitting analysis is widely used to infer seismic anisotropy, which can be interpreted in terms of lattice-preferred orientation of mantle minerals, shape-preferred orientation caused by fluid-filled cracks or alternating layers. Seismic anisotropy provides a unique link between directly observable surface structures and the more elusive dynamic processes in the mantle below. Thus, resolving the seismic anisotropy of the lithosphere/asthenosphere is of particular importance for geodynamic modeling and interpretations. The increasing number of seismic stations from temporary experiments and permanent installations creates a new basis for comprehensive studies of seismic anisotropy world-wide. However, the increasingly large data sets pose new challenges for the rapid and reliably analysis of teleseismic waveforms and for the interpretation of the measurements. Well-established routines and programs are available but are often impractical for analyzing large data sets from hundreds of stations. Additionally, shear wave splitting results are seldom evaluated using the same well-defined quality criteria which may complicate comparison with results from different studies. SplitRacer has been designed to overcome these challenges by incorporation of the following processing steps: i) downloading of waveform data from multiple stations in mseed-format using FDSNWS tools; ii) automated initial screening and categorizing of XKS-waveforms using a pre-set SNR-threshold; iii) particle-motion analysis of selected phases at longer periods to detect and correct for sensor misalignment; iv) splitting analysis of selected phases based on transverse-energy minimization for multiple, randomly-selected, relevant time windows; v) one and two-layer joint-splitting analysis for all phases at one station by simultaneously minimizing their transverse energy - this includes the analysis of null measurements. vi) comparison of results with theoretical splitting parameters determined for one, two, or continuously-varying anisotropic layer(s). Examples for the application of SplitRacer will be presented.

Aseismic deep subduction of the Philippine Sea plate and slab window

NASA Astrophysics Data System (ADS)

Huang, Zhouchuan; Zhao, Dapeng; Hasegawa, Akira; Umino, Norihito; Park, Jung-Ho; Kang, Ik-Bum

2013-10-01

We have made great efforts to collect and combine a large number of high-quality data from local earthquakes and teleseismic events recorded by the dense seismic networks in both South Korea and West Japan. This is the first time that a large number of Korean and Japanese seismic data sets are analyzed jointly. As a result, a high-resolution 3-D P-wave velocity model down to 700-km depth is determined, which clearly shows that the Philippine Sea (PHS) plate has subducted aseismically down to ˜460 km depth under the Japan Sea, Tsushima Strait and East China Sea. The aseismic PHS slab is visible in two areas: one is under the Japan Sea off western Honshu, and the other is under East China Sea off western Kyushu. However, the aseismic PHS slab is not visible between the two areas, where a slab window has formed. The slab window is located beneath the center of the present study region where many teleseismic rays crisscross. Detailed synthetic tests were conducted, which indicate that both the aseismic PHS slab and the slab window are robust features. Using the teleseismic data recorded by the Japanese stations alone, the aseismic PHS slab and the slab window were also revealed (Zhao et al., 2012), though the ray paths in the Japanese data set crisscross less well offshore. The slab window may be caused by the subducted Kyushu-Palau Ridge and Kinan Seamount Chain where the PHS slab may be segmented. Hot mantle upwelling is revealed in the big mantle wedge above the Pacific slab under the present study region, which may have facilitated the formation of the PHS slab window. These novel findings may shed new light on the subduction history of the PHS plate and the dynamic evolution of the Japan subduction zone.

Deep structure of Pyrenees range (SW Europe) imaged by joint inversion of gravity and teleseismic delay time

NASA Astrophysics Data System (ADS)

Dufréchou, G.; Tiberi, C.; Martin, R.; Bonvalot, S.; Chevrot, S.; Seoane, L.

2018-04-01

We present a new model of the lithosphere and asthenosphere structure down to 300 km depth beneath the Pyrenees from the joint inversion of recent gravity and teleseismic data. Unlike previous studies, crustal correction were not applied on teleseismic data in order (i) to preserve the consistency between gravity data, which are mainly sensitive to the density structure of the crust.lithosphere, and travel time data, and (ii) to avoid the introduction of biases resulting from crustal reductions. The density model down to 100 km depth is preferentially used here to discuss the lithospheric structure of the Pyrenees, whereas the asthenospheric structure from 100 km to 300 km depth is discussed from our velocity model. The absence of a high density anomaly in our model between 30-100 km depth (except the Labourd density anomaly) in the northern part of the Pyrenees seems to preclude eclogitization of the subducted Iberian crust at the scale of the entire Pyrenean range. Local eclogitization of the deep Pyrenean crust beneath the western part of the Axial Zone (West of Andorra) associated with the positive Central density anomaly is proposed. The Pyrenean lithosphere in density and velocity models appears segmented from East to West. No clear relation between the along-strike segmentation and mapped major faults is visible in our models. The Pyrenees' lithosphere segments are associated to different seismicity pattern in the Pyrenees suggesting a possible relation between the deep structure of the Pyrenees and its seismicity in the upper crust. The concentration of earthquakes localized just straight up the Central density anomaly can result of the subsidence and/or delamination of an eclogitized Pyrenean deep root. The velocity model in the asthenosphere is similar to previous studies. The absence of a high-velocity anomaly in the upper mantle and transition zone (i.e. 125 to 225 km depth) seems to preclude the presence of a detached oceanic lithosphere beneath the European lithosphere.

Mantle dynamics and Cretaceous magmatism in east-central China: Insight from teleseismic tomograms

NASA Astrophysics Data System (ADS)

Jiang, Guoming; Zhang, Guibin; Zhao, Dapeng; Lü, Qingtian; Li, Hongyi; Li, Xinfu

2015-11-01

Both the rich mineralization in the Lower Yangtze Block (LYB) and the post-collisional mafic rocks in the Dabie Orogen (DBO) are closely related to the Cretaceous magmatism in east-central China. Various geodynamic models have been proposed for explaining the mechanism of the Cretaceous magmatism, but these models are controversial and even contradictory with each other, especially on the mechanism of adakites. A unified geodynamic model is required for explaining the magmatism in east-central China, in particular, the spatial and temporal correlations of magmatic activity in the DBO and that in the LYB. For this purpose, we apply teleseismic tomography to study P-wave velocity structure down to 800 km depth beneath east-central China. A modified multiple-channel cross-correlation method is used to collect 28,805 high-quality P-wave arrival-time data from seismograms of distant earthquakes recorded by permanent seismic stations and our temporary stations in the study region. To remove the influence of crustal heterogeneity on the mantle tomography, we used the CRUST1.0 model to correct the teleseismic relative residuals. Our tomography revealed distinct high-velocity (high-V) anomalies beneath the DBO and two flanks of the LYB, and low-velocity (low-V) anomalies above the high-V zones. Combining our tomographic images with previous geological, geochemical and geophysical results, we infer that these high-V and low-V anomalies reflect the detached lithosphere and upwelling asthenospheric materials, respectively, which are associated with the Late Mesozoic dynamic process and the Cretaceous magmatism. We propose a double-slab subduction model that a ridge subduction yielded the adakitic rocks in the LYB during 150-135 Ma and the subsequent Pacific Plate subduction played a crucial role in not only the formation of igneous rocks in the LYB but also remelting of the subducted South China Block beneath the DBO during 135-101 Ma.

Large-scale trench-normal mantle flow beneath central South America

NASA Astrophysics Data System (ADS)

Reiss, M. C.; Rümpker, G.; Wölbern, I.

2018-01-01

We investigate the anisotropic properties of the fore-arc region of the central Andean margin between 17-25°S by analyzing shear-wave splitting from teleseismic and local earthquakes from the Nazca slab. With partly over ten years of recording time, the data set is uniquely suited to address the long-standing debate about the mantle flow field at the South American margin and in particular whether the flow field beneath the slab is parallel or perpendicular to the trench. Our measurements suggest two anisotropic layers located within the crust and mantle beneath the stations, respectively. The teleseismic measurements show a moderate change of fast polarizations from North to South along the trench ranging from parallel to subparallel to the absolute plate motion and, are oriented mostly perpendicular to the trench. Shear-wave splitting measurements from local earthquakes show fast polarizations roughly aligned trench-parallel but exhibit short-scale variations which are indicative of a relatively shallow origin. Comparisons between fast polarization directions from local earthquakes and the strike of the local fault systems yield a good agreement. To infer the parameters of the lower anisotropic layer we employ an inversion of the teleseismic waveforms based on two-layer models, where the anisotropy of the upper (crustal) layer is constrained by the results from the local splitting. The waveform inversion yields a mantle layer that is best characterized by a fast axis parallel to the absolute plate motion which is more-or-less perpendicular to the trench. This orientation is likely caused by a combination of the fossil crystallographic preferred orientation of olivine within the slab and entrained mantle flow beneath the slab. The anisotropy within the crust of the overriding continental plate is explained by the shape-preferred orientation of micro-cracks in relation to local fault zones which are oriented parallel to the overall strike of the Andean range. Our results do not provide any evidence for a significant contribution of trench-parallel mantle flow beneath the subducting slab.

Lithospheric architecture of the South-Western Alps revealed by multiparameter teleseismic full-waveform inversion

NASA Astrophysics Data System (ADS)

Beller, S.; Monteiller, V.; Operto, S.; Nolet, G.; Paul, A.; Zhao, L.

2018-02-01

The Western Alps, although being intensively investigated, remains elusive when it comes to determining its lithospheric structure. New inferences on the latter are important for the understanding of processes and mechanisms of orogeny needed to unravel the dynamic evolution of the Alps. This situation led to the deployment of the CIFALPS temporary experiment, conducted to address the lack of seismological data amenable to high-resolution seismic imaging of the crust and the upper mantle. We perform a 3-D isotropic full-waveform inversion (FWI) of nine teleseismic events recorded by the CIFALPS experiment to infer 3-D models of both density and P- and S-wave velocities of the Alpine lithosphere. Here, by FWI is meant the inversion of the full seismograms including phase and amplitude effects within a time window following the first arrival up to a frequency of 0.2 Hz. We show that the application of the FWI at the lithospheric scale is able to generate images of the lithosphere with unprecedented resolution and can furnish a reliable density model of the upper lithosphere. In the shallowest part of the crust, we retrieve the shape of the fast/dense Ivrea body anomaly and detect the low velocities of the Po and SE France sedimentary basins. The geometry of the Ivrea body as revealed by our density model is consistent with the Bouguer anomaly. A sharp Moho transition is followed from the external part (30 km depth) to the internal part of the Alps (70-80 km depth), giving clear evidence of a continental subduction event during the formation of the Alpine Belt. A low-velocity zone in the lower lithosphere of the S-wave velocity model supports the hypothesis of a slab detachment in the western part of the Alps that is followed by asthenospheric upwelling. The application of FWI to teleseismic data helps to fill the gap of resolution between traditional imaging techniques, and enables integrated interpretations of both upper and lower lithospheric structures.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Nugraha, Andri Dian, E-mail: andridn104@gmail.com; Widiyantoro, Sri; Shiddiqi, Hasbi Ash

Indonesian archipelago region is located in active tectonic setting and high seismicity zone. During the last decade, Indonesian was experienced with destructive major earthquakes causing damage and victims. The information of precise earthquake location parameters are very important in partular for earthquake early warning to the society and for advance seismic studies. In this study, we attempted to improve hypocenter location compiled by BMKG for time periods of April, 2009 up to June, 2014 for about 22,000 earthquake events around Indonesian region. For the firts time, we applied teleseismic double-difference relocation algorithm (teletomoDD) to improve hypocenter region in Indonesia regionmore » combining regional and teleseismic stations. Hypocenter relocation was performed utilizing local, regional, and teleseismic P-wave arrival time data. Our relocation result show that travel-time RMS errors were greatly reduced compared to the BMKG catalog. Seismicity at shallower depth (less than 50 km) shows significantly improvement especially in depth, and refined shallow geological structures, e.g. trench and major strike slip faults. Clustered seismicity is also detected beneath volcanic region, and probably related volcano activities and also major faults nearby. In the Sunda arc region, seismicity at shallower depth centered at two major distributions parallel to the trench strike direction, i.e. around fore-arc and in mainland that related to major fault, e.g. the Sumatran fault, and volcanic fronts. Below Central Java region, relocated hypocenter result showed double seismic zone pattern. A seismic gap is detected around the Sunda-Banda transition zone where transition between oceanic subduction to continental crust collision of Australian plate occurs. In Eastern Indonesia region, shallow earthquakes are observed related to major strike slip faults, e.g. Sorong and Palu-Koro fault, volcanism, and shallow part of subduction and collision zones. We also compare our result in the Sunda Arc region with slab1.0 model and our relocated seismicity shows good agreement with the previous slab geometry. Horizontal position shift of relocated events are mostly perpendicular to the trench directions.« less

Layered Crustal and Mantle Structure and Anisotropy beneath the Afar Depression and Malawi Rift Zone

NASA Astrophysics Data System (ADS)

Reed, Cory Alexander

Although a wealth of geophysical data sets have been acquired within the vicinity of continental rift zones, the mechanisms responsible for the breakup of stable continental lithosphere are ambiguous. Eastern Africa is host to the largest contemporary rift zone on Earth, and is thus the most prominent site with which to investigate the processes which govern the rupture of continental lithosphere. The studies herein represent teleseismic analyses of the velocity and thermomechanical structure of the crust and mantle beneath the Afar Depression and Malawi Rift Zone (MRZ) of the East African Rift System. Within the Afar Depression, the first densely-spaced receiver function investigation of crustal thickness and inferred velocity attenuation across the Tendaho Graben is conducted, and the largest to-date study of the topography of the mantle transition zone (MTZ) beneath NE Africa is provided, which reveals low upper-mantle velocities beneath the Afar concordant with a probable mantle plume traversing the MTZ beneath the western Ethiopian Plateau. In the vicinity of the MRZ, a data set comprised of 35 seismic stations is employed that was deployed over a two year period from mid-2012 to mid-2014, belonging to the SAFARI (Seismic Arrays For African Rift Initiation) experiment. Accordingly, the first MTZ topography and shear wave splitting analyses were conducted in the region. The latter reveals largely plate motion-parallel anisotropy that is locally modulated by lithospheric thickness abnormalities adjacent to the MRZ, while the former reveals normal MTZ thicknesses and shallow discontinuities that support the presence of a thick lithospheric keel within the MRZ region. These evidences strongly argue for the evolution of the MRZ via passive rifting mechanisms absent lower-mantle influences.

Receiver function studies of crustal structure, composition, and evolution beneath the Afar Depression, Ethiopia

NASA Astrophysics Data System (ADS)

Almadani, Sattam Abdulkareem

The dissertation utilizes a set of sophisticated computer programs developed at the Geophysics group at Missouri S&T to characterize crustal properties beneath the Afar Depression in Ethiopia where extensional tectonics dominates. In this study, measurements of crustal thickness (H), crustal mean V p/Vs [which is related to Poisson's ratio (sigma)], and the sharpness of the Moho (R) were determined using teleseismic data from 18 broadband seismic sensors that we deployed along a profile of 250 km long with a station spacing of ˜ 10 km. The stations had been recording continuously for an entire year from December 2009 until December 2010. The measurements were determined by stacking P-to-S converted waves (PmS) and their multiples (PPmS and PSmS). Results suggest that the average crustal thickness beneath the Afar Depression is about 28.56+/-0.28 km and the crust is characterized by large Vp/Vs of 1.93+/-0.017 and smaller-than-normal overall stacking amplitude of the P-to-S converted phases beneath most stations. Our results suggest that the crust beneath the entire study area is significantly thinned and extensively intruded by mafic dikes, representing a transitional stage between continental and ocean crust. The Tendaho Graben has the thinnest and most mafic crust, which is also supported by the observation of gravity data which suggest that the active magmatic areas are characterized by higher gravity anomalies while the thicker crusts have smaller and negative anomalies. Thus, the crust beneath the center of the Tendaho Graben is likely to be oceanic-type, and becomes progressively more continental away from the center.

Reworked crustal of early Paleozoic WuYi Orogen revealed by receiver function data

NASA Astrophysics Data System (ADS)

Wei, Y.; Duan, Y.; Tian, X.; Zhao, Y.

2017-12-01

Intraplate orogenic belt, which occurs at the rigid and undeformable plate interiors, is a distinct new type of orogen rather than an interplate or plate marginal orogenic belt, whose deformation occurs exclusively at plate margins. Therefore, intraplate orogenic belts are the most obvious exception to the plate-tectonic paradigm, they are uncommon in Earth's history. The early Paleozoic Wuyi orogen in South China is one of the few examples of intraplate orogen, and is a key to understanding the process of intraplate orogenesis and global early Paleozoic geodynamics. In this study, we select teleseismic records from 45 mobile linear seismic stations deployed in Wuyi Mountain and 58 permanent stations setting in Jiangxi and Fujian provinces, from January 2011 to December 2012, and calculate the crustal thickness and average crustal Vp/Vs ratio using the H-κ stacking method. The main results include the following: 1) the crustal average Poission's ratio shows an increase tendency from land to sea, the interior of Wuyi orogen belt with an low ration less than 0.23, and the coastline with high ration which is up to 0.28, which indicate a very heterogeneous crustal structure and composition in Wuyi orogen and coast belt. 2) the crustal thickness ranges 28-34 km and shows a tendency of thinning from inland to coast in the region of SE China margin, which maight mean the eastern Eurasia lithospheric is extension and thinning induced by the subducted paleo-Pacific slab. To conclusion, we assume that Wuyi orogen experienced upper crustal thickening, lower crust and lithosphere delamination during the early Paleozoic orogeny, and lithosphere extension in Mesozoic. This research is founded by the Natural Science Foundation of China (41174052 and 41604048).

Relating the Seismic Character of the Crust and Upper Mantle to Late-Cenozoic Extension in Southwestern N.A.

NASA Astrophysics Data System (ADS)

Thurner, S.; Frassetto, A.; Porter, R.; Zandt, G.

2008-12-01

A recent tectonic reconstruction (McQuarrie and Wernicke, 2005) places detailed constraints on the magnitude and scope of late-Cenozoic extension throughout Southwestern North America. This project seeks to better understand the distribution of extension throughout the crust and upper mantle and elucidate the transition from the highly extended Basin and Range to the relatively unextended Colorado Plateau. To this end, we present teleseismic receiver functions generated from 31 broadband seismometers associated with EarthScope's BigFoot Array, TriNet, and PASSCAL stations deployed across Southern California and Arizona. We employ the common-conversion-point stacking method to analyze variations in lithospheric structure. Additionally, in regions with clear converted wave reverberations we analyze the trade-off between crustal thickness and bulk Vp/Vs to improve our view of how crustal thickness and Vp/Vs relate to different tectonic environments and degree of extension. Our preliminary estimates indicate crustal thicknesses of ~25-30 km in eastern California increasing to ~40- 45 km within the southern Colorado Plateau. The transition between thin to thick crust appears to occur over as little as 20 km. Crustal Vp/Vs varies considerably, with Vp/Vs greater than 1.8 near the Transverse Ranges and Colorado Plateau, and less than 1.8 in the southern Basin and Range. We also view a change in the nature of the Moho approaching the Colorado Plateau. Initial calculations indicate the amplitude of the converted wave from the Moho is twice as strong beneath the Mojave and Southern Basin and Range than the Colorado Plateau. Additionally, we observe laminated crust in the western Mojave Desert approaching the Transverse Ranges.

Seismic probing of continental subduction zones

NASA Astrophysics Data System (ADS)

Zhao, Liang; Xu, Xiaobing; Malusà, Marco G.

2017-09-01

High-resolution images of Earth's interior provide pivotal information for the understanding of a range of geodynamic processes, including continental subduction and exhumation of ultrahigh-pressure (UHP) metamorphic rocks. Here we present a synthesis of available global seismic observations on continental subduction zones, and selected examples of seismic probing from the European Alps, the Himalaya-Tibet and the Qinling-Dabie orogenic belts. Our synthesis and examples show that slabs recognized beneath exhumed continental UHP terranes generally have shallow dip angles (<45°) at depths <100 km, to become much steeper at depths >100 km. Slabs underlined by a clear high velocity anomaly from Earth's surface to the mantle are generally Cenozoic in age. Some of these slabs are continuous, whereas other continental subduction zones are located above discontinuous high velocity anomalies possibly suggesting slab breakoff. The density of seismic stations and the quality of recordings are of primary importance to get high-resolution images of the upper mantle to be used as a starting point to provide reliable geodynamic interpretations. In some cases, areas previously indicated as possible site of slab breakoff, such as the European Alps, have been later proven to be located above a continuous slab by using higher quality travel time data from denser seismic arrays. Discriminating between oceanic and continental slabs can be challenging, but valuable information can be provided by combining teleseismic tomography and receiver function analysis. The upper mantle beneath most continental UHP terranes generally shows complex seismic anisotropy patterns that are potentially preserved even in pre-Cenozoic subduction zones. These patterns can be used to provide information on continental slabs that are no longer highlighted by a clear high-velocity anomaly.

Improved Seismic Images of the Pacific Northwest Interior, With a Focus on the Region of the Columbia River Flood Basalts and Central Idaho

NASA Astrophysics Data System (ADS)

Stanciu, A. C.; Humphreys, E.; Clayton, R. W.

2017-12-01

We construct a P-wave model of the upper mantle based on new and previously acquired data from the USArray-TA stations and regional deployments, including the HLP, ID-OR, and the currently recording Wallowa stations. Our teleseismic arrival times are corrected for crustal structure (based on surface wave, receiver function, and controlled-source models from the region). Our modeling incorporates 3-D ray tracing and several simple considerations of radial anisotropy on travel time. As imaged previously, we find high P-wave velocity anomalies located beneath the Wallowa Mountains and beneath the Idaho Batholith in central west Idaho. Our improved imaging finds that these two anomalies are located down to 350 km depth, and are clearly separated from one another and from a shallower fast anomaly in the uppermost mantle beneath the westernmost Snake River Plain. Our preferred interpretation includes a combination of delamination and slab fragments in this region. As fast (and presumably cool) structures, these upper-mantle anomalies are thought to have a lithospheric origin. The anomaly beneath central Idaho is interpreted as the leading edge of the Farallon slab associated with the accretion of Siletzia terrane to North America. This anomaly may include some North American lithosphere that delaminated from the Laramide-thickened lithospheric mantle, perhaps related to Challis magmatism. The Wallowa anomaly is likely to represent Farallon lithosphere that delaminated during the Columbia River flood basalt event. The small anomaly between the two deeper fast anomalies, occurring at depths above 150km, could represent an isolated lithospheric fragment or a structure created by the Columbia River flood basalt event.

The crustal structure of Ellesmere Island, Arctic Canada—teleseismic mapping across a remote intraplate orogenic belt

NASA Astrophysics Data System (ADS)

Schiffer, Christian; Stephenson, Randell; Oakey, Gordon N.; Jacobsen, Bo H.

2016-03-01

Ellesmere Island in Arctic Canada displays a complex geological evolution. The region was affected by two distinct orogenies, the Palaeozoic Ellesmerian orogeny (the Caledonian equivalent in Arctic Canada and Northern Greenland) and the Palaeogene Eurekan orogeny, related to the opening of Baffin Bay and the consequent convergence of the Greenland plate. The details of this complex evolution and the present-day deep structure are poorly constrained in this remote area and deep geophysical data are sparse. Receiver function analysis of seven temporary broad-band seismometers of the Ellesmere Island Lithosphere Experiment complemented by two permanent stations provides important data on the crustal velocity structure of Ellesmere Island. The crustal expression of the northernmost tectonic block of Ellesmere Island (˜82°-83°N), Pearya, which was accreted during the Ellesmerian orogeny, is similar to that at the southernmost part, which is part of the Precambrian Laurentian (North America-Greenland) craton. Both segments have thick crystalline crust (˜35-36 km) and comparable velocity-depth profiles. In contrast, crustal thickness in central Ellesmere Island decreases from ˜24-30 km in the Eurekan fold and thrust belt (˜79.7°-80.6°N) to ˜16-20 km in the Hazen Stable Block (HSB; ˜80.6°-81.4°N) and is covered by a thick succession of metasediments. A deep crustal root (˜48 km) at ˜79.6°N is interpreted as cratonic crust flexed beneath the Eurekan fold and thrust belt. The Carboniferous to Palaeogene sedimentary succession of the Sverdrup Basin is inferred to be up to 1-4 km thick, comparable to geologically-based estimates, near the western margin of the HSB.

Preliminary results of the Source China Sea passive source OBS array experiment

NASA Astrophysics Data System (ADS)

Yang, T.; Liu, C.; Pei, Y.; Xia, S.

2013-12-01

The Scarborough, or Huangyan, Seamount chain in South China Sea (SCS) represents an extreme case of the global mid-ocean ridge system where the magmatism continues for many million years after the cessation of spreading. To understand this unique process, the South China Sea Deep (SCSD) program funded an experiment deploying a passive source OBS array to image the lithospheric structure beneath the extinct ridge. In April 2012, 18 passive source OBSs, including 15 Guralp CMG-40T OBS and 3 I-4C OBS, were deployed around the Huangyan Island for one year. 11 OBSs were successfully recovered this April, and their data are being processed. Here we present some preliminary results from analyses of this dataset, including the general quality of three-component seismograms, characteristics of seafloor ambient noise spectra, determining the OBS orientation from the Rayleigh wave polarization, and the dispersion analysis of Rayleigh waves. We found that, for most stations, seismograms from teleseismic, regional and local events are generally good with the horizontal records being comparable with vertical component. The noise levels in these seafloor stations are much higher than land-based stations, especially in shorter periods, likely suggesting the direct and stronger impact from the tempestuous SCS. Applications of more sophisticated seismic techniques such as surface wave tomography, seismic anisotropy, receiver function and ambient noise cross-correlation are underway. In addition to the low recovery rate, there are other lessons learned from this experiment. For example, at least two stations have detectable timing problems; Airgun shots should have been used to constrain the timings and orientations in both deployment and recovery. It is still challenging and costly to carry out long-term passive source seismic observations in deep sea.

Discriminating different type waves from pressure and ground motion observation in the seafloor by DONET cabled observation network.

NASA Astrophysics Data System (ADS)

Araki, E.; Kawaguchi, K.; Kaneda, Y.

2011-12-01

We developed and deployed seafloor cabled observatory called "Dense Ocean-floor Network for Earthquake and Tsunamis (DONET)" in the Nankai Trough, south of Japan. The main purpose of the DONET network is to observe large earthquake such as Tonankai earthquake in the deployed seafloor and associate Tsunamis in real-time to help disaster mitigation, and as well to monitor inter-seismic crustal activities such as micro earthquakes, very low frequency earthquakes, and slower crustal deformation. In each DONET seafloor observatory, high-sensitive broadband set of instruments for seismic and seafloor pressure monitoring, consisted from Guralp CMG3T broadband seismometer, Metrozet TSA100S accelerometer, Paroscientific 8B7000-2 pressure gauge, a deep-sea differential pressure gauge, a hydrophone, and a seawater thermometer, are installed. The density of seafloor observatories are 20 observatories distributed in 15-30 km interval which is optimized for monitoring of events in the plate boundary beneath the network. DONET may be regarded as a large-scale, high sensitive high density seismic array for monitoring teleseismic events in the Philippine Sea and the Pacific Ocean. The DONET seafloor observatories are situated in wide range of seafloor depth between 1800m and 4500m, from the seafloor basin about 50 km off Japanese Island through the slope of accerecionary prism to the deep trench axis 150 km off the coast, that may also regarded as a vertical array in the 4.5km thick ocean. This variation of depths helps identify T-phases from the array record. In data analysis, it is necessary to identify propagation mode of each observed wave which may often be mixed together. In our design of DONET observation system, we took care to help identification of seismic phase by obtaining both ground motion and seafloor pressure in the same location. This is simply achieved by combining seafloor pressure gauges and seismometer in a single observatory package, but care was taken to observe both in the similar level of sensitivity and dynamic range in wide frequencies from near DC to over 100 Hz. In the case of DONET, the broadband seismometer and the differential pressure gauge have similar level of sensitivity in 0.005 - 10 Hz, and similarly the accelerometer and the hydrophone cover between 1-100Hz, in total covering most frequencies of our interest, 0.005 Hz to 100 Hz. With both ground motion and seafloor pressure measurement, we may distinguish types of waves relatively easily, and it is also possible to filter particular types of waves from the array dataset to help our data analysis. For example, it has been commonly practiced to distinguish up-going and down-going seismic phases from pressure and ground motion, but this is relatively difficult only with sparse seismometer array. This technique may also be applied to correct teleseismic record with sea surface reflection in receiver function analysis for exploring deep crustal structure.

Hydrophone calibration for MERMAID seismic network

NASA Astrophysics Data System (ADS)

Joubert, C.; Nolet, G.; Sukhovich, A.; Ogé, A.; Argentino, J.; Hello, Y.

2013-12-01

The MERMAID float (Mobile Earthquake Recorder in Marine Areas by Independent Divers) is a new oceanic seismometer which has already successfully recorded P-waves from teleseismic events, when deployed in the Mediterranean Sea and the Indian Ocean. The frequency band of teleseismic acquisition is for frequencies up to 2 Hz. The P-waves are recorded with a Rafos II hydrophone. The hydrophone has a flat frequency response from 5 Hz to 10 kHz but its behavior below 5 Hz is not documented. In this work we determine the Rafos II response with the electronic card used for MERMAID in the frequency band of 0.1 to 2 Hz. A simple and low-cost calibration method at low frequencies was developed and applied to the Rafos II but can be used for any hydrophone. In this calibration method a brief pressure increase of 1000 Pa is applied to the hydrophone. We record response after filtering and digitizing by the electronic card. To create the pressure increase, the hydrophone is placed in a calibration chamber filled with water and making sure no air bubbles are present. By opening a solenoid valve connected to a tube with 10 cm extra water on top of the chamber, an abrupt pressure of 1000 Pa is applied. The output signal is fitted to an empirical response function, that characterizes it with four parameters: A, B, α and τ which control the shape of the signal: h(t) = t^(ατ) e^(-αt) (A + Bt). A represents the magnification, α defines the exponential relaxation of the signal, B models the overshoot and τ allows for a slightly delayed response due to the low-pass filtering in the electronics. A set of 20 experiments is used to characterize the Rafos II instrumental response in association with the electronic card. The method developed here offers a good and simple way to estimate the response at low frequencies. The MERMAID hydrophone response to the step function input of 1000 Pa can be defined by A = 0.36 × 0.02 mV/Pa, B = - 0.08 × 0.01 mV Pa^(-1) s^(-1), α = 0.28 × 0.02 s^(-1) and τ = 0.41 × 0.14 s.

Mapping apparent stress and energy radiation over fault zones of major earthquakes

USGS Publications Warehouse

McGarr, A.; Fletcher, Joe B.

2002-01-01

Using published slip models for five major earthquakes, 1979 Imperial Valley, 1989 Loma Prieta, 1992 Landers, 1994 Northridge, and 1995 Kobe, we produce maps of apparent stress and radiated seismic energy over their fault surfaces. The slip models, obtained by inverting seismic and geodetic data, entail the division of the fault surfaces into many subfaults for which the time histories of seismic slip are determined. To estimate the seismic energy radiated by each subfault, we measure the near-fault seismic-energy flux from the time-dependent slip there and then multiply by a function of rupture velocity to obtain the corresponding energy that propagates into the far-field. This function, the ratio of far-field to near-fault energy, is typically less than 1/3, inasmuch as most of the near-fault energy remains near the fault and is associated with permanent earthquake deformation. Adding the energy contributions from all of the subfaults yields an estimate of the total seismic energy, which can be compared with independent energy estimates based on seismic-energy flux measured in the far-field, often at teleseismic distances. Estimates of seismic energy based on slip models are robust, in that different models, for a given earthquake, yield energy estimates that are in close agreement. Moreover, the slip-model estimates of energy are generally in good accord with independent estimates by others, based on regional or teleseismic data. Apparent stress is estimated for each subfault by dividing the corresponding seismic moment into the radiated energy. Distributions of apparent stress over an earthquake fault zone show considerable heterogeneity, with peak values that are typically about double the whole-earthquake values (based on the ratio of seismic energy to seismic moment). The range of apparent stresses estimated for subfaults of the events studied here is similar to the range of apparent stresses for earthquakes in continental settings, with peak values of about 8 MPa in each case. For earthquakes in compressional tectonic settings, peak apparent stresses at a given depth are substantially greater than corresponding peak values from events in extensional settings; this suggests that crustal strength, inferred from laboratory measurements, may be a limiting factor. Lower bounds on shear stresses inferred from the apparent stress distribution of the 1995 Kobe earthquake are consistent with tectonic-stress estimates reported by Spudich et al. (1998), based partly on slip-vector rake changes.

Regional W-Phase Source Inversion for Moderate to Large Earthquakes in China and Neighboring Areas

NASA Astrophysics Data System (ADS)

Zhao, Xu; Duputel, Zacharie; Yao, Zhenxing

2017-12-01

Earthquake source characterization has been significantly speeded up in the last decade with the development of rapid inversion techniques in seismology. Among these techniques, the W-phase source inversion method quickly provides point source parameters of large earthquakes using very long period seismic waves recorded at teleseismic distances. Although the W-phase method was initially developed to work at global scale (within 20 to 30 min after the origin time), faster results can be obtained when seismological data are available at regional distances (i.e., Δ ≤ 12°). In this study, we assess the use and reliability of regional W-phase source estimates in China and neighboring areas. Our implementation uses broadband records from the Chinese network supplemented by global seismological stations installed in the region. Using this data set and minor modifications to the W-phase algorithm, we show that reliable solutions can be retrieved automatically within 4 to 7 min after the earthquake origin time. Moreover, the method yields stable results down to Mw = 5.0 events, which is well below the size of earthquakes that are rapidly characterized using W-phase inversions at teleseismic distances.

Remote Love Wave Triggering of Tremor in the Nankai Subduction Zone: New Observations and Dynamic Stress Modeling

NASA Astrophysics Data System (ADS)

Enescu, B.; Chao, K.; Obara, K.; Peng, Z.; Matsuzawa, T.; Yagi, Y.

2013-12-01

The triggering of deep non-volcanic tremor (NVT) in the Nankai region, southwest Japan, by the surface waves of several large teleseismic earthquakes has been well documented (e.g., Miyazawa & Mori, 2005). These previous studies report that the Nankai NVT is primarily triggered by the passage of Rayleigh waves from the teleseismic events (e.g., Miyazawa & Brodsky, 2008). The relative lack of Love wave triggering in Nankai would be, however, an exception to the general observation that triggered tremor shows a positive correlation with the triggering potential, defined using the Coulomb failure criteria (Hill, 2012). To clarify the Nankai NVT triggering mechanism, we have systematically searched for triggered tremor due to large teleseismic events (Mw ≥ 7.5) occurred from 2001 to 2012. Our present analysis focuses on western Shikoku, where triggered NVT has been previously reported (e.g., Miyazawa & Mori, 2006). From a total of 55 teleseismic events, 18 show associated triggered NVT. Our analysis presents clear evidence of triggered NVT that correlates well with the passage of Love waves. The most outstanding example is that of the 2012 M8.6 Sumatra earthquake, a strike-slip event characterized by relatively large amplitude Love waves. The incoming surface waves from this earthquake are almost strike-parallel to the Nankai subduction zone, which corresponds to a higher Love wave triggering potential (Hill, 2012). The 2001 M7.8 Kunlun, the 2003 M8.3 Tokachi-oki, the 2004 M9.2 & 2007 M8.5 Sumatra, the 2006 M8.3 Kuril-Islands and the 2008 M7.9 Wenchuan earthquakes show as well Love-wave associated NVT triggering. In most of these cases the tremor is initiated by the incoming, faster-traveling Love waves and continues during the latter, larger-amplitude Rayleigh waves. We are also conducting dynamic stress modeling to better understand the triggering mechanism of tremor. Our approach builds up on the methods of Gonzalez-Huizar & Velasco (2011) and Obara (2012). In the case of the 2012 Sumatra earthquake, we found a high correlation between the Love waves dynamic Coulomb stress change at the tremor source and the triggered NVT, for a time period of about 400s, which starts from the first Love wave cycles. Afterwards, the tremor bursts have slightly larger amplitudes and the correlation with the surface waves becomes poor. Preliminary results indicate a shallower location for these later tremors. Our results indicate that the triggering mechanism of NVT in western Shikoku is essentially the same with the one operating (e.g., Hill, 2012) in other subduction regions around the world (e.g., Cascadia). The tremor responds to excitation by both Love and Rayleigh waves according to the Coulomb failure criterion; failure, once underway, might be controlled by other mechanisms (e.g., some form of rate-state friction), which we plan to address in future studies.

Crust and Upper Mantle Structure beneath Isparta Angle in SW Turkey from P and S Receiver Functions

NASA Astrophysics Data System (ADS)

Kahraman, M.; Turkelli, N.; Özacar, A.; Sandvol, E. A.; Teoman, U.

2015-12-01

Isparta Angle (IA) constitutes a triangular shape elevated tectonic domain in SW Turkey which contains units stacked with opposing thrust vergences during Late Cretaceous to Miocene. The region which is located at the junction between Aegean and Cyprus arcs separated by a slab tear is now bounded by Fethiye-Burdur Fault Zone (FBFZ) in the west and Akşehir-Afyon Fault Zones (AAFZ) in the east. In the area, seismicity displays ongoing extension along active grabens oriented at different directions. In the past, many competing geodynamic scenarios had been proposed to explain the complex tectonic evolution of the area. In this study, we used both P and S receiver functions (RFs) to present high resolution crustal and upper mantle images down to 200 km. Moho and upper crustal discontinuities were well resolved by P Rfs; however S RFs were utilized to image lithospheric-asthenospheric boundaries having the benefit of being free of multiple conversions. RFs were calculated from 916 teleseismic earthquakes (Mw ≥ 5.5) recorded by 42 permanent and temporary broadband stations BU-KOERI/NEMC, DEMP/ERD and Isparta Angle Seismic Experiment deployed by collaboration of BU-KOERI and University of Missouri. Totally, 4501 P and 946 S RFs with the cut-off frequencies of ~1.0 Hz and ~0.5 Hz, respectively, were obtained by applying iterative-time domain deconvolution. Crustal thickness and Vp/Vs ratios were calculated by grid search of maximum amplitude of P RFs(Ps,PpPs and PsPs+PpSs) in depth and Vp/Vs domain. Then, we created 2-D P and S migrated cross-sections to observe crustal and lithospheric-asthenospheric discontinuities beneath the region. P RFs indicates that, average crustal thickness and Vp/Vs ratio is ~36 km and 1.78 in the region with small changing values close to the edges. Migrated P RFs cross-sections revealed a sharp change in Moho (Moho offset) on the western boundary that spatially correlates with the FBFZ. We also found a relatively flat Moho in the center and what appears to be imaged northern tip of slab at ~45 km depth. Finally, ~30km crustal thickness released in southeast beneath the Cyprus. On the other hand; preliminary results of S RFs cross-sections present the LAB boundary between ~60 to ~90 km depth range, observed almost beneath all profiles and clear positive phase arrivals right below the LAB depths.

Complex Anisotropic Structure of the Mantle Wedge Beneath Kamchatka Volcanoes

NASA Astrophysics Data System (ADS)

Levin, V.; Park, J.; Gordeev, E.; Droznin, D.

2002-12-01

A wedge of mantle material above the subducting lithospheric plate at a convergent margin is among the most dynamic environments of the Earth's interior. Deformation and transport of solid and volatile phases within this region control the fundamental process of elemental exchange between the surficial layers and the interior of the planet. A helpful property in the study of material deformation and transport within the upper mantle is seismic anisotropy, which may reflect both microscopic effects of preferentialy aligned crystals of olivine and orthopyroxene and macroscopic effects of systematic cracks, melt lenses, layering etc. Through the mapping of anisotropic properties within the mantle wedge we can establish patterns of deformation. Volatile content affects olivine alignment, so regions of anomalous volatile content may be evident. Indicators of seismic anisotropy commonly employed in upper mantle studies include shear wave birefringence and mode-conversion between compressional and shear body waves. When combined together, these techniques offer complementary constraints on the location and intensity of anisotropic properties. The eastern coast of southern Kamchatka overlies a vigorous convergent margin where the Pacific plate descends at a rate of almost 80 mm/yr towards the northwest. We extracted seismic anisotropy indicators from two data sets sensitive to the anisotropic properties of the uppermost mantle. Firstly, we evaluated teleseismic receiver functions for a number of sites, and found ample evidence for anisotropicaly-influenced P-to-S mode conversion. Secondly, we measured splitting in S waves of earthquakes with sources within the downgoing slab. The first set of observations provides constraints on the depth ranges where strong changes in anisotropic properties take place. The local splitting data provides constraints on the cumulative strength of anisotropic properties along specific pathways through the mantle wedge and possibly parts of the slab. To explain the vertical stratification of anisotropy implied from receiver functions, and the strong lateral dependence of shear-wave splitting observations, we cannot rely on simple models of mantle wedge behaviour e.g., olivine-crystal alignment through subduction-driven corner flow. Diverse mechanisms can contribute to the observed pattern of anisotropic properties, with volatiles likely being a key influence. For instance, we find evidence in favor of a slow-symmetry-axis anisotropy within the uppermost 10-20 km of the mantle wedge, implying either excessive hydration of the mantle or else a presence of systematically aligned volatile-filled cracks or lenses. Also, shear-wave splitting is weak beneath the Avachinsky-Koryaksky volcanic center, suggesting either vertical flow or the influence of volatiles and/or thermally-enhanced diffusion creep.

Study of the Triggering Level of Dynamic Stress Induces Non-Volcanic Tremor in Longitudinal Valley in Eastern Taiwan

NASA Astrophysics Data System (ADS)

Sun, W. F.; Chang, W. Y.; Chen, H. Y.

2015-12-01

Taiwan is located at the margin of the Eurasian Plate and the Philippine Sea Plate, which is a subduction zone between these two plates and the fault structures are rather complicated and dense seismicity, especially the Longitudinal Valley (LV) in eastern Taiwan. Non-volcanic tremor (NVT) is a seismic signal with low amplitude and long duration. NVT is often occurred below the seismogenic zone, which is between the lower crust and upper mantle, and the arrival time data of the body wave is difficult to be collected. Therefore, this study aims to investigate the physical mechanisms of NVT in several steps. First, in the investigation of the teleseismic earthquake data from the U.S. Geological Survey in 2005 to 2014, thirty-five potential teleseismic earthquakes are selected. Second, the seismograms are collected from the Broadband Array in Taiwan for Seismology (BATS) and Central Weather Bureau Seismic Network (CWBSN) for these thirty-five potential teleseismic earthquakes. Third, the Seismic Analysis Code is used to select the seismograms from seven possible events which satisfied the conditions of triggering tremor during the passage of the surface wave. Forth, a band-pass filter is applied to retain the frequency with the range of 2-8 HZ of the surface waveform. Finally, visually determination for the tremor signals. The experimental results show that five certainly NVT events and two potential triggered events were found in the LV zone of eastern Taiwan. The locations of the hypocenters were then estimated using HYPO71 for these five certain events. According to the estimated hypocenters, the sources of NVT are possibly beneath the southern region of LV, close to the Chih-Shang fault. Moreover, these estimated hypocenters are within the high Vp/Vs ratio region and in depth of 30-40 km. The further analysis found that the amplitude of the surface wave is one of the key factors that when the peak ground velocity > 0.02cm/s, which equivalents to 2-3kPa dynamic stress, might trigger tremors.

Long Indian Slab in the Mantle Transition Zone Under Eastern Tibet: Evidence from Teleseismic Tomography

NASA Astrophysics Data System (ADS)

Lei, J.; Zhao, D.; Zha, X.

2014-12-01

We present a new 3-D P-wave velocity model of the upper mantle under eastern Tibet determined from 113,831 high-quality teleseismic arrival-time data. Our data are hand-picked from seismograms of 784 teleseismic events (30o-90o) with magnitudes of 5.2 or greater. These events were recorded by 21 portable seismic stations deployed in Yunnan during April 2010 to July 2011 and 259 permanent stations of Chinese provincial seismic networks during September 2008 to December 2011 in the study region. Our results provide new insights into the mantle structure and dynamics of eastern Tibet. High-velocity (high-V) anomalies are revealed down to 200 km depth under stable cratonic regions, such as Sichuan basin, Ordos and Alashan blocks. Prominent low-velocity (low-V) anomalies are revealed in the upper mantle under the Kunlun-Qinling fold zone, Songpan-Ganzi, Qiangtang, Lahsa, and Chuan-Dian diamond blocks, suggesting that the eastward moving low-V materials are obstructed by Sichuan basin, Ordos and Alashan blocks, and they could be extruded through the Qinling fold zone and the Chuan-Dian block to eastern China. In addition, the extent and thickness of these low-V anomalies are well correlated with the surface topography, suggesting that uplift of eastern Tibet is closely related to the low-V anomalies which may reflect hot materials and have strong buoyancy. In the mantle transition zone, broad high-V anomalies are visible from the Burma arc northward to the Kunlun fault and eastward to the Xiaojiang fault, which extend for a total of approximately 700 km. The high-V anomalies are connected upward to the Wadati-Benioff seismic zone beneath the Burma arc. These results suggest that the Indian slab has subducted horizontally for a long distance in the mantle transition zone after it descended into the mantle, and its deep dehydration has contributed to forming the low-V anomalies in the big mantle wedge above the slab. Our present results shed new light on the formation and evolution of the Tibetan plateau.

Local Earthquake Tomography in the Eifel Region, Middle Europe

NASA Astrophysics Data System (ADS)

Gaensicke, H.

2001-12-01

The aim of the Eifel Plume project is to verify the existence of an assumed mantle plume responsible for the Tertiary and Quaternary volcanism in the Eifel region of midwest Germany. During a large passive and semi-active seismological experiment (November 1997 - June 1998) about 160 mobil broadband and short period stations were operated in addition to about 100 permanent stations in the area of interest. The stations registered teleseismic and local events. Local events are used to obtain a threedimensional tomographic model of seismic velocities in the crust. Since local earthquake tomography requires a large set of crustal travel paths, seismograms of local events recorded from July 1998 to June 2001 by permanent stations were added to the Eifel Plume data set. In addition to travel time corrections for the teleseismic tomography of the upper mantle, the new 3D velocity model should improve the precision for location of local events. From a total of 832 local seismic events, 172 were identified as tectonic earthquakes. The other events were either quarry blasts or shallow mine-induced seismic events. The locations of 60 quarry blasts are known and for 30 of them the firing time was measured during the field experiment. Since the origin time and location of these events are known with high precision, they are used to validate inverted velocity models. Station corrections from simultaneous 1D-inversion of local earthquake traveltimes and hypocenters are in good agreement with travel time residuals calculated from teleseismic rays. A strong azimuthal dependency of travel time residuals resulting from a 1D velocity model was found for quarry blasts with hypocenters in the volcanic field in the center of the Eifel. Simultaneous 3D-inversion calculations show strong heterogeneities in the upper crust and a negative anomaly for p-wave velocities in the lower crust. The latter either could indicate a low velocity zone close to the Moho or subsidence of the Moho. We present preliminary results obtained by simultaneous inversion of earthquake and velocity parameters constrained by known geological parameters and the controlled source information from calibrated quarry blasts.

Eastern North American finite-frequency, compressional and shear tomographic models

NASA Astrophysics Data System (ADS)

Savage, B.; Shen, Y.

2017-12-01

The Eastern North American margin and continental interior is imaged using a finite-frequency, tomographic method. Each of the P and S teleseismic body wave date sets consists of over 80,000 usable measurements recorded on the Transportable Array (TA). Sensitivity kernels are computed from a 1D model with grid spacing of 50 x 50 x 25 km. Measurements are performed automatically at three individual frequency bands, allowing a more effective use of the available broadband data. Imaged shear and compressional wave speeds show similar long-wavelength features of reduced wave speeds along the continent-ocean margin and increased wave speeds within the stable interior. Wave speeds throughout the model are highly variable at the scale of 100 to 200 km. Large wave speed reductions are present near New England, the Mid-Atlantic states, and the Gulf Coast states; these variations are present in previous models. Interestingly, the strongly reduced wave speeds near South Carolina are absent at depths greater than of 150 km within this model and recent teleseismic body-wave models. This result is contrary to a variety of surface wave models which contain an intense, reduced wave speed anomaly extending past 250 km depth and interpreted as a mantle upwelling associated with edge driven convection. An anomaly along the West Virginia-Virginia border, associated with volcanism and mantle upwelling, is also present, tightly constrained, and extends to 200 km depth. Moreover, the interior of the continent contains significant, regional wave speed variations. Variation of this style is present in other surface and body wave models and is not consistent with a massive, homogeneous continent with no internal variations. These internal continental variations suggest a compositional influence as temperature, melt and water are thought to have minimal effect. Unlike surface wave models that include a distinct continental base around 175 km, teleseismic body wave models, including this one, do not show this base. However, this model does include the deep, positive wave speed anomaly within the mantle transition zone interpreted as a slab fragment, agreeing with previous models.

Investigation of the 27 February 2010 Mw 8.8 Chilean earthquake integrating aftershock analysis, back-projection imaging and cGPS results

NASA Astrophysics Data System (ADS)

Clévédé, E.; Satriano, C.; Bukchin, B.; Lancieri, M.; Fuenzalida, A.; Vilotte, J.; Lyon-Caen, H.; Vigny, C.; Socquet, A.; Aranda, C.; Campos, J. A.; Scientific Team of the Lia Montessus de Ballore (Cnrs-Insu, U. Chile)

2010-12-01

The Mw 8.8 earthquake in central Chile ruptured more than 400 km along the subduction bound between the Nazca and the South American plates. The aftershock distribution clearly shows that this earthquake filled a well-known seismic gap, corresponding to rupture extension of the 1835 earthquake. The triggered post-seismic activity extends farther north of the gap, partially overlapping the 1985 and the 1960 Valparaiso earthquakes. However, the analysis of continuous GPS (cGPS) recordings, and back projection imaging of teleseismic body wave energy, indicate that the rupture stopped south of Valparaiso, around -33.5 degrees of latitude. An important question is how far the rupture actually extended to the north and the potential relation between the northernmost aftershock activity and remaining asperities within the ruptured zone of the previous Valparaiso earthquakes. The extension of the rupture offshore, towards west, also deserves further investigation. The aftershock distribution and the back propagation analysis support the hypothesis that, in the northern part, the rupture may have reached the surface at the trench. In this work, we performed a CMT and depth location study for more than 10 of the immediate largest aftershocks using teleseismic surface wave analysis constrained by P-wave polarity. In parallel, a detailed analysis of aftershocks in the northern part of the rupture, between 2010-03-11 and 2010-05-13, have been performed using the data from the station of the Chilean Servicio Sismológico Nacional (SSN), and of the post-seismic network, deployed by the French CNRS-INSU, GFZ, IRIS, and Caltech. We accurately hand-picked 153 larger events, which have been located using a non-linear probabilistic code, with improved depth location. Focal mechanisms have been computed for the larger events. Those results have been integrated with the analysis of cGPS and teleseismic back projection, and the overall kinematic of the Maule earthquake is discussed as well as the extension of the rupture along strike and dip.

The 2015 April 25 Gorkha Earthquake and its Aftershocks: Implications for lateral heterogeneity on the Main Himalayan Thrust

NASA Astrophysics Data System (ADS)

Mitra, S.; Kumar, A.; Priestley, K. F.

2016-12-01

The 2015 Gorkha earthquake (Mw 7.8) occurred by thrust faulting on a ˜150 km long and ˜70 km wide, locked downdip segment of the Main Himalayan Thrust (MHT), causing the Himalaya to slip SSW over the Indian Plate, and was followed by major-to-moderate aftershocks. Back projection of teleseismic P-wave and inversion of teleseismic body waves provide constraints on the geometry and kinematics of the mainshock rupture and source mechanism of aftershocks. The mainshock initiated ˜80 km west of Katmandu, close to the locking line on the MHT and propagated eastwards, along ˜117° azimuth, for a duration of ˜70 s, in multi-stage rupture. The mainshock has been modeled using four sub-events, propagating from west-to-east. The first sub-event (0-20 s) ruptured at a velocity of ˜3.5 km/s on a ˜6° N dipping flat segment of the MHT with thrust motion. The second sub-event (20-35 s) ruptured a ˜18° W dipping lateral ramp on the MHT in oblique thrust motion. The rupture velocity dropped from 3.5 km/s to 2.5 km/s, as a result of updip propagation of the rupture. The third sub-event (35-50 s) ruptured a ˜7° N dipping, eastward flat segment of the MHT with thrust motion and resulted in the largest amplitude arrivals at teleseismic distances. The fourth sub-event (50-70 s) occurred by left-lateral strike-slip motion on a steeply dipping transverse fault, at high angle to the MHT and arrested the eastward propagation of the mainshock rupture. Eastward stress build-up following the mainshock resulted in the largest aftershock (Mw 7.3), which occurred on the MHT, immediately east of the mainshock rupture. Source mechanism of moderate aftershocks reveal stress adjustment at the edges of the mainshock fault, flexural faulting on top of the downgoing Indian Plate and extensional faulting in the hanging wall of the MHT.

The 2015 April 25 Gorkha (Nepal) earthquake and its aftershocks: implications for lateral heterogeneity on the Main Himalayan Thrust

NASA Astrophysics Data System (ADS)

Kumar, Ajay; Singh, Shashwat K.; Mitra, S.; Priestley, K. F.; Dayal, Shankar

2017-02-01

The 2015 Gorkha earthquake (Mw 7.8) occurred by thrust faulting on a ˜150 km long and ˜70 km wide, locked downdip segment of the Main Himalayan Thrust (MHT), causing the Himalaya to slip SSW over the Indian Plate, and was followed by major-to-moderate aftershocks. Back projection of teleseismic P-wave and inversion of teleseismic body waves provide constraints on the geometry and kinematics of the main-shock rupture and source mechanism of aftershocks. The main-shock initiated ˜80 km west of Katmandu, close to the locking line on the MHT and propagated eastwards along ˜117° azimuth for a duration of ˜70 s, with varying rupture velocity on a heterogeneous fault surface. The main-shock has been modelled using four subevents, propagating from west-to-east. The first subevent (0-20 s) ruptured at a velocity of ˜3.5 km s- 1 on a ˜6°N dipping flat segment of the MHT with thrust motion. The second subevent (20-35 s) ruptured a ˜18° W dipping lateral ramp on the MHT in oblique thrust motion. The rupture velocity dropped from 3.5 km s- 1 to 2.5 km s- 1, as a result of updip propagation of the rupture. The third subevent (35-50 s) ruptured a ˜7°N dipping, eastward flat segment of the MHT with thrust motion and resulted in the largest amplitude arrivals at teleseismic distances. The fourth subevent (50-70 s) occurred by left-lateral strike-slip motion on a steeply dipping transverse fault, at high angle to the MHT and arrested the eastward propagation of the main-shock rupture. Eastward stress build-up following the main-shock resulted in the largest aftershock (Mw 7.3), which occurred on the MHT, immediately east of the main-shock rupture. Source mechanisms of moderate aftershocks reveal stress adjustment at the edges of the main-shock fault, flexural faulting on top of the downgoing Indian Plate and extensional faulting in the hanging wall of the MHT.

Teleseismic Lg of Semipalatinsk and Novaya Zemlya Nuclear Explosions Recorded by the GRF (Gräfenberg) Array: Comparison with Regional Lg (BRV) and their Potential for Accurate Yield Estimation

NASA Astrophysics Data System (ADS)

Schlittenhardt, J.

- A comparison of regional and teleseismic log rms (root-mean-square) Lg amplitude measurements have been made for 14 underground nuclear explosions from the East Kazakh test site recorded both by the BRV (Borovoye) station in Kazakhstan and the GRF (Gräfenberg) array in Germany. The log rms Lg amplitudes observed at the BRV regional station at a distance of 690km and at the teleseismic GRF array at a distance exceeding 4700km show very similar relative values (standard deviation 0.048 magnitude units) for underground explosions of different sizes at the Shagan River test site. This result as well as the comparison of BRV rms Lg magnitudes (which were calculated from the log rms amplitudes using an appropriate calibration) with magnitude determinations for P waves of global seismic networks (standard deviation 0.054 magnitude units) point to a high precision in estimating the relative source sizes of explosions from Lg-based single station data. Similar results were also obtained by other investigators (Patton, 1988; Ringdaletal., 1992) using Lg data from different stations at different distances.Additionally, GRF log rms Lg and P-coda amplitude measurements were made for a larger data set from Novaya Zemlya and East Kazakh explosions, which were supplemented with mb(Lg) amplitude measurements using a modified version of Nuttli's (1973, 1986a) method. From this test of the relative performance of the three different magnitude scales, it was found that the Lg and P-coda based magnitudes performed equally well, whereas the modified Nuttli mb(Lg) magnitudes show greater scatter when compared to the worldwide mb reference magnitudes. Whether this result indicates that the rms amplitude measurements are superior to the zero-to-peak amplitude measurement of a single cycle used for the modified Nuttli method, however, cannot be finally assessed, since the calculated mb(Lg) magnitudes are only preliminary until appropriate attenuation corrections are available for the specific path to GRF.

Investigation of Earthquake and Geyser Events in the Upper Geyser Basin of Yellowstone National Park from a Nodal Array

NASA Astrophysics Data System (ADS)

Garcia, S.; Karplus, M. S.; Farrell, J.; Lin, F. C.; Smith, R. B.

2017-12-01

A large seismic nodal array incorporating 133 three-component, 5-Hz geophones deployed for two weeks in early November 2015 in the Upper Geyser Basin recorded earthquake and hydrothermal activity. The University of Utah, the University of Texas at El Paso, and Yellowstone National Park collaborated to deploy Fairfield Nodal ZLand 3-C geophones concentrically centered around the Old Faithful Geyser with an average station spacing of 50 m and an aperture of 1 km. The array provided a unique dataset to investigate wave propagation through various fractures and active geysers in a hydrothermal field located over the Yellowstone hotspot. The complicated sub-surface features associated with the hydrothermal field appear to impact earthquake wave propagation in the Upper Geyser Basin and to generate seismic signals. Previous work using ambient noise cross-correlation has found an intricately fractured sub-surface that provides pathways for water beneath parts of the Upper Geyser Basin that likely feed Old Faithful and other nearby geysers and hot springs. For this study, we used the data to create visualizations of local earthquake, teleseismic earthquake, and hydrothermal events as they propagate through the array. These ground motion visualizations allow observation of wave propagation through the geyser field, which may indicate the presence of anomalous structure impacting seismic velocities and attenuation. Three teleseismic events were observed in the data, two 6.9MW earthquakes that occurred off the coast of Coquimbo, Colombia 9,000km from the array and one 6.5MW near the Aleutian Islands 4,500km from the array. All three teleseismic events observed in the data exhibited strong direct P-wave arrivals and several additional phases. One local earthquake event (2.5ML) 100km from the Upper Geyser Basin was also well-recorded by the array. Time-domain spectrograms show the dominant frequencies present in the recordings of these events. The two 6.9MW earthquakes in Chile were one hour apart and offered interesting signals that also included a geyser tremor between the two events.

Large earthquake rupture process variations on the Middle America megathrust

NASA Astrophysics Data System (ADS)

Ye, Lingling; Lay, Thorne; Kanamori, Hiroo

2013-11-01

The megathrust fault between the underthrusting Cocos plate and overriding Caribbean plate recently experienced three large ruptures: the August 27, 2012 (Mw 7.3) El Salvador; September 5, 2012 (Mw 7.6) Costa Rica; and November 7, 2012 (Mw 7.4) Guatemala earthquakes. All three events involve shallow-dipping thrust faulting on the plate boundary, but they had variable rupture processes. The El Salvador earthquake ruptured from about 4 to 20 km depth, with a relatively large centroid time of ˜19 s, low seismic moment-scaled energy release, and a depleted teleseismic short-period source spectrum similar to that of the September 2, 1992 (Mw 7.6) Nicaragua tsunami earthquake that ruptured the adjacent shallow portion of the plate boundary. The Costa Rica and Guatemala earthquakes had large slip in the depth range 15 to 30 km, and more typical teleseismic source spectra. Regional seismic recordings have higher short-period energy levels for the Costa Rica event relative to the El Salvador event, consistent with the teleseismic observations. A broadband regional waveform template correlation analysis is applied to categorize the focal mechanisms for larger aftershocks of the three events. Modeling of regional wave spectral ratios for clustered events with similar mechanisms indicates that interplate thrust events have corner frequencies, normalized by a reference model, that increase down-dip from anomalously low values near the Middle America trench. Relatively high corner frequencies are found for thrust events near Costa Rica; thus, variations along strike of the trench may also be important. Geodetic observations indicate trench-parallel motion of a forearc sliver extending from Costa Rica to Guatemala, and low seismic coupling on the megathrust has been inferred from a lack of boundary-perpendicular strain accumulation. The slip distributions and seismic radiation from the large regional thrust events indicate relatively strong seismic coupling near Nicoya, Costa Rica, patchy zones of strong seismic coupling in the shallowest megathrust region along Nicaragua and El Salvador, and small deeper patchy zones of strong seismic coupling near Guatemala, which can be reconciled with the geodetic observations as long as the strong coupling is limited to a small fraction of the megathrust area.

Regional Seismic Arrays and Nuclear Test Ban Verification

DTIC Science & Technology

1990-12-01

estimation has been difficult to automate, at least for regional and teleseismic signals. A neural network approach might be applicable here. The data must...use of trained neural networks . Of the 95 events examined, 66 were selected for the classification study based on high signal-to-noise ratio and...the International Joint Conference on Neural Networks , Washington, D.C., June, 1989. Menke, W. Geophysical Data Analysis : Discrete Inverse Theory

Insights into the lithospheric architecture of Iberia and Morocco from teleseismic body-wave attenuation

NASA Astrophysics Data System (ADS)

Bezada, Maximiliano J.

2017-11-01

The long and often complicated tectonic history of continental lithosphere results in lateral strength heterogeneities which in turn affect the style and localization of deformation. In this study, we produce a model for the attenuation structure of Iberia and northern Morocco using a waveform-matching approach on P-wave data from teleseismic deep-focus earthquakes. We find that attenuation is correlated with zones of intraplate deformation and seismicity, but do not find a consistent relationship between attenuation and recent volcanism. The main features of our model are low to moderate Δt* in the undeformed Tertiary basins of Spain and high Δt* in areas deformed by the Alpine orogeny. Additionally, low Δt* is found in areas where the Alboran slab is thought to be attached to the Iberian and African lithosphere, and high Δt* where it has detached. These features are robust with respect to inversion parameters, and are consistent with independent data. Very mild backazimuthal dependence of the measurements and comparison with previous results suggest that the source of the attenuation is sub-crustal. In line with other recent studies, the range of Δt* we observe is much larger than can be expected from lithospheric thickness or temperature variations.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gunawan, H.; Puspito, N. T.; Ibrahim, G.

The new approach method to determine the magnitude by using amplitude displacement relationship (A), epicenter distance ({Delta}) and duration of high frequency radiation (t) has been investigated for Tasikmalaya earthquake, on September 2, 2009, and their aftershock. Moment magnitude scale commonly used seismic surface waves with the teleseismic range of the period is greater than 200 seconds or a moment magnitude of the P wave using teleseismic seismogram data and the range of 10-60 seconds. In this research techniques have been developed a new approach to determine the displacement amplitude and duration of high frequency radiation using near earthquake. Determinationmore » of the duration of high frequency using half of period of P waves on the seismograms displacement. This is due tothe very complex rupture process in the near earthquake. Seismic data of the P wave mixing with other wave (S wave) before the duration runs out, so it is difficult to separate or determined the final of P-wave. Application of the 68 earthquakes recorded by station of CISI, Garut West Java, the following relationship is obtained: Mw = 0.78 log (A) + 0.83 log {Delta}+ 0.69 log (t) + 6.46 with: A (m), d (km) and t (second). Moment magnitude of this new approach is quite reliable, time processing faster so useful for early warning.« less

Upper mantle anisotropic attenuation of the Sierra Nevada and surroundings

NASA Astrophysics Data System (ADS)

Bernardino, M. J.; Jones, C. H.; Monsalve, G.

2016-12-01

We investigate the contribution of anelasticity in the generation of seismic velocity variations within the upper mantle of the Sierra Nevada and surrounding regions through teleseismic shear-wave attenuation. Given that anelastic effects are most sensitive to temperature and hydration and less to composition and small degrees of partial melt, we aim constrain the thermal structure beneath this region and identify locations where elevated upper mantle temperatures dominate. We also investigate the dependence of shear-wave attenuation on direction by accounting for seismic anisotropy in our measurements. S-wave t* values are determined from teleseismic S- and SKS- phases recorded on permanent and temporary deployments within the California region with particular focus on the Sierra Nevada Earthscope Project (SNEP) and the Sierran Paradox Experiment (SPE) stations. S-waveforms are rotated into the Sierran SFast, N75°E, and SSlow, N15°W, components. Following the method of Stachnik et al., (2004), S-wave spectra for each event are jointly inverted for a single seismic moment, M0k, and corner frequency, fck, for each event, and separate t* for each ray path. The resulting t*Fast and t*Slow measurements are then inverted for three-dimensional variations in (1/QFast) and (1/QSlow). Results are compared with previous magnetotelluric, surface heat flow, and body-wave velocity inversion studies.

Seismic anisotropy of northeastern Algeria from shear-wave splitting analysis

NASA Astrophysics Data System (ADS)

Radi, Zohir; Yelles-Chaouche, Abdelkrim; Bokelmann, Götz

2015-11-01

There are few studies of internal deformation under northern Africa; here we present such a study. We analyze teleseismic shear-wave splitting for northeast Algeria, to improve our knowledge of lithospheric and asthenospheric deformation mechanisms in this region. We study waveform data generated by tens of teleseismic events recorded at five recently installed broadband (BB) stations in Algeria. These stations cover an area 2° across, extending from the Tellian geological units in the North to the Saharan Atlas units in the South. Analysis of SKS-wave splitting results insignificant spatial variations in fast polarization orientation, over a scale length of at most 100 km. The seismic anisotropy shows three clear spatial patterns. A general ENE-WSW orientation is observed under the stations in the north. This polarization orientation follows the direction of the Tell Atlas mountain chain, which is perpendicular to the convergence direction between Africa and Eurasia. Delay times vary significantly across the region, between 0.6 and 2.0 s. At several stations there is an indication of a WNW-ESE polarization orientation, which is apparently related to a later geodynamic evolutionary phase in this region. A third pattern of seismic anisotropy emerges in the South, with an orientation of roughly N-S. We discuss these observations in light of geodynamic models and present-day geodetic motion.

Comparative study of two tsunamigenic earthquakes in the Solomon Islands: 2015 Mw 7.0 normal-fault and 2013 Santa Cruz Mw 8.0 megathrust earthquakes

NASA Astrophysics Data System (ADS)

Heidarzadeh, Mohammad; Harada, Tomoya; Satake, Kenji; Ishibe, Takeo; Gusman, Aditya Riadi

2016-05-01

The July 2015 Mw 7.0 Solomon Islands tsunamigenic earthquake occurred ~40 km north of the February 2013 Mw 8.0 Santa Cruz earthquake. The proximity of the two epicenters provided unique opportunities for a comparative study of their source mechanisms and tsunami generation. The 2013 earthquake was an interplate event having a thrust focal mechanism at a depth of 30 km while the 2015 event was a normal-fault earthquake occurring at a shallow depth of 10 km in the overriding Pacific Plate. A combined use of tsunami and teleseismic data from the 2015 event revealed the north dipping fault plane and a rupture velocity of 3.6 km/s. Stress transfer analysis revealed that the 2015 earthquake occurred in a region with increased Coulomb stress following the 2013 earthquake. Spectral deconvolution, assuming the 2015 tsunami as empirical Green's function, indicated the source periods of the 2013 Santa Cruz tsunami as 10 and 22 min.

The 2015 Illapel earthquake, central Chile: A type case for a characteristic earthquake?

NASA Astrophysics Data System (ADS)

Tilmann, F.; Zhang, Y.; Moreno, M.; Saul, J.; Eckelmann, F.; Palo, M.; Deng, Z.; Babeyko, A.; Chen, K.; Baez, J. C.; Schurr, B.; Wang, R.; Dahm, T.

2016-01-01

On 16 September 2015, the MW = 8.2 Illapel megathrust earthquake ruptured the Central Chilean margin. Combining inversions of displacement measurements and seismic waveforms with high frequency (HF) teleseismic backprojection, we derive a comprehensive description of the rupture, which also predicts deep ocean tsunami wave heights. We further determine moment tensors and obtain accurate depth estimates for the aftershock sequence. The earthquake nucleated near the coast but then propagated to the north and updip, attaining a peak slip of 5-6 m. In contrast, HF seismic radiation is mostly emitted downdip of the region of intense slip and arrests earlier than the long period rupture, indicating smooth slip along the shallow plate interface in the final phase. A superficially similar earthquake in 1943 with a similar aftershock zone had a much shorter source time function, which matches the duration of HF seismic radiation in the recent event, indicating that the 1943 event lacked the shallow slip.

Interaction between the Dauki and the Indo-Burman convergence boundaries from teleseismic and locally recorded earthquake data

NASA Astrophysics Data System (ADS)

Howe, M.; Moulik, P.; Seeber, L.; Kim, W.; Steckler, M. S.

2012-12-01

The Himalayan and the Burma Arcs converge onto the Indian plate from opposite sides near their syntaxial juncture and have reduced it to a sliver. Both geology and seismicity point to recent internal deformation and high seismogenic potential within this sliver. Large historical earthquakes, including the Great Indian earthquake of 1897 (Mw ~8.1), along with the recent seismicity, suggest that the cratonic blocks in the region are bounded by active faults. The most prominent is the E-W trending Dauki Fault, a deeply-rooted, north-dipping thrust fault, situated between the Shillong massif to the north and the Sylhet Basin to the south. Along the Burma Arc, the subducted seismogenic slab of the Indian plate is continuous north to the syntaxis. Yet the Naga and Tripura segments of the accretionary fold belt, respectively north and south of the easterly extrapolation of the Dauki fault, are distinct. Accretion has advanced far westward into the foredeep of the Dauki structure along the front of the Tripura segment, while it has remained stunted facing the uplifted Shillong massif along the Naga segment. Moreover, the Dauki topographic front can be traced eastwards across the Burma Arc separating the two segments. Recent earthquakes support the hypothesis that the Dauki convergence structure continues below the Burma accretionary belt. Using teleseismic and regional data from the deployment of a local network, we explore the interaction of the Dauki thrust fault with the Burma Arc subduction zone. Preliminary observations include: While seismicity is concentrated in the slab at the eastward extrapolation of the Dauki fault, shallow seismicity is diffuse and does not illuminate the Dauki fault itself. P-axes in moment-tensor solutions of earthquakes within the Indian plate tend to be directed N-S and are locally parallel to the India-Burma boundary, particularly in the slab. T-axes tend to be oriented E-W with a strong tendency to follow the slab down dip. This pattern is remarkably consistent, despite the scattered seismicity, and suggests that the stress in the Indian plate, including the subducted oceanic portion of the plate, is still primarily controlled by the Himalayan collision to the north and down-dip pull by the Burma slab. Moment tensor solutions for some of the shallow earthquakes along the fold belt are consistent with geodetic results, showing partitioning of the oblique India-Burma convergence between belt-parallel dextral faults and belt-normal shortening by thrust faults. Relocations of the events using the double-difference algorithm may provide additional constraints on the geometry of the slab. In addition to the analysis of teleseismic data, a network of six seismic stations was also installed in Bangladesh in the region surrounding Sylhet, south of the Shillong Plateau during 2007-2008. Over 200 regional and local events are detected and located by the Sylhet array. About a dozen events are large enough allowing us to determine focal depths and mechanisms that will augment the catalog of the teleseismic events, providing additional insights into the tectonics in the region.

A probabilistic framework for single-station location of seismicity on Earth and Mars

NASA Astrophysics Data System (ADS)

Böse, M.; Clinton, J. F.; Ceylan, S.; Euchner, F.; van Driel, M.; Khan, A.; Giardini, D.; Lognonné, P.; Banerdt, W. B.

2017-01-01

Locating the source of seismic energy from a single three-component seismic station is associated with large uncertainties, originating from challenges in identifying seismic phases, as well as inevitable pick and model uncertainties. The challenge is even higher for planets such as Mars, where interior structure is a priori largely unknown. In this study, we address the single-station location problem by developing a probabilistic framework that combines location estimates from multiple algorithms to estimate the probability density function (PDF) for epicentral distance, back azimuth, and origin time. Each algorithm uses independent and complementary information in the seismic signals. Together, the algorithms allow locating seismicity ranging from local to teleseismic quakes. Distances and origin times of large regional and teleseismic events (M > 5.5) are estimated from observed and theoretical body- and multi-orbit surface-wave travel times. The latter are picked from the maxima in the waveform envelopes in various frequency bands. For smaller events at local and regional distances, only first arrival picks of body waves are used, possibly in combination with fundamental Rayleigh R1 waveform maxima where detectable; depth phases, such as pP or PmP, help constrain source depth and improve distance estimates. Back azimuth is determined from the polarization of the Rayleigh- and/or P-wave phases. When seismic signals are good enough for multiple approaches to be used, estimates from the various methods are combined through the product of their PDFs, resulting in an improved event location and reduced uncertainty range estimate compared to the results obtained from each algorithm independently. To verify our approach, we use both earthquake recordings from existing Earth stations and synthetic Martian seismograms. The Mars synthetics are generated with a full-waveform scheme (AxiSEM) using spherically-symmetric seismic velocity, density and attenuation models of Mars that incorporate existing knowledge of Mars internal structure, and include expected ambient and instrumental noise. While our probabilistic framework is developed mainly for application to Mars in the context of the upcoming InSight mission, it is also relevant for locating seismic events on Earth in regions with sparse instrumentation.

Surface wave imaging of the Lithosphere-Asthenosphere system beneath 0-80 My seafloor of the equatorial Mid-Atlantic Ridge from the PI-LAB Experiment

NASA Astrophysics Data System (ADS)

Rychert, C.; Harmon, N.; Kendall, J. M.; Agius, M. R.; Tharimena, S.

2017-12-01

Oceanic lithosphere is the simplest realization of the tectonic plate, yet there are several indications that the evolution of oceanic lithosphere is more complicated than simple half space cooling models, i.e. sharp seismic discontinuities at 60-80 km depth, flattening of bathymetry at > 80 My. A deeper understanding of the complexities of oceanic lithosphere requires in situ measurements, and to date much work has focused on the Pacific ocean. The PI-LAB (Passive Imaging of the Lithosphere-Asthenosphere Boundary) experiment deployed 39 ocean bottom seismometers and 39 ocean bottom magnetotelluric instruments around the equatorial Mid Atlantic ridge from 0-80 My old seafloor. We analysed Rayleigh wave dispersion at 18-143 s period using teleseismic events and Rayleigh wave and Love wave dispersion from 5-22 s period using ambient noise. We observe both fundamental mode and first higher mode Rayleigh waves at 5 - 18 s periods, with average phase velocities that range from 1.5 km/s at 5 s period to 4.31 km/s at 143 s, and fundamental mode Love waves, with average phase velocities ranging from 4.00 km/s at 5 s to 4.51 at 22 s. We invert these phase velocities for radially anisotropic shear velocity structure and find a 60 km thick fast lid for the region with velocities of 4.62 km/s, and x values up to 1.08 indicating radial anisotropy is required in the upper 200 km. We also examined the variation in phase velocity as function seafloor age across the region using the teleseismic Rayleigh wave dataset. From 25-81 s period we find low velocities beneath young seafloor ages. We find velocity systematically increases with seafloor age. At 40 My old seafloor, the phase velocities stop increasing and flatten out. At the longest periods (> 81 s) we observe no clear relationship with seafloor age, suggesting that lithospheric thickening ceases beneath seafloor > 50 My old.

Source Mechanisms of Destructive Tsunamigenic Earthquakes occurred along the Major Subduction Zones

NASA Astrophysics Data System (ADS)

Yolsal-Çevikbilen, Seda; Taymaz, Tuncay; Ulutaş, Ergin

2016-04-01

Subduction zones, where an oceanic plate is subducted down into the mantle by tectonic forces, are potential tsunami locations. Many big, destructive and tsunamigenic earthquakes (Mw > 7.5) and high amplitude tsunami waves are observed along the major subduction zones particularly near Indonesia, Japan, Kuril and Aleutan Islands, Gulf of Alaska, Southern America. Not all earthquakes are tsunamigenic; in order to generate a tsunami, the earthquake must occur under or near the ocean, be large, and create significant vertical movements of the seafloor. It is also known that tsunamigenic earthquakes release their energy over a couple of minutes, have long source time functions and slow-smooth ruptures. In this study, we performed point-source inversions by using teleseismic long-period P- and SH- and broad-band P-waveforms recorded by the Federation of Digital Seismograph Networks (FDSN) and the Global Digital Seismograph Network (GDSN) stations. We obtained source mechanism parameters and finite-fault slip distributions of recent destructive ten earthquakes (Mw ≥ 7.5) by comparing the shapes and amplitudes of long period P- and SH-waveforms, recorded in the distance range of 30° - 90°, with synthetic waveforms. We further obtained finite-fault rupture histories of those earthquakes to determine the faulting area (fault length and width), maximum displacement, rupture duration and stress drop. We applied a new back-projection method that uses teleseismic P-waveforms to integrate the direct P-phase with reflected phases from structural discontinuities near the source, and customized it to estimate the spatio-temporal distribution of the seismic energy release of earthquakes. Inversion results exhibit that recent tsunamigenic earthquakes show dominantly thrust faulting mechanisms with small amount of strike-slip components. Their focal depths are also relatively shallow (h < 40 km). As an example, the September 16, 2015 Illapel (Chile) earthquake (Mw: 8.3; h: 26 km) reflects the major characteristics of the Peru-Chile subduction zone between the Nazca and South America Plates. The size, location, depth and focal mechanism of this earthquake are consistent with its occurrence on the megathrust interface in this region. This study is supported by the Scientific and Technological Research Council of Turkey (TUBITAK, Project No: CAYDAG - 114Y066).

Teleseismic Array Studies of Earth's Core-Mantle Boundary

NASA Astrophysics Data System (ADS)

Alexandrakis, Catherine

2011-12-01

The core mantle boundary (CMB) is an inaccessible and complex region, knowledge of which is vital to our understanding of many Earth processes. Above it is the heterogeneous lower-mantle. Below the boundary is the outer-core, composed of liquid iron, and/or nickel and some lighter elements. Elucidation of how these two distinct layers interact may enable researchers to better understand the geodynamo, global tectonics, and overall Earth history. One parameter that can be used to study structure and limit potential chemical compositions is seismic-wave velocity. Current global-velocity models have significant uncertainties in the 200 km above and below the CMB. In this thesis, these regions are studied using three methods. The upper outer core is studied using two seismic array methods. First, a modified vespa, or slant-stack method is applied to seismic observations at broadband seismic arrays, and at large, dense groups of broadband seismic stations dubbed 'virtual' arrays. Observations of core-refracted teleseismic waves, such as SmKS, are used to extract relative arrivaltimes. As with previous studies, lower -mantle heterogeneities influence the extracted arrivaltimes, giving significant scatter. To remove raypath effects, a new method was developed, called Empirical Transfer Functions (ETFs). When applied to SmKS waves, this method effectively isolates arrivaltime perturbations caused by outer core velocities. By removing raypath effects, the signals can be stacked further reducing scatter. The results of this work were published as a new 1D outer-core model, called AE09. This model describes a well-mixed outer core. Two array methods are used to detect lower mantle heterogeneities, in particular Ultra-Low Velocity Zones (ULVZs). The ETF method and beam forming are used to isolate a weak P-wave that diffracts along the CMB. While neither the ETF method nor beam forming could adequately image the low-amplitude phase, beam forms of two events indicate precursors to the SKS and SKKS phase, which may be ULVZ indicators. Finally, cross-correlated observed and modelled beams indicate a tendency towards a ULVZ-like lower mantle in the study region.

Near Source Contributions to Teleseismic P Wave Coda and Regional Phases

DTIC Science & Technology

1991-04-27

Pasadena, CA 91-125 Mr. William J. Best Prof. F. A. Dahlen 907 Westwood Drive Geological and Geophysical Sciences Vienna, VA 22180 Princeton...Station S-CUBED University of California A Division of Maxwell Laboratory Berkeley, CA 94720 P.O.Box 1620 La Jolla, CA 92038-1620 2 Prof. William ...Geosciences- Building #77 University of Arizona Tucson, AZ 85721 Dr. William Wortman Mission Research Corporation 8560 Cinderbed Rd. Suite # 700 Newington

Arms Control and Nonproliferation Technologies, First Quarter 1994

DTIC Science & Technology

1994-01-01

earthquake show detectable signals PATWINGSPAC, U.S. Navy, (3) an array of the U.S. for magnitude 3 events at 320-360 kilometers. Initial Navy SOSUS system...30 explosions shots, in generating seismic signals at regional or greater than 50 tons, including one shot greater than teleseismic distances). These... general Rainier Mesa earthquakes from Rock Valley at NTS appear to be events (Area 12) of roughly the same yield had signif- intermediate between the

Advances in Mixed Signal Processing for Regional and Teleseismic Arrays

DTIC Science & Technology

2006-08-15

1: Mixture of signals from two earthquakes from south of Africa and the Philippines observed at USAEDS long-period seismic array in Korea. Correct...window where the detector will miss valid signals . 2 Approaches to detecting signals on arrays all focus on the basic model that expresses the observed...possible use in detecting infrasound signals . The approach is based on orthogonal- ity properties of the eigen vectors of the spectral matrix under a

Calculation of Source and Structure Parameters at Regional and Teleseismic Distances

DTIC Science & Technology

1989-04-13

Vedder. Am.. 67. 1029-1050, 1977. Geology of the Los Angeles Basin area, California-An Introduc- Langston. C. A.. Structure under Mount Rainier , Washington...Assistant Professor of Geophysics. David H. Eggler, Professor of Petrology , Chair of Graduate Program in Geosciences. ’)7 We approve the thesis of Rotert... petrologic , and tectonic models of this region. Data from other areas indicate that upper mantle P-wave velocities and structure correlate with surface

Broadband Studies of Semsmic Sources at Regional and Teleseismic Distances Using Advanced Time Series Analysis Methods. Volume 1.

DTIC Science & Technology

1991-03-21

discussion of spectral factorability and motivations for broadband analysis, the report is subdivided into four main sections. In Section 1.0, we...estimates. The motivation for developing our multi-channel deconvolution method was to gain information about seismic sources, most notably, nuclear...with complex constraints for estimating the rupture history. Such methods (applied mostly to data sets that also include strong rmotion data), were

Seismic Tomography of the Arabian-Eurasian Collision Zone and Surrounding Areas

DTIC Science & Technology

2010-05-20

zone. The crustal models correlate well with geologic and tectonic features. The upper mantle tomograms show the images of the subducted Neotethys...We first obtain Pn and Sn velocities using local and regional arrival time data. Second, we obtain the 3-D crustal P and S velocity models...teleseismic tomography provides a high-resolution, 3-D P-wave velocity model for the crust, upper mantle, and the transition zone. The crustal models

Rapid finite-fault inversions in Southern California using Cybershake Green's functions

NASA Astrophysics Data System (ADS)

Thio, H. K.; Polet, J.

2017-12-01

We have developed a system for rapid finite fault inversion for intermediate and large Southern California earthquakes using local, regional and teleseismic seismic waveforms as well as geodetic data. For modeling the local seismic data, we use 3D Green's functions from the Cybershake project, which were made available to us courtesy of the Southern California Earthquake Center (SCEC). The use of 3D Green's functions allows us to extend the inversion to higher frequency waveform data and smaller magnitude earthquakes, in addition to achieving improved solutions in general. The ultimate aim of this work is to develop the ability to provide high quality finite fault models within a few hours after any damaging earthquake in Southern California, so that they may be used as input to various post-earthquake assessment tools such as ShakeMap, as well as by the scientific community and other interested parties. Additionally, a systematic determination of finite fault models has value as a resource for scientific studies on detailed earthquake processes, such as rupture dynamics and scaling relations. We are using an established least-squares finite fault inversion method that has been applied extensively both on large as well as smaller regional earthquakes, in conjunction with the 3D Green's functions, where available, as well as 1D Green's functions for areas for which the Cybershake library has not yet been developed. We are carrying out validation and calibration of this system using significant earthquakes that have occurred in the region over the last two decades, spanning a range of locations and magnitudes (5.4 and higher).

Waveform complexity caused by near trench structure and its impact on earthquake source study: application to the 2015 Illapel earthquake sequence

NASA Astrophysics Data System (ADS)

Qian, Y.; Wei, S.; Wu, W.; Ni, S.

2017-12-01

Among various types of 3D heterogeneity in the Earth, trench might be the most complex systems, which includes rapidly varying bathymetry and usually thick sediment below water layer. These structure complexities can cause substantial waveform complexities on seismograms, but their corresponding impact on the earthquake source studies has not yet been well understood. Here we explore those effects via studies of two moderate aftershocks (one near the coast while the other close to the Peru-Chile trench axis) in the 2015 Illapel earthquake sequence. The horizontal locations and depths of these two events are poorly constrained and the reported results of various agencies display substantial variations. Thus, we first relocated the epicenters using the P-wave first arrivals and determined other parameters by waveform fitting. In a jackknifing way, we found that the trench event has large differences between regional and teleseismic solutions, in particular for depth, while the coastal event shows consistent results. The teleseismic P/Pdiff waves between these two events also display distinctly different features. More specifically, the trench event has more complex P/Pdiff waves and stronger coda waves, in terms of amplitude and duration (longer than 100s). The coda waves are coherent across stations at different distances and azimuths, indicating a more likely origin of scattering waves due to 3D heterogeneity near trench. To quantitatively model those 3D effects, we adopted a hybrid waveform simulation approach that computes the 3D wavefield in the source region by the Spectral Element Method (SEM) and then propagates the wavefield to teleseismic and shadow zone distances through the Direct Solution Method (DSM). We incorporated the GEBCO bathymetry and water layer into the SEM simulations and assumed the IASP91 1D model for DSM computation. Comparing with the poor 1D synthetics fitting to the data, we do obtain dramatic improvement in 3D waveform fittings across a series of frequency bands. With sensitivity tests of 3D waveform modeling, the centroid longitude and depth for the near trench event are refined. Our study suggests that the complex trench structure must be taken into account for a reliable analysis of shallow earthquake near trench, in particular for the shallowest tsunamigenic earthquakes.

Teleseismic Upper-mantle Tomography of the Tanlu Fault Zone in East China

NASA Astrophysics Data System (ADS)

Lei, J., Sr.; Zhao, D.; Du, M.; Mi, Q.; Lu, M.

2017-12-01

The Tanlu fault zone, NNE-SSW oriented with strike-slip motions, is the most significant active fault in East China. The great 1668 Tancheng earthquake (Ms 8.5) occurred on this fault zone, which is located above the stagnant Pacific slab in the mantle transition zone (MTZ). To the east of the Tancheng earthquake epicenter and under the southernmost Korean Peninsula to westernmost Japan, the subducting Pacific slab exhibits a sharp change in its geometry. However, the relationship between the Pacific slab and the great earthquake on the Tanlu fault is unclear. To address this issue, we conduct teleseismic P-wave tomography using 44,715 relative arrival times. These data are collected from high-quality seismograms of 838 teleseismic events (M > 5.5; epicenter distances of 30-90 degrees) recorded at 126 provincial seismic stations around the Tanlu fault zone in East China. Our results show that at depths < 150 km, high velocity (high-V) anomalies appear to the west of the Tanlu fault, whereas some low velocity (low-V) anomalies are visible to the east of the fault zone. Strong lateral heterogeneities are revealed along the fault zone. At depths of 230-470 km, to the northwest of the Tanlu fault, there are obvious low-V anomalies which may reflect hot and wet mantle upwelling, whereas to the east, some high-V anomalies are visible, which may reflect the detached Eurasian lithosphere. In the MTZ, both high-V and low-V anomalies are visible, and the widespread high-V anomalies may reflect the stagnant Pacific slab. Beneath the hypocenter of the 1668 Tancheng earthquake, a prominent low-V anomaly is revealed in the upper mantle down the MTZ depth, which may reflect upwelling flow of hot and wet materials. Fluids from the upwelling mantle flow may have played a key role in the generation of the Tancheng earthquake. Integrating with previous findings, our present results suggest that the Tancheng earthquake could be related to the sharp change in the Pacific slab geometry, the eastward retreat of the Pacific slab, as well as some slab-materials collapsing down to the lower mantle resulted from the gravity effect and/or phase transition, which may cause the low-V anomaly in the MTZ. This work is supported by NSFC (41530212 and 41674091).

High-resolution Teleseismic Tomography Reveals a Complex Lithospheric Structure Beneath the North Anatolian Fault

NASA Astrophysics Data System (ADS)

Papaleo, E.; Cornwell, D. G.; Rawlinson, N.

2016-12-01

We present high-resolution tomography images of a major active continental strike slip fault zone, the North Anatolian Fault (NAF) in northern Turkey. Historical seismic records show that the NAF, with a length of 1500 km and a current slip rate of 25 mm/yr, is capable of producing large magnitude earthquakes that have activated different segments of the fault in a westward progression towards the study region, where the devastating Izmit and Düzce events occurred in 1999. The NAF poses a major seismic hazard to the city of Istanbul, situated close to one of the two strands into which the fault splays east of the Sea of Marmara. In order to improve our understanding of the lower crust and upper mantle properties that influence fault dynamics throughout the seismic cycle, we constrain NAF structure across the Moho in unprecedented detail by applying teleseismic tomography to data recorded by an array of 70 temporary seismic stations deployed with 7 km spacing (Dense Array for North Anatolia, DANA). High quality recordings of teleseismic earthquakes combined with the dense nature of the array allow high-resolution (i.e. horizontal and vertical resolution of 8 and 15 km, respectively) 3D seismic imaging of the velocity structure beneath the NAF. The northern branch of the NAF coincides with an abrupt change between opposite polarity velocity anomalies and can be traced to at least Moho depths ( 36 km) with a width of ≤8 km. A similar pattern of antithetic anomalies occurs over a horizontal distance of 30-50 km below the Moho and may indicate a widening shear zone as it passes from the crust into the upper mantle. We find evidence for significant along-strike variation in NAF structure over distances of ≤20 km and interpret an east-to-west narrowing of upper mantle slow velocity anomalies (from 50 to 30 km) to represent laterally variable strain focussing within the lithosphere. Our observations are consistent with the notion that the NAF marks the boundary between compositionally distinct lithospheres with different tectonic histories and reactivates the pre-existing Intra-Pontide suture zone. We discuss our results in terms of the influence of lithosphere heterogeneity on the development and evolution of global continental strike-slip fault zones and assess the applicability of current shear zone deformation models.

Teleseismic Investigations of the Malawi and Luangwa Rift Zones: Ongoing Observations From the SAFARI Experiment

NASA Astrophysics Data System (ADS)

Reed, C. A.; Gao, S. S.; Liu, K. H.; Yu, Y.; Chindandali, P. R. N.; Massinque, B.; Mdala, H. S.; Mutamina, D. M.

2015-12-01

In order to evaluate the influence of crustal and mantle heterogeneities upon the initiation of the Malawi rift zone (MRZ) and reactivation of the Zambian Luangwa rift zone (LRZ) subject to Cenozoic plate boundary stress fields and mantle buoyancy forces, we installed and operated 33 Seismic Arrays For African Rift Initiation (SAFARI) three-component broadband seismic stations in Malawi, Mozambique, and Zambia from 2012 to 2014. During the twenty-four month acquisition period, nearly 6200 radial receiver functions (RFs) were recorded. Stations situated within the MRZ, either along the coastal plains or within the Shire Graben toward the south, report an average crustal thickness of 42 km relative to approximately 46 km observed at stations located along the rift flanks. This implies the juvenile MRZ is characterized by a stretching factor not exceeding 1.1. Meanwhile, P-to-S velocity ratios within the MRZ increase from 1.71 to 1.82 in southernmost Malawi, indicating a substantial modification of the crust during Recent rifting. Time-series stacking of approximately 5500 RFs recorded by the SAFARI and 44 neighboring network stations reveals an apparent uplift of 10 to 15 km along both the 410- and 660-km mantle transition zone (MTZ) discontinuities beneath the MRZ and LRZ which, coupled with an apparently normal 250-km MTZ thickness, implies a first-order high-velocity contribution from thickened lithosphere. Preliminary manual checking of SAFARI shear-wave splitting (SWS) measurements provides roughly 650 high-quality XKS phases following a component re-orientation to correct station misalignments. Regional azimuthal variations in SWS fast orientations are observed, from rift-parallel in the vicinity of the LRZ to rift-oblique in the MRZ. A major 60° rotation in the fast orientation occurs at approximately 31°E, possibly resulting from the modulation of mantle flow around a relatively thick lithospheric keel situated between the two rift zones.

Topography of the lithosphere-asthenosphere boundary below the Upper Rhine Graben Rift and the volcanic Eifel region, Central Europe

NASA Astrophysics Data System (ADS)

Seiberlich, C. K. A.; Ritter, J. R. R.; Wawerzinek, B.

2013-09-01

We study the crust-mantle and lithosphere-asthenosphere boundaries (Moho and LAB) in Central Europe, specifically below the Upper Rhine Graben (URG) rift, the Eifel volcanic region and their surrounding areas. Teleseismic recordings at permanent and mobile stations are analysed to search for shear (S) wave to compressional (P) wave converted phases. After a special processing these phases are identified in shear wave receiver functions (S-RFs). Conversions from the Moho at 2.9-3.3 s arrival time are the clearest signals in the S-RFs and indicate a relatively flat Moho at 27-30 km depth. A negative polarity conversion signal at 7-9 s arrival time can be explained with a low shear wave velocity zone (LVsZ) in the upper mantle. We use forward S-RF waveform modelling and Monte-Carlo techniques to determine shear wave velocity (vs)-depth (z) profiles which explain the observed S-RF and which outline variations of the lithospheric thickness in the study region. Across the URG rift and its surrounding mountain ranges (Black Forest, Odenwald etc.) the LAB is at a depth of about 60 ± 5 km. This depth is found for the rift itself as well as for the rift shoulders. Southeast and southwest of the URG, in the regions of the Swabian Alb and Vosges Mountains, the LAB dips to about 78 ± 5 km depth. In the volcanic Eifel region the LAB is at a much shallower depth of just 41 ± 5 km. There an upwelling mantle plume thermally eroded the lower lithosphere. The reduction of vs is about 2%-4% in the upper asthenosphere compared to the lower lithosphere. This vs contrast may be explained with a low portion of partial melt or hydrous minerals in the asthenosphere.

Moho Depth and Geometry in the Illinois Basin Region Based on Gravity and Seismic Data from an EarthScope FlexArray Experiment

NASA Astrophysics Data System (ADS)

Curcio, D. D.; Pavlis, G. L.; Yang, X.; Hamburger, M. W.; Zhang, H.; Ravat, D.

2017-12-01

We present results from a combined analysis of seismic and gravity in the Illinois Basin region that demonstrate the presence of an unusually deep and highly variable Moho discontinuity. We construct a new, high-resolution image of the Earth's crust beneath the Illinois Basin using teleseismic P-wave receiver functions from the EarthScope OIINK (Ozarks, Illinois, INdiana, Kentucky) Flexible Array and the USArray Transportable Array. Our seismic analyses involved data from 143 OIINK stations and 80 USArray stations, using 3D plane-wave migration and common conversion point (CCP) stacking of P-to-S conversion data. Seismic interpretation has been done using the seismic exploration software package Petrel. One of the most surprising results is the anomalous depth of the Moho in this area, ranging from 41 to 63 km, with an average depth of 50 km. This thickened crust is unexpected in the Illinois Basin area, which has not been subject to convergence and mountain building processes in the last 900 Ma. This anomalously thick crust in combination with the minimal topography requires abnormally dense lower crust or unusually light upper mantle in order to retain gravitational equilibrium. Combining gravity modeling with the seismically identified Moho and a ubiquitous lower crustal boundary, we solve for the density variation of the middle and lower crust. We test the hypothesis that the anomalously thick crust and its high lower crustal layer observed in most of the central and southeastern Illinois Basin predates the formation and development of the current Illinois Basin. Post-formation tectonic activity, such as late Precambrian rifting or underplating are inferred to have modified the crustal thickness as well. The combination of high-resolution seismic data analysis and gravity modeling promises to provide additional insight into the geometry and composition of the lower crust in the Illinois Basin area.

Apulian crust: Top to bottom

NASA Astrophysics Data System (ADS)

Amato, Alessandro; Bianchi, Irene; Agostinetti, Nicola Piana

2014-12-01

We investigate the crustal seismic structure of the Adria plate using teleseismic receiver functions (RF) recorded at 12 broadband seismic stations in the Apulia region. Detailed models of the Apulian crust, e.g. the structure of the Apulian Multi-layer Platform (AMP), are crucial for assessing the presence of potential décollements at different depth levels that may play a role in the evolution of the Apenninic orogen. We reconstruct S-wave velocity profiles applying a trans-dimensional Monte Carlo method for the inversion of RF data. Using this method, the resolution at the different depth level is completely dictated by the data and we avoid introducing artifacts in the crustal structure. We focus our study on three different key-elements: the Moho depth, the lower crust S-velocity, and the fine-structure of the AMP. We find a well defined and relatively flat Moho discontinuity below the region at 28-32 km depth, possibly indicating that the original Moho is still preserved in the area. The lower crust appears as a generally low velocity layer (average Vs = 3.7 km/s in the 15-26 km depth interval), likely suggestive of a felsic composition, with no significant velocity discontinuities except for its upper and lower boundaries where we find layering. Finally, for the shallow structure, the comparison of RF results with deep well stratigraphic and sonic log data allowed us to constrain the structure of the AMP and the presence of underlying Permo-Triassic (P-T) sediments. We find that the AMP structure displays small-scale heterogeneities in the region, with a thickness of the carbonates layers varying between 4 and 12 km, and is underlain by a thin, discontinuous layer of P-T terrigenous sediments, that are lacking in some areas. This fact may be due to the roughness in the original topography of the continental margins or to heterogeneities in its shallow structure due to the rifting process.

Insights on the lithospheric structure of the Zagros mountain belt from seismological data analysis

NASA Astrophysics Data System (ADS)

Paul, A.; Kaviani, A.; Vergne, J.; Hatzfeld, D.; Mokhtari, M.

2003-04-01

As part of a French-Iranian collaboration, we installed a temporary seismological network across the Zagros for 4.5 months in 2000-2001 to investigate the lithospheric structure of the mountain belt. The network included 65 stations located along a 600-km long line (average spacing of ˜10 km) from the coast of the Persian Gulf to the stable block of Central Iran. A migrated depth cross-section computed from radial receiver functions displays clear P-to-S conversions at the Moho beneath most of the profile. The average Moho depth is 45 to 50 km beneath the folded belt. It deepens rather abruptly beneath the suture zone of the MZT (Main Zagros Thrust) and the Sanandaj-Sirjan (SS) metamorphic zone. The maximum crustal thickness of ˜65 km is reached 50 km NE of the surface trace of the MZT. The region of over-thickened crust is shifted to the NE with respect to the areas of highest elevations and the strongest negative Bouguer anomaly. To the NE, the crust of the block of Central Iran is 40-km thick on average. Two patches of Ps converted energy can be seen below the Moho in the northern half of the transect that cannot be attributed to multiple reflections. Teleseismic P residual travel time curves display lateral variations as large as 1.5 s with both long (faster arrivals in the SW than in the NE) and short-scale variations (in the MZT region). They were inverted for variations of P wave velocity with the ACH technique. The crustal layer exhibits rather strong lateral variations of Vp with lower velocities under the MZT and the Urumieh-Dokhtar magmatic assemblage, and faster velocities under the SS zone. In the mantle, a clear difference appears between the faster P wave velocities of the Arabian craton and the relatively lower velocities of the mantle of Central Iran.

Crustal and uppermost mantle structure and deformation in east-central China

NASA Astrophysics Data System (ADS)

Li, H.; Yang, X.; Ouyang, L.; Li, J.

2017-12-01

We conduct a non-linear joint inversion of receiver functions and Rayleigh wave dispersions to obtain the crustal and upper mantle velocity structure in east-central China. In the meanwhile, the lithosphere and upper mantle deformation beneath east-central China is also evaluated with teleseismic shear wave splitting measurements. The resulting velocity model reveals that to the east of the North-South Gravity Lineament, the crust and the lithosphere are significantly thinned. Furthermore, three extensive crustal/lithospheric thinning sub-regions are clearly identified within the study area. This indicates that the modification of the crust and lithosphere in central-eastern China is non-uniform due to the heterogeneity of the lithospheric strength. Extensive crustal and lithospheric thinning could occur in some weak zones such as the basin-range junction belts and large faults. The structure beneath the Dabie orogenic belt is complex due to the collision between the North and South China Blocks during the Late Paleozoic-Triassic. The Dabie orogenic belt is generally delineated by a thick crust with a mid-crust low-velocity zone and a two-directional convergence in the lithospheric scale. Obvious velocity contrast exhibits in the crust and upper mantle at both sides of the Tanlu fault, which suggests the deep penetration of this lithospheric-scale fault. Most of our splitting measurements show nearly E-W trending fast polarization direction which is slightly deviating from the direction of plate motion. The similar present-day lithosphere structure and upper mantle deformation may imply that the eastern NCC and the eastern SCB were dominated by a common dynamic process after late Mesozoic, i.e., the westward subduction of Pacific plate and the retreat of the subduction plate. The westward subduction of the Philippine plate and the long-range effects of the collision between the Indian plate and Eurasia plate during Cenozoic may have also contributed to the present velocity structure and stress environment of eastern China.

New insights on regional tectonics and crustal composition of the eastern Sierras Pampeanas in the Andean back arc region, Argentina (31-32ºS)

NASA Astrophysics Data System (ADS)

Ammirati, J. B.; Venerdini, A. L.; Alvarado, P. M.; Gilbert, H. J.

2017-12-01

Within the flat slab region of the south central Andes, the eastern Sierras Pampeanas (ESP) are the easternmost expression of a series of foreland uplifts affecting the Argentine back arc region ( 31-32ºS). This important crustal deformation has been related to the subduction of the Juan Fernández Ridge (JFR) under the South American plate. Geological observations suggest that the regional crustal structure is inherited from the accretion of different terranes during the Ordovician and later reactivated since the Miocene during the Andean compression. Geophysical experiments allowed to image how the structure observed at the surface behave in depth as décollement levels that accommodate regional crustal shortening. In order to get new insights on the mechanisms that control crustal regional tectonics, we computed teleseismic receiver functions (RF) and jointly invert them with Rayleigh-wave phase velocity dispersion curves. RFs allow resolving crustal thickness and intra crustal velocity variations with a good vertical resolution whereas surface wave information helps to constrain absolute seismic wave velocities. Our results show how the crustal thickness is increasing to the west with an important step in Moho depth. We observe that this step presents a NW-SE orientation, parallel to the trace at the surface of the Valle Fértil - La Huerta (SVF-LH) fault which suggest that this Moho step marks the transition in depth between the Pampia terrane (east) and the Cuyania terrane (west). Our images also reveal the presence of a high wave velocity lower crust west of this Moho step, beneath the eastern Sierras Pampeanas. This observation suggests that the SVF-LH fault is underthrusting the Cuyania lower crust under the Pampia terrane. Finally, our seismic images show very localized low velocity zones located at 10 km beneath late Cenozoic volcanic fields. We believe that these low velocity zones correspond to old magma chambers associated to the recent flat slab-related volcanism in the ESP.

BASE Flexible Array Preliminary Lithospheric Structure Analysis

NASA Astrophysics Data System (ADS)

Yeck, W. L.; Sheehan, A. F.; Anderson, M. L.; Siddoway, C. S.; Erslev, E.; Harder, S. H.; Miller, K. C.

2009-12-01

The Bighorns Arch Seismic Experiment (BASE) is a Flexible Array experiment integrated with EarthScope. The goal of BASE is to develop a better understanding of how basement-involved foreland arches form and what their link is to plate tectonic processes. To achieve this goal, the crustal structure under the Bighorn Mountain range, Bighorn Basin, and Powder River Basin of northern Wyoming and southern Montana are investigated through the deployment of 35 broadband seismometers, 200 short period seismometers, 1600 “Texan” instruments using active sources and 800 “Texan” instruments monitoring passive sources, together with field structural analysis of brittle structures. The novel combination of these approaches and anticipated simultaneous data inversion will give a detailed structural crustal image of the Bighorn region at all levels of the crust. Four models have been proposed for the formation of the Bighorn foreland arch: subhorizontal detachment within the crust, lithospheric buckling, pure shear lithospheric thickening, and fault blocks defined by lithosphere-penetrating thrust faults. During the summer of 2009, we deployed 35 broadband instruments, which have already recorded several magnitude 7+ teleseismic events. Through P wave receiver function analysis of these 35 stations folded in with many EarthScope Transportable Array stations in the region, we present a preliminary map of the Mohorovicic discontinuity. This crustal map is our first test of how the unique Moho geometries predicted by the four hypothesized models of basement involved arches fit seismic observations for the Bighorn Mountains. In addition, shear-wave splitting analysis for our first few recorded teleseisms helps us determine if strong lithospheric deformation is preserved under the range. These analyses help lead us to our final goal, a complete 4D (3D spatial plus temporal) lithospheric-scale model of arch formation which will advance our understanding of the mechanisms accommodating and driving basement-involved arch formation as well as continental lithospheric rheology.

Inverting Coseismic TEC Disturbances for Neutral Atmosphere Pressure Wave

NASA Astrophysics Data System (ADS)

Lee, R. F.; Mikesell, D.; Rolland, L.

2017-12-01

Research from the past 20 years has shown that we can detect coseismic disturbances in the total electron content (TEC) using global navigation space systems (GNSS). In the near field, TEC disturbances are created by the direct wave from rupture on the surface. This pressure wave travels through the neutral atmosphere to the ionosphere within about 10 minutes. This provides the opportunity to almost immediately characterize the source of the acoustic disturbance on the surface using methods from seismology. In populated areas, this could provide valuable information to first responders. To retrieve the surface motion amplitude information we must account for changes in the waveform caused by the geomagnetic field, motion of the satellites and the geometry of the satellites and receivers. One method is to use a transfer function to invert for the neutral atmosphere pressure wave. Gómez et al (2015) first employed an analytical model to invert for acoustic waves produced by Rayleigh waves propagating along the Earth's surface. Here, we examine the same model in the near field using the TEC disturbances from the direct wave produced by rupture at the surface. We compare results from the forward model against a numerical model that has been shown to be in good agreement with observations from the 2011 Van (Turkey) earthquake. We show the forward model predictions using both methods for the Van earthquake. We then analyze results for hypothetical events at different latitudes and discuss the reliability of the analytical model in each scenario. Gómez, D., R. Jr. Smalley, C. A. Langston, T. J. Wilson, M. Bevis, I. W. D. Dalziel, E. C. Kendrick, S. A. Konfal, M. J. Willis, D. A. Piñón, et al. (2015), Virtual array beamforming of GPS TEC observations of coseismic ionospheric disturbances near the Geomagnetic South Pole triggered by teleseismic megathrusts, J. Geophys. Res. Space Physics, 120, 9087-9101, doi:10.1002/2015JA021725.

Investigation of mantle kinematics beneath the Hellenic-subduction zone with teleseismic direct shear waves

NASA Astrophysics Data System (ADS)

Confal, Judith M.; Eken, Tuna; Tilmann, Frederik; Yolsal-Çevikbilen, Seda; Çubuk-Sabuncu, Yeşim; Saygin, Erdinc; Taymaz, Tuncay

2016-12-01

The subduction and roll-back of the African plate beneath the Eurasian plate along the arcuate Hellenic trench is the dominant geodynamic process in the Aegean and western Anatolia. Mantle flow and lithospheric kinematics in this region can potentially be understood better by mapping seismic anisotropy. This study uses direct shear-wave splitting measurements based on the Reference Station Technique in the southern Aegean Sea to reveal seismic anisotropy in the mantle. The technique overcomes possible contamination from source-side anisotropy on direct S-wave signals recorded at a station pair by maximizing the correlation between the seismic traces at reference and target stations after correcting the reference stations for known receiver-side anisotropy and the target stations for arbitrary splitting parameters probed via a grid search. We obtained splitting parameters at 35 stations with good-quality S-wave signals extracted from 81 teleseismic events. Employing direct S-waves enabled more stable and reliable splitting measurements than previously possible, based on sparse SKS data at temporary stations, with one to five events for local SKS studies, compared with an average of 12 events for each station in this study. The fast polarization directions mostly show NNE-SSW orientation with splitting time delays between 1.15 s and 1.62 s. Two stations in the west close to the Hellenic Trench and one in the east show N-S oriented fast polarizations. In the back-arc region three stations exhibit NE-SW orientation. The overall fast polarization variations tend to be similar to those obtained from previous SKS splitting studies in the region but indicate a more consistent pattern, most likely due to the usage of a larger number of individual observations in direct S-wave derived splitting measurements. Splitting analysis on direct shear waves typically resulted in larger split time delays compared to previous studies, possibly because S-waves travel along a longer path in the same anisotropic structure. Considering the S-derived splitting measurements of this study together with earlier SKS and Rayleigh wave anisotropy modelling results we suggest that the very consistent direct S-derived fast shear wave directions can be explained by the lattice-preferred orientation of olivine in the asthenospheric mantle due to mantle flow induced by the roll-back of the slab. It is possible that a small contribution originated in the lower crust beneath the study region where anisotropic fabric might have formed in response to extension in the Miocene.

Pipeline Processing with an Iterative, Context-Based Detection Model

DTIC Science & Technology

2016-01-22

25: Teleseismic paths from earthquakes in Myanmar to three North American arrays. The path length to ILAR (the nearest array) is about 8950...kilometers. ................................. 57 Figure 26: Waveforms of Myanmar calibration event (left) and target event (right), recorded at ILAR...one Myanmar event (2007 5/16 8:56:16.0, Mw 6.3; 20.47°N 100.69°E) as a calibration for a second event occurring nearly 4 years later (2011 3/24 13:55

Imaging the density distributions at the regional scale using full waveform and gravity data inversion - Application to the Pyrenees

NASA Astrophysics Data System (ADS)

Martin, Roland; Chevrot, Sébastien; Wang, Yi; Spangenberg, Hannah; Goubet, Marie; Monteiller, Vadim; Komatitsch, Dimitri; Seoane, Lucia; Dufréchou, Grégory

2017-04-01

We present a hybrid inversion method that allows us to image density distributions at the regional scale using both seismic and gravity data. One main goal is to obtain densities and seismic wave velocities (P and S) in the lithosphere with a fine resolution to get important constraints on the mineralogic composition and thermal state of the lithosphere. In the context of the Pyrenees (located between Spain and France), accurate Vp and Vs seismic velocity models are computed first on a 3D spectral element grid at the scale of the Pyrenees by inverting teleseismic full waveforms. In a second step, Vp velocities are mapped to densities using empirical relations to build an a priori density model. BGI and BRGM Bouguer gravity anomaly data sets are then inverted on the same 3D spectral element grid as the Vp model at a resolution of 1-2 km by using high-order numerical integration formulae. Solutions are compared to those obtained using classical semi-analytical techniques. This procedure opens the possibility to invert both teleseismic and gravity data on the same finite-element grid. It can handle topography of the free surface in the same spectral-element distorted mesh that is used to solve the wave equation, without performing extra interpolations between different grids and models. WGS84 curvature, SRTM or ETOPO1 topographies are used.

Upper mantle structure beneath southern African cratons from seismic finite-frequency P- and S-body wave tomography

NASA Astrophysics Data System (ADS)

Youssof, M.; Thybo, H.; Artemieva, I. M.; Levander, A.

2015-06-01

We present a 3D high-resolution seismic model of the southern African cratonic region from teleseismic tomographic inversion of the P- and S-body wave dataset recorded by the Southern African Seismic Experiment (SASE). Utilizing 3D sensitivity kernels, we invert traveltime residuals of teleseismic body waves to calculate velocity anomalies in the upper mantle down to a 700 km depth with respect to the ak135 reference model. Various resolution tests allow evaluation of the extent of smearing effects and help defining the optimum inversion parameters (i.e., damping and smoothness) for regularizing the inversion calculations. The fast lithospheric keels of the Kaapvaal and Zimbabwe cratons reach depths of 300-350 km and 200-250 km, respectively. The paleo-orogenic Limpopo Belt is represented by negative velocity perturbations down to a depth of ˜ 250 km, implying the presence of chemically fertile material with anomalously low wave speeds. The Bushveld Complex has low velocity down to ˜ 150 km, which is attributed to chemical modification of the cratonic mantle. In the present model, the finite-frequency sensitivity kernels allow to resolve relatively small-scale anomalies, such as the Colesberg Magnetic Lineament in the suture zone between the eastern and western blocks of the Kaapvaal Craton, and a small northern block of the Kaapvaal Craton, located between the Limpopo Belt and the Bushveld Complex.

Teleseismic P-wave Attenuation beneath the Eastern Tibetan Plateau

NASA Astrophysics Data System (ADS)

Gao, S. S.; Peng, Y.; Liu, K. H.

2017-12-01

Numerous laboratory, theoretical, and observational studies have demonstrated that the teleseismic body-wave attenuation factor (or t*), which is quantified by the travel time over the quality factor Q, can provide robust constraints on the thermal and physical state of the crust and upper mantle, and thus is ideal for investigating crustal and mantle dynamics in tectonically active areas such as the Eastern Tibetan Plateau, where pervasively distributed lower crustal flow has been regarded as a mechanism for its shortening and uplift. For this study, broadband seismic data recorded by 256 stations are used to compute the t* relative to a station in the Sichuan Basin. We have developed a set of procedures to reliably measure the P-wave t* values using the spectral ratio method through manually adjusting the time window and visually determining the quality of the measurements. Anomalously high t* values are found beneath active orogenic zones such as Qinling and Longmenshan, with magnitude of about 0.4 s. The Longmenshan block shows the most significant and spatially-consistent high t* measurements, probably caused by the accumulation of a thick and partially melted lower crustal layer. This high attenuation zone continues toward the south to the Songpan-Ganzi terrane to the south, but with a greatly reduced magnitude, suggesting a thinner low-viscosity lower crustal layer. The observations provided independent constraints on the spatial distribution and thickness of the proposed system of lower crustal flow.

A joint local and teleseismic tomography study of the Mississippi Embayment and New Madrid Seismic Zone

NASA Astrophysics Data System (ADS)

Nyamwandha, Cecilia A.; Powell, Christine A.; Langston, Charles A.

2016-05-01

Detailed, upper mantle P and S wave velocity (Vp and Vs) models are developed for the northern Mississippi Embayment (ME), a major physiographic feature in the Central United States (U.S.) and the location of the active New Madrid Seismic Zone (NMSZ). This study incorporates local earthquake and teleseismic data from the New Madrid Seismic Network, the Earthscope Transportable Array, and the FlexArray Northern Embayment Lithospheric Experiment stations. The Vp and Vs solutions contain anomalies with similar magnitudes and spatial distributions. High velocities are present in the lower crust beneath the NMSZ. A pronounced low-velocity anomaly of ~ -3%--5% is imaged at depths of 100-250 km. High-velocity anomalies of ~ +3%-+4% are observed at depths of 80-160 km and are located along the sides and top of the low-velocity anomaly. The low-velocity anomaly is attributed to the presence of hot fluids upwelling from a flat slab segment stalled in the transition zone below the Central U.S.; the thinned and weakened ME lithosphere, still at slightly higher temperatures from the passage of the Bermuda hotspot in mid-Cretaceous, provides an optimal pathway for the ascent of the fluids. The observed high-velocity anomalies are attributed to the presence of mafic rocks emplaced beneath the ME during initial rifting in the early Paleozoic and to remnants of the depleted, lower portion of the lithosphere.

Upper mantle anisotropy from long-period P polarization

NASA Astrophysics Data System (ADS)

Schulte-Pelkum, Vera; Masters, Guy; Shearer, Peter M.

2001-10-01

We introduce a method to infer upper mantle azimuthal anisotropy from the polarization, i.e., the direction of particle motion, of teleseismic long-period P onsets. The horizontal polarization of the initial P particle motion can deviate by >10° from the great circle azimuth from station to source despite a high degree of linearity of motion. Recent global isotropic three-dimensional mantle models predict effects that are an order of magnitude smaller than our observations. Stations within regional distances of each other show consistent azimuthal deviation patterns, while the deviations seem to be independent of source depth and near-source structure. We demonstrate that despite this receiver-side spatial coherence, our polarization data cannot be fit by a large-scale joint inversion for whole mantle structure. However, they can be reproduced by azimuthal anisotropy in the upper mantle and crust. Modeling with an anisotropic reflectivity code provides bounds on the magnitude and depth range of the anisotropy manifested in our data. Our method senses anisotropy within one wavelength (250 km) under the receiver. We compare our inferred fast directions of anisotropy to those obtained from Pn travel times and SKS splitting. The results of the comparison are consistent with azimuthal anisotropy situated in the uppermost mantle, with SKS results deviating from Pn and Ppol in some regions with probable additional deeper anisotropy. Generally, our fast directions are consistent with anisotropic alignment due to lithospheric deformation in tectonically active regions and to absolute plate motion in shield areas. Our data provide valuable additional constraints in regions where discrepancies between results from different methods exist since the effect we observe is local rather than cumulative as in the case of travel time anisotropy and shear wave splitting. Additionally, our measurements allow us to identify stations with incorrectly oriented horizontal components.

Seismic anisotropy in localized shear zones versus distributed tectonic fabrics: examples from geologic and seismic observations in western North America and the European Alps

NASA Astrophysics Data System (ADS)

Mahan, Kevin H.; Schulte-Pelkum, Vera; Condit, Cailey; Leydier, Thomas; Goncalves, Philippe; Raju, Anissha; Brownlee, Sarah; Orlandini, Omero F.

2017-04-01

Modern methods for detecting seismic anisotropy offer an array of promising tools for imaging deep crustal deformation but also present challenges, especially with respect to potential biases in both the detection methods themselves as well as in competing processes for localized versus distributed deformation. We address some of these issues from the geophysical perspective by employing azimuthally dependent amplitude and polarity variations in teleseismic receiver functions combined with a compilation of published rock elasticity tensors from middle and deep crustal rocks, and from the geological perspective through studies of shear zone deformation processes. Examples are highlighted at regional and outcrop scales from western North America and the European Alps. First, in regional patterns, strikes of seismically detected fabric from receiver functions in California show a strong alignment with current strike-slip motion between the Pacific and North American plates, with high signal strength near faults and from depths below the brittle-ductile transition suggesting these faults have deep ductile roots. In contrast, despite NE-striking shear zones being the most prominent features portrayed on Proterozoic tectonic maps of the southwestern USA, receiver function anisotropy from the central Rocky Mountain region appears to more prominently reflect broadly distributed Proterozoic fabric domains that preceded late-stage localized shear zones. Possible causes for the discrepancy fall into two categories: those that involve a) bias in seismic sampling and/or b) deformation processes that lead to either weaker anisotropy in the shear zones compared to adjacent domains or to a symmetry that is different from that conventionally assumed. Most of these explanations imply that the seismically sampled domains contain important structural information that is distinct from the shear zones. The second set of examples stem from studies of outcrop-scale shear zones in upper amphibolite-facies (0.9-1.0 GPa, 700 °C) mafic metagabbro from Precambrian exposures in Montana (USA) and in greenschist-facies (0.7-0.8 GPa, 450-500 °C) metagranites from the External Crystalline Massifs of the European Central Alps. The shear zones are characterized by strain gradients from undeformed coarse-grained protoliths to very fine grained ultramylonite, and by microstructures dominated by CPO-producing deformation mechanisms in the protomylonite and CPO-weakening mechanisms such as dissolution-precipitation creep and grain boundary sliding in the ultramylonite. In the mafic mylonites, the result is a lower seismic anisotropy ( 2%) in the core of the shear zones despite a well-developed hornblende shape-preferred orientation. Preliminary observations of these examples suggest that marginal gradients may contribute as much or more to the bulk anisotropy signal compared to the higher strained cores of these structures. If true, a similar effect could explain some otherwise puzzling anisotropy studies of larger scale shear zones such as from the Himalaya where anisotropy tilt proximal to the Main Himalayan Thrust is notably steeper than expected. In conclusion, while some anisotropy studies of crustal scale deformation patterns are relatively straightforward, others will require careful consideration of the limitations and potential future improvements to seismic detection methods, including ground truthing based on samples and exposures as well as a better understanding of physical processes involved in deformation localization.

Mapping lithosphere thickness beneath the Southern Caribbean and Venezuela using body wave reflectivity and surface wave tomography

NASA Astrophysics Data System (ADS)

Masy, J.; Niu, F.; Levander, A.; Schmitz, M.

2012-12-01

The Caribbean (CAR) and South American (SA) plate boundary in Venezuela is a broad zone of diffuse deformation and faulting. GPS measurements indicate that the CAR is moving approximately 2 cm/yr respect to SA, parallel to the strike slip fault system in the east, but with an oblique convergence component in the west (Weber et al., 2001). Along the central and eastern Venezuela coast, most of the motion is accommodated by both transpression and transtension along the right lateral strike-slip San Sebastian- El Pilar fault system. The main tectonic features of the area include accretionary wedges and coastal thrust belts with their associated foreland basins (e.g. Sierra del Interior and Espino Graben). Southern of the plate boundary is located the Guayana Shield, which is part of the Amazonian Craton, and is an elevated plain consisting of Precambrian rocks. BOLIVAR (Broadband Onshore-Offshore Lithospheric Investigation of Venezuela and the Antilles Arc Region) was a multidisciplinary, international investigation to determine the evolution of the CAR-SA plate boundary (Levander et al., 2006) that included a 47 station broadband seismic array to complement the 40 station Venezuelan national array operated by FUNVISIS. The goal of this study is to map out lithosphere thickness across the region in order to understand its role for the various types of deformations observed at surface. We combined surface wave tomography and body wave reflectivity to locate the depth of the lithosphere-asthenosphere boundary (LAB). To generate a coherent 3D reflectivity volume of the study area, we used both P- and S-wave receiver-function data, as well as the ScS reverberation records of two deep earthquakes occurring in South America. We also measured Rayleigh phase velocities in the frequency range of 20-100 s using the two plane-wave method to remove multi-pathing effects (Forsyth and Li, 2005). Finite-frequency kernels were computed for a total of 63 teleseismic events to improve lateral resolution (Yang and Forsyth, 2006). The phase velocities were inverted for 1D shear velocity structure on a 0.5 by 0.5 degree grid. Crustal thickness for the starting models was previously determined from BOLIVAR and other wide-angle seismic experiments and receiver function analysis (Schmitz et al., 2001; Niu et al., 2007; Bezada et al., 2007; Clark et al., 2008; Guedez, 2008; Magnani et al., 2009; Bezada et al., 2010a). The resulting 3D shear velocity model was then used to determine the depth of the LAB in conjunction with Ps and Sp receiver functions. LAB depth is approximately 120-140 km beneath the Archean-Proterozoic Guayana Shield, in reasonable agreement with body wave tomography. The lithosphere thins to the west beneath the Barinas Apure Basin to about 90 km, and to the north beneath the Sierra del Interior to 80 to 90 km. Offshore beneath the Cariaco basin the LAB is ~60 km. At depths up to 200 km beneath the Peninsula the Paria, there are high velocities interepreted as the subducting oceanic part of the South American Plate, a result that is consistent with finite-frequency P wave tomography (Bezada et al., 2010b).

Mantle wedge anisotropy beneath the Western Alps: insights from Receiver Function analysis

NASA Astrophysics Data System (ADS)

Piana Agostinetti, Nicola; Salimbeni, Simone; Pondrelli, Silvia; Malusa', Marco; Zhao, Liang; Eva, Elena; Solarino, Stefano; Paul, Anne; Guillot, Stéphane; Schwartz, Stéphane; Dumont, Thierry; Aubert, Coralie; Wang, Qingchen; Zhu, Rixiang

2017-04-01

Orogens and subductions zones are the locus where crustal materials are recycled into the upper mantle. Such rocks undergo to several metamorphic reactions during which their seismic properties vary due to the changes in P-T conditions. Metamorphic reactions can imply: (a) the formation of schist-like materials, and (b) a pronounced water flux from the subducted crust. Both these processes should generate highly anisotropic volumes at upper mantle depths. Thus, unveiling the presence of seismic anisotropy at such depth level can put constraints on the metamorphic reactions and the P-T conditions of the subducted materials. The Alpine orogen is composed of three main regions where different geodynamic processes shaped a highly heterogeneous mountain chain. Beneath the Alps, a high velocity body has been imaged sinking in the upper mantle, indicating the presence of a relict of subduction. Such subduction process has been probably terminated with the closure of the Piemont-Liguria Ocean, but evidence of continental subduction has been found beneath the Western Alps. Seismic anisotropy is likely to develop both in the subducted materials and in the mantle wedge, where serpentinized materials could be found due to the low T conditions. We analysed P receiver function (RF) from 46 seismic stations deployed along a linear array crossing the Western Alps, where previous studies revealed the presence of the subducted European lower crust to 80 km depth. RF is a widely used tool for reconstructing subsurface seismic structures, based on the recognition of P-to-S converted phases in teleseismic P-wave coda. The RF data-set is migrated at depth and decomposed into azimuthal harmonics. Computing the first, k=0, and the second, k=1, harmonics allows to separate the "isotropic" contribution, due to the change of the isotropic properties of the sampled materials (recorded on the k=0 harmonics), from the "anisotropic" contribution, where the energy is related to the propagation of the P-wave through anisotropic materials (recorded on the k=1 harmonics). Preliminary results show the presence of a Ps phase on the k=0 harmonics along the western portion of the profile, with increasing time-delay toward East. This phase is interpreted as the European Moho Ps, confirming the geometry of the European Moho beneath the Western Alps. Beneath the internal portion of the orogen, the k=1 harmonics display energetic pulses between 3-7 s, indicating the development of anisotropy within a broad volume of rocks, at lower crustal and upper mantle depths. The presence of anisotropic materials is jointly interpreted with the depicted geometry of the main seismic discontinuities and the location of the intermediate-depth seismicity recorded in the region.

Crustal and Upper Mantle Structure from Joint Inversion of Body Wave and Gravity Data

DTIC Science & Technology

2012-09-01

CRUSTAL AND UPPER MANTLE STRUCTURE FROM JOINT INVERSION OF BODY WAVE AND GRAVITY DATA Eric A. Bergman1, Charlotte Rowe2, and Monica Maceira2...for these events include many readings of direct crustal P and S phases, as well as regional (Pn and Sn) and teleseismic phases. These data have been...the usefulness of the gravity data, we apply high-pass filtering, yielding gravity anomalies that possess higher resolving power for crustal and

Effects of shallow-layer reverberation on measurement of teleseismic P-wave travel times for ocean bottom seismograph data

NASA Astrophysics Data System (ADS)

Obayashi, Masayuki; Ishihara, Yasushi; Suetsugu, Daisuke

2017-03-01

We conducted synthetic experiments to evaluate the effects of shallow-layer reverberation in oceanic regions on P-wave travel times measured by waveform cross-correlation. Time shift due to waveform distortion by the reverberation was estimated as a function of period. Reverberations in the crystalline crust advance the P-waves by a frequency-independent time shift of about 0.3 s in oceans. Sediment does not affect the time shifts in the mid-ocean regions, but effects as large as -0.8 s or more occur where sediment thickness is greater than 600 m for periods longer than 15 s. The water layer causes time delays (+0.3 s) in the relatively shallow (<3500 m) water region for periods longer than 20 s. The time shift may influence mantle images obtained if the reverberation effects are not accounted for in seismic tomography. We propose a simple method to correct relative P-wave travel times at two sites for shallow-layer reverberation by the cross-convolution of the crustal responses at the two sites. [Figure not available: see fulltext. Caption: .

Frequency-Dependent Rupture Processes for the 2011 Tohoku Earthquake

NASA Astrophysics Data System (ADS)

Miyake, H.

2012-12-01

The 2011 Tohoku earthquake is characterized by frequency-dependent rupture process [e.g., Ide et al., 2011; Wang and Mori, 2011; Yao et al., 2011]. For understanding rupture dynamics of this earthquake, it is extremely important to investigate wave-based source inversions for various frequency bands. The above frequency-dependent characteristics have been derived from teleseismic analyses. This study challenges to infer frequency-dependent rupture processes from strong motion waveforms of K-NET and KiK-net stations. The observations suggested three or more S-wave phases, and ground velocities at several near-source stations showed different arrivals of their long- and short-period components. We performed complex source spectral inversions with frequency-dependent phase weighting developed by Miyake et al. [2002]. The technique idealizes both the coherent and stochastic summation of waveforms using empirical Green's functions. Due to the limitation of signal-to-noise ratio of the empirical Green's functions, the analyzed frequency bands were set within 0.05-10 Hz. We assumed a fault plane with 480 km in length by 180 km in width with a single time window for rupture following Koketsu et al. [2011] and Asano and Iwata [2012]. The inversion revealed source ruptures expanding from the hypocenter, and generated sharp slip-velocity intensities at the down-dip edge. In addition to test the effects of empirical/hybrid Green's functions and with/without rupture front constraints on the inverted solutions, we will discuss distributions of slip-velocity intensity and a progression of wave generation with increasing frequency.

Hear it, See it, Explore it: Visualizations and Sonifications of Seismic Signals

NASA Astrophysics Data System (ADS)

Fisher, M.; Peng, Z.; Simpson, D. W.; Kilb, D. L.

2010-12-01

Sonification of seismic data is an innovative way to represent seismic data in the audible range (Simpson, 2005). Seismic waves with different frequency and temporal characteristics, such as those from teleseismic earthquakes, deep “non-volcanic” tremor and local earthquakes, can be easily discriminated when time-compressed to the audio range. Hence, sonification is particularly useful for presenting complicated seismic signals with multiple sources, such as aftershocks within the coda of large earthquakes, and remote triggering of earthquakes and tremor by large teleseismic earthquakes. Previous studies mostly focused on converting the seismic data into audible files by simple time compression or frequency modulation (Simpson et al., 2009). Here we generate animations of the seismic data together with the sounds. We first read seismic data in the SAC format into Matlab, and generate a sequence of image files and an associated WAV sound file. Next, we use a third party video editor, such as the QuickTime Pro, to combine the image sequences and the sound file into an animation. We have applied this simple procedure to generate animations of remotely triggered earthquakes, tremor and low-frequency earthquakes in California, and mainshock-aftershock sequences in Japan and California. These animations clearly demonstrate the interactions of earthquake sequences and the richness of the seismic data. The tool developed in this study can be easily adapted for use in other research applications and to create sonification/animation of seismic data for education and outreach purpose.

Gaps, tears and seismic anisotropy around the subducting slabs of the Antilles

NASA Astrophysics Data System (ADS)

Schlaphorst, David; Kendall, J.-Michael; Baptie, Brian; Latchman, Joan L.; Tait, Steve

2017-02-01

Seismic anisotropy in and beneath the subducting slabs of the Antilles is investigated using observations of shear-wave splitting. We use a combination of teleseismic and local events recorded at three-component broadband seismic stations on every major island in the area to map anisotropy in the crust, the mantle wedge and the slab/sub-slab mantle. To date this is the most comprehensive study of anisotropy in this region, involving 52 stations from 8 seismic networks. Local event delay times (0.21 ± 0.12 s) do not increase with depth, indicating a crustal origin in anisotropy and an isotropic mantle wedge. Teleseismic delay times are much larger (1.34 ± 0.47 s), with fast shear-wave polarisations that are predominantly parallel to trend of the arc. These observations can be interpreted three ways: (1) the presence of pre-existing anisotropy in the subducting slab; (2) anisotropy due to sub-slab mantle flow around the eastern margin of the nearly stationary Caribbean plate; (3) some combination of both mechanisms. However, there are two notable variations in the trench-parallel pattern of anisotropy - trench-perpendicular alignment is observed in narrow regions east of Puerto Rico and south of Martinique. These observations support previously proposed ideas of eastward sublithospheric mantle flow through gaps in the slab. Furthermore, the pattern of anisotropy south of Martinique, near Saint Lucia is consistent with a previously proposed location for the boundary between the North and South American plates.

The temporal distribution of seismic radiation during deep earthquake rupture

USGS Publications Warehouse

Houston, H.; Vidale, J.E.

1994-01-01

The time history of energy release during earthquakes illuminates the process of failure, which remains enigmatic for events deeper than about 100 kilometers. Stacks of teleseismic records from regional arrays for 122 intermediate (depths of 100 to 350 kilometers) and deep (depths of 350 to 700 kilometers) earthquakes show that the temporal pattern of short-period seismic radiation has a systematic variation with depth. On average, for intermediate depth events more radiation is released toward the beginning of the rupture than near the end, whereas for deep events radiation is released symmetrically over the duration of the event, with an abrupt beginning and end of rupture. These findings suggest a variation in the style of rupture related to decreasing fault heterogeneity with depth.The time history of energy release during earthquakes illuminates the process of failure, which remains enigmatic for events deeper than about 100 kilometers. Stacks of teleseismic records from regional arrays for 122 intermediate (depths of 100 to 350 kilometers) and deep (depths of 350 to 700 kilometers) earthquakes show that the temporal pattern of short-period seismic radiation has a systematic variation with depth. On average, for intermediate depth events more radiation is released toward the beginning of the rupture than near the end, whereas for deep events radiation is released symmetrically over the duration of the event, with an abrupt beginning and end of rupture. These findings suggest a variation in the style of rupture related to decreasing fault heterogeneity with depth.

High-Resolution Body Wave Tomography of the Ross Sea Embayment, Antarctica

NASA Astrophysics Data System (ADS)

White-Gaynor, A.; Nyblade, A.; Wiens, D. A.; Aster, R. C.; Gerstoft, P.; Bromirski, P. D.; Stephen, R. A.

2017-12-01

The West Antarctic Rift System (WARS) is one of the least understood continental rift system on the planet. The 1000 km wide WARS includes the Ross Sea Embayment between Marie Byrd Land and the Transantarctic Mountains (TAMS). Active volcanism on Ross Island continues to challenge our understanding of the generally quiescent rift system. Previous regional-scale body wave tomographic investigations have identified areas of low seismic wave speeds to 200 km depth beneath Ross Island. However, the spatial extent of the low velocity structure across the entirety of the WARS remains poorly constrained due to the insufficient resolution of upper mantle structure under the Ross Sea Embayment away from Ross Island. We utilize teleseismic P wave observations recorded on the RIS/DRIS network, which consists of 34 seismometers deployed across the Ross Ice Shelf, along with data from nearby POLENET and TAMSEIS stations to better resolve this region. Relative P wave travel time residuals from 1300 teleseismic events, obtained using a multichannel cross-correlation method, have been inverted for a seismic velocity model of the upper mantle throughout the Ross Sea Embayment. Our results suggest that the low wave speed structure under Ross Island extends roughly halfway across the Embayment and south along the Transantarctic Mountains. This observation is consistent with a two-phase rifting history for the WARS in which broad, late Cretaceous rifting between Marie Byrd Land and the TAMS transitioned to more focused rifting along the TAMS margin in the Cenozoic.

Cenozoic volcanism in the Bohemian Massif in the context of P- and S-velocity high-resolution teleseismic tomography of the upper mantle

NASA Astrophysics Data System (ADS)

Plomerová, Jaroslava; Munzarová, Helena; Vecsey, Luděk.; Kissling, Eduard; Achauer, Ulrich; Babuška, Vladislav

2016-08-01

New high-resolution tomographic models of P- and S-wave isotropic-velocity perturbations for the Bohemian upper mantle are estimated from carefully preprocessed travel-time residuals of teleseismic P, PKP and S waves recorded during the BOHEMA passive seismic experiment. The new data resolve anomalies with scale lengths 30-50 km. The models address whether a small mantle plume in the western Bohemian Massif is responsible for this geodynamically active region in central Europe, as expressed in recurrent earthquake swarms. Velocity-perturbations of the P- and S-wave models show similar features, though their resolutions are different. No model resolves a narrow subvertical low-velocity anomaly, which would validate the "baby-plume" concept. The new tomographic inferences complement previous studies of the upper mantle beneath the Bohemian Massif, in a broader context of the European Cenozoic Rift System (ECRIS) and of other Variscan Massifs in Europe. The low-velocity perturbations beneath the Eger Rift, observed in about 200km-broad zone, agree with shear-velocity models from full-waveform inversion, which also did not identify a mantle plume beneath the ECRIS. Boundaries between mantle domains of three tectonic units that comprise the region, determined from studies of seismic anisotropy, represent weak zones in the otherwise rigid continental mantle lithosphere. In the past, such zones could have channeled upwelling of hot mantle material, which on its way could have modified the mantle domain boundaries and locally thinned the lithosphere.

Rupture Characteristics of the 25 November 2016 Aketao Earthquake ( M w 6.6) in Eastern Pamir Revealed by GPS and Teleseismic Data

NASA Astrophysics Data System (ADS)

Li, Jie; Liu, Gang; Qiao, Xuejun; Xiong, Wei; Wang, Xiaoqiang; Liu, Daiqin; Sun, Jianing; Yushan, Ailixiati; Yusan, Sulitan; Fang, Wei; Wang, Qi

2018-02-01

The 25 November 2016 Aketao, Xinjiang earthquake occurred on the northeastern margin of the Pamir plateau, rupturing the Muji fault on the northern segment of the Kongur Extensional System. We collected coseismic offsets at 7 GPS sites, which show that the fault experienced significate dextral slip with a near-field geodetic displacement of up to 12 cm along the strike. The joint inversion of GPS data and teleseismic P waveforms suggests a complex rupture pattern characterized by the unilateral propagation slip from the epicenter to the southeast for 60 km with a total seismic moment of 1.3 × 1019 Nm, corresponding to a magnitude of M w 6.7 earthquake. Our model of slip distribution shows two major slip patches with a slip amplitude up to 0.6 m, one located at shallow depths of 0-8 km close to the hypocenter with apparent surface breaks and the other, 40 km to the southeast, buried at a greater depth of 12 km. The rupture is dominated by a right-lateral strike slip with significant normal-slip components. The near-field GPS data enhances the spatial resolution of source model. Based on the preferred slip model, the static Coulomb Failure Stress change caused by 2016 Aketao earthquake suggests that the unzipped western and eastern ends of Muji fault and the northern segment of Kungai Fault are significantly promoted.

Internal structure of the San Jacinto fault zone in the trifurcation area southeast of Anza, California, from data of dense seismic arrays

NASA Astrophysics Data System (ADS)

Qin, L.; Ben-Zion, Y.; Qiu, H.; Share, P.-E.; Ross, Z. E.; Vernon, F. L.

2018-04-01

We image the internal structure of the San Jacinto fault zone (SJFZ) in the trifurcation area southeast of Anza, California, with seismic records from dense linear and rectangular arrays. The examined data include recordings from more than 20 000 local earthquakes and nine teleseismic events. Automatic detection algorithms and visual inspection are used to identify P and S body waves, along with P- and S-types fault zone trapped waves (FZTW). The location at depth of the main branch of the SJFZ, the Clark fault, is identified from systematic waveform changes across lines of sensors within the dense rectangular array. Delay times of P arrivals from teleseismic and local events indicate damage asymmetry across the fault, with higher damage to the NE, producing a local reversal of the velocity contrast in the shallow crust with respect to the large-scale structure. A portion of the damage zone between the main fault and a second mapped surface trace to the NE generates P- and S-types FZTW. Inversions of high-quality S-type FZTW indicate that the most likely parameters of the trapping structure are width of ˜70 m, S-wave velocity reduction of 60 per cent, Q value of 60 and depth of ˜2 km. The local reversal of the shallow velocity contrast across the fault with respect to large-scale structure is consistent with preferred propagation of earthquake ruptures in the area to the NW.

Teleseismic P wave tomography of South Island, New Zealand upper mantle: Evidence of subduction of Pacific lithosphere since 45 Ma

NASA Astrophysics Data System (ADS)

Zietlow, Daniel W.; Molnar, Peter H.; Sheehan, Anne F.

2016-06-01

A P wave speed tomogram produced from teleseismic travel time measurements made on and offshore the South Island of New Zealand shows a nearly vertical zone with wave speeds that are 4.5% higher than the background average reaching to depths of approximately 450 km under the northwestern region of the island. This structure is consistent with oblique west-southwest subduction of Pacific lithosphere since about 45 Ma, when subduction beneath the region began. The high-speed zone reaches about 200-300 km below the depths of the deepest intermediate-depth earthquakes (subcrustal to ~200 km) and therefore suggests that ~200-300 km of slab below them is required to produce sufficient weight to induce the intermediate-depth seismicity. In the southwestern South Island, high P wave speeds indicate subduction of the Australian plate at the Puysegur Trench to approximately 200 km depth. A band with speeds ~2-3.5% lower than the background average is found along the east coast of the South Island to depths of ~150-200 km and underlies Miocene or younger volcanism; these low speeds are consistent with thinned lithosphere. A core of high speeds under the Southern Alps associated with a convergent margin and mountain building imaged in previous investigations is not well resolved in this study. This could suggest that such high speeds are limited in both width and depth and not resolvable by our data.

Detection and analysis of a transient energy burst with beamforming of multiple teleseismic phases

NASA Astrophysics Data System (ADS)

Retailleau, Lise; Landès, Matthieu; Gualtieri, Lucia; Shapiro, Nikolai M.; Campillo, Michel; Roux, Philippe; Guilbert, Jocelyn

2018-01-01

Seismological detection methods are traditionally based on picking techniques. These methods cannot be used to analyse emergent signals where the arrivals cannot be picked. Here, we detect and locate seismic events by applying a beamforming method that combines multiple body-wave phases to USArray data. This method explores the consistency and characteristic behaviour of teleseismic body waves that are recorded by a large-scale, still dense, seismic network. We perform time-slowness analysis of the signals and correlate this with the time-slowness equivalent of the different body-wave phases predicted by a global traveltime calculator, to determine the occurrence of an event with no a priori information about it. We apply this method continuously to one year of data to analyse the different events that generate signals reaching the USArray network. In particular, we analyse in detail a low-frequency secondary microseismic event that occurred on 2010 February 1. This event, that lasted 1 d, has a narrow frequency band around 0.1 Hz, and it occurred at a distance of 150° to the USArray network, South of Australia. We show that the most energetic phase observed is the PKPab phase. Direct amplitude analysis of regional seismograms confirms the occurrence of this event. We compare the seismic observations with models of the spectral density of the pressure field generated by the interferences between oceanic waves. We attribute the observed signals to a storm-generated microseismic event that occurred along the South East Indian Ridge.

Subduction and collision processes in the Central Andes constrained by converted seismic phases.

PubMed

Yuan, X; Sobolev, S V; Kind, R; Oncken, O; Bock, G; Asch, G; Schurr, B; Graeber, F; Rudloff, A; Hanka, W; Wylegalla, K; Tibi, R; Haberland, C; Rietbrock, A; Giese, P; Wigger, P; Röwer, P; Zandt, G; Beck, S; Wallace, T; Pardo, M; Comte, D

The Central Andes are the Earth's highest mountain belt formed by ocean-continent collision. Most of this uplift is thought to have occurred in the past 20 Myr, owing mainly to thickening of the continental crust, dominated by tectonic shortening. Here we use P-to-S (compressional-to-shear) converted teleseismic waves observed on several temporary networks in the Central Andes to image the deep structure associated with these tectonic processes. We find that the Moho (the Mohorovicić discontinuity--generally thought to separate crust from mantle) ranges from a depth of 75 km under the Altiplano plateau to 50 km beneath the 4-km-high Puna plateau. This relatively thin crust below such a high-elevation region indicates that thinning of the lithospheric mantle may have contributed to the uplift of the Puna plateau. We have also imaged the subducted crust of the Nazca oceanic plate down to 120 km depth, where it becomes invisible to converted teleseismic waves, probably owing to completion of the gabbro-eclogite transformation; this is direct evidence for the presence of kinetically delayed metamorphic reactions in subducting plates. Most of the intermediate-depth seismicity in the subducting plate stops at 120 km depth as well, suggesting a relation with this transformation. We see an intracrustal low-velocity zone, 10-20 km thick, below the entire Altiplano and Puna plateaux, which we interpret as a zone of continuing metamorphism and partial melting that decouples upper-crustal imbrication from lower-crustal thickening.

Effects of subduction and slab gaps on mantle flow beneath the Lesser Antilles based on observations of seismic anisotropy

NASA Astrophysics Data System (ADS)

Schlaphorst, David; Kendall, J.-Michael; Baptie, Brian; Latchman, Joan L.; Bouin, Marie-Paule

2016-04-01

Subduction is a key process in the formation of continental crust. However, the interaction of the mantle with the subducting slab is not fully understood and varies between subduction zones. The flow geometry and stress patterns influence seismic anisotropy; since anisotropic layers lead to variations in the speed of seismic waves as a function of the direction of wave propagation, mantle flow can be constrained by investigating the structure of these anisotropic layers. In this study we investigate seismic anisotropy in the eastern Greater and the Lesser Antilles along a subduction environment, including the crust and the upper mantle as regions of interest. We use a combination of teleseismic and local events recorded at three-component broadband seismic stations on every major island in the area to observe and distinguish between anisotropy in the crust, the mantle wedge and the sub-slab mantle. Local event delay times (0.21±0.12s) do not increase with depth, indicating a crustal origin and an isotropic mantle wedge. Teleseismic delay times are larger (1.34±0.47s), indicating sub-slab anisotropy. The results suggest trench-parallel mantle flow, with the exception of trench-perpendicular alignment in narrow regions east of Puerto Rico and south of Martinique, suggesting mantle flow through gaps in the slab. This agrees with the continuous northward mantle flow that is caused by the subducting slab proposed by previous studies of that region. We were able to identify a pattern previously unseen by other studies; on St. Lucia a trench-perpendicular trend also indicated by the stations around can be observed. This pattern can be explained by a mantle flow through a gap induced by the subduction of the boundary zone between the North and South American plates. This feature has been proposed for that area using tomographic modelling (van Benthem et al., 2013). It is based on previous results by Wadge & Shepherd (1984), who observed a vertical gap in the Wadati-Benioff zone at that location using a seismicity catalogue from local seismic networks. This work strengthens the argument for that location to be the plate boundary between the North and South American plates.

W phase source inversion for moderate to large earthquakes (1990-2010)

USGS Publications Warehouse

Duputel, Zacharie; Rivera, Luis; Kanamori, Hiroo; Hayes, Gavin P.

2012-01-01

Rapid characterization of the earthquake source and of its effects is a growing field of interest. Until recently, it still took several hours to determine the first-order attributes of a great earthquake (e.g. Mw≥ 7.5), even in a well-instrumented region. The main limiting factors were data saturation, the interference of different phases and the time duration and spatial extent of the source rupture. To accelerate centroid moment tensor (CMT) determinations, we have developed a source inversion algorithm based on modelling of the W phase, a very long period phase (100–1000 s) arriving at the same time as the P wave. The purpose of this work is to finely tune and validate the algorithm for large-to-moderate-sized earthquakes using three components of W phase ground motion at teleseismic distances. To that end, the point source parameters of all Mw≥ 6.5 earthquakes that occurred between 1990 and 2010 (815 events) are determined using Federation of Digital Seismograph Networks, Global Seismographic Network broad-band stations and STS1 global virtual networks of the Incorporated Research Institutions for Seismology Data Management Center. For each event, a preliminary magnitude obtained from W phase amplitudes is used to estimate the initial moment rate function half duration and to define the corner frequencies of the passband filter that will be applied to the waveforms. Starting from these initial parameters, the seismic moment tensor is calculated using a preliminary location as a first approximation of the centroid. A full CMT inversion is then conducted for centroid timing and location determination. Comparisons with Harvard and Global CMT solutions highlight the robustness of W phase CMT solutions at teleseismic distances. The differences in Mw rarely exceed 0.2 and the source mechanisms are very similar to one another. Difficulties arise when a target earthquake is shortly (e.g. within 10 hr) preceded by another large earthquake, which disturbs the waveforms of the target event. To deal with such difficult situations, we remove the perturbation caused by earlier disturbing events by subtracting the corresponding synthetics from the data. The CMT parameters for the disturbed event can then be retrieved using the residual seismograms. We also explore the feasibility of obtaining source parameters of smaller earthquakes in the range 6.0 ≤Mw w= 6 or larger.

Importance of model parameterization in finite fault inversions: Application to the 1974 Mw 8.0 Peru Earthquake

USGS Publications Warehouse

Hartzell, Stephen; Langer, Charley

1993-01-01

The spatial and temporal slip distributions for the October 3, 1974 (Mw = 8.0), Peru subduction zone earthquake and its largest aftershock on November 9 (Ms = 7.1) are calculated and analyzed in terms of the inversion parameterization and tectonic significance. Teleseismic, long-period World-Wide Standard Seismograph Network, P and SH waveforms are inverted to obtain the rupture histories. We demonstrate that erroneous results are obtained if a parameterization is used that does not allow for a sufficiently complex source, involving spatial variation in slip amplitude, risetime, and rupture time. The inversion method utilizes a parameterization of the fault that allows for a discretized source risetime and rupture time. Well-located aftershocks recorded on a local network have the same general pattern as teleseismically determined hypocenters and help to constrain the geometry of the subduction zone. For the main shock a hinged fault is preferred having a shallow plane with a dip of 11° and a deeper, landward plane with a dip of 30°. The preferred nucleation depth lies between 11 and 15 km. A bilateral rupture is obtained with two major concentrations of slip, one 60 to 70 km to the northwest of the epicenter and a second 80 to 100 km to the south and southeast of the epicenter. For these source regions, risetimes vary from 6 to 18 s. Our estimates of risetimes are consistent with the time for the rupture to traverse the dominant local asperity. The slip distribution for the November 9 aftershock falls within a conspicuous hole in the main shock rupture pattern, near the hypocenter of the main shock. The November 9 event has a simple risetime function with a duration of 2 s. Aftershocks recorded by the local network are shown to cluster near the hypocenter of the impending November 9 event and downdip from the largest main shock source region. Slip during the main shock is concentrated at shallow depths above 15 km and extends updip from the hypocenter to near the plate boundary at the trench axis. The large amount of slip at shallow depths is attributed to the absence of any significant accretionary wedge of sediments, and the relatively young age and high convergence rate of the subducted plate, which results in good seismic coupling near the trench axis.

Kinematic source inversion of the 2017 Puebla-Morelos, Mexico earthquake (2017/09/19, Mw.7.1)

NASA Astrophysics Data System (ADS)

Iglesias, A.; Castro-Artola, O.; Hjorleifsdottir, V.; Singh, S. K.; Ji, C.; Franco-Sánchez, S. I.

2017-12-01

On September 19th 2017, an Mw 7.1 earthquake struck Central Mexico, causing severe damage in the epicentral region, especially in several small and medium size houses as well as historical buildings like churches and government offices. In Mexico City, at a distance of 100km from the epicenter, 38 buildings collapsed. Authorities reported that 369 persons were killed by the earthquake (> 60% in the Mexico City). We determined the hypocentral location (18.406N, 98.706W, d=57km), from regional data, situating this earthquake inside the subducted Cocos Plate, with a normal fault mechanism (Globalcmt: =300°, =44°, and =-82°). In this presentation we show the the slip on the fault plane, determined by 1) a frequency-domain inversion using local and regional acceleration records that have been numerically integrated twice and bandpass filtered between 2 and 30, and 2) a wavelet domain inversion using teleseismic body and surface-waves, filtered between 1-100 s and 50-150 s respectively, as well as static offsets. In both methods the fault plane is divided into subfaults, and for each subfault we invert for the average slip, and timing of initiation of slip. In the first method the slip direction is fixed to the ? direction and we invert for the rise time. In the second method the direction of slip is estimated, with values between -90 and +90 allowed, and the time history is an asymmetric cosine time function, for which we determine the "rise" and "fall" durations. For both methods, synthetic seismograms, based on the GlobalCMT focal mechanism, are computed for each subfault-station pair and for three components (Z, N-S, EW). Preliminary results, using local data, show some slip concentrated close to the hypocentral location and a large patch 20 km in NW direction far from the origin. Using teleseismic data, it is difficult to distinguish between the two fault planes, as the waveforms are equally well fit using either one of them. However, both are consistent with a simple rupture patch of 25x20 km and maximum slip of 2 m. Solutions based on both fault planes suggest directivity towards the NW. There is some evidence of two short pulses superimposed on a longer duration pulse in the body wave seismograms, but they are difficult to resolve with the inversion

Rupture History of the 2001 Nisqually Washington Earthquake

NASA Astrophysics Data System (ADS)

Xu, Q.; Creager, K. C.; Crosson, R. S.

2001-12-01

We analyze the temporal-spatial rupture history of the magnitude 6.8 February 28, 2001 Nisqually earthquake using about two dozen 3-component strong-motion records from the Pacific Northwest Seismic Network (PNSN) and the USGS National Strong Motion Program (NSMP) network. We employ a finite-fault inversion scheme similar to Hartzell and Heaton [Bull. Seism. Soc. Am., 1983] to recover the slip history. We assume rupture initiates at the epicenter and origin time determined using PNSN P arrival times and a high-resolution 3-D velocity model. Hypocentral depth is 54 km based on our analysis of teleseismic pP-P times and the regional 3-D model. Using the IASP91 standard Earth model to explain the pP-P times gives a depth of 58 km. Three-component strong motion accelerograms are integrated to obtain velocity, low-pass filtered at 4 s period and windowed to include the direct P- and S- wave arrivals. Theoretical Green's functions are calculated using the Direct Solution Method (DSM) [Cummins, etal, Geophys. Res. Lett., 1994] for each of 169, 4km x 4km, subfaults which lie on one of the two fault plates specified by the Harvard CMT solution. A unique 1-D model that gives an adequate representation of velocity structure for each station is obtained by path averaging the 3-D tomographic model. The S velocity model is generated from the P velocity model. For Vp larger than 4.5 km/s, We use the linear relationship Vs=0.18+0.52Vp obtained from laboratory measurements of local mafic rock samples. For slower velocities, probably associated with sedimentary rocks, we derived Vs=Vp/2.04 which best fits the strong-motion S-arrival times. The resulting source model indicates unilateral rupture along a fault that is elongated in the north-south direction. Inversion for the near vertical (strike 1° , dip 72° ) and horizontal (strike 183° , dip 18° ) fault planes reveal the same source directivity, however, the horizontal fault plane gives a slightly better fit to the data than the vertical one. We will also incorporate teleseismic P pP and sP waves into the waveform modeling to provide additional constraints on vertical source directivity.

Heterogeneous seismic anisotropy in the transition zone and uppermost lower mantle: evidence from South America, Izu-Bonin and Japan

NASA Astrophysics Data System (ADS)

Lynner, Colton; Long, Maureen D.

2015-06-01

Measurements of seismic anisotropy are commonly used to constrain deformation in the upper mantle. Observations of anisotropy at mid-mantle depths are, however, relatively sparse. In this study we probe the anisotropic structure of the mid-mantle (transition zone and uppermost lower mantle) beneath the Japan, Izu-Bonin, and South America subduction systems. We present source-side shear wave splitting measurements for direct teleseismic S phases from earthquakes deeper than 300 km that have been corrected for the effects of upper mantle anisotropy beneath the receiver. In each region, we observe consistent splitting with delay times as large as 1 s, indicating the presence of anisotropy at mid-mantle depths. Clear splitting of phases originating from depths as great as ˜600 km argues for a contribution from anisotropy in the uppermost lower mantle as well as the transition zone. Beneath Japan, fast splitting directions are perpendicular or oblique to the slab strike and do not appear to depend on the propagation direction of the waves. Beneath South America and Izu-Bonin, splitting directions vary from trench-parallel to trench-perpendicular and have an azimuthal dependence, indicating lateral heterogeneity. Our results provide evidence for the presence of laterally variable anisotropy and are indicative of variable deformation and dynamics at mid-mantle depths in the vicinity of subducting slabs.

Scattering - a probe to Earth's small scale structure

NASA Astrophysics Data System (ADS)

Rost, S.; Earle, P.

2009-05-01

Much of the short-period teleseismic wavefield shows strong evidence for scattered waves in extended codas trailing the main arrivals predicted by ray theory. This energy mainly originates from high-frequency body waves interacting with fine-scale volumetric heterogeneities in the Earth. Studies of this energy revealed much of what we know about Earth's structure at scale lengths around 10 km throughout the Earth from crust to core. From these data we can gain important information about the mineral-physical and geochemical constitution of the Earth that is inaccessible to many other seismic imaging techniques. Previous studies used scattered energy related to PKP, PKiKP, and Pdiff to identify and map the small-scale structure of the mantle and core. We will present observations related to the core phases PKKP and P'P' to study fine-scale mantle heterogeneities. These phases are maximum travel-time phases with respect to perturbations at their reflection points. This allows observation of the scattered energy as precursors to the main phase avoiding common problems with traditional coda phases which arrive after the main pulse. The precursory arrival of the scattered energy allows the separation between deep Earth and crustal contributions to the scattered wavefield for certain source-receiver configurations. Using the information from these scattered phases we identify regions of the mantle that shows increased scattering potential likely linked to larger scale mantle structure identified in seismic tomography and geodynamical models.

Regional-Scale Differential Time Tomography Methods: Development and Application to the Sichuan, China, Dataset

NASA Astrophysics Data System (ADS)

Zhang, H.; Thurber, C.; Wang, W.; Roecker, S. W.

2008-12-01

We extended our recent development of double-difference seismic tomography [Zhang and Thurber, BSSA, 2003] to the use of station-pair residual differences in addition to event-pair residual differences. Tomography using station- pair residual differences is somewhat akin to teleseismic tomography but with the sources contained within the model region. Synthetic tests show that the inversion using both event- and station-pair residual differences has advantages in terms of more accurately recovering higher-resolution structure in both the source and receiver regions. We used the Spherical-Earth Finite-Difference (SEFD) travel time calculation method in the tomographic system. The basic concept is the extension of a standard Cartesian FD travel time algorithm [Vidale, 1990] to the spherical case by developing a mesh in radius, co-latitude, and longitude, expressing the FD derivatives in a form appropriate to the spherical mesh, and constructing"stencil" to calculate extrapolated travel times. The SEFD travel time calculation method is more advantageous in dealing with heterogeneity and sphericity of the Earth than the simple Earth flattening transformation and the"sphere-in-a-bo" approach [Flanagan et al., 2007]. We applied this method to the Sichuan, China data set for the period of 2001 to 2004. The Vp, Vs and Vp/Vs models show that there is a clear contrast across the Longmenshan Fault, where the 2008 M8 Wenchuan earthquake initiated.

Magnitude determination using duration of high frequency energy radiation and displacement amplitude: application to waveform data recorded in regional distance range

NASA Astrophysics Data System (ADS)

Hara, T.

2012-12-01

Hara (2007. EPS, 59, 227 - 231) developed a method to determine earthquake magnitudes using durations of high frequency energy radiation and displacement amplitudes of tele-seismic events, and showed that it was applicable to huge events such as the 2004 Sumatra earthquake (Mw 9.0 after the Global CMT catalog. In the following the moment magnitude are from their estimates). Since Hara (2007) developed this method, we have been applying it to large shallow events, and confirmed its effectiveness. The results for several events are available at the web site of our institute (http://iisee.kenken.go.jp/quakes.htm). Also, Hara (2011. EPS, 63, 525-528) applied this method to the 2011 Off the Pacific Coast of Tohoku Earthquake (Mw 9.1), and showed that it worked well. In these applications, we used only waveform data recorded in the tele-seismic distance range (30 - 85 degrees). In order to have a magnitude estimate faster, it is necessary to analyze regional distance range data. In this study, we applied the method of Hara (2007) to waveform data recorded in the regional distance range (8 - 30 degrees) to investigate its applicability. We slightly modified the method by changing durations of times series used for analysis considering arrivals of high amplitude Rayleigh waves. We selected the six recent huge (their moment magnitude are equal to or greater than 8.5) earthquakes; they are the December 26, 2004 Sumatra (Mw 9.0), the March 28, 2005 Northern Sumatra (Mw 8,6), the September 12, 2007 Southern Sumatra (Mw 8.5), the February 27, 2010 Chile (Mw 8.8), the March 11, 2011 off the Pacific Coast of Tohoku (Mw 9.1), the April 11, 2012 off West Coast of Northern Sumatra (Mw 8.6). We retrieved BHZ channel waveform data from IRIS DMC. For the 2004 Sumatra and 2010 Chile earthquakes, only a few waveform data are available. The estimated magnitudes are 9.16, 8.66, 8.53, 8.83, 9.15, and 8.70, respectively. Also, the estimated high frequency energy radiation durations are consistent with the centroid time shifts of the Global CMT catalog. These preliminary results suggest that the method of Hara (2007) is applicable to waveform data recorded in the regional distance range. We plan to apply this method to smaller events to investigate a possible systematic deviation from analyses of tele-seismic records.

Time functions of deep earthquakes from broadband and short-period stacks

USGS Publications Warehouse

Houston, H.; Benz, H.M.; Vidale, J.E.

1998-01-01

To constrain dynamic source properties of deep earthquakes, we have systematically constructed broadband time functions of deep earthquakes by stacking and scaling teleseismic P waves from U.S. National Seismic Network, TERRAscope, and Berkeley Digital Seismic Network broadband stations. We examined 42 earthquakes with depths from 100 to 660 km that occurred between July 1, 1992 and July 31, 1995. To directly compare time functions, or to group them by size, depth, or region, it is essential to scale them to remove the effect of moment, which varies by more than 3 orders of magnitude for these events. For each event we also computed short-period stacks of P waves recorded by west coast regional arrays. The comparison of broadband with short-period stacks yields a considerable advantage, enabling more reliable measurement of event duration. A more accurate estimate of the duration better constrains the scaling procedure to remove the effect of moment, producing scaled time functions with both correct timing and amplitude. We find only subtle differences in the broadband time-function shape with moment, indicating successful scaling and minimal effects of attenuation at the periods considered here. The average shape of the envelopes of the short-period stacks is very similar to the average broadband time function. The main variations seen with depth are (1) a mild decrease in duration with increasing depth, (2) greater asymmetry in the time functions of intermediate events compared to deep ones, and (3) unexpected complexity and late moment release for events between 350 and 550 km, with seven of the eight events in that depth interval displaying markedly more complicated time functions with more moment release late in the rupture than most events above or below. The first two results are broadly consistent with our previous studies, while the third is reported here for the first time. The greater complexity between 350 and 550 km suggests greater heterogeneity in the failure process in that depth range. Copyright 1998 by the American Geophysical Union.

Acceleration spectra for subduction zone earthquakes

USGS Publications Warehouse

Boatwright, J.; Choy, G.L.

1989-01-01

We estimate the source spectra of shallow earthquakes from digital recordings of teleseismic P wave groups, that is, P+pP+sP, by making frequency dependent corrections for the attenuation and for the interference of the free surface. The correction for the interference of the free surface assumes that the earthquake radiates energy from a range of depths. We apply this spectral analysis to a set of 12 subduction zone earthquakes which range in size from Ms = 6.2 to 8.1, obtaining corrected P wave acceleration spectra on the frequency band from 0.01 to 2.0 Hz. Seismic moment estimates from surface waves and normal modes are used to extend these P wave spectra to the frequency band from 0.001 to 0.01 Hz. The acceleration spectra of large subduction zone earthquakes, that is, earthquakes whose seismic moments are greater than 1027 dyn cm, exhibit intermediate slopes where u(w)???w5/4 for frequencies from 0.005 to 0.05 Hz. For these earthquakes, spectral shape appears to be a discontinuous function of seismic moment. Using reasonable assumptions for the phase characteristics, we transform the spectral shape observed for large earthquakes into the time domain to fit Ekstrom's (1987) moment rate functions for the Ms=8.1 Michoacan earthquake of September 19, 1985, and the Ms=7.6 Michoacan aftershock of September 21, 1985. -from Authors

Broadband Studies of Seismic Sources at Regional and Teleseismic Distances Using Advanced Time Series Analysis Methods

DTIC Science & Technology

1990-02-01

transform the waveforms of this event to those of the Titanial must be a band limited representation of the firing sequence. Therefore, we decided to...design a Wiener filter to transform Pn waveforms of Event Titania4 into those of Event Titanial at all sensors of NORESS. Prior to applying this technique...for transforming the Pn phases of event Titania 4 into those of event Titanial . 28 T’tania4 -* Titania3 Titania3 B5 T’tania4 Titania4 - Titania3

Receiver function HV ratio: a new measurement for reducing non-uniqueness of receiver function waveform inversion

NASA Astrophysics Data System (ADS)

Chong, Jiajun; Chu, Risheng; Ni, Sidao; Meng, Qingjun; Guo, Aizhi

2018-02-01

It is known that a receiver function has relatively weak constraint on absolute seismic wave velocity, and that joint inversion of the receiver function with surface wave dispersion has been widely applied to reduce the trade-off of velocity with interface depth. However, some studies indicate that the receiver function itself is capable for determining the absolute shear-wave velocity. In this study, we propose to measure the receiver function HV ratio which takes advantage of the amplitude information of the receiver function to constrain the shear-wave velocity. Numerical analysis indicates that the receiver function HV ratio is sensitive to the average shear-wave velocity in the depth range it samples, and can help to reduce the non-uniqueness of receiver function waveform inversion. A joint inversion scheme has been developed, and both synthetic tests and real data application proved the feasibility of the joint inversion.

Mitigating artifacts in back-projection source imaging with implications for frequency-dependent properties of the Tohoku-Oki earthquake

NASA Astrophysics Data System (ADS)

Meng, Lingsen; Ampuero, Jean-Paul; Luo, Yingdi; Wu, Wenbo; Ni, Sidao

2012-12-01

Comparing teleseismic array back-projection source images of the 2011 Tohoku-Oki earthquake with results from static and kinematic finite source inversions has revealed little overlap between the regions of high- and low-frequency slip. Motivated by this interesting observation, back-projection studies extended to intermediate frequencies, down to about 0.1 Hz, have suggested that a progressive transition of rupture properties as a function of frequency is observable. Here, by adapting the concept of array response function to non-stationary signals, we demonstrate that the "swimming artifact", a systematic drift resulting from signal non-stationarity, induces significant bias on beamforming back-projection at low frequencies. We introduce a "reference window strategy" into the multitaper-MUSIC back-projection technique and significantly mitigate the "swimming artifact" at high frequencies (1 s to 4 s). At lower frequencies, this modification yields notable, but significantly smaller, artifacts than time-domain stacking. We perform extensive synthetic tests that include a 3D regional velocity model for Japan. We analyze the recordings of the Tohoku-Oki earthquake at the USArray and at the European array at periods from 1 s to 16 s. The migration of the source location as a function of period, regardless of the back-projection methods, has characteristics that are consistent with the expected effect of the "swimming artifact". In particular, the apparent up-dip migration as a function of frequency obtained with the USArray can be explained by the "swimming artifact". This indicates that the most substantial frequency-dependence of the Tohoku-Oki earthquake source occurs at periods longer than 16 s. Thus, low-frequency back-projection needs to be further tested and validated in order to contribute to the characterization of frequency-dependent rupture properties.

Statistical characterization of Earth’s heterogeneities from seismic scattering

NASA Astrophysics Data System (ADS)

Zheng, Y.; Wu, R.

2009-12-01

The distortion of a teleseismic wavefront carries information about the heterogeneities through which the wave propagates and it is manifestited as logarithmic amplitude (logA) and phase fluctuations of the direct P wave recorded by a seismic network. By cross correlating the fluctuations (e.g., logA-logA or phase-phase), we obtain coherence functions, which depend on spatial lags between stations and incident angles between the incident waves. We have mathematically related the depth-dependent heterogeneity spectrum to the observable coherence functions using seismic scattering theory. We will show that our method has sharp depth resolution. Using the HiNet seismic network data in Japan, we have inverted power spectra for two depth ranges, ~0-120km and below ~120km depth. The coherence functions formed by different groups of stations or by different groups of earthquakes at different back azimuths are similar. This demonstrates that the method is statistically stable and the inhomogeneities are statistically stationary. In both depth intervals, the trend of the spectral amplitude decays from large scale to small scale in a power-law fashion with exceptions at ~50km for the logA data. Due to the spatial spacing of the seismometers, only information from length scale 15km to 200km is inverted. However our scattering method provides new information on small to intermediate scales that are comparable to scales of the recycled materials and thus is complimentary to the global seismic tomography which reveals mainly large-scale heterogeneities on the order of ~1000km. The small-scale heterogeneities revealed here are not likely of pure thermal origin. Therefore, the length scale and strength of heterogeneities as a function of depth may provide important constraints in mechanical mixing of various components in the mantle convection.

A search in strainmeter data for slow slip associated with triggered and ambient tremor near Parkfield, California

USGS Publications Warehouse

Smith, E.F.; Gomberg, J.

2009-01-01

We test the hypothesis that, as in subduction zones, slow slip facilitates triggered and ambient tremor in the transform boundary setting of California. Our study builds on the study of Peng et al. (2009) of triggered and ambient tremor near Parkfield, California during time intervals surrounding 31, potentially triggering, M ≥ 7.5 teleseismic earthquakes; waves from 10 of these triggered tremor and 29 occurred in periods of ambient tremor activity. We look for transient slow slip during 3-month windows that include 11 of these triggering and nontriggering teleseisms, using continuous strain data recorded on two borehole Gladwin tensor strainmeters (GTSM) located within the distribution of tremor epicenters. We model the GTSM data assuming only tidal and “drift” signals are present and find no detectable slow slip, either ongoing when the teleseismic waves passed or triggered by them. We infer a conservative detection threshold of about 5 nanostrain for abrupt changes and about twice this for slowly evolving signals. This could be lowered slightly by adding analyses of other data types, modeled slow slip signals, and GTSM data calibration. Detection of slow slip also depends on the slipping fault's location and size, which we describe in terms of equivalent earthquake moment magnitude, M. In the best case of the GTSM above a very shallow slipping fault, detectable slip events must exceed M~2, and if the slow slip is beneath the seismogenic zone (below ~15 km depth), even M~5 events are likely to remain hidden.

Teleseismic P-wave tomography and mantle dynamics beneath Eastern Tibet

NASA Astrophysics Data System (ADS)

Lei, Jianshe; Zhao, Dapeng

2016-05-01

We determined a new 3-D P-wave velocity model of the upper mantle beneath eastern Tibet using 112,613 high-quality arrival-time data collected from teleseismic seismograms recorded by a new portable seismic array in Yunnan and permanent networks in southwestern China. Our results provide new insights into the mantle structure and dynamics of eastern Tibet. High-velocity (high-V) anomalies are revealed down to 200 km depth under the Sichuan basin and the Ordos and Alashan blocks. Low-velocity (low-V) anomalies are imaged in the upper mantle under the Kunlun-Qilian and Qinling fold zones, and the Songpan-Ganzi, Qiangtang, Lhasa and Chuan-Dian diamond blocks, suggesting that eastward moving low-V materials are extruded to eastern China after the obstruction by the Sichuan basin, and the Ordos and Alashan blocks. Furthermore, the extent and thickness of these low-V anomalies are correlated with the surface topography, suggesting that the uplift of eastern Tibet could be partially related to these low-V materials having a higher temperature and strong positive buoyancy. In the mantle transition zone (MTZ), broad high-V anomalies are visible from the Burma arc northward to the Kunlun fault and eastward to the Xiaojiang fault, and they are connected upward with the Wadati-Benioff seismic zone. These results suggest that the subducted Indian slab has traveled horizontally for a long distance after it descended into the MTZ, and return corner flow and deep slab dehydration have contributed to forming the low-V anomalies in the big mantle wedge. Our results shed new light on the dynamics of the eastern Tibetan plateau.

Short-Period Surface Wave Based Seismic Event Relocation

NASA Astrophysics Data System (ADS)

White-Gaynor, A.; Cleveland, M.; Nyblade, A.; Kintner, J. A.; Homman, K.; Ammon, C. J.

2017-12-01

Accurate and precise seismic event locations are essential for a broad range of geophysical investigations. Superior location accuracy generally requires calibration with ground truth information, but superb relative location precision is often achievable independently. In explosion seismology, low-yield explosion monitoring relies on near-source observations, which results in a limited number of observations that challenges our ability to estimate any locations. Incorporating more distant observations means relying on data with lower signal-to-noise ratios. For small, shallow events, the short-period (roughly 1/2 to 8 s period) fundamental-mode and higher-mode Rayleigh waves (including Rg) are often the most stable and visible portion of the waveform at local distances. Cleveland and Ammon [2013] have shown that teleseismic surface waves are valuable observations for constructing precise, relative event relocations. We extend the teleseismic surface wave relocation method, and apply them to near-source distances using Rg observations from the Bighorn Arche Seismic Experiment (BASE) and the Earth Scope USArray Transportable Array (TA) seismic stations. Specifically, we present relocation results using short-period fundamental- and higher-mode Rayleigh waves (Rg) in a double-difference relative event relocation for 45 delay-fired mine blasts and 21 borehole chemical explosions. Our preliminary efforts are to explore the sensitivity of the short-period surface waves to local geologic structure, source depth, explosion magnitude (yield), and explosion characteristics (single-shot vs. distributed source, etc.). Our results show that Rg and the first few higher-mode Rayleigh wave observations can be used to constrain the relative locations of shallow low-yield events.

A Self-Consistent Fault Slip Model for the 2011 Tohoku Earthquake and Tsunami

NASA Astrophysics Data System (ADS)

Yamazaki, Yoshiki; Cheung, Kwok Fai; Lay, Thorne

2018-02-01

The unprecedented geophysical and hydrographic data sets from the 2011 Tohoku earthquake and tsunami have facilitated numerous modeling and inversion analyses for a wide range of dislocation models. Significant uncertainties remain in the slip distribution as well as the possible contribution of tsunami excitation from submarine slumping or anelastic wedge deformation. We seek a self-consistent model for the primary teleseismic and tsunami observations through an iterative approach that begins with downsampling of a finite fault model inverted from global seismic records. Direct adjustment of the fault displacement guided by high-resolution forward modeling of near-field tsunami waveform and runup measurements improves the features that are not satisfactorily accounted for by the seismic wave inversion. The results show acute sensitivity of the runup to impulsive tsunami waves generated by near-trench slip. The adjusted finite fault model is able to reproduce the DART records across the Pacific Ocean in forward modeling of the far-field tsunami as well as the global seismic records through a finer-scale subfault moment- and rake-constrained inversion, thereby validating its ability to account for the tsunami and teleseismic observations without requiring an exotic source. The upsampled final model gives reasonably good fits to onshore and offshore geodetic observations albeit early after-slip effects and wedge faulting that cannot be reliably accounted for. The large predicted slip of over 20 m at shallow depth extending northward to 39.7°N indicates extensive rerupture and reduced seismic hazard of the 1896 tsunami earthquake zone, as inferred to varying extents by several recent joint and tsunami-only inversions.

Hypocenter relocation along the Sunda arc in Indonesia, using a 3D seismic velocity model

USGS Publications Warehouse

Nugraha, Andri Dian; Shiddiqi, Hasbi A.; Widiyantoro, Sri; Thurber, Clifford H.; Pesicek, Jeremy D.; Zhang, Haijiang; Wiyono, Samsul H.; Ramadhan, Mohamad; Wandano,; Irsyam, Mahsyur

2018-01-01

The tectonics of the Sunda arc region is characterized by the junction of the Eurasian and Indo‐Australian tectonic plates, causing complex dynamics to take place. High‐seismicity rates in the Indonesian region occur due to the interaction between these tectonic plates. The availability of a denser network of seismometers after the earthquakes of Mw">Mw 9.1 in 2004 and  Mw">Mw 8.6 in 2005 supports various seismic studies, one of which regards the precise relocation of the hypocenters. In this study, hypocenter relocation was performed using a teleseismic double‐difference (DD) relocation method (teletomoDD) combining arrival times of P and S waves from stations at local, regional, and teleseismic distances. The catalog data were taken from the Agency of Meteorology, Climatology, and Geophysics (BMKG) of Indonesia, and the International Seismological Centre (ISC) for the time period of April 2009 to May 2015. The 3D seismic‐wave velocity model with a grid size 1°×1°">1°×1° was used in the travel‐time calculations. Relocation results show a reduction in travel‐time residuals compared with the initial locations. The relocation results better illuminate subducted slabs and active faults in the region such as the Mentawai back thrust and the outer rise in the subduction zone south of Java. Focal mechanisms from the Global Centroid Moment Tensor catalog are analyzed in conjunction with the relocation results, and our synthesis of the results provides further insight into seismogenesis in the region.

Teleseismic shear-wave splitting in SE Tibet: Insight into complex crust and upper-mantle deformation

NASA Astrophysics Data System (ADS)

Huang, Zhouchuan; Wang, Liangshu; Xu, Mingjie; Ding, Zhifeng; Wu, Yan; Wang, Pan; Mi, Ning; Yu, Dayong; Li, Hua

2015-12-01

We measured shear-wave splitting of teleseismic XKS phases (i.e., SKS, SKKS and PKS) recorded by more than 300 temporary ChinArray stations in Yunnan of SE Tibet. The first-order pattern of XKS splitting measurements shows that the fast polarization directions (φ) change (at ∼26-27°N) from dominant N-S in the north to E-W in the south. While splitting observations around the eastern Himalayan syntax well reflect anisotropy in the lithosphere under left-lateral shear deformation, the dominant E-W φ to the south of ∼26°N is consistent with the maximum extension in the crust and suggest vertically coherent pure-shear deformation throughout the lithosphere in Yunnan. However, the thin lithosphere (<80 km) could account for only part (<0.7 s) of the observed splitting delay times (δt, 0.9-1.5 s). Anisotropy in the asthenosphere is necessary to explain the NW-SE and nearly E-W φ in these regions. The NE-SW φ can be explained by the counter flow caused by the subduction and subsequent retreat of the Burma slab. The E-W φ is consistent with anisotropy due to the absolute plate motion in SE Tibet and the eastward asthenospheric flow from Tibet to eastern China accompanying the tectonic evolution of the plateau. Our results provide new information on different deformation fields in different layers under SE Tibet, which improves our understanding on the complex geodynamics related to the tectonic uplift and southeastward expansion of Tibetan material under the plateau.

Imaging Crustal Structure with Waveform and HV Ratio of Body-wave Receiver Function

NASA Astrophysics Data System (ADS)

Chong, J.; Chu, R.; Ni, S.; Meng, Q.; Guo, A.

2017-12-01

It is known that receiver function has less constraint on the absolute velocity, and joint inversion of receiver function and surface wave dispersion has been widely applied to reduce the non-uniqueness of velocity and interface depth. However, some studies indicate that the receiver function itself is capable for determining the absolute shear wave velocity. In this study, we propose to measure the receiver function HV ratio which takes advantage of the amplitude information of the radial and vertical receiver functions to constrain the shear-wave velocity. Numerical analysis indicates that the receiver function HV ratio is sensitive to the average shear wave velocity in the depth range it samples, and can help to reduce the non-uniqueness of receiver function waveform inversion. A joint inversion scheme has been developed, and both synthetic tests and real data application proved the feasibility of the joint inversion. The method has been applied to the dense seismic array of ChinArray program in SE Tibet during the time period from August 2011 to August 2012 in SE Tibet (ChinArray-Himalaya, 2011). The measurements of receiver function HV ratio reveals the lateral variation of the tectonics in of the study region. And main features of the velocity structure imagined by the new joint inversion method are consistent with previous studies. KEYWORDS: receiver function HV ratio, receiver function waveform inversion, crustal structure ReferenceChinArray-Himalaya. 2011. China Seismic Array waveform data of Himalaya Project. Institute of Geophysics, China Earthquake Administration. doi:10.12001/ChinArray.Data. Himalaya. Jiajun Chong, Risheng Chu*, Sidao Ni, Qingjun Meng, Aizhi Guo, 2017. Receiver Function HV Ratio, a New Measurement for Reducing Non-uniqueness of Receiver Function Waveform Inversion. (under revision)

GENERAL EARTHQUAKE-OBSERVATION SYSTEM (GEOS).

USGS Publications Warehouse

Borcherdt, R.D.; Fletcher, Joe B.; Jensen, E.G.; Maxwell, G.L.; VanSchaack, J.R.; Warrick, R.E.; Cranswick, E.; Johnston, M.J.S.; McClearn, R.

1985-01-01

Microprocessor technology has permitted the development of a General Earthquake-Observation System (GEOS) useful for most seismic applications. Central-processing-unit control via robust software of system functions that are isolated on hardware modules permits field adaptability of the system to a wide variety of active and passive seismic experiments and straightforward modification for incorporation of improvements in technology. Various laboratory tests and numerous deployments of a set of the systems in the field have confirmed design goals, including: wide linear dynamic range (16 bit/96 dB); broad bandwidth (36 hr to 600 Hz; greater than 36 hr available); selectable sensor-type (accelerometer, seismometer, dilatometer); selectable channels (1 to 6); selectable record mode (continuous, preset, trigger); large data capacity (1. 4 to 60 Mbytes); selectable time standard (WWVB, master, manual); automatic self-calibration; simple field operation; full capability to adapt system in the field to a wide variety of experiments; low power; portability; and modest costs. System design goals for a microcomputer-controlled system with modular software and hardware components as implemented on the GEOS are presented. The systems have been deployed for 15 experiments, including: studies of near-source strong motion; high-frequency microearthquakes; crustal structure; down-hole wave propagation; teleseismicity; and earth-tidal strains.

Crustal and Upper Mantle Velocity Structure beneath Northwestern South America revealed by the CARMArray

NASA Astrophysics Data System (ADS)

Miao, W.; Cornthwaite, J.; Levander, A.; Niu, F.; Schmitz, M.; Dionicio, V.; Nader-Nieto, M. F.

2017-12-01

The Caribbean plate (CAR) is a fragment of the Farallon plate heavily modified by igneous processes that created the Caribbean large igneous province (CLIP) between 110 and 80 Ma.The CAR collided with and initiated subduction beneath northwestern South America plate (SA) at about 60-55 Ma as a narrow flat-slab subduction zone with an accretionary prism offshore, but no volcanic arc. Large scale regional tomography suggests that 1000 km of the CAR has been subducted (Van Benthem et al., 2013, JGR). The flat slab has caused Laramide-style basement uplifts of the Merida Andes, Sierra de la Perija, and Santa Marta ranges with elevations >5 km. The details of subduction geometry of the CAR plate beneath northeastern Colombia and northwestern Venezuela are complicated and remain unclear. The region of slab steepening lies below the triangular Maracaibo block (Bezada et al, 2010, JGR), bounded by major strike slip faults and currently escaping to the north over the CAR. Geodetic data suggests the this region has the potential for a magnitude 8+ earthquake (Bilham and Mencin, 2013, AGU Abstract). To better understand the subduction geometry, we deployed 65 broadband (BB) stations across northeastern Colombia and northwestern Venezuela in April of 2016. The 65 stations interweave with the 32 existing Colombian and Venezuelan BB stations, forming a 2-D array (hereafter referred to as CARMArray) with a station spacing of 35-100 km that covers an area of 600 km by 400 km extending from the Caribbean coast in Colombia to the interior plains of Venezuela. With data from the first year of operation, we have measured the Rayleigh wave phase velocities and Z/H ratios in the period range of 8-40 s using both ambient noise and earthquake data recorded by the CARMArray. We also generated Ps receiver functions from waveform data of teleseismic events recorded by the array. We then jointly inverted the three datasets to construct a 3-D S-wave velocity model beneath the array. We will report the initial results of the inversion and discuss the lateral variations of crustal and upper mantle structure and their potential links with surface geology and regional tectonics.

New constraints on the crustal structure beneath northern Tyrrhenian Sea

NASA Astrophysics Data System (ADS)

Levin, V. L.; Park, J. J.

2009-12-01

We present new seismological data on the seismic structure beneath the Tyrrhenian Sea between Corsica and the coast of Italy. Teleseismic receiver functions from two Tyrrhenian islands (Elba and Gorgona) identify clear P-to-S mode-converted waves from two distinct interfaces, at ~20 and ~45 km depth. Both interfaces are characterized by an increase of seismic wavespeed with depth. Using a summation of direct and multiply-reflected body waves within the P wave coda we estimate the mean ratio of compressional and shear wave speeds above the 45 km interface to be 1.75-1.80. Using reflectivity computations in 1D layered models we develop a model of seismic wavespeed distribution that yields synthetic seismograms very similar to those observed. We apply a Ps-multiple summation procedure to the synthetic waveforms to further verify the match between observed and predicted wavefields. The lower layer of our model, between 20 and 45 km, has Vp ~ 7.5 km/sec, a value that can be ascribed to either very fast crustal rocks or very slow upper mantle rocks. The Vp/Vs ratio is ~1.8 in this intermediate layer. On the basis of a well-constrained downward increase in seismic wave speed beneath this second layer, we interpret it as the magmatically reworked lower crust, a lithology that has been proposed to explain high-Vp layers in the crustal roots of island-arc terranes and volcanically altered continental margins, as well as lower-crustal high-Vp features sometimes seen beneath continental rifts. The presence of a thick layer of high-Vp, but crustal, lithology beneath the Tyrrhenian Sea differs considerably from previous estimates that interpreted the interface at ~20 km as the Moho. Our new interpretation obviates a need for a crustal thickness change of over 20 km at the crest of the Apennines orogen. We propose an alteration in the properties of the lower crust instead. We argue that ongoing convergent subduction of the Adriatic lithospehre is not required beneath northern Apennines, and that a delamination or vertical "drip" of detached lithosphere would fit the observations well.

A Preliminary Look at the Crust and Upper Mantle of North Africa Using Libyan Seismic Data

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pasyanos, M

2005-08-05

In recent years, LLNL has been developing methods to jointly invert both surface wave dispersion data and teleseismic receiver functions. The technique holds great promise in accurately estimating seismic structure, including important tectonic parameters such as basin thickness, crustal thickness, upper mantle velocity, etc. We proposed applying this method to some recently available data from several Libyan stations, as we believe the technique has not been applied to any stations in Libya. The technique holds the promise of improving our understanding of the crust and upper mantle in Libya and North Africa. We recently requested seismic data from stations GHARmore » (Gharyan) and MARJ (Al Marj) in Libya for about 20 events. The events were large events at regional distances suitable for making dispersion measurements. An example of waveforms recorded at the two stations from an earthquake in Italy is shown in Figure 1. The paths traverse the Ionian Sea. Notice the slow short period group velocities of the surface waves across the Mediterranean, particularly to the easternmost station MARJ. However, because of data availability, signal-to-noise ratio, etc. we were unable to make measurements for every one of these events at both stations. Figure 2 shows a map of paths for 20 sec Rayleigh waves in the eastern Mediterranean region. Paths measured at the two Libyan stations are shown in green. Rayleigh wave dispersion measurements at 20 sec period are sensitive to velocities in the upper 20 km or so, and reveal sediment thickness, crustal velocity, and crustal thickness. Tomographic inversions reveal the sharp group velocity contrast between regions with deep sedimentary basins and those without. Figure 3, the result of an inversion made before adding the new dispersion measurements, shows slow group velocities in the Black Sea, Adriatic Sea, and Eastern Mediterranean. In general, these features correspond well with the sediment thickness model from Laske, shown in Figure 4. Details in and around the Sirt (Sirte) Basin in northern Libya, however, are poorly defined.« less

Seismic and thermal structure of the crust and uppermost mantle beneath Antarctica from inversion of multiple seismic datasets

NASA Astrophysics Data System (ADS)

Wiens, D.; Shen, W.; Anandakrishnan, S.; Aster, R. C.; Gerstoft, P.; Bromirski, P. D.; Dalziel, I.; Hansen, S. E.; Heeszel, D.; Huerta, A. D.; Nyblade, A.; Stephen, R. A.; Wilson, T. J.; Winberry, J. P.; Stern, T. A.

2017-12-01

Since the last decade of the 20th century, over 200 broadband seismic stations have been deployed across Antarctica (e.g., temporary networks such as TAMSEIS, AGAP/GAMSEIS, POLENET/ANET, TAMNNET and RIS/DRIS by U.S. geoscientists as well as stations deployed by Japan, Britain, China, Norway, and other countries). In this presentation, we discuss our recent efforts to build reference crustal and uppermost mantle shear velocity (Vs) and thermal models for continental Antarctica based on those seismic arrays. By combing the high resolution Rayleigh wave dispersion maps derived from both ambient noise and teleseismic earthquakes, together with P receiver function waveforms, we develop a 3-D Vs model for the crust and uppermost mantle beneath Central and West Antarctica to a depth of 200 km. Additionally, using this 3-D seismic model to constrain the crustal structure, we re-invert for the upper mantle thermal structure using the surface wave data within a thermodynamic framework and construct a 3-D thermal model for the Antarctic lithosphere. The final product, a high resolution thermal model together with associated uncertainty estimates from the Monte Carlo inversion, allows us to derive lithospheric thickness and surface heat flux maps for much of the continent. West Antarctica shows a much thinner lithosphere ( 50-90 km) than East Antarctica ( 130-230 km), with a sharp transition along the Transantarctic Mountains (TAM). A variety of geological features, including a slower/hotter but highly heterogeneous West Antarctica and a much faster/colder East Antarctic craton, are present in the 3-D seismic/thermal models. Notably, slow seismic velocities observed in the uppermost mantle beneath the southern TAM are interpreted as a signature of lithospheric foundering and replacement with hot asthenosphere. The high resolution image of these features from the 3-D models helps further investigation of the dynamic state of Antarctica's lithosphere and underlying asthenosphere and provides key constraints on the interaction between the solid Earth and the West Antarctic Ice Sheet.

Intracontinental Deformation in the NW Iranian Plateau and Comparisons with the Northern Margin of the Tibetan Plateau

NASA Astrophysics Data System (ADS)

Chen, L.; Jiang, M.; Talebian, M.; Wan, B.; Ai, Y.; Ghods, A.; Sobouti, F.; Xiao, W.; Zhu, R.

2017-12-01

This study investigates the intracontinental deformation and its relationship with the structure of the crust and uppermost mantle in the NW Iranian plateau by combining new seismic and geological observations, to understand how this part of the plateau deformed to accommodate the Arabia-Eurasia plate collision and how the property of the lithosphere controls the deformation pattern. In contrast to the adjacent Anatolian block that exhibits westward large-scale extrusion, the northwesternmost part of the Iranian plateau shows dispersed intracontinental deformations with the development of numerous small-scale and discontinuous right-lateral strike-slip faults. The dispersed surface structures and deformation pattern correspond well to the active volcanism and seismically slow crust and uppermost mantle, and hence a weak lithosphere of the area. Further to the southeast are the western part of the Alborz Mountains and the southern Caspian Sea, both of which are characterized by stronger and more rigid lithosphere with relatively fast crust and uppermost mantle and absence of Quaternary volcanoes. A sharp Moho offset of 18 km has been imaged at the border of the Alborz and southern Caspian Sea using teleseismic receiver function data from a dense seismic array deployed under a collaborative project named "China-Iran Geological and Geophysical Survey in the Iranian Plateau (CIGSIP)". The sharp Moho offset and the minor undulations of the Moho on both sides indicate insignificant intracrustal deformation but mainly relative crustal movements between the Alborz Mountains and southern Caspian Sea, a behavior consistent with the relatively rigid nature of the lithosphere. Similar Moho offsets and lithospheric structures have been reported at the borders between the Kunlun Mountains and Qaidam or Tarim Basins in the northern margin of the Tibetan plateau, suggesting the occurrence of relative crustal movements with the effects of rigid continental lithosphere in the region. The new observations in the NW Iranian plateau combined with those in the Tibetan plateau thus provide solid evidence that intracontinental deformation is primarily controlled by the structure and properties of the continental lithosphere that may or may not have been severely altered by the collisional processes at plate margins.

Finite-Frequency Seismic Imaging of Upper-Mantle Velocity Structures Beneath the South China Continent

NASA Astrophysics Data System (ADS)

Qu, P.; Chen, Y. J.; Yu, Y.

2017-12-01

South China Continent is major formed from the Paleo-South China plate. The continent has experienced complicated tectonic history after Neoproterozoic. Previous studies suggested some possible model for the collision between South China Continent and North China Continent. Body wave tomography and surface wave tomography are widely used to inverse upper mantle velocity structure. In our study, finite frequency tomography were carried on to get explanation more correctly. We gathered nearly 60000 pieces of teleseismic event records by 166 broad band seismic stations with Mw > 5.5. Here sensitive kernel of ak135 velocity structure was calculated, which is based on Born approximation, and then we applied multi-channel cross-correlation to pick arrival time difference under 3 frequency band. Combining with crust thickness correct from receiver function, we solve the inversion matrix by LSQR method, and get accurate upper mantle structure of P, S velocity. For more accurate results, we apply a method to calculate Vp/Vs ratio, to help to verify the velocity anomaly. The result in this research shows: 1. A strong velocity anomaly exists in the northern of South China Continent, in an area 31°N between 112°-118°E. The anomaly is about . We suggest that, this anomaly is related to the collision from North China Continent. It implies the collision underthrusted to southward. 2. A clearly slow velocity anomaly exists in the northern of Cathaysia block. This low velocity anomaly exist on the boundary of Yangtz block and Cathysian block, it is related to the left over of block collision in early phanerozoic. 3. We recognized some little velocity anomaly exit in the research area. Comparing these velocity anomaly with U-Pb zircon ages, we suggest complicated orogenesis in Phanerozoic is the cause of the formation of these little anomaly. The result in our study support the collision model, which shows the underthrust direction is southward, on the south of Qinling-Dabie Orogen. The anomaly mass is larger than the composite orogenic in Yangtze block.

DOE Office of Scientific and Technical Information (OSTI.GOV)

bedle, H; Matzel, E; Flanagan, M

This report summarizes the data analysis achieved during Heather Bedle's eleven-week Technical Scholar internship at Lawrence Livermore National Labs during the early summer 2006. The work completed during this internship resulted in constraints on the crustal and upper mantle S-velocity structure in Northern Africa, the Mediterranean, the Middle East, and Europe, through the fitting of regional waveform data. This data extends current raypath coverage and will be included in a joint inversion along with data from surface wave group velocity measurements, S and P teleseismic arrival time data, and receiver function data to create an improved velocity model of themore » upper mantle in this region. The tectonic structure of the North African/Mediterranean/Europe/Middle Eastern study region is extremely heterogeneous. This region consists of, among others, stable cratons and platforms such as the West Africa Craton, and Baltica in Northern Europe; oceanic subduction zones throughout the Mediterranean Sea where the African and Eurasian plate collide; regions of continental collision as the Arabian Plate moves northward into the Turkish Plate; and rifting in the Red Sea, separating the Arabian and Nubian shields. With such diverse tectonic structures, many of the waveforms were difficult to fit. This is not unexpected as the waveforms are fit using an averaged structure. In many cases the raypaths encounter several tectonic features, complicating the waveform, and making it hard for the software to converge on a 1D average structure. Overall, the quality of the waveform data was average, with roughly 30% of the waveforms being discarded due to excessive noise that interfered with the frequency ranges of interest. An inversion for the 3D S-velocity structure of this region was also performed following the methodology of Partitioned Waveform Inversion (Nolet, 1990; Van der Lee and Nolet, 1997). The addition of the newly fit waveforms drastically extends the range of the model. The model now extends as far east in Africa to cover Chad and Niger, and reaches south to cover Zambia. The model is also stretched eastward to cover the eastern half of India, and northward to cover the southern portion of Scandinavia.« less

Tremors Triggered along the Queen Charlotte Fault

NASA Astrophysics Data System (ADS)

Aiken, C.; Peng, Z.; Chao, K.

2012-12-01

In the past decade, deep tectonic tremors have been observed in numerous tectonic environments surrounding the Pacific and Caribbean plates. In these regions, tremors triggered by both regional and distant earthquakes have also been observed. Despite the ubiquitous observations of triggered tremors, tremors triggered in differing strike-slip environments are less understood. Here, we conduct a preliminary search of tremors triggered by teleseismic earthquakes along the transpressive Queen Charlotte Fault (QCF) located between the Cascadia subduction zone and Alaska. Tectonic tremors have not been previously reported along the QCF. We select teleseismic earthquakes during the 1990-2012 period as having magnitude M ≥ 6.5 and occurring at least 1,000 km away from the region. We reduce the number of mainshocks by selecting those that generate greater than 1 kPa dynamic stress estimated from surface-wave magnitude equations [e.g. van der Elst and Brodsky, 2010]. Our mainshock waveforms are retrieved from the Canadian National Seismograph Network (CNSN), processed, and filtered for triggered tremor observations. We characterize triggered tremors as high-frequency signals visible among several stations and coincident with broadband surface wave peaks. So far, we have found tremors triggered along the QCF by surface waves of five great earthquakes - the 2002/11/03 Mw7.9 Denali Fault, 2004/12/26 Mw9.0 Sumatra, 2010/02/27 Mw8.8 Chile, 2011/03/11 Mw9.0 Japan, and 2012/04/11 Mw8.6 Sumatra earthquakes. We compare our results to tremors triggered by teleseismic earthquakes on strike-slip faults in central and southern California, as well as Cuba [Peng et al., 2012]. Among strike-slip faults in these regions, we also compare triggered tremor amplitudes to peak ground velocities from the mainshocks and compute dynamic stresses to determine a triggering threshold for the QCF. We find that in most cases tremors in the QCF are triggered primarily by the Love waves, and additional tremors are triggered by the subsequent Rayleigh waves. This is consistent with the near strike-parallel incidence for many triggering earthquakes, which tends to produce maximum triggering potential for vertical strike-slip faults. These results suggest a shear faulting mechanism is responsible for the triggered tremor on the QCF. The triggering threshold of dynamic stress is higher than that found at the Parkfield-Cholame section of the San Andreas Fault (2-3 KPa). This could be due to the sparse network coverage in the QCF, which may miss weak tremor signals triggered by smaller-size events. Our observations suggest that triggered tremor could occur in many places on major strike-slip faults around the world, although the necessary conditions for tremor generation are still not clear at this stage.

Teleseismic P wave coda from oceanic trench and other bathymetric features

NASA Astrophysics Data System (ADS)

Wu, W.; Ni, S.

2012-12-01

Teleseismic P waves are essential for studying rupture processes of great earthquakes, either in the back projection method or in finite fault inversion method involving of quantitative waveform modeling. In these studies, P waves are assumed to be direct P waves generated by localized patches of the ruptured fault. However, for some oceanic earthquakes happening near the subductiontrenches or mid-ocean ridges, we observed strong signals between P and PP are often observed on theat telseseismic networkdistances. These P wave coda signals show strong coherence and their amplitudes are sometimes comparable with those of the direct P wave or even higher for some special frequenciesfrequency band. With array analysis, we find that the coda's slowness is very close to that of the direct P wave, suggesting that they are generated near the source region. As the earthquakes occur near the trenches or mid-ocean ridges which are both featured by rapid variation of bathymetry, the coda waves are very probably generated by the scattered surface wave or S wave at the irregular bathymetry. Then, we apply the realistic bathymetry data to calculate the 3D synthetics and the coda can be well predicted by the synthetics. So the topography/bathymetry is confirmed to be the main source of the coda. The coda waves are so strong that it may affect the imaging rupture processes of ocean earthquakes, so the topography/bathymetry effect should be taken into account. However, these strong coda waves can also be used utilized to locate the oceanic earthquakes. The 3D synthetics demonstrate that the coda waves are dependent on both the specific bathymetry and the location of the earthquake. Given the determined bathymetry, the earthquake location can be constrained by the coda, e.g. the distance between trench and the earthquake can be determine from the relative arrival between the P wave and its coda which is generated by the trench. In order to locate the earthquakes using the bathymetry, it is indispensible to get all the 3D synthetics with possible different horizontal locations and depths of the earthquakes. However, the computation will be very expensive if using the numerical simulation in the whole medium. Considering that the complicated structure is only near the source region, we apply ray theory to interface full wave field from spectral-element simulation to get the teleseismic P waves. With this approach, computation efficiency is greatly improved and the relocation of the earthquake can be completed more efficiently. As for the relocation accuracy, it can be as high as 10km for the earthquakes near the trench. So it provides us another, sometimes most favorable, method to locate the ocean earthquakes with ground-truth accuracy.

Studies of earthquakes and microearthquakes using near-field seismic and geodetic observations

NASA Astrophysics Data System (ADS)

O'Toole, Thomas Bartholomew

The Centroid-Moment Tensor (CMT) method allows an optimal point-source description of an earthquake to be recovered from a set of seismic observations, and, for over 30 years, has been routinely applied to determine the location and source mechanism of teleseismically recorded earthquakes. The CMT approach is, however, entirely general: any measurements of seismic displacement fields could, in theory, be used within the CMT inversion formulation, so long as the treatment of the earthquake as a point source is valid for that data. We modify the CMT algorithm to enable a variety of near-field seismic observables to be inverted for the source parameters of an earthquake. The first two data types that we implement are provided by Global Positioning System receivers operating at sampling frequencies of 1,Hz and above. When deployed in the seismic near field, these instruments may be used as long-period-strong-motion seismometers, recording displacement time series that include the static offset. We show that both the displacement waveforms, and static displacements alone, can be used to obtain CMT solutions for moderate-magnitude earthquakes, and that performing analyses using these data may be useful for earthquake early warning. We also investigate using waveform recordings - made by conventional seismometers deployed at the surface, or by geophone arrays placed in boreholes - to determine CMT solutions, and their uncertainties, for microearthquakes induced by hydraulic fracturing. A similar waveform inversion approach could be applied in many other settings where induced seismicity and microseismicity occurs..

Coherent Seismic Arrivals in the P Wave Coda of the 2012 Mw 7.2 Sumatra Earthquake: Water Reverberations or an Early Aftershock?

NASA Astrophysics Data System (ADS)

Fan, Wenyuan; Shearer, Peter M.

2018-04-01

Teleseismic records of the 2012 Mw 7.2 Sumatra earthquake contain prominent phases in the P wave train, arriving about 50 to 100 s after the direct P arrival. Azimuthal variations in these arrivals, together with back-projection analysis, led Fan and Shearer (https://doi.org/10.1002/2016GL067785) to conclude that they originated from early aftershock(s), located ˜150 km northeast of the mainshock and landward of the trench. However, recently, Yue et al. (https://doi.org/10.1002/2017GL073254) argued that the anomalous arrivals are more likely water reverberations from the mainshock, based mostly on empirical Green's function analysis of a M6 earthquake near the mainshock and a water phase synthetic test. Here we present detailed back-projection and waveform analyses of three M6 earthquakes within 100 km of the Mw 7.2 earthquake, including the empirical Green's function event analyzed in Yue et al. (https://doi.org/10.1002/2017GL073254). In addition, we examine the waveforms of three M5.5 reverse-faulting earthquakes close to the inferred early aftershock location in Fan and Shearer (https://doi.org/10.1002/2016GL067785). These results suggest that the reverberatory character of the anomalous arrivals in the mainshock coda is consistent with water reverberations, but the origin of this energy is more likely an early aftershock rather than delayed and displaced water reverberations from the mainshock.

3D Crust and Uppermost Mantle Structure beneath Tian Shan Region from ambient noise and earthquake surface waves

NASA Astrophysics Data System (ADS)

Xiao, X.; Wen, L.

2017-12-01

As a typical active intracontinental mountain range in Central Asia, Tian Shan Mt serves as the prototype in studying geodynamic processes and mechanism of intracontinental mountain building. We study 3D crust and the uppermost mantle structure beneath Tian Shan region using ambient noise and earthquake surface waves. Our dataset includes vertical component records of 62 permanent broadband seismic stations operated by the Earthquake Administration of China. Firstly, we calculate two-year stacked Cross-Correlation Functions (CCFs) of ambient noise records between the stations. The CCFs are treated as the Empirical Green's Functions (EGFs) of each station pair, from which we measured phase velocities of fundamental-mode Rayleigh wave in the period of 3-40 s using a frequency-time analysis method. Secondly, we collect surface wave data from tele-seismic events with Mw > 5.5 and depth shallower than 200 km and measure phase velocities of the fundamental-mode of Rayleigh wave in the period of 30-150 s using a two-station method. Finally, we combine the phase velocity measurements from ambient noise and earthquake surface waves, obtain lateral isotropic phase velocity maps at different periods based on tomography and invert a 3D Vsv model of crust and uppermost mantle down to about 150 km using a Monte Carlo Inversion method. We will discuss our inversion results in detail, as well as their implications to the tectonics in the region.

The 13 January 2001 El Salvador earthquake: A multidata analysis

NASA Astrophysics Data System (ADS)

ValléE, Martin; Bouchon, Michel; Schwartz, Susan Y.

2003-04-01

On 13 January 2001, a large normal faulting intermediate depth event (Mw = 7.7) occurred 40 km off the El Salvadorian coast (Central America). We analyze this earthquake using teleseismic, regional, and local data. We first build a kinematic source model by simultaneously inverting P and SH displacement waveforms and source time functions derived from surface waves using an empirical Green's function analysis. In an attempt to discriminate between the two nodal planes (30° trenchward dipping and 60° landward dipping), we perform identical inversions using both possible fault planes. After relocating the hypocentral depth at 54 km, we retrieve the kinematic features of the rupture using a combination of the Neighborhood algorithm of [1999] and the Simplex method allowing for variable rupture velocity and slip. We find updip rupture propagation yielding a centroid depth around 47 km for both assumed fault planes with a larger variance reduction obtained using the 60° landward dipping nodal plane. We test the two possible fault models using regional broadband data and near-field accelerograms provided by [2001]. Near-field data confirm that the steeper landward dipping nodal plane is preferred. Rupture propagated mostly updip and to the northwest, resulting in a main moment release zone of approximately 25 km × 50 km with an average slip of ˜3.5 m. The large slip occurs near the interplate interface at a location where the slab steepens dip significantly. The occurrence of this event is well-explained by bending of the subducting plate.

Imaging the Alaskan subduction zone with joint inversion of ambient noise and teleseismic surface waves

NASA Astrophysics Data System (ADS)

Martin-Short, R.; Allen, R. M.; Porritt, R.

2017-12-01

Alaska consists of a complex arrangement of terranes of various geological affinities, mostof which have been accreted to the margin of North America over the last 200Myr. Today,the southern margin of Alaska is a site of active subduction, displaying a myriad ofenigmatic tectonic features. These include transition from compressional to strike-slipdominated deformation, accretion of the over-thickened Yakutat terrane, termination ofAleutian arc magnetism and the Wrangell Volcanic Field, whose magma source remainsdebated. The ongoing deployment of Transportable Array (TA) seismometers across Alaskaprovides an unprecedented opportunity to image these features in detail and learn moreabout the tectonic history of the region. Here we present a three dimensional model ofshear wave (Vsv) velocity beneath Alaska constructed using joint inversion of phasevelocity maps derived from ambient noise and teleseismic surface wave tomography. Thismodel possesses good resolution from the upper crust to about 150km depth, thuscomplementing recent body wave models of the region, which lack resolution above 100km.In the upper crust, we are able to distinguish major sedimentary basins and the cores ofmountain belts. At mid-crustal depths, we see a sharp velocity contrast across the Denalifault, suggesting that it marks a significant step in crustal thickness. In the mantle wedgeabove the subducting Yakutat terrane we observe a high velocity anomaly that may berelated to paucity of volcanism in this region. At greater depths, we image the subductingPacific-Yakutat slab as an elongate, high velocity anomaly that terminates abruptly at 145ºW, slightly further east than suggested by the Wadati-Benioff zone alone. There is alarge, low velocity anomaly beneath the Wrangell Volcanic Field, hinting that magmatismhere may be related to mantle upwelling around the slab edge.

Teleseismic P-wave Tomography and Mantle Dynamics beneath Eastern Tibet: Insight into Tengchong Volcano and Large Earthquakes

NASA Astrophysics Data System (ADS)

Lei, J., Sr.; Zhao, D.

2016-12-01

We determined a new 3-D P-wave velocity model of the upper mantle beneath eastern Tibet using 112,613 high-quality arrival-time data collected from teleseismic seismograms recorded by a new portable seismic array in Yunnan and permanent networks in southwestern China. Our results provide new insights into the mantle structure and dynamics of eastern Tibet. High-velocity (high-V) anomalies are revealed down to 200 km depth under the Sichuan basin and the Ordos and Alashan blocks. Low-velocity (low-V) anomalies are imaged in the upper mantle under the Kunlun-Qilian and Qinling fold zones, and the Songpan-Ganzi, Qiangtang, Lhasa and Chuan-Dian diamond blocks, suggesting that eastward moving low-V materials are extruded to eastern China after the obstruction by the Sichuan basin, and the Ordos and Alashan blocks. Furthermore, the extent and thickness of these low-V anomalies are correlated with the surface topography, suggesting that the uplift of eastern Tibet could be partially related to these low-V materials having a higher temperature and strong positive buoyancy. In the mantle transition zone (MTZ), broad high-V anomalies are visible from the Burma arc northward to the Kunlun fault and eastward to the Xiaojiang fault, and they are connected upward with the Wadati-Benioff seismic zone. These results suggest that the subducted Indian slab has traveled horizontally for a long distance after it descended into the MTZ, and return corner flow and deep slab dehydration have contributed to forming the low-V anomalies in the big mantle wedge. Our results shed new light on the deep origin of Tengchong volcano and large crustal earthquakes as well as the mantle dynamics of the eastern Tibetan plateau.

Determination of broadband moment magnitude (Mwp) for August 11, 2009 Suruga-Bay earthquake (MJMA=6.5)

NASA Astrophysics Data System (ADS)

Tsuboi, S.; Hirshorn, B. F.

2009-12-01

We have determined Mwp for the August 11, 2009 Suruga-Bay earthquake (MJMA=6.5) using broadband seismograms recorded at close epicentral distance stations. We have used two broadband seismograph stations: JHJ2 (epicentral distance 1.9 degree) and FUJ (epicentral distance 0.44 degree). Because of the close epicentral distance of FUJ, the seismogram is clipped at about 10 second after the P-wave arrival. However, it was possible to use the first 10 second of this seismogram to compute Mwp. We get Mwp=6.4 for JHJ2 and 6.8 for FUJ(figure 1). After we apply Whitmore et al (2000)’s correction and average these two stations, we get Mwp=6.6 for this event. The epicentral distance of 0.44 degree for magnitude 6.5 earthquake is marginal to treat this seismogram as far-field. However, considering the aftershock distribution, the fault area seems to be limited to within the Suruga-Bay, which may confirm the fact that Mwp can be successfully computed at FUJ based on the far-field approximation. This result is significant in using Mwp from close epicentral distance seismograms to issue early tsunami warning. A large earthquake with Mw=7.5 (GCMT) occurred in Andaman Island, India, 10 minutes before this Suruga-Bay event. This made it very difficult to estimate Mwp for the Suruga-Bay event from broadband seismograms at teleseismic distances because of the large amplitude of Mw7.5 Andaman Island earthquake. In this case, it is therefore difficult to issue accurate tsunami warnings based on the teleseismic stations. We used broadband seismograms recorded by F-net operated by the National Research Institute for Earth Science and Disaster Prevention.

Global Search of Triggered Tectonic Tremor

NASA Astrophysics Data System (ADS)

Peng, Z.; Aiken, C.; Chao, K.; Gonzalez-Huizar, H.; Wang, B.; Ojha, L.; Yang, H.

2013-05-01

Deep tectonic tremor has been observed at major plate-boundary faults around the Pacific Rim. While regular or ambient tremor occurs spontaneously or accompanies slow-slip events, tremor could be also triggered by large distant earthquakes and solid earth tides. Because triggered tremor occurs on the same fault patches as ambient tremor and is relatively easy to identify, a systematic global search of triggered tremor could help to identify the physical mechanisms and necessary conditions for tremor generation. Here we conduct a global search of tremor triggered by large teleseismic earthquakes. We mainly focus on major faults with significant strain accumulations where no tremor has been reported before. These includes subduction zones in Central and South America, strike-slip faults around the Caribbean plate, the Queen Charlotte-Fairweather fault system and the Denali fault in the western Canada and Alaska, the Sumatra-Java subduction zone, the Himalaya frontal thrust faults, as well as major strike-slip faults around Tibet. In each region, we first compute the predicted dynamic stresses σd from global earthquakes with magnitude>=5.0 in the past 20 years, and select events with σd > 1 kPa. Next, we download seismic data recorded by stations from local or global seismic networks, and identify triggered tremor as a high-frequency non-impulsive signal that is in phase with the large-amplitude teleseismic waves. In cases where station distributions are dense enough, we also locate tremor based on the standard envelope cross-correlation techniques. Finally, we calculate the triggering potential for the Love and Rayleigh waves with the local fault orientation and surface-wave incident angles. So far we have found several new places that are capable of generating triggered tremor. We will summarize these observations and discuss their implications on physical mechanisms of tremor and remote triggering.

Rupture process of the 2010 Mw 7.8 Mentawai tsunami earthquake from joint inversion of near-field hr-GPS and teleseismic body wave recordings constrained by tsunami observations

NASA Astrophysics Data System (ADS)

Yue, Han; Lay, Thorne; Rivera, Luis; Bai, Yefei; Yamazaki, Yoshiki; Cheung, Kwok Fai; Hill, Emma M.; Sieh, Kerry; Kongko, Widjo; Muhari, Abdul

2014-07-01

The 25 October 2010 Mentawai tsunami earthquake (Mw 7.8) ruptured the shallow portion of the Sunda megathrust seaward of the Mentawai Islands, offshore of Sumatra, Indonesia, generating a strong tsunami that took 509 lives. The rupture zone was updip of those of the 12 September 2007 Mw 8.5 and 7.9 underthrusting earthquakes. High-rate (1 s sampling) GPS instruments of the Sumatra GPS Array network deployed on the Mentawai Islands and Sumatra mainland recorded time-varying and static ground displacements at epicentral distances from 49 to 322 km. Azimuthally distributed tsunami recordings from two deepwater sensors and two tide gauges that have local high-resolution bathymetric information provide additional constraints on the source process. Finite-fault rupture models, obtained by joint inversion of the high-rate (hr)-GPS time series and numerous teleseismic broadband P and S wave seismograms together with iterative forward modeling of the tsunami recordings, indicate rupture propagation ~50 km up dip and ~100 km northwest along strike from the hypocenter, with a rupture velocity of ~1.8 km/s. Subregions with large slip extend from 7 to 10 km depth ~80 km northwest from the hypocenter with a maximum slip of 8 m and from ~5 km depth to beneath thin horizontal sedimentary layers beyond the prism deformation front for ~100 km along strike, with a localized region having >15 m of slip. The seismic moment is 7.2 × 1020 N m. The rupture model indicates that local heterogeneities in the shallow megathrust can accumulate strain that allows some regions near the toe of accretionary prisms to fail in tsunami earthquakes.

Tremor, remote triggering and earthquake cycle

NASA Astrophysics Data System (ADS)

Peng, Z.

2012-12-01

Deep tectonic tremor and episodic slow-slip events have been observed at major plate-boundary faults around the Pacific Rim. These events have much longer source durations than regular earthquakes, and are generally located near or below the seismogenic zone where regular earthquakes occur. Tremor and slow-slip events appear to be extremely stress sensitive, and could be instantaneously triggered by distant earthquakes and solid earth tides. However, many important questions remain open. For example, it is still not clear what are the necessary conditions for tremor generation, and how remote triggering could affect large earthquake cycle. Here I report a global search of tremor triggered by recent large teleseismic earthquakes. We mainly focus on major subduction zones around the Pacific Rim. These include the southwest and northeast Japan subduction zones, the Hikurangi subduction zone in New Zealand, the Cascadia subduction zone, and the major subduction zones in Central and South America. In addition, we examine major strike-slip faults around the Caribbean plate, the Queen Charlotte fault in northern Pacific Northwest Coast, and the San Andreas fault system in California. In each place, we first identify triggered tremor as a high-frequency non-impulsive signal that is in phase with the large-amplitude teleseismic waves. We also calculate the dynamic stress and check the triggering relationship with the Love and Rayleigh waves. Finally, we calculate the triggering potential with the local fault orientation and surface-wave incident angles. Our results suggest that tremor exists at many plate-boundary faults in different tectonic environments, and could be triggered by dynamic stress as low as a few kPas. In addition, we summarize recent observations of slow-slip events and earthquake swarms triggered by large distant earthquakes. Finally, we propose several mechanisms that could explain apparent clustering of large earthquakes around the world.

What role did the Hikurangi subduction zone play in the M7.8 Kaikoura earthquake?

NASA Astrophysics Data System (ADS)

Wallace, L. M.; Hamling, I. J.; Kaneko, Y.; Fry, B.; Clark, K.; Bannister, S. C.; Ellis, S. M.; Francois-Holden, C.; Hreinsdottir, S.; Mueller, C.

2017-12-01

The 2016 M7.8 Kaikoura earthquake ruptured at least a dozen faults in the northern South Island of New Zealand, within the transition from the Hikurangi subduction zone (in the North Island) to the transpressive Alpine Fault (in the central South Island). The role that the southern end of the Hikurangi subduction zone played (or did not play) in the Kaikoura earthquake remains one of the most controversial aspects of this spectacularly complex earthquake. Investigations using near-field seismological and geodetic data suggest a dominantly crustal faulting source for the event, while studies relying on teleseismic data propose that a large portion of the moment release is due to rupture of the Hikurangi subduction interface beneath the northern South Island. InSAR and GPS data also show that a large amount of afterslip (up to 0.5 m) occurred on the subduction interface beneath the crustal faults that ruptured in the M7.8 earthquake, during the months following the earthquake. Modeling of GPS velocities for the 20 year period prior to the earthquake indicate that interseismic coupling was occurring on the Hikurangi subduction interface beneath the northern South Island, in a similar location to the suggested coseismic and postseismic slip on the subduction interface. We will integrate geodetic, seismological, tsunami, and geological observations in an attempt to balance the seemingly conflicting views from local and teleseismic data regarding the role that the southern Hikurangi subduction zone played in the earthquake. We will also discuss the broader implications of the observed coseismic and postseismic deformation for understanding the kinematics of the southern termination of the Hikurangi subduction zone, and its role in the transition from subduction to strike-slip in the central New Zealand region.

Rapid and Robust Cross-Correlation-Based Seismic Phase Identification Using an Approximate Nearest Neighbor Method

NASA Astrophysics Data System (ADS)

Tibi, R.; Young, C. J.; Gonzales, A.; Ballard, S.; Encarnacao, A. V.

2016-12-01

The matched filtering technique involving the cross-correlation of a waveform of interest with archived signals from a template library has proven to be a powerful tool for detecting events in regions with repeating seismicity. However, waveform correlation is computationally expensive, and therefore impractical for large template sets unless dedicated distributed computing hardware and software are used. In this study, we introduce an Approximate Nearest Neighbor (ANN) approach that enables the use of very large template libraries for waveform correlation without requiring a complex distributed computing system. Our method begins with a projection into a reduced dimensionality space based on correlation with a randomized subset of the full template archive. Searching for a specified number of nearest neighbors is accomplished by using randomized K-dimensional trees. We used the approach to search for matches to each of 2700 analyst-reviewed signal detections reported for May 2010 for the IMS station MKAR. The template library in this case consists of a dataset of more than 200,000 analyst-reviewed signal detections for the same station from 2002-2014 (excluding May 2010). Of these signal detections, 60% are teleseismic first P, and 15% regional phases (Pn, Pg, Sn, and Lg). The analyses performed on a standard desktop computer shows that the proposed approach performs the search of the large template libraries about 20 times faster than the standard full linear search, while achieving recall rates greater than 80%, with the recall rate increasing for higher correlation values. To decide whether to confirm a match, we use a hybrid method involving a cluster approach for queries with two or more matches, and correlation score for single matches. Of the signal detections that passed our confirmation process, 52% were teleseismic first P, and 30% were regional phases.

Discrimination of nuclear explosions and earthquakes from teleseismic distances with a local network of short period seismic stations using artificial neural networks

NASA Astrophysics Data System (ADS)

Tiira, Timo

1996-10-01

Seismic discrimination capability of artificial neural networks (ANNs) was studied using earthquakes and nuclear explosions from teleseismic distances. The events were selected from two areas, which were analyzed separately. First, 23 nuclear explosions from Semipalatinsk and Lop Nor test sites were compared with 46 earthquakes from adjacent areas. Second, 39 explosions from Nevada test site were compared with 27 earthquakes from close-by areas. The basic discriminants were complexity, spectral ratio and third moment of frequency. The spectral discriminants were computed in five different ways to obtain all the information embedded in the signals, some of which were relatively weak. The discriminants were computed using data from six short period stations in Central and southern Finland. The spectral contents of the signals of both classes varied considerably between the stations. The 66 discriminants were formed into 65 optimum subsets of different sizes by using stepwise linear regression. A type of ANN called multilayer perceptron (MLP) was applied to each of the subsets. As a comparison the classification was repeated using linear discrimination analysis (LDA). Since the number of events was small the testing was made with the leave-one-out method. The ANN gave significantly better results than LDA. As a final tool for discrimination a combination of the ten neural nets with the best performance were used. All events from Central Asia were clearly discriminated and over 90% of the events from Nevada region were confidently discriminated. The better performance of ANNs was attributed to its ability to form complex decision regions between the groups and to its highly non-linear nature.