Full waveform inversion of combined towed streamer and limited OBS seismic data: a theoretical study

NASA Astrophysics Data System (ADS)

Yang, Huachen; Zhang, Jianzhong

2018-06-01

In marine seismic oil exploration, full waveform inversion (FWI) of towed-streamer data is used to reconstruct velocity models. However, the FWI of towed-streamer data easily converges to a local minimum solution due to the lack of low-frequency content. In this paper, we propose a new FWI technique using towed-streamer data, its integrated data sets and limited OBS data. Both integrated towed-streamer seismic data and OBS data have low-frequency components. Therefore, at early iterations in the new FWI technique, the OBS data combined with the integrated towed-streamer data sets reconstruct an appropriate background model. And the towed-streamer seismic data play a major role in later iterations to improve the resolution of the model. The new FWI technique is tested on numerical examples. The results show that when starting models are not accurate enough, the models inverted using the new FWI technique are superior to those inverted using conventional FWI.

Full-waveform seismic tomography of the Vrancea, Romania, subduction region

NASA Astrophysics Data System (ADS)

Baron, Julie; Morelli, Andrea

2017-12-01

The Vrancea region is one of the few locations of deep seismicity in Europe. Seismic tomography has been able to map lithospheric downwelling, but has not been able yet to clearly discriminate between competing geodynamic interpretations of the geological and geophysical evidence available. We study the seismic structure of the Vrancea subduction zone, using adjoint-based, full-waveform tomography to map the 3D vP and vS structure in detail. We use the database that was built during the CALIXTO (Carpathian Arc Lithosphere X-Tomography) temporary experiment, restricted to the broadband sensors and local intermediate-depth events. We fit waveforms with a cross-correlation misfit criterion in separate time windows around the expected P and S arrivals, and perform 17 iterations of vP and vS model updates (altogether, requiring about 16 million CPU hours) before reaching stable convergence. Among other features, our resulting model shows a nearly vertical, high-velocity body, that overlaps with the distribution of seismicity in its northeastern part. In its southwestern part, a slab appears to dip less steeply to the NW, and is suggestive of ongoing - or recently concluded - subduction geodynamic processes. Joint inversion for vP and vS allow us to address the vP/vS ratio distribution, that marks high vP/vS in the crust beneath the Focsani sedimentary basin - possibly due to high fluid pressure - and a low vP/vS edge along the lower plane of the subducting lithosphere, that in other similar environment has been attributed to dehydration of serpentine in the slab. In spite of the restricted amount of data available, and limitations on the usable frequency pass-band, full-waveform inversion reveals its potential to improve the general quality of imaging with respect to other tomographic techniques - although at a sensible cost in terms of computing resources. Our study also shows that re-analysis of legacy data sets with up-to-date techniques may bring new, useful, information.

Seismic Waveform Modeling of Broadband Data From a Temporary High-Density Deployment in the Los Angeles Basin

NASA Astrophysics Data System (ADS)

Herrman, M.; Polet, J.

2016-12-01

A total of 73 broadband seismometers were deployed for a passive source seismic experiment called the Los Angeles Syncline Seismic Interferometry Experiment (LASSIE) from September to November of 2014. The purpose of this experiment was to collect high density seismic data for the Los Angeles Basin (LAB) to better understand basin structure and response. This research will use the data collected from LASSIE to assess and refine current velocity models of the LAB using a full waveform modeling approach. To this end we will compare seismograms recorded by LASSIE for a subset of the 53 earthquakes and quarry blasts located by the Southern California Seismic Network (SCSN) that occurred within or near the LAB during the deployment period to synthetic seismograms generated by the Frequency-Wavenumber (FK) code developed by Zhu and Rivera (2002). A first analysis of the data indicates that roughly 25 of the 53 events have waveforms with sufficiently high signal to noise ratio, providing approximately 500 seismograms that are of suitable quality for comparison. We observe significant changes in waveform characteristics between stations with a very small separation distance of approximately 1 km. Focal mechanisms for most of these events have been obtained from Dr. Egill Hauksson (personal communication). We will show comparisons between the broadband velocity waveforms recorded by stations across the LASSIE array and FK synthetics determined for a variety of 1D velocity models that have been developed for the LAB area (such as Hadley and Kanamori, 1977; Hauksson, 1989, 1995 and Magistrale, 1992). The results of these comparisons will be analyzed to provide additional constraints on the subsurface seismic velocity structure within the Los Angeles basin.

Application of Carbonate Reservoir using waveform inversion and reverse-time migration methods

NASA Astrophysics Data System (ADS)

Kim, W.; Kim, H.; Min, D.; Keehm, Y.

2011-12-01

Recent exploration targets of oil and gas resources are deeper and more complicated subsurface structures, and carbonate reservoirs have become one of the attractive and challenging targets in seismic exploration. To increase the rate of success in oil and gas exploration, it is required to delineate detailed subsurface structures. Accordingly, migration method is more important factor in seismic data processing for the delineation. Seismic migration method has a long history, and there have been developed lots of migration techniques. Among them, reverse-time migration is promising, because it can provide reliable images for the complicated model even in the case of significant velocity contrasts in the model. The reliability of seismic migration images is dependent on the subsurface velocity models, which can be extracted in several ways. These days, geophysicists try to obtain velocity models through seismic full waveform inversion. Since Lailly (1983) and Tarantola (1984) proposed that the adjoint state of wave equations can be used in waveform inversion, the back-propagation techniques used in reverse-time migration have been used in waveform inversion, which accelerated the development of waveform inversion. In this study, we applied acoustic waveform inversion and reverse-time migration methods to carbonate reservoir models with various reservoir thicknesses to examine the feasibility of the methods in delineating carbonate reservoir models. We first extracted subsurface material properties from acoustic waveform inversion, and then applied reverse-time migration using the inverted velocities as a background model. The waveform inversion in this study used back-propagation technique, and conjugate gradient method was used in optimization. The inversion was performed using the frequency-selection strategy. Finally waveform inversion results showed that carbonate reservoir models are clearly inverted by waveform inversion and migration images based on the inversion results are quite reliable. Different thicknesses of reservoir models were also described and the results revealed that the lower boundary of the reservoir was not delineated because of energy loss. From these results, it was noted that carbonate reservoirs can be properly imaged and interpreted by waveform inversion and reverse-time migration methods. This work was supported by the Energy Resources R&D program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (No. 2009201030001A, No. 2010T100200133) and the Brain Korea 21 project of Energy System Engineering.

ASKI: A modular toolbox for scattering-integral-based seismic full waveform inversion and sensitivity analysis utilizing external forward codes

NASA Astrophysics Data System (ADS)

Schumacher, Florian; Friederich, Wolfgang

Due to increasing computational resources, the development of new numerically demanding methods and software for imaging Earth's interior remains of high interest in Earth sciences. Here, we give a description from a user's and programmer's perspective of the highly modular, flexible and extendable software package ASKI-Analysis of Sensitivity and Kernel Inversion-recently developed for iterative scattering-integral-based seismic full waveform inversion. In ASKI, the three fundamental steps of solving the seismic forward problem, computing waveform sensitivity kernels and deriving a model update are solved by independent software programs that interact via file output/input only. Furthermore, the spatial discretizations of the model space used for solving the seismic forward problem and for deriving model updates, respectively, are kept completely independent. For this reason, ASKI does not contain a specific forward solver but instead provides a general interface to established community wave propagation codes. Moreover, the third fundamental step of deriving a model update can be repeated at relatively low costs applying different kinds of model regularization or re-selecting/weighting the inverted dataset without need to re-solve the forward problem or re-compute the kernels. Additionally, ASKI offers the user sensitivity and resolution analysis tools based on the full sensitivity matrix and allows to compose customized workflows in a consistent computational environment. ASKI is written in modern Fortran and Python, it is well documented and freely available under terms of the GNU General Public License (http://www.rub.de/aski).

Source mechanism of small long-period events at Mount St. Helens in July 2005 using template matching, phase-weighted stacking, and full-waveform inversion

USGS Publications Warehouse

Matoza, Robin S.; Chouet, Bernard A.; Dawson, Phillip B.; Shearer, Peter M.; Haney, Matthew M.; Waite, Gregory P.; Moran, Seth C.; Mikesell, T. Dylan

2015-01-01

Long-period (LP, 0.5-5 Hz) seismicity, observed at volcanoes worldwide, is a recognized signature of unrest and eruption. Cyclic LP “drumbeating” was the characteristic seismicity accompanying the sustained dome-building phase of the 2004–2008 eruption of Mount St. Helens (MSH), WA. However, together with the LP drumbeating was a near-continuous, randomly occurring series of tiny LP seismic events (LP “subevents”), which may hold important additional information on the mechanism of seismogenesis at restless volcanoes. We employ template matching, phase-weighted stacking, and full-waveform inversion to image the source mechanism of one multiplet of these LP subevents at MSH in July 2005. The signal-to-noise ratios of the individual events are too low to produce reliable waveform-inversion results, but the events are repetitive and can be stacked. We apply network-based template matching to 8 days of continuous velocity waveform data from 29 June to 7 July 2005 using a master event to detect 822 network triggers. We stack waveforms for 359 high-quality triggers at each station and component, using a combination of linear and phase-weighted stacking to produce clean stacks for use in waveform inversion. The derived source mechanism pointsto the volumetric oscillation (~10 m3) of a subhorizontal crack located at shallow depth (~30 m) in an area to the south of Crater Glacier in the southern portion of the breached MSH crater. A possible excitation mechanism is the sudden condensation of metastable steam from a shallow pressurized hydrothermal system as it encounters cool meteoric water in the outer parts of the edifice, perhaps supplied from snow melt.

Localized time-lapse elastic waveform inversion using wavefield injection and extrapolation: 2-D parametric studies

NASA Astrophysics Data System (ADS)

Yuan, Shihao; Fuji, Nobuaki; Singh, Satish; Borisov, Dmitry

2017-06-01

We present a methodology to invert seismic data for a localized area by combining source-side wavefield injection and receiver-side extrapolation method. Despite the high resolving power of seismic full waveform inversion, the computational cost for practical scale elastic or viscoelastic waveform inversion remains a heavy burden. This can be much more severe for time-lapse surveys, which require real-time seismic imaging on a daily or weekly basis. Besides, changes of the structure during time-lapse surveys are likely to occur in a small area rather than the whole region of seismic experiments, such as oil and gas reservoir or CO2 injection wells. We thus propose an approach that allows to image effectively and quantitatively the localized structure changes far deep from both source and receiver arrays. In our method, we perform both forward and back propagation only inside the target region. First, we look for the equivalent source expression enclosing the region of interest by using the wavefield injection method. Second, we extrapolate wavefield from physical receivers located near the Earth's surface or on the ocean bottom to an array of virtual receivers in the subsurface by using correlation-type representation theorem. In this study, we present various 2-D elastic numerical examples of the proposed method and quantitatively evaluate errors in obtained models, in comparison to those of conventional full-model inversions. The results show that the proposed localized waveform inversion is not only efficient and robust but also accurate even under the existence of errors in both initial models and observed data.

Waveform-based Bayesian full moment tensor inversion and uncertainty determination for the induced seismicity in an oil/gas field

NASA Astrophysics Data System (ADS)

Gu, Chen; Marzouk, Youssef M.; Toksöz, M. Nafi

2018-03-01

Small earthquakes occur due to natural tectonic motions and are induced by oil and gas production processes. In many oil/gas fields and hydrofracking processes, induced earthquakes result from fluid extraction or injection. The locations and source mechanisms of these earthquakes provide valuable information about the reservoirs. Analysis of induced seismic events has mostly assumed a double-couple source mechanism. However, recent studies have shown a non-negligible percentage of non-double-couple components of source moment tensors in hydraulic fracturing events, assuming a full moment tensor source mechanism. Without uncertainty quantification of the moment tensor solution, it is difficult to determine the reliability of these source models. This study develops a Bayesian method to perform waveform-based full moment tensor inversion and uncertainty quantification for induced seismic events, accounting for both location and velocity model uncertainties. We conduct tests with synthetic events to validate the method, and then apply our newly developed Bayesian inversion approach to real induced seismicity in an oil/gas field in the sultanate of Oman—determining the uncertainties in the source mechanism and in the location of that event.

The Modularized Software Package ASKI - Full Waveform Inversion Based on Waveform Sensitivity Kernels Utilizing External Seismic Wave Propagation Codes

NASA Astrophysics Data System (ADS)

Schumacher, F.; Friederich, W.

2015-12-01

We present the modularized software package ASKI which is a flexible and extendable toolbox for seismic full waveform inversion (FWI) as well as sensitivity or resolution analysis operating on the sensitivity matrix. It utilizes established wave propagation codes for solving the forward problem and offers an alternative to the monolithic, unflexible and hard-to-modify codes that have typically been written for solving inverse problems. It is available under the GPL at www.rub.de/aski. The Gauss-Newton FWI method for 3D-heterogeneous elastic earth models is based on waveform sensitivity kernels and can be applied to inverse problems at various spatial scales in both Cartesian and spherical geometries. The kernels are derived in the frequency domain from Born scattering theory as the Fréchet derivatives of linearized full waveform data functionals, quantifying the influence of elastic earth model parameters on the particular waveform data values. As an important innovation, we keep two independent spatial descriptions of the earth model - one for solving the forward problem and one representing the inverted model updates. Thereby we account for the independent needs of spatial model resolution of forward and inverse problem, respectively. Due to pre-integration of the kernels over the (in general much coarser) inversion grid, storage requirements for the sensitivity kernels are dramatically reduced.ASKI can be flexibly extended to other forward codes by providing it with specific interface routines that contain knowledge about forward code-specific file formats and auxiliary information provided by the new forward code. In order to sustain flexibility, the ASKI tools must communicate via file output/input, thus large storage capacities need to be accessible in a convenient way. Storing the complete sensitivity matrix to file, however, permits the scientist full manual control over each step in a customized procedure of sensitivity/resolution analysis and full waveform inversion.

Python Waveform Cross-Correlation

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

Templeton, Dennise

PyWCC is a tool to compute seismic waveform cross-correlation coefficients on single-component or multiple-component seismic data across a network of seismic sensors. PyWCC compares waveform data templates with continuous seismic data, associates the resulting detections, identifies the template with the highest cross-correlation coefficient, and outputs a catalog of detections above a user-defined absolute cross-correlation threshold value.

Full moment tensors with uncertainties for the 2017 North Korea declared nuclear test and for a collocated, subsequent event

NASA Astrophysics Data System (ADS)

Alvizuri, C. R.; Tape, C.

2017-12-01

A seismic moment tensor is a 3×3 symmetric matrix that characterizes the far-field seismic radiation from a source, whether it be an earthquake, volcanic event, explosion. We estimate full moment tensors and their uncertainties for the North Korea declared nuclear test and for a collocated event that occurred eight minutes later. The nuclear test and the subsequent event occurred on September 3, 2017 at around 03:30 and 03:38 UTC time. We perform a grid search over the six-dimensional space of moment tensors, generating synthetic waveforms at each moment tensor grid point and then evaluating a misfit function between the observed and synthetic waveforms. The synthetic waveforms are computed using a 1-D structure model for the region; this approximation requires careful assessment of time shifts between data and synthetics, as well as careful choice of the bandpass for filtering. For each moment tensor we characterize its uncertainty in terms of waveform misfit, a probability function, and a confidence curve for the probability that the true moment tensor lies within the neighborhood of the optimal moment tensor. For each event we estimate its moment tensor using observed waveforms from all available seismic stations within a 2000-km radius. We use as much of the waveform as possible, including surface waves for all stations, and body waves above 1 Hz for some of the closest stations. Our preliminary magnitude estimates are Mw 5.1-5.3 for the first event and Mw 4.7 for the second event. Our results show a dominantly positive isotropic moment tensor for the first event, and a dominantly negative isotropic moment tensor for the subsequent event. As expected, the details of the probability density, waveform fit, and confidence curves are influenced by the structural model, the choice of filter frequencies, and the selection of stations.

Subsurface fault geometries in Southern California illuminated through Full-3D Seismic Waveform Tomography (F3DT)

NASA Astrophysics Data System (ADS)

Lee, En-Jui; Chen, Po

2017-04-01

More precise spatial descriptions of fault systems play an essential role in tectonic interpretations, deformation modeling, and seismic hazard assessments. The recent developed full-3D waveform tomography techniques provide high-resolution images and are able to image the material property differences across faults to assist the understanding of fault systems. In the updated seismic velocity model for Southern California, CVM-S4.26, many velocity gradients show consistency with surface geology and major faults defined in the Community Fault Model (CFM) (Plesch et al. 2007), which was constructed by using various geological and geophysical observations. In addition to faults in CFM, CVM-S4.26 reveals a velocity reversal mainly beneath the San Gabriel Mountain and Western Mojave Desert regions, which is correlated with the detachment structure that has also been found in other independent studies. The high-resolution tomographic images of CVM-S4.26 could assist the understanding of fault systems in Southern California and therefore benefit the development of fault models as well as other applications, such as seismic hazard analysis, tectonic reconstructions, and crustal deformation modeling.

Full Seismic Waveform Tomography of the Japan region using Adjoint Methods

NASA Astrophysics Data System (ADS)

Steptoe, Hamish; Fichtner, Andreas; Rickers, Florian; Trampert, Jeannot

2013-04-01

We present a full-waveform tomographic model of the Japan region based on spectral-element wave propagation, adjoint techniques and seismic data from dense station networks. This model is intended to further our understanding of both the complex regional tectonics and the finite rupture processes of large earthquakes. The shallow Earth structure of the Japan region has been the subject of considerable tomographic investigation. The islands of Japan exist in an area of significant plate complexity: subduction related to the Pacific and Philippine Sea plates is responsible for the majority of seismicity and volcanism of Japan, whilst smaller micro-plates in the region, including the Okhotsk, and Okinawa and Amur, part of the larger North America and Eurasia plates respectively, contribute significant local intricacy. In response to the need to monitor and understand the motion of these plates and their associated faults, numerous seismograph networks have been established, including the 768 station high-sensitivity Hi-net network, 84 station broadband F-net and the strong-motion seismograph networks K-net and KiK-net in Japan. We also include the 55 station BATS network of Taiwan. We use this exceptional coverage to construct a high-resolution model of the Japan region from the full-waveform inversion of over 15,000 individual component seismograms from 53 events that occurred between 1997 and 2012. We model these data using spectral-element simulations of seismic wave propagation at a regional scale over an area from 120°-150°E and 20°-50°N to a depth of around 500 km. We quantify differences between observed and synthetic waveforms using time-frequency misfits allowing us to separate both phase and amplitude measurements whilst exploiting the complete waveform at periods of 15-60 seconds. Fréchet kernels for these misfits are calculated via the adjoint method and subsequently used in an iterative non-linear conjugate-gradient optimization. Finally, we employ custom smoothing algorithms to remove the singularities of the Fréchet kernels and artifacts introduced by the heterogeneous coverage in oceanic regions of the model.

A waveform detector that targets template–decorrelated signals and achieves its predicted performance, Part I: Demonstration with IMS data

DOE PAGES

Carmichael, Joshua Daniel

2016-01-01

Here, waveform correlation detectors used in seismic monitoring scan multichannel data to test two competing hypotheses: that data contain (1) a noisy, amplitude-scaled version of a template waveform, or, (2) only noise. In reality, seismic wavefields include signals triggered by non-target sources (background seismicity) and targets signals that are only partially correlated with the waveform template.

Detecting and Locating Seismic Events Without Phase Picks or Velocity Models

NASA Astrophysics Data System (ADS)

Arrowsmith, S.; Young, C. J.; Ballard, S.; Slinkard, M.

2015-12-01

The standard paradigm for seismic event monitoring is to scan waveforms from a network of stations and identify the arrival time of various seismic phases. A signal association algorithm then groups the picks to form events, which are subsequently located by minimizing residuals between measured travel times and travel times predicted by an Earth model. Many of these steps are prone to significant errors which can lead to erroneous arrival associations and event locations. Here, we revisit a concept for event detection that does not require phase picks or travel time curves and fuses detection, association and location into a single algorithm. Our pickless event detector exploits existing catalog and waveform data to build an empirical stack of the full regional seismic wavefield, which is subsequently used to detect and locate events at a network level using correlation techniques. Because the technique uses more of the information content of the original waveforms, the concept is particularly powerful for detecting weak events that would be missed by conventional methods. We apply our detector to seismic data from the University of Utah Seismograph Stations network and compare our results with the earthquake catalog published by the University of Utah. We demonstrate that the pickless detector can detect and locate significant numbers of events previously missed by standard data processing techniques.

Measuring the misfit between seismograms using an optimal transport distance: application to full waveform inversion

NASA Astrophysics Data System (ADS)

Métivier, L.; Brossier, R.; Mérigot, Q.; Oudet, E.; Virieux, J.

2016-04-01

Full waveform inversion using the conventional L2 distance to measure the misfit between seismograms is known to suffer from cycle skipping. An alternative strategy is proposed in this study, based on a measure of the misfit computed with an optimal transport distance. This measure allows to account for the lateral coherency of events within the seismograms, instead of considering each seismic trace independently, as is done generally in full waveform inversion. The computation of this optimal transport distance relies on a particular mathematical formulation allowing for the non-conservation of the total energy between seismograms. The numerical solution of the optimal transport problem is performed using proximal splitting techniques. Three synthetic case studies are investigated using this strategy: the Marmousi 2 model, the BP 2004 salt model, and the Chevron 2014 benchmark data. The results emphasize interesting properties of the optimal transport distance. The associated misfit function is less prone to cycle skipping. A workflow is designed to reconstruct accurately the salt structures in the BP 2004 model, starting from an initial model containing no information about these structures. A high-resolution P-wave velocity estimation is built from the Chevron 2014 benchmark data, following a frequency continuation strategy. This estimation explains accurately the data. Using the same workflow, full waveform inversion based on the L2 distance converges towards a local minimum. These results yield encouraging perspectives regarding the use of the optimal transport distance for full waveform inversion: the sensitivity to the accuracy of the initial model is reduced, the reconstruction of complex salt structure is made possible, the method is robust to noise, and the interpretation of seismic data dominated by reflections is enhanced.

Full-Waveform Envelope Templates for Low Magnitude Discrimination and Yield Estimation at Local and Regional Distances with Application to the North Korean Nuclear Tests

NASA Astrophysics Data System (ADS)

Yoo, S. H.

2017-12-01

Monitoring seismologists have successfully used seismic coda for event discrimination and yield estimation for over a decade. In practice seismologists typically analyze long-duration, S-coda signals with high signal-to-noise ratios (SNR) at regional and teleseismic distances, since the single back-scattering model reasonably predicts decay of the late coda. However, seismic monitoring requirements are shifting towards smaller, locally recorded events that exhibit low SNR and short signal lengths. To be successful at characterizing events recorded at local distances, we must utilize the direct-phase arrivals, as well as the earlier part of the coda, which is dominated by multiple forward scattering. To remedy this problem, we have developed a new hybrid method known as full-waveform envelope template matching to improve predicted envelope fits over the entire waveform and account for direct-wave and early coda complexity. We accomplish this by including a multiple forward-scattering approximation in the envelope modeling of the early coda. The new hybrid envelope templates are designed to fit local and regional full waveforms and produce low-variance amplitude estimates, which will improve yield estimation and discrimination between earthquakes and explosions. To demonstrate the new technique, we applied our full-waveform envelope template-matching method to the six known North Korean (DPRK) underground nuclear tests and four aftershock events following the September 2017 test. We successfully discriminated the event types and estimated the yield for all six nuclear tests. We also applied the same technique to the 2015 Tianjin explosions in China, and another suspected low-yield explosion at the DPRK test site on May 12, 2010. Our results show that the new full-waveform envelope template-matching method significantly improves upon longstanding single-scattering coda prediction techniques. More importantly, the new method allows monitoring seismologists to extend coda-based techniques to lower magnitude thresholds and low-yield local explosions.

Nature and Role of Subducting Sediments on the Megathrust and Forearc Evolution in the 2004 Great Sumatra Earthquake Rupture Zone: Results from Full Waveform Inversion of Long Offset Seismic Data

NASA Astrophysics Data System (ADS)

Singh, S. C.; Qin, Y.

2015-12-01

On active accretionary margins, the nature of incoming sediments defines the locking mechanism on the megathrust, and the development and evolution of the accretionary wedge. Drilling is the most direct method to characterise the nature of these sediments, but the drilling is very expensive, and provide information at only a few locations. In north Sumatra, an IODP drilling is programmed to take place in July-August 2016. We have performed seismic full waveform inversion of 12 km long offset seismic reflection data acquired by WesternGeco in 2006 over a 35 km zone near the subduction front in the 2004 earthquake rupture zone area that provide detailed quantitative information on the characteristics of the incoming sediments. We first downward continue the surface streamer data to the seafloor, which removes the effect of deep water (~5 km) and brings out the refraction arrivals as the first arrivals. We carry out travel time tomography, and then performed full waveform inversion of seismic refraction data followed by the full waveform inversion of reflection data providing detailed (10-20 m) velocity structure. The sediments in this area are 3-5 km thick where the P-wave velocity increases from 1.6 km/s near the seafloor to more than 4.5 km/s above the oceanic crust. The high velocity of sediments above the basement suggests that the sediments are highly compacted, strengthened the coupling near the subduction front, which might have been responsible for 2004 earthquake rupture propagation up to the subduction front, enhancing the tsunami. We also find several thin velocity layers within the sediments, which might be due to high pore-pressure fluid or free gas. These layers might be responsible for the formation of pseudo-decollement within the forearc sediments that acts as a conveyer belt between highly compacted subducting lower sediments and accreted sediments above. The presence of well intact sediments on the accretionary prism supports this interpretation. Our results provide first hand information about the sediments properties, which will be ground toothed by drilling.

Global and local waveform simulations using the VERCE platform

NASA Astrophysics Data System (ADS)

Garth, Thomas; Saleh, Rafiq; Spinuso, Alessandro; Gemund, Andre; Casarotti, Emanuele; Magnoni, Federica; Krischner, Lion; Igel, Heiner; Schlichtweg, Horst; Frank, Anton; Michelini, Alberto; Vilotte, Jean-Pierre; Rietbrock, Andreas

2017-04-01

In recent years the potential to increase resolution of seismic imaging by full waveform inversion has been demonstrated on a range of scales from basin to continental scales. These techniques rely on harnessing the computational power of large supercomputers, and running large parallel codes to simulate the seismic wave field in a three-dimensional geological setting. The VERCE platform is designed to make these full waveform techniques accessible to a far wider spectrum of the seismological community. The platform supports the two widely used spectral element simulation programs SPECFEM3D Cartesian, and SPECFEM3D globe, allowing users to run a wide range of simulations. In the SPECFEM3D Cartesian implementation the user can run waveform simulations on a range of pre-loaded meshes and velocity models for specific areas, or upload their own velocity model and mesh. In the new SPECFEM3D globe implementation, the user will be able to select from a number of continent scale model regions, or perform waveform simulations for the whole earth. Earthquake focal mechanisms can be downloaded within the platform, for example from the GCMT catalogue, or users can upload their own focal mechanism catalogue through the platform. The simulations can be run on a range of European supercomputers in the PRACE network. Once a job has been submitted and run through the platform, the simulated waveforms can be manipulated or downloaded for further analysis. The misfit between the simulated and recorded waveforms can then be calculated through the platform through three interoperable workflows, for raw-data access (FDSN) and caching, pre-processing and finally misfit. The last workflow makes use of the Pyflex analysis software. In addition, the VERCE platform can be used to produce animations of waveform propagation through the velocity model, and synthetic shakemaps. All these data-products are made discoverable and re-usable thanks to the VERCE data and metadata management layer. We demonstrate the functionality of the VERCE platform with two use cases, one using the pre-loaded velocity model and mesh for the Maule area of Chile using the SPECFEM3D Cartesian workflow, and one showing the output of a global simulation using the SPECFEM3D globe workflow. It is envisioned that this tool will allow a much greater range of seismologists to access these full waveform inversion tools, and aid full waveform tomographic and source inversion, synthetic shakemap production and other full waveform applications, in a wide range of tectonic settings.

Frozen Gaussian approximation for 3D seismic tomography

NASA Astrophysics Data System (ADS)

Chai, Lihui; Tong, Ping; Yang, Xu

2018-05-01

Three-dimensional (3D) wave-equation-based seismic tomography is computationally challenging in large scales and high-frequency regime. In this paper, we apply the frozen Gaussian approximation (FGA) method to compute 3D sensitivity kernels and seismic tomography of high-frequency. Rather than standard ray theory used in seismic inversion (e.g. Kirchhoff migration and Gaussian beam migration), FGA is used to compute the 3D high-frequency sensitivity kernels for travel-time or full waveform inversions. Specifically, we reformulate the equations of the forward and adjoint wavefields for the purpose of convenience to apply FGA, and with this reformulation, one can efficiently compute the Green’s functions whose convolutions with source time function produce wavefields needed for the construction of 3D kernels. Moreover, a fast summation method is proposed based on local fast Fourier transform which greatly improves the speed of reconstruction as the last step of FGA algorithm. We apply FGA to both the travel-time adjoint tomography and full waveform inversion (FWI) on synthetic crosswell seismic data with dominant frequencies as high as those of real crosswell data, and confirm again that FWI requires a more sophisticated initial velocity model for the convergence than travel-time adjoint tomography. We also numerically test the accuracy of applying FGA to local earthquake tomography. This study paves the way to directly apply wave-equation-based seismic tomography methods into real data around their dominant frequencies.

Seismic waveform sensitivity to global boundary topography

NASA Astrophysics Data System (ADS)

Colombi, Andrea; Nissen-Meyer, Tarje; Boschi, Lapo; Giardini, Domenico

2012-09-01

We investigate the implications of lateral variations in the topography of global seismic discontinuities, in the framework of high-resolution forward modelling and seismic imaging. We run 3-D wave-propagation simulations accurate at periods of 10 s and longer, with Earth models including core-mantle boundary topography anomalies of ˜1000 km spatial wavelength and up to 10 km height. We obtain very different waveform signatures for PcP (reflected) and Pdiff (diffracted) phases, supporting the theoretical expectation that the latter are sensitive primarily to large-scale structure, whereas the former only to small scale, where large and small are relative to the frequency. PcP at 10 s seems to be well suited to map such a small-scale perturbation, whereas Pdiff at the same frequency carries faint signatures that do not allow any tomographic reconstruction. Only at higher frequency, the signature becomes stronger. We present a new algorithm to compute sensitivity kernels relating seismic traveltimes (measured by cross-correlation of observed and theoretical seismograms) to the topography of seismic discontinuities at any depth in the Earth using full 3-D wave propagation. Calculation of accurate finite-frequency sensitivity kernels is notoriously expensive, but we reduce computational costs drastically by limiting ourselves to spherically symmetric reference models, and exploiting the axial symmetry of the resulting propagating wavefield that collapses to a 2-D numerical domain. We compute and analyse a suite of kernels for upper and lower mantle discontinuities that can be used for finite-frequency waveform inversion. The PcP and Pdiff sensitivity footprints are in good agreement with the result obtained cross-correlating perturbed and unperturbed seismogram, validating our approach against full 3-D modelling to invert for such structures.

Earthquake detection through computationally efficient similarity search

PubMed Central

Yoon, Clara E.; O’Reilly, Ossian; Bergen, Karianne J.; Beroza, Gregory C.

2015-01-01

Seismology is experiencing rapid growth in the quantity of data, which has outpaced the development of processing algorithms. Earthquake detection—identification of seismic events in continuous data—is a fundamental operation for observational seismology. We developed an efficient method to detect earthquakes using waveform similarity that overcomes the disadvantages of existing detection methods. Our method, called Fingerprint And Similarity Thresholding (FAST), can analyze a week of continuous seismic waveform data in less than 2 hours, or 140 times faster than autocorrelation. FAST adapts a data mining algorithm, originally designed to identify similar audio clips within large databases; it first creates compact “fingerprints” of waveforms by extracting key discriminative features, then groups similar fingerprints together within a database to facilitate fast, scalable search for similar fingerprint pairs, and finally generates a list of earthquake detections. FAST detected most (21 of 24) cataloged earthquakes and 68 uncataloged earthquakes in 1 week of continuous data from a station located near the Calaveras Fault in central California, achieving detection performance comparable to that of autocorrelation, with some additional false detections. FAST is expected to realize its full potential when applied to extremely long duration data sets over a distributed network of seismic stations. The widespread application of FAST has the potential to aid in the discovery of unexpected seismic signals, improve seismic monitoring, and promote a greater understanding of a variety of earthquake processes. PMID:26665176

Full seismic waveform tomography for upper-mantle structure in the Australasian region using adjoint methods

NASA Astrophysics Data System (ADS)

Fichtner, Andreas; Kennett, Brian L. N.; Igel, Heiner; Bunge, Hans-Peter

2009-12-01

We present a full seismic waveform tomography for upper-mantle structure in the Australasian region. Our method is based on spectral-element simulations of seismic wave propagation in 3-D heterogeneous earth models. The accurate solution of the forward problem ensures that waveform misfits are solely due to as yet undiscovered Earth structure and imprecise source descriptions, thus leading to more realistic tomographic images and source parameter estimates. To reduce the computational costs, we implement a long-wavelength equivalent crustal model. We quantify differences between the observed and the synthetic waveforms using time-frequency (TF) misfits. Their principal advantages are the separation of phase and amplitude misfits, the exploitation of complete waveform information and a quasi-linear relation to 3-D Earth structure. Fréchet kernels for the TF misfits are computed via the adjoint method. We propose a simple data compression scheme and an accuracy-adaptive time integration of the wavefields that allows us to reduce the storage requirements of the adjoint method by almost two orders of magnitude. To minimize the waveform phase misfit, we implement a pre-conditioned conjugate gradient algorithm. Amplitude information is incorporated indirectly by a restricted line search. This ensures that the cumulative envelope misfit does not increase during the inversion. An efficient pre-conditioner is found empirically through numerical experiments. It prevents the concentration of structural heterogeneity near the sources and receivers. We apply our waveform tomographic method to ~1000 high-quality vertical-component seismograms, recorded in the Australasian region between 1993 and 2008. The waveforms comprise fundamental- and higher-mode surface and long-period S body waves in the period range from 50 to 200 s. To improve the convergence of the algorithm, we implement a 3-D initial model that contains the long-wavelength features of the Australasian region. Resolution tests indicate that our algorithm converges after around 10 iterations and that both long- and short-wavelength features in the uppermost mantle are well resolved. There is evidence for effects related to the non-linearity in the inversion procedure. After 11 iterations we fit the data waveforms acceptably well; with no significant further improvements to be expected. During the inversion the total fitted seismogram length increases by 46 per cent, providing a clear indication of the efficiency and consistency of the iterative optimization algorithm. The resulting SV-wave velocity model reveals structural features of the Australasian upper mantle with great detail. We confirm the existence of a pronounced low-velocity band along the eastern margin of the continent that can be clearly distinguished against Precambrian Australia and the microcontinental Lord Howe Rise. The transition from Precambrian to Phanerozoic Australia (the Tasman Line) appears to be sharp down to at least 200 km depth. It mostly occurs further east of where it is inferred from gravity and magnetic anomalies. Also clearly visible are the Archean and Proterozoic cratons, the northward continuation of the continent and anomalously low S-wave velocities in the upper mantle in central Australia. This is, to the best of our knowledge, the first application of non-linear full seismic waveform tomography to a continental-scale problem.

Waveform Retrieval and Phase Identification for Seismic Data from the CASS Experiment

NASA Astrophysics Data System (ADS)

Li, Zhiwei; You, Qingyu; Ni, Sidao; Hao, Tianyao; Wang, Hongti; Zhuang, Cantao

2013-05-01

The little destruction to the deployment site and high repeatability of the Controlled Accurate Seismic Source (CASS) shows its potential for investigating seismic wave velocities in the Earth's crust. However, the difficulty in retrieving impulsive seismic waveforms from the CASS data and identifying the seismic phases substantially prevents its wide applications. For example, identification of the seismic phases and accurate measurement of travel times are essential for resolving the spatial distribution of seismic velocities in the crust. Until now, it still remains a challenging task to estimate the accurate travel times of different seismic phases from the CASS data which features extended wave trains, unlike processing of the waveforms from impulsive events such as earthquakes or explosive sources. In this study, we introduce a time-frequency analysis method to process the CASS data, and try to retrieve the seismic waveforms and identify the major seismic phases traveling through the crust. We adopt the Wigner-Ville Distribution (WVD) approach which has been used in signal detection and parameter estimation for linear frequency modulation (LFM) signals, and proves to feature the best time-frequency convergence capability. The Wigner-Hough transform (WHT) is applied to retrieve the impulsive waveforms from multi-component LFM signals, which comprise seismic phases with different arrival times. We processed the seismic data of the 40-ton CASS in the field experiment around the Xinfengjiang reservoir with the WVD and WHT methods. The results demonstrate that these methods are effective in waveform retrieval and phase identification, especially for high frequency seismic phases such as PmP and SmS with strong amplitudes in large epicenter distance of 80-120 km. Further studies are still needed to improve the accuracy on travel time estimation, so as to further promote applicability of the CASS for and imaging the seismic velocity structure.

Development of real time monitor system displaying seismic waveform data observed at seafloor seismic network, DONET, for disaster management information

NASA Astrophysics Data System (ADS)

Horikawa, H.; Takaesu, M.; Sueki, K.; Takahashi, N.; Sonoda, A.; Miura, S.; Tsuboi, S.

2014-12-01

Mega-thrust earthquakes are anticipated to occur in the Nankai Trough in southwest Japan. In the source areas, we have deployed seafloor seismic network, DONET (Dense Ocean-floor Network System for Earthquake and Tsunamis), in 2010 in order to monitor seismicity, crustal deformations, and tsunamis. DONET system consists of totally 20 stations, which is composed of six kinds of sensors, including strong-motion seismometers　and quartz pressure gauges. Those stations are densely distributed with an average spatial interval of 15-20 km and cover near the trench axis to coastal areas. Observed data are transferred to a land station through a fiber-optical cable and then to JAMSTEC (Japan Agency for Marine-Earth Science and Technology) data management center through a private network in real time. After 2011 off the Pacific coast of Tohoku Earthquake, each local government close to Nankai Trough try to plan disaster prevention scheme. JAMSTEC will disseminate DONET data combined with research accomplishment so that they will be widely recognized as important earthquake information. In order to open DONET data observed for research to local government, we have developed a web application system, REIS　(Real-time Earthquake Information System). REIS is providing seismic waveform data to some local governments close to Nankai Trough as a pilot study. As soon as operation of DONET is ready, REIS will start full-scale operation. REIS can display seismic waveform data of DONET in real-time, users can select strong motion and pressure data, and configure the options of trace view arrangement, time scale, and amplitude. In addition to real-time monitoring, REIS can display past seismic waveform data and show earthquake epicenters on the map. In this presentation, we briefly introduce DONET system and then show our web　application system. We also discuss our future plans for further developments of REIS.

ASDF: An Adaptable Seismic Data Format with Full Provenance

NASA Astrophysics Data System (ADS)

Smith, J. A.; Krischer, L.; Tromp, J.; Lefebvre, M. P.

2015-12-01

In order for seismologists to maximize their knowledge of how the Earth works, they must extract the maximum amount of useful information from all recorded seismic data available for their research. This requires assimilating large sets of waveform data, keeping track of vast amounts of metadata, using validated standards for quality control, and automating the workflow in a careful and efficient manner. In addition, there is a growing gap between CPU/GPU speeds and disk access speeds that leads to an I/O bottleneck in seismic workflows. This is made even worse by existing seismic data formats that were not designed for performance and are limited to a few fixed headers for storing metadata.The Adaptable Seismic Data Format (ASDF) is a new data format for seismology that solves the problems with existing seismic data formats and integrates full provenance into the definition. ASDF is a self-describing format that features parallel I/O using the parallel HDF5 library. This makes it a great choice for use on HPC clusters. The format integrates the standards QuakeML for seismic sources and StationXML for receivers. ASDF is suitable for storing earthquake data sets, where all waveforms for a single earthquake are stored in a one file, ambient noise cross-correlations, and adjoint sources. The format comes with a user-friendly Python reader and writer that gives seismologists access to a full set of Python tools for seismology. There is also a faster C/Fortran library for integrating ASDF into performance-focused numerical wave solvers, such as SPECFEM3D_GLOBE. Finally, a GUI tool designed for visually exploring the format exists that provides a flexible interface for both research and educational applications. ASDF is a new seismic data format that offers seismologists high-performance parallel processing, organized and validated contents, and full provenance tracking for automated seismological workflows.

Application of Adjoint Method and Spectral-Element Method to Tomographic Inversion of Regional Seismological Structure Beneath Japanese Islands

NASA Astrophysics Data System (ADS)

Tsuboi, S.; Miyoshi, T.; Obayashi, M.; Tono, Y.; Ando, K.

2014-12-01

Recent progress in large scale computing by using waveform modeling technique and high performance computing facility has demonstrated possibilities to perform full-waveform inversion of three dimensional (3D) seismological structure inside the Earth. We apply the adjoint method (Liu and Tromp, 2006) to obtain 3D structure beneath Japanese Islands. First we implemented Spectral-Element Method to K-computer in Kobe, Japan. We have optimized SPECFEM3D_GLOBE (Komatitsch and Tromp, 2002) by using OpenMP so that the code fits hybrid architecture of K-computer. Now we could use 82,134 nodes of K-computer (657,072 cores) to compute synthetic waveform with about 1 sec accuracy for realistic 3D Earth model and its performance was 1.2 PFLOPS. We use this optimized SPECFEM3D_GLOBE code and take one chunk around Japanese Islands from global mesh and compute synthetic seismograms with accuracy of about 10 second. We use GAP-P2 mantle tomography model (Obayashi et al., 2009) as an initial 3D model and use as many broadband seismic stations available in this region as possible to perform inversion. We then use the time windows for body waves and surface waves to compute adjoint sources and calculate adjoint kernels for seismic structure. We have performed several iteration and obtained improved 3D structure beneath Japanese Islands. The result demonstrates that waveform misfits between observed and theoretical seismograms improves as the iteration proceeds. We now prepare to use much shorter period in our synthetic waveform computation and try to obtain seismic structure for basin scale model, such as Kanto basin, where there are dense seismic network and high seismic activity. Acknowledgements: This research was partly supported by MEXT Strategic Program for Innovative Research. We used F-net seismograms of the National Research Institute for Earth Science and Disaster Prevention.

Salvus: A scalable software suite for full-waveform modelling & inversion

NASA Astrophysics Data System (ADS)

Afanasiev, M.; Boehm, C.; van Driel, M.; Krischer, L.; Fichtner, A.

2017-12-01

Full-waveform inversion (FWI), whether at the lab, exploration, or planetary scale, requires the cooperation of five principal components. (1) The geometry of the domain needs to be properly discretized and an initial guess of the model parameters must be projected onto it; (2) Large volumes of recorded waveform data must be collected, organized, and processed; (3) Synthetic waveform data must be efficiently and accurately computed through complex domains; (4) Suitable misfit functions and optimization techniques must be used to relate discrepancies in data space to perturbations in the model; and (5) Some form of workflow management must be employed to schedule and run (1) - (4) in the correct order. Each one of these components can represent a formidable technical challenge which redirects energy from the true task at hand: using FWI to extract new information about some underlying continuum.In this presentation we give an overview of the current status of the Salvus software suite, which was introduced to address the challenges listed above. Specifically, we touch on (1) salvus_mesher, which eases the discretization of complex Earth models into hexahedral meshes; (2) salvus_seismo, which integrates with LASIF and ObsPy to streamline the processing and preparation of seismic data; (3) salvus_wave, a high-performance and scalable spectral-element solver capable of simulating waveforms through general unstructured 2- and 3-D domains, and (4) salvus_opt, an optimization toolbox specifically designed for full-waveform inverse problems. Tying everything together, we also discuss (5) salvus_flow: a workflow package designed to orchestrate and manage the rest of the suite. It is our hope that these developments represent a step towards the automation of large-scale seismic waveform inversion, while also lowering the barrier of entry for new applications. We include several examples of Salvus' use in (extra-) planetary seismology, non-destructive testing, and medical imaging.

Analysis of low-frequency seismic signals generated during a multiple-iceberg calving event at Jakobshavn Isbræ, Greenland

USGS Publications Warehouse

Walter, Fabian; Amundson, Jason M.; O'Neel, Shad; Truffer, Martin; Fahnestock, Mark; Fricker, Helen A.

2012-01-01

We investigated seismic signals generated during a large-scale, multiple iceberg calving event that occurred at Jakobshavn Isbræ, Greenland, on 21 August 2009. The event was recorded by a high-rate time-lapse camera and five broadband seismic stations located within a few hundred kilometers of the terminus. During the event two full-glacier-thickness icebergs calved from the grounded (or nearly grounded) terminus and immediately capsized; the second iceberg to calve was two to three times smaller than the first. The individual calving and capsize events were well-correlated with the radiation of low-frequency seismic signals (<0.1 Hz) dominated by Love and Rayleigh waves. In agreement with regional records from previously published ‘glacial earthquakes’, these low-frequency seismic signals had maximum power and/or signal-to-noise ratios in the 0.05–0.1 Hz band. Similarly, full waveform inversions indicate that these signals were also generated by horizontal single forces acting at the glacier terminus. The signals therefore appear to be local manifestations of glacial earthquakes, although the magnitudes of the signals (twice-time integrated force histories) were considerably smaller than previously reported glacial earthquakes. We thus speculate that such earthquakes may be a common, if not pervasive, feature of all full-glacier-thickness calving events from grounded termini. Finally, a key result from our study is that waveform inversions performed on low-frequency, calving-generated seismic signals may have only limited ability to quantitatively estimate mass losses from calving. In particular, the choice of source time function has little impact on the inversion but dramatically changes the earthquake magnitude. Accordingly, in our analysis, it is unclear whether the smaller or larger of the two calving icebergs generated a larger seismic signal.

Accumulated energy norm for full waveform inversion of marine data

NASA Astrophysics Data System (ADS)

Shin, Changsoo; Ha, Wansoo

2017-12-01

Macro-velocity models are important for imaging the subsurface structure. However, the conventional objective functions of full waveform inversion in the time and the frequency domain have a limited ability to recover the macro-velocity model because of the absence of low-frequency information. In this study, we propose new objective functions that can recover the macro-velocity model by minimizing the difference between the zero-frequency components of the square of seismic traces. Instead of the seismic trace itself, we use the square of the trace, which contains low-frequency information. We apply several time windows to the trace and obtain zero-frequency information of the squared trace for each time window. The shape of the new objective functions shows that they are suitable for local optimization methods. Since we use the acoustic wave equation in this study, this method can be used for deep-sea marine data, in which elastic effects can be ignored. We show that the zero-frequency components of the square of the seismic traces can be used to recover macro-velocities from synthetic and field data.

Reducing disk storage of full-3D seismic waveform tomography (F3DT) through lossy online compression

NASA Astrophysics Data System (ADS)

Lindstrom, Peter; Chen, Po; Lee, En-Jui

2016-08-01

Full-3D seismic waveform tomography (F3DT) is the latest seismic tomography technique that can assimilate broadband, multi-component seismic waveform observations into high-resolution 3D subsurface seismic structure models. The main drawback in the current F3DT implementation, in particular the scattering-integral implementation (F3DT-SI), is the high disk storage cost and the associated I/O overhead of archiving the 4D space-time wavefields of the receiver- or source-side strain tensors. The strain tensor fields are needed for computing the data sensitivity kernels, which are used for constructing the Jacobian matrix in the Gauss-Newton optimization algorithm. In this study, we have successfully integrated a lossy compression algorithm into our F3DT-SI workflow to significantly reduce the disk space for storing the strain tensor fields. The compressor supports a user-specified tolerance for bounding the error, and can be integrated into our finite-difference wave-propagation simulation code used for computing the strain fields. The decompressor can be integrated into the kernel calculation code that reads the strain fields from the disk and compute the data sensitivity kernels. During the wave-propagation simulations, we compress the strain fields before writing them to the disk. To compute the data sensitivity kernels, we read the compressed strain fields from the disk and decompress them before using them in kernel calculations. Experiments using a realistic dataset in our California statewide F3DT project have shown that we can reduce the strain-field disk storage by at least an order of magnitude with acceptable loss, and also improve the overall I/O performance of the entire F3DT-SI workflow significantly. The integration of the lossy online compressor may potentially open up the possibilities of the wide adoption of F3DT-SI in routine seismic tomography practices in the near future.

Reducing Disk Storage of Full-3D Seismic Waveform Tomography (F3DT) Through Lossy Online Compression

DOE PAGES

Lindstrom, Peter; Chen, Po; Lee, En-Jui

2016-05-05

Full-3D seismic waveform tomography (F3DT) is the latest seismic tomography technique that can assimilate broadband, multi-component seismic waveform observations into high-resolution 3D subsurface seismic structure models. The main drawback in the current F3DT implementation, in particular the scattering-integral implementation (F3DT-SI), is the high disk storage cost and the associated I/O overhead of archiving the 4D space-time wavefields of the receiver- or source-side strain tensors. The strain tensor fields are needed for computing the data sensitivity kernels, which are used for constructing the Jacobian matrix in the Gauss-Newton optimization algorithm. In this study, we have successfully integrated a lossy compression algorithmmore » into our F3DT SI workflow to significantly reduce the disk space for storing the strain tensor fields. The compressor supports a user-specified tolerance for bounding the error, and can be integrated into our finite-difference wave-propagation simulation code used for computing the strain fields. The decompressor can be integrated into the kernel calculation code that reads the strain fields from the disk and compute the data sensitivity kernels. During the wave-propagation simulations, we compress the strain fields before writing them to the disk. To compute the data sensitivity kernels, we read the compressed strain fields from the disk and decompress them before using them in kernel calculations. Experiments using a realistic dataset in our California statewide F3DT project have shown that we can reduce the strain-field disk storage by at least an order of magnitude with acceptable loss, and also improve the overall I/O performance of the entire F3DT-SI workflow significantly. The integration of the lossy online compressor may potentially open up the possibilities of the wide adoption of F3DT-SI in routine seismic tomography practices in the near future.« less

Velocity structure of a bottom simulating reflector offshore Peru: Results from full waveform inversion

USGS Publications Warehouse

Pecher, I.A.; Minshull, T.A.; Singh, S.C.; von Huene, Roland E.

1996-01-01

Much of our knowledge of the worldwide distribution of submarine gas hydrates comes from seismic observations of Bottom Simulating Reflectors (BSRs). Full waveform inversion has proven to be a reliable technique for studying the fine structure of BSRs using the compressional wave velocity. We applied a non-linear full waveform inversion technique to a BSR at a location offshore Peru. We first determined the large-scale features of seismic velocity variations using a statistical inversion technique to maximise coherent energy along travel-time curves. These velocities were used for a starting velocity model for the full waveform inversion, which yielded a detailed velocity/depth model in the vicinity of the BSR. We found that the data are best fit by a model in which the BSR consists of a thin, low-velocity layer. The compressional wave velocity drops from 2.15 km/s down to an average of 1.70 km/s in an 18m thick interval, with a minimum velocity of 1.62 km/s in a 6 m interval. The resulting compressional wave velocity was used to estimate gas content in the sediments. Our results suggest that the low velocity layer is a 6-18 m thick zone containing a few percent of free gas in the pore space. The presence of the BSR coincides with a region of vertical uplift. Therefore, we suggest that gas at this BSR is formed by a dissociation of hydrates at the base of the hydrate stability zone due to uplift and subsequently a decrease in pressure.

Duration of Tsunami Generation Longer than Duration of Seismic Wave Generation in the 2011 Mw 9.0 Tohoku-Oki Earthquake

NASA Astrophysics Data System (ADS)

Fujihara, S.; Korenaga, M.; Kawaji, K.; Akiyama, S.

2013-12-01

We try to compare and evaluate the nature of tsunami generation and seismic wave generation in occurrence of the 2011 Tohoku-Oki earthquake (hereafter, called as TOH11), in terms of two type of moment rate functions, inferred from finite source imaging of tsunami waveforms and seismic waveforms. Since 1970's, the nature of "tsunami earthquakes" has been discussed in many researches (e.g. Kanamori, 1972; Kanamori and Kikuchi, 1993; Kikuchi and Kanamori, 1995; Ide et al., 1993; Satake, 1994) mostly based on analysis of seismic waveform data , in terms of the "slow" nature of tsunami earthquakes (e.g., the 1992 Nicaragura earthquake). Although TOH11 is not necessarily understood as a tsunami earthquake, TOH11 is one of historical earthquakes that simultaneously generated large seismic waves and tsunami. Also, TOH11 is one of earthquakes which was observed both by seismic observation network and tsunami observation network around the Japanese islands. Therefore, for the purpose of analyzing the nature of tsunami generation, we try to utilize tsunami waveform data as much as possible. In our previous studies of TOH11 (Fujihara et al., 2012a; Fujihara et al., 2012b), we inverted tsunami waveforms at GPS wave gauges of NOWPHAS to image the spatio-temporal slip distribution. The "temporal" nature of our tsunami source model is generally consistent with the other tsunami source models (e.g., Satake et al, 2013). For seismic waveform inversion based on 1-D structure, here we inverted broadband seismograms at GSN stations based on the teleseismic body-wave inversion scheme (Kikuchi and Kanamori, 2003). Also, for seismic waveform inversion considering the inhomogeneous internal structure, we inverted strong motion seismograms at K-NET and KiK-net stations, based on 3-D Green's functions (Fujihara et al., 2013a; Fujihara et al., 2013b). The gross "temporal" nature of our seismic source models are generally consistent with the other seismic source models (e.g., Yoshida et al., 2011; Ide at al., 2011; Yagi and Fukahata, 2011; Suzuki et al., 2011). The comparison of two type of moment rate functions, inferred from finite source imaging of tsunami waveforms and seismic waveforms, suggested that there was the time period common to both seismic wave generation and tsunami generation followed by the time period unique to tsunami generation. At this point, we think that comparison of the absolute values of moment rates is not so meaningful between tsunami waveform inversion and seismic waveform inversion, because of general ambiguity of rigidity values of each subfault in the fault region (assuming the rigidity value of 30 GPa of Yoshida et al (2011)). Considering this, the normalized value of moment rate function was also evaluated and it does not change the general feature of two moment rate functions in terms of duration property. Furthermore, the results suggested that tsunami generation process apparently took more time than seismic wave generation process did. Tsunami can be generated even by "extra" motions resulting from many suggested abnormal mechanisms. These extra motions may be attribute to the relatively larger-scale tsunami generation than expected from the magnitude level from seismic ground motion, and attribute to the longer duration of tsunami generation process.

Using 3D Simulation of Elastic Wave Propagation in Laplace Domain for Electromagnetic-Seismic Inverse Modeling

NASA Astrophysics Data System (ADS)

Petrov, P.; Newman, G. A.

2010-12-01

Quantitative imaging of the subsurface objects is essential part of modern geophysical technology important in oil and gas exploration and wide-range engineering applications. A significant advancement in developing a robust, high resolution imaging technology is concerned with using the different geophysical measurements (gravity, EM and seismic) sense the subsurface structure. A joint image of the subsurface geophysical attributes (velocity, electrical conductivity and density) requires the consistent treatment of the different geophysical data (electromagnetic and seismic) due to their differing physical nature - diffusive and attenuated propagation of electromagnetic energy and nonlinear, multiple scattering wave propagation of seismic energy. Recent progress has been reported in the solution of this problem by reducing the complexity of seismic wave field. Works formed by Shin and Cha (2009 and 2008) suggests that low-pass filtering the seismic trace via Laplace-Fourier transformation can be an effective approach for obtaining seismic data that has similar spatial resolution to EM data. The effect of Laplace- Fourier transformation on the low-pass filtered trace changes the modeling of the seismic wave field from multi-wave propagation to diffusion. The key benefit of transformation is that diffusive wave-field inversion works well for both data sets seismic (Shin and Cha, 2008) and electromagnetic (Commer and Newman 2008, Newman et al., 2010). Moreover the different data sets can also be matched for similar and consistent resolution. Finally, the low pass seismic image is also an excellent choice for a starting model when analyzing the entire seismic waveform to recover the high spatial frequency components of the seismic image; its reflectivity (Shin and Cha, 2009). Without a good starting model full waveform seismic imaging and migration can encounter serious difficulties. To produce seismic wave fields consistent for joint imaging in the Laplace-Fourier domain we had developed 3D code for full-wave field simulation in the elastic media which take into account nonlinearity introduced by free-surface effects. Our approach is based on the velocity-stress formulation. In the contrast to conventional formulation we defined the material properties such as density and Lame constants not at nodal points but within cells. This second order finite differences method formulated in the cell-based grid, generate numerical solutions compatible with analytical ones within the range errors determinate by dispersion analysis. Our simulator will be embedded in an inversion scheme for joint seismic- electromagnetic imaging. It also offers possibilities for preconditioning the seismic wave propagation problems in the frequency domain. References. Shin, C. & Cha, Y. (2009), Waveform inversion in the Laplace-Fourier domain, Geophys. J. Int. 177(3), 1067- 1079. Shin, C. & Cha, Y. H. (2008), Waveform inversion in the Laplace domain, Geophys. J. Int. 173(3), 922-931. Commer, M. & Newman, G. (2008), New advances in three-dimensional controlled-source electromagnetic inversion, Geophys. J. Int. 172(2), 513-535. Newman, G. A., Commer, M. & Carazzone, J. J. (2010), Imaging CSEM data in the presence of electrical anisotropy, Geophysics, in press.

Including Short Period Constraints In the Construction of Full Waveform Tomographic Models

NASA Astrophysics Data System (ADS)

Roy, C.; Calo, M.; Bodin, T.; Romanowicz, B. A.

2015-12-01

Thanks to the introduction of the Spectral Element Method (SEM) in seismology, which allows accurate computation of the seismic wavefield in complex media, the resolution of regional and global tomographic models has improved in recent years. However, due to computational costs, only long period waveforms are considered, and only long wavelength structure can be constrained. Thus, the resulting 3D models are smooth, and only represent a small volumetric perturbation around a smooth reference model that does not include upper-mantle discontinuities (e.g. MLD, LAB). Extending the computations to shorter periods, necessary for the resolution of smaller scale features, is computationally challenging. In order to overcome these limitations and to account for layered structure in the upper mantle in our full waveform tomography, we include information provided by short period seismic observables (receiver functions and surface wave dispersion), sensitive to sharp boundaries and anisotropic structure respectively. In a first step, receiver functions and dispersion curves are used to generate a number of 1D radially anisotropic shear velocity profiles using a trans-dimensional Markov-chain Monte Carlo (MCMC) algorithm. These 1D profiles include both isotropic and anisotropic discontinuities in the upper mantle (above 300 km depth) beneath selected stationsand are then used to build a 3D starting model for the full waveform tomographic inversion. This model is built after 1) interpolation between the available 1D profiles, and 2) homogeneization of the layered 1D models to obtain an equivalent smooth 3D starting model in the period range of interest for waveform inversion. The waveforms used in the inversion are collected for paths contained in the region of study and filtered at periods longer than 40s. We use the spectral element code "RegSEM" (Cupillard et al., 2012) for forward computations and a quasi-Newton inversion approach in which kernels are computed using normal mode perturbation theory. We present here the first reults of such an approach after successive iterations of a full waveform tomography of the North American continent.

Detailed Velocity and Density models of the Cascadia Subduction Zone from Prestack Full-Waveform Inversion

NASA Astrophysics Data System (ADS)

Fortin, W.; Holbrook, W. S.; Mallick, S.; Everson, E. D.; Tobin, H. J.; Keranen, K. M.

2014-12-01

Understanding the geologic composition of the Cascadia Subduction Zone (CSZ) is critically important in assessing seismic hazards in the Pacific Northwest. Despite being a potential earthquake and tsunami threat to millions of people, key details of the structure and fault mechanisms remain poorly understood in the CSZ. In particular, the position and character of the subduction interface remains elusive due to its relative aseismicity and low seismic reflectivity, making imaging difficult for both passive and active source methods. Modern active-source reflection seismic data acquired as part of the COAST project in 2012 provide an opportunity to study the transition from the Cascadia basin, across the deformation front, and into the accretionary prism. Coupled with advances in seismic inversion methods, this new data allow us to produce detailed velocity models of the CSZ and accurate pre-stack depth migrations for studying geologic structure. While still computationally expensive, current computing clusters can perform seismic inversions at resolutions that match that of the seismic image itself. Here we present pre-stack full waveform inversions of the central seismic line of the COAST survey offshore Washington state. The resultant velocity model is produced by inversion at every CMP location, 6.25 m laterally, with vertical resolution of 0.2 times the dominant seismic frequency. We report a good average correlation value above 0.8 across the entire seismic line, determined by comparing synthetic gathers to the real pre-stack gathers. These detailed velocity models, both Vp and Vs, along with the density model, are a necessary step toward a detailed porosity cross section to be used to determine the role of fluids in the CSZ. Additionally, the P-velocity model is used to produce a pre-stack depth migration image of the CSZ.

anisotropic microseismic focal mechanism inversion by waveform imaging matching

NASA Astrophysics Data System (ADS)

Wang, L.; Chang, X.; Wang, Y.; Xue, Z.

2016-12-01

The focal mechanism is one of the most important parameters in source inversion, for both natural earthquakes and human-induced seismic events. It has been reported to be useful for understanding stress distribution and evaluating the fracturing effect. The conventional focal mechanism inversion method picks the first arrival waveform of P wave. This method assumes the source as a Double Couple (DC) type and the media isotropic, which is usually not the case for induced seismic focal mechanism inversion. For induced seismic events, the inappropriate source and media model in inversion processing, by introducing ambiguity or strong simulation errors, will seriously reduce the inversion effectiveness. First, the focal mechanism contains significant non-DC source type. Generally, the source contains three components: DC, isotropic (ISO) and the compensated linear vector dipole (CLVD), which makes focal mechanisms more complicated. Second, the anisotropy of media will affect travel time and waveform to generate inversion bias. The common way to describe focal mechanism inversion is based on moment tensor (MT) inversion which can be decomposed into the combination of DC, ISO and CLVD components. There are two ways to achieve MT inversion. The wave-field migration method is applied to achieve moment tensor imaging. This method can construct elements imaging of MT in 3D space without picking the first arrival, but the retrieved MT value is influenced by imaging resolution. The full waveform inversion is employed to retrieve MT. In this method, the source position and MT can be reconstructed simultaneously. However, this method needs vast numerical calculation. Moreover, the source position and MT also influence each other in the inversion process. In this paper, the waveform imaging matching (WIM) method is proposed, which combines source imaging with waveform inversion for seismic focal mechanism inversion. Our method uses the 3D tilted transverse isotropic (TTI) elastic wave equation to approximate wave propagating in anisotropic media. First, a source imaging procedure is employed to obtain the source position. Second, we refine a waveform inversion algorithm to retrieve MT. We also use a microseismic data set recorded in surface acquisition to test our method.

3D Acoustic Full Waveform Inversion for Engineering Purpose

NASA Astrophysics Data System (ADS)

Lim, Y.; Shin, S.; Kim, D.; Kim, S.; Chung, W.

2017-12-01

Seismic waveform inversion is the most researched data processing technique. In recent years, with an increase in marine development projects, seismic surveys are commonly conducted for engineering purposes; however, researches for application of waveform inversion are insufficient. The waveform inversion updates the subsurface physical property by minimizing the difference between modeled and observed data. Furthermore, it can be used to generate an accurate subsurface image; however, this technique consumes substantial computational resources. Its most compute-intensive step is the calculation of the gradient and hessian values. This aspect gains higher significance in 3D as compared to 2D. This paper introduces a new method for calculating gradient and hessian values, in an effort to reduce computational overburden. In the conventional waveform inversion, the calculation area covers all sources and receivers. In seismic surveys for engineering purposes, the number of receivers is limited. Therefore, it is inefficient to construct the hessian and gradient for the entire region (Figure 1). In order to tackle this problem, we calculate the gradient and the hessian for a single shot within the range of the relevant source and receiver. This is followed by summing up of these positions for the entire shot (Figure 2). In this paper, we demonstrate that reducing the area of calculation of the hessian and gradient for one shot reduces the overall amount of computation and therefore, the computation time. Furthermore, it is proved that the waveform inversion can be suitably applied for engineering purposes. In future research, we propose to ascertain an effective calculation range. This research was supported by the Basic Research Project(17-3314) of the Korea Institute of Geoscience and Mineral Resources(KIGAM) funded by the Ministry of Science, ICT and Future Planning of Korea.

Seismic waveform tomography with shot-encoding using a restarted L-BFGS algorithm.

PubMed

Rao, Ying; Wang, Yanghua

2017-08-17

In seismic waveform tomography, or full-waveform inversion (FWI), one effective strategy used to reduce the computational cost is shot-encoding, which encodes all shots randomly and sums them into one super shot to significantly reduce the number of wavefield simulations in the inversion. However, this process will induce instability in the iterative inversion regardless of whether it uses a robust limited-memory BFGS (L-BFGS) algorithm. The restarted L-BFGS algorithm proposed here is both stable and efficient. This breakthrough ensures, for the first time, the applicability of advanced FWI methods to three-dimensional seismic field data. In a standard L-BFGS algorithm, if the shot-encoding remains unchanged, it will generate a crosstalk effect between different shots. This crosstalk effect can only be suppressed by employing sufficient randomness in the shot-encoding. Therefore, the implementation of the L-BFGS algorithm is restarted at every segment. Each segment consists of a number of iterations; the first few iterations use an invariant encoding, while the remainder use random re-coding. This restarted L-BFGS algorithm balances the computational efficiency of shot-encoding, the convergence stability of the L-BFGS algorithm, and the inversion quality characteristic of random encoding in FWI.

Time-lapse seismic waveform modelling and attribute analysis using hydromechanical models for a deep reservoir undergoing depletion

NASA Astrophysics Data System (ADS)

He, Y.-X.; Angus, D. A.; Blanchard, T. D.; Wang, G.-L.; Yuan, S.-Y.; Garcia, A.

2016-04-01

Extraction of fluids from subsurface reservoirs induces changes in pore pressure, leading not only to geomechanical changes, but also perturbations in seismic velocities and hence observable seismic attributes. Time-lapse seismic analysis can be used to estimate changes in subsurface hydromechanical properties and thus act as a monitoring tool for geological reservoirs. The ability to observe and quantify changes in fluid, stress and strain using seismic techniques has important implications for monitoring risk not only for petroleum applications but also for geological storage of CO2 and nuclear waste scenarios. In this paper, we integrate hydromechanical simulation results with rock physics models and full-waveform seismic modelling to assess time-lapse seismic attribute resolution for dynamic reservoir characterization and hydromechanical model calibration. The time-lapse seismic simulations use a dynamic elastic reservoir model based on a North Sea deep reservoir undergoing large pressure changes. The time-lapse seismic traveltime shifts and time strains calculated from the modelled and processed synthetic data sets (i.e. pre-stack and post-stack data) are in a reasonable agreement with the true earth models, indicating the feasibility of using 1-D strain rock physics transform and time-lapse seismic processing methodology. Estimated vertical traveltime shifts for the overburden and the majority of the reservoir are within ±1 ms of the true earth model values, indicating that the time-lapse technique is sufficiently accurate for predicting overburden velocity changes and hence geomechanical effects. Characterization of deeper structure below the overburden becomes less accurate, where more advanced time-lapse seismic processing and migration is needed to handle the complex geometry and strong lateral induced velocity changes. Nevertheless, both migrated full-offset pre-stack and near-offset post-stack data image the general features of both the overburden and reservoir units. More importantly, the results from this study indicate that integrated seismic and hydromechanical modelling can help constrain time-lapse uncertainty and hence reduce risk due to fluid extraction and injection.

Resolving the fine-scale velocity structure of continental hyperextension at the Deep Galicia Margin using full-waveform inversion

NASA Astrophysics Data System (ADS)

Davy, R. G.; Morgan, J. V.; Minshull, T. A.; Bayrakci, G.; Bull, J. M.; Klaeschen, D.; Reston, T. J.; Sawyer, D. S.; Lymer, G.; Cresswell, D.

2018-01-01

Continental hyperextension during magma-poor rifting at the Deep Galicia Margin is characterized by a complex pattern of faulting, thin continental fault blocks and the serpentinization, with local exhumation, of mantle peridotites along the S-reflector, interpreted as a detachment surface. In order to understand fully the evolution of these features, it is important to image seismically the structure and to model the velocity structure to the greatest resolution possible. Traveltime tomography models have revealed the long-wavelength velocity structure of this hyperextended domain, but are often insufficient to match accurately the short-wavelength structure observed in reflection seismic imaging. Here, we demonstrate the application of 2-D time-domain acoustic full-waveform inversion (FWI) to deep-water seismic data collected at the Deep Galicia Margin, in order to attain a high-resolution velocity model of continental hyperextension. We have used several quality assurance procedures to assess the velocity model, including comparison of the observed and modeled waveforms, checkerboard tests, testing of parameter and inversion strategy and comparison with the migrated reflection image. Our final model exhibits an increase in the resolution of subsurface velocities, with particular improvement observed in the westernmost continental fault blocks, with a clear rotation of the velocity field to match steeply dipping reflectors. Across the S-reflector, there is a sharpening in the velocity contrast, with lower velocities beneath S indicative of preferential mantle serpentinization. This study supports the hypothesis that normal faulting acts to hydrate the upper-mantle peridotite, observed as a systematic decrease in seismic velocities, consistent with increased serpentinization. Our results confirm the feasibility of applying the FWI method to sparse, deep-water crustal data sets.

First results from a full-waveform inversion of the African continent using Salvus

NASA Astrophysics Data System (ADS)

van Herwaarden, D. P.; Afanasiev, M.; Krischer, L.; Trampert, J.; Fichtner, A.

2017-12-01

We present the initial results from an elastic full-waveform inversion (FWI) of the African continent which is melded together within the framework of the Collaborative Seismic Earth Model (CSEM) project. The continent of Africa is one of the most geophysically interesting regions on the planet. More specifically, Africa contains the Afar Depression, which is the only place on Earth where incipient seafloor spreading is sub-aerially exposed, along with other anomalous features such as the topography in the south, and several smaller surface expressions such as the Cameroon Volcanic Line and Congo Basin. Despite its significance, relatively few tomographic images exist of Africa, and, as a result, the debate on the geophysical origins of Africa's anomalies is rich and ongoing. Tomographic images of Africa present unique challenges due to uneven station coverage: while tectonically active areas such as the Afar rift are well sampled, much of the continent exhibits a severe lack of seismic stations. And, while Africa is mostly surrounded by tectonically active spreading plate boundaries, the interior of the continent is seismically quiet. To mitigate such issues, our simulation domain is extended to include earthquakes occurring in the South Atlantic and along the western edge of South America. Waveform modelling and inversion is performed using Salvus, a flexible and high-performance software suite based on the spectral-element method. Recently acquired recordings from the AfricaArray and NARS seismic networks are used to complement data obtained from global networks. We hope that this new model presents a fresh high-resolution image of African geodynamic structure, and helps advance the debate regarding the causative mechanisms of its surface anomalies.

Interpretaion of synthetic seismic time-lapse monitoring data for Korea CCS project based on the acoustic-elastic coupled inversion

NASA Astrophysics Data System (ADS)

Oh, J.; Min, D.; Kim, W.; Huh, C.; Kang, S.

2012-12-01

Recently, the CCS (Carbon Capture and Storage) is one of the promising methods to reduce the CO2 emission. To evaluate the success of the CCS project, various geophysical monitoring techniques have been applied. Among them, the time-lapse seismic monitoring is one of the effective methods to investigate the migration of CO2 plume. To monitor the injected CO2 plume accurately, it is needed to interpret seismic monitoring data using not only the imaging technique but also the full waveform inversion, because subsurface material properties can be estimated through the inversion. However, previous works for interpreting seismic monitoring data are mainly based on the imaging technique. In this study, we perform the frequency-domain full waveform inversion for synthetic data obtained by the acoustic-elastic coupled modeling for the geological model made after Ulleung Basin, which is one of the CO2 storage prospects in Korea. We suppose the injection layer is located in fault-related anticlines in the Dolgorae Deformed Belt and, for more realistic situation, we contaminate the synthetic monitoring data with random noise and outliers. We perform the time-lapse full waveform inversion in two scenarios. One scenario is that the injected CO2 plume migrates within the injection layer and is stably captured. The other scenario is that the injected CO2 plume leaks through the weak part of the cap rock. Using the inverted P- and S-wave velocities and Poisson's ratio, we were able to detect the migration of the injected CO2 plume. Acknowledgment This work was financially supported by the Brain Korea 21 project of Energy Systems Engineering, the "Development of Technology for CO2 Marine Geological Storage" program funded by the Ministry of Land, Transport and Maritime Affairs (MLTM) of Korea and the Korea CCS R&D Center (KCRC) grant funded by the Korea government (Ministry of Education, Science and Technology) (No. 2012-0008926).

Line-source simulation for shallow-seismic data. Part 2: full-waveform inversion—a synthetic 2-D case study

NASA Astrophysics Data System (ADS)

Schäfer, M.; Groos, L.; Forbriger, T.; Bohlen, T.

2014-09-01

Full-waveform inversion (FWI) of shallow-seismic surface waves is able to reconstruct lateral variations of subsurface elastic properties. Line-source simulation for point-source data is required when applying algorithms of 2-D adjoint FWI to recorded shallow-seismic field data. The equivalent line-source response for point-source data can be obtained by convolving the waveforms with √{t^{-1}} (t: traveltime), which produces a phase shift of π/4. Subsequently an amplitude correction must be applied. In this work we recommend to scale the seismograms with √{2 r v_ph} at small receiver offsets r, where vph is the phase velocity, and gradually shift to applying a √{t^{-1}} time-domain taper and scaling the waveforms with r√{2} for larger receiver offsets r. We call this the hybrid transformation which is adapted for direct body and Rayleigh waves and demonstrate its outstanding performance on a 2-D heterogeneous structure. The fit of the phases as well as the amplitudes for all shot locations and components (vertical and radial) is excellent with respect to the reference line-source data. An approach for 1-D media based on Fourier-Bessel integral transformation generates strong artefacts for waves produced by 2-D structures. The theoretical background for both approaches is presented in a companion contribution. In the current contribution we study their performance when applied to waves propagating in a significantly 2-D-heterogeneous structure. We calculate synthetic seismograms for 2-D structure for line sources as well as point sources. Line-source simulations obtained from the point-source seismograms through different approaches are then compared to the corresponding line-source reference waveforms. Although being derived by approximation the hybrid transformation performs excellently except for explicitly back-scattered waves. In reconstruction tests we further invert point-source synthetic seismograms by a 2-D FWI to subsurface structure and evaluate its ability to reproduce the original structural model in comparison to the inversion of line-source synthetic data. Even when applying no explicit correction to the point-source waveforms prior to inversion only moderate artefacts appear in the results. However, the overall performance is best in terms of model reproduction and ability to reproduce the original data in a 3-D simulation if inverted waveforms are obtained by the hybrid transformation.

Comparison of the Cut-and-Paste and Full Moment Tensor Methods for Estimating Earthquake Source Parameters

NASA Astrophysics Data System (ADS)

Templeton, D.; Rodgers, A.; Helmberger, D.; Dreger, D.

2008-12-01

Earthquake source parameters (seismic moment, focal mechanism and depth) are now routinely reported by various institutions and network operators. These parameters are important for seismotectonic and earthquake ground motion studies as well as calibration of moment magnitude scales and model-based earthquake-explosion discrimination. Source parameters are often estimated from long-period three- component waveforms at regional distances using waveform modeling techniques with Green's functions computed for an average plane-layered models. One widely used method is waveform inversion for the full moment tensor (Dreger and Helmberger, 1993). This method (TDMT) solves for the moment tensor elements by performing a linearized inversion in the time-domain that minimizes the difference between the observed and synthetic waveforms. Errors in the seismic velocity structure inevitably arise due to either differences in the true average plane-layered structure or laterally varying structure. The TDMT method can account for errors in the velocity model by applying a single time shift at each station to the observed waveforms to best match the synthetics. Another method for estimating source parameters is the Cut-and-Paste (CAP) method. This method breaks the three-component regional waveforms into five windows: vertical and radial component Pnl; vertical and radial component Rayleigh wave; and transverse component Love waves. The CAP method performs a grid search over double-couple mechanisms and allows the synthetic waveforms for each phase (Pnl, Rayleigh and Love) to shift in time to account for errors in the Green's functions. Different filtering and weighting of the Pnl segment relative to surface wave segments enhances sensitivity to source parameters, however, some bias may be introduced. This study will compare the TDMT and CAP methods in two different regions in order to better understand the advantages and limitations of each method. Firstly, we will consider the northeastern China/Korean Peninsula region where average plane-layered structure is well known and relatively laterally homogenous. Secondly, we will consider the Middle East where crustal and upper mantle structure is laterally heterogeneous due to recent and ongoing tectonism. If time allows we will investigate the efficacy of each method for retrieving source parameters from synthetic data generated using a three-dimensional model of seismic structure of the Middle East, where phase delays are known to arise from path-dependent structure.

Full Waveform Adjoint Seismic Tomography of the Antarctic Plate

NASA Astrophysics Data System (ADS)

Lloyd, A. J.; Wiens, D.; Zhu, H.; Tromp, J.; Nyblade, A.; Anandakrishnan, S.; Aster, R. C.; Huerta, A. D.; Winberry, J. P.; Wilson, T. J.; Dalziel, I. W. D.; Hansen, S. E.; Shore, P.

2017-12-01

Recent studies investigating the response and influence of the solid Earth on the evolution of the cryosphere demonstrate the need to account for 3D rheological structure to better predict ice sheet dynamics, stability, and future sea level impact, as well as to improve glacial isostatic adjustment models and more accurately measure ice mass loss. Critical rheological properties like mantle viscosity and lithospheric thickness may be estimated from shear wave velocity models that, for Antarctica, would ideally possess regional-scale resolution extending down to at least the base of the transition zone (i.e. 670 km depth). However, current global- and continental-scale seismic velocity models are unable to obtain both the resolution and spatial coverage necessary, do not take advantage of the full set of available Antarctic data, and, in most instance, employ traditional seismic imaging techniques that utilize limited seismogram information. We utilize 3-component earthquake waveforms from almost 300 Antarctic broadband seismic stations and 26 southern mid-latitude stations from 270 earthquakes (5.5 ≤ Mw ≤ 7.0) between 2001-2003 and 2007-2016 to conduct a full-waveform adjoint inversion for Antarctica and surrounding regions of the Antarctic plate. Necessary forward and adjoint wavefield simulations are performed utilizing SPECFEM3D_GLOBE with the aid of the Texas Advanced Computing Center. We utilize phase observations from seismogram segments containing P, S, Rayleigh, and Love waves, including reflections and overtones, which are autonomously identified using FLEXWIN. The FLEXWIN analysis is carried out over a short (15-50 s) and long (initially 50-150 s) period band that target body waves, or body and surface waves, respectively. As our model is iteratively refined, the short-period corner of the long period band is gradually reduced to 25 s as the model converges over 20 linearized inversion iterations. We will briefly present this new high-resolution transverse isotropic seismic model of the Antarctic upper mantle and transition zone, which will be broadly valuable to advance cryosphere studies and improve understanding of the tectonic structure and geodynamic processes of Antarctica.

Real-Time seismic waveforms monitoring with BeiDou Navigation Satellite System (BDS) observations for the 2015 Mw 7.8 Nepal earthquake

NASA Astrophysics Data System (ADS)

Geng, T.

2015-12-01

Nowadays more and more high-rate Global Navigation Satellite Systems (GNSS) data become available in real time, which provide more opportunities to monitor the seismic waveforms. China's GNSS, BeiDou Navigation Satellite System (BDS), has already satisfied the requirement of stand-alone precise positioning in Asia-Pacific region with 14 in-orbit satellites, which promisingly suggests that BDS could be applied to the high-precision earthquake monitoring as GPS. In the present paper, real-time monitoring of seismic waveforms using BDS measurements is assessed. We investigate a so-called "variometric" approach to measure real-time seismic waveforms with high-rate BDS observations. This approach is based on time difference technique and standard broadcast products which are routinely available in real time. The 1HZ BDS data recorded by Beidou Experimental Tracking Stations (BETS) during the 2015 Mw 7.8 Nepal earthquake is analyzed. The results indicate that the accuracies of velocity estimation from BDS are 2-3 mm/s in horizontal components and 8-9 mm/s in vertical component, respectively, which are consistent with GPS. The seismic velocity waveforms during earthquake show good agreement between BDS and GPS. Moreover, the displacement waveforms is reconstructed by an integration of velocity time series with trend removal. The displacement waveforms with the accuracy of 1-2 cm are derived by comparing with post-processing GPS precise point positioning (PPP).

P-wave velocity model of mud volcano on the continental slope of the Canadian Beaufort Sea from frequency-domain full waveform inversion

NASA Astrophysics Data System (ADS)

Jang, U. G.; Kang, S. G.; Hong, J. K.; Jin, Y. K.; Dallimore, S.; Riedel, M.; Paull, C. K.

2017-12-01

2014 Expedition ARA05C was a multidisciplinary undertaking conducted in the Canadian Beaufort Sea, Arctic Ocean on the Korean ice breaker IBRV ARAON from August 30 to September 19, 2014. The program was carried out as collaboration between the Korea Polar Research Institute (KOPRI), Geological Survey of Canada (GSC), Monterey Bay Aquarium Research Institute (MBARI), Department of Fisheries and Ocean (DFO) with participation by Bremen University (BARUM). During this expedition, multi-channel seismic (MCS) data were acquired on the outer continental shelf and upper slope of the Canadian Beaufort Sea, totaling 20 lines with 1,000 line-kilometers from September 1 to September 13, 2014. Three MCS survey lines was designed to cross the three submarine mud volcanoes found in the slope at approximate water depth of 290 m, 460 m and 740 m. Submarine mud volcanoes are seafloor structures with positive topography formed by a combination of mud eruption, gas emission, and water seepage from the subsurface. MCS data will allow image subsurface structures of mud volcanoes as identification of fluid migration pathways, however, imaging its subsurface structure is difficult by using conventional seismic data processing procedure, because it is seismically characterized by acoustically transparent facies. Full waveform inversion (FWI) is non-linear data-fitting procedure to estimate the physical properties of the subsurface by minimizing the difference between the observed and modelled data. FWI uses the two-wave wave equation to compute forward/backward wavefield to calculate the gradient direction, therefore it can derive more detailed velocity model beyond travel-time tomography techniques, which use only the kinematics of seismic data, by additional information provided by the amplitude and phase of the seismic waveform. In this study, we suggest P-wave structure of mud volcanos, which were inverted by 2D acoustic FWI. It will be useful to understand the characterization of mud volcanoes on the slope of Canadian Beaufort Sea.

A frozen Gaussian approximation-based multi-level particle swarm optimization for seismic inversion

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

Li, Jinglai, E-mail: jinglaili@sjtu.edu.cn; Lin, Guang, E-mail: lin491@purdue.edu; Computational Sciences and Mathematics Division, Pacific Northwest National Laboratory, Richland, WA 99352

2015-09-01

In this paper, we propose a frozen Gaussian approximation (FGA)-based multi-level particle swarm optimization (MLPSO) method for seismic inversion of high-frequency wave data. The method addresses two challenges in it: First, the optimization problem is highly non-convex, which makes hard for gradient-based methods to reach global minima. This is tackled by MLPSO which can escape from undesired local minima. Second, the character of high-frequency of seismic waves requires a large number of grid points in direct computational methods, and thus renders an extremely high computational demand on the simulation of each sample in MLPSO. We overcome this difficulty by threemore » steps: First, we use FGA to compute high-frequency wave propagation based on asymptotic analysis on phase plane; Then we design a constrained full waveform inversion problem to prevent the optimization search getting into regions of velocity where FGA is not accurate; Last, we solve the constrained optimization problem by MLPSO that employs FGA solvers with different fidelity. The performance of the proposed method is demonstrated by a two-dimensional full-waveform inversion example of the smoothed Marmousi model.« less

Time-domain full-waveform inversion of Rayleigh and Love waves in presence of free-surface topography

NASA Astrophysics Data System (ADS)

Pan, Yudi; Gao, Lingli; Bohlen, Thomas

2018-05-01

Correct estimation of near-surface seismic-wave velocity when encountering lateral heterogeneity and free surface topography is one of the challenges to current shallow seismic. We propose to use time-domain full-waveform inversion (FWI) of surface waves, including both Rayleigh and Love waves, to solve this problem. We adopt a 2D time-domain finite-difference method with an improved vacuum formulation (IVF) to simulate shallow-seismic Rayleigh wave in presence of free-surface topography. We modify the IVF for SH-wave equation for the simulation of Love wave in presence of topographic free surface and prove its accuracy by benchmark tests. Checkboard model tests are performed in both cases when free-surface topography is included or neglected in FWI. Synthetic model containing a dipping planar free surface and lateral heterogeneity was then tested, in both cases of considering and neglecting free-surface topography. Both checkerboard and synthetic models show that Rayleigh- and Love-wave FWI have similar ability of reconstructing near-surface structures when free-surface topography is considered, while Love-wave FWI could reconstruct near-surface structures better than Rayleigh-wave when free-surface topography is neglected.

2D Seismic Imaging of Elastic Parameters by Frequency Domain Full Waveform Inversion

NASA Astrophysics Data System (ADS)

Brossier, R.; Virieux, J.; Operto, S.

2008-12-01

Thanks to recent advances in parallel computing, full waveform inversion is today a tractable seismic imaging method to reconstruct physical parameters of the earth interior at different scales ranging from the near- surface to the deep crust. We present a massively parallel 2D frequency-domain full-waveform algorithm for imaging visco-elastic media from multi-component seismic data. The forward problem (i.e. the resolution of the frequency-domain 2D PSV elastodynamics equations) is based on low-order Discontinuous Galerkin (DG) method (P0 and/or P1 interpolations). Thanks to triangular unstructured meshes, the DG method allows accurate modeling of both body waves and surface waves in case of complex topography for a discretization of 10 to 15 cells per shear wavelength. The frequency-domain DG system is solved efficiently for multiple sources with the parallel direct solver MUMPS. The local inversion procedure (i.e. minimization of residuals between observed and computed data) is based on the adjoint-state method which allows to efficiently compute the gradient of the objective function. Applying the inversion hierarchically from the low frequencies to the higher ones defines a multiresolution imaging strategy which helps convergence towards the global minimum. In place of expensive Newton algorithm, the combined use of the diagonal terms of the approximate Hessian matrix and optimization algorithms based on quasi-Newton methods (Conjugate Gradient, LBFGS, ...) allows to improve the convergence of the iterative inversion. The distribution of forward problem solutions over processors driven by a mesh partitioning performed by METIS allows to apply most of the inversion in parallel. We shall present the main features of the parallel modeling/inversion algorithm, assess its scalability and illustrate its performances with realistic synthetic case studies.

Probabilistic Tsunami Hazard Analysis

NASA Astrophysics Data System (ADS)

Thio, H. K.; Ichinose, G. A.; Somerville, P. G.; Polet, J.

2006-12-01

The recent tsunami disaster caused by the 2004 Sumatra-Andaman earthquake has focused our attention to the hazard posed by large earthquakes that occur under water, in particular subduction zone earthquakes, and the tsunamis that they generate. Even though these kinds of events are rare, the very large loss of life and material destruction caused by this earthquake warrant a significant effort towards the mitigation of the tsunami hazard. For ground motion hazard, Probabilistic Seismic Hazard Analysis (PSHA) has become a standard practice in the evaluation and mitigation of seismic hazard to populations in particular with respect to structures, infrastructure and lifelines. Its ability to condense the complexities and variability of seismic activity into a manageable set of parameters greatly facilitates the design of effective seismic resistant buildings but also the planning of infrastructure projects. Probabilistic Tsunami Hazard Analysis (PTHA) achieves the same goal for hazards posed by tsunami. There are great advantages of implementing such a method to evaluate the total risk (seismic and tsunami) to coastal communities. The method that we have developed is based on the traditional PSHA and therefore completely consistent with standard seismic practice. Because of the strong dependence of tsunami wave heights on bathymetry, we use a full waveform tsunami waveform computation in lieu of attenuation relations that are common in PSHA. By pre-computing and storing the tsunami waveforms at points along the coast generated for sets of subfaults that comprise larger earthquake faults, we can efficiently synthesize tsunami waveforms for any slip distribution on those faults by summing the individual subfault tsunami waveforms (weighted by their slip). This efficiency make it feasible to use Green's function summation in lieu of attenuation relations to provide very accurate estimates of tsunami height for probabilistic calculations, where one typically computes thousands of earthquake scenarios. We have carried out preliminary tsunami hazard calculations for different return periods for western North America and Hawaii based on thousands of earthquake scenarios around the Pacific rim and along the coast of North America. We will present tsunami hazard maps for several return periods and also discuss how to use these results for probabilistic inundation and runup mapping. Our knowledge of certain types of tsunami sources is very limited (e.g. submarine landslides), but a probabilistic framework for tsunami hazard evaluation can include even such sources and their uncertainties and present the overall hazard in a meaningful and consistent way.

Exploring Large-Scale Cross-Correlation for Teleseismic and Regional Seismic Event Characterization

NASA Astrophysics Data System (ADS)

Dodge, Doug; Walter, William; Myers, Steve; Ford, Sean; Harris, Dave; Ruppert, Stan; Buttler, Dave; Hauk, Terri

2013-04-01

The decrease in costs of both digital storage space and computation power invites new methods of seismic data processing. At Lawrence Livermore National Laboratory(LLNL) we operate a growing research database of seismic events and waveforms for nuclear explosion monitoring and other applications. Currently the LLNL database contains several million events associated with tens of millions of waveforms at thousands of stations. We are making use of this database to explore the power of seismic waveform correlation to quantify signal similarities, to discover new events not in catalogs, and to more accurately locate events and identify source types. Building on the very efficient correlation methodologies of Harris and Dodge (2011) we computed the waveform correlation for event pairs in the LLNL database in two ways. First we performed entire waveform cross-correlation over seven distinct frequency bands. The correlation coefficient exceeds 0.6 for more than 40 million waveform pairs for several hundred thousand events at more than a thousand stations. These correlations reveal clusters of mining events and aftershock sequences, which can be used to readily identify and locate events. Second we determine relative pick times by correlating signals in time windows for distinct seismic phases. These correlated picks are then used to perform very high accuracy event relocations. We are examining the percentage of events that correlate as a function of magnitude and observing station distance in selected high seismicity regions. Combining these empirical results and those using synthetic data, we are working to quantify relationships between correlation and event pair separation (in epicenter and depth) as well as mechanism differences. Our exploration of these techniques on a large seismic database is in process and we will report on our findings in more detail at the meeting.

Exploring Large-Scale Cross-Correlation for Teleseismic and Regional Seismic Event Characterization

NASA Astrophysics Data System (ADS)

Dodge, D.; Walter, W. R.; Myers, S. C.; Ford, S. R.; Harris, D.; Ruppert, S.; Buttler, D.; Hauk, T. F.

2012-12-01

The decrease in costs of both digital storage space and computation power invites new methods of seismic data processing. At Lawrence Livermore National Laboratory (LLNL) we operate a growing research database of seismic events and waveforms for nuclear explosion monitoring and other applications. Currently the LLNL database contains several million events associated with tens of millions of waveforms at thousands of stations. We are making use of this database to explore the power of seismic waveform correlation to quantify signal similarities, to discover new events not in catalogs, and to more accurately locate events and identify source types. Building on the very efficient correlation methodologies of Harris and Dodge (2011) we computed the waveform correlation for event pairs in the LLNL database in two ways. First we performed entire waveform cross-correlation over seven distinct frequency bands. The correlation coefficient exceeds 0.6 for more than 40 million waveform pairs for several hundred thousand events at more than a thousand stations. These correlations reveal clusters of mining events and aftershock sequences, which can be used to readily identify and locate events. Second we determine relative pick times by correlating signals in time windows for distinct seismic phases. These correlated picks are then used to perform very high accuracy event relocations. We are examining the percentage of events that correlate as a function of magnitude and observing station distance in selected high seismicity regions. Combining these empirical results and those using synthetic data, we are working to quantify relationships between correlation and event pair separation (in epicenter and depth) as well as mechanism differences. Our exploration of these techniques on a large seismic database is in process and we will report on our findings in more detail at the meeting.

The analysis and interpretation of very-long-period seismic signals on volcanoes

NASA Astrophysics Data System (ADS)

Sindija, Dinko; Neuberg, Jurgen; Smith, Patrick

2017-04-01

The study of very long period (VLP) seismic signals became possible with the widespread use of broadband instruments. VLP seismic signals are caused by transients of pressure in the volcanic edifice and have periods ranging from several seconds to several minutes. For the VLP events recorded in March 2012 and 2014 at Soufriere Hills Volcano, Montserrat, we model the ground displacement using several source time functions: a step function using Richards growth equation, Küpper wavelet, and a damped sine wave to which an instrument response is then applied. This way we get a synthetic velocity seismogram which is directly comparable to the data. After the full vector field of ground displacement is determined, we model the source mechanism to determine the relationship between the source mechanism and the observed VLP waveforms. Emphasis of the research is on how different VLP waveforms are related to the volcano environment and the instrumentation used and on the processing steps in this low frequency band to get most out of broadband instruments.

Moment Inversion of the DPRK Nuclear Tests Using Finite-Difference Three-dimensional Strain Green's Tensors

NASA Astrophysics Data System (ADS)

Bao, X.; Shen, Y.; Wang, N.

2017-12-01

Accurate estimation of the source moment is important for discriminating underground explosions from earthquakes and other seismic sources. In this study, we invert for the full moment tensors of the recent seismic events (since 2016) at the Democratic People's Republic of Korea (PRRK) Punggye-ri test site. We use waveform data from broadband seismic stations located in China, Korea, and Japan in the inversion. Using a non-staggered-grid, finite-difference algorithm, we calculate the strain Green's tensors (SGT) based on one-dimensional (1D) and three-dimensional (3D) Earth models. Taking advantage of the source-receiver reciprocity, a SGT database pre-calculated and stored for the Punggye-ri test site is used in inversion for the source mechanism of each event. With the source locations estimated from cross-correlation using regional Pn and Pn-coda waveforms, we obtain the optimal source mechanism that best fits synthetics to the observed waveforms of both body and surface waves. The moment solutions of the first three events (2016-01-06, 2016-09-09, and 2017-09-03) show dominant isotropic components, as expected from explosions, though there are also notable non-isotropic components. The last event ( 8 minutes after the mb6.3 explosion in 2017) contained mainly implosive component, suggesting a collapse following the explosion. The solutions from the 3D model can better fit observed waveforms than the corresponding solutions from the 1D model. The uncertainty in the resulting moment solution is influenced by heterogeneities not resolved by the Earth model according to the waveform misfit. Using the moment solutions, we predict the peak ground acceleration at the Punggye-ri test site and compare the prediction with corresponding InSAR and other satellite images.

Full Waveform Inversion of Diving & Reflected Waves based on Scale Separation for Velocity and Impedance Imaging

NASA Astrophysics Data System (ADS)

Brossier, Romain; Zhou, Wei; Operto, Stéphane; Virieux, Jean

2015-04-01

Full Waveform Inversion (FWI) is an appealing method for quantitative high-resolution subsurface imaging (Virieux et al., 2009). For crustal-scales exploration from surface seismic, FWI generally succeeds in recovering a broadband of wavenumbers in the shallow part of the targeted medium taking advantage of the broad scattering-angle provided by both reflected and diving waves. In contrast, deeper targets are often only illuminated by short-spread reflections, which favor the reconstruction of the short wavelengths at the expense of the longer ones, leading to a possible notch in the intermediate part of the wavenumber spectrum. To update the velocity macromodel from reflection data, image-domain strategies (e.g., Symes & Carazzone, 1991) aim to maximize a semblance criterion in the migrated domain. Alternatively, recent data-domain strategies (e.g., Xu et al., 2012, Ma & Hale, 2013, Brossier et al., 2014), called Reflection FWI (RFWI), inspired by Chavent et al. (1994), rely on a scale separation between the velocity macromodel and prior knowledge of the reflectivity to emphasize the transmission regime in the sensitivity kernel of the inversion. However, all these strategies focus on reflected waves only, discarding the low-wavenumber information carried out by diving waves. With the current development of very long-offset and wide-azimuth acquisitions, a significant part of the recorded energy is provided by diving waves and subcritical reflections, and high-resolution tomographic methods should take advantage of all types of waves. In this presentation, we will first review the issues of classical FWI when applied to reflected waves and how RFWI is able to retrieve the long wavelength of the model. We then propose a unified formulation of FWI (Zhou et al., 2014) to update the low wavenumbers of the velocity model by the joint inversion of diving and reflected arrivals, while the impedance model is updated thanks to reflected wave only. An alternate inversion of high wavenumber impedance model and low wavenumber velocity model is performed to iteratively improve subsurface models. References : Brossier, R., Operto, S. & Virieux, J., 2014. Velocity model building from seismic reflection data by full waveform inversion, Geophysical Prospecting, doi:10.1111/1365-2478.12190 Chavent, G., Clément, F. & Gomez, S., 1994.Automatic determination of velocities via migration-based traveltime waveform inversion: A synthetic data example, SEG Technical Program Expanded Abstracts 1994, pp. 1179--1182. Ma, Y. & Hale, D., 2013. Wave-equation reflection traveltime inversion with dynamic warping and full waveform inversion, Geophysics, 78(6), R223--R233. Symes, W.W. & Carazzone, J.J., 1991. Velocity inversion by differential semblance optimization, Geophysics, 56, 654--663. Virieux, J. & Operto, S., 2009. An overview of full waveform inversion in exploration geophysics, Geophysics, 74(6), WCC1--WCC26. Xu, S., Wang, D., Chen, F., Lambaré, G. & Zhang, Y., 2012. Inversion on reflected seismic wave, SEG Technical Program Expanded Abstracts 2012, pp. 1--7. Zhou, W., Brossier, R., Operto, S., & Virieux, J., 2014. Acoustic multiparameter full-waveform inversion through a hierachical scheme, in SEG Technical Program Expanded Abstracts 2014, pp. 1249--1253

The source parameters of 2013 Mw6.6 Lushan earthquake constrained with the restored local clipped seismic waveforms

NASA Astrophysics Data System (ADS)

Hao, J.; Zhang, J. H.; Yao, Z. X.

2017-12-01

We developed a method to restore the clipped seismic waveforms near epicenter using projection onto convex sets method (Zhang et al, 2016). This method was applied to rescue the local clipped waveforms of 2013 Mw 6.6 Lushan earthquake. We restored 88 out of 93 clipped waveforms of 38 broadband seismic stations of China Earthquake Networks (CEN). The epicenter distance of the nearest station to the epicenter that we can faithfully restore is only about 32 km. In order to investigate if the source parameters of earthquake could be determined exactly with the restored data, restored waveforms are utilized to get the mechanism of Lushan earthquake. We apply the generalized reflection-transmission coefficient matrix method to calculate the synthetic seismic records and simulated annealing method in inversion (Yao and Harkrider, 1983; Hao et al., 2012). We select 5 stations of CEN with the epicenter distance about 200km whose records aren't clipped and three-component velocity records are used. The result shows the strike, dip and rake angles of Lushan earthquake are 200o, 51o and 87o respectively, hereinafter "standard result". Then the clipped and restored seismic waveforms are applied respectively. The strike, dip and rake angles of clipped seismic waveforms are 184o, 53o and 72o respectively. The largest misfit of angle is 16o. In contrast, the strike, dip and rake angles of restored seismic waveforms are 198o, 51o and 87o respectively. It is very close to the "standard result". We also study the rupture history of Lushan earthquake constrained with the restored local broadband and teleseismic waves based on finite fault method (Hao et al., 2013). The result consists with that constrained with the strong motion and teleseismic waves (Hao et al., 2013), especially the location of the patch with larger slip. In real-time seismology, determining the source parameters as soon as possible is important. This method will help us to determine the mechanism of earthquake using the local clipped waveforms. Strong motion stations in China don't have good coverage at present. This method will help us to investigate the rupture history of large earthquake in China using the local clipped data of broadband stations.

Frequency Domain Full-Waveform Inversion in Imaging Thrust Related Features

NASA Astrophysics Data System (ADS)

Jaiswal, P.; Zelt, C. A.

2010-12-01

Seismic acquisition in rough terrain such as mountain belts suffers from problems related to near-surface conditions such as statics, inconsistent energy penetration, rapid decay of signal, and imperfect receiver coupling. Moreover in the presence of weakly compacted soil, strong ground roll may obscure the reflection arrivals at near offsets further diminishing the scope of estimating a reliable near surface image though conventional processing. Traveltime and waveform inversion not only overcome the simplistic assumptions inherent in conventional processing such as hyperbolic moveout and convolution model, but also use parts of the seismic coda, such as the direct arrival and refractions, that are discarded in the latter. Traveltime and waveform inversion are model-based methods that honour the physics of wave propagation. Given the right set of preconditioned data and starting model, waveform inversion in particular has been realized as a powerful tool for velocity model building. This paper examines two case studies on waveform inversion using real data from the Naga Thrust Belt in the Northeast India. Waveform inversion in this paper is performed in the frequency domain and is multiscale in nature i.e., the inversion progressively ascends from the lower to the higher end of the frequency spectra increasing the wavenumber content of the recovered model. Since the real data are band limited, the success of waveform inversion depends on how well the starting model can account for the missing low wavenumbers. In this paper it is observed that the required starting model can be prepared using the regularized inversion of direct and reflected arrival times.

Large-scale seismic waveform quality metric calculation using Hadoop

NASA Astrophysics Data System (ADS)

Magana-Zook, S.; Gaylord, J. M.; Knapp, D. R.; Dodge, D. A.; Ruppert, S. D.

2016-09-01

In this work we investigated the suitability of Hadoop MapReduce and Apache Spark for large-scale computation of seismic waveform quality metrics by comparing their performance with that of a traditional distributed implementation. The Incorporated Research Institutions for Seismology (IRIS) Data Management Center (DMC) provided 43 terabytes of broadband waveform data of which 5.1 TB of data were processed with the traditional architecture, and the full 43 TB were processed using MapReduce and Spark. Maximum performance of 0.56 terabytes per hour was achieved using all 5 nodes of the traditional implementation. We noted that I/O dominated processing, and that I/O performance was deteriorating with the addition of the 5th node. Data collected from this experiment provided the baseline against which the Hadoop results were compared. Next, we processed the full 43 TB dataset using both MapReduce and Apache Spark on our 18-node Hadoop cluster. These experiments were conducted multiple times with various subsets of the data so that we could build models to predict performance as a function of dataset size. We found that both MapReduce and Spark significantly outperformed the traditional reference implementation. At a dataset size of 5.1 terabytes, both Spark and MapReduce were about 15 times faster than the reference implementation. Furthermore, our performance models predict that for a dataset of 350 terabytes, Spark running on a 100-node cluster would be about 265 times faster than the reference implementation. We do not expect that the reference implementation deployed on a 100-node cluster would perform significantly better than on the 5-node cluster because the I/O performance cannot be made to scale. Finally, we note that although Big Data technologies clearly provide a way to process seismic waveform datasets in a high-performance and scalable manner, the technology is still rapidly changing, requires a high degree of investment in personnel, and will likely require significant changes in other parts of our infrastructure. Nevertheless, we anticipate that as the technology matures and third-party tool vendors make it easier to manage and operate clusters, Hadoop (or a successor) will play a large role in our seismic data processing.

Retrieval of P wave Basin Response from Autocorrelation of Seismic Noise-Jakarta, Indonesia

NASA Astrophysics Data System (ADS)

Saygin, E.; Cummins, P. R.; Lumley, D. E.

2016-12-01

Indonesia's capital city, Jakarta, is home to a very large (over 10 million), vulnerable population and is proximate to known active faults, as well as to the subduction of Australian plate, which has a megathrust at abut 300 km distance, as well as intraslab seismicity extending to directly beneath the city. It is also located in a basin filled with a thick layer of unconsolidated and poorly consolidated sediment, which increases the seismic hazard the city is facing. Therefore, the information on the seismic velocity structure of the basin is crucial for increasing our knowledge of the seismic risk. We undertook a passive deployment of broadband seismographs throughout the city over a 3-month interval in 2013-2014, recording ambient seismic noise at over 90 sites for intervals of 1 month or more. Here we consider autocorrelations of the vertical component of the continuously recorded seismic wavefield across this dense network to image the shallow P wave velocity structure of Jakarta, Indonesia. Unlike the surface wave Green's functions used in ambient noise tomography, the vertical-component autocorrelograms are dominated by body wave energy that is potentially sensitive to sharp velocity contrasts, which makes them useful in seismic imaging. Results show autocorrelograms at different seismic stations with travel time variations that largely reflect changes in sediment thickness across the basin. We also confirm the validity our interpretation of the observed autocorrelation waveforms by conducting 2D finite difference full waveform numerical modeling for randomly distributed seismic sources to retrieve the reflection response through autocorrelation.

Elastic-Waveform Inversion with Compressive Sensing for Sparse Seismic Data

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

Lin, Youzuo; Huang, Lianjie

2015-01-28

Accurate velocity models of compressional- and shear-waves are essential for geothermal reservoir characterization and microseismic imaging. Elastic-waveform inversion of multi-component seismic data can provide high-resolution inversion results of subsurface geophysical properties. However, the method requires seismic data acquired using dense source and receiver arrays. In practice, seismic sources and/or geophones are often sparsely distributed on the surface and/or in a borehole, such as 3D vertical seismic profiling (VSP) surveys. We develop a novel elastic-waveform inversion method with compressive sensing for inversion of sparse seismic data. We employ an alternating-minimization algorithm to solve the optimization problem of our new waveform inversionmore » method. We validate our new method using synthetic VSP data for a geophysical model built using geologic features found at the Raft River enhanced-geothermal-system (EGS) field. We apply our method to synthetic VSP data with a sparse source array and compare the results with those obtained with a dense source array. Our numerical results demonstrate that the velocity models produced with our new method using a sparse source array are almost as accurate as those obtained using a dense source array.« less

A recent Mw 4.3 earthquake proving activity of a shallow strike-slip fault in the northern part of the Western Desert, Egypt

NASA Astrophysics Data System (ADS)

Ezzelarab, Mohamed; Ebraheem, Mohamed O.; Zahradník, Jiří

2018-03-01

The Mw 4.3 earthquake of September 2015 is the first felt earthquake since 1900 A.D in the northern part of the Western Desert, Egypt, south of the El-Alamein City. The available waveform data observed at epicentral distances 52-391 km was collected and carefully evaluated. Nine broad-band stations were selected to invert full waveforms for the centroid position (horizontal and vertical) and for the focal mechanism solution. The first-arrival travel times, polarities and low-frequency full waveforms (0.03-0.08 Hz) are consistently explained in this paper as caused by a shallow source of the strike-slip mechanism. This finding indicates causal relation of this earthquake to the W-E trending South El-Alamein fault, which developed in Late Cretaceous as dextral strike slip fault. Recent activity of this fault, proven by the studied rare earthquake, is of fundamental importance for future seismic hazard evaluations, underlined by proximity (∼65 km) of the source zone to the first nuclear power plant planned site in Egypt. Safe exploration and possible future exploitation of hydrocarbon reserves, reported around El-Alamein fault in the last decade, cannot be made without considering the seismic potential of this fault.

A Synthetic Study on the Resolution of 2D Elastic Full Waveform Inversion

NASA Astrophysics Data System (ADS)

Cui, C.; Wang, Y.

2017-12-01

Gradient based full waveform inversion is an effective method in seismic study, it makes full use of the information given by seismic records and is capable of providing a more accurate model of the interior of the earth at a relatively low computational cost. However, the strong non-linearity of the problem brings about many difficulties in the assessment of its resolution. Synthetic inversions are therefore helpful before an inversion based on real data is made. Checker-board test is a commonly used method, but it is not always reliable due to the significant difference between a checker-board and the true model. Our study aims to provide a basic understanding of the resolution of 2D elastic inversion by examining three main factors that affect the inversion result respectively: 1. The structural characteristic of the model; 2. The level of similarity between the initial model and the true model; 3. The spacial distribution of sources and receivers. We performed about 150 synthetic inversions to demonstrate how each factor contributes to quality of the result, and compared the inversion results with those achieved by checker-board tests. The study can be a useful reference to assess the resolution of an inversion in addition to regular checker-board tests, or to determine whether the seismic data of a specific region is sufficient for a successful inversion.

Seismic facies analysis based on self-organizing map and empirical mode decomposition

NASA Astrophysics Data System (ADS)

Du, Hao-kun; Cao, Jun-xing; Xue, Ya-juan; Wang, Xing-jian

2015-01-01

Seismic facies analysis plays an important role in seismic interpretation and reservoir model building by offering an effective way to identify the changes in geofacies inter wells. The selections of input seismic attributes and their time window have an obvious effect on the validity of classification and require iterative experimentation and prior knowledge. In general, it is sensitive to noise when waveform serves as the input data to cluster analysis, especially with a narrow window. To conquer this limitation, the Empirical Mode Decomposition (EMD) method is introduced into waveform classification based on SOM. We first de-noise the seismic data using EMD and then cluster the data using 1D grid SOM. The main advantages of this method are resolution enhancement and noise reduction. 3D seismic data from the western Sichuan basin, China, are collected for validation. The application results show that seismic facies analysis can be improved and better help the interpretation. The powerful tolerance for noise makes the proposed method to be a better seismic facies analysis tool than classical 1D grid SOM method, especially for waveform cluster with a narrow window.

Stochastic Seismic Inversion and Migration for Offshore Site Investigation in the Northern Gulf of Mexico

NASA Astrophysics Data System (ADS)

Son, J.; Medina-Cetina, Z.

2017-12-01

We discuss the comparison between deterministic and stochastic optimization approaches to the nonlinear geophysical full-waveform inverse problem, based on the seismic survey data from Mississippi Canyon in the Northern Gulf of Mexico. Since the subsea engineering and offshore construction projects actively require reliable ground models from various site investigations, the primary goal of this study is to reconstruct the accurate subsurface information of the soil and rock material profiles under the seafloor. The shallow sediment layers have naturally formed heterogeneous formations which may cause unwanted marine landslides or foundation failures of underwater infrastructure. We chose the quasi-Newton and simulated annealing as deterministic and stochastic optimization algorithms respectively. Seismic forward modeling based on finite difference method with absorbing boundary condition implements the iterative simulations in the inverse modeling. We briefly report on numerical experiments using a synthetic data as an offshore ground model which contains shallow artificial target profiles of geomaterials under the seafloor. We apply the seismic migration processing and generate Voronoi tessellation on two-dimensional space-domain to improve the computational efficiency of the imaging stratigraphical velocity model reconstruction. We then report on the detail of a field data implementation, which shows the complex geologic structures in the Northern Gulf of Mexico. Lastly, we compare the new inverted image of subsurface site profiles in the space-domain with the previously processed seismic image in the time-domain at the same location. Overall, stochastic optimization for seismic inversion with migration and Voronoi tessellation show significant promise to improve the subsurface imaging of ground models and improve the computational efficiency required for the full waveform inversion. We anticipate that by improving the inversion process of shallow layers from geophysical data will better support the offshore site investigation.

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.

2-D traveltime and waveform inversion for improved seismic imaging: Naga Thrust and Fold Belt, India

NASA Astrophysics Data System (ADS)

Jaiswal, Priyank; Zelt, Colin A.; Bally, Albert W.; Dasgupta, Rahul

2008-05-01

Exploration along the Naga Thrust and Fold Belt in the Assam province of Northeast India encounters geological as well as logistic challenges. Drilling for hydrocarbons, traditionally guided by surface manifestations of the Naga thrust fault, faces additional challenges in the northeast where the thrust fault gradually deepens leaving subtle surface expressions. In such an area, multichannel 2-D seismic data were collected along a line perpendicular to the trend of the thrust belt. The data have a moderate signal-to-noise ratio and suffer from ground roll and other acquisition-related noise. In addition to data quality, the complex geology of the thrust belt limits the ability of conventional seismic processing to yield a reliable velocity model which in turn leads to poor subsurface image. In this paper, we demonstrate the application of traveltime and waveform inversion as supplements to conventional seismic imaging and interpretation processes. Both traveltime and waveform inversion utilize the first arrivals that are typically discarded during conventional seismic processing. As a first step, a smooth velocity model with long wavelength characteristics of the subsurface is estimated through inversion of the first-arrival traveltimes. This velocity model is then used to obtain a Kirchhoff pre-stack depth-migrated image which in turn is used for the interpretation of the fault. Waveform inversion is applied to the central part of the seismic line to a depth of ~1 km where the quality of the migrated image is poor. Waveform inversion is performed in the frequency domain over a series of iterations, proceeding from low to high frequency (11-19 Hz) using the velocity model from traveltime inversion as the starting model. In the end, the pre-stack depth-migrated image and the waveform inversion model are jointly interpreted. This study demonstrates that a combination of traveltime and waveform inversion with Kirchhoff pre-stack depth migration is a promising approach for the interpretation of geological structures in a thrust belt.

Analysis of stress drops and rupture lengths along the northern segment of the New Madrid seismic zone

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

Mickus, K.L.

1993-03-01

The New Madrid seismic zone is the most seismically active region in the central US. The seismic zone consists of three linear trends bounded by latitudes 35.5[degree] to 37[degree] N and longitudes 89[degree] to 90.5[degree] W. This study is concerned with the most northern segment that trends northeast from New Madrid, Missouri to Charleston, Missouri. The purpose of this study is to determine stress drops and rupture lengths of small earthquakes (M less than 3.5). To determine the stress drops and rupture lengths, the author used waveforms collected by the St. Louis University seismic network. He used small events (Mmore » between 1.0 and 2.0) as empirical Green's functions to deconvolve out site, path and instrument effects on the P-waveforms on larger events (M between 2.0 and 3.6). Examining the seismic records from 1980 to the present, he found five larger events that had colocated (within 1 km) smaller events. To insure that the larger and smaller were colocated the events were relocated using a three-dimensional velocity model. After insuring the events were relocated, the deconvolved waveforms were used to determine the seismic moment and hence the stress drops and rupture lengths by estimating the area of the deconvolved waveforms and the rise time of each pulse.« less

Full waveform inversion using a decomposed single frequency component from a spectrogram

NASA Astrophysics Data System (ADS)

Ha, Jiho; Kim, Seongpil; Koo, Namhyung; Kim, Young-Ju; Woo, Nam-Sub; Han, Sang-Mok; Chung, Wookeen; Shin, Sungryul; Shin, Changsoo; Lee, Jaejoon

2018-06-01

Although many full waveform inversion methods have been developed to construct velocity models of subsurface, various approaches have been presented to obtain an inversion result with long-wavelength features even though seismic data lacking low-frequency components were used. In this study, a new full waveform inversion algorithm was proposed to recover a long-wavelength velocity model that reflects the inherent characteristics of each frequency component of seismic data using a single-frequency component decomposed from the spectrogram. We utilized the wavelet transform method to obtain the spectrogram, and the decomposed signal from the spectrogram was used as transformed data. The Gauss-Newton method with the diagonal elements of an approximate Hessian matrix was used to update the model parameters at each iteration. Based on the results of time-frequency analysis in the spectrogram, numerical tests with some decomposed frequency components were performed using a modified SEG/EAGE salt dome (A-A‧) line to demonstrate the feasibility of the proposed inversion algorithm. This demonstrated that a reasonable inverted velocity model with long-wavelength structures can be obtained using a single frequency component. It was also confirmed that when strong noise occurs in part of the frequency band, it is feasible to obtain a long-wavelength velocity model from the noise data with a frequency component that is less affected by the noise. Finally, it was confirmed that the results obtained from the spectrogram inversion can be used as an initial velocity model in conventional inversion methods.

Waveform classification and statistical analysis of seismic precursors to the July 2008 Vulcanian Eruption of Soufrière Hills Volcano, Montserrat

NASA Astrophysics Data System (ADS)

Rodgers, Mel; Smith, Patrick; Pyle, David; Mather, Tamsin

2016-04-01

Understanding the transition between quiescence and eruption at dome-forming volcanoes, such as Soufrière Hills Volcano (SHV), Montserrat, is important for monitoring volcanic activity during long-lived eruptions. Statistical analysis of seismic events (e.g. spectral analysis and identification of multiplets via cross-correlation) can be useful for characterising seismicity patterns and can be a powerful tool for analysing temporal changes in behaviour. Waveform classification is crucial for volcano monitoring, but consistent classification, both during real-time analysis and for retrospective analysis of previous volcanic activity, remains a challenge. Automated classification allows consistent re-classification of events. We present a machine learning (random forest) approach to rapidly classify waveforms that requires minimal training data. We analyse the seismic precursors to the July 2008 Vulcanian explosion at SHV and show systematic changes in frequency content and multiplet behaviour that had not previously been recognised. These precursory patterns of seismicity may be interpreted as changes in pressure conditions within the conduit during magma ascent and could be linked to magma flow rates. Frequency analysis of the different waveform classes supports the growing consensus that LP and Hybrid events should be considered end members of a continuum of low-frequency source processes. By using both supervised and unsupervised machine-learning methods we investigate the nature of waveform classification and assess current classification schemes.

Waveform classification of volcanic low-frequency earthquake swarms and its implication at Soufrière Hills Volcano, Montserrat

NASA Astrophysics Data System (ADS)

Green, David N.; Neuberg, Jürgen

2006-05-01

Low-frequency volcanic earthquakes are indicators of magma transport and activity within shallow conduit systems. At a number of volcanoes, these events exhibit a high degree of waveform similarity providing a criterion for classification. Using cross-correlation techniques to quantify the degree of similarity, we develop a method to sort events into families containing comparable waveforms. Events within a family have been triggered within one small source volume from which the seismic wave has then travelled along an identical path to the receiver. This method was applied to a series of 16 low-frequency earthquake swarms, well correlated with cyclic deformation recorded by tiltmeters, at Soufrière Hills Volcano, Montserrat, in June 1997. Nine waveform groups were identified containing more than 45 events each. The families are repeated across swarms with only small changes in waveform, indicating that the seismic source location is stable with time. The low-frequency seismic swarms begin prior to the point at which inflation starts to decelerate, suggesting that the seismicity indicates or even initiates a depressurisation process. A major dome collapse occurred within the time window considered, removing the top 100 m of the dome. This event caused activity within some families to pause for several cycles before reappearing. This shows that the collapse did not permanently disrupt the source mechanism or the path of the seismic waves.

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.

Construction and development of IGP DMC of China National Seismological Network

NASA Astrophysics Data System (ADS)

Zheng, X.; Zheng, J.; Lin, P.; Yao, Z.; Liang, J.

2011-12-01

In 2003, CEA (China Earthquake Administration) commenced the construction of China Digital Seismological Observation Network. By the end of 2007, a new-generation digital seismological observation system had been established, which consists of 1 National Seismic Network, 32 regional seismic networks, 2 small-aperture seismic arrays, 6 volcano monitoring networks and 19 mobile seismic networks, as well as CENC (China Earthquake Network Center) DMC (Data Management Centre) and IGP (Institute of Geophysics) DMC. Since then, the seismological observation system of China has completely entered a digital time. For operational, data backup and data security considerations, the DMC at the Institute of Geophysics (IGP), CEA was established at the end of 2007. IGP DMC now receives and archives waveform data from more than 1000 permanent seismic stations around China in real-time. After the great Wenchuan and Yushu earthquakes, the real-time waveform data from 56 and 8 portable seismic stations deployed in the aftershock area are added to IGP DMC. The technical system of IGP DMC is designed to conduct data management, processing and service through the network of CEA. We developed and integrated a hardware system with high-performance servers, large-capacity disc arrays, tape library and other facilities, as well as software packages for real-time waveform data receiving, storage, quality control, processing and service. Considering the demands from researchers for large quantities of seismic event waveform data, IGP DMC adopts an innovative "user order" method to extract event waveform data. Users can specify seismic stations, epicenter distance and record length. In a short period of 3 years, IGP DMC has supplied about 350 Terabytes waveform data to over 200 researches of more than 40 academic institutions. According to incomplete statistics, over 40 papers have been published in professional journals, in which 30 papers were indexed by SCI. Now, IGP DMC has become an important platform of promoting seismological researches in China. In the future, IGP DMC will continue to improve its technical system with powerful ability of waveform data processing, management and service, and to provide better and more data service to the research community. We expect IGP DMC to become an exchange and collaboration platform for geo-scientific researchers around the world.

Assessing waveform predictions of recent three-dimensional velocity models of the Tibetan Plateau

NASA Astrophysics Data System (ADS)

Bao, Xueyang; Shen, Yang

2016-04-01

Accurate velocity models are essential for both the determination of earthquake locations and source moments and the interpretation of Earth structures. With the increasing number of three-dimensional velocity models, it has become necessary to assess the models for accuracy in predicting seismic observations. Six models of the crustal and uppermost mantle structures in Tibet and surrounding regions are investigated in this study. Regional Rayleigh and Pn (or Pnl) waveforms from two ground truth events, including one nuclear explosion and one natural earthquake located in the study area, are simulated by using a three-dimensional finite-difference method. Synthetics are compared to observed waveforms in multiple period bands of 20-75 s for Rayleigh waves and 1-20 s for Pn/Pnl waves. The models are evaluated based on the phase delays and cross-correlation coefficients between synthetic and observed waveforms. A model generated from full-wave ambient noise tomography best predicts Rayleigh waves throughout the data set, as well as Pn/Pnl waves traveling from the Tarim Basin to the stations located in central Tibet. In general, the models constructed from P wave tomography are not well suited to predict Rayleigh waves, and vice versa. Possible causes of the differences between observed and synthetic waveforms, and frequency-dependent variations of the "best matching" models with the smallest prediction errors are discussed. This study suggests that simultaneous prediction for body and surface waves requires an integrated velocity model constructed with multiple seismic waveforms and consideration of other important properties, such as anisotropy.

Rapid and Robust Cross-Correlation-Based Seismic Signal Identification Using an Approximate Nearest Neighbor Method

DOE PAGES

Tibi, Rigobert; Young, Christopher; Gonzales, Antonio; ...

2017-07-04

The matched filtering technique that uses 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 paper, we introduce an approximate nearest neighbor (ANN) approach that enables the use of very large template libraries for waveform correlation. Our method begins with a projection into a reduced dimensionality space, based on correlation with a randomized subset ofmore » the full template archive. Searching for a specified number of nearest neighbors for a query waveform is accomplished by iteratively comparing it with the neighbors of its immediate neighbors. We used the approach to search for matches to each of ~2300 analyst-reviewed signal detections reported in May 2010 for the International Monitoring System station MKAR. The template library in this case consists of a data set of more than 200,000 analyst-reviewed signal detections for the same station from February 2002 to July 2016 (excluding May 2010). Of these signal detections, 73% are teleseismic first P and 17% regional phases (Pn, Pg, Sn, and Lg). Finally, the analyses performed on a standard desktop computer show that the proposed ANN approach performs a search of the large template libraries about 25 times faster than the standard full linear search and achieves recall rates greater than 80%, with the recall rate increasing for higher correlation thresholds.« less

Rapid and Robust Cross-Correlation-Based Seismic Signal Identification Using an Approximate Nearest Neighbor Method

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

Tibi, Rigobert; Young, Christopher; Gonzales, Antonio

The matched filtering technique that uses 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 paper, we introduce an approximate nearest neighbor (ANN) approach that enables the use of very large template libraries for waveform correlation. Our method begins with a projection into a reduced dimensionality space, based on correlation with a randomized subset ofmore » the full template archive. Searching for a specified number of nearest neighbors for a query waveform is accomplished by iteratively comparing it with the neighbors of its immediate neighbors. We used the approach to search for matches to each of ~2300 analyst-reviewed signal detections reported in May 2010 for the International Monitoring System station MKAR. The template library in this case consists of a data set of more than 200,000 analyst-reviewed signal detections for the same station from February 2002 to July 2016 (excluding May 2010). Of these signal detections, 73% are teleseismic first P and 17% regional phases (Pn, Pg, Sn, and Lg). Finally, the analyses performed on a standard desktop computer show that the proposed ANN approach performs a search of the large template libraries about 25 times faster than the standard full linear search and achieves recall rates greater than 80%, with the recall rate increasing for higher correlation thresholds.« less

Alaska Volcano Observatory Seismic Network Data Availability

NASA Astrophysics Data System (ADS)

Dixon, J. P.; Haney, M. M.; McNutt, S. R.; Power, J. A.; Prejean, S. G.; Searcy, C. K.; Stihler, S. D.; West, M. E.

2009-12-01

The Alaska Volcano Observatory (AVO) established in 1988 as a cooperative program of the U.S. Geological Survey, the Geophysical Institute at the University of Alaska Fairbanks, and the Alaska Division of Geological and Geophysical Surveys, monitors active volcanoes in Alaska. Thirty-three volcanoes are currently monitored by a seismograph network consisting of 193 stations, of which 40 are three-component stations. The current state of AVO’s seismic network, and data processing and availability are summarized in the annual AVO seismological bulletin, Catalog of Earthquake Hypocenters at Alaska Volcanoes, published as a USGS Data Series (most recent at http://pubs.usgs.gov/ds/467). Despite a rich seismic data set for 12 VEI 2 or greater eruptions, and over 80,000 located earthquakes in the last 21 years, the volcanic seismicity in the Aleutian Arc remains understudied. Initially, AVO seismic data were only provided via a data supplement as part of the annual bulletin, or upon request. Over the last few years, AVO has made seismic data more available with the objective of increasing volcano seismic research on the Aleutian Arc. The complete AVO earthquake catalog data are now available through the annual AVO bulletin and have been submitted monthly to the on-line Advanced National Seismic System (ANSS) composite catalog since 2008. Segmented waveform data for all catalog earthquakes are available upon request and efforts are underway to make this archive web accessible as well. Continuous data were first archived using a tape backup, but the availability of low cost digital storage media made a waveform backup of continuous data a reality. Currently the continuous AVO waveform data can be found in several forms. Since late 2002, AVO has burned all continuous waveform data to DVDs, as well as storing these data in Antelope databases at the Geophysical Institute. Beginning in 2005, data have been available through a Winston Wave Server housed at the USGS in Anchorage. AVO waveform data were added to the Incorporated Research Institutions for Seismology Data Management Center (IRIS-DMC) beginning in 2008 and now includes continuous waveform data from all available AVO seismograph stations in real time. Data coverage is available through the DMC’s Metadata Aggregator.

Seismic Evidence for Fluid/Gas Beneath the Mentawai Fore-Arc Basin, Central Sumatra

NASA Astrophysics Data System (ADS)

Huot, Gabriel; Singh, Satish C.

2018-02-01

Since 2004, there have been three great interplate earthquakes (Mw > 8.0) offshore Sumatra. In addition to rupturing the megathrust, these earthquakes might also have ruptured the backthrusts that bound the Andaman Islands to the Mentawai Islands toward the forearc basins. Here we apply a combination of traveltime tomography and seismic full waveform inversion to an ultralong offset seismic reflection profile from the Mentawai forearc basin, in the region of the 2007 Mw 8.4 Bengkulu earthquake. We perform a waveform inversion of far-offset data followed by a waveform inversion of near-offset data using the starting model derived from the traveltime tomography. Our results show the presence of a large, low-velocity anomaly above the backthrust. The seismic reflection image indicates that this low-velocity anomaly lies either within highly compacted sediments from the accretionary wedge or within highly deformed sediments from the forearc basin. The porosity estimation, using the effective medium theory, suggests that a small amount of gas (from 2 to 13%) or a significant amount of fluid (from 17 to 40%) could generate this low-velocity zone. The presence of fluids and the observation of bottom simulating reflector below a push-up ridge might be associated with mud diapirism. The fluids could originate locally from the dewatering of the sediments from the accretionary wedge or forearc basin. The high reflectivity of the backthrust in this region might also indicate deeper fluid origin, either from underplated sediments on the subduction interface or from the serpentinized mantle wedge.

Monitoring performance using synthetic data for induced microseismicity by hydrofracking at the Wysin site (Poland)

NASA Astrophysics Data System (ADS)

López-Comino, J. A.; Cesca, S.; Kriegerowski, M.; Heimann, S.; Dahm, T.; Mirek, J.; Lasocki, S.

2017-07-01

Ideally, the performance of a dedicated seismic monitoring installation should be assessed prior to the observation of target seismicity. This work is focused on a hydrofracking experiment monitored at Wysin, NE Poland. A microseismic synthetic catalogue is generated to assess the monitoring performance during the pre-operational phase, where seismic information only concerns the noise conditions and the potential background seismicity. Full waveform, accounting for the expected spatial, magnitude and focal mechanism distributions and a realistic local crustal model, are combined with real noise recording to produce either event based or continuous synthetic waveforms. The network detection performance is assessed in terms of the magnitude of completeness (Mc) through two different techniques. First, we use an amplitude threshold, taking into the ratio among the maximal amplitude of synthetic waveforms and station-dependent noise levels, for different values of signal-to-noise ratio. The detection probability at each station is estimated for the whole data set and extrapolated to a broader range of magnitude and distances. We estimate an Mc of about 0.55, when considering the distributed network, and can further decrease Mc to 0.45 using arrays techniques. The second approach, taking advantage on an automatic, coherence-based detection algorithm, can lower Mc to ∼ 0.1, at the cost of an increase of false detections. Mc experiences significant changes during day hours, in consequence of strongly varying noise conditions. Moreover, due to the radiation patterns and network geometry, double-couple like sources are better detected than tensile cracks, which may be induced during fracking.

Virtual Seismic Observation (VSO) with Sparsity-Promotion Inversion

NASA Astrophysics Data System (ADS)

Tiezhao, B.; Ning, J.; Jianwei, M.

2017-12-01

Large station interval leads to low resolution images, sometimes prevents people from obtaining images in concerned regions. Sparsity-promotion inversion, a useful method to recover missing data in industrial field acquisition, can be lent to interpolate seismic data on none-sampled sites, forming Virtual Seismic Observation (VSO). Traditional sparsity-promotion inversion suffers when coming up with large time difference in adjacent sites, which we concern most and use shift method to improve it. The procedure of the interpolation is that we first employ low-pass filter to get long wavelength waveform data and shift the waveforms of the same wave in different seismograms to nearly same arrival time. Then we use wavelet-transform-based sparsity-promotion inversion to interpolate waveform data on none-sampled sites and filling a phase in each missing trace. Finally, we shift back the waveforms to their original arrival times. We call our method FSIS (Filtering, Shift, Interpolation, Shift) interpolation. By this way, we can insert different virtually observed seismic phases into none-sampled sites and get dense seismic observation data. For testing our method, we randomly hide the real data in a site and use the rest to interpolate the observation on that site, using direct interpolation or FSIS method. Compared with directly interpolated data, interpolated data with FSIS can keep amplitude better. Results also show that the arrival times and waveforms of those VSOs well express the real data, which convince us that our method to form VSOs are applicable. In this way, we can provide needed data for some advanced seismic technique like RTM to illuminate shallow structures.

Nuclear Explosion Monitoring Advances and Challenges

NASA Astrophysics Data System (ADS)

Baker, G. E.

2015-12-01

We address the state-of-the-art in areas important to monitoring, current challenges, specific efforts that illustrate approaches addressing shortcomings in capabilities, and additional approaches that might be helpful. The exponential increase in the number of events that must be screened as magnitude thresholds decrease presents one of the greatest challenges. Ongoing efforts to exploit repeat seismic events using waveform correlation, subspace methods, and empirical matched field processing holds as much "game-changing" promise as anything being done, and further efforts to develop and apply such methods efficiently are critical. Greater accuracy of travel time, signal loss, and full waveform predictions are still needed to better locate and discriminate seismic events. Important developments include methods to model velocities using multiple types of data; to model attenuation with better separation of source, path, and site effects; and to model focusing and defocusing of surface waves. Current efforts to model higher frequency full waveforms are likely to improve source characterization while more effective estimation of attenuation from ambient noise holds promise for filling in gaps. Censoring in attenuation modeling is a critical problem to address. Quantifying uncertainty of discriminants is key to their operational use. Efforts to do so for moment tensor (MT) inversion are particularly important, and fundamental progress on the statistics of MT distributions is the most important advance needed in the near term in this area. Source physics is seeing great progress through theoretical, experimental, and simulation studies. The biggest need is to accurately predict the effects of source conditions on seismic generation. Uniqueness is the challenge here. Progress will depend on studies that probe what distinguishes mechanisms, rather than whether one of many possible mechanisms is consistent with some set of observations.

Accoustic waveform logging--Advances in theory and application

USGS Publications Warehouse

Paillet, F.L.; Cheng, C.H.; Pennington , W.D.

1992-01-01

Full-waveform acoustic logging has made significant advances in both theory and application in recent years, and these advances have greatly increased the capability of log analysts to measure the physical properties of formations. Advances in theory provide the analytical tools required to understand the properties of measured seismic waves, and to relate those properties to such quantities as shear and compressional velocity and attenuation, and primary and fracture porosity and permeability of potential reservoir rocks. The theory demonstrates that all parts of recorded waveforms are related to various modes of propagation, even in the case of dipole and quadrupole source logging. However, the theory also indicates that these mode properties can be used to design velocity and attenuation picking schemes, and shows how source frequency spectra can be selected to optimize results in specific applications. Synthetic microseismogram computations are an effective tool in waveform interpretation theory; they demonstrate how shear arrival picks and mode attenuation can be used to compute shear velocity and intrinsic attenuation, and formation permeability for monopole, dipole and quadrupole sources. Array processing of multi-receiver data offers the opportunity to apply even more sophisticated analysis techniques. Synthetic microseismogram data is used to illustrate the application of the maximum-likelihood method, semblance cross-correlation, and Prony's method analysis techniques to determine seismic velocities and attenuations. The interpretation of acoustic waveform logs is illustrated by reviews of various practical applications, including synthetic seismogram generation, lithology determination, estimation of geomechanical properties in situ, permeability estimation, and design of hydraulic fracture operations.

Long-period GPS waveforms. What can GPS bring to Earth seismic velocity models?

NASA Astrophysics Data System (ADS)

Kelevitz, Krisztina; Houlié, Nicolas; Boschi, Lapo; Nissen-Meyer, Tarje; Giardini, Domenico

2014-05-01

It is now commonly admitted that high rate GPS observations can provide reliable surface displacement waveforms (Cervelli, et al., 2001; Langbein, et al., 2006; Houlié, et al., 2006; Houlié et al., 2011). For long-period (T>5s) transients, it was shown that GPS and seismometer (STS-1) displacements are in agreement at least for vertical component (Houlié, et al., Sci. Rep. 2011). We propose here to supplement existing long-period seismic networks with high rate (>= 1Hz) GPS data in order to improve the resolution of global seismic velocity models. GPS measurements are providing a wide range of frequencies, going beyond the range of STS-1 in the low frequency end. Nowadays, almost 10.000 GPS receivers would be able to record data at 1 Hz with 3000+ stations already streaming data in Real-Time (RT). The reasons for this quick expansion are the price of receivers, their low maintenance, and the wide range of activities they can be used for (transport, science, public apps, navigation, etc.). We are presenting work completed on the 1Hz GPS records of the Hokkaido earthquake (25th of September, 2003, Mw=8.3). 3D Waveforms have been computed with an improved, stabilised inversion algorithm in order to constrain the ground motion history. Through the better resolution of inversion of the GPS phase observations, we determine displacement waveforms of frequencies ranging from 0.77 mHz to 330 mHz for a selection of sites. We compare inverted GPS waveforms with STS-1 waveforms and synthetic waveforms computed using 3D global wave propagation with SPECFEM. At co-located sites (STS-1 and GPS located within 10km) the agreement is good for the vertical component between seismic (both real and synthetic) and GPS waveforms.

Imaging The Shallow Velocity Structure Of The Hikurangi Megathrust Using Full-Waveform Inversion

NASA Astrophysics Data System (ADS)

Gray, M.; Bell, R. E.; Morgan, J. V.

2017-12-01

The Hikurangi margin, offshore North Island, New Zealand, exhibits a number of different slip behaviours, including shallow slow slip events (SSEs) (<2km to 15 km). There is also a strong contrast in geodetic coupling along the margin. While reflection data provides an image of the structure, no information about physical properties is provided. Full-waveform inversion (FWI) is an imaging technique which incorporates the full seismic wavelet rather than just the first arrivals, as in traditional tomography. By propagating synthetic seismic waves through a velocity model and comparing the synthetic wavelets to the field data, we update the velocity model until the real and synthetic wavelets match. In this way, we can resolve high-resolution physical property variations which influence the seismic wavefield. In our study, FWI was used to resolve the P-wave velocity structure at the Hikurangi megathrust up to 2km. This method enables investigation of how upper-plate structure may influence plate boundary slip behaviour. In 2005, a seismic survey was carried out over the Hikurangi megathrust. The data was acquired from a 12km streamer, allowing FWI analysis up to 2km below the seabed. The results show low velocity zones correlating to faults interpreted from reflection seismic imaging. We believe these low velocity zones, particularly near the frontal thrust resolve faulting in the area, and present these faults as possible fluid conduits. As the dataset was not collected specifically for FWI, the results show promise in resolving more information at depth. As such, both a 3D seismic survey and two drilling expeditions have been approved for the period November 2017 - May 2018. The seismic survey will be carried out with parameters optimal for FWI, allow imaging of the fault boundary, which is not possible with the current 2D data. The cores will provide direct geological evidence which can be used in conjunction with velocity models to discern lithology and structure. The current result identifies the existence of overpressure and aids in drilling safety when collecting these cores. In conjunction with the new IODP cores, the FWI model will improve understanding of properties of the shallow structure of the megathrust.

Zephyr: Open-source Parallel Seismic Waveform Inversion in an Integrated Python-based Framework

NASA Astrophysics Data System (ADS)

Smithyman, B. R.; Pratt, R. G.; Hadden, S. M.

2015-12-01

Seismic Full-Waveform Inversion (FWI) is an advanced method to reconstruct wave properties of materials in the Earth from a series of seismic measurements. These methods have been developed by researchers since the late 1980s, and now see significant interest from the seismic exploration industry. As researchers move towards implementing advanced numerical modelling (e.g., 3D, multi-component, anisotropic and visco-elastic physics), it is desirable to make use of a modular approach, minimizing the effort developing a new set of tools for each new numerical problem. SimPEG (http://simpeg.xyz) is an open source project aimed at constructing a general framework to enable geophysical inversion in various domains. In this abstract we describe Zephyr (https://github.com/bsmithyman/zephyr), which is a coupled research project focused on parallel FWI in the seismic context. The software is built on top of Python, Numpy and IPython, which enables very flexible testing and implementation of new features. Zephyr is an open source project, and is released freely to enable reproducible research. We currently implement a parallel, distributed seismic forward modelling approach that solves the 2.5D (two-and-one-half dimensional) viscoacoustic Helmholtz equation at a range modelling frequencies, generating forward solutions for a given source behaviour, and gradient solutions for a given set of observed data. Solutions are computed in a distributed manner on a set of heterogeneous workers. The researcher's frontend computer may be separated from the worker cluster by a network link to enable full support for computation on remote clusters from individual workstations or laptops. The present codebase introduces a numerical discretization equivalent to that used by FULLWV, a well-known seismic FWI research codebase. This makes it straightforward to compare results from Zephyr directly with FULLWV. The flexibility introduced by the use of a Python programming environment makes extension of the codebase with new methods much more straightforward. This enables comparison and integration of new efforts with existing results.

Velocity models and images using full waveform inversion and reverse time migration for the offshore permafrost in the Canadian shelf of Beaufort Sea, Arctic

NASA Astrophysics Data System (ADS)

Kang, S. G.; Hong, J. K.; Jin, Y. K.; Kim, S.; Kim, Y. G.; Dallimore, S.; Riedel, M.; Shin, C.

2015-12-01

During Expedition ARA05C (from Aug 26 to Sep 19, 2014) on the Korean icebreaker RV ARAON, the multi-channel seismic (MCS) data were acquired on the outer shelf and slope of the Canadian Beaufort Sea to investigate distribution and internal geological structures of the offshore ice-bonded permafrost and gas hydrates, totaling 998 km L-km with 19,962 shots. The MCS data were recorded using a 1500 m long solid-type streamer with 120 channels. Shot and group spacing were 50 m and 12.5 m, respectively. Most MCS survey lines were designed perpendicular and parallel to the strike of the shelf break. Ice-bonded permafrost or ice-bearing sediments are widely distributed under the Beaufort Sea shelf, which have formed during periods of lower sea level when portions of the shelf less than ~100m water depth were an emergent coastal plain exposed to very cold surface. The seismic P-wave velocity is an important geophysical parameter for identifying the distribution of ice-bonded permafrost with high velocity in this area. Recently, full waveform inversion (FWI) and reverse time migration (RTM) are commonly used to delineate detailed seismic velocity information and seismic image of geological structures. FWI is a data fitting procedure based on wave field modeling and numerical analysis to extract quantitative geophysical parameters such as P-, S-wave velocities and density from seismic data. RTM based on 2-way wave equation is a useful technique to construct accurate seismic image with amplitude preserving of field data. In this study, we suggest two-dimensional P-wave velocity model (Figure.1) using the FWI algorithm to delineate the top and bottom boundaries of ice-bonded permafrost in the Canadian shelf of Beaufort Sea. In addition, we construct amplitude preserving migrated seismic image using RTM to interpret the geological history involved with the evolution of permafrost.

Monitoring performance for hydraulic fracturing using synthetic microseismic catalogue at the Wysin site (Poland)

NASA Astrophysics Data System (ADS)

Ángel López Comino, José; Cesca, Simone; Kriegerowski, Marius; Heimann, Sebastian; Dahm, Torsten; Mirek, Janusz; Lasocky, Stanislaw

2017-04-01

Previous analysis to assess the monitoring performance of a dedicated seismic network are always useful to determine its capability of detecting, locating and characterizing target seismicity. This work focuses on a hydrofracking experiment in Poland, which is monitored in the framework of the SHEER (SHale gas Exploration and Exploitation induced Risks) EU project. The seismic installation is located near Wysin (Poland), in the central-western part of the Peribaltic synclise at Pomerania. The network setup includes a distributed network of six broadband stations, three shallow borehole stations and three small-scale arrays. We assess the monitoring performance prior operations, using synthetic seismograms. Realistic full waveform are generated and combined with real noise before fracking operations, to produce either event based or continuous synthetic waveforms. Background seismicity is modelled by double couple (DC) focal mechanisms. Non-DC sources resemble induced tensile fractures opening in the direction of the minimal compressive stress and closing in the same direction after the injection. Microseismic sources are combined with a realistic crustal model, distribution of hypocenters, magnitudes and source durations. The network detection performance is then assessed in terms of Magnitude of Completeness (Mc) through two different techniques: i) using an amplitude threshold approach, taking into account a station dependent noise level and different values of signal-to-noise ratio (SNR) and ii) through the application of an automatic detection algorithm to the continuous synthetic dataset. In the first case, we compare the maximal amplitude of noise free synthetic waveforms with the different noise levels. Imposing the simultaneous detection at e.g. 4 stations for a robust detection, the Mc is assessed and can be adjusted by empirical relationships for different SNR values. We find that different source mechanisms have different detection threshold. The background seismicity (DC sources) is better detectable than induced earthquakes (tensile cracks mechanisms). Assuming a SNR of 2, we estimate a Mc 0.55 around the fracking wells, with an increase of 0.05 during day hours. The value of Mc can be decreased to 0.45 around the fracking region, taking advantage by the array installations. The second approach applies a full waveform detection and location algorithm based on the stacking of smooth characteristic function and the identification of high coherence in the signals recorded at different stations. In this case the detection can be increased at the cost of increasing also false detections, with an acceptable compromise found for Mc 0.1.

Foundations for a multiscale collaborative Earth model

NASA Astrophysics Data System (ADS)

Afanasiev, Michael; Peter, Daniel; Sager, Korbinian; Simutė, Saulė; Ermert, Laura; Krischer, Lion; Fichtner, Andreas

2016-01-01

We present a computational framework for the assimilation of local to global seismic data into a consistent model describing Earth structure on all seismically accessible scales. This Collaborative Seismic Earth Model (CSEM) is designed to meet the following requirements: (i) Flexible geometric parametrization, capable of capturing topography and bathymetry, as well as all aspects of potentially resolvable structure, including small-scale heterogeneities and deformations of internal discontinuities. (ii) Independence of any particular wave equation solver, in order to enable the combination of inversion techniques suitable for different types of seismic data. (iii) Physical parametrization that allows for full anisotropy and for variations in attenuation and density. While not all of these parameters are always resolvable, the assimilation of data that constrain any parameter subset should be possible. (iv) Ability to accommodate successive refinements through the incorporation of updates on any scale as new data or inversion techniques become available. (v) Enable collaborative Earth model construction. The structure of the initial CSEM is represented on a variable-resolution tetrahedral mesh. It is assembled from a long-wavelength 3-D global model into which several regional-scale tomographies are embedded. We illustrate the CSEM workflow of successive updating with two examples from Japan and the Western Mediterranean, where we constrain smaller scale structure using full-waveform inversion. Furthermore, we demonstrate the ability of the CSEM to act as a vehicle for the combination of different tomographic techniques with a joint full-waveform and traveltime ray tomography of Europe. This combination broadens the exploitable frequency range of the individual techniques, thereby improving resolution. We perform two iterations of a whole-Earth full-waveform inversion using a long-period reference data set from 225 globally recorded earthquakes. At this early stage of the CSEM development, the broad global updates mostly act to remove artefacts from the assembly of the initial CSEM. During the future evolution of the CSEM, the reference data set will be used to account for the influence of small-scale refinements on large-scale global structure. The CSEM as a computational framework is intended to help bridging the gap between local, regional and global tomography, and to contribute to the development of a global multiscale Earth model. While the current construction serves as a first proof of concept, future refinements and additions will require community involvement, which is welcome at this stage already.

Source characterization of a small earthquake cluster at Edmond, Oklahoma using a very dense array

NASA Astrophysics Data System (ADS)

Ng, R.; Nakata, N.

2017-12-01

Recent seismicity in Oklahoma has caught the attention of the public in the last few years since seismicity is commonly related to loss in urban areas. To account for the increase in public interest, improve the understanding of damaging ground motions produced in earthquakes and develop better seismic hazard assessment, we must characterize the seismicity in Oklahoma and its associated structure and source parameters. Regional changes in subsurface stresses have increased seismic activities due to reactivation of faults in places such as central Oklahoma. It is imperative for seismic investigation and modeling to characterize subsurface structural features that may influence the damaging effects of ground motion. We analyze the full-waveform data collected from a temporary dense array of 72 portable seismometers with a 110 meter spacing that were active for a one-month period from May to June 2017, deployed at Edmond, Oklahoma. The data from this one-month duration array captured over 10,000 events and enabled us to make measurements of small-scale lateral variations of earthquake wavefields. We examine the waveform for events using advanced methods of detection, location and determine the source mechanism. We compare our results with selected events listed in the Oklahoma Geological Survey (OGS) and United States Geological Survey (USGS) catalogue. Based on the detection and located small events, we will discuss the causative fault structure at the area and present the results of the investigation.

W17_geowave “3D full waveform geophysical models”

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

Larmat, Carene; Maceira, Monica; Roy, Corinna

2018-02-12

Performance of the MCMC inversion according to the number of cores for the computation. A) 64 cores. B) 480 cores. C) 816 cores. The true model is represented by the black line. Vsv is the wave speed of S waves polarized in the vertical plane, ξ is an anisotropy parameter. The Earth is highly anisotropics; the wavespeed of seismic waves depends on the polarization of the wave. Seismic inversion of the elastic structure is usually limited to isotropic information such as Vsv. Our research looked at the inversion of Earth anisotropy.

ADVANCED WAVEFORM SIMULATION FOR SEISMIC MONITORING EVENTS

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

Helmberger, Donald V.; Tromp, Jeroen; Rodgers, Arthur J.

Earthquake source parameters underpin several aspects of nuclear explosion monitoring. Such aspects are: calibration of moment magnitudes (including coda magnitudes) and magnitude and distance amplitude corrections (MDAC); source depths; discrimination by isotropic moment tensor components; and waveform modeling for structure (including waveform tomography). This project seeks to improve methods for and broaden the applicability of estimating source parameters from broadband waveforms using the Cut-and-Paste (CAP) methodology. The CAP method uses a library of Green’s functions for a one-dimensional (1D, depth-varying) seismic velocity model. The method separates the main arrivals of the regional waveform into 5 windows: Pnl (vertical and radialmore » components), Rayleigh (vertical and radial components) and Love (transverse component). Source parameters are estimated by grid search over strike, dip, rake and depth and seismic moment or equivalently moment magnitude, MW, are adjusted to fit the amplitudes. Key to the CAP method is allowing the synthetic seismograms to shift in time relative to the data in order to account for path-propagation errors (delays) in the 1D seismic velocity model used to compute the Green’s functions. The CAP method has been shown to improve estimates of source parameters, especially when delay and amplitude biases are calibrated using high signal-to-noise data from moderate earthquakes, CAP+.« less

Closet to Cloud: The online archiving of tape-based continuous NCSN seismic data from 1993-2005

NASA Astrophysics Data System (ADS)

Neuhauser, D. S.; Aranha, M. A.; Kohler, W. M.; Oppenheimer, D.

2016-12-01

As earthquake monitoring systems in the 1980s moved from analog to digital recording systems, most seismic networks only archived digital waveforms from detected events due to lack of affordable online digital storage for continuous high-rate (100 sps) data. The Northern California Earthquake Data Center (NCEDC), established in 1991 by UC Berkeley and the USGS Menlo Park, archived 20 sps continuous data and triggerd high-rate from the sparse Berkeley seismic network, but could not afford the online storage for continuous high-rate data from the 300+ stations of the USGS Northern California Seismic Network (NCSN). The discovery of non-volcanic tremor and the use of continuous waveform correlation techniques for detecting repeating earthquakes combined with the increase in disk capacity capacity and significant reduction in disk costs led the Northern California Earthquake Data Center (NCEDC) to begin archiving continuous high-rate waveforms in 2004-2005. The USGS Menlo Park NCSN network had backup tapes of continuous high-rate waveform data since 1993 on the shelf, and the USGS and NCEDC embarked on a project to restore and archive all continuous NCSN data from 1993 through 2005. We will discuss the procedures and problems encountered when reading, transcribing, converting data formats, SEED channel naming, and archiving the 1993-2005 continuous NCSN waveforms. We will also illustrate new science enabled by these data. These and other northern California seismic and geophysical data are available via web services at http://service.ncedc.org

Full-waveform detection of non-impulsive seismic events based on time-reversal methods

NASA Astrophysics Data System (ADS)

Solano, Ericka Alinne; Hjörleifsdóttir, Vala; Liu, Qinya

2017-12-01

We present a full-waveform detection method for non-impulsive seismic events, based on time-reversal principles. We use the strain Green's tensor as a matched filter, correlating it with continuous observed seismograms, to detect non-impulsive seismic events. We show that this is mathematically equivalent to an adjoint method for detecting earthquakes. We define the detection function, a scalar valued function, which depends on the stacked correlations for a group of stations. Event detections are given by the times at which the amplitude of the detection function exceeds a given value relative to the noise level. The method can make use of the whole seismic waveform or any combination of time-windows with different filters. It is expected to have an advantage compared to traditional detection methods for events that do not produce energetic and impulsive P waves, for example glacial events, landslides, volcanic events and transform-fault earthquakes for events which velocity structure along the path is relatively well known. Furthermore, the method has advantages over empirical Greens functions template matching methods, as it does not depend on records from previously detected events, and therefore is not limited to events occurring in similar regions and with similar focal mechanisms as these events. The method is not specific to any particular way of calculating the synthetic seismograms, and therefore complicated structural models can be used. This is particularly beneficial for intermediate size events that are registered on regional networks, for which the effect of lateral structure on the waveforms can be significant. To demonstrate the feasibility of the method, we apply it to two different areas located along the mid-oceanic ridge system west of Mexico where non-impulsive events have been reported. The first study area is between Clipperton and Siqueiros transform faults (9°N), during the time of two earthquake swarms, occurring in March 2012 and May 2016. The second area of interest is the Gulf of California where two swarms took place during July and September of 2015. We show that we are able to detect previously non-reported, non-impulsive events and recommend that this method be used together with more traditional template matching methods to maximize the number of detected events.

Large-scale seismic waveform quality metric calculation using Hadoop

DOE PAGES

Magana-Zook, Steven; Gaylord, Jessie M.; Knapp, Douglas R.; ...

2016-05-27

Here in this work we investigated the suitability of Hadoop MapReduce and Apache Spark for large-scale computation of seismic waveform quality metrics by comparing their performance with that of a traditional distributed implementation. The Incorporated Research Institutions for Seismology (IRIS) Data Management Center (DMC) provided 43 terabytes of broadband waveform data of which 5.1 TB of data were processed with the traditional architecture, and the full 43 TB were processed using MapReduce and Spark. Maximum performance of ~0.56 terabytes per hour was achieved using all 5 nodes of the traditional implementation. We noted that I/O dominated processing, and that I/Omore » performance was deteriorating with the addition of the 5th node. Data collected from this experiment provided the baseline against which the Hadoop results were compared. Next, we processed the full 43 TB dataset using both MapReduce and Apache Spark on our 18-node Hadoop cluster. We conducted these experiments multiple times with various subsets of the data so that we could build models to predict performance as a function of dataset size. We found that both MapReduce and Spark significantly outperformed the traditional reference implementation. At a dataset size of 5.1 terabytes, both Spark and MapReduce were about 15 times faster than the reference implementation. Furthermore, our performance models predict that for a dataset of 350 terabytes, Spark running on a 100-node cluster would be about 265 times faster than the reference implementation. We do not expect that the reference implementation deployed on a 100-node cluster would perform significantly better than on the 5-node cluster because the I/O performance cannot be made to scale. Finally, we note that although Big Data technologies clearly provide a way to process seismic waveform datasets in a high-performance and scalable manner, the technology is still rapidly changing, requires a high degree of investment in personnel, and will likely require significant changes in other parts of our infrastructure. Nevertheless, we anticipate that as the technology matures and third-party tool vendors make it easier to manage and operate clusters, Hadoop (or a successor) will play a large role in our seismic data processing.« less

Large-scale seismic waveform quality metric calculation using Hadoop

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

Magana-Zook, Steven; Gaylord, Jessie M.; Knapp, Douglas R.

Here in this work we investigated the suitability of Hadoop MapReduce and Apache Spark for large-scale computation of seismic waveform quality metrics by comparing their performance with that of a traditional distributed implementation. The Incorporated Research Institutions for Seismology (IRIS) Data Management Center (DMC) provided 43 terabytes of broadband waveform data of which 5.1 TB of data were processed with the traditional architecture, and the full 43 TB were processed using MapReduce and Spark. Maximum performance of ~0.56 terabytes per hour was achieved using all 5 nodes of the traditional implementation. We noted that I/O dominated processing, and that I/Omore » performance was deteriorating with the addition of the 5th node. Data collected from this experiment provided the baseline against which the Hadoop results were compared. Next, we processed the full 43 TB dataset using both MapReduce and Apache Spark on our 18-node Hadoop cluster. We conducted these experiments multiple times with various subsets of the data so that we could build models to predict performance as a function of dataset size. We found that both MapReduce and Spark significantly outperformed the traditional reference implementation. At a dataset size of 5.1 terabytes, both Spark and MapReduce were about 15 times faster than the reference implementation. Furthermore, our performance models predict that for a dataset of 350 terabytes, Spark running on a 100-node cluster would be about 265 times faster than the reference implementation. We do not expect that the reference implementation deployed on a 100-node cluster would perform significantly better than on the 5-node cluster because the I/O performance cannot be made to scale. Finally, we note that although Big Data technologies clearly provide a way to process seismic waveform datasets in a high-performance and scalable manner, the technology is still rapidly changing, requires a high degree of investment in personnel, and will likely require significant changes in other parts of our infrastructure. Nevertheless, we anticipate that as the technology matures and third-party tool vendors make it easier to manage and operate clusters, Hadoop (or a successor) will play a large role in our seismic data processing.« less

Fracture characterization by means of attenuation and generation of tube waves in fractured crystalline rock at Mirror Lake, New Hampshire

USGS Publications Warehouse

Hardin, E.L.; Cheng, C.H.; Paillet, F.L.; Mendelson, J.D.

1987-01-01

Results are presented from experiments carried out in conjunction with the U. S. Geological Survey at the Hubbard Brook Experimental Forest near Mirror Lake, New Hampshire. The study focuses on our ability to obtain orientation and transmissivity estimates of naturally occurring fractures. The collected data set includes a four-offset hydrophone vertical seismic profile, full waveform acoustic logs at 5, 15, and 34 kHz, borehole televiewer, temperature, resistivity, and self-potential logs, and borehole-to-borehole pump test data. Borehole televiewer and other geophysical logs indicate that permeable fractures intersect the Mirror Lake boreholes at numerous depths, but less than half of these fractures appear to have significant permeability beyond the annulus of drilling disturbance on the basis of acoustic waveform log analysis. The vertical seismic profiling (VSP) data indicate a single major permeable fracture near a depth of 44 m, corresponding to one of the most permeable fractures identified in the acoustic waveform log analysis. VSP data also indicate a somewhat less permeable fracture at 220 m and possible fractures at depths of 103 and 135 m; all correspond to major permeable fractures in the acoustic waveform data set. Pump test data confirm the presence of a hydraulic connection between the Mirror Lake boreholes through a shallow dipping zone of permeability at 44 m in depth. Effective fracture apertures calculated from modeled transmissivities correspond to those estimated for the largest fractures indicated on acoustic waveform logs but are over an order of magnitude larger than effective apertures calculated from tube waves in the VSP data set. This discrepancy is attributed to the effect of fracture stiffness. A new model is presented to account for the mechanical strength of asperities in resisting fracture closure during the passage of seismic waves during the generation of VSPs.

Quantification of Uncertainty in Full-Waveform Moment Tensor Inversion for Regional Seismicity

NASA Astrophysics Data System (ADS)

Jian, P.; Hung, S.; Tseng, T.

2013-12-01

Routinely and instantaneously determined moment tensor solutions deliver basic information for investigating faulting nature of earthquakes and regional tectonic structure. The accuracy of full-waveform moment tensor inversion mostly relies on azimuthal coverage of stations, data quality and previously known earth's structure (i.e., impulse responses or Green's functions). However, intrinsically imperfect station distribution, noise-contaminated waveform records and uncertain earth structure can often result in large deviations of the retrieved source parameters from the true ones, which prohibits the use of routinely reported earthquake catalogs for further structural and tectonic interferences. Duputel et al. (2012) first systematically addressed the significance of statistical uncertainty estimation in earthquake source inversion and exemplified that the data covariance matrix, if prescribed properly to account for data dependence and uncertainty due to incomplete and erroneous data and hypocenter mislocation, cannot only be mapped onto the uncertainty estimate of resulting source parameters, but it also aids obtaining more stable and reliable results. Over the past decade, BATS (Broadband Array in Taiwan for Seismology) has steadily devoted to building up a database of good-quality centroid moment tensor (CMT) solutions for moderate to large magnitude earthquakes that occurred in Taiwan area. Because of the lack of the uncertainty quantification and reliability analysis, it remains controversial to use the reported CMT catalog directly for further investigation of regional tectonics, near-source strong ground motions, and seismic hazard assessment. In this study, we develop a statistical procedure to make quantitative and reliable estimates of uncertainty in regional full-waveform CMT inversion. The linearized inversion scheme adapting efficient estimation of the covariance matrices associated with oversampled noisy waveform data and errors of biased centroid positions is implemented and inspected for improving source parameter determination of regional seismicity in Taiwan. Synthetic inversion tests demonstrate the resolved moment tensors would better match the hypothetical CMT solutions, and tend to suppress unreal non-double-couple components and reduce the trade-off between focal mechanism and centroid depth if individual signal-to-noise ratios and correlation lengths for 3-component seismograms at each station and mislocation uncertainties are properly taken into account. We further testify the capability of our scheme in retrieving the robust CMT information for mid-sized (Mw~3.5) and offshore earthquakes in Taiwan, which offers immediate and broad applications in detailed modelling of regional stress field and deformation pattern and mapping of subsurface velocity structures.

Modularized seismic full waveform inversion based on waveform sensitivity kernels - The software package ASKI

NASA Astrophysics Data System (ADS)

Schumacher, Florian; Friederich, Wolfgang; Lamara, Samir; Gutt, Phillip; Paffrath, Marcel

2015-04-01

We present a seismic full waveform inversion concept for applications ranging from seismological to enineering contexts, based on sensitivity kernels for full waveforms. The kernels are derived from Born scattering theory as the Fréchet derivatives of linearized frequency-domain full waveform data functionals, quantifying the influence of elastic earth model parameters and density on the data values. For a specific source-receiver combination, the kernel is computed from the displacement and strain field spectrum originating from the source evaluated throughout the inversion domain, as well as the Green function spectrum and its strains originating from the receiver. By storing the wavefield spectra of specific sources/receivers, they can be re-used for kernel computation for different specific source-receiver combinations, optimizing the total number of required forward simulations. In the iterative inversion procedure, the solution of the forward problem, the computation of sensitivity kernels and the derivation of a model update is held completely separate. In particular, the model description for the forward problem and the description of the inverted model update are kept independent. Hence, the resolution of the inverted model as well as the complexity of solving the forward problem can be iteratively increased (with increasing frequency content of the inverted data subset). This may regularize the overall inverse problem and optimizes the computational effort of both, solving the forward problem and computing the model update. The required interconnection of arbitrary unstructured volume and point grids is realized by generalized high-order integration rules and 3D-unstructured interpolation methods. The model update is inferred solving a minimization problem in a least-squares sense, resulting in Gauss-Newton convergence of the overall inversion process. The inversion method was implemented in the modularized software package ASKI (Analysis of Sensitivity and Kernel Inversion), which provides a generalized interface to arbitrary external forward modelling codes. So far, the 3D spectral-element code SPECFEM3D (Tromp, Komatitsch and Liu, 2008) and the 1D semi-analytical code GEMINI (Friederich and Dalkolmo, 1995) in both, Cartesian and spherical framework are supported. The creation of interfaces to further forward codes is planned in the near future. ASKI is freely available under the terms of the GPL at www.rub.de/aski . Since the independent modules of ASKI must communicate via file output/input, large storage capacities need to be accessible conveniently. Storing the complete sensitivity matrix to file, however, permits the scientist full manual control over each step in a customized procedure of sensitivity/resolution analysis and full waveform inversion. In the presentation, we will show some aspects of the theory behind the full waveform inversion method and its practical realization by the software package ASKI, as well as synthetic and real-data applications from different scales and geometries.

Seismic source models for very-long period seismic signals on White Island, New Zealand

NASA Astrophysics Data System (ADS)

Jiwani-Brown, Elliot; Neuberg, Jurgen; Jolly, Art

2015-04-01

Very-long-period seismic signals (VLP) from White Island have a duration of only a few tens of seconds and a waveform that indicates an elastic (or viscoelastic) interaction of a source region with the surrounding medium; unlike VLP signals on some other volcanoes that indicate a step function recorded in the near field of the seismic source, White Island VLPs exhibit a Ricker waveform. We explore a set of isotropic, seismic source models based on the interaction between magma and water/brine in direct contact. Seismic amplitude measurements are taken into account to estimate the volume changes at depth that can produce the observed displacement at the surface. Furthermore, the influence of different fluid types are explored.

Determination of source parameters of the 2017 Mount Agung volcanic earthquake from moment-tensor inversion method using local broadband seismic waveforms

NASA Astrophysics Data System (ADS)

Madlazim; Prastowo, T.; Supardiyono; Hardy, T.

2018-03-01

Monitoring of volcanoes has been an important issue for many purposes, particularly hazard mitigation. With regard to this, the aims of the present work are to estimate and analyse source parameters of a volcanic earthquake driven by recent magmatic events of Mount Agung in Bali island that occurred on September 28, 2017. The broadband seismogram data consisting of 3 local component waveforms were recorded by the IA network of 5 seismic stations: SRBI, DNP, BYJI, JAGI, and TWSI (managed by BMKG). These land-based observatories covered a full 4-quadrant region surrounding the epicenter. The methods used in the present study were seismic moment-tensor inversions, where the data were all analyzed to extract the parameters, namely moment magnitude, type of a volcanic earthquake indicated by percentages of seismic components: compensated linear vector dipole (CLVD), isotropic (ISO), double-couple (DC), and source depth. The results are given in the forms of variance reduction of 65%, a magnitude of M W 3.6, a CLVD of 40%, an ISO of 33%, a DC of 27% and a centroid-depth of 9.7 km. These suggest that the unusual earthquake was dominated by a vertical CLVD component, implying the dominance of uplift motion of magmatic fluid flow inside the volcano.

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

Chen, Yu; Gao, Kai; Huang, Lianjie

Accurate imaging and characterization of fracture zones is crucial for geothermal energy exploration. Aligned fractures within fracture zones behave as anisotropic media for seismic-wave propagation. The anisotropic properties in fracture zones introduce extra difficulties for seismic imaging and waveform inversion. We have recently developed a new anisotropic elastic-waveform inversion method using a modified total-variation regularization scheme and a wave-energy-base preconditioning technique. Our new inversion method uses the parameterization of elasticity constants to describe anisotropic media, and hence it can properly handle arbitrary anisotropy. We apply our new inversion method to a seismic velocity model along a 2D-line seismic data acquiredmore » at Eleven-Mile Canyon located at the Southern Dixie Valley in Nevada for geothermal energy exploration. Our inversion results show that anisotropic elastic-waveform inversion has potential to reconstruct subsurface anisotropic elastic parameters for imaging and characterization of fracture zones.« less

Laboratory simulation of volcano seismicity.

PubMed

Benson, Philip M; Vinciguerra, Sergio; Meredith, Philip G; Young, R Paul

2008-10-10

The physical processes generating seismicity within volcanic edifices are highly complex and not fully understood. We report results from a laboratory experiment in which basalt from Mount Etna volcano (Italy) was deformed and fractured. The experiment was monitored with an array of transducers around the sample to permit full-waveform capture, location, and analysis of microseismic events. Rapid post-failure decompression of the water-filled pore volume and damage zone triggered many low-frequency events, analogous to volcanic long-period seismicity. The low frequencies were associated with pore fluid decompression and were located in the damage zone in the fractured sample; these events exhibited a weak component of shear (double-couple) slip, consistent with fluid-driven events occurring beneath active volcanoes.

Velocity and Attenuation Structure of the Earth's Inner Core Boundary From Semi-Automatic Waveform Modeling

NASA Astrophysics Data System (ADS)

Jin, J.; Song, X.; Sun, D.; Helmberger, D. V.

2013-12-01

The structure of the Earth's inner core boundary (ICB) is complex. Hemispherical differences and local variations of velocity and attenuation structures, as well as the ICB topography have been reported in previous studies. We are using an automatic waveform modeling method to improve the resolution of the ICB structures. The full waveforms of triplicated PKP phases at distance ranges from 120 to 165 degrees are used to model the lowermost 200 km of the outer core and the uppermost 600km of the inner core. Given a 1D velocity and attenuation model, synthetic seismograms are generated by Generalized Ray Theory. We are also experimenting 2D synthetic methods (WKM, AXISEM, and 2D FD) for 2D models (in the mantle and the inner core). The source time function is determined by observed seismic data. We use neighborhood algorithm to search for a group of models that minimize the misfit between predictions and observations. Tests on synthetic data show the efficiency of this method in resolving detailed velocity and attenuation structures of the ICB simultaneously. We are analyzing seismic record sections at dense arrays along different paths and will report our modeling and inversion results in the meeting.

Seismic Source Locations and Parameters for Sparce Networks by Matching Observed Seismograms to Semi-Empirical Synthetic Seismograms

NASA Astrophysics Data System (ADS)

Marshall, M. E.; Salzberg, D. H.

2006-05-01

The purpose of this study is to further demonstrate the accuracy of full-waveform earthquake location method using semi-empirical synthetic waveforms and received data from two or more regional stations. To test the method, well-constrained events from southern and central California are being used as a testbed. A suite of regional California events is being processed. Our focus is on aftershocks of the Parkfield event, the Hector Mine event, and the San Simian event. In all three cases, the aftershock locations are known to within 1 km. For Parkfield, with its extremely dense local network, the events are located to within 300 m or better. We are processing the data using a grid spacing of 0.5 km in three dimensions. Often, the minimum in residual from the semi-empirical waveform matching is within one grid point of the 'ground truth' location, which is as good as can be expected. We will present the results and compare those to the event locations reported in catalogs using the dense local seismic networks that are present in California. The preliminary results indicate that matched-waveform locations are able to resolve the locations with accuracies better than GT5, and possibly approaching GT1. These results only require two stations at regional distances and differing azimuths. One of the disadvantages of the California testbed is that all of the earthquakes in a particular region typically have very similar focal mechanisms. In theory, the semi-empirical approach should allow us to generate the well-matched synthetic waveforms regardless of the varying mechanisms. To verify this aspect, we apply the technique to relocate and simulate the JUNCTION nuclear test (March 26, 1992) using waveforms from the Little Skull Mountain mainshock.

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.

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.

Modern Workflow Full Waveform Inversion Applied to North America and the Northern Atlantic

NASA Astrophysics Data System (ADS)

Krischer, Lion; Fichtner, Andreas; Igel, Heiner

2015-04-01

We present the current state of a new seismic tomography model obtained using full waveform inversion of the crustal and upper mantle structure beneath North America and the Northern Atlantic, including the westernmost part of Europe. Parts of the eastern portion of the initial model consists of previous models by Fichtner et al. (2013) and Rickers et al. (2013). The final results of this study will contribute to the 'Comprehensive Earth Model' being developed by the Computational Seismology group at ETH Zurich. Significant challenges include the size of the domain, the uneven event and station coverage, and the strong east-west alignment of seismic ray paths across the North Atlantic. We use as much data as feasible, resulting in several thousand recordings per event depending on the receivers deployed at the earthquakes' origin times. To manage such projects in a reproducible and collaborative manner, we, as tomographers, should abandon ad-hoc scripts and one-time programs, and adopt sustainable and reusable solutions. Therefore we developed the LArge-scale Seismic Inversion Framework (LASIF - http://lasif.net), an open-source toolbox for managing seismic data in the context of non-linear iterative inversions that greatly reduces the time to research. Information on the applied processing, modelling, iterative model updating, what happened during each iteration, and so on are systematically archived. This results in a provenance record of the final model which in the end significantly enhances the reproducibility of iterative inversions. Additionally, tools for automated data download across different data centers, window selection, misfit measurements, parallel data processing, and input file generation for various forward solvers are provided.

Expanding the frontiers of waveform imaging with Salvus

NASA Astrophysics Data System (ADS)

Afanasiev, M.; Boehm, C.; van Driel, M.; Krischer, L.; Fichtner, A.

2017-12-01

Mechanical waves are natural harbingers of information. From medical ultrasound to the normal modes of Sun, wave motion is often our best window into the character of some underlying continuum. For over a century, geophysicists have been using this window to peer deep into the Earth, developing techniques that have gone on to underlie much of world's energy economy. As computers and numerical techniques have become more powerful over the last several decades, seismologists have begun to scale back classical simplifying approximations of wave propagation physics. As a result, we are now approaching the ideal of `full-waveform inversion'; maximizing the aperture of our window by taking the full complexity of wave motion into account.Salvus is a modern high-performance software suite which aims to bring recent developments in geophysical waveform inversion to new and exciting domains. In this short presentation we will look at the connections between these applications, with examples from non-destructive testing, medical imaging, seismic exploration, and (extra-) planetary seismology.

Computing the Sensitivity Kernels for 2.5-D Seismic Waveform Inversion in Heterogeneous, Anisotropic Media

NASA Astrophysics Data System (ADS)

Zhou, Bing; Greenhalgh, S. A.

2011-10-01

2.5-D modeling and inversion techniques are much closer to reality than the simple and traditional 2-D seismic wave modeling and inversion. The sensitivity kernels required in full waveform seismic tomographic inversion are the Fréchet derivatives of the displacement vector with respect to the independent anisotropic model parameters of the subsurface. They give the sensitivity of the seismograms to changes in the model parameters. This paper applies two methods, called `the perturbation method' and `the matrix method', to derive the sensitivity kernels for 2.5-D seismic waveform inversion. We show that the two methods yield the same explicit expressions for the Fréchet derivatives using a constant-block model parameterization, and are available for both the line-source (2-D) and the point-source (2.5-D) cases. The method involves two Green's function vectors and their gradients, as well as the derivatives of the elastic modulus tensor with respect to the independent model parameters. The two Green's function vectors are the responses of the displacement vector to the two directed unit vectors located at the source and geophone positions, respectively; they can be generally obtained by numerical methods. The gradients of the Green's function vectors may be approximated in the same manner as the differential computations in the forward modeling. The derivatives of the elastic modulus tensor with respect to the independent model parameters can be obtained analytically, dependent on the class of medium anisotropy. Explicit expressions are given for two special cases—isotropic and tilted transversely isotropic (TTI) media. Numerical examples are given for the latter case, which involves five independent elastic moduli (or Thomsen parameters) plus one angle defining the symmetry axis.

Pick- and waveform-based techniques for real-time detection of induced seismicity

NASA Astrophysics Data System (ADS)

Grigoli, Francesco; Scarabello, Luca; Böse, Maren; Weber, Bernd; Wiemer, Stefan; Clinton, John F.

2018-05-01

The monitoring of induced seismicity is a common operation in many industrial activities, such as conventional and non-conventional hydrocarbon production or mining and geothermal energy exploitation, to cite a few. During such operations, we generally collect very large and strongly noise-contaminated data sets that require robust and automated analysis procedures. Induced seismicity data sets are often characterized by sequences of multiple events with short interevent times or overlapping events; in these cases, pick-based location methods may struggle to correctly assign picks to phases and events, and errors can lead to missed detections and/or reduced location resolution and incorrect magnitudes, which can have significant consequences if real-time seismicity information are used for risk assessment frameworks. To overcome these issues, different waveform-based methods for the detection and location of microseismicity have been proposed. The main advantages of waveform-based methods is that they appear to perform better and can simultaneously detect and locate seismic events providing high-quality locations in a single step, while the main disadvantage is that they are computationally expensive. Although these methods have been applied to different induced seismicity data sets, an extensive comparison with sophisticated pick-based detection methods is still missing. In this work, we introduce our improved waveform-based detector and we compare its performance with two pick-based detectors implemented within the SeiscomP3 software suite. We test the performance of these three approaches with both synthetic and real data sets related to the induced seismicity sequence at the deep geothermal project in the vicinity of the city of St. Gallen, Switzerland.

Adaptive phase k-means algorithm for waveform classification

NASA Astrophysics Data System (ADS)

Song, Chengyun; Liu, Zhining; Wang, Yaojun; Xu, Feng; Li, Xingming; Hu, Guangmin

2018-01-01

Waveform classification is a powerful technique for seismic facies analysis that describes the heterogeneity and compartments within a reservoir. Horizon interpretation is a critical step in waveform classification. However, the horizon often produces inconsistent waveform phase, and thus results in an unsatisfied classification. To alleviate this problem, an adaptive phase waveform classification method called the adaptive phase k-means is introduced in this paper. Our method improves the traditional k-means algorithm using an adaptive phase distance for waveform similarity measure. The proposed distance is a measure with variable phases as it moves from sample to sample along the traces. Model traces are also updated with the best phase interference in the iterative process. Therefore, our method is robust to phase variations caused by the interpretation horizon. We tested the effectiveness of our algorithm by applying it to synthetic and real data. The satisfactory results reveal that the proposed method tolerates certain waveform phase variation and is a good tool for seismic facies analysis.

Seismic waveform classification using deep learning

NASA Astrophysics Data System (ADS)

Kong, Q.; Allen, R. M.

2017-12-01

MyShake is a global smartphone seismic network that harnesses the power of crowdsourcing. It has an Artificial Neural Network (ANN) algorithm running on the phone to distinguish earthquake motion from human activities recorded by the accelerometer on board. Once the ANN detects earthquake-like motion, it sends a 5-min chunk of acceleration data back to the server for further analysis. The time-series data collected contains both earthquake data and human activity data that the ANN confused. In this presentation, we will show the Convolutional Neural Network (CNN) we built under the umbrella of supervised learning to find out the earthquake waveform. The waveforms of the recorded motion could treat easily as images, and by taking the advantage of the power of CNN processing the images, we achieved very high successful rate to select the earthquake waveforms out. Since there are many non-earthquake waveforms than the earthquake waveforms, we also built an anomaly detection algorithm using the CNN. Both these two methods can be easily extended to other waveform classification problems.

Towards a Full Waveform Ambient Noise Inversion

NASA Astrophysics Data System (ADS)

Sager, K.; Ermert, L. A.; Boehm, C.; Fichtner, A.

2015-12-01

Noise tomography usually works under the assumption that the inter-station ambient noise correlation is equal to a scaled version of the Green's function between the two receivers. This assumption, however, is only met under specific conditions, for instance, wavefield diffusivity and equipartitioning, zero attenuation, etc., that are typically not satisfied in the Earth. This inconsistency inhibits the exploitation of the full waveform information contained in noise correlations regarding Earth structure and noise generation. To overcome this limitation we attempt to develop a method that consistently accounts for noise distribution, 3D heterogeneous Earth structure and the full seismic wave propagation physics in order to improve the current resolution of tomographic images of the Earth. As an initial step towards a full waveform ambient noise inversion we develop a preliminary inversion scheme based on a 2D finite-difference code simulating correlation functions and on adjoint techniques. With respect to our final goal, a simultaneous inversion for noise distribution and Earth structure, we address the following two aspects: (1) the capabilities of different misfit functionals to image wave speed anomalies and source distribution and (2) possible source-structure trade-offs, especially to what extent unresolvable structure could be mapped into the inverted noise source distribution and vice versa.

Waveform Fingerprinting for Efficient Seismic Signal Detection

NASA Astrophysics Data System (ADS)

Yoon, C. E.; OReilly, O. J.; Beroza, G. C.

2013-12-01

Cross-correlating an earthquake waveform template with continuous waveform data has proven a powerful approach for detecting events missing from earthquake catalogs. If templates do not exist, it is possible to divide the waveform data into short overlapping time windows, then identify window pairs with similar waveforms. Applying these approaches to earthquake monitoring in seismic networks has tremendous potential to improve the completeness of earthquake catalogs, but because effort scales quadratically with time, it rapidly becomes computationally infeasible. We develop a fingerprinting technique to identify similar waveforms, using only a few compact features of the original data. The concept is similar to human fingerprints, which utilize key diagnostic features to identify people uniquely. Analogous audio-fingerprinting approaches have accurately and efficiently found similar audio clips within large databases; example applications include identifying songs and finding copyrighted content within YouTube videos. In order to fingerprint waveforms, we compute a spectrogram of the time series, and segment it into multiple overlapping windows (spectral images). For each spectral image, we apply a wavelet transform, and retain only the sign of the maximum magnitude wavelet coefficients. This procedure retains just the large-scale structure of the data, providing both robustness to noise and significant dimensionality reduction. Each fingerprint is a high-dimensional, sparse, binary data object that can be stored in a database without significant storage costs. Similar fingerprints within the database are efficiently searched using locality-sensitive hashing. We test this technique on waveform data from the Northern California Seismic Network that contains events not detected in the catalog. We show that this algorithm successfully identifies similar waveforms and detects uncataloged low magnitude events in addition to cataloged events, while running to completion faster than a comparison waveform autocorrelation code.

Threshold magnitudes for a multichannel correlation detector in background seismicity

DOE PAGES

Carmichael, Joshua D.; Hartse, Hans

2016-04-01

Colocated explosive sources often produce correlated seismic waveforms. Multichannel correlation detectors identify these signals by scanning template waveforms recorded from known reference events against "target" data to find similar waveforms. This screening problem is challenged at thresholds required to monitor smaller explosions, often because non-target signals falsely trigger such detectors. Therefore, it is generally unclear what thresholds will reliably identify a target explosion while screening non-target background seismicity. Here, we estimate threshold magnitudes for hypothetical explosions located at the North Korean nuclear test site over six months of 2010, by processing International Monitoring System (IMS) array data with a multichannelmore » waveform correlation detector. Our method (1) accounts for low amplitude background seismicity that falsely triggers correlation detectors but is unidentifiable with conventional power beams, (2) adapts to diurnally variable noise levels and (3) uses source-receiver reciprocity concepts to estimate thresholds for explosions spatially separated from the template source. Furthermore, we find that underground explosions with body wave magnitudes m b = 1.66 are detectable at the IMS array USRK with probability 0.99, when using template waveforms consisting only of P -waves, without false alarms. We conservatively find that these thresholds also increase by up to a magnitude unit for sources located 4 km or more from the Feb.12, 2013 announced nuclear test.« less

Time-Lapse Monitoring with 4D Seismic Coda Waves in Active, Passive and Ambient Noise Data

NASA Astrophysics Data System (ADS)

Lumley, D. E.; Kamei, R.; Saygin, E.; Shragge, J. C.

2017-12-01

The Earth's subsurface is continuously changing, due to temporal variations in fluid flow, stress, temperature, geomechanics and geochemistry, for example. These physical changes occur at broad tectonic and earthquake scales, and also at very detailed near-surface and reservoir scales. Changes in the physical states of the earth cause time-varying changes in the physical properties of rocks and fluids, which can be monitored with natural or manmade seismic waves. Time-lapse (4D) seismic monitoring is important for applications related to natural and induced seismicity, hydrocarbon and groundwater reservoir depletion, CO2 sequestration etc. An exciting new research area involves moving beyond traditional methods in order to use the full complex time-lapse scattered wavefield (4D coda waves) for both manmade active-source 3D/4D seismic data, and also to use continuous recordings of natural-source passive seismic data, especially (micro) earthquakes and ocean ambient noise. This research involves full wave-equation approaches including waveform inversion (FWI), interferometry, Large N sensor arrays, "big data" information theory, and high performance supercomputing (HPC). I will present high-level concepts and recent data results that are quite spectacular and highly encouraging.

Simultaneous inversion of seismic velocity and moment tensor using elastic-waveform inversion of microseismic data: Application to the Aneth CO2-EOR field

NASA Astrophysics Data System (ADS)

Chen, Y.; Huang, L.

2017-12-01

Moment tensors are key parameters for characterizing CO2-injection-induced microseismic events. Elastic-waveform inversion has the potential to providing accurate results of moment tensors. Microseismic waveforms contains information of source moment tensors and the wave propagation velocity along the wavepaths. We develop an elastic-waveform inversion method to jointly invert the seismic velocity model and moment tensor. We first use our adaptive moment-tensor joint inversion method to estimate moment tensors of microseismic events. Our adaptive moment-tensor inversion method jointly inverts multiple microseismic events with similar waveforms within a cluster to reduce inversion uncertainty for microseismic data recorded using a single borehole geophone array. We use this inversion result as the initial model for our elastic-waveform inversion to minimize the cross-correlated-based data misfit between observed data and synthetic data. We verify our method using synthetic microseismic data and obtain improved results of both moment tensors and seismic velocity model. We apply our new inversion method to microseismic data acquired at a CO2-enhanced oil recovery field in Aneth, Utah, using a single borehole geophone array. The results demonstrate that our new inversion method significantly reduces the data misfit compared to the conventional ray-theory-based moment-tensor inversion.

High-resolution near-surface velocity model building using full-waveform inversion—a case study from southwest Sweden

NASA Astrophysics Data System (ADS)

Adamczyk, A.; Malinowski, M.; Malehmir, A.

2014-06-01

Full-waveform inversion (FWI) is an iterative optimization technique that provides high-resolution models of subsurface properties. Frequency-domain, acoustic FWI was applied to seismic data acquired over a known quick-clay landslide scar in southwest Sweden. We inverted data from three 2-D seismic profiles, 261-572 m long, two of them shot with small charges of dynamite and one with a sledgehammer. To our best knowledge this is the first published application of FWI to sledgehammer data. Both sources provided data suitable for waveform inversion, the sledgehammer data containing even wider frequency spectrum. Inversion was performed for frequency groups between 27.5 and 43.1 Hz for the explosive data and 27.5-51.0 Hz for the sledgehammer. The lowest inverted frequency was limited by the resonance frequency of the standard 28-Hz geophones used in the survey. High-velocity granitic bedrock in the area is undulated and very shallow (15-100 m below the surface), and exhibits a large P-wave velocity contrast to the overlying normally consolidated sediments. In order to mitigate the non-linearity of the inverse problem we designed a multiscale layer-stripping inversion strategy. Obtained P-wave velocity models allowed to delineate the top of the bedrock and revealed distinct layers within the overlying sediments of clays and coarse-grained materials. Models were verified in an extensive set of validating procedures and used for pre-stack depth migration, which confirmed their robustness.

Crustal velocity structure of central Gansu Province from regional seismic waveform inversion using firework algorithm

NASA Astrophysics Data System (ADS)

Chen, Yanyang; Wang, Yanbin; Zhang, Yuansheng

2017-04-01

The firework algorithm (FWA) is a novel swarm intelligence-based method recently proposed for the optimization of multi-parameter, nonlinear functions. Numerical waveform inversion experiments using a synthetic model show that the FWA performs well in both solution quality and efficiency. We apply the FWA in this study to crustal velocity structure inversion using regional seismic waveform data of central Gansu on the northeastern margin of the Qinghai-Tibet plateau. Seismograms recorded from the moment magnitude ( M W) 5.4 Minxian earthquake enable obtaining an average crustal velocity model for this region. We initially carried out a series of FWA robustness tests in regional waveform inversion at the same earthquake and station positions across the study region, inverting two velocity structure models, with and without a low-velocity crustal layer; the accuracy of our average inversion results and their standard deviations reveal the advantages of the FWA for the inversion of regional seismic waveforms. We applied the FWA across our study area using three component waveform data recorded by nine broadband permanent seismic stations with epicentral distances ranging between 146 and 437 km. These inversion results show that the average thickness of the crust in this region is 46.75 km, while thicknesses of the sedimentary layer, and the upper, middle, and lower crust are 3.15, 15.69, 13.08, and 14.83 km, respectively. Results also show that the P-wave velocities of these layers and the upper mantle are 4.47, 6.07, 6.12, 6.87, and 8.18 km/s, respectively.

A Waveform Detector that Targets Template-Decorrelated Signals and Achieves its Predicted Performance: Demonstration with IMS Data

NASA Astrophysics Data System (ADS)

Carmichael, J.

2016-12-01

Waveform correlation detectors used in seismic monitoring scan multichannel data to test two competing hypotheses: that data contain (1) a noisy, amplitude-scaled version of a template waveform, or, (2) only noise. In reality, seismic wavefields include signals triggered by non-target sources (background seismicity) and target signals that are only partially correlated with the waveform template. We reform the waveform correlation detector hypothesis test to accommodate deterministic uncertainty in template/target waveform similarity and thereby derive a new detector from convex set projections (the "cone detector") for use in explosion monitoring. Our analyses give probability density functions that quantify the detectors' degraded performance with decreasing waveform similarity. We then apply our results to three announced North Korean nuclear tests and use International Monitoring System (IMS) arrays to determine the probability that low magnitude, off-site explosions can be reliably detected with a given waveform template. We demonstrate that cone detectors provide (1) an improved predictive capability over correlation detectors to identify such spatially separated explosive sources, (2) competitive detection rates, and (3) reduced false alarms on background seismicity. Figure Caption: Observed and predicted receiver operating characteristic curves for correlation statistic r(x) (left) and cone statistic s(x) (right) versus semi-empirical explosion magnitude. a: Shaded region shows range of ROC curves for r(x) that give the predicted detection performance in noise conditions recorded over 24 hrs on 8 October 2006. Superimposed stair plot shows the empirical detection performance (recorded detections/total events) averaged over 24 hr of data. Error bars indicate the demeaned range in observed detection probability over the day; means are removed to avoid risk of misinterpreting range to indicate probabilities can exceed one. b: Shaded region shows range of ROC curves for s(x) that give the predicted detection performance for the cone detector. Superimposed stair plot show observed detection performance averaged over 24 hr of data analogous to that shown in a.

Lithospheric layering in the North American craton revealed by including Short Period Constraints in Full Waveform Tomography

NASA Astrophysics Data System (ADS)

Roy, C.; Calo, M.; Bodin, T.; Romanowicz, B. A.

2017-12-01

Recent receiver function studies of the North American craton suggest the presence of significant layering within the cratonic lithosphere, with significant lateral variations in the depth of the velocity discontinuities. These structural boundaries have been confirmed recently using a transdimensional Markov Chain Monte Carlo approach (TMCMC), inverting surface wave dispersion data and converted phases simultaneously (Calò et al., 2016; Roy and Romanowicz 2017). The lateral resolution of upper mantle structure can be improved with a high density of broadband seismic stations, or with a sparse network using full waveform inversion based on numerical wavefield computation methods such as the Spectral Element Method (SEM). However, inverting for discontinuities with strong topography such as MLDS's or LAB, presents challenges in an inversion framework, both computationally, due to the short periods required, and from the point of view of stability of the inversion. To overcome these limitations, and to improve resolution of layering in the upper mantle, we are developing a methodology that combines full waveform inversion tomography and information provided by short period seismic observables. We have extended the 30 1D radially anisotropic shear velocity profiles of Calò et al. 2016 to several other stations, for which we used a recent shear velocity model (Clouzet et al., 2017) as constraint in the modeling. These 1D profiles, including both isotropic and anisotropic discontinuities in the upper mantle (above 300 km depth) are then used to build a 3D starting model for the full waveform tomographic inversion. This model is built after 1) homogenization of the layered 1D models and 2) interpolation between the 1D smooth profiles and the model of Clouzet et al. 2017, resulting in a smooth 3D starting model. Waveforms used in the inversion are filtered at periods longer than 30s. We use the SEM code "RegSEM" for forward computations and a quasi-Newton inversion approach in which kernels are computed using normal mode perturbation theory. The resulting volumetric velocity perturbations around the homogenized starting model are then added to the discontinuous 3D starting model by dehomogenizing the model. We present here the first results of such an approach for refining structure in the North American continent.

Restoration of clipped seismic waveforms using projection onto convex sets method

PubMed Central

Zhang, Jinhai; Hao, Jinlai; Zhao, Xu; Wang, Shuqin; Zhao, Lianfeng; Wang, Weimin; Yao, Zhenxing

2016-01-01

The seismic waveforms would be clipped when the amplitude exceeds the upper-limit dynamic range of seismometer. Clipped waveforms are typically assumed not useful and seldom used in waveform-based research. Here, we assume the clipped components of the waveform share the same frequency content with the un-clipped components. We leverage this similarity to convert clipped waveforms to true waveforms by iteratively reconstructing the frequency spectrum using the projection onto convex sets method. Using artificially clipped data we find that statistically the restoration error is ~1% and ~5% when clipped at 70% and 40% peak amplitude, respectively. We verify our method using real data recorded at co-located seismometers that have different gain controls, one set to record large amplitudes on scale and the other set to record low amplitudes on scale. Using our restoration method we recover 87 out of 93 clipped broadband records from the 2013 Mw6.6 Lushan earthquake. Estimating that we recover 20 clipped waveforms for each M5.0+ earthquake, so for the ~1,500 M5.0+ events that occur each year we could restore ~30,000 clipped waveforms each year, which would greatly enhance useable waveform data archives. These restored waveform data would also improve the azimuthal station coverage and spatial footprint. PMID:27966618

Search for repeating events at the plate interface in the seismic sequence of the 2014 Mw8.1 Iquique earthquake, Chile

NASA Astrophysics Data System (ADS)

Kummerow, Joern; Asch, Guenter; Sens-Schönfelder, Christoph; Schurr, Bernd; Tilmann, Frederik; Shapiro, Serge A.

2017-04-01

The 2014 Mw8.1 Iquique earthquake occurred along a segment of the northern Chile- southern Peru seismic gap which had not ruptured for more than 100 years. A specific feature of this event is the observation of prominent foreshock clusters with successively increasing seismic moment releases starting several months before the main shock (e.g., Schurr et al., 2014). The entire seismic sequence, including also the aftershock seismicity, was monitored exceptionally well by the Integrated Plate Boundary Observatory Chile (IPOC). Here, we present results from a systematic, long-term search for repeating seismic events along the plate interface in the source region of the 1 April 2014 (Mw8.1) Iquique main shock. Repeating earthquakes are widely assumed to indicate recurrent ruptures on the same fault patch and to accommodate aseismic slip in the creeping portions around the seismic patch. According to this concept, the analysis of repeating events and of their temporal behaviour provides a tool to estimate the amount of creep. We use the IPOC and two additional local seismic networks and select recorded waveforms of several hundreds of located earthquakes within the foreshock and aftershock series as template events. Waveforms are windowed around the P and S phases and bandpass-filtered for different frequency bands. Window starts are defined by manually revised P onset times. We then run a newly implemented correlation detector on the resampled, continuous seismic data to find highly similar waveforms for each template event. Repeating earthquakes are finally identified by a combination of estimated source dimensions, high waveform similarity and precise relative relocations of the events within each multiplet group. The analysis of the spatial and temporal patterns of the detected repeating earthquake sequences allows to test the proposed idea of progressive unlocking of the plate boundary before the Iquique main shock.

FDM simulation of earthquakes off western Kyushu, Japan, using a land-ocean unified 3D structure model

NASA Astrophysics Data System (ADS)

Okamoto, Taro; Takenaka, Hiroshi; Nakamura, Takeshi; Hara, Tatsuhiko

2017-07-01

Seismic activity occurred off western Kyushu, Japan, at the northern end of the Okinawa Trough on May 6, 2016 (14:11 JST), 22 days after the onset of the 2016 Kumamoto earthquake sequence. The area is adjacent to the Beppu-Shimabara graben where the 2016 Kumamoto earthquake sequence occurred. In the area off western Kyushu, a M7.1 earthquake also occurred on November 14, 2015 (5:51 JST), and a tsunami with a height of 0.3 m was observed. In order to better understand these seismic activity and tsunamis, it is necessary to study the sources of, and strong motions due to, earthquakes in the area off western Kyushu. For such studies, validation of synthetic waveforms is important because of the presence of the oceanic water layer and thick sediments in the source area. We show the validation results for synthetic waveforms through nonlinear inversion analyses of small earthquakes ( M5). We use a land-ocean unified 3D structure model, 3D HOT finite-difference method ("HOT" stands for Heterogeneity, Ocean layer and Topography) and a multi-graphic processing unit (GPU) acceleration to simulate the wave propagations. We estimate the first-motion augmented moment tensor (FAMT) solution based on both the long-period surface waves and short-period body waves. The FAMT solutions systematically shift landward by about 13 km, on average, from the epicenters determined by the Japan Meteorological Agency. The synthetics provide good reproductions of the observed full waveforms with periods of 10 s or longer. On the other hand, for waveforms with shorter periods (down to 4 s), the later surface waves are not reproduced well, while the first parts of the waveforms (comprising P- and S-waves) are reproduced to some extent. These results indicate that the current 3D structure model around Kyushu is effective for generating full waveforms, including surface waves with periods of about 10 s or longer. Based on these findings, we analyze the 2015 M7.1 event using the cross-correlations between the observed and synthetic waveforms. The result suggests a rupture propagation toward the NNE, with a major radiation about 25 km north of the onset point.[Figure not available: see fulltext.

DEVELOPING AND EXPLOITING A UNIQUE SEISMIC DATA SET FROM SOUTH AFRICAN GOLD MINES FOR SOURCE CHARACTERIZATION AND WAVE PROPAGATION

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

Julia, J; Nyblade, A A; Gok, R

2008-07-08

In this project, we are developing and exploiting a unique seismic data set to address the characteristics of small seismic events and the associated seismic signals observed at local (< 200 km) and regional (< 2000 km) distances. The dataset is being developed using mining-induced events from 3 deep gold mines in South Africa recorded on inmine networks (< 1 km) comprised of tens of high-frequency sensors, a network of 4 broadband stations installed as part of this project at the surface around the mines (1-10 km), and a network of existing broadband seismic stations at local/regional distances (50-1000 km)more » from the mines. After 1 year of seismic monitoring of mine activity (2007), over 10,000 events in the range -3.4 < ML < 4.4 have been catalogued and recorded by the in-mine networks. Events with positive magnitudes are generally well recorded by the surface-mine stations, while magnitudes 3.0 and larger are seen at regional distances (up to {approx}600 km) in high-pass filtered recordings. We have analyzed in-mine recordings in detail at one of the South African mines (Savuka) to (i) improve on reported hypocentral locations, (ii) verify sensor orientations, and (iii) determine full moment tensor solutions. Hypocentral relocations on all catalogued events have been obtained from P- and S-wave travel-times reported by the mine network operator through an automated procedure that selects travel-times falling on Wadati lines with slopes in the 0.6-0.7 range; sensor orientations have been verified and, when possible, corrected by correlating P-, SV-, and SH-waveforms obtained from theoretical and empirical (polarization filter) rotation angles; full moment tensor solutions have been obtained by inverting P-, SV-, and SH- spectral amplitudes measured on the theoretically rotated waveforms with visually assigned polarities. The relocation procedure has revealed that origin times often necessitate a negative correction of a few tenths of second and that hypocentral locations may move a few hundreds of meters. The full moment tensor determination has revealed that the most common focal mechanism (47 out of 82 solutions for events in the 0.2 < ML < 4.1 range) consists of a similar percentage of isotropic (implosive) and deviatoric components, with a normal fault-type best double couple. We have also calibrated the regional stations for seismic coda derived source spectra and moment magnitude using the envelope methodology of Mayeda et al (2003). We tie the coda Mw to independent values from waveform modeling. The resulting coda-based source spectra of shallow mining-related events show significant spectral peaking that is not seen in deeper tectonic earthquakes. This coda peaking may be an independent method of identifying shallow events and is similar to coda peaking previously observed for Nevada explosions, where the frequency of the observed spectral peak correlates with depth of burial (Murphy et al., 2008).« less

Estimating uncertainty of Full Waveform Inversion with Ensemble-based methods

NASA Astrophysics Data System (ADS)

Thurin, J.; Brossier, R.; Métivier, L.

2017-12-01

Uncertainty estimation is one key feature of tomographic applications for robust interpretation. However, this information is often missing in the frame of large scale linearized inversions, and only the results at convergence are shown, despite the ill-posed nature of the problem. This issue is common in the Full Waveform Inversion community.While few methodologies have already been proposed in the literature, standard FWI workflows do not include any systematic uncertainty quantifications methods yet, but often try to assess the result's quality through cross-comparison with other results from seismic or comparison with other geophysical data. With the development of large seismic networks/surveys, the increase in computational power and the more and more systematic application of FWI, it is crucial to tackle this problem and to propose robust and affordable workflows, in order to address the uncertainty quantification problem faced for near surface targets, crustal exploration, as well as regional and global scales.In this work (Thurin et al., 2017a,b), we propose an approach which takes advantage of the Ensemble Transform Kalman Filter (ETKF) proposed by Bishop et al., (2001), in order to estimate a low-rank approximation of the posterior covariance matrix of the FWI problem, allowing us to evaluate some uncertainty information of the solution. Instead of solving the FWI problem through a Bayesian inversion with the ETKF, we chose to combine a conventional FWI, based on local optimization, and the ETKF strategies. This scheme allows combining the efficiency of local optimization for solving large scale inverse problems and make the sampling of the local solution space possible thanks to its embarrassingly parallel property. References:Bishop, C. H., Etherton, B. J. and Majumdar, S. J., 2001. Adaptive sampling with the ensemble transform Kalman filter. Part I: Theoretical aspects. Monthly weather review, 129(3), 420-436.Thurin, J., Brossier, R. and Métivier, L. 2017,a.: Ensemble-Based Uncertainty Estimation in Full Waveform Inversion. 79th EAGE Conference and Exhibition 2017, (12 - 15 June, 2017)Thurin, J., Brossier, R. and Métivier, L. 2017,b.: An Ensemble-Transform Kalman Filter - Full Waveform Inversion scheme for Uncertainty estimation; SEG Technical Program Expanded Abstracts 2012

Viterbi sparse spike detection and a compositional origin to ultralow-velocity zones

NASA Astrophysics Data System (ADS)

Brown, Samuel Paul

Accurate interpretation of seismic travel times and amplitudes in both the exploration and global scales is complicated by the band-limited nature of seismic data. We present a stochastic method, Viterbi sparse spike detection (VSSD), to reduce a seismic waveform into a most probable constituent spike train. Model waveforms are constructed from a set of candidate spike trains convolved with a source wavelet estimate. For each model waveform, a profile hidden Markov model (HMM) is constructed to represent the waveform as a stochastic generative model with a linear topology corresponding to a sequence of samples. The Viterbi algorithm is employed to simultaneously find the optimal nonlinear alignment between a model waveform and the seismic data, and to assign a score to each candidate spike train. The most probable travel times and amplitudes are inferred from the alignments of the highest scoring models. Our analyses show that the method can resolve closely spaced arrivals below traditional resolution limits and that travel time estimates are robust in the presence of random noise and source wavelet errors. We applied the VSSD method to constrain the elastic properties of a ultralow- velocity zone (ULVZ) at the core-mantle boundary beneath the Coral Sea. We analyzed vertical component short period ScP waveforms for 16 earthquakes occurring in the Tonga-Fiji trench recorded at the Alice Springs Array (ASAR) in central Australia. These waveforms show strong pre and postcursory seismic arrivals consistent with ULVZ layering. We used the VSSD method to measure differential travel-times and amplitudes of the post-cursor arrival ScSP and the precursor arrival SPcP relative to ScP. We compare our measurements to a database of approximately 340,000 synthetic seismograms finding that these data are best fit by a ULVZ model with an S-wave velocity reduction of 24%, a P-wave velocity reduction of 23%, a thickness of 8.5 km, and a density increase of 6%. We simultaneously constrain both P- and S-wave velocity reductions as a 1:1 ratio inside this ULVZ. This 1:1 ratio is not consistent with a partial melt origin to ULVZs. Rather, we demonstrate that a compositional origin is more likely.

Picking vs Waveform based detection and location methods for induced seismicity monitoring

NASA Astrophysics Data System (ADS)

Grigoli, Francesco; Boese, Maren; Scarabello, Luca; Diehl, Tobias; Weber, Bernd; Wiemer, Stefan; Clinton, John F.

2017-04-01

Microseismic monitoring is a common operation in various industrial activities related to geo-resouces, such as oil and gas and mining operations or geothermal energy exploitation. In microseismic monitoring we generally deal with large datasets from dense monitoring networks that require robust automated analysis procedures. The seismic sequences being monitored are often characterized by very many events with short inter-event times that can even provide overlapped seismic signatures. In these situations, traditional approaches that identify seismic events using dense seismic networks based on detections, phase identification and event association can fail, leading to missed detections and/or reduced location resolution. In recent years, to improve the quality of automated catalogues, various waveform-based methods for the detection and location of microseismicity have been proposed. These methods exploit the coherence of the waveforms recorded at different stations and do not require any automated picking procedure. Although this family of methods have been applied to different induced seismicity datasets, an extensive comparison with sophisticated pick-based detection and location methods is still lacking. We aim here to perform a systematic comparison in term of performance using the waveform-based method LOKI and the pick-based detection and location methods (SCAUTOLOC and SCANLOC) implemented within the SeisComP3 software package. SCANLOC is a new detection and location method specifically designed for seismic monitoring at local scale. Although recent applications have proved an extensive test with induced seismicity datasets have been not yet performed. This method is based on a cluster search algorithm to associate detections to one or many potential earthquake sources. On the other hand, SCAUTOLOC is more a "conventional" method and is the basic tool for seismic event detection and location in SeisComp3. This approach was specifically designed for regional and teleseismic applications, thus its performance with microseismic data might be limited. We analyze the performance of the three methodologies for a synthetic dataset with realistic noise conditions as well as for the first hour of continuous waveform data, including the Ml 3.5 St. Gallen earthquake, recorded by a microseismic network deployed in the area. We finally compare the results obtained all these three methods with a manually revised catalogue.

Detailed seismic velocity of the incoming subducting sediments in the 2004 great Sumatra earthquake rupture zone from full waveform inversion of long offset seismic data

NASA Astrophysics Data System (ADS)

Qin, Yanfang; Singh, Satish C.

2017-04-01

The nature of incoming sediments defines the locking mechanism on the megathrust, and the development and evolution of the accretionary wedge. Here we present results from seismic full waveform inversion of 12 km long offset seismic reflection data within the trench in the 2004 Sumatra earthquake rupture zone area that provide detailed quantitative information on the incoming oceanic sediments and the trench-fill sediments. The thickness of sediments in this area is 3-4 km, and P wave velocity is as much as 4.5 km/s just above the oceanic crust, suggesting the presence of silica-rich highly compacted and lithified sediments leading to a strong coupling up to the subduction front. We also find an 70-80 m thick low-velocity layer, capped by a high-velocity layer, at 0.8 km above the subducting plate. This low-velocity layer, previously identified as high-amplitude negative polarity reflection, could have porosity of up to 30% containing overpressured fluids, which could act as a protodécollement seaward from the accretionary prism and décollement beneath the forearc. This weak protodécollement combined with the high-velocity indurated sediments above the basement possibly facilitated the rupture propagating up to the front during the 2004 earthquake and enhancing the tsunami. We also find another low-velocity layer within the sediments that may act as a secondary décollement observed offshore central Sumatra, forming bivergent pop-up structures and acting as a conveyer belt in preserving these pop-up structures in the forearc region.

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.

Seismology-based early identification of dam-formation landquake events.

PubMed

Chao, Wei-An; Zhao, Li; Chen, Su-Chin; Wu, Yih-Min; Chen, Chi-Hsuan; Huang, Hsin-Hua

2016-01-12

Flooding resulting from the bursting of dams formed by landquake events such as rock avalanches, landslides and debris flows can lead to serious bank erosion and inundation of populated areas near rivers. Seismic waves can be generated by landquake events which can be described as time-dependent forces (unloading/reloading cycles) acting on the Earth. In this study, we conduct inversions of long-period (LP, period ≥20 s) waveforms for the landquake force histories (LFHs) of ten events, which provide quantitative characterization of the initiation, propagation and termination stages of the slope failures. When the results obtained from LP waveforms are analyzed together with high-frequency (HF, 1-3 Hz) seismic signals, we find a relatively strong late-arriving seismic phase (dubbed Dam-forming phase or D-phase) recorded clearly in the HF waveforms at the closest stations, which potentially marks the time when the collapsed masses sliding into river and perhaps even impacting the topographic barrier on the opposite bank. Consequently, our approach to analyzing the LP and HF waveforms developed in this study has a high potential for identifying five dam-forming landquake events (DFLEs) in near real-time using broadband seismic records, which can provide timely warnings of the impending floods to downstream residents.

Fault zone reverberations from cross-correlations of earthquake waveforms and seismic noise

NASA Astrophysics Data System (ADS)

Hillers, Gregor; Campillo, Michel

2016-03-01

Seismic wavefields interact with low-velocity fault damage zones. Waveforms of ballistic fault zone head waves, trapped waves, reflected waves and signatures of trapped noise can provide important information on structural and mechanical fault zone properties. Here we extend the class of observable fault zone waves and reconstruct in-fault reverberations or multiples in a strike-slip faulting environment. Manifestations of the reverberations are significant, consistent wave fronts in the coda of cross-correlation functions that are obtained from scattered earthquake waveforms and seismic noise recorded by a linear fault zone array. The physical reconstruction of Green's functions is evident from the high similarity between the signals obtained from the two different scattered wavefields. Modal partitioning of the reverberation wavefield can be tuned using different data normalization techniques. The results imply that fault zones create their own ambiance, and that the here reconstructed reverberations are a key seismic signature of wear zones. Using synthetic waveform modelling we show that reverberations can be used for the imaging of structural units by estimating the location, extend and magnitude of lateral velocity contrasts. The robust reconstruction of the reverberations from noise records suggests the possibility to resolve the response of the damage zone material to various external and internal loading mechanisms.

Full waveform seismic modelling of Chalk Group rocks from the Danish North Sea - implications for velocity analysis

NASA Astrophysics Data System (ADS)

Montazeri, Mahboubeh; Moreau, Julien; Uldall, Anette; Nielsen, Lars

2015-04-01

This study aims at understanding seismic wave propagation in the fine-layered Chalk Group, which constitutes the main reservoir for oil and gas production in the Danish North Sea. The starting point of our analysis is the Nana-1XP exploration well, which shows strong seismic contrasts inside the Chalk Group. For the purposes of seismic waveform modelling, we here assume a one-dimensional model with homogeneous and isotropic layers designed to capture the main fluctuations in petrophysical properties observed in the well logs. The model is representative of the stratigraphic sequences of the area and it illustrates highly contrasting properties of the Chalk Group. Finite-difference (FD) full wave technique, both acoustic and elastic equations are applied to the model. Velocity analysis of seismic data is a crucial step for stacking, multiple suppression, migration, and depth conversion of the seismic record. Semblance analysis of the synthetic seismic records shows strong amplitude peaks outside the expected range for the time interval representing the Chalk Group, especially at the base. The various synthetic results illustrate the occurrence and the impact of different types of waves including multiples, converted waves and refracted waves. The interference of these different wave types with the primary reflections can explain the strong anomalous amplitudes in the semblance plot. In particular, the effect of strongly contrasting thin beds plays an important role in the generation of the high anomalous amplitude values. If these anomalous amplitudes are used to pick the velocities, it would impede proper stacking of the data and may result in sub-optimal migration and depth conversion. Consequently this may lead to erroneous or sub-optimal seismic images of the Chalk Group and the underlying layers. Our results highlight the importance of detailed velocity analysis and proper picking of velocity functions in the Chalk Group intervals. We show that application of standard front mutes in the mid- and far-offset ranges does not significantly improve the results of the standard semblance analysis. These synthetic modelling results could be used as starting points for defining optimized processing flows for the seismic data sets acquired in the study area with the aim of improving the imaging of the Chalk Group.

Seismic Wavefield Imaging of Long-Period Ground Motion in the Tokyo Metropolitan Area, Japan

NASA Astrophysics Data System (ADS)

Nagao, H.; Kano, M.; Nagata, K.; Ito, S. I.; Sakai, S.; Nakagawa, S.; Hori, M.; Hirata, N.

2017-12-01

Long-period ground motions due to large earthquakes can cause devastating disasters, especially in urbanized areas located on sedimentary basins. To assess and mitigate such damage, it is essential to rapidly evaluate seismic hazards for infrastructures, which can be simulated by seismic response analyses that use waveforms at the base of each infrastructure as an input ground motion. The present study reconstructs the seismic wavefield in the Tokyo metropolitan area located on the Kanto sedimentary basin, Japan, from seismograms of the Metropolitan Seismic Observation network (MeSO-net). The obtained wavefield fully explains the observed waveforms in the frequency band of 0.10-0.20 Hz. This is attributed to the seismic wavefield imaging technique proposed by Kano et al. (2017), which implements the replica exchange Monte Carlo method to simultaneously estimate model parameters related to the subsurface structure and source information. Further investigation shows that the reconstructed seismic wavefield lower than 0.30 Hz is of high quality in terms of variance reduction (VR), which quantifies a misfit in waveforms but that the VR rapidly worsens in higher frequencies. Meanwhile, the velocity response spectra show good agreement with observations up to 0.90 Hz in terms of the combined goodness of fit (CGOF), which is a measure of misfit in the velocity response spectra. Inputting the reconstructed wavefield into seismic response analyses, we can rapidly assess the overall damage to infrastructures immediately after a large earthquake.

Source-independent full waveform inversion of seismic data

DOEpatents

Lee, Ki Ha

2006-02-14

A set of seismic trace data is collected in an input data set that is first Fourier transformed in its entirety into the frequency domain. A normalized wavefield is obtained for each trace of the input data set in the frequency domain. Normalization is done with respect to the frequency response of a reference trace selected from the set of seismic trace data. The normalized wavefield is source independent, complex, and dimensionless. The normalized wavefield is shown to be uniquely defined as the normalized impulse response, provided that a certain condition is met for the source. This property allows construction of the inversion algorithm disclosed herein, without any source or source coupling information. The algorithm minimizes the error between data normalized wavefield and the model normalized wavefield. The methodology is applicable to any 3-D seismic problem, and damping may be easily included in the process.

Rupture Dynamics and Seismic Radiation on Rough Faults for Simulation-Based PSHA

NASA Astrophysics Data System (ADS)

Mai, P. M.; Galis, M.; Thingbaijam, K. K. S.; Vyas, J. C.; Dunham, E. M.

2017-12-01

Simulation-based ground-motion predictions may augment PSHA studies in data-poor regions or provide additional shaking estimations, incl. seismic waveforms, for critical facilities. Validation and calibration of such simulation approaches, based on observations and GMPE's, is important for engineering applications, while seismologists push to include the precise physics of the earthquake rupture process and seismic wave propagation in 3D heterogeneous Earth. Geological faults comprise both large-scale segmentation and small-scale roughness that determine the dynamics of the earthquake rupture process and its radiated seismic wavefield. We investigate how different parameterizations of fractal fault roughness affect the rupture evolution and resulting near-fault ground motions. Rupture incoherence induced by fault roughness generates realistic ω-2 decay for high-frequency displacement amplitude spectra. Waveform characteristics and GMPE-based comparisons corroborate that these rough-fault rupture simulations generate realistic synthetic seismogram for subsequent engineering application. Since dynamic rupture simulations are computationally expensive, we develop kinematic approximations that emulate the observed dynamics. Simplifying the rough-fault geometry, we find that perturbations in local moment tensor orientation are important, while perturbations in local source location are not. Thus, a planar fault can be assumed if the local strike, dip, and rake are maintained. The dynamic rake angle variations are anti-correlated with local dip angles. Based on a dynamically consistent Yoffe source-time function, we show that the seismic wavefield of the approximated kinematic rupture well reproduces the seismic radiation of the full dynamic source process. Our findings provide an innovative pseudo-dynamic source characterization that captures fault roughness effects on rupture dynamics. Including the correlations between kinematic source parameters, we present a new pseudo-dynamic rupture modeling approach for computing broadband ground-motion time-histories for simulation-based PSHA

Characteristics of Induced and Tectonic Seismicity in Oklahoma Based on High-precision Earthquake Relocations and Focal mechanisms

NASA Astrophysics Data System (ADS)

Aziz Zanjani, F.; Lin, G.

2016-12-01

Seismic activity in Oklahoma has greatly increased since 2013, when the number of wastewater disposal wells associated with oil and gas production was significantly increased in the area. An M5.8 earthquake at about 5 km depth struck near Pawnee, Oklahoma on September 3, 2016. This earthquake is postulated to be related with the anthropogenic activity in Oklahoma. In this study, we investigate the seismic characteristics in Oklahoma by using high-precision earthquake relocations and focal mechanisms. We acquire the seismic data between January 2013 and October 2016 recorded by the local and regional (within 200 km distance from the Pawnee mainshock) seismic stations from the Incorporated Research Institutions for Seismology (IRIS). We relocate all the earthquakes by applying the source-specific station term method and a differential time relocation method based on waveform cross-correlation data. The high-precision earthquake relocation catalog is then used to perform full-waveform modeling. We use Muller's reflection method for Green's function construction and the mtinvers program for moment tensor inversion. The sensitivity of the solution to the station and component distribution is evaluated by carrying out the Jackknife resampling. These earthquake relocation and focal mechanism results will help constrain the fault orientation and the earthquake rupture length. In order to examine the static Coulomb stress change due to the 2016 Pawnee earthquake, we utilize the Coulomb 3 software in the vicinity of the mainshock and compare the aftershock pattern with the calculated stress variation. The stress change in the study area can be translated into probability of seismic failure on other parts of the designated fault.

Seismic source inversion using Green's reciprocity and a 3-D structural model for the Japanese Islands

NASA Astrophysics Data System (ADS)

Simutė, S.; Fichtner, A.

2015-12-01

We present a feasibility study for seismic source inversions using a 3-D velocity model for the Japanese Islands. The approach involves numerically calculating 3-D Green's tensors, which is made efficient by exploiting Green's reciprocity. The rationale for 3-D seismic source inversion has several aspects. For structurally complex regions, such as the Japan area, it is necessary to account for 3-D Earth heterogeneities to prevent unknown structure polluting source solutions. In addition, earthquake source characterisation can serve as a means to delineate existing faults. Source parameters obtained for more realistic Earth models can then facilitate improvements in seismic tomography and early warning systems, which are particularly important for seismically active areas, such as Japan. We have created a database of numerically computed 3-D Green's reciprocals for a 40°× 40°× 600 km size area around the Japanese Archipelago for >150 broadband stations. For this we used a regional 3-D velocity model, recently obtained from full waveform inversion. The model includes attenuation and radial anisotropy and explains seismic waveform data for periods between 10 - 80 s generally well. The aim is to perform source inversions using the database of 3-D Green's tensors. As preliminary steps, we present initial concepts to address issues that are at the basis of our approach. We first investigate to which extent Green's reciprocity works in a discrete domain. Considering substantial amounts of computed Green's tensors we address storage requirements and file formatting. We discuss the importance of the initial source model, as an intelligent choice can substantially reduce the search volume. Possibilities to perform a Bayesian inversion and ways to move to finite source inversion are also explored.

Resolvability of regional density structure

NASA Astrophysics Data System (ADS)

Plonka, A.; Fichtner, A.

2016-12-01

Lateral density variations are the source of mass transport in the Earth at all scales, acting as drivers of convectivemotion. However, the density structure of the Earth remains largely unknown since classic seismic observables and gravityprovide only weak constraints with strong trade-offs. Current density models are therefore often based on velocity scaling,making strong assumptions on the origin of structural heterogeneities, which may not necessarily be correct. Our goal is to assessif 3D density structure may be resolvable with emerging full-waveform inversion techniques. We have previously quantified the impact of regional-scale crustal density structure on seismic waveforms with the conclusion that reasonably sized density variations within thecrust can leave a strong imprint on both travel times and amplitudes, and, while this can produce significant biases in velocity and Q estimates, the seismic waveform inversion for density may become feasible. In this study we performprincipal component analyses of sensitivity kernels for P velocity, S velocity, and density. This is intended to establish theextent to which these kernels are linearly independent, i.e. the extent to which the different parameters may be constrainedindependently. Since the density imprint we observe is not exclusively linked to travel times and amplitudes of specific phases,we consider waveform differences between complete seismograms. We test the method using a known smooth model of the crust and seismograms with clear Love and Rayleigh waves, showing that - as expected - the first principal kernel maximizes sensitivity to SH and SV velocity structure, respectively, and that the leakage between S velocity, P velocity and density parameter spaces is minimal in the chosen setup. Next, we apply the method to data from 81 events around the Iberian Penninsula, registered in total by 492 stations. The objective is to find a principal kernel which would maximize the sensitivity to density, potentially allowing for independent density resolution, and, as the final goal, for direct density inversion.

Seismic waveform inversion using neural networks

NASA Astrophysics Data System (ADS)

De Wit, R. W.; Trampert, J.

2012-12-01

Full waveform tomography aims to extract all available information on Earth structure and seismic sources from seismograms. The strongly non-linear nature of this inverse problem is often addressed through simplifying assumptions for the physical theory or data selection, thus potentially neglecting valuable information. Furthermore, the assessment of the quality of the inferred model is often lacking. This calls for the development of methods that fully appreciate the non-linear nature of the inverse problem, whilst providing a quantification of the uncertainties in the final model. We propose to invert seismic waveforms in a fully non-linear way by using artificial neural networks. Neural networks can be viewed as powerful and flexible non-linear filters. They are very common in speech, handwriting and pattern recognition. Mixture Density Networks (MDN) allow us to obtain marginal posterior probability density functions (pdfs) of all model parameters, conditioned on the data. An MDN can approximate an arbitrary conditional pdf as a linear combination of Gaussian kernels. Seismograms serve as input, Earth structure parameters are the so-called targets and network training aims to learn the relationship between input and targets. The network is trained on a large synthetic data set, which we construct by drawing many random Earth models from a prior model pdf and solving the forward problem for each of these models, thus generating synthetic seismograms. As a first step, we aim to construct a 1D Earth model. Training sets are constructed using the Mineos package, which computes synthetic seismograms in a spherically symmetric non-rotating Earth by summing normal modes. We train a network on the body waveforms present in these seismograms. Once the network has been trained, it can be presented with new unseen input data, in our case the body waves in real seismograms. We thus obtain the posterior pdf which represents our final state of knowledge given the information in the training set and the real data.

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

Carmichael, Joshua D.; Hartse, Hans

Colocated explosive sources often produce correlated seismic waveforms. Multichannel correlation detectors identify these signals by scanning template waveforms recorded from known reference events against "target" data to find similar waveforms. This screening problem is challenged at thresholds required to monitor smaller explosions, often because non-target signals falsely trigger such detectors. Therefore, it is generally unclear what thresholds will reliably identify a target explosion while screening non-target background seismicity. Here, we estimate threshold magnitudes for hypothetical explosions located at the North Korean nuclear test site over six months of 2010, by processing International Monitoring System (IMS) array data with a multichannelmore » waveform correlation detector. Our method (1) accounts for low amplitude background seismicity that falsely triggers correlation detectors but is unidentifiable with conventional power beams, (2) adapts to diurnally variable noise levels and (3) uses source-receiver reciprocity concepts to estimate thresholds for explosions spatially separated from the template source. Furthermore, we find that underground explosions with body wave magnitudes m b = 1.66 are detectable at the IMS array USRK with probability 0.99, when using template waveforms consisting only of P -waves, without false alarms. We conservatively find that these thresholds also increase by up to a magnitude unit for sources located 4 km or more from the Feb.12, 2013 announced nuclear test.« less

Efficient blind search for similar-waveform earthquakes in years of continuous seismic data

NASA Astrophysics Data System (ADS)

Yoon, C. E.; Bergen, K.; Rong, K.; Elezabi, H.; Bailis, P.; Levis, P.; Beroza, G. C.

2017-12-01

Cross-correlating an earthquake waveform template with continuous seismic data has proven to be a sensitive, discriminating detector of small events missing from earthquake catalogs, but a key limitation of this approach is that it requires advance knowledge of the earthquake signals we wish to detect. To overcome this limitation, we can perform a blind search for events with similar waveforms, comparing waveforms from all possible times within the continuous data (Brown et al., 2008). However, the runtime for naive blind search scales quadratically with the duration of continuous data, making it impractical to process years of continuous data. The Fingerprint And Similarity Thresholding (FAST) detection method (Yoon et al., 2015) enables a comprehensive blind search for similar-waveform earthquakes in a fast, scalable manner by adapting data-mining techniques originally developed for audio and image search within massive databases. FAST converts seismic waveforms into compact "fingerprints", which are efficiently organized and searched within a database. In this way, FAST avoids the unnecessary comparison of dissimilar waveforms. To date, the longest duration of continuous data used for event detection with FAST was 3 months at a single station near Guy-Greenbrier, Arkansas, which revealed microearthquakes closely correlated with stages of hydraulic fracturing (Yoon et al., 2017). In this presentation we introduce an optimized, parallel version of the FAST software with improvements to the fingerprinting algorithm and the ability to detect events using continuous data from a network of stations (Bergen et al., 2016). We demonstrate its ability to detect low-magnitude earthquakes within several years of continuous data at locations of interest in California.

Hydraulic Fracturing Induced Seismicity at Preese Hall, UK: Moment Tensors, Uncertainties and Implications for Microseismic Monitoring Strategies

NASA Astrophysics Data System (ADS)

O'toole, T. B.; Woodhouse, J. H.; Verdon, J.; Kendall, J.

2012-12-01

Hydraulic fracturing operations carried out in April and May 2011 by Cuadrilla Resources Ltd. during the exploration of a shale gas reservoir at Preese Hall, near Blackpool, UK, induced a series of microseismic events. The largest of these, with magnitude ML = 2.3, was felt at the surface and recorded by the British Geological Survey regional seismic network. Subsequently, two local seismic stations were installed, which continued to detect seismicity with ML ≤ 1.5 until the hydraulic fracture treatment was suspended due to the anomalously large magnitudes of the induced earthquakes. Here, we present the results of moment tensor inversions of seismic waveforms recorded by these two near-field stations. We determine the best point source description of an event by minimising the least-squares difference between observed and synthetic waveforms. In contrast to source mechanisms obtained from body wave polarity and amplitude picks, which require a good sampling of the focal sphere and typically assume a pure double-couple mechanism, using the whole waveform allows us to place good constraints on the moment tensor even when only a few seismograms are available, and also enables the investigation of possible non-double-couple components and volume changes associated with a source. We discuss our results in the context of the studies commissioned by Cuadrilla after the suspension of hydraulic fracturing operations at Preese Hall. Using synthetic waveform data, we investigate how different monitoring geometries can be used to reduce uncertainties in source parameters of induced microseisms. While our focus is on the monitoring of hydraulic fracturing operations, the methods developed here are general and could equally be applied to determine moment tensors from surface and borehole observations of seismicity induced by other activities.

Complete Moment Tensor Determination of Induced Seismicity in Unconventional and Conventional Oil/Gas Fields

NASA Astrophysics Data System (ADS)

Gu, C.; Li, J.; Toksoz, M. N.

2013-12-01

Induced seismicity occurs both in conventional oil/gas fields due to production and water injection and in unconventional oil/gas fields due to hydraulic fracturing. Source mechanisms of these induced earthquakes are of great importance for understanding their causes and the physics of the seismic processes in reservoirs. Previous research on the analysis of induced seismic events in conventional oil/gas fields assumed a double couple (DC) source mechanism. However, recent studies have shown a non-negligible percentage of a non-double-couple (non-DC) component of source moment tensor in hydraulic fracturing events (Šílený et al., 2009; Warpinski and Du, 2010; Song and Toksöz, 2011). In this study, we determine the full moment tensor of the induced seismicity data in a conventional oil/gas field and for hydrofrac events in an unconventional oil/gas field. Song and Toksöz (2011) developed a full waveform based complete moment tensor inversion method to investigate a non-DC source mechanism. We apply this approach to the induced seismicity data from a conventional gas field in Oman. In addition, this approach is also applied to hydrofrac microseismicity data monitored by downhole geophones in four wells in US. We compare the source mechanisms of induced seismicity in the two different types of gas fields and explain the differences in terms of physical processes.

Full waveform approach for the automatic detection and location of acoustic emissions from hydraulic fracturing at Äspö (Sweden)

NASA Astrophysics Data System (ADS)

Ángel López Comino, José; Cesca, Simone; Heimann, Sebastian; Grigoli, Francesco; Milkereit, Claus; Dahm, Torsten; Zang, Arno

2017-04-01

A crucial issue to analyse the induced seismicity for hydraulic fracturing is the detection and location of massive microseismic or acoustic emissions (AE) activity, with robust and sufficiently accurate automatic algorithms. Waveform stacking and coherence analysis have been tested for local seismic monitoring and mining induced seismicity improving the classical detection and location methods (e.g. short-term-average/long-term-average and automatic picking of the P and S waves first arrivals). These techniques are here applied using a full waveform approach for a hydraulic fracturing experiment (Nova project 54-14-1) that took place 410 m below surface in the Äspö Hard Rock Laboratory (Sweden). Continuous waveform recording with a near field network composed by eleven AE sensors are processed. The piezoelectric sensors have their highest sensitive in the frequency range 1 to 100 kHz, but sampling rates were extended to 1 MHz. We present the results obtained during the conventional, continuous water-injection experiment HF2 (Hydraulic Fracture 2). The event detector is based on the stacking of characteristic functions. It follows a delay-and-stack approach, where the likelihood of the hypocenter location in a pre-selected seismogenic volume is mapped by assessing the coherence of the P onset times at different stations. A low detector threshold is chosen, in order not to loose weaker events. This approach also increases the number of false detections. Therefore, the dataset has been revised manually, and detected events classified in terms of true AE events related to the fracturing process, electronic noise related to 50 Hz overtones, long period and other signals. The location of the AE events is further refined using a more accurate waveform stacking method which uses both P and S phases. A 3D grid is generated around the hydraulic fracturing volume and we retrieve a multidimensional matrix, whose absolute maximum corresponds to the spatial coordinates of the seismic event. The relative location accuracy is improved using a master event approach to correct for travel time perturbations. The master event is selected based on a good signal to noise ratio leading to a robust location with small uncertainties. Relative magnitudes are finally estimated upon the decay of the maximal recorded amplitude from the AE location. The resulting catalogue is composed of more than 4000 AEs. Their hypocenters are spatially clustered in a planar region, resembling the main fracture plane; its orientation and size are estimated from the spatial distribution of AEs. This work is funded by the EU H2020 SHEER project. Nova project 54-14-1 was financially supported by the GFZ German Research Center for Geosciences (75%), the KIT Karlsruhe Institute of Technology (15%) and the Nova Center for University Studies, Research and Development (10%). An additional in-kind contribution of SKB for using Äspö Hard Rock Laboratory as test site for geothermal research is greatly acknowledged.

Approximate solutions of acoustic 3D integral equation and their application to seismic modeling and full-waveform inversion

NASA Astrophysics Data System (ADS)

Malovichko, M.; Khokhlov, N.; Yavich, N.; Zhdanov, M.

2017-10-01

Over the recent decades, a number of fast approximate solutions of Lippmann-Schwinger equation, which are more accurate than classic Born and Rytov approximations, were proposed in the field of electromagnetic modeling. Those developments could be naturally extended to acoustic and elastic fields; however, until recently, they were almost unknown in seismology. This paper presents several solutions of this kind applied to acoustic modeling for both lossy and lossless media. We evaluated the numerical merits of those methods and provide an estimation of their numerical complexity. In our numerical realization we use the matrix-free implementation of the corresponding integral operator. We study the accuracy of those approximate solutions and demonstrate, that the quasi-analytical approximation is more accurate, than the Born approximation. Further, we apply the quasi-analytical approximation to the solution of the inverse problem. It is demonstrated that, this approach improves the estimation of the data gradient, comparing to the Born approximation. The developed inversion algorithm is based on the conjugate-gradient type optimization. Numerical model study demonstrates that the quasi-analytical solution significantly reduces computation time of the seismic full-waveform inversion. We also show how the quasi-analytical approximation can be extended to the case of elastic wavefield.

Low frequency full waveform seismic inversion within a tree based Bayesian framework

NASA Astrophysics Data System (ADS)

Ray, Anandaroop; Kaplan, Sam; Washbourne, John; Albertin, Uwe

2018-01-01

Limited illumination, insufficient offset, noisy data and poor starting models can pose challenges for seismic full waveform inversion. We present an application of a tree based Bayesian inversion scheme which attempts to mitigate these problems by accounting for data uncertainty while using a mildly informative prior about subsurface structure. We sample the resulting posterior model distribution of compressional velocity using a trans-dimensional (trans-D) or Reversible Jump Markov chain Monte Carlo method in the wavelet transform domain of velocity. This allows us to attain rapid convergence to a stationary distribution of posterior models while requiring a limited number of wavelet coefficients to define a sampled model. Two synthetic, low frequency, noisy data examples are provided. The first example is a simple reflection + transmission inverse problem, and the second uses a scaled version of the Marmousi velocity model, dominated by reflections. Both examples are initially started from a semi-infinite half-space with incorrect background velocity. We find that the trans-D tree based approach together with parallel tempering for navigating rugged likelihood (i.e. misfit) topography provides a promising, easily generalized method for solving large-scale geophysical inverse problems which are difficult to optimize, but where the true model contains a hierarchy of features at multiple scales.

Estimation of full moment tensors, including uncertainties, for earthquakes, volcanic events, and nuclear explosions

NASA Astrophysics Data System (ADS)

Alvizuri, Celso R.

We present a catalog of full seismic moment tensors for 63 events from Uturuncu volcano in Bolivia. The events were recorded during 2011-2012 in the PLUTONS seismic array of 24 broadband stations. Most events had magnitudes between 0.5 and 2.0 and did not generate discernible surface waves; the largest event was Mw 2.8. For each event we computed the misfit between observed and synthetic waveforms, and we used first-motion polarity measurements to reduce the number of possible solutions. Each moment tensor solution was obtained using a grid search over the six-dimensional space of moment tensors. For each event we show the misfit function in eigenvalue space, represented by a lune. We identify three subsets of the catalog: (1) 6 isotropic events, (2) 5 tensional crack events, and (3) a swarm of 14 events southeast of the volcanic center that appear to be double couples. The occurrence of positively isotropic events is consistent with other published results from volcanic and geothermal regions. Several of these previous results, as well as our results, cannot be interpreted within the context of either an oblique opening crack or a crack-plus-double-couple model. Proper characterization of uncertainties for full moment tensors is critical for distinguishing among physical models of source processes. A seismic moment tensor is a 3x3 symmetric matrix that provides a compact representation of a seismic source. We develop an algorithm to estimate moment tensors and their uncertainties from observed seismic data. For a given event, the algorithm performs a grid search over the six-dimensional space of moment tensors by generating synthetic waveforms for each moment tensor and then evaluating a misfit function between the observed and synthetic waveforms. 'The' moment tensor M0 for the event is then the moment tensor with minimum misfit. To describe the uncertainty associated with M0, we first convert the misfit function to a probability function. The uncertainty, or rather the confidence, is then given by the 'confidence curve' P( V), where P(V) is the probability that the true moment tensor for the event lies within the neighborhood of M that has fractional volume V. The area under the confidence curve provides a single, abbreviated 'confidence parameter' for M0. We apply the method to data from events in different regions and tectonic settings: 63 small (M w 4) earthquakes in the southern Alaska subduction zone, and 12 earthquakes and 17 nuclear explosions at the Nevada Test Site. Characterization of moment tensor uncertainties puts us in better position to discriminate among moment tensor source types and to assign physical processes to the events.

Static Corrections to Improve Seismic Monitoring of the North Korean Nuclear Test Site with Regional Arrays

NASA Astrophysics Data System (ADS)

Wilkins, N.; Wookey, J. M.; Selby, N. D.

2017-12-01

Seismology is an important part of the International Monitoring System (IMS) installed to detect, identify, and locate nuclear detonations in breach of the Comprehensive nuclear Test Ban Treaty (CTBT) prior to and after its entry into force. Seismic arrays in particular provide not only a means of detecting and locating underground nuclear explosions, but in discriminating them from naturally occurring earthquakes of similar magnitude. One potential discriminant is the amplitude ratio of high frequency (> 2 Hz) P waves to S waves (P/S) measured at regional distances (3 - 17 °). Accurate measurement of such discriminants, and the ability to detect low-magnitude seismicity from a suspicious event relies on high signal-to-noise ratio (SNR) data. A correction to the slowness vector of the incident seismic wavefield, and static corrections applied to the waveforms recorded at each receiver within the array can be shown to improve the SNR. We apply codes we have developed to calculate slowness-azimuth station corrections (SASCs) and static corrections to the arrival time and amplitude of the seismic waveform to seismic arrays regional to the DPRK nuclear test site at Punggye-ri, North Korea. We use the F-statistic to demonstrate the SNR improvement to data from the nuclear tests and other seismic events in the vicinity of the test site. We also make new measurements of P/S with the corrected waveforms and compare these with existing measurements.

Lower Crstal Reflectity bands and Magma Emplacement in Norweigian sea, NE Atlantic

NASA Astrophysics Data System (ADS)

Rai, A.; Breivik, A. J.; Mjelde, R.

2013-12-01

In this study we present the OBS data collected along seismic profiles in the norweigian sea. The traveltime modelling of the OBS data provides first-hand information about seismic structure of the subsurface. However, waveform modelling is used to further constrain the fine scale structure, velocity constrast and velocity gradients. By forward modelling and inversion of the seismic waveforms, we show that the multiple bands of reflectivity could be due to multiple episodes of magma emplacements that might have frozen in the form of sills. These mafic intrusions probably intruded into the ductile lower crust during the main rifting phase of Europe and Greenland.

Seismic moment tensor inversion using 3D velocity model and its application to the 2013 Lushan earthquake sequence

NASA Astrophysics Data System (ADS)

Zhu, Lupei; Zhou, Xiaofeng

2016-10-01

Source inversion of small-magnitude events such as aftershocks or mine collapses requires use of relatively high frequency seismic waveforms which are strongly affected by small-scale heterogeneities in the crust. In this study, we developed a new inversion method called gCAP3D for determining general moment tensor of a seismic source using Green's functions of 3D models. It inherits the advantageous features of the ;Cut-and-Paste; (CAP) method to break a full seismogram into the Pnl and surface-wave segments and to allow time shift between observed and predicted waveforms. It uses grid search for 5 source parameters (relative strengths of the isotropic and compensated-linear-vector-dipole components and the strike, dip, and rake of the double-couple component) that minimize the waveform misfit. The scalar moment is estimated using the ratio of L2 norms of the data and synthetics. Focal depth can also be determined by repeating the inversion at different depths. We applied gCAP3D to the 2013 Ms 7.0 Lushan earthquake and its aftershocks using a 3D crustal-upper mantle velocity model derived from ambient noise tomography in the region. We first relocated the events using the double-difference method. We then used the finite-differences method and reciprocity principle to calculate Green's functions of the 3D model for 20 permanent broadband seismic stations within 200 km from the source region. We obtained moment tensors of the mainshock and 74 aftershocks ranging from Mw 5.2 to 3.4. The results show that the Lushan earthquake is a reverse faulting at a depth of 13-15 km on a plane dipping 40-47° to N46° W. Most of the aftershocks occurred off the main rupture plane and have similar focal mechanisms to the mainshock's, except in the proximity of the mainshock where the aftershocks' focal mechanisms display some variations.

A Comparison of Earthquake Back-Projection Imaging Methods for Dense Local Arrays, and Application to the 2011 Virginia Aftershock Sequence

NASA Astrophysics Data System (ADS)

Beskardes, G. D.; Hole, J. A.; Wang, K.; Wu, Q.; Chapman, M. C.; Davenport, K. K.; Michaelides, M.; Brown, L. D.; Quiros, D. A.

2016-12-01

Back-projection imaging has recently become a practical method for local earthquake detection and location due to the deployment of densely sampled, continuously recorded, local seismograph arrays. Back-projection is scalable to earthquakes with a wide range of magnitudes from very tiny to very large. Local dense arrays provide the opportunity to capture very tiny events for a range applications, such as tectonic microseismicity, source scaling studies, wastewater injection-induced seismicity, hydraulic fracturing, CO2 injection monitoring, volcano studies, and mining safety. While back-projection sometimes utilizes the full seismic waveform, the waveforms are often pre-processed to overcome imaging issues. We compare the performance of back-projection using four previously used data pre-processing methods: full waveform, envelope, short-term averaging / long-term averaging (STA/LTA), and kurtosis. The goal is to identify an optimized strategy for an entirely automated imaging process that is robust in the presence of real-data issues, has the lowest signal-to-noise thresholds for detection and for location, has the best spatial resolution of the energy imaged at the source, preserves magnitude information, and considers computational cost. Real data issues include aliased station spacing, low signal-to-noise ratio (to <1), large noise bursts and spatially varying waveform polarity. For evaluation, the four imaging methods were applied to the aftershock sequence of the 2011 Virginia earthquake as recorded by the AIDA array with 200-400 m station spacing. These data include earthquake magnitudes from -2 to 3 with highly variable signal to noise, spatially aliased noise, and large noise bursts: realistic issues in many environments. Each of the four back-projection methods has advantages and disadvantages, and a combined multi-pass method achieves the best of all criteria. Preliminary imaging results from the 2011 Virginia dataset will be presented.

Effects of volcano topography on seismic broad-band waveforms

NASA Astrophysics Data System (ADS)

Neuberg, Jürgen; Pointer, Tim

2000-10-01

Volcano seismology often deals with rather shallow seismic sources and seismic stations deployed in their near field. The complex stratigraphy on volcanoes and near-field source effects have a strong impact on the seismic wavefield, complicating the interpretation techniques that are usually employed in earthquake seismology. In addition, as most volcanoes have a pronounced topography, the interference of the seismic wavefield with the stress-free surface results in severe waveform perturbations that affect seismic interpretation methods. In this study we deal predominantly with the surface effects, but take into account the impact of a typical volcano stratigraphy as well as near-field source effects. We derive a correction term for plane seismic waves and a plane-free surface such that for smooth topographies the effect of the free surface can be totally removed. Seismo-volcanic sources radiate energy in a broad frequency range with a correspondingly wide range of different Fresnel zones. A 2-D boundary element method is employed to study how the size of the Fresnel zone is dependent on source depth, dominant wavelength and topography in order to estimate the limits of the plane wave approximation. This approximation remains valid if the dominant wavelength does not exceed twice the source depth. Further aspects of this study concern particle motion analysis to locate point sources and the influence of the stratigraphy on particle motions. Furthermore, the deployment strategy of seismic instruments on volcanoes, as well as the direct interpretation of the broad-band waveforms in terms of pressure fluctuations in the volcanic plumbing system, are discussed.

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.

Characterizing Micro- and Macro-Scale Seismicity from Bayou Corne, Louisiana

NASA Astrophysics Data System (ADS)

Baig, A. M.; Urbancic, T.; Karimi, S.

2013-12-01

The initiation of felt seismicity in Bayou Corne, Louisiana, coupled with other phenomena detected by residents on the nearby housing development, prompted a call to install a broadband seismic network to monitor subsurface deformation. The initial deployment was in place to characterize the deformation contemporaneous with the formation of a sinkhole located in close proximity to a salt dome. Seismic events generated during this period followed a swarm-like behaviour with moment magnitudes culminating around Mw2.5. However, the seismic data recorded during this sequence suffer from poor signal to noise, onsets that are very difficult to pick, and the presence of a significant amount of energy arriving later in the waveforms. Efforts to understand the complexity in these waveforms are ongoing, and involve invoking the complexities inherent in recording in a highly attenuating swamp overlying a complex three-dimensional structure with the strong material property contrast of the salt dome. In order to understand the event character, as well as to locally lower the completeness threshold of the sequence, a downhole array of 15 Hz sensors was deployed in a newly drilled well around the salt dome. Although the deployment lasted a little over a month in duration, over 1000 events were detected down to moment magnitude -Mw3. Waveform quality tended to be excellent, with very distinct P and S wave arrivals observable across the array for most events. The highest magnitude events were seen as well on the surface network and allowed for the opportunity to observe the complexities introduced by the site effects, while overcoming the saturation effects on the higher-frequency downhole geophones. This hybrid downhole and surface array illustrates how a full picture of subsurface deformation is only made possible by combining the high-frequency downhole instrumentation to see the microseismicity complemented with a broadband array to accurately characterize the source parameters for the larger magnitude events. Our presentation is focused on investigating this deformation, characterizing the scaling behaviour and the other source processes by taking advantage of the wide-band afforded to us through the deployment.

Seismological investigation of earthquakes in the New Madrid Seismic Zone. Final report, September 1986--December 1992

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

Herrmann, R.B.; Nguyen, B.

Earthquake activity in the New Madrid Seismic Zone had been monitored by regional seismic networks since 1975. During this time period, over 3,700 earthquakes have been located within the region bounded by latitudes 35{degrees}--39{degrees}N and longitudes 87{degrees}--92{degrees}W. Most of these earthquakes occur within a 1.5{degrees} x 2{degrees} zone centered on the Missouri Bootheel. Source parameters of larger earthquakes in the zone and in eastern North America are determined using surface-wave spectral amplitudes and broadband waveforms for the purpose of determining the focal mechanism, source depth and seismic moment. Waveform modeling of broadband data is shown to be a powerful toolmore » in defining these source parameters when used complementary with regional seismic network data, and in addition, in verifying the correctness of previously published focal mechanism solutions.« less

Sparse decomposition of seismic data and migration using Gaussian beams with nonzero initial curvature

NASA Astrophysics Data System (ADS)

Liu, Peng; Wang, Yanfei

2018-04-01

We study problems associated with seismic data decomposition and migration imaging. We first represent the seismic data utilizing Gaussian beam basis functions, which have nonzero curvature, and then consider the sparse decomposition technique. The sparse decomposition problem is an l0-norm constrained minimization problem. In solving the l0-norm minimization, a polynomial Radon transform is performed to achieve sparsity, and a fast gradient descent method is used to calculate the waveform functions. The waveform functions can subsequently be used for sparse Gaussian beam migration. Compared with traditional sparse Gaussian beam methods, the seismic data can be properly reconstructed employing fewer Gaussian beams with nonzero initial curvature. The migration approach described in this paper is more efficient than the traditional sparse Gaussian beam migration.

Characterizing Geological Facies using Seismic Waveform Classification in Sarawak Basin

NASA Astrophysics Data System (ADS)

Zahraa, Afiqah; Zailani, Ahmad; Prasad Ghosh, Deva

2017-10-01

Numerous effort have been made to build relationship between geology and geophysics using different techniques throughout the years. The integration of these two most important data in oil and gas industry can be used to reduce uncertainty in exploration and production especially for reservoir productivity enhancement and stratigraphic identification. This paper is focusing on seismic waveform classification to different classes using neural network and to link them according to the geological facies which are established using the knowledge on lithology and log motif of well data. Seismic inversion is used as the input for the neural network to act as the direct lithology indicator reducing dependency on well calibration. The interpretation of seismic facies classification map provides a better understanding towards the lithology distribution, depositional environment and help to identify significant reservoir rock

Anisotropic full waveform ambient noise and earthquake tomography of the Ontong Java Plateau and surrounding Pacific upper mantle

NASA Astrophysics Data System (ADS)

Hirsch, A. C.; Savage, B.; Shen, Y.

2017-12-01

The Ontong Java (OJP) and Manihiki plateau (MP) large igneous provinces (LIP) of the Southwest Pacific took shape from a complicated, but poorly understood geological history. Unraveling the formation and deformation of these Pacific LIPs is not straightforward due to limited available data, remote location, and atypical geology. Origin hypotheses include melting of a plume or a fast-spreading triple junction, but distinguishing between these requires a further understanding of 120 Ma of deformation of each LIP. A previous tomographic model of OJP observed highly abnormal Rayleigh shear wave speeds, >4.75km/s, and attributed these to an unusual composition, garnet and clinopyroxene residual from melting pyroxenite entrained within a rising plume. Unfortunately, this model lacks constraints on the horizontally polarized shear wave speeds, SH or Love waves, anisotropy, and attenuation. We therefore perform a transverse-isotropic, scattering-integral, full-waveform tomography between periods of 25 and 200 seconds utilizing both ambient noise empirical Green's functions and seismic data from regional earthquakes. Our tomographic model improves upon previous work using permanent and temporary seismic stations, increased model space, and utilizing three components of seismic data (vertical, radial, and tangential). Included is also an assessment of the anelastic attenuation in the western Pacific using both surface waves and multiple core reflections. Our results will improve the tomographic resolution around OJP and the Pacific upper mantle between 35 and 300 km depth. This improved model will enhance our understanding of the tectonic history of the OJP and MP regions, and the Pacific Indo-Australian plate boundary.

Nonlinear 1D and 2D waveform inversions of SS precursors and their applications in mantle seismic imaging

NASA Astrophysics Data System (ADS)

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

2016-12-01

Seismic velocities and the topography of mantle discontinuities are crucial for the understanding of mantle structure, dynamics and mineralogy. While these two observables are closely linked, the vast majority of high-resolution seismic images are retrieved under the assumption of horizontally stratified mantle interfaces. This conventional correction-based process could lead to considerable errors due to the inherent trade-off between velocity and discontinuity depth. In this study, we introduce a nonlinear joint waveform inversion method that simultaneously recovers discontinuity depths and seismic velocities using the waveforms of SS precursors. Our target region is the upper mantle and transition zone beneath Northeast Asia. In this region, the inversion outcomes clearly delineate a westward dipping high-velocity structure in association with the subducting Pacific plate. Above the flat part of the slab west of the Japan sea, our results show a shear wave velocity reduction of 1.5% in the upper mantle and 10-15 km depression of the 410 km discontinuity beneath the Changbaishan volcanic field. We also identify the maximum correlation between shear velocity and transition zone thickness at an approximate slab dip of 30 degrees, which is consistent with previously reported values in this region.To validate the results of the 1D waveform inversion of SS precursors, we discretize the mantle beneath the study region and conduct a 2D waveform tomographic survey using the same nonlinear approach. The problem is simplified by adopting the discontinuity depths from the 1D inversion and solving only for perturbations in shear velocities. The resulting models obtained from the 1D and 2D approaches are self-consistent. Low-velocities beneath the Changbai intraplate volcano likely persist to a depth of 500 km. Collectively, our seismic observations suggest that the active volcanoes in eastern China may be fueled by a hot thermal anomaly originating from the mantle transition zone.

The OSCAR experiment: using full-waveform inversion in the analysis of young oceanic crust

NASA Astrophysics Data System (ADS)

Silverton, Akela; Morgan, Joanna; Wilson, Dean; Hobbs, Richard

2017-04-01

The OSCAR experiment aims to derive an integrated model to better explain the effects of heat loss and alteration by hydrothermal fluids, associated with the cooling of young oceanic crust at an axial ridge. High-resolution seismic imaging of the sediments and basaltic basement can be used to map fluid flow pathways between the oceanic crust and the surrounding ocean. To obtain these high-resolution images, we undertake full-waveform inversion (FWI), an advanced seismic imaging technique capable of resolving velocity heterogeneities at a wide range of length scales, from background trends to fine-scale geological/crustal detail, in a fully data-driven automated manner. This technology is widely used within the petroleum sector due to its potential to obtain high-resolution P-wave velocity models that lead to improvements in migrated seismic images of the subsurface. Here, we use the P-wave velocity model obtained from travel-time tomography as the starting model in the application of acoustic, time-domain FWI to a multichannel streamer field dataset acquired in the east Pacific along a profile between the Costa Rica spreading centre and the Ocean Drilling Program (ODP) borehole 504B, where the crust is approximately six million years old. FWI iteratively improves the velocity model by minimizing the misfit between the predicted data and the field data. It seeks to find a high-fidelity velocity model that is capable of matching individual seismic waveforms of the original raw field dataset, with an initial focus on matching the low-frequency components of the early arriving energy. Quality assurance methods adopted during the inversion ensure convergence in the direction of the global minimum. We demonstrate that FWI is able to recover fine-scale, high-resolution velocity heterogeneities within the young oceanic crust along the profile. The highly resolved FWI velocity model is useful in the identification of the layer 2A/2B interface and low-velocity layers that may represent zones of enhanced fluid movement. With FWI we are able to better explain the non-linear changes in velocity as the crust evolves with distance from the spreading centre and image the effects of any alteration by hydrothermal fluids. This model provides valuable insight and new constraints on the thermal processes involved, at spreading centres, setting a new benchmark for integrated multi-physics experiments at similar ocean ridge systems. This research is part of a major, interdisciplinary NERC-funded collaboration entitled: Oceanographic and Seismic Characterisation of heat dissipation and alteration by hydrothermal fluids at an Axial Ridge (OSCAR).

Resolvability of regional density structure and the road to direct density inversion - a principal-component approach to resolution analysis

NASA Astrophysics Data System (ADS)

Płonka, Agnieszka; Fichtner, Andreas

2017-04-01

Lateral density variations are the source of mass transport in the Earth at all scales, acting as drivers of convective motion. However, the density structure of the Earth remains largely unknown since classic seismic observables and gravity provide only weak constraints with strong trade-offs. Current density models are therefore often based on velocity scaling, making strong assumptions on the origin of structural heterogeneities, which may not necessarily be correct. Our goal is to assess if 3D density structure may be resolvable with emerging full-waveform inversion techniques. We have previously quantified the impact of regional-scale crustal density structure on seismic waveforms with the conclusion that reasonably sized density variations within the crust can leave a strong imprint on both travel times and amplitudes, and, while this can produce significant biases in velocity and Q estimates, the seismic waveform inversion for density may become feasible. In this study we perform principal component analyses of sensitivity kernels for P velocity, S velocity, and density. This is intended to establish the extent to which these kernels are linearly independent, i.e. the extent to which the different parameters may be constrained independently. We apply the method to data from 81 events around the Iberian Penninsula, registered in total by 492 stations. The objective is to find a principal kernel which would maximize the sensitivity to density, potentially allowing for as independent as possible density resolution. We find that surface (mosty Rayleigh) waves have significant sensitivity to density, and that the trade-off with velocity is negligible. We also show the preliminary results of the inversion.

Developing a Near Real-time System for Earthquake Slip Distribution Inversion

NASA Astrophysics Data System (ADS)

Zhao, Li; Hsieh, Ming-Che; Luo, Yan; Ji, Chen

2016-04-01

Advances in observational and computational seismology in the past two decades have enabled completely automatic and real-time determinations of the focal mechanisms of earthquake point sources. However, seismic radiations from moderate and large earthquakes often exhibit strong finite-source directivity effect, which is critically important for accurate ground motion estimations and earthquake damage assessments. Therefore, an effective procedure to determine earthquake rupture processes in near real-time is in high demand for hazard mitigation and risk assessment purposes. In this study, we develop an efficient waveform inversion approach for the purpose of solving for finite-fault models in 3D structure. Full slip distribution inversions are carried out based on the identified fault planes in the point-source solutions. To ensure efficiency in calculating 3D synthetics during slip distribution inversions, a database of strain Green tensors (SGT) is established for 3D structural model with realistic surface topography. The SGT database enables rapid calculations of accurate synthetic seismograms for waveform inversion on a regular desktop or even a laptop PC. We demonstrate our source inversion approach using two moderate earthquakes (Mw~6.0) in Taiwan and in mainland China. Our results show that 3D velocity model provides better waveform fitting with more spatially concentrated slip distributions. Our source inversion technique based on the SGT database is effective for semi-automatic, near real-time determinations of finite-source solutions for seismic hazard mitigation purposes.

Remote detection of weak aftershocks of the DPRK underground explosions using waveform cross correlation

NASA Astrophysics Data System (ADS)

Le Bras, R.; Rozhkov, M.; Bobrov, D.; Kitov, I. O.; Sanina, I.

2017-12-01

Association of weak seismic signals generated by low-magnitude aftershocks of the DPRK underground tests into event hypotheses represent a challenge for routine automatic and interactive processing at the International Data Centre (IDC) of the Comprehensive Nuclear-Test-Ban Treaty Organization, due to the relatively low station density of the International Monitoring System (IMS) seismic network. Since 2011, as an alternative, the IDC has been testing various prototype techniques of signal detection and event creation based on waveform cross correlation. Using signals measured by seismic stations of the IMS from DPRK explosions as waveform templates, the IDC detected several small (estimated mb between 2.2 and 3.6) seismic events after two DPRK tests conducted on September 9, 2016 and September 3, 2017. The obtained detections were associated with reliable event hypothesis and then used to locate these events relative to the epicenters of the DPRK explosions. We observe high similarity of the detected signals with the corresponding waveform templates. The newly found signals also correlate well between themselves. In addition, the values of the signal-to-noise ratios (SNR) estimated using the traces of cross correlation coefficients, increase with template length (from 5 s to 150 s), providing strong evidence in favour of their spatial closeness, which allows interpreting them as explosion aftershocks. We estimated the relative magnitudes of all aftershocks using the ratio of RMS amplitudes of the master and slave signal in the cross correlation windows characterized by the highest SNR. Additional waveform data from regional non-IMS stations MDJ and SEHB provide independent validation of these aftershock hypotheses. Since waveform templates from any single master event may be sub-efficient at some stations, we have also developed a method of joint usage of the DPRK and the biggest aftershocks templates to build more robust event hypotheses.

Blast waves from violent explosive activity at Yasur Volcano, Vanuatu

NASA Astrophysics Data System (ADS)

Marchetti, E.; Ripepe, M.; Delle Donne, D.; Genco, R.; Finizola, A.; Garaebiti, E.

2013-11-01

and seismic waveforms were collected during violent strombolian activity at Yasur Volcano (Vanuatu). Averaging ~3000 seismic events showed stable waveforms, evidencing a low-frequency (0.1-0.3 Hz) signal preceding ~5-6 s the explosion. Infrasonic waveforms were mostly asymmetric with a sharp compressive (5-106 Pa) onset, followed by a small long-lasting rarefaction phase. Regardless of the pressure amplitude, the ratio between the positive and negative phases was constant. These waveform characteristics closely resembled blast waves. Infrared imagery showed an apparent cold spherical front ~20 m thick, which moved between 342 and 405 m/s before the explosive hot gas/fragments cloud. We interpret this cold front as that produced by the vapor condensation induced by the passage of the shock front. We suggest that violent strombolian activity at Yasur was driven by supersonic dynamics with gas expanding at 1.1 Mach number inside the conduit.

Visco-elastic controlled-source full waveform inversion without surface waves

NASA Astrophysics Data System (ADS)

Paschke, Marco; Krause, Martin; Bleibinhaus, Florian

2016-04-01

We developed a frequency-domain visco-elastic full waveform inversion for onshore seismic experiments with topography. The forward modeling is based on a finite-difference time-domain algorithm by Robertsson that uses the image-method to ensure a stress-free condition at the surface. The time-domain data is Fourier-transformed at every point in the model space during the forward modeling for a given set of frequencies. The motivation for this approach is the reduced amount of memory when computing kernels, and the straightforward implementation of the multiscale approach. For the inversion, we calculate the Frechet derivative matrix explicitly, and we implement a Levenberg-Marquardt scheme that allows for computing the resolution matrix. To reduce the size of the Frechet derivative matrix, and to stabilize the inversion, an adapted inverse mesh is used. The node spacing is controlled by the velocity distribution and the chosen frequencies. To focus the inversion on body waves (P, P-coda, and S) we mute the surface waves from the data. Consistent spatiotemporal weighting factors are applied to the wavefields during the Fourier transform to obtain the corresponding kernels. We test our code with a synthetic study using the Marmousi model with arbitrary topography. This study also demonstrates the importance of topography and muting surface waves in controlled-source full waveform inversion.

Full Waveform Inversion of Reflection Seismic Data for Ocean Temperature Profiles

DTIC Science & Technology

2008-01-01

ographic processes and properties, such as internal-wave spectra [Holbrook and Fer, 2005; Krahmann et al, 2006] and temperature contrasts [ Paramo and...contribute little to the reflectance in the Norwegian Sea data set used here [ Paramo and Holbrook, 2005], so we assume for this study that all...bathyther- mograph) profiles presented by Paramo and Holbrook [2005], displayed here in the intercept time-slowness (Tau-p) domain, rather than the

Earthquake Fingerprints: Representing Earthquake Waveforms for Similarity-Based Detection

NASA Astrophysics Data System (ADS)

Bergen, K.; Beroza, G. C.

2016-12-01

New earthquake detection methods, such as Fingerprint and Similarity Thresholding (FAST), use fast approximate similarity search to identify similar waveforms in long-duration data without templates (Yoon et al. 2015). These methods have two key components: fingerprint extraction and an efficient search algorithm. Fingerprint extraction converts waveforms into fingerprints, compact signatures that represent short-duration waveforms for identification and search. Earthquakes are detected using an efficient indexing and search scheme, such as locality-sensitive hashing, that identifies similar waveforms in a fingerprint database. The quality of the search results, and thus the earthquake detection results, is strongly dependent on the fingerprinting scheme. Fingerprint extraction should map similar earthquake waveforms to similar waveform fingerprints to ensure a high detection rate, even under additive noise and small distortions. Additionally, fingerprints corresponding to noise intervals should have mutually dissimilar fingerprints to minimize false detections. In this work, we compare the performance of multiple fingerprint extraction approaches for the earthquake waveform similarity search problem. We apply existing audio fingerprinting (used in content-based audio identification systems) and time series indexing techniques and present modified versions that are specifically adapted for seismic data. We also explore data-driven fingerprinting approaches that can take advantage of labeled or unlabeled waveform data. For each fingerprinting approach we measure its ability to identify similar waveforms in a low signal-to-noise setting, and quantify the trade-off between true and false detection rates in the presence of persistent noise sources. We compare the performance using known event waveforms from eight independent stations in the Northern California Seismic Network.

Multi-parameter Full-waveform Inversion for Acoustic VTI Medium with Surface Seismic Data

NASA Astrophysics Data System (ADS)

Cheng, X.; Jiao, K.; Sun, D.; Huang, W.; Vigh, D.

2013-12-01

Full-waveform Inversion (FWI) attracts wide attention recently in oil and gas industry as a new promising tool for high resolution subsurface velocity model building. While the traditional common image point gather based tomography method aims to focus post-migrated data in depth domain, FWI aims to directly fit the observed seismic waveform in either time or frequency domain. The inversion is performed iteratively by updating the velocity fields to reduce the difference between the observed and the simulated data. It has been shown the inversion is very sensitive to the starting velocity fields, and data with long offsets and low frequencies is crucial for the success of FWI to overcome this sensitivity. Considering the importance of data with long offsets and low frequencies, in most geologic environment, anisotropy is an unavoidable topic for FWI especially at long offsets, since anisotropy tends to have more pronounced effects on waves traveled for a great distance. In VTI medium, this means more horizontal velocity will be registered in middle-to-long offset data, while more vertical velocity will be registered in near-to-middle offset data. Up to date, most of real world applications of FWI still remain in isotropic medium, and only a few studies have been shown to account for anisotropy. And most of those studies only account for anisotropy in waveform simulation, but not invert for those anisotropy fields. Multi-parameter inversion for anisotropy fields, even in VTI medium, remains as a hot topic in the field. In this study, we develop a strategy for multi-parameter FWI for acoustic VTI medium with surface seismic data. Because surface seismic data is insensitivity to the delta fields, we decide to hold the delta fields unchanged during our inversion, and invert only for vertical velocity and epsilon fields. Through parameterization analysis and synthetic tests, we find that it is more feasible to invert for the parameterization as vertical and horizontal velocities instead of inverting for the parameterization as vertical velocity and epsilon fields. We develop a hierarchical approach to invert for vertical velocity first but hold epsilon unchanged and only switch to simultaneous inversion when vertical velocity inversion are approaching convergence. During simultaneous inversion, we observe significant acceleration in the convergence when incorporates second order information and preconditioning into inversion. We demonstrate the success of our strategy for VTI FWI using synthetic and real data examples from the Gulf of Mexico. Our results show that incorporation of VTI FWI improves migration of large offset acquisition data, and produces better focused migration images to be used in exploration, production and development of oil fields.

Seismicity detection around the subduting seamount off Ibaraki the Japan Trench using dense OBS array data

NASA Astrophysics Data System (ADS)

Nakatani, Y.; Mochizuki, K.; Shinohara, M.; Yamada, T.; Hino, R.; Ito, Y.; Murai, Y.; Sato, T.

2013-12-01

A subducting seamount which has a height of about 3 km was revealed off Ibaraki in the Japan Trench by a seismic survey (Mochizuki et al., 2008). Mochizuki et al. (2008) also interpreted that interplate coupling was weak over the seamount because seismicity was low and the slip of the recent large earthquake did not propagate over it. To carry out further investigation, we deployed dense ocean bottom seismometers (OBSs) array around the seamount for about a year. During the observation period, seismicity off Ibaraki was activated due to the occurrence of the 2011 Tohoku earthquake. The southern edge of the mainshock rupture area was considered to be located around off Ibaraki by many source analyses. Moreover, Kubo et al. (2013) proposes the seamount played an important role in the rupture termination of the largest aftershock. Therefore, in this study, we try to understand about spatiotemporal variation of seismicity around the seamount before and after the Mw 9.0 event as a first step to elucidate relationship between the subducting seamount and seismogenic behavior. We used velocity waveforms of 1 Hz long-term OBSs which were densely deployed at station intervals of about 6 km. The sampling rate is 200 Hz and the observation period is from October 16, 2010 to September 19, 2011. Because of the ambient noise and effects of thick seafloor sediments, it is difficult to apply methods which have been used to on-land observational data for detecting seismicity to OBS data and to handle continuous waveforms automatically. We therefore apply back-projection method (e.g., Kiser and Ishii, 2012) to OBS waveform data which estimate energy-release source by stacking waveforms. Among many back-projection methods, we adopt a semblance analysis (e.g., Honda et al., 2008) which can detect feeble waves. First of all, we constructed a 3-D velocity structure model off Ibaraki by compiling the results of marine seismic surveys (e.g., Nakahigashi et al., 2012). Then, we divided a target area into small areas and calculated P-wave traveltimes between each station and all small areas by fast marching method (Rawlinson et al., 2006). After constructing theoretical travel-time tables, we applied a proper frequency filter to the observed waveforms and estimated seismic energy release by projecting semblance values. As the result of applying our method, we could successfully detect magnitude 2-3 earthquakes.

Full Waveform Inversion for Seismic Velocity And Anelastic Losses in Heterogeneous Structures

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

Askan, A.; /Carnegie Mellon U.; Akcelik, V.

2009-04-30

We present a least-squares optimization method for solving the nonlinear full waveform inverse problem of determining the crustal velocity and intrinsic attenuation properties of sedimentary valleys in earthquake-prone regions. Given a known earthquake source and a set of seismograms generated by the source, the inverse problem is to reconstruct the anelastic properties of a heterogeneous medium with possibly discontinuous wave velocities. The inverse problem is formulated as a constrained optimization problem, where the constraints are the partial and ordinary differential equations governing the anelastic wave propagation from the source to the receivers in the time domain. This leads to amore » variational formulation in terms of the material model plus the state variables and their adjoints. We employ a wave propagation model in which the intrinsic energy-dissipating nature of the soil medium is modeled by a set of standard linear solids. The least-squares optimization approach to inverse wave propagation presents the well-known difficulties of ill posedness and multiple minima. To overcome ill posedness, we include a total variation regularization functional in the objective function, which annihilates highly oscillatory material property components while preserving discontinuities in the medium. To treat multiple minima, we use a multilevel algorithm that solves a sequence of subproblems on increasingly finer grids with increasingly higher frequency source components to remain within the basin of attraction of the global minimum. We illustrate the methodology with high-resolution inversions for two-dimensional sedimentary models of the San Fernando Valley, under SH-wave excitation. We perform inversions for both the seismic velocity and the intrinsic attenuation using synthetic waveforms at the observer locations as pseudoobserved data.« less

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.

Seismic Full Waveform Modeling & Imaging in Attenuating Media

NASA Astrophysics Data System (ADS)

Guo, Peng

Seismic attenuation strongly affects seismic waveforms by amplitude loss and velocity dispersion. Without proper inclusion of Q parameters, errors can be introduced for seismic full waveform modeling and imaging. Three different (Carcione's, Robertsson's, and the generalized Robertsson's) isotropic viscoelastic wave equations based on the generalized standard linear solid (GSLS) are evaluated. The second-order displacement equations are derived, and used to demonstrate that, with the same stress relaxation times, these viscoelastic formulations are equivalent. By introducing separate memory variables for P and S relaxation functions, Robertsson's formulation is generalized to allow different P and S wave stress relaxation times, which improves the physical consistency of the Qp and Qs modelled in the seismograms.The three formulations have comparable computational cost. 3D seismic finite-difference forward modeling is applied to anisotropic viscoelastic media. The viscoelastic T-matrix (a dynamic effective medium theory) relates frequency-dependent anisotropic attenuation and velocity to reservoir properties in fractured HTI media, based on the meso-scale fluid flow attenuation mechanism. The seismic signatures resulting from changing viscoelastic reservoir properties are easily visible. Analysis of 3D viscoelastic seismograms suggests that anisotropic attenuation is a potential tool for reservoir characterization. To compensate the Q effects during reverse-time migration (RTM) in viscoacoustic and viscoelastic media, amplitudes need to be compensated during wave propagation; the propagation velocity of the Q-compensated wavefield needs to be the same as in the attenuating wavefield, to restore the phase information. Both amplitude and phase can be compensated when the velocity dispersion and the amplitude loss are decoupled. For wave equations based on the GSLS, because Q effects are coupled in the memory variables, Q-compensated wavefield propagates faster than the attenuating wavefield, and introduce unwanted phase shift. Numerical examples show that there are phase (depth) shifts in the Q-compensated RTM images from the GSLS equation. An adjoint-based least-squares reverse-time migration is proposed for viscoelastic media (Q-LSRTM), to compensate the attenuation losses in P and S images. The viscoelastic adjoint operator, and the P and S modulus perturbation imaging conditions are derived using the adjoint-state method and an augmented Lagrangian functional. Q-LSRTM solves the viscoelastic linearized modeling operator for synthetic data, and the adjoint operator is used for back propagating the data residual. Q-LSRTM is capable of iteratively updating the P and S modulus perturbations,in the direction of minimizing data residuals, and attenuation loss is iteratively compensated. A novel Q compensation approach is developed for adjoint seismic imaging by pseudodifferential scaling. With a correct Q model included in the migration algorithm, propagation effects, including the Q effects, can be compensated with the application of the inverse Hessian to the RTM image. Pseudodifferential scaling is used to efficiently approximate the action of the inverse Hessian. Numerical examples indicate that the adjoint RTM images with pseudodifferential scaling approximate the true model perturbation, and can be used as well-conditioned gradients for least-squares imaging.

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.

MyShake: Initial observations from a global smartphone seismic network

NASA Astrophysics Data System (ADS)

Kong, Qingkai; Allen, Richard M.; Schreier, Louis

2016-09-01

MyShake is a global smartphone seismic network that harnesses the power of crowdsourcing. In the first 6 months since the release of the MyShake app, there were almost 200,000 downloads. On a typical day about 8000 phones provide acceleration waveform data to the MyShake archive. The on-phone app can detect and trigger on P waves and is capable of recording magnitude 2.5 and larger events. More than 200 seismic events have been recorded so far, including events in Chile, Argentina, Mexico, Morocco, Nepal, New Zealand, Taiwan, Japan, and across North America. The largest number of waveforms from a single earthquake to date comes from the M5.2 Borrego Springs earthquake in Southern California, for which MyShake collected 103 useful three-component waveforms. The network continues to grow with new downloads from the Google Play store everyday and expands rapidly when public interest in earthquakes peaks such as during an earthquake sequence.

A long source area of the 1906 Colombia-Ecuador earthquake estimated from observed tsunami waveforms

NASA Astrophysics Data System (ADS)

Yamanaka, Yusuke; Tanioka, Yuichiro; Shiina, Takahiro

2017-12-01

The 1906 Colombia-Ecuador earthquake induced both strong seismic motions and a tsunami, the most destructive earthquake in the history of the Colombia-Ecuador subduction zone. The tsunami propagated across the Pacific Ocean, and its waveforms were observed at tide gauge stations in countries including Panama, Japan, and the USA. This study conducted slip inverse analysis for the 1906 earthquake using these waveforms. A digital dataset of observed tsunami waveforms at the Naos Island (Panama) and Honolulu (USA) tide gauge stations, where the tsunami was clearly observed, was first produced by consulting documents. Next, the two waveforms were applied in an inverse analysis as the target waveform. The results of this analysis indicated that the moment magnitude of the 1906 earthquake ranged from 8.3 to 8.6. Moreover, the dominant slip occurred in the northern part of the assumed source region near the coast of Colombia, where little significant seismicity has occurred, rather than in the southern part. The results also indicated that the source area, with significant slip, covered a long distance, including the southern, central, and northern parts of the region.[Figure not available: see fulltext.

Multi-Station Broad Regional Event Detection Using Waveform Correlation

NASA Astrophysics Data System (ADS)

Slinkard, M.; Stephen, H.; Young, C. J.; Eckert, R.; Schaff, D. P.; Richards, P. G.

2013-12-01

Previous waveform correlation studies have established the occurrence of repeating seismic events in various regions, and the utility of waveform-correlation event-detection on broad regional or even global scales to find events currently not included in traditionally-prepared bulletins. The computational burden, however, is high, limiting previous experiments to relatively modest template libraries and/or processing time periods. We have developed a distributed computing waveform correlation event detection utility that allows us to process years of continuous waveform data with template libraries numbering in the thousands. We have used this system to process several years of waveform data from IRIS stations in East Asia, using libraries of template events taken from global and regional bulletins. Detections at a given station are confirmed by 1) comparison with independent bulletins of seismicity, and 2) consistent detections at other stations. We find that many of the detected events are not in traditional catalogs, hence the multi-station comparison is essential. In addition to detecting the similar events, we also estimate magnitudes very precisely based on comparison with the template events (when magnitudes are available). We have investigated magnitude variation within detected families of similar events, false alarm rates, and the temporal and spatial reach of templates.

Relocation of micro-earthquakes in the Yeongdeok offshore area, Korea using local and Ocean bottom seismometers

NASA Astrophysics Data System (ADS)

HAN, M.; Kim, K. H.; Park, S. C.; Lin, P. P.; Chen, P.; Chang, H.; Jang, J. P.; Kuo, B. Y.; Liao, Y. C.

2016-12-01

Seismicity in the East Sea of Korea has been relatively high during the last four decades of instrumental earthquake observation period. Yeongdeok offshore area is probably the most seismically active area in the East Sea. This study analyzes seismic signals to detect micro-earthquakes and determine their precise earthquake hypocenters in the Yeoungdeok offshore area using data recorded by the Korea National Seismic Network (KNSN) and a temporary ocean bottom seismographic network (OBSN-PNU) operated by Korea Meteorological Administration and Pusan National University, respectively. Continuous waveform data recorded at four seismic stations in the study area of KNSN between January 2007 and July 2016 are inspected to detect any repeating earthquakes by applying a waveform cross-correlation detector. More than 1,600 events are triggered. Events outside the study area or in poor waveform quality are removed from further analysis. Approximately 500 earthquakes are selected, most of which have gone unreported because their magnitudes are lower than the detection threshold of the routine earthquake monitoring. Events in the study area are also under bad azimuthal coverage because all stations are located on land and thus biased to the west. OBSN-PNU comprised three ocean bottom seismometers and operated to observe micro-earthquakes in the study area between February and August 2016. The same technique applied to the KNSN data has been applied to the OBSN-PNU data to detect micro-earthquakes. Precise earthquake hypocenters are determined using phase arrival times and waveform similarities. Resultant hypocenters are clustered to form a few lineaments. They are compared to the local geological and geophysical features to understand micro-earthquake activity in the area.

A Study for Anisotropic Wavefield Analysis with Elastic Layered Models

NASA Astrophysics Data System (ADS)

Yoneki, R.; Mikada, H.; Takekawa, J.

2015-12-01

Subsurface materials are generally anisotropic due to complicated geological conditions, for example, sedimentary materials, fractures reflecting various stress conditions in the past and present in the subsurface. There are many studies on seismic wave propagation in TI (transversely isotropic) and orthorhombic media (e.g., Thomsen, 1986; Alkhalifah, 2000; Bansal and Sen, 2008). In most of those studies, the magnitude of anisotropy is assumed to be weak. Therefore, it may be not appropriate to apply their theories directly to strongly anisotropic subsurface media in seismic exploration. It is necessary to understand the effects of the anisotropy on the behavior of seismic wave propagation in strongly anisotropic media in the seismic exploration. In this study, we investigate the influence of strong anisotropy on received seismic waveforms using three-dimensional numerical models, and verified capability of detecting subsurface anisotropy. Our numerical models contain an isotropic and an anisotropic (VTI, transversely isotropic media with vertical symmetry axis) layer, respectively, in the isotropic background subsurface. Since the difference between the two models is only the anisotropy in the vertical propagation velocity, we could look at the influence of anisotropy in the residual wavefield that is the difference in the observed wavefields of two models. We analyzed the orbital motions of the residual wavefield to see what kind of wave motions the waveforms show. We found that the residual waveforms generated by the anisotropic layer include the orbital motions of shear waves right after the first arrival, i.e., mode conversion from the compressional waves due to the anisotropy. The residual waveforms could be exploited to estimate both the order of anisotropy and the thickness of anisotropic layer in subsurface.

Frequency-domain elastic full waveform inversion using encoded simultaneous sources

NASA Astrophysics Data System (ADS)

Jeong, W.; Son, W.; Pyun, S.; Min, D.

2011-12-01

Currently, numerous studies have endeavored to develop robust full waveform inversion and migration algorithms. These processes require enormous computational costs, because of the number of sources in the survey. To avoid this problem, the phase encoding technique for prestack migration was proposed by Romero (2000) and Krebs et al. (2009) proposed the encoded simultaneous-source inversion technique in the time domain. On the other hand, Ben-Hadj-Ali et al. (2011) demonstrated the robustness of the frequency-domain full waveform inversion with simultaneous sources for noisy data changing the source assembling. Although several studies on simultaneous-source inversion tried to estimate P- wave velocity based on the acoustic wave equation, seismic migration and waveform inversion based on the elastic wave equations are required to obtain more reliable subsurface information. In this study, we propose a 2-D frequency-domain elastic full waveform inversion technique using phase encoding methods. In our algorithm, the random phase encoding method is employed to calculate the gradients of the elastic parameters, source signature estimation and the diagonal entries of approximate Hessian matrix. The crosstalk for the estimated source signature and the diagonal entries of approximate Hessian matrix are suppressed with iteration as for the gradients. Our 2-D frequency-domain elastic waveform inversion algorithm is composed using the back-propagation technique and the conjugate-gradient method. Source signature is estimated using the full Newton method. We compare the simultaneous-source inversion with the conventional waveform inversion for synthetic data sets of the Marmousi-2 model. The inverted results obtained by simultaneous sources are comparable to those obtained by individual sources, and source signature is successfully estimated in simultaneous source technique. Comparing the inverted results using the pseudo Hessian matrix with previous inversion results provided by the approximate Hessian matrix, it is noted that the latter are better than the former for deeper parts of the model. This work was financially supported by the Brain Korea 21 project of Energy System Engineering, by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0006155), by the Energy Efficiency & Resources of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (No. 2010T100200133).

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..

Detecting Earthquakes over a Seismic Network using Single-Station Similarity Measures

NASA Astrophysics Data System (ADS)

Bergen, Karianne J.; Beroza, Gregory C.

2018-03-01

New blind waveform-similarity-based detection methods, such as Fingerprint and Similarity Thresholding (FAST), have shown promise for detecting weak signals in long-duration, continuous waveform data. While blind detectors are capable of identifying similar or repeating waveforms without templates, they can also be susceptible to false detections due to local correlated noise. In this work, we present a set of three new methods that allow us to extend single-station similarity-based detection over a seismic network; event-pair extraction, pairwise pseudo-association, and event resolution complete a post-processing pipeline that combines single-station similarity measures (e.g. FAST sparse similarity matrix) from each station in a network into a list of candidate events. The core technique, pairwise pseudo-association, leverages the pairwise structure of event detections in its network detection model, which allows it to identify events observed at multiple stations in the network without modeling the expected move-out. Though our approach is general, we apply it to extend FAST over a sparse seismic network. We demonstrate that our network-based extension of FAST is both sensitive and maintains a low false detection rate. As a test case, we apply our approach to two weeks of continuous waveform data from five stations during the foreshock sequence prior to the 2014 Mw 8.2 Iquique earthquake. Our method identifies nearly five times as many events as the local seismicity catalog (including 95% of the catalog events), and less than 1% of these candidate events are false detections.

Earthquake Monitoring: SeisComp3 at the Swiss National Seismic Network

NASA Astrophysics Data System (ADS)

Clinton, J. F.; Diehl, T.; Cauzzi, C.; Kaestli, P.

2011-12-01

The Swiss Seismological Service (SED) has an ongoing responsibility to improve the seismicity monitoring capability for Switzerland. This is a crucial issue for a country with low background seismicity but where a large M6+ earthquake is expected in the next decades. With over 30 stations with spacing of ~25km, the SED operates one of the densest broadband networks in the world, which is complimented by ~ 50 realtime strong motion stations. The strong motion network is expected to grow with an additional ~80 stations over the next few years. Furthermore, the backbone of the network is complemented by broadband data from surrounding countries and temporary sub-networks for local monitoring of microseismicity (e.g. at geothermal sites). The variety of seismic monitoring responsibilities as well as the anticipated densifications of our network demands highly flexible processing software. We are transitioning all software to the SeisComP3 (SC3) framework. SC3 is a fully featured automated real-time earthquake monitoring software developed by GeoForschungZentrum Potsdam in collaboration with commercial partner, gempa GmbH. It is in its core open source, and becoming a community standard software for earthquake detection and waveform processing for regional and global networks across the globe. SC3 was originally developed for regional and global rapid monitoring of potentially tsunamagenic earthquakes. In order to fulfill the requirements of a local network recording moderate seismicity, SED has tuned configurations and added several modules. In this contribution, we present our SC3 implementation strategy, focusing on the detection and identification of seismicity on different scales. We operate several parallel processing "pipelines" to detect and locate local, regional and global seismicity. Additional pipelines with lower detection thresholds can be defined to monitor seismicity within dense subnets of the network. To be consistent with existing processing procedures, the nonlinloc algorithm was implemented for manual and automatic locations using 1D and 3D velocity models; plugins for improved automatic phase picking and Ml computation were developed; and the graphical user interface for manual review was extended (including pick uncertainty definition; first motion focal mechanisms; interactive review of station magnitude waveforms; full inclusion of strong motion data). SC3 locations are fully compatible with those derived from the existing in-house processing tools and are stored in a database derived from the QuakeML data model. The database is shared with the SED alerting software, which merges origins from both SC3 and external sources in realtime and handles the alerting procedure. With the monitoring software being transitioned to SeisComp3, acquisition, archival and dissemination of SED waveform data now conforms to the seedlink and ArcLink protocols and continuous archives can be accessed via SED and all EIDA (European Integrated Data Archives) web-sites. Further, a SC3 module for waveform parameterisation has been developed, allowing rapid computation of peak values of ground motion and other engineering parameters within minutes of a new event. An output of this module is USGS ShakeMap XML. n minutes of a new event. An output of this module is USGS ShakeMap XML.

The Waveform Suite: A robust platform for accessing and manipulating seismic waveforms in MATLAB

NASA Astrophysics Data System (ADS)

Reyes, C. G.; West, M. E.; McNutt, S. R.

2009-12-01

The Waveform Suite, developed at the University of Alaska Geophysical Institute, is an open-source collection of MATLAB classes that provide a means to import, manipulate, display, and share waveform data while ensuring integrity of the data and stability for programs that incorporate them. Data may be imported from a variety of sources, such as Antelope, Winston databases, SAC files, SEISAN, .mat files, or other user-defined file formats. The waveforms being manipulated in MATLAB are isolated from their stored representations, relieving the overlying programs from the responsibility of understanding the specific format in which data is stored or retrieved. The waveform class provides an object oriented framework that simplifies manipulations to waveform data. Playing with data becomes easier because the tedious aspects of data manipulation have been automated. The user is able to change multiple waveforms simultaneously using standard mathematical operators and other syntactically familiar functions. Unlike MATLAB structs or workspace variables, the data stored within waveform class objects are protected from modification, and instead are accessed through standardized functions, such as get and set; these are already familiar to users of MATLAB’s graphical features. This prevents accidental or nonsensical modifications to the data, which in turn simplifies troubleshooting of complex programs. Upgrades to the internal structure of the waveform class are invisible to applications which use it, making maintenance easier. We demonstrate the Waveform Suite’s capabilities on seismic data from Okmok and Redoubt volcanoes. Years of data from Okmok were retrieved from Antelope and Winston databases. Using the Waveform Suite, we built a tremor-location program. Because the program was built on the Waveform Suite, modifying it to operate on real-time data from Redoubt involved only minimal code changes. The utility of the Waveform Suite as a foundation for large developments is demonstrated with the Correlation Toolbox for MATLAB. This mature package contains 50+ codes for carrying out various type of waveform correlation analyses (multiplet analysis, clustering, interferometry, …) This package is greatly strengthened by delegating numerous book-keeping and signal processing tasks to the underlying Waveform Suite. The Waveform Suite’s built-in tools for searching arbitrary directory/file structures is demonstrated with matched video and audio from the recent eruption of Redoubt Volcano. These tools were used to find subsets of photo images corresponding to specific seismic traces. Using Waveform’s audio file routines, matched video and audio were assembled to produce outreach-quality eruption products. The Waveform Suite is not designed as a ready-to-go replacement for more comprehensive packages such as SAC or AH. Rather, it is a suite of classes which provide core time series functionality in a MATLAB environment. It is designed to be a more robust alternative to the numerous ad hoc MATLAB formats that exist. Complex programs may be created upon the Waveform Suite’s framework, while existing programs may be modified to take advantage of the Waveform Suites capabilities.

Adding seismic broadband analysis to characterize Andean backarc seismicity in Argentina

NASA Astrophysics Data System (ADS)

Alvarado, P.; Giuliano, A.; Beck, S.; Zandt, G.

2007-05-01

Characterization of the highly seismically active Andean backarc is crucial for assessment of earthquake hazards in western Argentina. Moderate-to-large crustal earthquakes have caused several deaths, damage and drastic economic consequences in Argentinean history. We have studied the Andean backarc crust between 30°S and 36°S using seismic broadband data available from a previous ("the CHARGE") IRIS-PASSCAL experiment. We collected more than 12 terabytes of continuous seismic data from 22 broadband instruments deployed across Chile and Argentina during 1.5 years. Using free software we modeled full regional broadband waveforms and obtained seismic moment tensor inversions of crustal earthquakes testing for the best focal depth for each event. We also mapped differences in the Andean backarc crustal structure and found a clear correlation with different types of crustal seismicity (i.e. focal depths, focal mechanisms, magnitudes and frequencies of occurrence) and previously mapped terrane boundaries. We now plan to use the same methodology to study other regions in Argentina using near-real time broadband data available from the national seismic (INPRES) network and global seismic networks operating in the region. We will re-design the national seismic network to optimize short-period and broadband seismic station coverage for different network purposes. This work is an international effort that involves researchers and students from universities and national government agencies with the goal of providing more information about earthquake hazards in western Argentina.

Estimation of full moment tensors, including uncertainties, for earthquakes, volcanic events, and nuclear explosions

NASA Astrophysics Data System (ADS)

Alvizuri, Celso; Silwal, Vipul; Krischer, Lion; Tape, Carl

2017-04-01

A seismic moment tensor is a 3 × 3 symmetric matrix that provides a compact representation of seismic events within Earth's crust. We develop an algorithm to estimate moment tensors and their uncertainties from observed seismic data. For a given event, the algorithm performs a grid search over the six-dimensional space of moment tensors by generating synthetic waveforms at each grid point and then evaluating a misfit function between the observed and synthetic waveforms. 'The' moment tensor M for the event is then the moment tensor with minimum misfit. To describe the uncertainty associated with M, we first convert the misfit function to a probability function. The uncertainty, or rather the confidence, is then given by the 'confidence curve' P(V ), where P(V ) is the probability that the true moment tensor for the event lies within the neighborhood of M that has fractional volume V . The area under the confidence curve provides a single, abbreviated 'confidence parameter' for M. We apply the method to data from events in different regions and tectonic settings: small (Mw < 2.5) events at Uturuncu volcano in Bolivia, moderate (Mw > 4) earthquakes in the southern Alaska subduction zone, and natural and man-made events at the Nevada Test Site. Moment tensor uncertainties allow us to better discriminate among moment tensor source types and to assign physical processes to the events.

Convergence acceleration in scattering series and seismic waveform inversion using nonlinear Shanks transformation

NASA Astrophysics Data System (ADS)

Eftekhar, Roya; Hu, Hao; Zheng, Yingcai

2018-06-01

Iterative solution process is fundamental in seismic inversions, such as in full-waveform inversions and some inverse scattering methods. However, the convergence could be slow or even divergent depending on the initial model used in the iteration. We propose to apply Shanks transformation (ST for short) to accelerate the convergence of the iterative solution. ST is a local nonlinear transformation, which transforms a series locally into another series with an improved convergence property. ST works by separating the series into a smooth background trend called the secular term versus an oscillatory transient term. ST then accelerates the convergence of the secular term. Since the transformation is local, we do not need to know all the terms in the original series which is very important in the numerical implementation. The ST performance was tested numerically for both the forward Born series and the inverse scattering series (ISS). The ST has been shown to accelerate the convergence in several examples, including three examples of forward modeling using the Born series and two examples of velocity inversion based on a particular type of the ISS. We observe that ST is effective in accelerating the convergence and it can also achieve convergence even for a weakly divergent scattering series. As such, it provides a useful technique to invert for a large-contrast medium perturbation in seismic inversion.

Double-Difference Global Adjoint Tomography

NASA Astrophysics Data System (ADS)

Orsvuran, R.; Bozdag, E.; Lei, W.; Tromp, J.

2017-12-01

The adjoint method allows us to incorporate full waveform simulations in inverse problems. Misfit functions play an important role in extracting the relevant information from seismic waveforms. In this study, our goal is to apply the Double-Difference (DD) methodology proposed by Yuan et al. (2016) to global adjoint tomography. Dense seismic networks, such as USArray, lead to higher-resolution seismic images underneath continents. However, the imbalanced distribution of stations and sources poses challenges in global ray coverage. We adapt double-difference multitaper measurements to global adjoint tomography. We normalize each DD measurement by its number of pairs, and if a measurement has no pair, as may frequently happen for data recorded at oceanic stations, classical multitaper measurements are used. As a result, the differential measurements and pair-wise weighting strategy help balance uneven global kernel coverage. Our initial experiments with minor- and major-arc surface waves show promising results, revealing more pronounced structure near dense networks while reducing the prominence of paths towards cluster of stations. We have started using this new measurement in global adjoint inversions, addressing azimuthal anisotropy in upper mantle. Meanwhile, we are working on combining the double-difference approach with instantaneous phase measurements to emphasize contributions of scattered waves in global inversions and extending it to body waves. We will present our results and discuss challenges and future directions in the context of global tomographic inversions.

Historic and Instrumental Records of Repeating Seismicity in the Gyeongju Area, Southeastern Korea

NASA Astrophysics Data System (ADS)

HAN, M.; Kim, K. H.; Kang, S. Y.; Son, M.; Park, J. H.; LI, Z.

2015-12-01

Gyeongju area located in southeastern Korea has experienced repeated seismicity. Historic records during the last 2000 years in the area indicate the earthquake with magnitude 6.7 caused damages of human life and property in 779. During the period of modern instrumental seismic records, the area also experienced numerous small- and moderate-magnitude earthquakes. For example, an earthquake with magnitude 4.3 occurring in 1997 provided a chance for nationwide evaluations of earthquake safety and the renewal of earthquake monitoring system in Korea. The area is still experiencing small earthquakes including magnitude 3.5 in September 2014. We applied waveform correlation detector to continuously recorded seismic data from July 2014 to December 2014 to identify any repeating earthquakes. Detected waveforms are carefully inspected and more than 230 potential events are identified. Eighty three earthquakes among them have been selected for precise determination of earthquake hypocenters. Focal mechanism solutions for representative events were also determined. We further compared the results with those obtained using earthquakes prior to 2013. It has been confirmed the earthquakes in the area are clustered in space. Similar waveforms, earthquake locations, and focal mechanism solutions identified in the study indicates an active faults in the area. Since the area hosts many critical infra-structures, micro-seismicity in the area requires extensive study to address earthquake hazard issues.

Elastic Wave Imaging of in-Situ Bio-Alterations in a Contaminated Aquifer

NASA Astrophysics Data System (ADS)

Jaiswal, P.; Raj, R.; Atekwana, E. A.; Briand, B.; Alam, I.

2014-12-01

We present a pioneering report on the utility of seismic methods in imaging bio-induced elastic property changes within a contaminated aquifer. To understand physical properties of contaminated soil, we acquired 48 meters long multichannel seismic profile over the Norman landfill leachate plume in Norman Oklahoma, USA. We estimated both the P- and S- wave velocities respectively using full-waveform inversion of the transmission and the ground-roll coda. The resulting S-wave model showed distinct velocity anomaly (~10% over background) within the water table fluctuation zone bounded by the historical minimum and maximum groundwater table. In comparison, the P-wave velocity anomaly within the same zone was negligible. The Environmental Scanning Electron Microscope (ESEM) images of samples from a core located along the seismic profile clearly shows presence of biofilms in the water table fluctuation zone and their absence both above and below the fluctuation zone. Elemental chemistry further indicates that the sediment composition throughout the core is fairly constant. We conclude that the velocity anomaly in S-wave is due to biofilms. As a next step, we develop mechanistic modeling to gain insights into the petro-physical behavior of biofilm-bearing sediments. Preliminary results suggest that a plausible model could be biofilms acting as contact cement between sediment grains. The biofilm cement can be placed in two ways - (i) superficial non-contact deposition on sediment grains, and (ii) deposition at grain contacts. Both models explain P- and S- wave velocity structure at reasonable (~5-10%) biofilm saturation and are equivocally supported by the ESEM images. Ongoing attenuation modeling from full-waveform inversion and its mechanistic realization, may be able to further discriminate between the two cement models. Our study strongly suggests that as opposed to the traditional P-wave seismic, S-wave acquisition and imaging can be a more powerful tool for in-situ imaging of biofilm formation in field settings with significant implication for bioremediation and microbial enhanced oil recovery monitoring.

Change Detection via Cross-Borehole and VSP Seismic Surveys for the Source Physics Experiments (SPE) at the Nevada National Security Site (NNSS)

NASA Astrophysics Data System (ADS)

Knox, H. A.; Abbott, R. E.; Bonal, N. D.; Aldridge, D. F.; Preston, L. A.; Ober, C.

2012-12-01

In support of the Source Physics Experiment (SPE) at the Nevada National Security Site (NNSS), we have conducted two cross-borehole seismic experiments in the Climax Stock. The first experiment was conducted prior to the third shot in this multi-detonation program using two available boreholes and the shot hole, while the second experiment was conducted after the shot using four of the available boreholes. The first study focused on developing a well-characterized 2D pre-explosion Vp model including two VSPs and a seismic refraction survey, as well as quantifying baseline waveform similarity at reoccupied sites. This was accomplished by recording both "sparker" and accelerated weight drop sources on a hydrophone string and surface geophones. In total more than 18,500 unique source-receiver pairs were acquired during this testing. In the second experiment, we reacquired aproximately 8,800 source-receiver pairs and performed a cross-line survey allowing for a 3D post-explosion Vp model. The data acquired from the reoccupied sites was processed using cross-correlation methods and change detection methodologies, including comparison of the tomographic images. The survey design and subsequent processing provided an opportunity to investigate seismic wave propagation through damaged rock. We also performed full waveform forward modelling for a granitic body hosting a perched aquifer. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Homogenized moment tensor and the effect of near-field heterogeneities on nonisotropic radiation in nuclear explosion

NASA Astrophysics Data System (ADS)

Burgos, Gaël.; Capdeville, Yann; Guillot, Laurent

2016-06-01

We investigate the effect of small-scale heterogeneities close to a seismic explosive source, at intermediate periods (20-50 s), with an emphasis on the resulting nonisotropic far-field radiation. First, using a direct numerical approach, we show that small-scale elastic heterogeneities located in the near-field of an explosive source, generate unexpected phases (i.e., long period S waves). We then demonstrate that the nonperiodic homogenization theory applied to 2-D and 3-D elastic models, with various pattern of small-scale heterogeneities near the source, leads to accurate waveforms at a reduced computational cost compared to direct modeling. Further, it gives an interpretation of how nearby small-scale features interact with the source at low frequencies, through an explicit correction to the seismic moment tensor. In 2-D simulations, we find a deviatoric contribution to the moment tensor, as high as 21% for near-source heterogeneities showing a 25% contrast of elastic values (relative to a homogeneous background medium). In 3-D this nonisotropic contribution reaches 27%. Second, we analyze intermediate-periods regional seismic waveforms associated with some underground nuclear explosions conducted at the Nevada National Security Site and invert for the full moment tensor, in order to quantify the relative contribution of the isotropic and deviatoric components of the tensor. The average value of the deviatoric part is about 35%. We conclude that the interactions between an explosive source and small-scale local heterogeneities of moderate amplitude may lead to a deviatoric contribution to the seismic moment, close to what is observed using regional data from nuclear test explosions.

Seismic tomography of the southern California crust based on spectral-element and adjoint methods

NASA Astrophysics Data System (ADS)

Tape, Carl; Liu, Qinya; Maggi, Alessia; Tromp, Jeroen

2010-01-01

We iteratively improve a 3-D tomographic model of the southern California crust using numerical simulations of seismic wave propagation based on a spectral-element method (SEM) in combination with an adjoint method. The initial 3-D model is provided by the Southern California Earthquake Center. The data set comprises three-component seismic waveforms (i.e. both body and surface waves), filtered over the period range 2-30 s, from 143 local earthquakes recorded by a network of 203 stations. Time windows for measurements are automatically selected by the FLEXWIN algorithm. The misfit function in the tomographic inversion is based on frequency-dependent multitaper traveltime differences. The gradient of the misfit function and related finite-frequency sensitivity kernels for each earthquake are computed using an adjoint technique. The kernels are combined using a source subspace projection method to compute a model update at each iteration of a gradient-based minimization algorithm. The inversion involved 16 iterations, which required 6800 wavefield simulations. The new crustal model, m16, is described in terms of independent shear (VS) and bulk-sound (VB) wave speed variations. It exhibits strong heterogeneity, including local changes of +/-30 per cent with respect to the initial 3-D model. The model reveals several features that relate to geological observations, such as sedimentary basins, exhumed batholiths, and contrasting lithologies across faults. The quality of the new model is validated by quantifying waveform misfits of full-length seismograms from 91 earthquakes that were not used in the tomographic inversion. The new model provides more accurate synthetic seismograms that will benefit seismic hazard assessment.

Discriminating Induced-Microearthquakes Using New Seismic Features

NASA Astrophysics Data System (ADS)

Mousavi, S. M.; Horton, S.

2016-12-01

We studied characteristics of induced-microearthquakes on the basis of the waveforms recorded on a limited number of surface receivers using machine-learning techniques. Forty features in the time, frequency, and time-frequency domains were measured on each waveform, and several techniques such as correlation-based feature selection, Artificial Neural Networks (ANNs), Logistic Regression (LR) and X-mean were used as research tools to explore the relationship between these seismic features and source parameters. The results show that spectral features have the highest correlation to source depth. Two new measurements developed as seismic features for this study, spectral centroids and 2D cross-correlations in the time-frequency domain, performed better than the common seismic measurements. These features can be used by machine learning techniques for efficient automatic classification of low energy signals recorded at one or more seismic stations. We applied the technique to 440 microearthquakes-1.7Reference: Mousavi, S.M., S.P. Horton, C. A. Langston, B. Samei, (2016) Seismic features and automatic discrimination of deep and shallow induced-microearthquakes using neural network and logistic regression, Geophys. J. Int. doi: 10.1093/gji/ggw258.

ASDF - A Modern Data Format for Seismology

NASA Astrophysics Data System (ADS)

Krischer, Lion; Smith, James; Lei, Wenjie; Lefebvre, Matthieu; Ruan, Youyi; Sales de Andrade, Elliot; Podhorszki, Norbert; Bozdag, Ebru; Tromp, Jeroen

2017-04-01

Seismology as a science is driven by observing and understanding data and it is thus vital to make this as easy and accessible as possible. The growing volume of freely available data coupled with ever expanding computational power enables scientists to take on new and bigger problems. This evolution is to some part hindered as existing data formats have not been designed with it in mind. We present ASDF (http://seismic-data.org), the Adaptable Seismic Data Format, a novel, modern, and especially practical data format for all branches of seismology with particular focus on how it is incorporated into seismic full waveform inversion workflows. The format aims to solve five key issues: Efficiency: Fast I/O operations especially in high performance computing environments, especially limiting the total number of files. Data organization: Different types of data are needed for a variety of tasks. This results in ad hoc data organization and formats that are hard to maintain, integrate, reproduce, and exchange. Data exchange: We want to exchange complex and complete data sets. Reproducibility: Oftentimes just not existing but crucial to advance our science. Mining, visualization, and understanding of data: As data volumes grow, more complex, new techniques to query and visualize large datasets are needed. ASDF tackles these by defining a structure on top of HDF5 reusing as many existing standards (QuakeML, StationXML, PROV) as possible. An essential trait of ASDF is that it empowers the construction of completely self-describing data sets including waveform, station, and event data together with non-waveform data and a provenance description of everything. This for example for the first time enables the proper archival and exchange of processed or synthetic waveforms. To aid community adoption we developed mature tools in Python as well as in C and Fortran. Additionally we provide a formal definition of the format, a validation tool, and integration into widely used tools like ObsPy (http://obspy.org), SPECFEM GLOBE (https://geodynamics.org/cig/software/specfem3d_globe/), and Salvus (http://salvus.io).

Elastic full waveform inversion based on the homogenization method: theoretical framework and 2-D numerical illustrations

NASA Astrophysics Data System (ADS)

Capdeville, Yann; Métivier, Ludovic

2018-05-01

Seismic imaging is an efficient tool to investigate the Earth interior. Many of the different imaging techniques currently used, including the so-called full waveform inversion (FWI), are based on limited frequency band data. Such data are not sensitive to the true earth model, but to a smooth version of it. This smooth version can be related to the true model by the homogenization technique. Homogenization for wave propagation in deterministic media with no scale separation, such as geological media, has been recently developed. With such an asymptotic theory, it is possible to compute an effective medium valid for a given frequency band such that effective waveforms and true waveforms are the same up to a controlled error. In this work we make the link between limited frequency band inversion, mainly FWI, and homogenization. We establish the relation between a true model and an FWI result model. This relation is important for a proper interpretation of FWI images. We numerically illustrate, in the 2-D case, that an FWI result is at best the homogenized version of the true model. Moreover, it appears that the homogenized FWI model is quite independent of the FWI parametrization, as long as it has enough degrees of freedom. In particular, inverting for the full elastic tensor is, in each of our tests, always a good choice. We show how the homogenization can help to understand FWI behaviour and help to improve its robustness and convergence by efficiently constraining the solution space of the inverse problem.

Improved phase arrival estimate and location for local earthquakes in South Korea

NASA Astrophysics Data System (ADS)

Morton, E. A.; Rowe, C. A.; Begnaud, M. L.

2012-12-01

The Korean Institute of Geoscience and Mineral Resources (KIGAM) and the Korean Meteorological Agency (KMA) regularly report local (distance < ~1200 km) seismicity recorded with their networks; we obtain preliminary event location estimates as well as waveform data, but no phase arrivals are reported, so the data are not immediately useful for earthquake location. Our goal is to identify seismic events that are sufficiently well-located to provide accurate seismic travel-time information for events within the KIGAM and KMA networks, and also recorded by some regional stations. Toward that end, we are using a combination of manual phase identification and arrival-time picking, with waveform cross-correlation, to cluster events that have occurred in close proximity to one another, which allows for improved phase identification by comparing the highly correlating waveforms. We cross-correlate the known events with one another on 5 seismic stations and cluster events that correlate above a correlation coefficient threshold of 0.7, which reveals few clusters containing few events each. The small number of repeating events suggests that the online catalogs have had mining and quarry blasts removed before publication, as these can contribute significantly to repeating seismic sources in relatively aseismic regions such as South Korea. The dispersed source locations in our catalog, however, are ideal for seismic velocity modeling by providing superior sampling through the dense seismic station arrangement, which produces favorable event-to-station ray path coverage. Following careful manual phase picking on 104 events chosen to provide adequate ray coverage, we re-locate the events to obtain improved source coordinates. The re-located events are used with Thurber's Simul2000 pseudo-bending local tomography code to estimate the crustal structure on the Korean Peninsula, which is an important contribution to ongoing calibration for events of interest in the region.

Anisotropy effects on 3D waveform inversion

NASA Astrophysics Data System (ADS)

Stekl, I.; Warner, M.; Umpleby, A.

2010-12-01

In the recent years 3D waveform inversion has become achievable procedure for seismic data processing. A number of datasets has been inverted and presented (Warner el al 2008, Ben Hadj at all, Sirgue et all 2010) using isotropic 3D waveform inversion. However the question arises will the results be affected by isotropic assumption. Full-wavefield inversion techniques seek to match field data, wiggle-for-wiggle, to synthetic data generated by a high-resolution model of the sub-surface. In this endeavour, correctly matching the travel times of the principal arrivals is a necessary minimal requirement. In many, perhaps most, long-offset and wide-azimuth datasets, it is necessary to introduce some form of p-wave velocity anisotropy to match the travel times successfully. If this anisotropy is not also incorporated into the wavefield inversion, then results from the inversion will necessarily be compromised. We have incorporated anisotropy into our 3D wavefield tomography codes, characterised as spatially varying transverse isotropy with a tilted axis of symmetry - TTI anisotropy. This enhancement approximately doubles both the run time and the memory requirements of the code. We show that neglect of anisotropy can lead to significant artefacts in the recovered velocity models. We will present inversion results of inverting anisotropic 3D dataset by assuming isotropic earth and compare them with anisotropic inversion result. As a test case Marmousi model extended to 3D with no velocity variation in third direction and with added spatially varying anisotropy is used. Acquisition geometry is assumed as OBC with sources and receivers everywhere at the surface. We attempted inversion using both 2D and full 3D acquisition for this dataset. Results show that if no anisotropy is taken into account although image looks plausible most features are miss positioned in depth and space, even for relatively low anisotropy, which leads to incorrect result. This may lead to misinterpretation of results. However if correct physics is used results agree with correct model. Our algorithm is relatively affordable and runs on standard pc clusters in acceptable time. Refferences: H. Ben Hadj Ali, S. Operto and J. Virieux. Velocity model building by 3D frequency-domain full-waveform inversion of wide-aperture seismic data, Geophysics (Special issue: Velocity Model Building), 73(6), P. VE101-VE117 (2008). L. Sirgue, O.I. Barkved, J. Dellinger, J. Etgen, U. Albertin, J.H. Kommedal, Full waveform inversion: the next leap forward in imaging at Valhall, First Brake April 2010 - Issue 4 - Volume 28 M. Warner, I. Stekl, A. Umpleby, Efficient and Effective 3D Wavefield Tomography, 70th EAGE Conference & Exhibition (2008)

Intelligent earthquake data processing for global adjoint tomography

NASA Astrophysics Data System (ADS)

Chen, Y.; Hill, J.; Li, T.; Lei, W.; Ruan, Y.; Lefebvre, M. P.; Tromp, J.

2016-12-01

Due to the increased computational capability afforded by modern and future computing architectures, the seismology community is demanding a more comprehensive understanding of the full waveform information from the recorded earthquake seismograms. Global waveform tomography is a complex workflow that matches observed seismic data with synthesized seismograms by iteratively updating the earth model parameters based on the adjoint state method. This methodology allows us to compute a very accurate model of the earth's interior. The synthetic data is simulated by solving the wave equation in the entire globe using a spectral-element method. In order to ensure the inversion accuracy and stability, both the synthesized and observed seismograms must be carefully pre-processed. Because the scale of the inversion problem is extremely large and there is a very large volume of data to both be read and written, an efficient and reliable pre-processing workflow must be developed. We are investigating intelligent algorithms based on a machine-learning (ML) framework that will automatically tune parameters for the data processing chain. One straightforward application of ML in data processing is to classify all possible misfit calculation windows into usable and unusable ones, based on some intelligent ML models such as neural network, support vector machine or principle component analysis. The intelligent earthquake data processing framework will enable the seismology community to compute the global waveform tomography using seismic data from an arbitrarily large number of earthquake events in the fastest, most efficient way.

Detecting earthquakes over a seismic network using single-station similarity measures

NASA Astrophysics Data System (ADS)

Bergen, Karianne J.; Beroza, Gregory C.

2018-06-01

New blind waveform-similarity-based detection methods, such as Fingerprint and Similarity Thresholding (FAST), have shown promise for detecting weak signals in long-duration, continuous waveform data. While blind detectors are capable of identifying similar or repeating waveforms without templates, they can also be susceptible to false detections due to local correlated noise. In this work, we present a set of three new methods that allow us to extend single-station similarity-based detection over a seismic network; event-pair extraction, pairwise pseudo-association, and event resolution complete a post-processing pipeline that combines single-station similarity measures (e.g. FAST sparse similarity matrix) from each station in a network into a list of candidate events. The core technique, pairwise pseudo-association, leverages the pairwise structure of event detections in its network detection model, which allows it to identify events observed at multiple stations in the network without modeling the expected moveout. Though our approach is general, we apply it to extend FAST over a sparse seismic network. We demonstrate that our network-based extension of FAST is both sensitive and maintains a low false detection rate. As a test case, we apply our approach to 2 weeks of continuous waveform data from five stations during the foreshock sequence prior to the 2014 Mw 8.2 Iquique earthquake. Our method identifies nearly five times as many events as the local seismicity catalogue (including 95 per cent of the catalogue events), and less than 1 per cent of these candidate events are false detections.

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).

Unsupervised Approaches for Post-Processing in Computationally Efficient Waveform-Similarity-Based Earthquake Detection

NASA Astrophysics Data System (ADS)

Bergen, K.; Yoon, C. E.; OReilly, O. J.; Beroza, G. C.

2015-12-01

Recent improvements in computational efficiency for waveform correlation-based detections achieved by new methods such as Fingerprint and Similarity Thresholding (FAST) promise to allow large-scale blind search for similar waveforms in long-duration continuous seismic data. Waveform similarity search applied to datasets of months to years of continuous seismic data will identify significantly more events than traditional detection methods. With the anticipated increase in number of detections and associated increase in false positives, manual inspection of the detection results will become infeasible. This motivates the need for new approaches to process the output of similarity-based detection. We explore data mining techniques for improved detection post-processing. We approach this by considering similarity-detector output as a sparse similarity graph with candidate events as vertices and similarities as weighted edges. Image processing techniques are leveraged to define candidate events and combine results individually processed at multiple stations. Clustering and graph analysis methods are used to identify groups of similar waveforms and assign a confidence score to candidate detections. Anomaly detection and classification are applied to waveform data for additional false detection removal. A comparison of methods will be presented and their performance will be demonstrated on a suspected induced and non-induced earthquake sequence.

Observations of changes in waveform character induced by the 1999 Mw7.6 Chi-Chi earthquake

USGS Publications Warehouse

Chen, K.H.; Furumura, T.; Rubinstein, J.; Rau, R.-J.

2011-01-01

We observe changes in the waveforms of repeating earthquakes in eastern Taiwan following the 1999 Mw7.6 Chi-Chi earthquake, while their recurrence intervals appear to be unaffected. There is a clear reduction in waveform similarity and velocity changes indicated by delayed phases at the time of the Chi-Chi event. These changes are limited to stations in and paths that cross the 70 × 100 km region surrounding the Chi-Chi source area, the area where seismic intensity and co-seismic surface displacements were largest. This suggests that damage at the near-surface is responsible for the observed waveform changes. Delays are largest in the late S-wave coda, reaching approximately 120 ms. This corresponds to a path averaged Swave velocity reduction of approximately 1%. There is also evidence that damage in the fault-zone caused changes in waveform character at sites in the footwall, where source-receiver paths propagate either along or across the rupture. The reduction in waveform similarity persists through the most recent repeating event in our study (November 15, 2007), indicating that the subsurface damage induced by the Chi-Chi earthquake did not fully heal within the first 8 years following the Chi-Chi earthquake.

Observations of changes in waveform character induced by the 1999 M w7.6 Chi-Chi earthquake

USGS Publications Warehouse

Chen, K.H.; Furumura, T.; Rubinstein, J.; Rau, R.-J.

2011-01-01

We observe changes in the waveforms of repeating earthquakes in eastern Taiwan following the 1999 Mw7.6 Chi-Chi earthquake, while their recurrence intervals appear to be unaffected. There is a clear reduction in waveform similarity and velocity changes indicated by delayed phases at the time of the Chi-Chi event. These changes are limited to stations in and paths that cross the 70 ?? 100 km region surrounding the Chi-Chi source area, the area where seismic intensity and co-seismic surface displacements were largest. This suggests that damage at the near-surface is responsible for the observed waveform changes. Delays are largest in the late S-wave coda, reaching approximately 120 ms. This corresponds to a path averaged S wave velocity reduction of approximately 1%. There is also evidence that damage in the fault-zone caused changes in waveform character at sites in the footwall, where source-receiver paths propagate either along or across the rupture. The reduction in waveform similarity persists through the most recent repeating event in our study (November 15, 2007), indicating that the subsurface damage induced by the Chi-Chi earthquake did not fully heal within the first 8 years following the Chi-Chi earthquake. ?? 2011 by the American Geophysical Union.

Optimizing measurement geometry for seismic near-surface full waveform inversion

NASA Astrophysics Data System (ADS)

Nuber, André; Manukyan, Edgar; Maurer, Hansruedi

2017-09-01

Full waveform inversion (FWI) is an increasingly popular tool for analysing seismic data. Current practise is to record seismic data sets that are suitable for reflection processing, that is, a very dense spatial sampling and a high fold are required. Using tools from optimized experimental design (ED), we demonstrate that such a dense sampling is not necessary for FWI purposes. With a simple noise-free acoustic example, we show that only a few suitably selected source positions are required for computing high-quality images. A second, more extensive study includes elastic FWI with noise-contaminated data and free-surface boundary conditions on a typical near-surface setup, where surface waves play a crucial role. The study reveals that it is sufficient to employ a receiver spacing in the order of the minimum shear wavelength expected. Furthermore, we show that horizontally oriented sources and multicomponent receivers are the preferred option for 2-D elastic FWI, and we found that with a small amount of carefully selected source positions, similarly good results can be achieved, as if as many sources as receivers would have been employed. For the sake of simplicity, we assume in our simulations that the full data information content is available, but data pre-processing and the presence of coloured noise may impose restrictions. Our ED procedure requires an a priori subsurface model as input, but tests indicate that a relatively crude approximation to the true model is adequate. A further pre-requisite of our ED algorithm is that a suitable inversion strategy exists that accounts for the non-linearity of the FWI problem. Here, we assume that such a strategy is available. For the sake of simplicity, we consider only 2-D FWI experiments in this study, but our ED algorithm is sufficiently general and flexible, such that it can be adapted to other configurations, such as crosshole, vertical seismic profiling or 3-D surface setups, also including larger scale exploration experiments. It also offers interesting possibilities for analysing existing large-scale data sets that are too large to be inverted. With our methodology, it is possible to extract a small (and thus invertible) subset that offers similar information content as the full data set.

Towards seismic waveform inversion of long-offset Ocean-Bottom Seismic data for deep crustal imaging offshore Western Australia

NASA Astrophysics Data System (ADS)

Monnier, S.; Lumley, D. E.; Kamei, R.; Goncharov, A.; Shragge, J. C.

2016-12-01

Ocean Bottom Seismic datasets have become increasingly used in recent years to develop high-resolution, wavelength-scale P-wave velocity models of the lithosphere from waveform inversion, due to their recording of long-offset transmitted phases. New OBS surveys evolve towards novel acquisition geometries involving longer offsets (several hundreds of km), broader frequency content (1-100 Hz), while receiver sampling often remains sparse (several km). Therefore, it is critical to assess the effects of such geometries on the eventual success and resolution of waveform inversion velocity models. In this study, we investigate the feasibility of waveform inversion on the Bart 2D OBS profile acquired offshore Western Australia, to investigate regional crustal and Moho structures. The dataset features 14 broadband seismometers (0.01-100 Hz) from AuScope's national OBS fleet, offsets in excess of 280 km, and a sparse receiver sampling (18 km). We perform our analysis in four stages: (1) field data analysis, (2) 2D P-wave velocity model building, synthetic data (3) modelling, and (4) waveform inversion. Data exploration shows high-quality active-source signal down to 2Hz, and usable first arrivals to offsets greater than 100 km. The background velocity model is constructed by combining crustal and Moho information in continental reference models (e.g., AuSREM, AusMoho). These low-resolution studies suggest a crustal thickness of 20-25 km along our seismic line and constitute a starting point for synthetic modelling and inversion. We perform synthetic 2D time-domain modelling to: (1) evaluate the misfit between synthetic and field data within the usable frequency band (2-10 Hz); (2) validate our velocity model; and (3) observe the effects of sparse OBS interval on data quality. Finally, we apply 2D acoustic frequency-domain waveform inversion to the synthetic data to generate velocity model updates. The inverted model is compared to the reference model to investigate the improved crustal resolution and Moho boundary delineation that could be realized using waveform inversion, and to evaluate the effects of the acquisition parameters. The inversion strategies developed through the synthetic tests will help the subsequent inversion of sparse, long-offset OBS field data.

Toward Near Real-Time Tomography of the Upper Mantle

NASA Astrophysics Data System (ADS)

Debayle, E.; Dubuffet, F.

2014-12-01

We added a layer of automation to the Debayle and Ricard (2012)'s waveform modeling scheme for fundamental and higher mode surface waves in the period range 50-160s. We processed all the Rayleigh waveforms recorded on the LHZ channel by the virtual networks GSN_broadband, FDSN_all, and US_backbone between January 1996 and December 2013. Six millions of waveforms were obtained from IRIS DMC. We check that all the necessary information (instrument response, global CMT determination) is available and that each record includes a velocity window which encompasses the surface wave. Selected data must also have a signal-to-noise ratio greater than 3 in a range covering at least the periods between 50 and 100 s. About 3 millions of waveforms are selected (92% of the rejections are due to the signal to noise ratio criterion) and processed using Debayle and Ricard (2012)'s scheme, which allows the successful modeling of about 1.5 millions of waveforms. We complete this database with 60,000 waveforms recorded between 1976 and 1996 or after 1996 during various temporary experiments and with 161,730 Rayleigh waveforms analyzed at longer period, between 120 and 360 s. The whole data set is inverted using Debayle and Sambridge (2004)'s scheme to produce a 3D shear velocity model. A simple shell command "update_tomo" can then update our seismic model in an entirely automated way. Currently, this command checks from the CMT catalog what are the potential data available at the GSN_broadband, FDSN_all, and US_backbone virtual networks, uses web services to request these data from IRIS DMC and applies the processing chain described above to update our seismic model. We plan to update our seismic model on a regular basis in a near future, and to make it available on the web. Our most recent seismic model includes azimuthal anisotropy, achieves a lateral resolution of few hundred kilometers and a vertical resolution of a few tens of kilometers. The correlation with surface tectonics is very strong in the uppermost 200 km. Regions deeper than 400 km show no velocity contrasts larger than 1%, except for high velocity slabs which produce broad high velocity regions within the transition zone. The use of higher modes and long period surface waves allows us to extract the shear velocity structure down to about 1000 km depth.

Theoretical simulation of the multipole seismoelectric logging while drilling

NASA Astrophysics Data System (ADS)

Guan, Wei; Hu, Hengshan; Zheng, Xiaobo

2013-11-01

Acoustic logging-while-drilling (LWD) technology has been commercially used in the petroleum industry. However it remains a rather difficult task to invert formation compressional and shear velocities from acoustic LWD signals due to the unwanted strong collar wave, which covers or interferes with signals from the formation. In this paper, seismoelectric LWD is investigated for solving that problem. The seismoelectric field is calculated by solving a modified Poisson's equation, whose source term is the electric disturbance induced electrokinetically by the travelling seismic wave. The seismic wavefield itself is obtained by solving Biot's equations for poroelastic waves. From the simulated waveforms and the semblance plots for monopole, dipole and quadrupole sources, it is found that the electric field accompanies the collar wave as well as other wave groups of the acoustic pressure, despite the fact that seismoelectric conversion occurs only in porous formations. The collar wave in the electric field, however, is significantly weakened compared with that in the acoustic pressure, in terms of its amplitude relative to the other wave groups in the full waveforms. Thus less and shallower grooves are required to damp the collar wave if the seismoelectric LWD signals are recorded for extracting formation compressional and shear velocities.

Inversion of ground-motion data from a seismometer array for rotation using a modification of Jaeger's method

USGS Publications Warehouse

Chi, Wu-Cheng; Lee, W.H.K.; Aston, J.A.D.; Lin, C.J.; Liu, C.-C.

2011-01-01

We develop a new way to invert 2D translational waveforms using Jaeger's (1969) formula to derive rotational ground motions about one axis and estimate the errors in them using techniques from statistical multivariate analysis. This procedure can be used to derive rotational ground motions and strains using arrayed translational data, thus providing an efficient way to calibrate the performance of rotational sensors. This approach does not require a priori information about the noise level of the translational data and elastic properties of the media. This new procedure also provides estimates of the standard deviations of the derived rotations and strains. In this study, we validated this code using synthetic translational waveforms from a seismic array. The results after the inversion of the synthetics for rotations were almost identical with the results derived using a well-tested inversion procedure by Spudich and Fletcher (2009). This new 2D procedure can be applied three times to obtain the full, three-component rotations. Additional modifications can be implemented to the code in the future to study different features of the rotational ground motions and strains induced by the passage of seismic waves.

Support Vector Machine Model for Automatic Detection and Classification of Seismic Events

NASA Astrophysics Data System (ADS)

Barros, Vesna; Barros, Lucas

2016-04-01

The automated processing of multiple seismic signals to detect, localize and classify seismic events is a central tool in both natural hazards monitoring and nuclear treaty verification. However, false detections and missed detections caused by station noise and incorrect classification of arrivals are still an issue and the events are often unclassified or poorly classified. Thus, machine learning techniques can be used in automatic processing for classifying the huge database of seismic recordings and provide more confidence in the final output. Applied in the context of the International Monitoring System (IMS) - a global sensor network developed for the Comprehensive Nuclear-Test-Ban Treaty (CTBT) - we propose a fully automatic method for seismic event detection and classification based on a supervised pattern recognition technique called the Support Vector Machine (SVM). According to Kortström et al., 2015, the advantages of using SVM are handleability of large number of features and effectiveness in high dimensional spaces. Our objective is to detect seismic events from one IMS seismic station located in an area of high seismicity and mining activity and classify them as earthquakes or quarry blasts. It is expected to create a flexible and easily adjustable SVM method that can be applied in different regions and datasets. Taken a step further, accurate results for seismic stations could lead to a modification of the model and its parameters to make it applicable to other waveform technologies used to monitor nuclear explosions such as infrasound and hydroacoustic waveforms. As an authorized user, we have direct access to all IMS data and bulletins through a secure signatory account. A set of significant seismic waveforms containing different types of events (e.g. earthquake, quarry blasts) and noise is being analysed to train the model and learn the typical pattern of the signal from these events. Moreover, comparing the performance of the support-vector network to various classical learning algorithms used before in seismic detection and classification is an essential final step to analyze the advantages and disadvantages of the model.

Back to the Future: Long-Term Seismic Archives Revisited

NASA Astrophysics Data System (ADS)

Waldhauser, F.; Schaff, D. P.

2007-12-01

Archives of digital seismic data recorded by seismometer networks around the world have grown tremendously over the last several decades helped by the deployment of seismic stations and their continued operation within the framework of monitoring seismic activity. These archives typically consist of waveforms of seismic events and associated parametric data such as phase arrival time picks and the location of hypocenters. Catalogs of earthquake locations are fundamental data in seismology, and even in the Earth sciences in general. Yet, these locations have notoriously low spatial resolution because of errors in both the picks and the models commonly used to locate events one at a time. This limits their potential to address fundamental questions concerning the physics of earthquakes, the structure and composition of the Earth's interior, and the seismic hazards associated with active faults. We report on the comprehensive use of modern waveform cross-correlation based methodologies for high- resolution earthquake location - as applied to regional and global long-term seismic databases. By simultaneous re-analysis of two decades of the digital seismic archive of Northern California, reducing pick errors via cross-correlation and model errors via double-differencing, we achieve up to three orders of magnitude resolution improvement over existing hypocenter locations. The relocated events image networks of discrete faults at seismogenic depths across various tectonic settings that until now have been hidden in location uncertainties. Similar location improvements are obtained for earthquakes recorded at global networks by re- processing 40 years of parametric data from the ISC and corresponding waveforms archived at IRIS. Since our methods are scaleable and run on inexpensive Beowulf clusters, periodic re-analysis of entire archives may thus become a routine procedure to continuously improve resolution in existing catalogs. We demonstrate the role of seismic archives in obtaining the precise location of new events in real-time. Such information has considerable social and economic impact in the evaluation and mitigation of seismic hazards, for example, and highlights the need for consistent long-term seismic monitoring and archiving of records.

Full Moment Tensor Analysis Using First Motion Data at The Geysers Geothermal Field

NASA Astrophysics Data System (ADS)

Boyd, O.; Dreger, D. S.; Lai, V. H.; Gritto, R.

2012-12-01

Seismicity associated with geothermal energy production at The Geysers Geothermal Field in northern California has been increasing during the last forty years. We investigate source models of over fifty earthquakes with magnitudes ranging from Mw 3.5 up to Mw 4.5. We invert three-component, complete waveform data from broadband stations of the Berkeley Digital Seismic Network, the Northern California Seismic Network and the USA Array deployment (2005-2007) for the complete, six-element moment tensor. Some solutions are double-couple while others have substantial non-double-couple components. To assess the stability and significance of non-double-couple components, we use a suite of diagnostic tools including the F-test, Jackknife test, bootstrap and network sensitivity solution (NSS). The full moment tensor solutions of the studied events tend to plot in the upper half of the Hudson source type diagram where the fundamental source types include +CLVD, +LVD, tensile-crack, DC and explosion. Using the F-test to compare the goodness-of-fit values between the full and deviatoric moment tensor solutions, most of the full moment tensor solutions do not show a statistically significant improvement in fit over the deviatoric solutions. Because a small isotropic component may not significantly improve the fit, we include first motion polarity data to better constrain the full moment tensor solutions.

Moon meteoritic seismic hum: Steady state prediction

USGS Publications Warehouse

Lognonne, P.; Feuvre, M.L.; Johnson, C.L.; Weber, R.C.

2009-01-01

We use three different statistical models describing the frequency of meteoroid impacts on Earth to estimate the seismic background noise due to impacts on the lunar surface. Because of diffraction, seismic events on the Moon are typically characterized by long codas, lasting 1 h or more. We find that the small but frequent impacts generate seismic signals whose codas overlap in time, resulting in a permanent seismic noise that we term the "lunar hum" by analogy with the Earth's continuous seismic background seismic hum. We find that the Apollo era impact detection rates and amplitudes are well explained by a model that parameterizes (1) the net seismic impulse due to the impactor and resulting ejecta and (2) the effects of diffraction and attenuation. The formulation permits the calculation of a composite waveform at any point on the Moon due to simulated impacts at any epicentral distance. The root-mean-square amplitude of this waveform yields a background noise level that is about 100 times lower than the resolution of the Apollo long-period seismometers. At 2 s periods, this noise level is more than 1000 times lower than the low noise model prediction for Earth's microseismic noise. Sufficiently sensitive seismometers will allow the future detection of several impacts per day at body wave frequencies. Copyright 2009 by the American Geophysical Union.

Fast in-memory elastic full-waveform inversion using consumer-grade GPUs

NASA Astrophysics Data System (ADS)

Sivertsen Bergslid, Tore; Birger Raknes, Espen; Arntsen, Børge

2017-04-01

Full-waveform inversion (FWI) is a technique to estimate subsurface properties by using the recorded waveform produced by a seismic source and applying inverse theory. This is done through an iterative optimization procedure, where each iteration requires solving the wave equation many times, then trying to minimize the difference between the modeled and the measured seismic data. Having to model many of these seismic sources per iteration means that this is a highly computationally demanding procedure, which usually involves writing a lot of data to disk. We have written code that does forward modeling and inversion entirely in memory. A typical HPC cluster has many more CPUs than GPUs. Since FWI involves modeling many seismic sources per iteration, the obvious approach is to parallelize the code on a source-by-source basis, where each core of the CPU performs one modeling, and do all modelings simultaneously. With this approach, the GPU is already at a major disadvantage in pure numbers. Fortunately, GPUs can more than make up for this hardware disadvantage by performing each modeling much faster than a CPU. Another benefit of parallelizing each individual modeling is that it lets each modeling use a lot more RAM. If one node has 128 GB of RAM and 20 CPU cores, each modeling can use only 6.4 GB RAM if one is running the node at full capacity with source-by-source parallelization on the CPU. A parallelized per-source code using GPUs can use 64 GB RAM per modeling. Whenever a modeling uses more RAM than is available and has to start using regular disk space the runtime increases dramatically, due to slow file I/O. The extremely high computational speed of the GPUs combined with the large amount of RAM available for each modeling lets us do high frequency FWI for fairly large models very quickly. For a single modeling, our GPU code outperforms the single-threaded CPU-code by a factor of about 75. Successful inversions have been run on data with frequencies up to 40 Hz for a model of 2001 by 600 grid points with 5 m grid spacing and 5000 time steps, in less than 2.5 minutes per source. In practice, using 15 nodes (30 GPUs) to model 101 sources, each iteration took approximately 9 minutes. For reference, the same inversion run with our CPU code uses two hours per iteration. This was done using only a very simple wavefield interpolation technique, saving every second timestep. Using a more sophisticated checkpointing or wavefield reconstruction method would allow us to increase this model size significantly. Our results show that ordinary gaming GPUs are a viable alternative to the expensive professional GPUs often used today, when performing large scale modeling and inversion in geophysics.

Computational Approach for Improving Three-Dimensional Sub-Surface Earth Structure for Regional Earthquake Hazard Simulations in the San Francisco Bay Area

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

Rodgers, A. J.

In our Exascale Computing Project (ECP) we seek to simulate earthquake ground motions at much higher frequency than is currently possible. Previous simulations in the SFBA were limited to 0.5-1 Hz or lower (Aagaard et al. 2008, 2010), while we have recently simulated the response to 5 Hz. In order to improve confidence in simulated ground motions, we must accurately represent the three-dimensional (3D) sub-surface material properties that govern seismic wave propagation over a broad region. We are currently focusing on the San Francisco Bay Area (SFBA) with a Cartesian domain of size 120 x 80 x 35 km, butmore » this area will be expanded to cover a larger domain. Currently, the United States Geologic Survey (USGS) has a 3D model of the SFBA for seismic simulations. However, this model suffers from two serious shortcomings relative to our application: 1) it does not fit most of the available low frequency (< 1 Hz) seismic waveforms from moderate (magnitude M 3.5-5.0) earthquakes; and 2) it is represented with much lower resolution than necessary for the high frequency simulations (> 5 Hz) we seek to perform. The current model will serve as a starting model for full waveform tomography based on 3D sensitivity kernels. This report serves as the deliverable for our ECP FY2017 Quarter 4 milestone to FY 2018 “Computational approach to developing model updates”. We summarize the current state of 3D seismic simulations in the SFBA and demonstrate the performance of the USGS 3D model for a few selected paths. We show the available open-source waveform data sets for model updates, based on moderate earthquakes recorded in the region. We present a plan for improving the 3D model utilizing the available data and further development of our SW4 application. We project how the model could be improved and present options for further improvements focused on the shallow geotechnical layers using dense passive recordings of ambient and human-induced noise.« less

Investigating source processes of isotropic events

NASA Astrophysics Data System (ADS)

Chiang, Andrea

This dissertation demonstrates the utility of the complete waveform regional moment tensor inversion for nuclear event discrimination. I explore the source processes and associated uncertainties for explosions and earthquakes under the effects of limited station coverage, compound seismic sources, assumptions in velocity models and the corresponding Green's functions, and the effects of shallow source depth and free-surface conditions. The motivation to develop better techniques to obtain reliable source mechanism and assess uncertainties is not limited to nuclear monitoring, but they also provide quantitative information about the characteristics of seismic hazards, local and regional tectonics and in-situ stress fields of the region . This dissertation begins with the analysis of three sparsely recorded events: the 14 September 1988 US-Soviet Joint Verification Experiment (JVE) nuclear test at the Semipalatinsk test site in Eastern Kazakhstan, and two nuclear explosions at the Chinese Lop Nor test site. We utilize a regional distance seismic waveform method fitting long-period, complete, three-component waveforms jointly with first-motion observations from regional stations and teleseismic arrays. The combination of long period waveforms and first motion observations provides unique discrimination of these sparsely recorded events in the context of the Hudson et al. (1989) source-type diagram. We examine the effects of the free surface on the moment tensor via synthetic testing, and apply the moment tensor based discrimination method to well-recorded chemical explosions. These shallow chemical explosions represent rather severe source-station geometry in terms of the vanishing traction issues. We show that the combined waveform and first motion method enables the unique discrimination of these events, even though the data include unmodeled single force components resulting from the collapse and blowout of the quarry face immediately following the initial explosion. In contrast, recovering the announced explosive yield using seismic moment estimates from moment tensor inversion remains challenging but we can begin to put error bounds on our moment estimates using the NSS technique. The estimation of seismic source parameters is dependent upon having a well-calibrated velocity model to compute the Green's functions for the inverse problem. Ideally, seismic velocity models are calibrated through broadband waveform modeling, however in regions of low seismicity velocity models derived from body or surface wave tomography may be employed. Whether a velocity model is 1D or 3D, or based on broadband seismic waveform modeling or the various tomographic techniques, the uncertainty in the velocity model can be the greatest source of error in moment tensor inversion. These errors have not been fully investigated for the nuclear discrimination problem. To study the effects of unmodeled structures on the moment tensor inversion, we set up a synthetic experiment where we produce synthetic seismograms for a 3D model (Moschetti et al., 2010) and invert these data using Green's functions computed with a 1D velocity mode (Song et al., 1996) to evaluate the recoverability of input solutions, paying particular attention to biases in the isotropic component. The synthetic experiment results indicate that the 1D model assumption is valid for moment tensor inversions at periods as short as 10 seconds for the 1D western U.S. model (Song et al., 1996). The correct earthquake mechanisms and source depth are recovered with statistically insignificant isotropic components as determined by the F-test. Shallow explosions are biased by the theoretical ISO-CLVD tradeoff but the tectonic release component remains low, and the tradeoff can be eliminated with constraints from P wave first motion. Path-calibration to the 1D model can reduce non-double-couple components in earthquakes, non-isotropic components in explosions and composite sources and improve the fit to the data. When we apply the 3D model to real data, at long periods (20-50 seconds), we see good agreement in the solutions between the 1D and 3D models and slight improvement in waveform fits when using the 3D velocity model Green's functions. (Abstract shortened by ProQuest.).

Fully probabilistic seismic source inversion - Part 2: Modelling errors and station covariances

NASA Astrophysics Data System (ADS)

Stähler, Simon C.; Sigloch, Karin

2016-11-01

Seismic source inversion, a central task in seismology, is concerned with the estimation of earthquake source parameters and their uncertainties. Estimating uncertainties is particularly challenging because source inversion is a non-linear problem. In a companion paper, Stähler and Sigloch (2014) 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. A prerequisite for Bayesian inference is the proper characterisation of the noise afflicting the measurements, a problem we address here. 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 = 1 - CC 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. By identifying and quantifying this likelihood function, we make D and thus waveform cross-correlation measurements usable for fully probabilistic sampling strategies, in source inversion and related applications such as seismic tomography.

Microseismic Full Waveform Modeling in Anisotropic Media with Moment Tensor Implementation

NASA Astrophysics Data System (ADS)

Shi, Peidong; Angus, Doug; Nowacki, Andy; Yuan, Sanyi; Wang, Yanyan

2018-03-01

Seismic anisotropy which is common in shale and fractured rocks will cause travel-time and amplitude discrepancy in different propagation directions. For microseismic monitoring which is often implemented in shale or fractured rocks, seismic anisotropy needs to be carefully accounted for in source location and mechanism determination. We have developed an efficient finite-difference full waveform modeling tool with an arbitrary moment tensor source. The modeling tool is suitable for simulating wave propagation in anisotropic media for microseismic monitoring. As both dislocation and non-double-couple source are often observed in microseismic monitoring, an arbitrary moment tensor source is implemented in our forward modeling tool. The increments of shear stress are equally distributed on the staggered grid to implement an accurate and symmetric moment tensor source. Our modeling tool provides an efficient way to obtain the Green's function in anisotropic media, which is the key of anisotropic moment tensor inversion and source mechanism characterization in microseismic monitoring. In our research, wavefields in anisotropic media have been carefully simulated and analyzed in both surface array and downhole array. The variation characteristics of travel-time and amplitude of direct P- and S-wave in vertical transverse isotropic media and horizontal transverse isotropic media are distinct, thus providing a feasible way to distinguish and identify the anisotropic type of the subsurface. Analyzing the travel-times and amplitudes of the microseismic data is a feasible way to estimate the orientation and density of the induced cracks in hydraulic fracturing. Our anisotropic modeling tool can be used to generate and analyze microseismic full wavefield with full moment tensor source in anisotropic media, which can help promote the anisotropic interpretation and inversion of field data.

Microseismic Full Waveform Modeling in Anisotropic Media with Moment Tensor Implementation

NASA Astrophysics Data System (ADS)

Shi, Peidong; Angus, Doug; Nowacki, Andy; Yuan, Sanyi; Wang, Yanyan

2018-07-01

Seismic anisotropy which is common in shale and fractured rocks will cause travel-time and amplitude discrepancy in different propagation directions. For microseismic monitoring which is often implemented in shale or fractured rocks, seismic anisotropy needs to be carefully accounted for in source location and mechanism determination. We have developed an efficient finite-difference full waveform modeling tool with an arbitrary moment tensor source. The modeling tool is suitable for simulating wave propagation in anisotropic media for microseismic monitoring. As both dislocation and non-double-couple source are often observed in microseismic monitoring, an arbitrary moment tensor source is implemented in our forward modeling tool. The increments of shear stress are equally distributed on the staggered grid to implement an accurate and symmetric moment tensor source. Our modeling tool provides an efficient way to obtain the Green's function in anisotropic media, which is the key of anisotropic moment tensor inversion and source mechanism characterization in microseismic monitoring. In our research, wavefields in anisotropic media have been carefully simulated and analyzed in both surface array and downhole array. The variation characteristics of travel-time and amplitude of direct P- and S-wave in vertical transverse isotropic media and horizontal transverse isotropic media are distinct, thus providing a feasible way to distinguish and identify the anisotropic type of the subsurface. Analyzing the travel-times and amplitudes of the microseismic data is a feasible way to estimate the orientation and density of the induced cracks in hydraulic fracturing. Our anisotropic modeling tool can be used to generate and analyze microseismic full wavefield with full moment tensor source in anisotropic media, which can help promote the anisotropic interpretation and inversion of field data.

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

Rapid earthquake detection through GPU-Based template matching

NASA Astrophysics Data System (ADS)

Mu, Dawei; Lee, En-Jui; Chen, Po

2017-12-01

The template-matching algorithm (TMA) has been widely adopted for improving the reliability of earthquake detection. The TMA is based on calculating the normalized cross-correlation coefficient (NCC) between a collection of selected template waveforms and the continuous waveform recordings of seismic instruments. In realistic applications, the computational cost of the TMA is much higher than that of traditional techniques. In this study, we provide an analysis of the TMA and show how the GPU architecture provides an almost ideal environment for accelerating the TMA and NCC-based pattern recognition algorithms in general. So far, our best-performing GPU code has achieved a speedup factor of more than 800 with respect to a common sequential CPU code. We demonstrate the performance of our GPU code using seismic waveform recordings from the ML 6.6 Meinong earthquake sequence in Taiwan.

Extraction of Pn seismic signals from air-gun shots recorded by the Cascadia Amphibious seismic experiment

NASA Astrophysics Data System (ADS)

Rathnayaka, S.; Gao, H.

2017-12-01

The goal of this study is to extract Pn (head wave) seismic waveforms recorded by both offshore and onshore (broadband and short period) seismic stations and evaluate the data quality. Two offshore active-source seismic experiments, MGL 1211 and MGL 1212, were conducted from 13th June to 24th July 2012, during the first year deployment of the Cascadia Initiative Amphibious Array. In total, we choose 110 ocean bottom seismometers and 209 inland stations that are located along the entire Cascadia subduction zone. We first remove the instrument response, and then explore the potential frequency ranges and the diurnal effect. We make the common receiver gathering for each seismic station and filter the seismic waveforms at multiple frequency bands, ranging from 3-5 Hz, 5-10 Hz, 10-20 Hz, to 20-40 Hz, respectively. To quantitatively evaluate the data quality, we calculate the signal-to-noise ratio (SNR) of the waveforms for usable stations that record clear Pn arrivals at multiple frequency bands. Our results show that most offshore stations located at deep water (>1.5 km) record clear air-gun shot signals at frequencies higher than 3 Hz and up to 550 km away from the source. For most stations located on the shallow continental shelf, the seismic recordings appear much noisier at all the frequencies compared to stations at deep water. Three general trends are observed for the SNR distribution; First, the SNR ratio increases from lower to higher frequency bands; Second, the ratio decreases with the increasing source-to-receiver distance; And third, the ratio increases from shallow to deep water. We also observe a rough negative relationship of the signal-to-noise ratio with the thickness of the marine sediment. Only 5 inland stations record clear air-gun shot arrivals up to 200 km away from the source. More detailed data quality analysis with more results will also be present.

Forward and adjoint spectral-element simulations of seismic wave propagation using hardware accelerators

NASA Astrophysics Data System (ADS)

Peter, Daniel; Videau, Brice; Pouget, Kevin; Komatitsch, Dimitri

2015-04-01

Improving the resolution of tomographic images is crucial to answer important questions on the nature of Earth's subsurface structure and internal processes. Seismic tomography is the most prominent approach where seismic signals from ground-motion records are used to infer physical properties of internal structures such as compressional- and shear-wave speeds, anisotropy and attenuation. Recent advances in regional- and global-scale seismic inversions move towards full-waveform inversions which require accurate simulations of seismic wave propagation in complex 3D media, providing access to the full 3D seismic wavefields. However, these numerical simulations are computationally very expensive and need high-performance computing (HPC) facilities for further improving the current state of knowledge. During recent years, many-core architectures such as graphics processing units (GPUs) have been added to available large HPC systems. Such GPU-accelerated computing together with advances in multi-core central processing units (CPUs) can greatly accelerate scientific applications. There are mainly two possible choices of language support for GPU cards, the CUDA programming environment and OpenCL language standard. CUDA software development targets NVIDIA graphic cards while OpenCL was adopted mainly by AMD graphic cards. In order to employ such hardware accelerators for seismic wave propagation simulations, we incorporated a code generation tool BOAST into an existing spectral-element code package SPECFEM3D_GLOBE. This allows us to use meta-programming of computational kernels and generate optimized source code for both CUDA and OpenCL languages, running simulations on either CUDA or OpenCL hardware accelerators. We show here applications of forward and adjoint seismic wave propagation on CUDA/OpenCL GPUs, validating results and comparing performances for different simulations and hardware usages.

The damping of seismic waves and its determination from reflection seismograms

NASA Technical Reports Server (NTRS)

Engelhard, L.

1979-01-01

The damping in theoretical waveforms is described phenomenologically and a classification is proposed. A method for studying the Earth's crust was developed which includes this damping as derived from reflection seismograms. Seismic wave propagation by absorption, attenuation of seismic waves by scattering, and dispersion relations are considered. Absorption of seismic waves within the Earth as well as reflection and transmission of elastic waves seen through boundary layer absorption are also discussed.

An integrated investigation of the induced seismicity near Crooked Lake, Alberta, Canada in 2016

NASA Astrophysics Data System (ADS)

Wang, R.; Gu, Y. J.; Shen, J.; Schultz, R.

2016-12-01

In the past three years, the Crooked Lake (or Fox Creek) region has become one of the most seismically active areas in the Western Canada Sedimentary Basin (WCSB), mostly attributable to hydraulic-fracturing operations on shale gas. Among the human-related earthquakes, the January 12, 2016 event (M = 4.1) not only triggered the "red light" provincial protocol, leading to the temporary suspension of a near-by injection well, but also set a new magnitude record for earthquakes in Alberta in the last decade. In this study, we determine the source parameters (e.g., magnitude, hypocenter location) of this earthquake and its aftershocks using full moment tensor inversions. Our findings are consistent with the anthropogenic origin of this earthquake and the source solution of the main shock shows a strike-slip mechanism with limited non-double-couple components ( 22%). The candidate fault orientations, which are predominantly N-S and E-W trending, are consistent with those of earlier events in this region but different from induced events in other parts in the WCSB. The inferred compressional axis is supported by crustal stress orientations extracted from bore-hole breakouts and the right-lateral fault is preferred by both seismic and aeromagnetic data. A further analysis of the waveforms from the near-source stations (<10 km) detected nearly 100 pre-/aftershocks within a week of this earthquake. Systematic differences in the waveforms between earthquake multiples before and after the master event suggest moderate changes of seismic velocity structures at the injection depth around the source area, possibly a reflection of fluid migration and/or changes in stress field. In short, our integrated study on the January 2016 earthquake cluster offers critical insights on the nature of induced earthquakes in the Crooked Lake region and other parts of the WCSB.

Improving slowness estimate stability and visualization using limited sensor pair correlation on seismic arrays

NASA Astrophysics Data System (ADS)

Gibbons, Steven J.; Näsholm, S. P.; Ruigrok, E.; Kværna, T.

2018-04-01

Seismic arrays enhance signal detection and parameter estimation by exploiting the time-delays between arriving signals on sensors at nearby locations. Parameter estimates can suffer due to both signal incoherence, with diminished waveform similarity between sensors, and aberration, with time-delays between coherent waveforms poorly represented by the wave-front model. Sensor-to-sensor correlation approaches to parameter estimation have an advantage over direct beamforming approaches in that individual sensor-pairs can be omitted without necessarily omitting entirely the data from each of the sensors involved. Specifically, we can omit correlations between sensors for which signal coherence in an optimal frequency band is anticipated to be poor or for which anomalous time-delays are anticipated. In practice, this usually means omitting correlations between more distant sensors. We present examples from International Monitoring System seismic arrays with poor parameter estimates resulting when classical f-k analysis is performed over the full array aperture. We demonstrate improved estimates and slowness grid displays using correlation beamforming restricted to correlations between sufficiently closely spaced sensors. This limited sensor-pair correlation (LSPC) approach has lower slowness resolution than would ideally be obtained by considering all sensor-pairs. However, this ideal estimate may be unattainable due to incoherence and/or aberration and the LSPC estimate can often exploit all channels, with the associated noise-suppression, while mitigating the complications arising from correlations between very distant sensors. The greatest need for the method is for short-period signals on large aperture arrays although we also demonstrate significant improvement for secondary regional phases on a small aperture array. LSPC can also provide a robust and flexible approach to parameter estimation on three-component seismic arrays.

SIG-VISA: Signal-based Vertically Integrated Seismic Monitoring

NASA Astrophysics Data System (ADS)

Moore, D.; Mayeda, K. M.; Myers, S. C.; Russell, S.

2013-12-01

Traditional seismic monitoring systems rely on discrete detections produced by station processing software; however, while such detections may constitute a useful summary of station activity, they discard large amounts of information present in the original recorded signal. We present SIG-VISA (Signal-based Vertically Integrated Seismic Analysis), a system for seismic monitoring through Bayesian inference on seismic signals. By directly modeling the recorded signal, our approach incorporates additional information unavailable to detection-based methods, enabling higher sensitivity and more accurate localization using techniques such as waveform matching. SIG-VISA's Bayesian forward model of seismic signal envelopes includes physically-derived models of travel times and source characteristics as well as Gaussian process (kriging) statistical models of signal properties that combine interpolation of historical data with extrapolation of learned physical trends. Applying Bayesian inference, we evaluate the model on earthquakes as well as the 2009 DPRK test event, demonstrating a waveform matching effect as part of the probabilistic inference, along with results on event localization and sensitivity. In particular, we demonstrate increased sensitivity from signal-based modeling, in which the SIGVISA signal model finds statistical evidence for arrivals even at stations for which the IMS station processing failed to register any detection.

A Study of Regional Waveform Calibration in the Eastern Mediterranean Region.

NASA Astrophysics Data System (ADS)

di Luccio, F.; Pino, A.; Thio, H.

2002-12-01

We modeled Pnl phases from several moderate magnitude events in the eastern Mediterranean to test methods and to develop path calibrations for source determination. The study region spanning from the eastern part of the Hellenic arc to the eastern Anatolian fault is mostly interested by moderate earthquakes, that can produce relevant damages. The selected area consists of several tectonic environment, which produces increased level of difficulty in waveform modeling. The results of this study are useful for the analysis of regional seismicity and for seismic hazard as well, in particular because very few broadband seismic stations are available in the selected area. The obtained velocity model gives a 30 km crustal tickness and low upper mantle velocities. The applied inversion procedure to determine the source mechanism has been successful, also in terms of discrimination of depth, for the entire range of selected paths. We conclude that using the true calibration of the seismic structure and high quality broadband data, it is possible to determine the seismic source in terms of mechanism, even with a single station.

Seismpol_ a visual-basic computer program for interactive and automatic earthquake waveform analysis

NASA Astrophysics Data System (ADS)

Patanè, Domenico; Ferrari, Ferruccio

1997-11-01

A Microsoft Visual-Basic computer program for waveform analysis of seismic signals is presented. The program combines interactive and automatic processing of digital signals using data recorded by three-component seismic stations. The analysis procedure can be used in either an interactive earthquake analysis or an automatic on-line processing of seismic recordings. The algorithm works in the time domain using the Covariance Matrix Decomposition method (CMD), so that polarization characteristics may be computed continuously in real time and seismic phases can be identified and discriminated. Visual inspection of the particle motion in hortogonal planes of projection (hodograms) reduces the danger of misinterpretation derived from the application of the polarization filter. The choice of time window and frequency intervals improves the quality of the extracted polarization information. In fact, the program uses a band-pass Butterworth filter to process the signals in the frequency domain by analysis of a selected signal window into a series of narrow frequency bands. Significant results supported by well defined polarizations and source azimuth estimates for P and S phases are also obtained for short-period seismic events (local microearthquakes).

EDDIE Seismology: Introductory spectral analysis for undergraduates

NASA Astrophysics Data System (ADS)

Soule, D. C.; Gougis, R.; O'Reilly, C.

2016-12-01

We present a spectral seismology lesson in which students use spectral analysis to describe the frequency of seismic arrivals based on a conceptual presentation of waveforms and filters. The goal is for students to surpass basic waveform terminology and relate a time domain signals to their conjugates in the frequency domain. Although seismology instruction commonly engages students in analysis of authentic seismological data, this is less true for lower-level undergraduate seismology instruction due to coding barriers to many seismological analysis tasks. To address this, our module uses Seismic Canvas (Kroeger, 2015; https://seiscode.iris.washington.edu/projects/seismiccanvas), a graphically interactive application for accessing, viewing and analyzing waveform data, which we use to plot earthquake data in the time domain. Once students are familiarized with the general components of the waveform (i.e. frequency, wavelength, amplitude and period), they use Seismic Canvas to transform the data into the frequency domain. Bypassing the mathematics of Fourier Series allows focus on conceptual understanding by plotting and manipulating seismic data in both time and frequency domains. Pre/post-tests showed significant improvements in students' use of seismograms and spectrograms to estimate the frequency content of the primary wave, which demonstrated students' understanding of frequency and how data on the spectrogram and seismogram are related. Students were also able to identify the time and frequency of the largest amplitude arrival, indicating understanding of amplitude and use of a spectrogram as an analysis tool. Students were also asked to compare plots of raw data and the same data filtered with a high-pass filter, and identify the filter used to create the second plot. Students demonstrated an improved understanding of how frequency content can be removed from a signal in the spectral domain.

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.

Earthquake source properties of a shallow induced seismic sequence in SE Brazil

NASA Astrophysics Data System (ADS)

Agurto-Detzel, Hans; Bianchi, Marcelo; Prieto, Germán. A.; Assumpção, Marcelo

2017-04-01

We study source parameters of a cluster of 21 very shallow (<1 km depth) small-magnitude (Mw < 2) earthquakes induced by percolation of water by gravity in SE Brazil. Using a multiple empirical Green's functions (meGf) approach, we estimate seismic moments, corner frequencies, and static stress drops of these events by inversion of their spectral ratios. For the studied magnitude range (-0.3 < Mw < 1.9), we found an increase of stress drop with seismic moment. We assess associated uncertainties by considering different signal time windows and by performing a jackknife resampling of the spectral ratios. We also calculate seismic moments by full waveform inversion to independently validate our moments from spectral analysis. We propose repeated rupture on a fault patch at shallow depth, following continuous inflow of water, as the cause for the observed low absolute stress drop values (<1 MPa) and earthquake size dependency. To our knowledge, no other study on earthquake source properties of shallow events induced by water injection with no added pressure is available in the literature. Our study suggests that source parameter characterization may provide additional information of induced seismicity by hydraulic stimulation.

A Comprehensive Seismic Characterization of the Cove Fort-Sulphurdale Geothermal Site, Utah

NASA Astrophysics Data System (ADS)

Zhang, H.; Li, J.; Zhang, X.; Liu, Y.; Kuleli, H. S.; Toksoz, M. N.

2012-12-01

The Cove Fort-Sulphurdale geothermal area is located in the transition zone between the extensional Basin and Range Province to the west and the uplifted Colorado Plateau to the east. The region around the geothermal site has the highest heat flow values of over 260 mWm-2 in Utah. To better understand the structure around the geothermal site, the MIT group deployed 10 seismic stations for a period of one year from August 2010. The local seismic network detected over 500 local earthquakes, from which ~200 events located within the network were selected for further analysis. Our seismic analysis is focused on three aspects: seismic velocity and attenuation tomography, seismic event focal mechanism analysis, and seismic shear wave splitting analysis. First P- and S-wave arrivals are picked manually and then the waveform cross-correlation technique is applied to obtain more accurate differential times between event pairs observed on common stations. The double-difference tomography method of Zhang and Thurber (2003) is used to simultaneously determine Vp and Vs models and seismic event locations. For the attenuation tomography, we first calculate t* values from spectrum fitting and then invert them to get Q models based on known velocity models and seismic event locations. Due to the limited station coverage and relatively low signal to noise ratio, many seismic waveforms do not have clear first P arrival polarities and as a result the conventional focal mechanism determination method relying on the polarity information is not applicable. Therefore, we used the full waveform matching method of Li et al. (2010) to determine event focal mechanisms. For the shear wave splitting analysis, we used the cross-correlation method to determine the delay times between fast and slow shear waves and the polarization angles of fast shear waves. The delay times are further taken to image the anisotropy percentage distribution in three dimensions using the shear wave splitting tomography method of Zhang et al. (2007). For the study region, overall the velocity is lower and attenuation is higher in the western part. Correspondingly, the anisotropy is also stronger, indicating the fractures may be more developed in the western part. The average fast polarization directions of fast shear waves at each station mostly point NNE. From the focal mechanism analysis from selected events, it shows that the normal faulting events have strikes in NNE direction, and the events with strike slip mechanism have strikes either parallel with the NNE trending faults or their conjugate ones. Assuming the maximum horizontal stress (SHmax) is parallel with the strike of the normal faulting events and bisects the two fault planes of the strike-slip events, the inverted source mechanism suggests a NNE oriented maximum horizontal stress regime. This area is under W-E tensional stress, which means maximum compressional stress should be in the N-E or NNE direction in general. The combination of shear wave splitting and focal mechanism analysis suggests that in this region the faults and fractures are aligned in the NNE direction.

The recent seismicity of Teide volcano, Tenerife (Canary Islands, Spain)

NASA Astrophysics Data System (ADS)

D'Auria, L.; Albert, G. W.; Calvert, M. M.; Gray, A.; Vidic, C.; Barrancos, J.; Padilla, G.; García-Hernández, R.; Perez, N. M.

2017-12-01

Tenerife is an active volcanic island which experienced several eruptions of moderate intensity in historical times, and few explosive eruptions in the Holocene. The increasing population density and the consistent number of tourists are constantly raising the volcanic risk of the island.On 02/10/2016 a remarkable swarm of long-period events was recorded and was interpreted as the effect of a transient massive fluid discharge episode occurring within the deep hydrothermal system of Teide volcano. Actually, since Oct. 2016, the hydrothermal system of the volcano underwent a progressive pressurization, testified by the marked variation of different geochemical parameters. The most striking observation is the increase in the diffuse CO2 emission from the summit crater of Teide volcano which started increasing from a background value of about 20 tons/day and reaching a peak of 175 tons/day in Feb. 2017.The pressurization process has been accompanied by an increase in the volcano-tectonic seismicity of. Teide volcano, recorded by the Red Sísmica Canaria, managed by Instituto Volcanológico de Canarias (INVOLCAN). The network began its full operativity in Nov. 2016 and currently consists of 15 broadband seismic stations. Since Nov. 2016 the network detected more than 100 small magnitude earthquakes, located beneath Teide volcano at depths usually ranging between 5 and 15 km. On January 6th 2017 a M=2.5 earthquake was recorded in the area, being one of the strongest ever recorded since decades. Most of the events show typical features of the microseismicity of hydrothermal systems: high spatial and temporal clustering and similar waveforms of individual events which often are overlapped.We present the spatial and temporal distribution of the seismicity of Teide volcano since Nov. 2016, comparing it also with the past seismicity of the volcano. Furthermore we analyze the statistical properties of the numerous swarms recorded until now with the aid of a template-matching technique to detect a large amount of small events whose waveforms are hidden within the background noise.

Regional Moment Tensor Analysis of Earthquakes in Iran for 2010 to 2017 Using In-Country Data

NASA Astrophysics Data System (ADS)

Graybeal, D.; Braunmiller, J.

2017-12-01

Located in the middle of the Arabia-Eurasia continental collision, Iran is one of the most tectonically diverse and seismically active countries in the world. Until recently, however, seismic source parameter studies had to rely on teleseismic data or on data from temporary local arrays, which limited the scope of investigations. Relatively new broadband seismic networks operated by the Iranian Institute of Engineering Seismology (IIEES) and the Iranian Seismological Center (IRSC) currently consist of more than 100 stations and allow, for the first time, routine three-component full-waveform regional moment tensor analysis of the numerous M≥4.0 earthquakes that occur throughout the country. We use openly available, in-country data and include data from nearby permanent broadband stations available through IRIS and EIDA to improve azimuthal coverage for events in border regions. For the period from 2010 to 2017, we have obtained about 500 moment tensors for earthquakes ranging from Mw=3.6 to 7.8. The resulting database provides a unique, detailed view of deformation styles and earthquake depths in Iran. Overall, we find mainly thrust and strike-slip mechanisms as expected considering the convergent tectonic setting. Our magnitudes (Mw) are slightly smaller than ML and mb but comparable to Mw as reported in global catalogs (USGS ANSS). Event depths average about 3 km shallower than in global catalogs and are well constrained considering the capability of regional waveforms to resolve earthquake depth. Our dataset also contains several large magnitude main shock-aftershock sequences from different tectonic provinces, including the 2012 Ahar-Varzeghan (Mw=6.4), 2013 Kaki (Mw=6.5), and 2014 Murmuri (Mw=6.2) earthquakes. The most significant result in terms of seismogenesis and seismic hazard is that the vast majority of earthquakes occur at shallow depth, not in deeper basement. Our findings indicate that more than 80% of crustal seismicity in Iran likely occurs at depths of 12 km or less.

Iceberg calving as a primary source of regional‐scale glacier‐generated seismicity in the St. Elias Mountains, Alaska

USGS Publications Warehouse

O'Neel, Shad; Larsen, Christopher F.; Rupert, Natalia; Hansen, Roger

2010-01-01

Since the installation of the Alaska Regional Seismic Network in the 1970s, data analysts have noted nontectonic seismic events thought to be related to glacier dynamics. While loose associations with the glaciers of the St. Elias Mountains have been made, no detailed study of the source locations has been undertaken. We performed a two-step investigation surrounding these events, beginning with manual locations that guided an automated detection and event sifting routine. Results from the manual investigation highlight characteristics of the seismic waveforms including single-peaked (narrowband) spectra, emergent onsets, lack of distinct phase arrivals, and a predominant cluster of locations near the calving termini of several neighboring tidewater glaciers. Through these locations, comparison with previous work, analyses of waveform characteristics, frequency-magnitude statistics and temporal patterns in seismicity, we suggest calving as a source for the seismicity. Statistical properties and time series analysis of the event catalog suggest a scale-invariant process that has no single or simple forcing. These results support the idea that calving is often a response to short-lived or localized stress perturbations. Our results demonstrate the utility of passive seismic instrumentation to monitor relative changes in the rate and magnitude of iceberg calving at tidewater glaciers that may be volatile or susceptible to ensuing rapid retreat, especially when existing seismic infrastructure can be used.

Prospective testing of neo-deterministic seismic hazard scenarios for the Italian territory

NASA Astrophysics Data System (ADS)

Peresan, Antonella; Magrin, Andrea; Vaccari, Franco; Kossobokov, Vladimir; Panza, Giuliano F.

2013-04-01

A reliable and comprehensive characterization of expected seismic ground shaking, eventually including the related time information, is essential in order to develop effective mitigation strategies and increase earthquake preparedness. Moreover, any effective tool for SHA must demonstrate its capability in anticipating the ground shaking related with large earthquake occurrences, a result that can be attained only through rigorous verification and validation process. So far, the major problems in classical probabilistic methods for seismic hazard assessment, PSHA, consisted in the adequate description of the earthquake recurrence, particularly for the largest and sporadic events, and of the attenuation models, which may be unable to account for the complexity of the medium and of the seismic sources and are often weekly constrained by the available observations. Current computational resources and physical knowledge of the seismic waves generation and propagation processes allow nowadays for viable numerical and analytical alternatives to the use of attenuation relations. Accordingly, a scenario-based neo-deterministic approach, NDSHA, to seismic hazard assessment has been proposed, which allows considering a wide range of possible seismic sources as the starting point for deriving scenarios by means of full waveforms modeling. The method does not make use of attenuation relations and naturally supplies realistic time series of ground shaking, including reliable estimates of ground displacement readily applicable to seismic isolation techniques. Based on NDSHA, an operational integrated procedure for seismic hazard assessment has been developed, that allows for the definition of time dependent scenarios of ground shaking, through the routine updating of formally defined earthquake predictions. The integrated NDSHA procedure for seismic input definition, which is currently applied to the Italian territory, combines different pattern recognition techniques, designed for the space-time identification of strong earthquakes, with algorithms for the realistic modeling of ground motion. Accordingly, a set of deterministic scenarios of ground motion at bedrock, which refers to the time interval when a strong event is likely to occur within the alerted area, can be defined by means of full waveform modeling, both at regional and local scale. CN and M8S predictions, as well as the related time-dependent ground motion scenarios associated with the alarmed areas, are regularly updated every two months since 2006. The routine application of the time-dependent NDSHA approach provides information that can be useful in assigning priorities for timely mitigation actions and, at the same time, allows for a rigorous prospective testing and validation of the proposed methodology. As an example, for sites where ground shaking values greater than 0.2 g are estimated at bedrock, further investigations can be performed taking into account the local soil conditions, to assess the performances of relevant structures, such as historical and strategic buildings. The issues related with prospective testing and validation of the time-dependent NDSHA scenarios will be discussed, illustrating the results obtained for the recent strong earthquakes in Italy, including the May 20, 2012 Emilia earthquake.

Waveform Modeling Reveals Important Features of the Subduction Zone Seismic Structure Beneath the Tyrrhenian Sea, Italy

NASA Astrophysics Data System (ADS)

Di Luccio, F.; Persaud, P.; Pino, N. A.; Clayton, R. W.; Helmberger, D. V.; Li, D.

2016-12-01

Seismic images of the slab in southern Italy indicate a complex geodynamic system, although these images are strongly affected by limitations due to instrumental coverage, in terms of depth resolution and lateral extent. To help improve our knowledge of the structure of the Calabrian subduction zone, we analyze waveforms of regional events that occurred between 2001 and 2015 beneath the Tyrrhenian sea in the western Mediterranean. The selected events are deeper than 200 km and they were recorded at the Italian seismic network managed by Istituto Nazionale di Geofisica e Vulcanologia in Italy. We have also included recordings at ocean bottom seismometers and hydrophones, which were installed for a few months in 2000-2001, 2004-2005 and 2007-2008. Accurate selection of the source-to receiver raypaths can reveal significant differences at receivers, which are perpendicular to the trench with respect to other stations. P-wave complexity, converted phases and frequency content are some of the features we have observed for selected events. To investigate the slab structure, we model the waveforms using the 2D staggered grid Finite Difference method on graphics processing units developed by Li et al. (Geophys. J. Int., 2014).

Anatomy of Old Faithful hydrothermal system from subsurface seismic imaging of the Yellowstone Upper Geyser Basin

NASA Astrophysics Data System (ADS)

Wu, S. M.; Lin, F. C.; Farrell, J.; Ward, K. M.; Karplus, M. S.; Smith, R. B.

2017-12-01

The Upper Geyser Basin (UGB) in Yellowstone National Park contains one of the highest concentrations of hydrothermal features on Earth including the iconic Old Faithful Geyser (OFG). Although this system has been the focus of many geological, geochemical, and geophysical studies, the shallow (<200 m) subsurface structure and the hydrothermal tremor behavior remain poorly characterized. To probe the detailed structure that relates to the hydrothermal plumbing of the UGB, we deployed dense arrays of 3-C 5-Hz geophones in both November of 2015 and 2016, composed of 133 stations with 50 m spacing, and 519 station locations, with an 20 m spacing, respectively. By applying seismic interferometry techniques, we extracted Rayleigh-wave signals between 1-10 Hz via seismic signals excited by nearby hydrothermal features (e.g. geysers and pools). We observe a clear lateral velocity boundary at 3.3 Hz frequency that delineates a higher phase velocity of 1.6 km/sec in the NE and a lower phase velocity of 1.0 km/sec in the SW corresponding to the local geologic formation of rhyolitic and glacial deposits, respectively. We also image a relatively shallow (20-60 m deep) large reservoir with an estimated porosity 30% located 100 meters southwest of the OFG from the significant spatial-dependent waveform distortions and delays between 5-10 Hz frequency. This reservoir is likely controlled by the local geology with a rhyolitic deposit in the NE acting as a relatively impermeable barrier to vertical fluid ascent. To understand the pre-eruption tremor signals from OFG, we first study the seismic waveforms recorded at the closest station to the OFG cone. Many highly repetitive seismic pulses associated with bubble collapse, which compose the tremor signal, can be identified. Using a reference event template and the cross-correlation method, we can determine the onset of each individual bubbling event using a cross-correlation coefficient threshold of 0.8. Based on the detected timing, we then inspect the spatial and temporal variation of the event waveforms across the dense arrays. Clear correlation between temporal waveform variation and air temperature is observed. In this presentation, we will discuss the potential mechanisms of tremor waveform variation and how that can be used to improve our understanding of geyser dynamics.

Variable-period surface-wave magnitudes: A rapid and robust estimator of seismic moments

USGS Publications Warehouse

Bonner, J.; Herrmann, R.; Benz, H.

2010-01-01

We demonstrate that surface-wave magnitudes (Ms), measured at local, regional, and teleseismic distances, can be used as a rapid and robust estimator of seismic moment magnitude (Mw). We used the Russell (2006) variable-period surface-wave magnitude formula, henceforth called Ms(VMAX), to estimate the Ms for 165 North American events with 3.2

Moment tensor inversions using strong motion waveforms of Taiwan TSMIP data, 1993–2009

USGS Publications Warehouse

Chang, Kaiwen; Chi, Wu-Cheng; Gung, Yuancheng; Dreger, Douglas; Lee, William H K.; Chiu, Hung-Chie

2011-01-01

Earthquake source parameters are important for earthquake studies and seismic hazard assessment. Moment tensors are among the most important earthquake source parameters, and are now routinely derived using modern broadband seismic networks around the world. Similar waveform inversion techniques can also apply to other available data, including strong-motion seismograms. Strong-motion waveforms are also broadband, and recorded in many regions since the 1980s. Thus, strong-motion data can be used to augment moment tensor catalogs with a much larger dataset than that available from the high-gain, broadband seismic networks. However, a systematic comparison between the moment tensors derived from strong motion waveforms and high-gain broadband waveforms has not been available. In this study, we inverted the source mechanisms of Taiwan earthquakes between 1993 and 2009 by using the regional moment tensor inversion method using digital data from several hundred stations in the Taiwan Strong Motion Instrumentation Program (TSMIP). By testing different velocity models and filter passbands, we were able to successfully derive moment tensor solutions for 107 earthquakes of Mw >= 4.8. The solutions for large events agree well with other available moment tensor catalogs derived from local and global broadband networks. However, for Mw = 5.0 or smaller events, we consistently over estimated the moment magnitudes by 0.5 to 1.0. We have tested accelerograms, and velocity waveforms integrated from accelerograms for the inversions, and found the results are similar. In addition, we used part of the catalogs to study important seismogenic structures in the area near Meishan Taiwan which was the site of a very damaging earthquake a century ago, and found that the structures were dominated by events with complex right-lateral strike-slip faulting during the recent decade. The procedures developed from this study may be applied to other strong-motion datasets to compliment or fill gaps in catalogs from regional broadband networks and teleseismic networks.

The evolution of periodic seismicity, waveform similarity, and conduit processes during unrest episodes at Tungurahua volcano, Ecuador, in 2015

NASA Astrophysics Data System (ADS)

Bell, Andrew; Hernandez, Stephen; Gaunt, Elizabeth; Mothes, Patricia; Hidalgo, Silvana; Ruiz, Mario

2016-04-01

Tungurahua is a large andesitic stratovolcano located in the Andes of Ecuador. The current eruptive phase at Tungurahua began in 1999, and has been characterised by episodes of vulcanian and strombolian activity, interspersed by periods of relative quiescence. Despite showing only modest eruptive activity in 2015, seismic data revealed a pronounced change in the behaviour of the magma-conduit system compared to the preceding 15 years of activity. The change is most notable in the periodicity of interevent-times of volcanic earthquakes. Previous seismicity at Tungurahua is characterised by interevent-time periodicities typical of a Poisson process, or modestly clustered, with slightly elevated (anti-clustered) periodicities observed only rarely during vulcanian episodes. However, activity in 2015 saw a series of unrest episodes characterised by highly-periodic interevent-times, and including several notable episodes of 'drumbeat' earthquakes. Here we report seismic and associated geophysical signals recorded at Tungurahua in 2015 by the monitoring network of the Instituto Geofisico of Ecuador, their relation to conduit processes, and implications for the origins of unrest and likely future activity. Although the nature of the low-frequency seismic signals change both within and between unrest episodes, the underlying periodicity is more consistent and gradually evolving. Waveform similarity is high within phases, resulting from the repeated activation of persistent sources, but low between different episodes, suggesting the emergence of new sources and locations. The strength of periodicity is correlated with the average waveform similarity for all unrest episodes, with the relatively low waveform similarities observed for the highly periodic drumbeat earthquakes in April due to contamination from coexisting continuous tremor. Eruptive activity consisted of a few minor explosions and ash emission events. Notably, a short-lived episode of Strombolian activity in November with juvenile magma was quickly followed by a resumption of periodic low-frequency seismicity. The changes in the seismicity of Tungurahua in 2015 suggest a significant change in the magma-conduit system. Elevated periodicity may indicate the presence of a slowly upward moving plug at a depth of 1-2km below the summit crater, likely associated with the unusually long repose period since the last major vulcanian episode in October 2014. Evolution in the periodicity and type of seismic signals within and between unrest episodes will be controlled by a combination of the gas flux and permeability, and a balance between thermo-mechanical plug degradation and time-dependent healing processes. These factors are also likely to determine the nature of future eruptive activity.

An efficient repeating signal detector to investigate earthquake swarms

NASA Astrophysics Data System (ADS)

Skoumal, Robert J.; Brudzinski, Michael R.; Currie, Brian S.

2016-08-01

Repetitive earthquake swarms have been recognized as key signatures in fluid injection induced seismicity, precursors to volcanic eruptions, and slow slip events preceding megathrust earthquakes. We investigate earthquake swarms by developing a Repeating Signal Detector (RSD), a computationally efficient algorithm utilizing agglomerative clustering to identify similar waveforms buried in years of seismic recordings using a single seismometer. Instead of relying on existing earthquake catalogs of larger earthquakes, RSD identifies characteristic repetitive waveforms by rapidly identifying signals of interest above a low signal-to-noise ratio and then grouping based on spectral and time domain characteristics, resulting in dramatically shorter processing time than more exhaustive autocorrelation approaches. We investigate seismicity in four regions using RSD: (1) volcanic seismicity at Mammoth Mountain, California, (2) subduction-related seismicity in Oaxaca, Mexico, (3) induced seismicity in Central Alberta, Canada, and (4) induced seismicity in Harrison County, Ohio. In each case, RSD detects a similar or larger number of earthquakes than existing catalogs created using more time intensive methods. In Harrison County, RSD identifies 18 seismic sequences that correlate temporally and spatially to separate hydraulic fracturing operations, 15 of which were previously unreported. RSD utilizes a single seismometer for earthquake detection which enables seismicity to be quickly identified in poorly instrumented regions at the expense of relying on another method to locate the new detections. Due to the smaller computation overhead and success at distances up to ~50 km, RSD is well suited for real-time detection of low-magnitude earthquake swarms with permanent regional networks.

Focal mechanisms and moment magnitudes of micro-earthquakes in central Brazil by waveform inversion with quality assessment and inference of the local stress field

NASA Astrophysics Data System (ADS)

Carvalho, Juraci; Barros, Lucas Vieira; Zahradník, Jiří

2016-11-01

This paper documents an investigation on the use of full waveform inversion to retrieve focal mechanisms of 11 micro-earthquakes (Mw 0.8 to 1.4). The events represent aftershocks of a 5.0 mb earthquake that occurred on October 8, 2010 close to the city of Mara Rosa in the state of Goiás, Brazil. The main contribution of the work lies in demonstrating the feasibility of waveform inversion of such weak events. The inversion was made possible thanks to recordings available at 8 temporary seismic stations in epicentral distances of less than 8 km, at which waveforms can be successfully modeled at relatively high frequencies (1.5-2.0 Hz). On average, the fault-plane solutions obtained are in agreement with a composite focal mechanism previously calculated from first-motion polarities. They also agree with the fault geometry inferred from precise relocation of the Mara Rosa aftershock sequence. The focal mechanisms provide an estimate of the local stress field. This paper serves as a pilot study for similar investigations in intraplate regions where the stress-field investigations are difficult due to rare earthquake occurrences, and where weak events must be studied with a detailed quality assessment.

Full-waveform inversion of surface waves in exploration geophysics

NASA Astrophysics Data System (ADS)

Borisov, D.; Gao, F.; Williamson, P.; Tromp, J.

2017-12-01

Full-waveform inversion (FWI) is a data fitting approach to estimate high-resolution properties of the Earth from seismic data by minimizing the misfit between observed and calculated seismograms. In land seismics, the source on the ground generates high-amplitude surface waves, which generally represent most of the energy recorded by ground sensors. Although surface waves are widely used in global seismology and engineering studies, they are typically treated as noise within the seismic exploration community since they mask deeper reflections from the intervals of exploration interest. This is mainly due to the fact that surface waves decay exponentially with depth and for a typical frequency range (≈[5-50] Hz) sample only the very shallow part of the subsurface, but also because they are much more sensitive to S-wave than P-wave velocities. In this study, we invert surface waves in the hope of using them as additional information for updating the near surface. In a heterogeneous medium, the main challenge of surface wave inversion is associated with their dispersive character, which makes it difficult to define a starting model for conventional FWI which can avoid cycle-skipping. The standard approach to dealing with this is by inverting the dispersion curves in the Fourier (f-k) domain to generate locally 1-D models, typically for the shear wavespeeds only. However this requires that the near-surface zone be more or less horizontally invariant over a sufficient distance for the spatial Fourier transform to be applicable. In regions with significant topography, such as foothills, this is not the case, so we revert to the time-space domain, but aim to minimize the differences of envelopes in the early stages of the inversion to resolve the cycle-skipping issue. Once the model is good enough, we revert to the classic waveform-difference inversion. We first present a few synthetic examples. We show that classical FWI might be trapped in a local minimum even for relatively simple scenario, while FWI with envelopes is stable and can converge using an inaccurate starting model. We also perform resolution analysis using a checkerboard test. We then present a field example. The final shear wavespeed model is compared to the results from the inversion of dispersion curves.

Joint inversion of time-lapse VSP data for monitoring CO2 injection at the Farnsworth EOR field in Texas

NASA Astrophysics Data System (ADS)

Zhang, M.; Gao, K.; Balch, R. S.; Huang, L.

2016-12-01

During the Development Phase (Phase III) of the U.S. Southwest Regional Partnership on Carbon Sequestration (SWP), time-lapse 3D vertical seismic profiling (VSP) data were acquired to monitor CO2 injection/migration at the Farnsworth Enhanced Oil Recovery (EOR) field, in partnership with the industrial partner Chaparral Energy. The project is to inject a million tons of carbon dioxide into the target formation, the deep oil-bearing Morrow Formation in the Farnsworth Unit EOR field. Quantitative time-lapse seismic monitoring has the potential to track CO2 movement in geologic carbon storage sites. Los Alamos National Laboratory (LANL) has recently developed new full-waveform inversion methods to jointly invert time-lapse seismic data for changes in elastic and anisotropic parameters in target monitoring regions such as a CO2 reservoir. We apply our new joint inversion methods to time-lapse VSP data acquired at the Farnsworth EOR filed, and present some preliminary results showing geophysical properties changes in the reservoir.

Is the Local Seismicity in Haiti Capable of Imaging the Northern Caribbean Subduction?

NASA Astrophysics Data System (ADS)

Corbeau, J.; Clouard, V.; Rolandone, F.; Leroy, S. D.; de Lepinay, B. M.

2017-12-01

The boundary between the Caribbean (CA) and North American (NAM) plates in the Hispaniola region is the western prolongation of the NAM plate subduction evolving from a frontal subduction in the Lesser Antilles to an oblique collision against the Bahamas platform in Cuba. We analyze P-waveforms arriving at 27 broadband seismic temporary stations deployed along a 200 km-long N-S transect across Haiti, during the Trans-Haiti project. We compute teleseismic receiver functions using the ETMTRF method, and determine crustal thickness and bulk composition (Vp/Vs) using the H-k stacking method. Three distinctive crustal domains are imaged. We relate these domains to crustal terranes that have been accreted along the plate boundary during the northeastwards displacement of the CA plate. We propose a N-S crustal profile across Haiti accounting for the surface geology, shallow structural history and these new seismological constraints. Local seismicity recorded by the temporary network from April 2013 to June 2014 is used to relocate the seismicity. A total of 593 events were identified with magnitudes ranging from 1.6 to 4.5. This local seismicity, predominantly shallow (< 20 km) and situated in the southern part of Haiti along the major Enriquillo-Plantain-Garden strike-slip fault zone (EPGFZ) and offshore in Gonâve Bay, helps us to image deep active structures. Moment tensors for earthquakes with magnitudes between 3 and 4 were calculated by full waveform inversion using the ISOLA software. The analysis of the new moment tensors for the Haiti upper lithosphere indicates that normal, thrust and strike-slip faulting are equitably distributed. We found strike-slip events along the EPGFZ, near the location of the January 12th, 2010 earthquake. Most of the normal events are located in the area of Enriquillo and Azuei lakes, while the thrust events are located on both sides of the southern Peninsula of Haiti. The preliminary seismic data of our Haitian network, even noisy, tend to confirm that the North American slab in western Hispaniola is disappearing and that the scarcity of the seismic events could not be only the effect of the lack of a seismic network. Due to the geometry of the plate boundary, the deformation is predominantly strike-slip and there is no accommodation of an important part of convergence in this area.

Imaging Faults in Carbonate Reservoir using Full Waveform Inversion and Reverse Time Migration of Walkaway VSP Data

NASA Astrophysics Data System (ADS)

Takam Takougang, E. M.; Bouzidi, Y.

2016-12-01

Multi-offset Vertical Seismic Profile (walkaway VSP) data were collected in an oil field located in a shallow water environment dominated by carbonate rocks, offshore the United Arab Emirates. The purpose of the survey was to provide structural information of the reservoir, around and away from the borehole. Five parallel lines were collected using an air gun at 25 m shot interval and 4 m source depth. A typical recording tool with 20 receivers spaced every 15.1 m, and located in a deviated borehole with an angle varying between 0 and 24 degree from the vertical direction, was used to record the data. The recording tool was deployed at different depths for each line, from 521 m to 2742 m depth. Smaller offsets were used for shallow receivers and larger offsets for deeper receivers. The lines merged to form the input dataset for waveform tomography. The total length of the combined lines was 9 km, containing 1344 shots and 100 receivers in the borehole located half-way down. Acoustic full waveform inversion was applied in the frequency domain to derive a high resolution velocity model. The final velocity model derived after the inversion using the frequencies 5-40 Hz, showed good correlation with velocities estimated from vertical incidence VSP and sonic log, confirming the success of the inversion. The velocity model showed anomalous low values in areas that correlate with known location of hydrocarbon reservoir. Pre-stack depth Reverse time migration was then applied using the final velocity model from waveform inversion and the up-going wavefield from the input data. The final estimated source signature from waveform inversion was used as input source for reverse time migration. To save computational memory and time, every 3 shots were used during reverse time migration and the data were low-pass filtered to 30 Hz. Migration artifacts were attenuated using a second order derivative filter. The final migration image shows a good correlation with the waveform tomography velocity model, and highlights a complex network of faults in the reservoir, that could be useful in understanding fluid and hydrocarbon movements. This study shows that the combination of full waveform tomography and reverse time migration can provide high resolution images that can enhance interpretation and characterization of oil reservoirs.

Integrating long-offset transient electromagnetics (LOTEM) with seismics in an exploration environment

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

Strack, K.M.; Vozoff, K.

The applications of electromagnetics have increased in the past two decades because of an improved understanding of the methods, improves service availability, and the increased focus of exploration in the more complex reservoir characterization issues. For electromagnetic methods surface applications for hydrocarbon Exploration and Production are still a special case, while applications in borehole and airborne research and for engineering and environmental objectives are routine. In the past, electromagnetic techniques, in particular deep transient electromagnetics, made up a completely different discipline in geophysics, although many of the principles are similar to the seismic one. With an understanding of the specificmore » problems related to data processing initially and then acquisition, the inclusion of principles learned from seismics happened almost naturally. Initially, the data processing was very similar to seismic full-waveform processing. The hardware was also changed to include multichannel acquisition systems, and the field procedures became very similar to seismic surveying. As a consequence, the integration and synergism of the interpretation process is becoming almost automatic. The long-offset transient electromagnetic (LOTEM) technique will be summarized from the viewpoint of its similarity to seismics. The complete concept of the method will also be reviewed. An interpretation case history that integrates seismic and LOTEM from a hydrocarbon area in China clearly demonstrates the limitations and benefits of the method.« less

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

Dreger, Douglas S.; Ford, Sean R.; Walter, William R.

Research was carried out investigating the feasibility of using a regional distance seismic waveform moment tensor inverse procedure to estimate source parameters of nuclear explosions and to use the source inversion results to develop a source-type discrimination capability. The results of the research indicate that it is possible to robustly determine the seismic moment tensor of nuclear explosions, and when compared to natural seismicity in the context of the a Hudson et al. (1989) source-type diagram they are found to separate from populations of earthquakes and underground cavity collapse seismic sources.

Numerical Procedure to Forecast the Tsunami Parameters from a Database of Pre-Simulated Seismic Unit Sources

NASA Astrophysics Data System (ADS)

Jiménez, César; Carbonel, Carlos; Rojas, Joel

2018-04-01

We have implemented a numerical procedure to forecast the parameters of a tsunami, such as the arrival time of the front of the first wave and the maximum wave height in real and virtual tidal stations along the Peruvian coast, with this purpose a database of pre-computed synthetic tsunami waveforms (or Green functions) was obtained from numerical simulation of seismic unit sources (dimension: 50 × 50 km2) for subduction zones from southern Chile to northern Mexico. A bathymetry resolution of 30 arc-sec (approximately 927 m) was used. The resulting tsunami waveform is obtained from the superposition of synthetic waveforms corresponding to several seismic unit sources contained within the tsunami source geometry. The numerical procedure was applied to the Chilean tsunami of April 1, 2014. The results show a very good correlation for stations with wave amplitude greater than 1 m, in the case of the Arica tide station an error (from the maximum height of the observed and simulated waveform) of 3.5% was obtained, for Callao station the error was 12% and the largest error was in Chimbote with 53.5%, however, due to the low amplitude of the Chimbote wave (<1 m), the overestimated error, in this case, is not important for evacuation purposes. The aim of the present research is tsunami early warning, where speed is required rather than accuracy, so the results should be taken as preliminary.

Numerical Procedure to Forecast the Tsunami Parameters from a Database of Pre-Simulated Seismic Unit Sources

NASA Astrophysics Data System (ADS)

Jiménez, César; Carbonel, Carlos; Rojas, Joel

2017-09-01

We have implemented a numerical procedure to forecast the parameters of a tsunami, such as the arrival time of the front of the first wave and the maximum wave height in real and virtual tidal stations along the Peruvian coast, with this purpose a database of pre-computed synthetic tsunami waveforms (or Green functions) was obtained from numerical simulation of seismic unit sources (dimension: 50 × 50 km2) for subduction zones from southern Chile to northern Mexico. A bathymetry resolution of 30 arc-sec (approximately 927 m) was used. The resulting tsunami waveform is obtained from the superposition of synthetic waveforms corresponding to several seismic unit sources contained within the tsunami source geometry. The numerical procedure was applied to the Chilean tsunami of April 1, 2014. The results show a very good correlation for stations with wave amplitude greater than 1 m, in the case of the Arica tide station an error (from the maximum height of the observed and simulated waveform) of 3.5% was obtained, for Callao station the error was 12% and the largest error was in Chimbote with 53.5%, however, due to the low amplitude of the Chimbote wave (<1 m), the overestimated error, in this case, is not important for evacuation purposes. The aim of the present research is tsunami early warning, where speed is required rather than accuracy, so the results should be taken as preliminary.

Southern Africa seismic structure and source studies

NASA Astrophysics Data System (ADS)

Zhao, Ming

1998-09-01

The upper mantle seismic velocity structure beneath southern Africa is investigated using travel time and waveform data. Waveform and travel time data used in this study come mainly from a large mine tremor in South Africa (msb{b} 5.6) recorded on stations of the southern Africa and the Tanzania Broadband Seismic Experiment. Auxiliary data along similar profiles are obtained from other moderate events within eastern and southern Africa. The waveform data from the large tremor show upper mantle triplications for both the 400 and 670-km discontinuities between 18sp° and 27sp° distance. The most notable feature of the data is a large, late P phase that propagates to at least 27sp°. This phase is striking because of its late arrival time (as much as 15 seconds after direct P at 27sp°) and high amplitude relative to the first arrival. Travel times from all available stations are used to invert for the P wave velocity structure down to 800 km depth and S wave velocity structure down to 200 km using the Wiechert-Herglotz (W-H) inversion technique. The P wave velocities from the uppermost mantle down to 300 km are as much as 3% higher than the global average and are slightly slower than the global average between 300 and 400 km depths. The velocity gradient between 300 and 400 km is 0.0015 1/s. The S wave travel time data yield fast velocities above 200-km depth. The S wave velocity structure appears inconsistent with the P wave structure model indicating varying Poisson's ratio in the upper mantle. Little evidence is found for a pronounced upper mantle low velocity zone. Both sharp and gradual-change 400-km discontinuities are favored by the waveform data. The 670-km discontinuity appears as a gradual-change zone. The source mechanism of the mb 5.6 mining tremor itself is important for seismic discrimination and insight into mining tremor sources. Source parameters for this event as well as some other large mining tremors from the South African gold mines are studied using detailed waveform modeling. All these events (mb > 4.8) indicate normal-faulting slip with P wave nodal planes striking approximately NS. Tectonic stress is essential to control the mining seismicity of large magnitude. Mining geometry also plays an important role in influencing the seismicity. The crustal velocity structure at the study area is investigated in detail using teleseismic receiver function and regional surface wave dispersion data. The results indicate some lateral variation in the shallow crust. The thickness of the crust beneath the GSN station BOSA is 33-36 km. Gradually increasing velocities with depth in the crust are preferred. A thin layer with rather low velocity at the top of the crust beneath BOSA is important for generating the regional waveforms. The crust beneath LBTB is a few kilometers thicker than at BOSA and the Moho there is likely to be dipping. (Abstract shortened by UMI.)

Viscoelastic Finite Difference Modeling Using Graphics Processing Units

NASA Astrophysics Data System (ADS)

Fabien-Ouellet, G.; Gloaguen, E.; Giroux, B.

2014-12-01

Full waveform seismic modeling requires a huge amount of computing power that still challenges today's technology. This limits the applicability of powerful processing approaches in seismic exploration like full-waveform inversion. This paper explores the use of Graphics Processing Units (GPU) to compute a time based finite-difference solution to the viscoelastic wave equation. The aim is to investigate whether the adoption of the GPU technology is susceptible to reduce significantly the computing time of simulations. The code presented herein is based on the freely accessible software of Bohlen (2002) in 2D provided under a General Public License (GNU) licence. This implementation is based on a second order centred differences scheme to approximate time differences and staggered grid schemes with centred difference of order 2, 4, 6, 8, and 12 for spatial derivatives. The code is fully parallel and is written using the Message Passing Interface (MPI), and it thus supports simulations of vast seismic models on a cluster of CPUs. To port the code from Bohlen (2002) on GPUs, the OpenCl framework was chosen for its ability to work on both CPUs and GPUs and its adoption by most of GPU manufacturers. In our implementation, OpenCL works in conjunction with MPI, which allows computations on a cluster of GPU for large-scale model simulations. We tested our code for model sizes between 1002 and 60002 elements. Comparison shows a decrease in computation time of more than two orders of magnitude between the GPU implementation run on a AMD Radeon HD 7950 and the CPU implementation run on a 2.26 GHz Intel Xeon Quad-Core. The speed-up varies depending on the order of the finite difference approximation and generally increases for higher orders. Increasing speed-ups are also obtained for increasing model size, which can be explained by kernel overheads and delays introduced by memory transfers to and from the GPU through the PCI-E bus. Those tests indicate that the GPU memory size and the slow memory transfers are the limiting factors of our GPU implementation. Those results show the benefits of using GPUs instead of CPUs for time based finite-difference seismic simulations. The reductions in computation time and in hardware costs are significant and open the door for new approaches in seismic inversion.

Uncertainty Estimation in Elastic Full Waveform Inversion by Utilising the Hessian Matrix

NASA Astrophysics Data System (ADS)

Hagen, V. S.; Arntsen, B.; Raknes, E. B.

2017-12-01

Elastic Full Waveform Inversion (EFWI) is a computationally intensive iterative method for estimating elastic model parameters. A key element of EFWI is the numerical solution of the elastic wave equation which lies as a foundation to quantify the mismatch between synthetic (modelled) and true (real) measured seismic data. The misfit between the modelled and true receiver data is used to update the parameter model to yield a better fit between the modelled and true receiver signal. A common approach to the EFWI model update problem is to use a conjugate gradient search method. In this approach the resolution and cross-coupling for the estimated parameter update can be found by computing the full Hessian matrix. Resolution of the estimated model parameters depend on the chosen parametrisation, acquisition geometry, and temporal frequency range. Although some understanding has been gained, it is still not clear which elastic parameters can be reliably estimated under which conditions. With few exceptions, previous analyses have been based on arguments using radiation pattern analysis. We use the known adjoint-state technique with an expansion to compute the Hessian acting on a model perturbation to conduct our study. The Hessian is used to infer parameter resolution and cross-coupling for different selections of models, acquisition geometries, and data types, including streamer and ocean bottom seismic recordings. Information about the model uncertainty is obtained from the exact Hessian, and is essential when evaluating the quality of estimated parameters due to the strong influence of source-receiver geometry and frequency content. Investigation is done on both a homogeneous model and the Gullfaks model where we illustrate the influence of offset on parameter resolution and cross-coupling as a way of estimating uncertainty.

Local Wave Propagation and Crustal Structure Tomography in Northern Mississippi Embayment

NASA Astrophysics Data System (ADS)

Yang, Y.; Langston, C. A.

2016-12-01

Several datasets in the vicinity of the New Madrid Seismic Zone (NMSZ) are used to study local wave propagation and crustal structure in this region, including data collected for the Northern Embayment Lithosphere Experiment (NELE) project, Transportable Array, New Madrid Cooperative Network and Embayment Seismic Excitation Experiment (ESEE). Focal mechanisms and focal depths are determined with the help of synthetic seismograms for earthquakes with magnitude larger than 3. The thick unconsolidated sediment complicates waveforms inside the Mississippi Embayment by producing large converted PS, SP phases and reverberations that mask important near-source depth phases. Modeling events with well-constrained focal mechanisms using synthetic seismograms reveals a variety of waveguide propagation effects including P and S sediment reverberations as well as leaky mode P wave trains. Substantial differences in the travel time of the mid-crustal reflection are observed for waves traveling in different directions. The travel time of the mid-crustal reflection waves and direct waves are then used in a tomography for the crustal structure. The result reveals that there is a significant southwest dip to the top of the mid-crust in the vicinity of the NMSZ. Resulting image and the determined source parameters are essential for full waveform inversion to determine high-resolution crustal structure of the Northern Mississippi Embayment.

Resolution of VTI anisotropy with elastic full-waveform inversion: theory and basic numerical examples

NASA Astrophysics Data System (ADS)

Podgornova, O.; Leaney, S.; Liang, L.

2018-07-01

Extracting medium properties from seismic data faces some limitations due to the finite frequency content of the data and restricted spatial positions of the sources and receivers. Some distributions of the medium properties make low impact on the data (including none). If these properties are used as the inversion parameters, then the inverse problem becomes overparametrized, leading to ambiguous results. We present an analysis of multiparameter resolution for the linearized inverse problem in the framework of elastic full-waveform inversion. We show that the spatial and multiparameter sensitivities are intertwined and non-sensitive properties are spatial distributions of some non-trivial combinations of the conventional elastic parameters. The analysis accounts for the Hessian information and frequency content of the data; it is semi-analytical (in some scenarios analytical), easy to interpret and enhances results of the widely used radiation pattern analysis. Single-type scattering is shown to have limited sensitivity, even for full-aperture data. Finite-frequency data lose multiparameter sensitivity at smooth and fine spatial scales. Also, we establish ways to quantify a spatial-multiparameter coupling and demonstrate that the theoretical predictions agree well with the numerical results.

Data-Intensive Discovery Methods for Seismic Monitoring

NASA Astrophysics Data System (ADS)

Richards, P. G.; Schaff, D. P.; Young, C. J.; Slinkard, M.; Heck, S.; Ammon, C. J.; Cleveland, M.

2011-12-01

For most regions of our planet, earthquakes and explosions are still located one-at-a-time using seismic phase picks-a procedure that has not fundamentally changed for more than a century. But methods that recognize and use seismogram archives as a major resource, enabling comparisons of waveforms recorded from neighboring events and relocating numerous events relative to each other, have been successfully demonstrated, especially for California, where they have enabled new insights into earthquake physics and Earth structure, and have raised seismic monitoring to new levels. We are beginning a series of projects to evaluate such data-intensive methods on ever-larger scales, using cross correlation (CC) to analyze seismicity in three different ways: (1) to find repeating earthquakes (whose waveforms are very similar, so the CC value measured over long windows must be high); (2) to measure time differences and amplitude differences to enable precise relocations and relative amplitude studies, of seismic events with respect to their neighboring events (then CC can be much lower, yet still give a better estimate of arrival time differences and relative amplitudes, compared to differencing phase picks and magnitudes); and, perhaps most importantly, (3) as a detector, to find new events in current data streams that are similar to events already in the archive, or to add to the number of detections of an already known event. Experience documented by Schaff and Waldhauser (2005) for California and Schaff (2009) for China indicates that the great majority of events in seismically active regions generate waveforms that are sufficiently similar to the waveforms of neighboring events to allow CC methods to be used to obtain relative locations. Schaff (2008, 2010) has demonstrated the capability of CC methods to achieve detections, with minimal false alarms, down to more than a magnitude unit below conventional STA/LTA detectors though CC methods are far more computationally-intensive. Elsewhere at this meeting Cleveland, Ammon, and Van DeMark report in more detail on greatly-improved event locations along oceanic fracture zones using CC methods applied to 40-80s Rayleigh waves; and Slinkard, Carr, Heck and Young at Sandia have reported greatly-improved computational approaches that reduce CPU demands from hours using a fast workstation to minutes using a GPU, when a continuous data stream lasting several days is searched (using CC methods) for seismic signals similar to those of hundreds of previously documented events. From diverse results such as these, it seems appropriate to consider the future possibility of radical improvement in monitoring virtually all seismically active areas, using archives of prior events as the major resource-though we recognize that such an approach does not directly help to characterize seismic events in inactive regions, or events in active regions which are dissimilar to previously recorded events.

A Bootstrap-Based Probabilistic Optimization Method to Explore and Efficiently Converge in Solution Spaces of Earthquake Source Parameter Estimation Problems: Application to Volcanic and Tectonic Earthquakes

NASA Astrophysics Data System (ADS)

Dahm, T.; Heimann, S.; Isken, M.; Vasyura-Bathke, H.; Kühn, D.; Sudhaus, H.; Kriegerowski, M.; Daout, S.; Steinberg, A.; Cesca, S.

2017-12-01

Seismic source and moment tensor waveform inversion is often ill-posed or non-unique if station coverage is poor or signals are weak. Therefore, the interpretation of moment tensors can become difficult, if not the full model space is explored, including all its trade-offs and uncertainties. This is especially true for non-double couple components of weak or shallow earthquakes, as for instance found in volcanic, geothermal or mining environments.We developed a bootstrap-based probabilistic optimization scheme (Grond), which is based on pre-calculated Greens function full waveform databases (e.g. fomosto tool, doi.org/10.5880/GFZ.2.1.2017.001). Grond is able to efficiently explore the full model space, the trade-offs and the uncertainties of source parameters. The program is highly flexible with respect to the adaption to specific problems, the design of objective functions, and the diversity of empirical datasets.It uses an integrated, robust waveform data processing based on a newly developed Python toolbox for seismology (Pyrocko, see Heimann et al., 2017, http://doi.org/10.5880/GFZ.2.1.2017.001), and allows for visual inspection of many aspects of the optimization problem. Grond has been applied to the CMT moment tensor inversion using W-phases, to nuclear explosions in Korea, to meteorite atmospheric explosions, to volcano-tectonic events during caldera collapse and to intra-plate volcanic and tectonic crustal events.Grond can be used to optimize simultaneously seismological waveforms, amplitude spectra and static displacements of geodetic data as InSAR and GPS (e.g. KITE, Isken et al., 2017, http://doi.org/10.5880/GFZ.2.1.2017.002). We present examples of Grond optimizations to demonstrate the advantage of a full exploration of source parameter uncertainties for interpretation.

Towards Full-Waveform Ambient Noise Inversion

NASA Astrophysics Data System (ADS)

Sager, Korbinian; Ermert, Laura; Afanasiev, Michael; Boehm, Christian; Fichtner, Andreas

2017-04-01

Noise tomography usually works under the assumption that the inter-station ambient noise correlation is equal to a scaled version of the Green function between the two receivers. This assumption, however, is only met under specific conditions, e.g. wavefield diffusivity and equipartitioning, or the isotropic distribution of both mono- and dipolar uncorrelated noise sources. These assumptions are typically not satisfied in the Earth. This inconsistency inhibits the exploitation of the full waveform information contained in noise correlations in order to constrain Earth structure and noise generation. To overcome this limitation, we attempt to develop a method that consistently accounts for the distribution of noise sources, 3D heterogeneous Earth structure and the full seismic wave propagation physics. This is intended to improve the resolution of tomographic images, to refine noise source distribution, and thereby to contribute to a better understanding of both Earth structure and noise generation. First, we develop an inversion strategy based on a 2D finite-difference code using adjoint techniques. To enable a joint inversion for noise sources and Earth structure, we investigate the following aspects: i) the capability of different misfit functionals to image wave speed anomalies and source distribution and ii) possible source-structure trade-offs, especially to what extent unresolvable structure can be mapped into the inverted noise source distribution and vice versa. In anticipation of real-data applications, we present an extension of the open-source waveform modelling and inversion package Salvus (http://salvus.io). It allows us to compute correlation functions in 3D media with heterogeneous noise sources at the surface and the corresponding sensitivity kernels for the distribution of noise sources and Earth structure. By studying the effect of noise sources on correlation functions in 3D, we validate the aforementioned inversion strategy and prepare the workflow necessary for the first application of full waveform ambient noise inversion to a global dataset, for which a model for the distribution of noise sources is already available.

Microtremor survey to investigate seismic vulnerability around the Seulimum Fault, Aceh Besar-Indonesia

NASA Astrophysics Data System (ADS)

Simanjuntak, Andrean V. H.; Muksin, Umar; Rahmayani, Febrina

2018-05-01

The Seulimeum Fault has generated inland earthquake with magnitude larger than M 6.5 that destroyed houses in the Lamteuba and Krueng Raya Villages. Earthquakes along the Seulimeum Fault are mostly right lateral strike-slip characterizing the Fault. The understanding of the seismic vulnerability around the highly populated Banda Aceh City and the villages in Aceh Besar is therefore very important since the city, and the villages are very close to the Seulimeum Fault. A microtremor survey has been conducted to investigate seismic vulnerability in the area closed to the Seulimeum Fault. The waveforms of the microtremor have been recorded in Lamteuba and Kreung Raya villages, Aceh Besar at 20 sites for 7 days from August 14, 2017 with the interval of measurement of 1 km. The waveforms recorded for 30 minutes at each site by using one Taurus Seismometer in miniseed format. The data has been analyzing by using Geopsy to obtain the Horizontal-Vertical Spectral Ratio for each site. The seismic vulnerability is considered to be high if the value of the Horizontal-Vertical Spectral Ratio is high. The HVSR values are then interpolated to obtain the seismic vulnerability map. The preliminary result shows high seismic vulnerability in the area around the first site.

Estimation of Dynamic Friction Process of the Akatani Landslide Based on the Waveform Inversion and Numerical Simulation

NASA Astrophysics Data System (ADS)

Yamada, M.; Mangeney, A.; Moretti, L.; Matsushi, Y.

2014-12-01

Understanding physical parameters, such as frictional coefficients, velocity change, and dynamic history, is important issue for assessing and managing the risks posed by deep-seated catastrophic landslides. Previously, landslide motion has been inferred qualitatively from topographic changes caused by the event, and occasionally from eyewitness reports. However, these conventional approaches are unable to evaluate source processes and dynamic parameters. In this study, we use broadband seismic recordings to trace the dynamic process of the deep-seated Akatani landslide that occurred on the Kii Peninsula, Japan, which is one of the best recorded large slope failures. Based on the previous results of waveform inversions and precise topographic surveys done before and after the event, we applied numerical simulations using the SHALTOP numerical model (Mangeney et al., 2007). This model describes homogeneous continuous granular flows on a 3D topography based on a depth averaged thin layer approximation. We assume a Coulomb's friction law with a constant friction coefficient, i. e. the friction is independent of the sliding velocity. We varied the friction coefficients in the simulation so that the resulting force acting on the surface agrees with the single force estimated from the seismic waveform inversion. Figure shows the force history of the east-west components after the band-pass filtering between 10-100 seconds. The force history of the simulation with frictional coefficient 0.27 (thin red line) the best agrees with the result of seismic waveform inversion (thick gray line). Although the amplitude is slightly different, phases are coherent for the main three pulses. This is an evidence that the point-source approximation works reasonably well for this particular event. The friction coefficient during the sliding was estimated to be 0.38 based on the seismic waveform inversion performed by the previous study and on the sliding block model (Yamada et al., 2013), whereas the frictional coefficient estimated from the numerical simulation was about 0.27. This discrepancy may be due to the digital elevation model, to the other forces such as pressure gradients and centrifugal acceleration included in the model. However, quantitative interpretation of this difference requires further investigation.

A Quantitative Evaluation of SCEC Community Velocity Model Version 3.0

NASA Astrophysics Data System (ADS)

Chen, P.; Zhao, L.; Jordan, T. H.

2003-12-01

We present a systematic methodology for evaluating and improving 3D seismic velocity models using broadband waveform data from regional earthquakes. The operator that maps a synthetic waveform into an observed waveform is expressed in the Rytov form D(ω ) = {exp}[{i} ω δ τ {p}(ω ) - ω δ τ {q}(ω )]. We measure the phase delay time δ τ p(ω ) and the amplitude reduction time δ τ q(ω ) as a function of frequency ω using Gee & Jordan's [1992] isolation-filter technique, and we correct the data for frequency-dependent interference and frequency-independent source statics. We have applied this procedure to a set of small events in Southern California. Synthetic seismograms were computed using three types of velocity models: the 1D Standard Southern California Crustal Model (SoCaL) [Dreger & Helmberger, 1993], the 3D SCEC Community Velocity Model, Version 3.0 (CVM3.0) [Magistrale et al., 2000], and a set of path-averaged 1D models (A1D) extracted from CVM3.0 by horizontally averaging wave slownesses along source-receiver paths. The 3D synthetics were computed using K. Olsen's finite difference code. More than 1000 measurements were made on both P and S waveforms at frequencies ranging from 0.2 to 1 Hz. Overall, the 3D model provided a substantially better fit to the waveform data than either laterally homogeneous or path-dependent 1D models. Relative to SoCaL, CVM3.0 provided a variance reduction of about 64% in δ τ p, and 41% in δ τ q. Relative to A1D, the variance reduction is about 46% and 20%, respectively. The same set of measurements can be employed to invert for both seismic source properties and seismic velocity structures. Fully numerical methods are being developed to compute the Fréchet kernels for these measurements [L. Zhao et. al., this meeting]. This methodology thus provides a unified framework for regional studies of seismic sources and Earth structure in Southern California and elsewhere.

Performance of 3-Component Nodes in the IRIS Community Wavefield Demonstration Experiment

NASA Astrophysics Data System (ADS)

Sweet, J. R.; Anderson, K. R.; Woodward, R.

2017-12-01

In June 2016, a field crew of 50 students, faculty, industry personnel, and IRIS staff deployed a total of 390 stations as part of a community seismic experiment above an active seismic lineament in north-central Oklahoma. The goals of the experiment were to test new instrumentation and deployment strategies that record the full seismic wavefield, and to advance understanding of earthquake source processes and regional lithospheric structure. The crew deployed 363 3-component, 5Hz Generation 2 Fairfield Z-Land nodes along three seismic lines and in a seven-layer nested gradiometer array. The seismic lines spanned a region 13 km long by 5 km wide. A broadband, 18 station "Golay 3x6" array with an aperture of approximately 5 km was deployed around the gradiometer and seismic lines to collect waveform data from local and regional events. In addition, 9 infrasound stations were deployed in order to capture and identify acoustic events that might be recorded by the seismic array. The variety and geometry of instrumentation deployed was intended to capture the full seismic wavefield generated by the local and regional seismicity beneath the array and the surrounding region. Additional details on the instrumentation and how it was deployed can be found by visiting our website www.iris.edu/wavefields. We present a detailed analysis of noise across the array—including station performance, as well as noise from nearby sources (wind turbines, automobiles, etc.). We report a clear reduction in noise for buried 3-component nodes compared to co-located surface nodes (see Figure). Using the IRIS DMC's ISPAQ client, we present a variety of metrics to evaluate the network's performance. We also present highlights from student projects at the recently-held IRIS advanced data processing short course, which focused on analyzing the wavefield dataset using array processing techniques.

Non-periodic homogenization of 3-D elastic media for the seismic wave equation

NASA Astrophysics Data System (ADS)

Cupillard, Paul; Capdeville, Yann

2018-05-01

Because seismic waves have a limited frequency spectrum, the velocity structure of the Earth that can be extracted from seismic records has a limited resolution. As a consequence, one obtains smooth images from waveform inversion, although the Earth holds discontinuities and small scales of various natures. Within the last decade, the non-periodic homogenization method shed light on how seismic waves interact with small geological heterogeneities and `see' upscaled properties. This theory enables us to compute long-wave equivalent density and elastic coefficients of any media, with no constraint on the size, the shape and the contrast of the heterogeneities. In particular, the homogenization leads to the apparent, structure-induced anisotropy. In this paper, we implement this method in 3-D and show 3-D tests for the very first time. The non-periodic homogenization relies on an asymptotic expansion of the displacement and the stress involved in the elastic wave equation. Limiting ourselves to the order 0, we show that the practical computation of an upscaled elastic tensor basically requires (i) to solve an elastostatic problem and (ii) to low-pass filter the strain and the stress associated with the obtained solution. The elastostatic problem consists in finding the displacements due to local unit strains acting in all directions within the medium to upscale. This is solved using a parallel, highly optimized finite-element code. As for the filtering, we rely on the finite-element quadrature to perform the convolution in the space domain. We end up with an efficient numerical tool that we apply on various 3-D models to test the accuracy and the benefit of the homogenization. In the case of a finely layered model, our method agrees with results derived from Backus. In a more challenging model composed by a million of small cubes, waveforms computed in the homogenized medium fit reference waveforms very well. Both direct phases and complex diffracted waves are accurately retrieved in the upscaled model, although it is smooth. Finally, our upscaling method is applied to a realistic geological model. The obtained homogenized medium holds structure-induced anisotropy. Moreover, full seismic wavefields in this medium can be simulated with a coarse mesh (no matter what the numerical solver is), which significantly reduces computation costs usually associated with discontinuities and small heterogeneities. These three tests show that the non-periodic homogenization is both accurate and tractable in large 3-D cases, which opens the path to the correct account of the effect of small-scale features on seismic wave propagation for various applications and to a deeper understanding of the apparent anisotropy.

Imaging paleoslabs in the D″ layer beneath Central America and the Caribbean using seismic waveform inversion.

PubMed

Borgeaud, Anselme F E; Kawai, Kenji; Konishi, Kensuke; Geller, Robert J

2017-11-01

D″ (Dee double prime), the lowermost layer of the Earth's mantle, is the thermal boundary layer (TBL) of mantle convection immediately above the Earth's liquid outer core. As the origin of upwelling of hot material and the destination of paleoslabs (downwelling cold slab remnants), D″ plays a major role in the Earth's evolution. D″ beneath Central America and the Caribbean is of particular geodynamical interest, because the paleo- and present Pacific plates have been subducting beneath the western margin of Pangaea since ~250 million years ago, which implies that paleoslabs could have reached the lowermost mantle. We conduct waveform inversion using a data set of ~7700 transverse component records to infer the detailed three-dimensional S-velocity structure in the lowermost 400 km of the mantle in the study region so that we can investigate how cold paleoslabs interact with the hot TBL above the core-mantle boundary (CMB). We can obtain high-resolution images because the lowermost mantle here is densely sampled by seismic waves due to the full deployment of the USArray broadband seismic stations during 2004-2015. We find two distinct strong high-velocity anomalies, which we interpret as paleoslabs, just above the CMB beneath Central America and Venezuela, respectively, surrounded by low-velocity regions. Strong low-velocity anomalies concentrated in the lowermost 100 km of the mantle suggest the existence of chemically distinct denser material connected to low-velocity anomalies in the lower mantle inferred by previous studies, suggesting that plate tectonics on the Earth's surface might control the modality of convection in the lower mantle.

Adjoint Tomography of the Southern California Crust (Invited) (Invited)

NASA Astrophysics Data System (ADS)

Tape, C.; Liu, Q.; Maggi, A.; Tromp, J.

2009-12-01

We iteratively improve a three-dimensional tomographic model of the southern California crust using numerical simulations of seismic wave propagation based on a spectral-element method (SEM) in combination with an adjoint method. The initial 3D model is provided by the Southern California Earthquake Center. The dataset comprises three-component seismic waveforms (i.e. both body and surface waves), filtered over the period range 2-30 s, from 143 local earthquakes recorded by a network of 203 stations. Time windows for measurements are automatically selected by the FLEXWIN algorithm. The misfit function in the tomographic inversion is based on frequency-dependent multitaper traveltime differences. The gradient of the misfit function and related finite-frequency sensitivity kernels for each earthquake are computed using an adjoint technique. The kernels are combined using a source subspace projection method to compute a model update at each iteration of a gradient-based minimization algorithm. The inversion involved 16 iterations, which required 6800 wavefield simulations and a total of 0.8 million CPU hours. The new crustal model, m16, is described in terms of independent shear (Vs) and bulk-sound (Vb) wavespeed variations. It exhibits strong heterogeneity, including local changes of ±30% with respect to the initial 3D model. The model reveals several features that relate to geologic observations, such as sedimentary basins, exhumed batholiths, and contrasting lithologies across faults. The quality of the new model is validated by quantifying waveform misfits of full-length seismograms from 91 earthquakes that were not used in the tomographic inversion. The new model provides more accurate synthetic seismograms that will benefit seismic hazard assessment.

Discriminating Mining Induced Seismicity from Natural Tectonic Earthquakes in the Wasatch Plateau Region of Central Utah

NASA Astrophysics Data System (ADS)

Stein, J. R.; Pankow, K. L.; Koper, K. D.; McCarter, M. K.

2014-12-01

On average, several hundred earthquakes are located each year within the Wasatch Plateau region of central Utah. This region includes the boundary between the relatively stable Colorado Plateau and the actively extending Basin and Range physiographic provinces. Earthquakes in this region tend to fall in the intermountain seismic belt (ISB), a continuous band of seismicity that extends from Montana to Arizona. While most of the earthquakes in the ISB are of tectonic origin, events in the Wasatch Plateau also include mining induced seismicity (MIS) from local underground coal mining operations. Using a catalog of 16,182 seismic events (-0.25 < M < 4.5) recorded from 1981 to 2011, we use double difference relocation and waveform cross correlation techniques to help discriminate between these two populations of events. Double difference relocation greatly improves the relative locations between the many events that occur in this area. From the relative relocations, spatial differences between event types are used to differentiate between shallow MIS and considerably deeper events associated with tectonic seismicity. Additionally, waveform cross-correlation is used to cluster events with similar waveforms—meaning that events in each cluster should have a similar source location and mechanism—in order to more finely group seismic events occurring in the Wasatch Plateau. The results of this study provide both an increased understanding of the influence mining induced seismicity has on the number of earthquakes detected within this region, as well as better constraints on the deeper tectonic structure.

Supervised machine learning on a network scale: application to seismic event classification and detection

NASA Astrophysics Data System (ADS)

Reynen, Andrew; Audet, Pascal

2017-09-01

A new method using a machine learning technique is applied to event classification and detection at seismic networks. This method is applicable to a variety of network sizes and settings. The algorithm makes use of a small catalogue of known observations across the entire network. Two attributes, the polarization and frequency content, are used as input to regression. These attributes are extracted at predicted arrival times for P and S waves using only an approximate velocity model, as attributes are calculated over large time spans. This method of waveform characterization is shown to be able to distinguish between blasts and earthquakes with 99 per cent accuracy using a network of 13 stations located in Southern California. The combination of machine learning with generalized waveform features is further applied to event detection in Oklahoma, United States. The event detection algorithm makes use of a pair of unique seismic phases to locate events, with a precision directly related to the sampling rate of the generalized waveform features. Over a week of data from 30 stations in Oklahoma, United States are used to automatically detect 25 times more events than the catalogue of the local geological survey, with a false detection rate of less than 2 per cent. This method provides a highly confident way of detecting and locating events. Furthermore, a large number of seismic events can be automatically detected with low false alarm, allowing for a larger automatic event catalogue with a high degree of trust.

Micro-seismic waveform matching inversion based on gravitational search algorithm and parallel computation

NASA Astrophysics Data System (ADS)

Jiang, Y.; Xing, H. L.

2016-12-01

Micro-seismic events induced by water injection, mining activity or oil/gas extraction are quite informative, the interpretation of which can be applied for the reconstruction of underground stress and monitoring of hydraulic fracturing progress in oil/gas reservoirs. The source characterises and locations are crucial parameters that required for these purposes, which can be obtained through the waveform matching inversion (WMI) method. Therefore it is imperative to develop a WMI algorithm with high accuracy and convergence speed. Heuristic algorithm, as a category of nonlinear method, possesses a very high convergence speed and good capacity to overcome local minimal values, and has been well applied for many areas (e.g. image processing, artificial intelligence). However, its effectiveness for micro-seismic WMI is still poorly investigated; very few literatures exits that addressing this subject. In this research an advanced heuristic algorithm, gravitational search algorithm (GSA) , is proposed to estimate the focal mechanism (angle of strike, dip and rake) and source locations in three dimension. Unlike traditional inversion methods, the heuristic algorithm inversion does not require the approximation of green function. The method directly interacts with a CPU parallelized finite difference forward modelling engine, and updating the model parameters under GSA criterions. The effectiveness of this method is tested with synthetic data form a multi-layered elastic model; the results indicate GSA can be well applied on WMI and has its unique advantages. Keywords: Micro-seismicity, Waveform matching inversion, gravitational search algorithm, parallel computation

A Modified Normalization Technique for Frequency-Domain Full Waveform Inversion

NASA Astrophysics Data System (ADS)

Hwang, J.; Jeong, G.; Min, D. J.; KIM, S.; Heo, J. Y.

2016-12-01

Full waveform inversion (FWI) is a technique to estimate subsurface material properties minimizing the misfit function built with residuals between field and modeled data. To achieve computational efficiency, FWI has been performed in the frequency domain by carrying out modeling in the frequency domain, whereas observed data (time-series data) are Fourier-transformed.One of the main drawbacks of seismic FWI is that it easily gets stuck in local minima because of lacking of low-frequency data. To compensate for this limitation, damped wavefields are used, as in the Laplace-domain waveform inversion. Using damped wavefield in FWI plays a role in generating low-frequency components and help recover long-wavelength structures. With these newly generated low-frequency components, we propose a modified frequency-normalization technique, which has an effect of boosting contribution of low-frequency components to model parameter update.In this study, we introduce the modified frequency-normalization technique which effectively amplifies low-frequency components of damped wavefields. Our method is demonstrated for synthetic data for the SEG/EAGE salt model. AcknowledgementsThis work was supported by the Korea Institute of Energy Technology Evaluation and Planning(KETEP) and the Ministry of Trade, Industry & Energy(MOTIE) of the Republic of Korea (No. 20168510030830) and by the Dual Use Technology Program, granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea.

Medium effect on the characteristics of the coupled seismic and electromagnetic signals.

PubMed

Huang, Qinghua; Ren, Hengxin; Zhang, Dan; Chen, Y John

2015-01-01

Recently developed numerical simulation technique can simulate the coupled seismic and electromagnetic signals for a double couple point source or a finite fault planar source. Besides the source effect, the simulation results showed that both medium structure and medium property could affect the coupled seismic and electromagnetic signals. The waveform of coupled signals for a layered structure is more complicated than that for a simple uniform structure. Different from the seismic signals, the electromagnetic signals are sensitive to the medium properties such as fluid salinity and fluid viscosity. Therefore, the co-seismic electromagnetic signals may be more informative than seismic signals.

Medium effect on the characteristics of the coupled seismic and electromagnetic signals

PubMed Central

HUANG, Qinghua; REN, Hengxin; ZHANG, Dan; CHEN, Y. John

2015-01-01

Recently developed numerical simulation technique can simulate the coupled seismic and electromagnetic signals for a double couple point source or a finite fault planar source. Besides the source effect, the simulation results showed that both medium structure and medium property could affect the coupled seismic and electromagnetic signals. The waveform of coupled signals for a layered structure is more complicated than that for a simple uniform structure. Different from the seismic signals, the electromagnetic signals are sensitive to the medium properties such as fluid salinity and fluid viscosity. Therefore, the co-seismic electromagnetic signals may be more informative than seismic signals. PMID:25743062

Coupling Hydrodynamic and Wave Propagation Codes for Modeling of Seismic Waves recorded at the SPE Test.

NASA Astrophysics Data System (ADS)

Larmat, C. S.; Rougier, E.; Delorey, A.; Steedman, D. W.; Bradley, C. R.

2016-12-01

The goal of the Source Physics Experiment (SPE) is to bring empirical and theoretical advances to the problem of detection and identification of underground nuclear explosions. For this, the SPE program includes a strong modeling effort based on first principles calculations with the challenge to capture both the source and near-source processes and those taking place later in time as seismic waves propagate within complex 3D geologic environments. In this paper, we report on results of modeling that uses hydrodynamic simulation codes (Abaqus and CASH) coupled with a 3D full waveform propagation code, SPECFEM3D. For modeling the near source region, we employ a fully-coupled Euler-Lagrange (CEL) modeling capability with a new continuum-based visco-plastic fracture model for simulation of damage processes, called AZ_Frac. These capabilities produce high-fidelity models of various factors believed to be key in the generation of seismic waves: the explosion dynamics, a weak grout-filled borehole, the surrounding jointed rock, and damage creation and deformations happening around the source and the free surface. SPECFEM3D, based on the Spectral Element Method (SEM) is a direct numerical method for full wave modeling with mathematical accuracy. The coupling interface consists of a series of grid points of the SEM mesh situated inside of the hydrodynamic code's domain. Displacement time series at these points are computed using output data from CASH or Abaqus (by interpolation if needed) and fed into the time marching scheme of SPECFEM3D. We will present validation tests with the Sharpe's model and comparisons of waveforms modeled with Rg waves (2-8Hz) that were recorded up to 2 km for SPE. We especially show effects of the local topography, velocity structure and spallation. Our models predict smaller amplitudes of Rg waves for the first five SPE shots compared to pure elastic models such as Denny &Johnson (1991).

Quantitative analysis of seismic fault zone waves in the rupture zone of the 1992 Landers, California, earthquake: Evidence for a shallow trapping structure

USGS Publications Warehouse

Peng, Z.; Ben-Zion, Y.; Michael, A.J.; Zhu, L.

2003-01-01

We analyse quantitatively a waveform data set of 238 earthquakes recorded by a dense seismic array across and along the rupture zone of the 1992 Landers earthquake. A grid-search method with station delay corrections is used to locate events that do not have catalogue locations. The quality of fault zone trapped waves generated by each event is determined from the ratios of seismic energy in time windows corresponding to trapped waves and direct S waves at stations close to and off the fault zone. Approximately 70 per cent of the events with S-P times of less than 2 s, including many clearly off the fault, produce considerable trapped wave energy. This distribution is in marked contrast with previous claims that trapped waves are generated only by sources close to or inside the Landers rupture zone. The time difference between the S arrival and trapped waves group does not grow systematically with increasing hypocentral distance and depth. The dispersion measured from the trapped waves is weak. These results imply that the seismic trapping structure at the Landers rupture zone is shallow and does not extend continuously along-strike by more than a few kilometres. Synthetic waveform modelling indicates that the fault zone waveguide has depth of approximately 2-4 km, a width of approximately 200 m, an S-wave velocity reduction relative to the host rock of approximately 30-40 per cent and an S-wave attenuation coefficient of approximately 20-30. The fault zone waveguide north of the array appears to be shallower and weaker than that south of the array. The waveform modelling also indicates that the seismic trapping structure below the array is centred approximately 100 m east of the surface break.

An efficient approach to detection of weak seismic events at the Groningen gas field in the Netherlands

NASA Astrophysics Data System (ADS)

Wyer, P.; Zurek, B.

2017-12-01

Extensive additions to the Royal Dutch Meteorological Institute (KNMI) seismic monitoring network over recent years have yielded corresponding gains in detection of low magnitude seismicity induced by production of the Groningen gas field. A review of the weakest events in the seismic catalog demonstrates that waveforms from individual stations in the 30 x 35 km network area overlap sufficiently for normalized analytic envelopes to be constructively stacked without compensation for moveout, detection of individual station triggers or the need for more advanced approaches such as template matching. This observation opens the possibility of updating the historical catalog to current detection levels without having to implement more computationally expensive steps when reprocessing the legacy continuous data. A more consistent long term catalog would better constrain the frequency-size distribution (Gutenberg-Richter relationship) and provide a richer dataset for calibration of geomechanical and seismological models. To test the viability of a direct stacking approach, normalized waveform envelopes are partitioned by station into two discrete RMS stacks. Candidate seismic events are then identified as simultaneous STA/LTA triggers on both stacks. This partitioning has a minor impact on signal, but avoids the majority of false detections otherwise obtained on a single stack. Undesired detection of anthropogenic sources and earthquakes occurring outside the field can be further minimized by tuning the waveform frequency filters and trigger configuration. After minimal optimization, data from as few as 14 legacy stations are sufficient for robust automatic detection of known events approaching ML0 from the recent catalog. Ongoing work will determine residual false detection rates and whether previously unknown past events can be detected with sensitivities comparable to the modern KNMI catalog.

Seismometer Self-Noise and Measuring Methods

USGS Publications Warehouse

Ringler, Adam; R. Sleeman,; Hutt, Charles R.; Gee, Lind S.

2014-01-01

Seismometer self-noise is usually not considered when selecting and using seismic waveform data in scientific research as it is typically assumed that the self-noise is negligibly small compared to seismic signals. However, instrumental noise is part of the noise in any seismic record, and in particular, at frequencies below a few mHz, the instrumental noise has a frequency-dependent character and may dominate the noise. When seismic noise itself is considered as a carrier of information, as in seismic interferometry (e.g., Chaput et al. 2012), it becomes extremely important to estimate the contribution of instrumental noise to the recordings.

Recent advancements on the development of web-based applications for the implementation of seismic analysis and surveillance systems

NASA Astrophysics Data System (ADS)

Friberg, P. A.; Luis, R. S.; Quintiliani, M.; Lisowski, S.; Hunter, S.

2014-12-01

Recently, a novel set of modules has been included in the Open Source Earthworm seismic data processing system, supporting the use of web applications. These include the Mole sub-system, for storing relevant event data in a MySQL database (see M. Quintiliani and S. Pintore, SRL, 2013), and an embedded webserver, Moleserv, for serving such data to web clients in QuakeML format. These modules have enabled, for the first time using Earthworm, the use of web applications for seismic data processing. These can greatly simplify the operation and maintenance of seismic data processing centers by having one or more servers providing the relevant data as well as the data processing applications themselves to client machines running arbitrary operating systems.Web applications with secure online web access allow operators to work anywhere, without the often cumbersome and bandwidth hungry use of secure shell or virtual private networks. Furthermore, web applications can seamlessly access third party data repositories to acquire additional information, such as maps. Finally, the usage of HTML email brought the possibility of specialized web applications, to be used in email clients. This is the case of EWHTMLEmail, which produces event notification emails that are in fact simple web applications for plotting relevant seismic data.Providing web services as part of Earthworm has enabled a number of other tools as well. One is ISTI's EZ Earthworm, a web based command and control system for an otherwise command line driven system; another is a waveform web service. The waveform web service serves Earthworm data to additional web clients for plotting, picking, and other web-based processing tools. The current Earthworm waveform web service hosts an advanced plotting capability for providing views of event-based waveforms from a Mole database served by Moleserve.The current trend towards the usage of cloud services supported by web applications is driving improvements in JavaScript, css and HTML, as well as faster and more efficient web browsers, including mobile. It is foreseeable that in the near future, web applications are as powerful and efficient as native applications. Hence the work described here has been the first step towards bringing the Open Source Earthworm seismic data processing system to this new paradigm.

Automatic recovery of aftershock sequences at the International Data Centre: from concept to pipeline

NASA Astrophysics Data System (ADS)

Kitov, I.; Bobrov, D.; Rozhkov, M.

2016-12-01

Aftershocks of larger earthquakes represent an important source of information on the distribution and evolution of stresses and deformations in pre-seismic, co-seismic and post-seismic phases. For the International Data Centre (IDC) of the Comprehensive Nuclear-Test-Ban Organization (CTBTO) largest aftershocks sequences are also a challenge for automatic and interactive processing. The highest rate of events recorded by two and more seismic stations of the International Monitoring System from a relatively small aftershock area may reach hundreds per hour (e.g. Sumatra 2004 and Tohoku 2011). Moreover, there are thousands of reflected/refracted phases per hour with azimuth and slowness within the uncertainty limits of the first P-waves. Misassociation of these later phases, both regular and site specific, as the first P-wave results in creation of numerous wrong event hypotheses in automatic IDC pipeline. In turn, interactive review of such wrong hypotheses is direct waste of analysts' resources. Waveform cross correlation (WCC) is a powerful tool to separate coda phases from actual P-wave arrivals and to fully utilize the repeat character of waveforms generated by events close in space. Array seismic stations of the IMS enhance the performance of the WCC in two important aspects - they reduce detection threshold and effectively suppress arrivals from all sources except master events. An IDC specific aftershock tool has been developed and merged with standard IDC pipeline. The tool includes several procedures: creation of master events consisting of waveform templates at ten and more IMS stations; cross correlation (CC) of real-time waveforms with these templates, association of arrivals detected at CC-traces in event hypotheses; building events matching IDC quality criteria; and resolution of conflicts between events hypotheses created by neighboring master-events. The final cross correlation standard event lists (XSEL) is a start point of interactive analysis. Since global monitoring of underground nuclear tests is based on historical and synthetic data, each aftershock sequence can be tested for the CTBT violation with big earthquakes as an evasion scenario.

The Effect of Flow Velocity on Waveform Inversion

NASA Astrophysics Data System (ADS)

Lee, D.; Shin, S.; Chung, W.; Ha, J.; Lim, Y.; Kim, S.

2017-12-01

The waveform inversion is a velocity modeling technique that reconstructs accurate subsurface physical properties. Therefore, using the model in its final, updated version, we generated data identical to modeled data. Flow velocity, like several other factors, affects observed data in seismic exploration. Despite this, there is insufficient research on its relationship with waveform inversion. In this study, the generated synthetic data considering flow velocity was factored in waveform inversion and the influence of flow velocity in waveform inversion was analyzed. Measuring the flow velocity generally requires additional equipment. However, for situations where only seismic data was available, flow velocity was calculated by fixed-point iteration method using direct wave in observed data. Further, a new waveform inversion was proposed, which can be applied to the calculated flow velocity. We used a wave equation, which can work with the flow velocities used in the study by Käser and Dumbser. Further, we enhanced the efficiency of computation by applying the back-propagation method. To verify the proposed algorithm, six different data sets were generated using the Marmousi2 model; each of these data sets used different flow velocities in the range 0-50, i.e., 0, 2, 5, 10, 25, and 50. Thereafter, the inversion results from these data sets along with the results without the use of flow velocity were compared and analyzed. In this study, we analyzed the results of waveform inversion after flow velocity has been factored in. It was demonstrated that the waveform inversion is not affected significantly when the flow velocity is of smaller value. However, when the flow velocity has a large value, factoring it in the waveform inversion produces superior results. This research was supported by the Basic Research Project(17-3312, 17-3313) of the Korea Institute of Geoscience and Mineral Resources(KIGAM) funded by the Ministry of Science, ICT and Future Planning of Korea.

Seismic waveform inversion best practices: regional, global and exploration test cases

NASA Astrophysics Data System (ADS)

Modrak, Ryan; Tromp, Jeroen

2016-09-01

Reaching the global minimum of a waveform misfit function requires careful choices about the nonlinear optimization, preconditioning and regularization methods underlying an inversion. Because waveform inversion problems are susceptible to erratic convergence associated with strong nonlinearity, one or two test cases are not enough to reliably inform such decisions. We identify best practices, instead, using four seismic near-surface problems, one regional problem and two global problems. To make meaningful quantitative comparisons between methods, we carry out hundreds of inversions, varying one aspect of the implementation at a time. Comparing nonlinear optimization algorithms, we find that limited-memory BFGS provides computational savings over nonlinear conjugate gradient methods in a wide range of test cases. Comparing preconditioners, we show that a new diagonal scaling derived from the adjoint of the forward operator provides better performance than two conventional preconditioning schemes. Comparing regularization strategies, we find that projection, convolution, Tikhonov regularization and total variation regularization are effective in different contexts. Besides questions of one strategy or another, reliability and efficiency in waveform inversion depend on close numerical attention and care. Implementation details involving the line search and restart conditions have a strong effect on computational cost, regardless of the chosen nonlinear optimization algorithm.

Towards the application of seismogeodesy in central Italy: a case study for the 2016 August 24 Mw 6.1 Italy earthquake modelling

NASA Astrophysics Data System (ADS)

Chen, Kejie; Liu, Zhen; Liang, Cunren; Song, Y. Tony

2018-06-01

Dense strong motion and high-rate Global Navigation Satellite Systems (GNSS) networks have been deployed in central Italy for rapid seismic source determination and corresponding hazard mitigation. Different from previous studies for the consistency between two kinds of sensor at collocated stations, here we focus on the combination of high-rate GNSS displacement waveforms with collocated seismic strong motion accelerators, and investigate its application to image rupture history. Taking the 2016 August 24 Mw 6.1 Central Italy earthquake as a case study, we first generate more accurate and longer period seismogeodetic displacement waveforms by a Kalman filter, then model the rupture behaviour through a joint inversion including seismogeodetic waveforms and InSAR observations. Our results reveal that strong motion data alone can overestimate the magnitude and mismatch the GNSS observations, while 1 Hz sampling rate GNSS is insufficient and the displacement is too noisy to depict rupture process. By contrast, seismogeodetic data enhances temporal resolution and maintains the static offsets that provide vital constraint to the reliable estimation of earthquake magnitude. The obtained model is close to the jointly inverted one. Our work demonstrates the unique usefulness of seismogeodesy for fast seismic hazard response.

Using discrete wavelet transform features to discriminate between noise and phases in seismic waveforms

NASA Astrophysics Data System (ADS)

Forrest, R.; Ray, J.; Hansen, C. W.

2017-12-01

Currently, simple polarization metrics such as the horizontal-to-vertical ratio are used to discriminate between noise and various phases in three-component seismic waveform data collected at regional distances. Accurately establishing the identity and arrival of these waves in adverse signal-to-noise environments is helpful in detecting and locating the seismic events. In this work, we explore the use of multiresolution decompositions to discriminate between noise and event arrivals. A segment of the waveform lying inside a time-window that spans the coda of an arrival is subjected to a discrete wavelet decomposition. Multi-resolution classification features as well as statistical tests are derived from these wavelet decomposition quantities to quantify their discriminating power. Furthermore, we move to streaming data and address the problem of false positives by introducing ensembles of classifiers. We describe in detail results of these methods tuned from data obtained from Coronel Fontana, Argentina (CFAA), as well as Stephens Creek, Australia (STKA). Acknowledgement: Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA-0003525.

Seismic waveform modeling over cloud

NASA Astrophysics Data System (ADS)

Luo, Cong; Friederich, Wolfgang

2016-04-01

With the fast growing computational technologies, numerical simulation of seismic wave propagation achieved huge successes. Obtaining the synthetic waveforms through numerical simulation receives an increasing amount of attention from seismologists. However, computational seismology is a data-intensive research field, and the numerical packages usually come with a steep learning curve. Users are expected to master considerable amount of computer knowledge and data processing skills. Training users to use the numerical packages, correctly access and utilize the computational resources is a troubled task. In addition to that, accessing to HPC is also a common difficulty for many users. To solve these problems, a cloud based solution dedicated on shallow seismic waveform modeling has been developed with the state-of-the-art web technologies. It is a web platform integrating both software and hardware with multilayer architecture: a well designed SQL database serves as the data layer, HPC and dedicated pipeline for it is the business layer. Through this platform, users will no longer need to compile and manipulate various packages on the local machine within local network to perform a simulation. By providing users professional access to the computational code through its interfaces and delivering our computational resources to the users over cloud, users can customize the simulation at expert-level, submit and run the job through it.

On the potential of seismic rotational motion measurements for extraterrestrial seismology

NASA Astrophysics Data System (ADS)

Schmelzbach, Cedric; Sollberger, David; Khan, Amir; Greenhalgh, Stewart; Van Renterghem, Cederic; Robertsson, Johan

2017-04-01

Classically, seismological recordings consist of measurements of translational ground motion only. However, in addition to three vector components of translation there are three components of rotation to consider, leading to six degrees of freedom. Of particular interest is thereby the fact that measuring rotational motion means isolating shear (S) waves. Recording the rotational motion requires dedicated rotational sensors. Alternatively, since the rotational motion is given by the curl of the vectorial displacements, the rotational motion around the two horizontal axes can be computed from the horizontal spatial gradients of vertical translational recordings if standard translational seismometers are placed in an areal array at the free surface. This follows from the zero stress free surface condition. Combining rotational and translational motion measurements opens up new ways of analyzing seismic data, such as facilitating much improved arrival identification and wavefield separation (e.g., P-/S-wave separation), and local slowness (arrival direction and velocity) determination. Such combined measurements maximize the seismic information content that a single six-component station or a small station array can provide, and are of particular interest for sparse or single-station measurements such as in extraterrestrial seismology. We demonstrate the value of the analysis of combined translational and rotational recordings by re-evaluating data from the Apollo 17 lunar seismic profiling experiment (LSPE). The LSPE setup consisted of four vertical-component geophones arranged in a star-like geometry. This areal receiver layout enables computing the horizontal spatial gradients by spatial finite differencing of the vertical-component data for two perpendicular directions and, hence, the estimation of rotational motion around two horizontal axes. Specifically, the recorded seismic waveform data originated from eight explosive packages as well as from continuously listening to the natural lunar seismic activity of moonquakes. As an example, the combined analysis of translational and rotational motion from the active-source LSPE data provides, for the first time, the possibility to extract S-wave information from the enigmatic and reverbatory lunar seismic waveform data, which hithertofore had masked later arriving S-waves. The identification of S-waves enables to characterize the shallow lunar crust in a full elastic sense. The resultant Poisson's ratio profile allows discriminating shallow basalt layers of different degree of fracturing. Our successful analysis of the Apollo 17 data highlights the anticipated significant value of rotational measurements for future extraterrestrial seismology missions.

Seismic Observations Indicating That the 2015 Ogasawara (Bonin) Earthquake Ruptured Beneath the 660 km Discontinuity

NASA Astrophysics Data System (ADS)

Kuge, Keiko

2017-11-01

The termination of deep earthquakes at a depth of 700 km is a key feature for understanding the physical mechanism of deep earthquakes. The 680 km deep 30 May 2015, Ogasawara (Bonin) earthquake (Mw 7.9) and its aftershocks were recorded by seismic stations at distances from 7° to 19°. Synthetic seismograms indicate that the P waveforms depend on whether the earthquake is located above or below the 660 km discontinuity. In this study, I show that broadband recordings indicate that the 2015 earthquake may have occurred below the 660 km velocity discontinuity. Recordings of the P wave from the strongest aftershock lack evidence for wave triplication expected when a subhorizontal discontinuity underlies the hypocenter. Theoretical waveforms computed with a 660 km discontinuity above the aftershock and mainshock match the observed waveforms more accurately. These observations may indicate earthquake ruptures due to mantle minerals other than olivine or strong deformation of the 660 km phase transition.

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.

Simulations of Ground Motion in Southern California based upon the Spectral-Element Method

NASA Astrophysics Data System (ADS)

Tromp, J.; Komatitsch, D.; Liu, Q.

2003-12-01

We use the spectral-element method to simulate ground motion generated by recent well-recorded small earthquakes in Southern California. Simulations are performed using a new sedimentary basin model that is constrained by hundreds of petroleum industry well logs and more than twenty thousand kilometers of seismic reflection profiles. The numerical simulations account for 3D variations of seismic wave speeds and density, topography and bathymetry, and attenuation. Simulations for several small recent events demonstrate that the combination of a detailed sedimentary basin model and an accurate numerical technique facilitates the simulation of ground motion at periods of 2 seconds and longer inside the Los Angeles basin and 6 seconds and longer elsewhere. Peak ground displacement, velocity and acceleration maps illustrate that significant amplification occurs in the basin. Centroid-Moment Tensor mechanisms are obtained based upon Pnl and surface waveforms and numerically calculated 3D Frechet derivatives. We use a combination of waveform and waveform-envelope misfit criteria, and facilitate pure double-couple or zero-trace moment-tensor inversions.

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).

Total variation regularization for seismic waveform inversion using an adaptive primal dual hybrid gradient method

NASA Astrophysics Data System (ADS)

Yong, Peng; Liao, Wenyuan; Huang, Jianping; Li, Zhenchuan

2018-04-01

Full waveform inversion is an effective tool for recovering the properties of the Earth from seismograms. However, it suffers from local minima caused mainly by the limited accuracy of the starting model and the lack of a low-frequency component in the seismic data. Because of the high velocity contrast between salt and sediment, the relation between the waveform and velocity perturbation is strongly nonlinear. Therefore, salt inversion can easily get trapped in the local minima. Since the velocity of salt is nearly constant, we can make the most of this characteristic with total variation regularization to mitigate the local minima. In this paper, we develop an adaptive primal dual hybrid gradient method to implement total variation regularization by projecting the solution onto a total variation norm constrained convex set, through which the total variation norm constraint is satisfied at every model iteration. The smooth background velocities are first inverted and the perturbations are gradually obtained by successively relaxing the total variation norm constraints. Numerical experiment of the projection of the BP model onto the intersection of the total variation norm and box constraints has demonstrated the accuracy and efficiency of our adaptive primal dual hybrid gradient method. A workflow is designed to recover complex salt structures in the BP 2004 model and the 2D SEG/EAGE salt model, starting from a linear gradient model without using low-frequency data below 3 Hz. The salt inversion processes demonstrate that wavefield reconstruction inversion with a total variation norm and box constraints is able to overcome local minima and inverts the complex salt velocity layer by layer.

Probabilistic joint inversion of waveforms and polarity data for double-couple focal mechanisms of local earthquakes

NASA Astrophysics Data System (ADS)

Wéber, Zoltán

2018-06-01

Estimating the mechanisms of small (M < 4) earthquakes is quite challenging. A common scenario is that neither the available polarity data alone nor the well predictable near-station seismograms alone are sufficient to obtain reliable focal mechanism solutions for weak events. To handle this situation we introduce here a new method that jointly inverts waveforms and polarity data following a probabilistic approach. The procedure called joint waveform and polarity (JOWAPO) inversion maps the posterior probability density of the model parameters and estimates the maximum likelihood double-couple mechanism, the optimal source depth and the scalar seismic moment of the investigated event. The uncertainties of the solution are described by confidence regions. We have validated the method on two earthquakes for which well-determined focal mechanisms are available. The validation tests show that including waveforms in the inversion considerably reduces the uncertainties of the usually poorly constrained polarity solutions. The JOWAPO method performs best when it applies waveforms from at least two seismic stations. If the number of the polarity data is large enough, even single-station JOWAPO inversion can produce usable solutions. When only a few polarities are available, however, single-station inversion may result in biased mechanisms. In this case some caution must be taken when interpreting the results. We have successfully applied the JOWAPO method to an earthquake in North Hungary, whose mechanism could not be estimated by long-period waveform inversion. Using 17 P-wave polarities and waveforms at two nearby stations, the JOWAPO method produced a well-constrained focal mechanism. The solution is very similar to those obtained previously for four other events that occurred in the same earthquake sequence. The analysed event has a strike-slip mechanism with a P axis oriented approximately along an NE-SW direction.

A comparison of earthquake backprojection imaging methods for dense local arrays

NASA Astrophysics Data System (ADS)

Beskardes, G. D.; Hole, J. A.; Wang, K.; Michaelides, M.; Wu, Q.; Chapman, M. C.; Davenport, K. K.; Brown, L. D.; Quiros, D. A.

2018-03-01

Backprojection imaging has recently become a practical method for local earthquake detection and location due to the deployment of densely sampled, continuously recorded, local seismograph arrays. While backprojection sometimes utilizes the full seismic waveform, the waveforms are often pre-processed and simplified to overcome imaging challenges. Real data issues include aliased station spacing, inadequate array aperture, inaccurate velocity model, low signal-to-noise ratio, large noise bursts and varying waveform polarity. We compare the performance of backprojection with four previously used data pre-processing methods: raw waveform, envelope, short-term averaging/long-term averaging and kurtosis. Our primary goal is to detect and locate events smaller than noise by stacking prior to detection to improve the signal-to-noise ratio. The objective is to identify an optimized strategy for automated imaging that is robust in the presence of real-data issues, has the lowest signal-to-noise thresholds for detection and for location, has the best spatial resolution of the source images, preserves magnitude, and considers computational cost. Imaging method performance is assessed using a real aftershock data set recorded by the dense AIDA array following the 2011 Virginia earthquake. Our comparisons show that raw-waveform backprojection provides the best spatial resolution, preserves magnitude and boosts signal to detect events smaller than noise, but is most sensitive to velocity error, polarity error and noise bursts. On the other hand, the other methods avoid polarity error and reduce sensitivity to velocity error, but sacrifice spatial resolution and cannot effectively reduce noise by stacking. Of these, only kurtosis is insensitive to large noise bursts while being as efficient as the raw-waveform method to lower the detection threshold; however, it does not preserve the magnitude information. For automatic detection and location of events in a large data set, we therefore recommend backprojecting kurtosis waveforms, followed by a second pass on the detected events using noise-filtered raw waveforms to achieve the best of all criteria.

Investigation of Seismic Waves from Non-Natural Sources: A Case Study for Building Collapse and Surface Explosion

NASA Astrophysics Data System (ADS)

Houng, S.; Hong, T.

2013-12-01

The nature and excitation mechanism of incidents or non-natural events have been widely investigated using seismological techniques. With introduction of dense seismic networks, small-sized non-natural events such as building collapse and chemical explosions are well recorded. Two representative non-natural seismic sources are investigated. A 5-story building in South Korea, Sampoong department store, was collapsed in June 25, 1995, causing casualty of 1445. This accident is known to be the second deadliest non-terror-related building collapse in the world. The event was well recorded by a local station in ~ 9 km away. P and S waves were recorded weak, while monotonic Rayleigh waves were observed well. The origin time is determined using surface-wave arrival time. The magnitude of event is determined to be 1.2, which coincides with a theoretical estimate based on the mass and volume of building. Synthetic waveforms are modeled for various combinations of velocity structures and source time functions, which allow us to constrain the process of building collapse. It appears that the building was collapsed once within a couple of seconds. We also investigate a M2.1 chemical explosion at a fertilizer plant in Texas on April 18, 2013. It was reported that more than one hundred people were dead or injured by the explosion. Seismic waveforms for nearby stations are collected from Incorporated Research Institution of Seismology (IRIS). The event was well recorded at stations in ~500 km away from the source. Strong acoustic signals were observed at stations in a certain great-circle direction. This observation suggests preferential propagation of acoustic waves depending on atmospheric environment. Waveform cross-correlation, spectral analysis and waveform modeling are applied to understand the source physics. We discuss the nature of source and source excitation mechanism.

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.

A Fusion Model of Seismic and Hydro-Acoustic Propagation for Treaty Monitoring

NASA Astrophysics Data System (ADS)

Arora, Nimar; Prior, Mark

2014-05-01

We present an extension to NET-VISA (Network Processing Vertically Integrated Seismic Analysis), which is a probabilistic generative model of the propagation of seismic waves and their detection on a global scale, to incorporate hydro-acoustic data from the IMS (International Monitoring System) network. The new model includes the coupling of seismic waves into the ocean's SOFAR channel, as well as the propagation of hydro-acoustic waves from underwater explosions. The generative model is described in terms of multiple possible hypotheses -- seismic-to-hydro-acoustic, under-water explosion, other noise sources such as whales singing or icebergs breaking up -- that could lead to signal detections. We decompose each hypothesis into conditional probability distributions that are carefully analyzed and calibrated. These distributions include ones for detection probabilities, blockage in the SOFAR channel (including diffraction, refraction, and reflection around obstacles), energy attenuation, and other features of the resulting waveforms. We present a study of the various features that are extracted from the hydro-acoustic waveforms, and their correlations with each other as well the source of the energy. Additionally, an inference algorithm is presented that concurrently infers the seismic and under-water events, and associates all arrivals (aka triggers), both from seismic and hydro-acoustic stations, to the appropriate event, and labels the path taken by the wave. Finally, our results demonstrate that this fusion of seismic and hydro-acoustic data leads to very good performance. A majority of the under-water events that IDC (International Data Center) analysts built in 2010 are correctly located, and the arrivals that correspond to seismic-to-hydroacoustic coupling, the T phases, are mostly correctly identified. There is no loss in the accuracy of seismic events, in fact, there is a slight overall improvement.

Using Seismic Interferometry to Investigate Seismic Swarms

NASA Astrophysics Data System (ADS)

Matzel, E.; Morency, C.; Templeton, D. C.

2017-12-01

Seismicity provides a direct means of measuring the physical characteristics of active tectonic features such as fault zones. Hundreds of small earthquakes often occur along a fault during a seismic swarm. This seismicity helps define the tectonically active region. When processed using novel geophysical techniques, we can isolate the energy sensitive to the fault, itself. Here we focus on two methods of seismic interferometry, ambient noise correlation (ANC) and the virtual seismometer method (VSM). ANC is based on the observation that the Earth's background noise includes coherent energy, which can be recovered by observing over long time periods and allowing the incoherent energy to cancel out. The cross correlation of ambient noise between a pair of stations results in a waveform that is identical to the seismogram that would result if an impulsive source located at one of the stations was recorded at the other, the Green function (GF). The calculation of the GF is often stable after a few weeks of continuous data correlation, any perturbations to the GF after that point are directly related to changes in the subsurface and can be used for 4D monitoring.VSM is a style of seismic interferometry that provides fast, precise, high frequency estimates of the Green's function (GF) between earthquakes. VSM illuminates the subsurface precisely where the pressures are changing and has the potential to image the evolution of seismicity over time, including changes in the style of faulting. With hundreds of earthquakes, we can calculate thousands of waveforms. At the same time, VSM collapses the computational domain, often by 2-3 orders of magnitude. This allows us to do high frequency 3D modeling in the fault region. Using data from a swarm of earthquakes near the Salton Sea, we demonstrate the power of these techniques, illustrating our ability to scale from the far field, where sources are well separated, to the near field where their locations fall within each other's uncertainty ellipse. We use ANC to create a 3D model of the crust in the region. VSM provides better illumination of the active fault zone. Measures of amplitude and shape are used to refine source properties and locations in space and waveform modeling allows us to estimate near-fault seismic structure.

Full moment tensor and source location inversion based on full waveform adjoint method

NASA Astrophysics Data System (ADS)

Morency, C.

2012-12-01

The development of high-performance computing and numerical techniques enabled global and regional tomography to reach high levels of precision, and seismic adjoint tomography has become a state-of-the-art tomographic technique. The method was successfully used for crustal tomography of Southern California (Tape et al., 2009) and Europe (Zhu et al., 2012). Here, I will focus on the determination of source parameters (full moment tensor and location) based on the same approach (Kim et al, 2011). The method relies on full wave simulations and takes advantage of the misfit between observed and synthetic seismograms. An adjoint wavefield is calculated by back-propagating the difference between observed and synthetics from the receivers to the source. The interaction between this adjoint wavefield and the regular forward wavefield helps define Frechet derivatives of the source parameters, that is, the sensitivity of the misfit with respect to the source parameters. Source parameters are then recovered by minimizing the misfit based on a conjugate gradient algorithm using the Frechet derivatives. First, I will demonstrate the method on synthetic cases before tackling events recorded at the Geysers. The velocity model used at the Geysers is based on the USGS 3D velocity model. Waveform datasets come from the Northern California Earthquake Data Center. Finally, I will discuss strategies to ultimately use this method to characterize smaller events for microseismic and induced seismicity monitoring. References: - Tape, C., Q. Liu, A. Maggi, and J. Tromp, 2009, Adjoint tomography of the Southern California crust: Science, 325, 988992. - Zhu, H., Bozdag, E., Peter, D., and Tromp, J., 2012, Structure of the European upper mantle revealed by adjoint method: Nature Geoscience, 5, 493-498. - Kim, Y., Q. Liu, and J. Tromp, 2011, Adjoint centroid-moment tensor inversions: Geophys. J. Int., 186, 264278. Prepared by LLNL under Contract DE-AC52-07NA27344.

Landquake dynamics inferred from seismic source inversion: Greenland and Sichuan events of 2017

NASA Astrophysics Data System (ADS)

Chao, W. A.

2017-12-01

In June 2017 two catastrophic landquake events occurred in Greenland and Sichuan. The Greenland event leads to tsunami hazard in the small town of Nuugaarsiaq. A landquake in Sichuan hit the town, which resulted in over 100 death. Both two events generated the strong seismic signals recorded by the real-time global seismic network. I adopt an inversion algorithm to derive the landquake force time history (LFH) using the long-period waveforms, and the landslide volume ( 76 million m3) can be rapidly estimated, facilitating the tsunami-wave modeling for early warning purpose. Based on an integrated approach involving tsunami forward simulation and seismic waveform inversion, this study has significant implications to issuing actionable warnings before hazardous tsunami waves strike populated areas. Two single-forces (SFs) mechanism (two block model) yields the best explanation for Sichuan event, which demonstrates that secondary event (seismic inferred volume: 8.2 million m3) may be mobilized by collapse-mass hitting from initial rock avalanches ( 5.8 million m3), likely causing a catastrophic disaster. The later source with a force magnitude of 0.9967×1011 N occurred 70 seconds after first mass-movement occurrence. In contrast, first event has the smaller force magnitude of 0.8116×1011 N. In conclusion, seismically inferred physical parameters will substantially contribute to improving our understanding of landquake source mechanisms and mitigating similar hazards in other parts of the world.

Plateau subduction, intraslab seismicity and the Denali Volcanic Gap

NASA Astrophysics Data System (ADS)

Bostock, M. G.; Chuang, L. Y.; Wech, A.; Plourde, A. P.

2017-12-01

Tectonic tremors in Alaska (USA) are associated with subduction of the Yakutat plateau, but their origins are unclear due to lack of depth constraints. We have processed tremor recordings to extract low-frequency earthquakes (LFEs), and generated a set of six LFE waveform templates via iterative network matched filtering and stacking. The timing of impulsive P (compressional) wave and S (shear) wave arrivals on template waveforms places LFEs at 40-58 km depth, near the upper envelope of intraslab seismicity and immediately updip of increased levels of intraslab seismicity. S waves at near-epicentral distances display polarities consistent with shear slip on the plate boundary. We compare characteristics of LFEs, seismicity, and tectonic structures in central Alaska with those in warm subduction zones, and propose a new model for the region's unusual intraslab seismicity and the enigmatic Denali volcanic gap (i.e., an area of no volcanism where expected). We argue that fluids in the Yakutat plate are confined to its upper crust, and that shallow subduction leads to hydromechanical conditions at the slab interface in central Alaska akin to those in warm subduction zones where similar LFEs and tremor occur. These conditions lead to fluid expulsion at shallow depths, explaining strike-parallel alignment of tremor occurrence with the Denali volcanic gap. Moreover, the lack of double seismic zone and restriction of deep intraslab seismicity to a persistent low-velocity zone are simple consequences of anhydrous conditions prevailing in the lower crust and upper mantle of the Yakutat plate.

Plateau subduction, intraslab seismicity, and the Denali (Alaska) volcanic gap

USGS Publications Warehouse

Chuang, Lindsay Yuling; Bostock, Michael; Wech, Aaron; Plourde, Alexandre

2018-01-01

Tectonic tremors in Alaska (USA) are associated with subduction of the Yakutat plateau, but their origins are unclear due to lack of depth constraints. We have processed tremor recordings to extract low-frequency earthquakes (LFEs), and generated a set of six LFE waveform templates via iterative network matched filtering and stacking. The timing of impulsive P (compressional) wave and S (shear) wave arrivals on template waveforms places LFEs at 40–58 km depth, near the upper envelope of intraslab seismicity and immediately updip of increased levels of intraslab seismicity. S waves at near-epicentral distances display polarities consistent with shear slip on the plate boundary. We compare characteristics of LFEs, seismicity, and tectonic structures in central Alaska with those in warm subduction zones, and propose a new model for the region’s unusual intraslab seismicity and the enigmatic Denali volcanic gap (i.e., an area of no volcanism where expected). We argue that fluids in the Yakutat plate are confined to its upper crust, and that shallow subduction leads to hydromechanical conditions at the slab interface in central Alaska akin to those in warm subduction zones where similar LFEs and tremor occur. These conditions lead to fluid expulsion at shallow depths, explaining strike-parallel alignment of tremor occurrence with the Denali volcanic gap. Moreover, the lack of double seismic zone and restriction of deep intraslab seismicity to a persistent low-velocity zone are simple consequences of anhydrous conditions prevailing in the lower crust and upper mantle of the Yakutat plate.

Seismic attenuation and scattering tomography of rock samples using stochastic wavefields: linking seismology, volcanology, and rock physics.

NASA Astrophysics Data System (ADS)

Fazio, Marco; De Siena, Luca; Benson, Phillip

2016-04-01

Seismic attenuation and scattering are two attributes that can be linked with porosity and permeability in laboratory experiments. When measuring these two quantities using seismic waveforms recorder at lithospheric and volcanic scales the areas of highest heterogeneity, as batches of melt and zones of high deformation, produce anomalous values of the measured quantities, the seismic quality factor and scattering coefficient. When employed as indicators of heterogeneity and absorption in volcanic areas these anomalous effects become strong indicators of magma accumulation and tectonic boundaries, shaping magmatic chambers and conduit systems. We perform attenuation and scattering measurements and imaging using seismic waveforms produced in laboratory experiments, at frequencies ranging between the kHz and MHz. As attenuation and scattering are measured from the shape of the envelopes, disregarding phases, we are able to connect the observations with the micro fracturing and petrological quantities previously measured on the sample. Connecting the imaging of dry and saturated samples via these novel attributes with the burst of low-period events with increasing saturation and deformation is a challenge. Its solution could plant the seed for better relating attenuation and scattering tomography measurements to the presence of fluids and gas, therefore creating a novel path for reliable porosity and permeability tomography. In particular for volcanoes, being able to relate attenuation/scattering measurements with low-period micro seismicity could deliver new data to settle the debate about if both source and medium can produce seismic resonance.

Evaluation of Cross-Correlation Methods on a Massive Scale for Accurate Relocation of Seismic Events in East Asia

DTIC Science & Technology

2006-04-21

purposes, such as scientific study of earthquake interactions in a fault zone or seismic sources associated with magma conduits in a volcano , relative... Kilauea , J. Geophys. Res., 99, 375-393. HARRIS, D.B. (1991), A waveform correlation method for identifying quarry explosions, Bull. Seismol. Soc. Am

Locating and Modeling Regional Earthquakes with Broadband Waveform Data

NASA Astrophysics Data System (ADS)

Tan, Y.; Zhu, L.; Helmberger, D.

2003-12-01

Retrieving source parameters of small earthquakes (Mw < 4.5), including mechanism, depth, location and origin time, relies on local and regional seismic data. Although source characterization for such small events achieves a satisfactory stage in some places with a dense seismic network, such as TriNet, Southern California, a worthy revisit to the historical events in these places or an effective, real-time investigation of small events in many other places, where normally only a few local waveforms plus some short-period recordings are available, is still a problem. To address this issue, we introduce a new type of approach that estimates location, depth, origin time and fault parameters based on 3-component waveform matching in terms of separated Pnl, Rayleigh and Love waves. We show that most local waveforms can be well modeled by a regionalized 1-D model plus different timing corrections for Pnl, Rayleigh and Love waves at relatively long periods, i.e., 4-100 sec for Pnl, and 8-100 sec for surface waves, except for few anomalous paths involving greater structural complexity, meanwhile, these timing corrections reveal similar azimuthal patterns for well-located cluster events, despite their different focal mechanisms. Thus, we can calibrate the paths separately for Pnl, Rayleigh and Love waves with the timing corrections from well-determined events widely recorded by a dense modern seismic network or a temporary PASSCAL experiment. In return, we can locate events and extract their fault parameters by waveform matching for available waveform data, which could be as less as from two stations, assuming timing corrections from the calibration. The accuracy of the obtained source parameters is subject to the error carried by the events used for the calibration. The detailed method requires a Green­_s function library constructed from a regionalized 1-D model together with necessary calibration information, and adopts a grid search strategy for both hypercenter and focal mechanism. We show that the whole process can be easily automated and routinely provide reliable source parameter estimates with a couple of broadband stations. Two applications in the Tibet Plateau and Southern California will be presented along with comparisons of results against other methods.

Velocity Structure Determination Through Seismic Waveform Modeling and Time Deviations

NASA Astrophysics Data System (ADS)

Savage, B.; Zhu, L.; Tan, Y.; Helmberger, D. V.

2001-12-01

Through the use of seismic waveforms recorded by TriNet, a dataset of earthquake focal mechanisms and deviations (time shifts) relative to a standard model facilitates the investigation of the crust and uppermost mantle of southern California. The CAP method of focal mechanism determination, in use by TriNet on a routine basis, provides time shifts for surface waves and Pnl arrivals independently relative to the reference model. These shifts serve as initial data for calibration of local and regional seismic paths. Time shifts from the CAP method are derived by splitting the Pnl section of the waveform, the first arriving Pn to just before the arrival of the S wave, from the much slower surface waves then cross-correlating the data with synthetic waveforms computed from a standard model. Surface waves interact with the entire crust, but the upper crust causes the greatest effect. Whereas, Pnl arrivals sample the deeper crust, upper mantle, and source region. This natural division separates the upper from lower crust for regional calibration and structural modeling and allows 3-D velocity maps to be created using the resulting time shifts. Further examination of Pnl and other arrivals which interact with the Moho illuminate the complex nature of this boundary. Initial attempts at using the first 10 seconds of the Pnl section to determine upper most mantle structure have proven insightful. Two large earthquakes north of southern California in Nevada and Mammoth Lakes, CA allow the creation of record sections from 200 to 600 km. As the paths swing from east to west across southern California, simple 1-D models turn into complex structure, dramatically changing the waveform character. Using finite difference models to explain the structure, we determine that a low velocity zone is present at the base of the crust and extends to 100 km in depth. Velocity variations of 5 percent of the mantle in combination with steeply sloping edges produces complex waveform variations. Characteristics of this complex propagation appear from the southern Sierra Nevada Mountains, in the west, to Death Valley in the east. The structure does not cross the Garlock fault to the south, but we are unsure of the structures northern extent.

Seismic Investigation of Magmatic Unrest Beneath Mammoth Mountain, California Using Waveform Cross-Correlation

NASA Astrophysics Data System (ADS)

Lin, G.

2012-12-01

We investigate the seismic and magmatic activity during an 11-month-long seismic swarm between 1989 and 1990 beneath Mammoth Mountain (MM) at the southwest rim of Long Valley caldera in eastern California. This swarm is believed to be results of a shallow intrusion of magma beneath MM. It was followed by the emissions of carbon dioxide (CO2) gas, which caused tree-killings in 1990 and posed a significant human health risk around MM. In this study, we develop a new three-dimensional (3-D) P-wave velocity model using first-arrival picks by applying the simul2000 tomographic algorithm. The resulting 3-D model is correlated with the surface geological features at shallow depths and is used to constrain absolute earthquake locations for all local events in our study. We compute both P- and S-wave differential times using a time-domain waveform cross-correlation method. We then apply similar event cluster analysis and differential time location approach to further improve relative event location accuracy. A dramatic sharpening of seismicity pattern is obtained after these processes. The estimated uncertainties are a few meters in relative location and ~100 meters in absolute location. We also apply a high-resolution approach to estimate in situ near-source Vp/Vs ratios using differential times from waveform cross-correlation. This method provides highly precise results because cross-correlation can measure differential times to within a few milliseconds and can achieve a precision of 0.001 in estimated Vp/Vs ratio. Our results show a circular ring-like seismicity pattern with a diameter of 2 km between 3 and 8 km depth. These events are distributed in an anomalous body with low Vp and high Vp/Vs, which may be caused by over-pressured magmatically derived fluids. At shallower depths, we observe very low Vp/Vs anomalies beneath MM from the surface to 1 km below sea level whose locations agree with the proposed CO2 reservoir in previous studies. The systematic spatial and temporal migration of seismicity suggests fluid involvement in the seismic swarm. Our results will provide more robust constraints on the crustal structure and volcanic processes beneath Mammoth Mountain.

Using P-wave Triplications to Constrain the Mantle Transition Zone beneath Central Iranian Plateau and Surrounding Area

NASA Astrophysics Data System (ADS)

Chi, H. C.; Tseng, T. L.

2014-12-01

The Iranian Plateau is a tectonically complex region resulting from the continental collision between the African and Eurasian plates. The convergence of the two continents created the Zagros Mountains, the high topography southwest of Iran, and active seismicity along the Zagros-Bitlis suture. Tomographic studies in Iran reveal low seismic speeds and high attenuation of Sn wave in the uppermost mantle beneath the Iranian Plateau relative to adjacent regions. The deeper structure, however, remains curiously inconclusive. By contrast, a prominent fast seismic anomaly is found under central Tibet near depth of 600 km in the mantle transition zone (TZ), and it is speculated to be the remnant of lithosphere detached during the continental collision. We conduct a comparative study that utilizes triplicate arrivals of high-resolution P waveforms to investigate the velocity structure of mantle beneath the central Iranian Plateau and surroundings. Due to the abrupt increase in seismic wave speeds and density across the 410- and 660-km discontinuities, seismic waves at epicentral distances of 15-30 degrees would form multiple arrivals and the relative times and amplitudes between them are most sensitive to the variations in seismic speeds near the TZ. We combine several broadband arrays to construct 8 seismic profiles, each about 800 km long, that mainly sample the TZ under central Iranian Plateau, Turan shield and part of South Caspian basin. Move-outs between arrivals are clear in the profiles. Relative timings suggest a slightly smaller 660-km contrast under stable Turan shield. In the next stage, it is necessary to model waveforms after the source effect being removed properly. Our preliminary tests show that the F-K method can efficiently calculate the synthetic seismograms. We will determine the 1D velocity model for each sampled sector by minimizing the overall misfits between observed and predicted waveforms. The lateral variations may be further explored by comparing adjacent sectors. The results are important for understanding the lithosphere-mantle interaction during the process of continental collision.

Sensitivity analysis of seismic waveforms to upper-mantle discontinuities using the adjoint method

NASA Astrophysics Data System (ADS)

Koroni, Maria; Bozdağ, Ebru; Paulssen, Hanneke; Trampert, Jeannot

2017-09-01

Using spectral-element simulations of wave propagation, we investigated the sensitivity of seismic waveforms, recorded on transverse components, to upper-mantle discontinuities in 1-D and 3-D background models. These sensitivity kernels, or Fréchet derivatives, illustrate the spatial sensitivity to model parameters, of which those for shear wave speed and the surface topography of internal boundaries are discussed in this paper. We focus on the boundaries at 400 and 670 km depth of the mantle transition zone. SS precursors have frequently been used to infer the topography of upper-mantle discontinuities. These seismic phases are underside reflections off these boundaries and are usually analysed in the distance range of 110°-160°. This distance range is chosen to minimize the interference from other waves. We show sensitivity kernels for consecutive time windows at three characteristic epicentral distances within the 110°-160° range. The sensitivity kernels are computed with the adjoint method using synthetic data. From our simulations we can draw three main conclusions: (i) The exact Fréchet derivatives show that in all time windows, and also in those centred on the SS precursors, there is interference from other waves. This explains the difficulty reported in the literature to correct for 3-D shear wave speed perturbations, even if the 3-D structure is perfectly known. (ii) All studies attempting to map the topography of the 400 and 670 km discontinuities to date assume that the traveltimes of SS precursors can be linearly decomposed into a 3-D elastic structure and a topography part. We recently showed that such a linear decomposition is not possible for SS precursors, and the sensitivity kernels presented in this paper explain why. (iii) In agreement with previous work, we show that other parts of the seismograms have greater sensitivity to upper-mantle discontinuities than SS precursors, especially multiply bouncing S waves exploiting the S-wave triplications due to the mantle transition zone. These phases can potentially improve the inference of global topographic variations of the upper-mantle discontinuities in the context of full waveform inversion in a joint inversion for (an)elastic parameters and topography.

ASDF: A New Adaptable Data Format for Seismology Suitable for Large-Scale Workflows

NASA Astrophysics Data System (ADS)

Krischer, L.; Smith, J. A.; Spinuso, A.; Tromp, J.

2014-12-01

Increases in the amounts of available data as well as computational power opens the possibility to tackle ever larger and more complex problems. This comes with a slew of new problems, two of which are the need for a more efficient use of available resources and a sensible organization and storage of the data. Both need to be satisfied in order to properly scale a problem and both are frequent bottlenecks in large seismic inversions using ambient noise or more traditional techniques.We present recent developments and ideas regarding a new data format, named ASDF (Adaptable Seismic Data Format), for all branches of seismology aiding with the aforementioned problems. The key idea is to store all information necessary to fully understand a set of data in a single file. This enables the construction of self-explaining and exchangeable data sets facilitating collaboration on large-scale problems. We incorporate the existing metadata standards FDSN StationXML and QuakeML together with waveform and auxiliary data into a common container based on the HDF5 standard. A further critical component of the format is the storage of provenance information as an extension of W3C PROV, meaning information about the history of the data, assisting with the general problem of reproducibility.Applications of the proposed new format are numerous. In the context of seismic tomography it enables the full description and storage of synthetic waveforms including information about the used model, the solver, the parameters, and other variables that influenced the final waveforms. Furthermore, intermediate products like adjoint sources, cross correlations, and receiver functions can be described and most importantly exchanged with others.Usability and tool support is crucial for any new format to gain acceptance and we additionally present a fully functional implementation of this format based on Python and ObsPy. It offers a convenient way to discover and analyze data sets as well as making it trivial to execute processing functionality on modern high performance machines utilizing parallel I/O even for users not familiar with the details. An open-source development and design model as well as a wiki aim to involve the community.

Seismicity rate increases associated with slow slip episodes prior to the 2012 Mw 7.4 Ometepec earthquake

NASA Astrophysics Data System (ADS)

Colella, Harmony V.; Sit, Stefany M.; Brudzinski, Michael R.; Graham, Shannon E.; DeMets, Charles; Holtkamp, Stephen G.; Skoumal, Robert J.; Ghouse, Noorulann; Cabral-Cano, Enrique; Kostoglodov, Vladimir; Arciniega-Ceballos, Alejandra

2017-04-01

The March 20, 2012 Mw 7.4 Ometepec earthquake in the Oaxaca region of Southern Mexico provides a unique opportunity to examine whether subtle changes in seismicity, tectonic tremor, or slow slip can be observed prior to a large earthquake that may illuminate changes in stress or background slip rate. Continuous Global Positioning System (cGPS) data reveal a 5-month-long slow slip event (SSE) between ∼20 and 35 km depth that migrated toward and reached the vicinity of the mainshock a few weeks prior to the earthquake. Seismicity in Oaxaca is examined using single station tectonic tremor detection and multi-station waveform template matching of earthquake families. An increase in seismic activity, detected with template matching using aftershock waveforms, is only observed in the weeks prior to the mainshock in the region between the SSE and mainshock. In contrast, a SSE ∼15 months earlier occurred at ∼25-40 km depth and was primarily associated with an increase in tectonic tremor. Together, these observations indicate that in the Oaxaca region of Mexico shallower slow slip promotes elevated seismicity rates, and deeper slow slip promotes tectonic tremor. Results from this study add to a growing number of published accounts that indicate slow slip may be a common pre-earthquake signature.

Why Waveform Correlation Sometimes Fails

NASA Astrophysics Data System (ADS)

Carmichael, J.

2015-12-01

Waveform correlation detectors used in explosion monitoring scan noisy geophysical data to test two competing hypotheses: either (1) an amplitude-scaled version of a template waveform is present, or, (2) no signal is present at all. In reality, geophysical wavefields that are monitored for explosion signatures include waveforms produced by non-target sources that are partially correlated with the waveform template. Such signals can falsely trigger correlation detectors, particularly at low thresholds required to monitor for smaller target explosions. This challenge is particularly formidable when monitoring known test sites for seismic disturbances, since uncatalogued natural seismicity is (generally) more prevalent at lower magnitudes, and could be mistaken for small explosions. To address these challenges, we identify real examples in which correlation detectors targeting explosions falsely trigger on both site-proximal earthquakes (Figure 1, below) and microseismic "noise". Motivated by these examples, we quantify performance loss when applying these detectors, and re-evaluate the correlation-detector's hypothesis test. We thereby derive new detectors from more general hypotheses that admit unknown background seismicity, and apply these to real data. From our treatment, we derive "rules of thumb'' for proper template and threshold selection in heavily cluttered signal environments. Last, we answer the question "what is the probability of falsely detecting an earthquake collocated at a test site?", using correlation detectors that include explosion-triggered templates. Figure Top: An eight-channel data stream (black) recorded from an earthquake near a mine. Red markers indicate a detection. Middle: The correlation statistic computed by scanning the template against the data stream at top. The red line indicates the threshold for event declaration, determined by a false-alarm on noise probability constraint, as computed from the signal-absent distribution using the Neyman Pearson criteria. Bottom: The histogram of the correlation statistic time series (gray) superimposed on the theoretical null distribution (black curve). The line shows the threshold, consistent with a right-tail probability, computed from the black curve.

Coupled Hydrodynamic and Wave Propagation Modeling for the Source Physics Experiment: Study of Rg Wave Sources for SPE and DAG series.

NASA Astrophysics Data System (ADS)

Larmat, C. S.; Delorey, A.; Rougier, E.; Knight, E. E.; Steedman, D. W.; Bradley, C. R.

2017-12-01

This presentation reports numerical modeling efforts to improve knowledge of the processes that affect seismic wave generation and propagation from underground explosions, with a focus on Rg waves. The numerical model is based on the coupling of hydrodynamic simulation codes (Abaqus, CASH and HOSS), with a 3D full waveform propagation code, SPECFEM3D. Validation datasets are provided by the Source Physics Experiment (SPE) which is a series of highly instrumented chemical explosions at the Nevada National Security Site with yields from 100kg to 5000kg. A first series of explosions in a granite emplacement has just been completed and a second series in alluvium emplacement is planned for 2018. The long-term goal of this research is to review and improve current existing seismic sources models (e.g. Mueller & Murphy, 1971; Denny & Johnson, 1991) by providing first principles calculations provided by the coupled codes capability. The hydrodynamic codes, Abaqus, CASH and HOSS, model the shocked, hydrodynamic region via equations of state for the explosive, borehole stemming and jointed/weathered granite. A new material model for unconsolidated alluvium materials has been developed and validated with past nuclear explosions, including the 10 kT 1965 Merlin event (Perret, 1971) ; Perret and Bass, 1975). We use the efficient Spectral Element Method code, SPECFEM3D (e.g. Komatitsch, 1998; 2002), and Geologic Framework Models to model the evolution of wavefield as it propagates across 3D complex structures. The coupling interface is a series of grid points of the SEM mesh situated at the edge of the hydrodynamic code domain. We will present validation tests and waveforms modeled for several SPE tests which provide evidence that the damage processes happening in the vicinity of the explosions create secondary seismic sources. These sources interfere with the original explosion moment and reduces the apparent seismic moment at the origin of Rg waves up to 20%.

Sparsity-optimized separation of body waves and ground-roll by constructing dictionaries using tunable Q-factor wavelet transforms with different Q-factors

NASA Astrophysics Data System (ADS)

Chen, Xin; Chen, Wenchao; Wang, Xiaokai; Wang, Wei

2017-10-01

Low-frequency oscillatory ground-roll is regarded as one of the main regular interference waves, which obscures primary reflections in land seismic data. Suppressing the ground-roll can reasonably improve the signal-to-noise ratio of seismic data. Conventional suppression methods, such as high-pass and various f-k filtering, usually cause waveform distortions and loss of body wave information because of their simple cut-off operation. In this study, a sparsity-optimized separation of body waves and ground-roll, which is based on morphological component analysis theory, is realized by constructing dictionaries using tunable Q-factor wavelet transforms with different Q-factors. Our separation model is grounded on the fact that the input seismic data are composed of low-oscillatory body waves and high-oscillatory ground-roll. Two different waveform dictionaries using a low Q-factor and a high Q-factor, respectively, are confirmed as able to sparsely represent each component based on their diverse morphologies. Thus, seismic data including body waves and ground-roll can be nonlinearly decomposed into low-oscillatory and high-oscillatory components. This is a new noise attenuation approach according to the oscillatory behaviour of the signal rather than the scale or frequency. We illustrate the method using both synthetic and field shot data. Compared with results from conventional high-pass and f-k filtering, the results of the proposed method prove this method to be effective and advantageous in preserving the waveform and bandwidth of reflections.

Seismic body wave separation in volcano-tectonic activity inferred by the Convolutive Independent Component Analysis

NASA Astrophysics Data System (ADS)

Capuano, Paolo; De Lauro, Enza; De Martino, Salvatore; Falanga, Mariarosaria; Petrosino, Simona

2015-04-01

One of the main challenge in volcano-seismological literature is to locate and characterize the source of volcano/tectonic seismic activity. This passes through the identification at least of the onset of the main phases, i.e. the body waves. Many efforts have been made to solve the problem of a clear separation of P and S phases both from a theoretical point of view and developing numerical algorithms suitable for specific cases (see, e.g., Küperkoch et al., 2012). Recently, a robust automatic procedure has been implemented for extracting the prominent seismic waveforms from continuously recorded signals and thus allowing for picking the main phases. The intuitive notion of maximum non-gaussianity is achieved adopting techniques which involve higher-order statistics in frequency domain., i.e, the Convolutive Independent Component Analysis (CICA). This technique is successful in the case of the blind source separation of convolutive mixtures. In seismological framework, indeed, seismic signals are thought as the convolution of a source function with path, site and the instrument response. In addition, time-delayed versions of the same source exist, due to multipath propagation typically caused by reverberations from some obstacle. In this work, we focus on the Volcano Tectonic (VT) activity at Campi Flegrei Caldera (Italy) during the 2006 ground uplift (Ciaramella et al., 2011). The activity was characterized approximately by 300 low-magnitude VT earthquakes (Md < 2; for the definition of duration magnitude, see Petrosino et al. 2008). Most of them were concentrated in distinct seismic sequences with hypocenters mainly clustered beneath the Solfatara-Accademia area, at depths ranging between 1 and 4 km b.s.l.. The obtained results show the clear separation of P and S phases: the technique not only allows the identification of the S-P time delay giving the timing of both phases but also provides the independent waveforms of the P and S phases. This is an enormous advantage for all the problems related to the source inversion and location In addition, the VT seismicity was accompanied by hundreds of LP events (characterized by spectral peaks in the 0.5-2-Hz frequency band) that were concentrated in a 7-day interval. The main interest is to establish whether the occurrence of LPs is only limited to the swarm that reached a climax on days 26-28 October as indicated by Saccorotti et al. (2007), or a longer period is experienced. The automatically extracted waveforms with improved signal-to-noise ratio via CICA coupled with automatic phase picking allowed to compile a more complete seismic catalog and to better quantify the seismic energy release including the presence of LP events from the beginning of October until mid of November. Finally, a further check of the volcanic nature of extracted signals is achieved by looking at the seismological properties and the content of entropy held in the traces (Falanga and Petrosino 2012; De Lauro et al., 2012). Our results allow us to move towards a full description of the complexity of the source, which can be used for hazard-model development and forecast-model testing, showing an illustrative example of the applicability of the CICA method to regions with low seismicity in high ambient noise

A Contribution to the Understanding of the Regional Seismic Structure in the Eastern Mediterranean

NASA Astrophysics Data System (ADS)

Di Luccio, F.; Thio, H.; Pino, N.

2001-12-01

Regional earthquakes recorded by two digital broadband stations (BGIO and KEG) located in the Eastern Mediterranean have been analyzed in order to study the seismic structure in this region. The area consists of different tectonic provinces, which complicate the modeling of the seismic wave propagation. We have modeled the Pnl arrivals using the FK-integration technique (Saikia, 1994) along different paths at the two stations, at several distances, ranging from 400 to 1500 km. Comparing the synthetics obtained by using several models compiled by other authors, we have constructed a velocity model, considering the informations deriving from group velocity distribution, in order to determine the finer structure in the analyzed paths. The model has been perturbed by trial and error until a compressional velocity profile has been found producing the shape of the observed waveforms. The crustal thickness, upper mantle P-wave velocity and 410-km discontinuity determine the shape of the observed waveform portions.

Comparison of Velocity Models for South America through Seismic Wave Modeling of Ten Andean Earthquakes Recorded by the Brazilian Seismographic Network using the Spectral Element Method

NASA Astrophysics Data System (ADS)

Ciardelli, C.; Assumpcao, M.

2016-12-01

In this work, we carried out simulations of seismic waves propagation for ten large earthquakes occurred in Chile between 2011 and 2016,using the SPECFEM3D Global software (Komatitsch and Tromp, 2000) and the Centroid Moment Tensor solutions from the global catalog (Dziewonski, Chou and Woodhouse, 1981; Ekström, Nettles and Dziewonski, 2012). For each event, the complete wave field was calculated using the spectral element method and recorded at the coordinates of the Brazilian Seismographic Network, thus we can compare the synthetic seismograms with the real data. Initially, we assess the differences between CRUST1.0 and CRUST2.0 models using the transversely isotropic PREM for the internal part of the planet. We will also compare the PREM velocity model plus CRUST1.0 with the Feng's velocity model for South America (Feng, Van der Lee and Assumpção, 2007), calculated using Partitioned Waveform Inversion. For each model, we will evaluate the misfit for all stations of the network. The similarity can be estimated by pure RMS or combining it with cross-correlation. Travel-time residuals can also be used to better constrain velocity anomalies and avoid cycle-skipping. The results will help to assess which model is more appropriated to start a Full-waveform Tomography of the South American continent and the surrounding oceans.

Seismotectonics at the junction of the Philippine Sea plate and the Eurasian plate, in light of the 1990 Hualien earthquake and the near-field waveform inversion

NASA Astrophysics Data System (ADS)

Cheng, Hou-Sheng; Mozziconacci, Laetitia; Chang, Emmy T. Y.; Huang, Bor-Shouh

2016-04-01

In eastern Taiwan, the Longitudinal Valley (LV) is the suture zone separating the Eurasian plate (EUP) to the West from the Philippine Sea plate (PSP) to the East. The northern tip of the LV (near Hualien city) is the junction point where the collision evolve northward to a subduction of the PSP under the EUP. As a result, a high seismic activity is observed. Based on the CWB (Central Weather Bureau, Taiwan) earthquake catalog, four distinct seismic clusters can be observed in this area since 1990. We restrict our effort to the cluster caused by a doublet events of 1990 with two moderate-large earthquakes. The first shock of these doublet occurred on 13rd December with ML 6.5. Seventeen hours later and 15 km to the southeast occurred the second shock of ML 6.7. A campaign seismic network of 15 short-period stations - the Hualien Temporary Seismic Network (HTSN) was deployed during 2 months to detect the aftershocks of the doublet. By applying the near-field waveform inversion to the HTSN records, we can retrieve the focal mechanism solutions (FMS) from 50 aftershocks of local magnitude ranging from 2.5 to 5.0. A modified version of the program "FMNEAR" is adopted in this study, which has been proven to be efficient to retrieve FMS for small-to-moderate earthquakes with a limited number of stations. In practice, the near-field waveforms, were processed by band-pass filter between 0.52 and 1.2 Hz. Synthetic waveforms are built from the discrete wave number method of Bouchon (1981). The inversion is done by grid searches on the FMS parameters while the rake is inverted, the best result gives the lowest waveform misfit. The waveform adjustment are improved by depth optimization and a specific 1D velocity model for each station. Focal depths of events are in average 10km deeper than the depth determined by the island-wide seismic stations that suffered from the lack of stations to the east due to the ocean. The FMS of the 50 aftershocks can be classified into three groups according to their mechanisms and the P- and T-axes. The 3 groups distribute from north to south. The northern one is the largest one and is located along the northern and middle part of the northern segment of the LVF (NLVF). It is mainly reverse in type and display homogeneous FMS. Our hypothesis is that the fault generated the doublet is related to the structure activated by this first group. In the middle part, the second group is dominantly normal while the last group spreads in the southern portion of the NLVF with more strike-slip events.

Ground truth seismic events and location capability at Degelen mountain, Kazakhstan

NASA Astrophysics Data System (ADS)

Trabant, Chad; Thurber, Clifford; Leith, William

2002-07-01

We utilized nuclear explosions from the Degelen Mountain sub-region of the Semipalatinsk Test Site (STS), Kazakhstan, to assess seismic location capability directly. Excellent ground truth information for these events was either known or was estimated from maps of the Degelen Mountain adit complex. Origin times were refined for events for which absolute origin time information was unknown using catalog arrival times, our ground truth location estimates, and a time baseline provided by fixing known origin times during a joint hypocenter determination (JHD). Precise arrival time picks were determined using a waveform cross-correlation process applied to the available digital data. These data were used in a JHD analysis. We found that very accurate locations were possible when high precision, waveform cross-correlation arrival times were combined with JHD. Relocation with our full digital data set resulted in a mean mislocation of 2 km and a mean 95% confidence ellipse (CE) area of 6.6 km 2 (90% CE: 5.1 km 2), however, only 5 of the 18 computed error ellipses actually covered the associated ground truth location estimate. To test a more realistic nuclear test monitoring scenario, we applied our JHD analysis to a set of seven events (one fixed) using data only from seismic stations within 40° epicentral distance. Relocation with these data resulted in a mean mislocation of 7.4 km, with four of the 95% error ellipses covering less than 570 km 2 (90% CE: 438 km 2), and the other two covering 1730 and 8869 km 2 (90% CE: 1331 and 6822 km 2). Location uncertainties calculated using JHD often underestimated the true error, but a circular region with a radius equal to the mislocation covered less than 1000 km 2 for all events having more than three observations.

Imaging paleoslabs in the D″ layer beneath Central America and the Caribbean using seismic waveform inversion

PubMed Central

Borgeaud, Anselme F. E.; Kawai, Kenji; Konishi, Kensuke; Geller, Robert J.

2017-01-01

D″ (Dee double prime), the lowermost layer of the Earth’s mantle, is the thermal boundary layer (TBL) of mantle convection immediately above the Earth’s liquid outer core. As the origin of upwelling of hot material and the destination of paleoslabs (downwelling cold slab remnants), D″ plays a major role in the Earth’s evolution. D″ beneath Central America and the Caribbean is of particular geodynamical interest, because the paleo- and present Pacific plates have been subducting beneath the western margin of Pangaea since ~250 million years ago, which implies that paleoslabs could have reached the lowermost mantle. We conduct waveform inversion using a data set of ~7700 transverse component records to infer the detailed three-dimensional S-velocity structure in the lowermost 400 km of the mantle in the study region so that we can investigate how cold paleoslabs interact with the hot TBL above the core-mantle boundary (CMB). We can obtain high-resolution images because the lowermost mantle here is densely sampled by seismic waves due to the full deployment of the USArray broadband seismic stations during 2004–2015. We find two distinct strong high-velocity anomalies, which we interpret as paleoslabs, just above the CMB beneath Central America and Venezuela, respectively, surrounded by low-velocity regions. Strong low-velocity anomalies concentrated in the lowermost 100 km of the mantle suggest the existence of chemically distinct denser material connected to low-velocity anomalies in the lower mantle inferred by previous studies, suggesting that plate tectonics on the Earth’s surface might control the modality of convection in the lower mantle. PMID:29209659

Seismological investigation of the National Data Centre Preparedness Exercise 2013

NASA Astrophysics Data System (ADS)

Gestermann, Nicolai; Hartmann, Gernot; Ross, J. Ole; Ceranna, Lars

2015-04-01

The Comprehensive Nuclear-Test-Ban Treaty (CTBT) prohibits all kinds of nuclear explosions conducted on Earth - underground, underwater or in the atmosphere. The verification regime of the CTBT is designed to detect any treaty violation. While the data of the International Monitoring System (IMS) is collected, processed and technically analyzed at the International Data Centre (IDC) of the CTBT-Organization, National Data Centres (NDC) of the member states provide interpretation and advice to their government concerning suspicious detections. The NDC Preparedness Exercises (NPE) are regularly performed dealing with fictitious treaty violations to practice the combined analysis of CTBT verification technologies. These exercises should help to evaluate the effectiveness of analysis procedures applied at NDCs and the quality, completeness and usefulness of IDC products for example. The exercise trigger of NPE2013 is a combination of a tempo-spatial indication pointing to a certain waveform event and simulated radionuclide concentrations generated by forward Atmospheric Transport Modelling based on a fictitious release. For the waveform event the date (4 Sept. 2013) is given and the region is communicated in a map showing the fictitious state of "Frisia" at the Coast of the North Sea in Central Europe. The potential connection between the waveform and radionuclide evidence remains unclear for exercise participants. The verification task was to identify the waveform event and to investigate potential sources of the radionuclide findings. The final question was whether the findings are CTBT relevant and justify a request for On-Site-Inspection in "Frisia". The seismic event was not included in the Reviewed Event Bulletin (REB) of the IDC. The available detections from the closest seismic IMS stations lead to a epicenter accuracy of about 24 km which is not sufficient to specify the 1000 km2 inspection area in case of an OSI. With use of data from local stations and adjusted velocity models the epicenter accuracy could be improved to less than 2 km, which demonstrates the crucial role of national technical means for verification tasks. The seismic NPE2013 event could be identified as induced from natural gas production in the source region. Similar waveforms and comparable spectral characteristic as a set of events in the same region are clear indications. The scenario of a possible treaty violation at the location of the seismic NPE2013 event could be disproved.

Scenario design and basic analysis of the National Data Centre Preparedness Exercise 2013

NASA Astrophysics Data System (ADS)

Ross, Ole; Ceranna, Lars; Hartmann, Gernot; Gestermann, Nicolai; Bönneman, Christian

2014-05-01

The Comprehensive Nuclear-Test-Ban Treaty (CTBT) prohibits all kinds of nuclear explosions. For the detection of treaty violations the International Monitoring System (IMS) operates stations observing seismic, hydroacoustic, and infrasound signals as well as radioisotopes in the atmosphere. While the IMS data is collected, processed and technically analyzed in the International Data Center (IDC) of the CTBT-Organization, National Data Centers (NDC) provide interpretation and advice to their government concerning suspicious detections occurring in IMS data. NDC Preparedness Exercises (NPE) are regularly performed dealing with fictitious treaty violations to practice the combined analysis of CTBT verification technologies and for the mutual exchange of information between NDC and also with the IDC. The NPE2010 and NPE2012 trigger scenarios were based on selected seismic events from the Reviewed Event Bulletin (REB) serving as starting point for fictitious Radionuclide dispersion. The main task was the identification of the original REB event and the discrimination between earthquakes and explosions as source. The scenario design of NPE2013 differs from those of previous NPEs. The waveform event selection is not constrained to events in the REB. The exercise trigger is a combination of a tempo-spatial indication pointing to a certain waveform event and simulated radionuclide concentrations generated by forward Atmospheric Transport Modelling based on a fictitious release. For the waveform event the date (4 Sept. 2013) is given and the region is communicated in a map showing the fictitious state of "Frisia" at the Coast of the North Sea in Central Europe. The synthetic radionuclide detections start in Vienna (8 Sept, I-131) and Schauinsland (11 Sept, Xe-133) with rather low activity concentrations and are most prominent in Stockholm and Spitsbergen mid of September 2013. Smaller concentrations in Asia follow later on. The potential connection between the waveform and radionuclide evidence remains unclear. The verification task is to identify the waveform event and to investigate potential sources of the radionuclide findings. Finally the potential conjunction between the sources and the CTBT-relevance of the whole picture has to be evaluated. The overall question is whether requesting an On-Site-Inspection in "Frisia" would be justified. The poster presents the NPE2013 scenario and gives a basic analysis of the initial situation concerning both waveform detections and atmospheric dispersion conditions in Central Europe in early September 2013. The full NPE2013 scenario will be presented at the NDC Workshop mid of May 2014.

Advanced Waveform Simulation for Seismic Monitoring

DTIC Science & Technology

2008-09-01

velocity model. The method separates the main arrivals of the regional waveform into 5 windows: Pnl (vertical and radial components), Rayleigh (vertical and...ranges out to 10°, including extensive observations of crustal thinning and thickening and various Pnl complexities. Broadband modeling in 1D, 2D...existing models perform in predicting the various regional phases, Rayleigh waves, Love waves, and Pnl waves. Previous events from this Basin-and-Range

Web-based access to near real-time and archived high-density time-series data: cyber infrastructure challenges & developments in the open-source Waveform Server

NASA Astrophysics Data System (ADS)

Reyes, J. C.; Vernon, F. L.; Newman, R. L.; Steidl, J. H.

2010-12-01

The Waveform Server is an interactive web-based interface to multi-station, multi-sensor and multi-channel high-density time-series data stored in Center for Seismic Studies (CSS) 3.0 schema relational databases (Newman et al., 2009). In the last twelve months, based on expanded specifications and current user feedback, both the server-side infrastructure and client-side interface have been extensively rewritten. The Python Twisted server-side code-base has been fundamentally modified to now present waveform data stored in cluster-based databases using a multi-threaded architecture, in addition to supporting the pre-existing single database model. This allows interactive web-based access to high-density (broadband @ 40Hz to strong motion @ 200Hz) waveform data that can span multiple years; the common lifetime of broadband seismic networks. The client-side interface expands on it's use of simple JSON-based AJAX queries to now incorporate a variety of User Interface (UI) improvements including standardized calendars for defining time ranges, applying on-the-fly data calibration to display SI-unit data, and increased rendering speed. This presentation will outline the various cyber infrastructure challenges we have faced while developing this application, the use-cases currently in existence, and the limitations of web-based application development.

European distributed seismological data archives infrastructure: EIDA

NASA Astrophysics Data System (ADS)

Clinton, John; Hanka, Winfried; Mazza, Salvatore; Pederson, Helle; Sleeman, Reinoud; Stammler, Klaus; Strollo, Angelo

2014-05-01

The European Integrated waveform Data Archive (EIDA) is a distributed Data Center system within ORFEUS that (a) securely archives seismic waveform data and related metadata gathered by European research infrastructures, and (b) provides transparent access to the archives for the geosciences research communities. EIDA was founded in 2013 by ORFEUS Data Center, GFZ, RESIF, ETH, INGV and BGR to ensure sustainability of a distributed archive system and the implementation of standards (e.g. FDSN StationXML, FDSN webservices) and coordinate new developments. Under the mandate of the ORFEUS Board of Directors and Executive Committee the founding group is responsible for steering and maintaining the technical developments and organization of the European distributed seismic waveform data archive and the integration within broader multidisciplanry frameworks like EPOS. EIDA currently offers uniform data access to unrestricted data from 8 European archives (www.orfeus-eu.org/eida), linked by the Arclink protocol, hosting data from 75 permanent networks (1800+ stations) and 33 temporary networks (1200+) stations). Moreover, each archive may also provide unique, restricted datasets. A webinterface, developed at GFZ, offers interactive access to different catalogues (EMSC, GFZ, USGS) and EIDA waveform data. Clients and toolboxes like arclink_fetch and ObsPy can connect directly to any EIDA node to collect data. Current developments are directed to the implementation of quality parameters and strong motion parameters.

Collaborative Research: Ground Truth of African and Eastern Mediterranean Shallow Seismicity Using SAR Interferometry and Gibbs Sampling Inversion

DTIC Science & Technology

2006-10-05

the likely existence of a small foreshock . 2. BACKGROUND 2.1. InSAR The most well-known examples of InSAR used as a geodetic tool involve...the event. We have used the seismic waveforms in the Sultan Dag event to identify a small foreshock preceding the main shock by about 3 seconds

New Version of SeismicHandler (SHX) based on ObsPy

NASA Astrophysics Data System (ADS)

Stammler, Klaus; Walther, Marcus

2016-04-01

The command line version of SeismicHandler (SH), a scientific analysis tool for seismic waveform data developed around 1990, has been redesigned in the recent years, based on a project funded by the Deutsche Forschungsgemeinschaft (DFG). The aim was to address new data access techniques, simplified metadata handling and a modularized software design. As a result the program was rewritten in Python in its main parts, taking advantage of simplicity of this script language and its variety of well developed software libraries, including ObsPy. SHX provides an easy access to waveforms and metadata via arclink and FDSN webservice protocols, also access to event catalogs is implemented. With single commands whole networks or stations within a certain area may be read in, the metadata are retrieved from the servers and stored in a local database. For data processing the large set of SH commands is available, as well as the SH scripting language. Via this SH language scripts or additional Python modules the command set of SHX is easily extendable. The program is open source, tested on Linux operating systems, documentation and download is found at URL "https://www.seismic-handler.org/".

SGRAPH (SeismoGRAPHer): Seismic waveform analysis and integrated tools in seismology

NASA Astrophysics Data System (ADS)

Abdelwahed, Mohamed F.

2012-03-01

Although numerous seismological programs are currently available, most of them suffer from the inability to manipulate different data formats and the lack of embedded seismological tools. SeismoGRAPHer, or simply SGRAPH, is a new system for maintaining and analyzing seismic waveform data in a stand-alone, Windows-based application that manipulates a wide range of data formats. SGRAPH was intended to be a tool sufficient for performing basic waveform analysis and solving advanced seismological problems. The graphical user interface (GUI) utilities and the Windows functionalities, such as dialog boxes, menus, and toolbars, simplify the user interaction with the data. SGRAPH supports common data formats, such as SAC, SEED, GSE, ASCII, and Nanometrics Y-format, and provides the ability to solve many seismological problems with built-in inversion tools. Loaded traces are maintained, processed, plotted, and saved as SAC, ASCII, or PS (post script) file formats. SGRAPH includes Generalized Ray Theory (GRT), genetic algorithm (GA), least-square fitting, auto-picking, fast Fourier transforms (FFT), and many additional tools. This program provides rapid estimation of earthquake source parameters, location, attenuation, and focal mechanisms. Advanced waveform modeling techniques are provided for crustal structure and focal mechanism estimation. SGRAPH has been employed in the Egyptian National Seismic Network (ENSN) as a tool assisting with routine work and data analysis. More than 30 users have been using previous versions of SGRAPH in their research for more than 3 years. The main features of this application are ease of use, speed, small disk space requirements, and the absence of third-party developed components. Because of its architectural structure, SGRAPH can be interfaced with newly developed methods or applications in seismology. A complete setup file, including the SGRAPH package with the online user guide, is available.

Next-generation seismic experiments: wide-angle, multi-azimuth, three-dimensional, full-waveform inversion

NASA Astrophysics Data System (ADS)

Morgan, Joanna; Warner, Michael; Bell, Rebecca; Ashley, Jack; Barnes, Danielle; Little, Rachel; Roele, Katarina; Jones, Charles

2013-12-01

Full-waveform inversion (FWI) is an advanced seismic imaging technique that has recently become computationally feasible in three dimensions, and that is being widely adopted and applied by the oil and gas industry. Here we explore the potential for 3-D FWI, when combined with appropriate marine seismic acquisition, to recover high-resolution high-fidelity P-wave velocity models for subsedimentary targets within the crystalline crust and uppermost mantle. We demonstrate that FWI is able to recover detailed 3-D structural information within a radially faulted dome using a field data set acquired with a standard 3-D petroleum-industry marine acquisition system. Acquiring low-frequency seismic data is important for successful FWI; we show that current acquisition techniques can routinely acquire field data from airguns at frequencies as low as 2 Hz, and that 1 Hz acquisition is likely to be achievable using ocean-bottom hydrophones in deep water. Using existing geological and geophysical models, we construct P-wave velocity models over three potential subsedimentary targets: the Soufrière Hills Volcano on Montserrat and its associated crustal magmatic system, the crust and uppermost mantle across the continent-ocean transition beneath the Campos Basin offshore Brazil, and the oceanic crust and uppermost mantle beneath the East Pacific Rise mid-ocean ridge. We use these models to generate realistic multi-azimuth 3-D synthetic seismic data, and attempt to invert these data to recover the original models. We explore resolution and accuracy, sensitivity to noise and acquisition geometry, ability to invert elastic data using acoustic inversion codes, and the trade-off between low frequencies and starting velocity model accuracy. We show that FWI applied to multi-azimuth, refracted, wide-angle, low-frequency data can resolve features in the deep crust and uppermost mantle on scales that are significantly better than can be achieved by any other geophysical technique, and that these results can be obtained using relatively small numbers (60-90) of ocean-bottom receivers combined with large numbers of airgun shots. We demonstrate that multi-azimuth 3-D FWI is robust in the presence of noise, that acoustic FWI can invert elastic data successfully, and that the typical errors to be expected in starting models derived using traveltimes will not be problematic for FWI given appropriately designed acquisition. FWI is a rapidly maturing technology; its transfer from the petroleum sector to tackle a much broader range of targets now appears to be entirely achievable.

Subduction zone guided waves in Northern Chile

NASA Astrophysics Data System (ADS)

Garth, Thomas; Rietbrock, Andreas

2016-04-01

Guided wave dispersion is observed in subduction zones as high frequency energy is retained and delayed by low velocity structure in the subducting slab, while lower frequency energy is able to travel at the faster velocities associated with the surrounding mantle material. As subduction zone guided waves spend longer interacting with the low velocity structure of the slab than any other seismic phase, they have a unique capability to resolve these low velocity structures. In Northern Chile, guided wave arrivals are clearly observed on two stations in the Chilean fore-arc on permanent stations of the IPOC network. High frequency (> 5 Hz) P-wave arrivals are delayed by approximately 2 seconds compared to the low frequency (< 2 Hz) P-wave arrivals. Full waveform finite difference modelling is used to test the low velocity slab structure that cause this P-wave dispersion. The synthetic waveforms produced by these models are compared to the recorded waveforms. Spectrograms are used to compare the relative arrival times of different frequencies, while the velocity spectra is used to constrain the relative amplitude of the arrivals. Constraining the waveform in these two ways means that the full waveform is also matched, and the low pass filtered observed and synthetic waveforms can be compared. A combined misfit between synthetic and observed waveforms is then calculated following Garth & Rietbrock (2014). Based on this misfit criterion we constrain the velocity model by using a grid search approach. Modelling the guided wave arrivals suggest that the observed dispersion cannot be solely accounted for by a single low velocity layer as suggested by previous guided wave studies. Including dipping low velocity normal fault structures in the synthetic model not only accounts for the observed strong P-wave coda, but also produces a clear first motion dispersion. We therefore propose that the lithospheric mantle of the subducting Nazca plate is highly hydrated at intermediate depths by dipping low velocity normal faults. Additionally, we show that the low velocity oceanic crust persists to depths of up to 200 km, well beyond the depth range where the eclogite transition is expected to have occurred. Our results suggest that young subducting lithosphere also has the potential to carry much larger amounts of water to the mantle than has previously been appreciated.

Southern California Seismic Network: New Design and Implementation of Redundant and Reliable Real-time Data Acquisition Systems

NASA Astrophysics Data System (ADS)

Saleh, T.; Rico, H.; Solanki, K.; Hauksson, E.; Friberg, P.

2005-12-01

The Southern California Seismic Network (SCSN) handles more than 2500 high-data rate channels from more than 380 seismic stations distributed across southern California. These data are imported real-time from dataloggers, earthworm hubs, and partner networks. The SCSN also exports data to eight different partner networks. Both the imported and exported data are critical for emergency response and scientific research. Previous data acquisition systems were complex and difficult to operate, because they grew in an ad hoc fashion to meet the increasing needs for distributing real-time waveform data. To maximize reliability and redundancy, we apply best practices methods from computer science for implementing the software and hardware configurations for import, export, and acquisition of real-time seismic data. Our approach makes use of failover software designs, methods for dividing labor diligently amongst the network nodes, and state of the art networking redundancy technologies. To facilitate maintenance and daily operations we seek to provide some separation between major functions such as data import, export, acquisition, archiving, real-time processing, and alarming. As an example, we make waveform import and export functions independent by operating them on separate servers. Similarly, two independent servers provide waveform export, allowing data recipients to implement their own redundancy. The data import is handled differently by using one primary server and a live backup server. These data import servers, run fail-over software that allows automatic role switching in case of failure from primary to shadow. Similar to the classic earthworm design, all the acquired waveform data are broadcast onto a private network, which allows multiple machines to acquire and process the data. As we separate data import and export away from acquisition, we are also working on new approaches to separate real-time processing and rapid reliable archiving of real-time data. Further, improved network security is an integral part of the new design. Redundant firewalls will provide secure data imports, exports, and acquisition as well as DMZ zones for web servers and other publicly available servers. We will present the detailed design of this new configuration that is currently being implemented by the SCSN at Caltech. The design principals are general enough to be of use to most regional seismic networks.

Spectral-Element Seismic Wave Propagation Codes for both Forward Modeling in Complex Media and Adjoint Tomography

NASA Astrophysics Data System (ADS)

Smith, J. A.; Peter, D. B.; Tromp, J.; Komatitsch, D.; Lefebvre, M. P.

2015-12-01

We present both SPECFEM3D_Cartesian and SPECFEM3D_GLOBE open-source codes, representing high-performance numerical wave solvers simulating seismic wave propagation for local-, regional-, and global-scale application. These codes are suitable for both forward propagation in complex media and tomographic imaging. Both solvers compute highly accurate seismic wave fields using the continuous Galerkin spectral-element method on unstructured meshes. Lateral variations in compressional- and shear-wave speeds, density, as well as 3D attenuation Q models, topography and fluid-solid coupling are all readily included in both codes. For global simulations, effects due to rotation, ellipticity, the oceans, 3D crustal models, and self-gravitation are additionally included. Both packages provide forward and adjoint functionality suitable for adjoint tomography on high-performance computing architectures. We highlight the most recent release of the global version which includes improved performance, simultaneous MPI runs, OpenCL and CUDA support via an automatic source-to-source transformation library (BOAST), parallel I/O readers and writers for databases using ADIOS and seismograms using the recently developed Adaptable Seismic Data Format (ASDF) with built-in provenance. This makes our spectral-element solvers current state-of-the-art, open-source community codes for high-performance seismic wave propagation on arbitrarily complex 3D models. Together with these solvers, we provide full-waveform inversion tools to image the Earth's interior at unprecedented resolution.

Multi-Phenomenological Analysis of the 12 August 2015 Tianjin, China Chemical Explosion

NASA Astrophysics Data System (ADS)

Pasyanos, M.; Kim, K.; Park, J.; Stump, B. W.; Hayward, C.; Che, I. Y.; Zhao, L.; Myers, S. C.

2016-12-01

We perform a multi-phenomenological analysis of the massive near-surface chemical explosions that occurred in Tianjin, China on 12 August 2015. A recent assessment of these events was performed by Zhao et al. (2016) using local (< 100 km) seismic data. This study considers a regional assessment of the same sequence in the absence of having any local data. We provide additional insight by combining regional seismic analysis with the use of infrasound signals and an assessment of the event crater. Event locations using infrasound signals recorded at Korean and IMS arrays are estimated based on the Bayesian Infrasonic Source Location (BISL) method (Modrak et al., 2010), and improved with azimuthal corrections using a raytracing (Blom and Waxler, 2012) and the Ground-to-Space (G2S) atmospheric models (Drob et al., 2003). The location information provided from the infrasound signals is then merged with the regional seismic arrivals to produce a joint event location. The yields of the events are estimated from seismic and infrasonic observations. Seismic waveform envelope method (Pasyanos et al., 2012) including the free surface effect (Pasyanos and Ford, 2015) is applied to regional seismic signals. Waveform inversion method (Kim and Rodgers, 2016) is used for infrasound signals. A combination of the seismic and acoustic signals can provide insights on the energy partitioning and break the tradeoffs between the yield and the depth/height of explosions, resulting in a more robust estimation of event yield. The yield information from the different phenomenologies are combined through the use of likelihood functions.

Unreported seismic events found far off-shore Mexico using full-waveform, cross-correlation detection method.

NASA Astrophysics Data System (ADS)

Solano, ErickaAlinne; Hjorleifsdottir, Vala; Perez-Campos, Xyoli

2015-04-01

A large subset of seismic events do not have impulsive arrivals, such as low frequency events in volcanoes, earthquakes in the shallow part of the subduction interface and further down dip from the traditional seismogenic part, glacial events, volcanic and non-volcanic tremors and landslides. A suite of methods can be used to detect these non-impulsive events. One of this methods is the full-waveform detection based on time reversal methods (Solano, et al , submitted to GJI). The method uses continuous observed seismograms, together with Greens functions and moment tensor responses calculated for an arbitrary 3D structure. This method was applied to the 2012 Ometepec-Pinotepa Nacional earthquake sequence in Guerrero, Mexico. During the span time of the study, we encountered three previously unknown events. One of this events was an impulsive earthquake in the Ometepec area, that only has clear arrivals on three stations and was therefore not located and reported by the SSN. The other two events are previously undetected events, very depleted in high frequencies, that occurred far outside the search area. A very rough estimate gives the location of this two events in the portion of the East Pacific Rise around 9 N. These two events are detected despite their distance from the search area, due to favorable move-out on the array of the Mexican National Seismological Service network (SSN). We are expanding the study area to the EPR and to a larger period of time, with the objective of finding more events in that region. We will present an analysis of the newly detected events, as well as any further findings at the meeting.

Phase Velocity and Full-Waveform Analysis of Co-located Distributed Acoustic Sensing (DAS) Channels and Geophone Sensor

NASA Astrophysics Data System (ADS)

Parker, L.; Mellors, R. J.; Thurber, C. H.; Wang, H. F.; Zeng, X.

2015-12-01

A 762-meter Distributed Acoustic Sensing (DAS) array with a channel spacing of one meter was deployed at the Garner Valley Downhole Array in Southern California. The array was approximately rectangular with dimensions of 180 meters by 80 meters. The array also included two subdiagonals within the rectangle along which three-component geophones were co-located. Several active sources were deployed, including a 45-kN, swept-frequency, shear-mass shaker, which produced strong Rayleigh waves across the array. Both DAS and geophone traces were filtered in 2-Hz steps between 4 and 20 Hz to obtain phase velocities as a function of frequency from fitting the moveout of travel times over distances of 35 meters or longer. As an alternative to this traditional means of finding phase velocity, it is theoretically possible to find the Rayleigh-wave phase velocity at each point of co-location as the ratio of DAS and geophone responses, because DAS is sensitive to ground strain and geophones are sensitive to ground velocity, after suitable corrections for instrument response (Mikumo & Aki, 1964). The concept was tested in WPP, a seismic wave propagation program, by first validating and then using a 3D synthetic, full-waveform seismic model to simulate the effect of increased levels of noise and uncertainty as data go from ideal to more realistic. The results obtained from this study provide a better understanding of the DAS response and its potential for being combined with traditional seismometers for obtaining phase velocity at a single location. This analysis is part of the PoroTomo project (Poroelastic Tomography by Adjoint Inverse Modeling of Data from Seismology, Geodesy, and Hydrology, http://geoscience.wisc.edu/feigl/porotomo).

Fine-scale thermohaline ocean structure retrieved with 2-D prestack full-waveform inversion of multichannel seismic data: Application to the Gulf of Cadiz (SW Iberia)

NASA Astrophysics Data System (ADS)

Dagnino, D.; Sallarès, V.; Biescas, B.; Ranero, C. R.

2016-08-01

This work demonstrates the feasibility of 2-D time-domain, adjoint-state acoustic full-waveform inversion (FWI) to retrieve high-resolution models of ocean physical parameters such as sound speed, temperature and salinity. The proposed method is first described and then applied to prestack multichannel seismic (MCS) data acquired in the Gulf of Cadiz (SW Iberia) in 2007 in the framework of the Geophysical Oceanography project. The inversion strategy flow includes specifically designed data preconditioning for acoustic noise reduction, followed by the inversion of sound speed in the shotgather domain. We show that the final sound speed model has a horizontal resolution of ˜ 70 m, which is two orders of magnitude better than that of the initial model constructed with coincident eXpendable Bathy Thermograph (XBT) data, and close to the theoretical resolution of O(λ). Temperature (T) and salinity (S) are retrieved with the same lateral resolution as sound speed by combining the inverted sound speed model with the thermodynamic equation of seawater and a local, depth-dependent T-S relation derived from regional conductivity-temperature-depth (CTD) measurements of the National Oceanic and Atmospheric Administration (NOAA) database. The comparison of the inverted T and S models with XBT and CTD casts deployed simultaneously to the MCS acquisition shows that the thermohaline contrasts are resolved with an accuracy of 0.18oC for temperature and 0.08 PSU for salinity. The combination of oceanographic and MCS data into a common, pseudo-automatic inversion scheme allows to quantitatively resolve submeso-scale features that ought to be incorporated into larger-scale ocean models of oceans structure and circulation.

Rapid Modeling of and Response to Large Earthquakes Using Real-Time GPS Networks (Invited)

NASA Astrophysics Data System (ADS)

Crowell, B. W.; Bock, Y.; Squibb, M. B.

2010-12-01

Real-time GPS networks have the advantage of capturing motions throughout the entire earthquake cycle (interseismic, seismic, coseismic, postseismic), and because of this, are ideal for real-time monitoring of fault slip in the region. Real-time GPS networks provide the perfect supplement to seismic networks, which operate with lower noise and higher sampling rates than GPS networks, but only measure accelerations or velocities, putting them at a supreme disadvantage for ascertaining the full extent of slip during a large earthquake in real-time. Here we report on two examples of rapid modeling of recent large earthquakes near large regional real-time GPS networks. The first utilizes Japan’s GEONET consisting of about 1200 stations during the 2003 Mw 8.3 Tokachi-Oki earthquake about 100 km offshore Hokkaido Island and the second investigates the 2010 Mw 7.2 El Mayor-Cucapah earthquake recorded by more than 100 stations in the California Real Time Network. The principal components of strain were computed throughout the networks and utilized as a trigger to initiate earthquake modeling. Total displacement waveforms were then computed in a simulated real-time fashion using a real-time network adjustment algorithm that fixes a station far away from the rupture to obtain a stable reference frame. Initial peak ground displacement measurements can then be used to obtain an initial size through scaling relationships. Finally, a full coseismic model of the event can be run minutes after the event, given predefined fault geometries, allowing emergency first responders and researchers to pinpoint the regions of highest damage. Furthermore, we are also investigating using total displacement waveforms for real-time moment tensor inversions to look at spatiotemporal variations in slip.

Adjoint tomography of the crust and upper mantle structure beneath the Kanto region using broadband seismograms

NASA Astrophysics Data System (ADS)

Miyoshi, Takayuki; Obayashi, Masayuki; Peter, Daniel; Tono, Yoko; Tsuboi, Seiji

2017-12-01

A three-dimensional seismic wave speed model in the Kanto region of Japan was developed using adjoint tomography for application in the effective reproduction of observed waveforms. Starting with a model based on previous travel time tomographic results, we inverted the waveforms obtained at seismic broadband stations from 140 local earthquakes in the Kanto region to obtain the P- and S-wave speeds V p and V s . Additionally, all centroid times of the source solutions were determined before the structural inversion. The synthetic displacements were calculated using the spectral-element method (SEM) in which the Kanto region was parameterized using 16 million grid points. The model parameters V p and V s were updated iteratively by Newton's method using the misfit and Hessian kernels until the misfit between the observed and synthetic waveforms was minimized. Computations of the forward and adjoint simulations were conducted on the K computer in Japan. The optimized SEM code required a total of 6720 simulations using approximately 62,000 node hours to obtain the final model after 16 iterations. The proposed model reveals several anomalous areas with extremely low- V s values in comparison with those of the initial model. These anomalies were found to correspond to geological features, earthquake sources, and volcanic regions with good data coverage and resolution. The synthetic waveforms obtained using the newly proposed model for the selected earthquakes showed better fit than the initial model to the observed waveforms in different period ranges within 5-30 s. This result indicates that the model can accurately predict actual waveforms. [Figure not available: see fulltext.

Numerical modeling of landslides and generated seismic waves: The Bingham Canyon Mine landslides

NASA Astrophysics Data System (ADS)

Miallot, H.; Mangeney, A.; Capdeville, Y.; Hibert, C.

2016-12-01

Landslides are important natural hazards and key erosion processes. They create long period surface waves that can be recorded by regional and global seismic networks. The seismic signals are generated by acceleration/deceleration of the mass sliding over the topography. They consist in a unique and powerful tool to detect, characterize and quantify the landslide dynamics. We investigate here the processes at work during the two massive landslides that struck the Bingham Canyon Mine on the 10th April 2013. We carry a combined analysis of the generated seismic signals and the landslide processes computed with a 3D modeling on a complex topography. Forces computed by broadband seismic waveform inversion are used to constrain the study and particularly the force-source and the bulk dynamic. The source time function are obtained by a 3D model (Shaltop) where rheological parameters can be adjusted. We first investigate the influence of the initial shape of the sliding mass which strongly affects the whole landslide dynamic. We also see that the initial shape of the source mass of the first landslide constrains pretty well the second landslide source mass. We then investigate the effect of a rheological parameter, the frictional angle, that strongly influences the resulted computed seismic source function. We test here numerous friction laws as the frictional Coulomb law and a velocity-weakening friction law. Our results show that the force waveform fitting the observed data is highly variable depending on these different choices.

The Iquique 2014 sequence: understanding its nucleation and propagation from the seismicity evolution

NASA Astrophysics Data System (ADS)

Fuenzalida, A.; Rietbrock, A.; Woollam, J.; Tavera, H.; Ruiz, S.

2017-12-01

The Northern Chile and Southern Peru region is well known for its high seismic hazard due to the lack of recent major ruptures along long segments of the subduction interface. For this reason the 2014 Iquique Mw 8.1 earthquake that occurred in the Northern Chile seismic gap was expected and high quality seismic and geodetic networks were operating at the time of the event recording the precursory phase of a mega-thrust event with unprecedented detail. In this study we used seismic data collected during the 2014 Iquique sequence to generate a detailed earthquake catalogue. This catalogue consists of more than 15,000 events identified in Northern Chile during the period between 1/3/14 and 31/5/14 and provides full coverage of the immediate foreshock sequence, the main-shock and early after-shock series. The initial catalogue was obtained by automatic data processing and only selecting events with at least two associate S phases to improve the reliability of initial locations. Subsequently, this subset of events was automatically processed again using an optimized STA/LTA triggering algorithm for both P and S-waves and constraining the detection times by estimated arrival times at each station calculated for the preliminary locations. Finally, all events were relocated using a recently developed 1D velocity model and associated station corrections. For events Mw 4 or larger that occurred between the 15/3/14 and 10/04/14, we estimated it regional moment tensor by full-waveform inversion. Our results confirm the seismic activation of the upper plate during the foreshock sequence, as well highlight a crustal activity on the fore-arc during the aftershock series. The seismicity distribution was compared to the previous inter-seismic coupling studies obtained in the region, in which we observe interplay between high and low coupling areas, which are correlated to the seismicity rate. The spatial distribution of the seismicity and the complexities on the mechanisms observed during the sequence can be associated to the observed seamounts belonging to the Iquique ridge by previous marine experiment. To conclude our study, we perform a space and time analysis of the seismicity and we propose several scenarios to explain the nucleation of the earthquake and the way on which the seismicity behave during the sequence.

Initial rupture of earthquakes in the 1995 Ridgecrest, California sequence

USGS Publications Warehouse

Mori, J.; Kanamori, H.

1996-01-01

Close examination of the P waves from earthquakes ranging in size across several orders of magnitude shows that the shape of the initiation of the velocity waveforms is independent of the magnitude of the earthquake. A model in which earthquakes of all sizes have similar rupture initiation can explain the data. This suggests that it is difficult to estimate the eventual size of an earthquake from the initial portion of the waveform. Previously reported curvature seen in the beginning of some velocity waveforms can be largely explained as the effect of anelastic attenuation; thus there is little evidence for a departure from models of simple rupture initiation that grow dynamically from a small region. The results of this study indicate that any "precursory" radiation at seismic frequencies must emanate from a source region no larger than the equivalent of a M0.5 event (i.e. a characteristic length of ???10 m). The size of the nucleation region for magnitude 0 to 5 earthquakes thus is not resolvable with the standard seismic instrumentation deployed in California. Copyright 1996 by the American Geophysical Union.

A Global Upper-Mantle Tomographic Model of Shear Attenuation

NASA Astrophysics Data System (ADS)

Karaoglu, H.; Romanowicz, B. A.

2016-12-01

Mapping anelastic 3D structure within the earth's mantle is key to understanding present day mantle dynamics, as it provides complementary constraints to those obtained from elastic structure, with the potential to distinguish between thermal and compositional heterogeneity. For this, we need to measure seismic wave amplitudes, which are sensitive to both elastic (through focusing and scattering) and anelastic structure. The elastic effects are less pronounced at long periods, so previous global upper-mantle attenuation models are based on teleseismic surface wave data, sometimes including overtones. In these studies, elastic effects are considered either indirectly, by eliminating data strongly contaminated by them (e.g. Romanowicz, 1995; Gung and Romanowicz, 2004), or by correcting for elastic focusing effects using an approximate linear approach (Dalton et al., 2008). Additionally, in these studies, the elastic structure is held fixed when inverting for intrinsic attenuation . The importance of (1) having a good starting elastic model, (2) accurate modeling of the seismic wavefield and (3) joint inversion for elastic and anelastic structure, becomes more evident as the targeted resolution level increases. Also, velocity dispersion effects due to anelasticity need to be taken into account. Here, we employ a hybrid full waveform inversion method, inverting jointly for global elastic and anelastic upper mantle structure, starting from the latest global 3D shear velocity model built by our group (French and Romanowicz, 2014), using the spectral element method for the forward waveform modeling (Capdeville et al., 2003), and normal-mode perturbation theory (NACT - Li and Romanowicz, 1995) for kernel computations. We present a 3D upper-mantle anelastic model built by using three component fundamental and overtone surface waveforms down to 60 s as well as long period body waveforms down to 30 s. We also include source and site effects to first order as frequency independent scalar factors. The robustness of the inversion method is assessed through synthetic and resolution tests. We discuss salient features of the resulting anelastic model and in particular the well-resolved strong correlation with tectonics observed in the first 200 km of the mantle.

New constraints on the magmatic system beneath Newberry Volcano from the analysis of active and passive source seismic data, and ambient noise

NASA Astrophysics Data System (ADS)

Heath, B.; Toomey, D. R.; Hooft, E. E. E.

2014-12-01

Magmatic systems beneath arc-volcanoes are often poorly resolved by seismic imaging due to the small spatial scale and large magnitude of crustal heterogeneity in combination with field experiments that sparsely sample the wavefield. Here we report on our continued analysis of seismic data from a line of densely-spaced (~300 m), three-component seismometers installed on Newberry Volcano in central Oregon for ~3 weeks; the array recorded an explosive shot, ~20 teleseismic events, and ambient noise. By jointly inverting both active and passive-source travel time data, the resulting tomographic image reveals a more detailed view of the presumed rhyolitic magma chamber at ~3-5 km depth, previously imaged by Achauer et al. (1988) and Beachly et al. (2012). The magma chamber is elongated perpendicular to the trend of extensional faulting and encircled by hypocenters of small (M < 2) earthquakes located by PNSN. We also model teleseismic waveforms using a 2-D synthetic seismogram code to recreate anomalous amplitudes observed in the P-wave coda for sites within the caldera. Autocorrelation of ambient noise data also reveals large amplitude waveforms for a small but spatially grouped set of stations, also located within the caldera. On the basis of these noise observations and 2-D synthetic models, which both require slow seismic speeds at depth, we conclude that our tomographic model underestimates low-velocity anomalies associated with the inferred crustal magma chamber; this is due in large part to wavefront healing, which reduces observed travel time anomalies, and regularization constraints, which minimize model perturbations. Only by using various methods that interrogate different aspects of the seismic data are we able to more realistically constrain the complicated, heterogeneous volcanic system. In particular, modeling of waveform characteristics provides a better measure of the spatial scale and magnitude of crustal velocities near magmatic systems.

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.

Characterizing waveform uncertainty due to ambient noise for the Global Seismic Network

NASA Astrophysics Data System (ADS)

Guandique, J. A.; Burdick, S.; Lekic, V.

2015-12-01

Ambient seismic noise is the vibration present on seismograms not due by any earthquake or discrete source. It can be caused by trees swaying in the wind or trucks rumbling on the freeway, but the main source of noise is the microseism caused by ocean waves. The frequency content and amplitude of seismic noise varies due to weather, season, and the location of a station, among other factors. Because noise affects recordings of earthquake waveforms, better understanding it could improve the detection of small earthquakes, reduce false positives in earthquake early warning, and quantify uncertainty in waveform-based studies In this study, we used two years of 3-component accelerograms from stations in the GSN. We eliminate days with major earthquakes, aggregate analysis by month, and calculate the mean power spectrum for each component and the transfer function between components. For each power spectrum, we determine the dominant frequency and amplitude of the primary (PM) and secondary (SM) microseisms which appear at periods of ~14s and ~7s, as well as any other prominent peaks. The cross-component terms show that noise recorded on different components cannot be treated as independent. Trends in coherence and phase delay suggest directionality in the noise and information about in which modes it propagates. Preliminary results show that the noise on island stations exhibits less monthly variability, and its PM peaks tend to be much weaker than the SM peaks. The continental stations show much less consistent behavior, with higher variability in the PM peaks between stations and higher frequency content during winter months. Stations that are further inland have smaller SM peaks compared to coastal stations, which are more similar to island stations. Using these spectra and cross-component results, we develop a method for generating realistic 3-component seismic noise and covariance matrices, which can be used across various seismic applications.

The ear, the eye, earthquakes and feature selection: listening to automatically generated seismic bulletins for clues as to the differences between true and false events.

NASA Astrophysics Data System (ADS)

Kuzma, H. A.; Arehart, E.; Louie, J. N.; Witzleben, J. L.

2012-04-01

Listening to the waveforms generated by earthquakes is not new. The recordings of seismometers have been sped up and played to generations of introductory seismology students, published on educational websites and even included in the occasional symphony. The modern twist on earthquakes as music is an interest in using state-of-the-art computer algorithms for seismic data processing and evaluation. Algorithms such as such as Hidden Markov Models, Bayesian Network models and Support Vector Machines have been highly developed for applications in speech recognition, and might also be adapted for automatic seismic data analysis. Over the last three years, the International Data Centre (IDC) of the Comprehensive Test Ban Treaty Organization (CTBTO) has supported an effort to apply computer learning and data mining algorithms to IDC data processing, particularly to the problem of weeding through automatically generated event bulletins to find events which are non-physical and would otherwise have to be eliminated by the hand of highly trained human analysts. Analysts are able to evaluate events, distinguish between phases, pick new phases and build new events by looking at waveforms displayed on a computer screen. Human ears, however, are much better suited to waveform processing than are the eyes. Our hypothesis is that combining an auditory representation of seismic events with visual waveforms would reduce the time it takes to train an analyst and the time they need to evaluate an event. Since it takes almost two years for a person of extraordinary diligence to become a professional analyst and IDC contracts are limited to seven years by Treaty, faster training would significantly improve IDC operations. Furthermore, once a person learns to distinguish between true and false events by ear, various forms of audio compression can be applied to the data. The compression scheme which yields the smallest data set in which relevant signals can still be heard is likely an excellent candidate from which to draw features that can be fed into machine learning algorithms since it contains a compact numerical representation of the information that humans need to evaluate events. The challenge in this work is that, although it is relatively easy to pick out earthquake arrivals in waveform data from a single station, when stations are combined the addition of background noise tends to confuse and overwhelm the listener. To solve this problem, we rely on techniques such as the slowing down of recordings without altering the pitch which are used by ethnomusicologists to understand highly complex rhythms and sounds. We work with professional musicians and recorders to mix the data from different seismic stations in a way which reduces noise and preserves the uniqueness of each station.

Non-Linear Seismic Velocity Estimation from Multiple Waveform Functionals and Formal Assessment of Constraints

DTIC Science & Technology

2011-09-01

tectonically active regions such as the Middle East. For example, we previously applied the code to determine the crust and upper mantle structure...Objective Optimization (MOO) for Multiple Datasets The primary goal of our current project is to develop a tool for estimating crustal structure that...be used to obtain crustal velocity structures by modeling broadband waveform, receiver function, and surface wave dispersion data. The code has been

Deghosting based on the transmission matrix method

NASA Astrophysics Data System (ADS)

Wang, Benfeng; Wu, Ru-Shan; Chen, Xiaohong

2017-12-01

As the developments of seismic exploration and subsequent seismic exploitation advance, marine acquisition systems with towed streamers become an important seismic data acquisition method. But the existing air-water reflective interface can generate surface related multiples, including ghosts, which can affect the accuracy and performance of the following seismic data processing algorithms. Thus, we derive a deghosting method from a new perspective, i.e. using the transmission matrix (T-matrix) method instead of inverse scattering series. The T-matrix-based deghosting algorithm includes all scattering effects and is convergent absolutely. Initially, the effectiveness of the proposed method is demonstrated using synthetic data obtained from a designed layered model, and its noise-resistant property is also illustrated using noisy synthetic data contaminated by random noise. Numerical examples on complicated data from the open SMAART Pluto model and field marine data further demonstrate the validity and flexibility of the proposed method. After deghosting, low frequency components are recovered reasonably and the fake high frequency components are attenuated, and the recovered low frequency components will be useful for the subsequent full waveform inversion. The proposed deghosting method is currently suitable for two-dimensional towed streamer cases with accurate constant depth information and its extension into variable-depth streamers in three-dimensional cases will be studied in the future.

Towards Full-Waveform Ambient Noise Inversion

NASA Astrophysics Data System (ADS)

Sager, K.; Ermert, L. A.; Boehm, C.; Fichtner, A.

2016-12-01

Noise tomography usually works under the assumption that the inter-station ambient noise correlation is equal to a scaled version of the Green function between the two receivers. This assumption, however, is only met under specific conditions, e.g. wavefield diffusivity and equipartitioning, or the isotropic distribution of both mono- and dipolar uncorrelated noise sources. These assumptions are typically not satisfied in the Earth. This inconsistency inhibits the exploitation of the full waveform information contained in noise correlations in order to constrain Earth structure and noise generation. To overcome this limitation, we attempt to develop a method that consistently accounts for the distribution of noise sources, 3D heterogeneous Earth structure and the full seismic wave propagation physics. This is intended to improve the resolution of tomographic images, to refine noise source location, and thereby to contribute to a better understanding of noise generation. We introduce an operator-based formulation for the computation of correlation functions and apply the continuous adjoint method that allows us to compute first and second derivatives of misfit functionals with respect to source distribution and Earth structure efficiently. Based on these developments we design an inversion scheme using a 2D finite-difference code. To enable a joint inversion for noise sources and Earth structure, we investigate the following aspects: The capability of different misfit functionals to image wave speed anomalies and source distribution. Possible source-structure trade-offs, especially to what extent unresolvable structure can be mapped into the inverted noise source distribution and vice versa. In anticipation of real-data applications, we present an extension of the open-source waveform modelling and inversion package Salvus, which allows us to compute correlation functions in 3D media with heterogeneous noise sources at the surface.

Full Waveform Modelling for Subsurface Characterization with Converted-Wave Seismic Reflection

NASA Astrophysics Data System (ADS)

Triyoso, Wahyu; Oktariena, Madaniya; Sinaga, Edycakra; Syaifuddin, Firman

2017-04-01

While a large number of reservoirs have been explored using P-waves seismic data, P-wave seismic survey ceases to provide adequate result in seismically and geologically challenging areas, like gas cloud, shallow drilling hazards, strong multiples, highly fractured, anisotropy. Most of these reservoir problems can be addressed using P and PS seismic data combination. Multicomponent seismic survey records both P-wave and S-wave unlike conventional survey that only records compressional P-wave. Under certain conditions, conventional energy source can be used to record P and PS data using the fact that compressional wave energy partly converts into shear waves at the reflector. Shear component can be recorded using down going P-wave and upcoming S-wave by placing a horizontal component geophone on the ocean floor. A synthetic model is created based on real data to analyze the effect of gas cloud existence to PP and PS wave reflections which has a similar characteristic to Sub-Volcanic imaging. The challenge within the multicomponent seismic is the different travel time between P-wave and S-wave, therefore the converted-wave seismic data should be processed with different approach. This research will provide a method to determine an optimum converted point known as Common Conversion Point (CCP) that can solve the Asymmetrical Conversion Point of PS data. The value of γ (Vp/Vs) is essential to estimate the right CCP that will be used in converted-wave seismic processing. This research will also continue to the advanced processing method of converted-wave seismic by applying Joint Inversion to PP&PS seismic. Joint Inversion is a simultaneous model-based inversion that estimates the P&S-wave impedance which are consistent with the PP&PS amplitude data. The result reveals a more complex structure mirrored in PS data below the gas cloud area. Through estimated γ section resulted from Joint Inversion, we receive a better imaging improvement below gas cloud area tribute to the converted-wave seismic as additional constrain.

Decomposition Techniques for Icesat/glas Full-Waveform Data

NASA Astrophysics Data System (ADS)

Liu, Z.; Gao, X.; Li, G.; Chen, J.

2018-04-01

The geoscience laser altimeter system (GLAS) on the board Ice, Cloud, and land Elevation Satellite (ICESat), is the first long-duration space borne full-waveform LiDAR for measuring the topography of the ice shelf and temporal variation, cloud and atmospheric characteristics. In order to extract the characteristic parameters of the waveform, the key step is to process the full waveform data. In this paper, the modified waveform decomposition method is proposed to extract the echo components from full-waveform. First, the initial parameter estimation is implemented through data preprocessing and waveform detection. Next, the waveform fitting is demonstrated using the Levenberg-Marquard (LM) optimization method. The results show that the modified waveform decomposition method can effectively extract the overlapped echo components and missing echo components compared with the results from GLA14 product. The echo components can also be extracted from the complex waveforms.

Preparing for InSight - using the continuous seismic data flow to investigate the deep interior of Mars

NASA Astrophysics Data System (ADS)

Hempel, S.; Garcia, R.; Weber, R. C.; Schmerr, N. C.; Panning, M. P.; Lognonne, P. H.; Banerdt, W. B.

2016-12-01

Complementary to investigating ray theoretically predictable parameters to explore the deep interior of Mars (see AGU contribution by R. Weber et al.), this paper presents the waveform approach to illuminate the lowermost mantle and core-mantle boundary of Mars. In preparation to the NASA discovery mission InSight, scheduled for launch in May, 2018, we produce synthetic waveforms considering realistic combinations of sources and a single receiver, as well as noise models. Due to a lack of constraints on the scattering properties of the Martian crust and mantle, we assume Earth-like scattering as a minimum and Moon-like scattering as a maximum possibility. Various seismic attenuation models are also investigated. InSight is set up to deliver event data as well as a continuous data flow. Where ray theoretical approaches will investigate the event data, the continuous data flow may contain signals reflected multiple times off the same reflector, e.g. the underside of the lithosphere, or the core-mantle boundary. It may also contain signals of individual events not detected or interfering wavefields radiated off multiple undetected events creating 'seismic noise'. We will use AxiSEM to simulate a continuous data flow for these cases for various 1D and 2D Mars models, and explore the possibilities of seismic interferometry to use seismic information hidden in the coda to investigate the deep interior of Mars.

Changes in seismic velocity during the first 14 months of the 2004-2008 eruption of Mount St. Helens, Washington

NASA Astrophysics Data System (ADS)

Hotovec-Ellis, A. J.; Vidale, J. E.; Gomberg, J.; Thelen, W.; Moran, S. C.

2015-09-01

Mount St. Helens began erupting in late 2004 following an 18 year quiescence. Swarms of repeating earthquakes accompanied the extrusion of a mostly solid dacite dome over the next 4 years. In some cases the waveforms from these earthquakes evolved slowly, likely reflecting changes in the properties of the volcano that affect seismic wave propagation. We use coda-wave interferometry to quantify small changes in seismic velocity structure (usually <1%) between two similar earthquakes and employed waveforms from several hundred families of repeating earthquakes together to create a continuous function of velocity change observed at permanent stations operated within 20 km of the volcano. The high rate of earthquakes allowed tracking of velocity changes on an hourly time scale. Changes in velocity were largest near the newly extruding dome and likely related to shallow deformation as magma first worked its way to the surface. We found strong correlation between velocity changes and the inverse of real-time seismic amplitude measurements during the first 3 weeks of activity, suggesting that fluctuations of pressure in the shallow subsurface may have driven both seismicity and velocity changes. Velocity changes during the remainder of the eruption likely result from a complex interplay of multiple effects and are not well explained by any single factor alone, highlighting the need for complementary geophysical data when interpreting velocity changes.

Structure of the California Coast Ranges and San Andreas Fault at SAFOD from seismic waveform inversion and reflection imaging

USGS Publications Warehouse

Bleibinhaus, F.; Hole, J.A.; Ryberg, T.; Fuis, G.S.

2007-01-01

A seismic reflection and refraction survey across the San Andreas Fault (SAF) near Parkfield provides a detailed characterization of crustal structure across the location of the San Andreas Fault Observatory at Depth (SAFOD). Steep-dip prestack migration and frequency domain acoustic waveform tomography were applied to obtain highly resolved images of the upper 5 km of the crust for 15 km on either side of the SAF. The resulting velocity model constrains the top of the Salinian granite with great detail. Steep-dip reflection seismic images show several strong-amplitude vertical reflectors in the uppermost crust near SAFOD that define an ???2-km-wide zone comprising the main SAF and two or more local faults. Another prominent subvertical reflector at 2-4 km depth ???9 km to the northeast of the SAF marks the boundary between the Franciscan terrane and the Great Valley Sequence. A deep seismic section of low resolution shows several reflectors in the Salinian crust west of the SAF. Two horizontal reflectors around 10 km depth correlate with strains of seismicity observed along-strike of the SAF. They represent midcrustal shear zones partially decoupling the ductile lower crust from the brittle upper crust. The deepest reflections from ???25 km depth are interpreted as crust-mantle boundary. Copyright 2007 by the American Geophysical Union.

Detecting Micro-seismicity and Long-duration Tremor-like Events from the Oklahoma Wavefield Experiment

NASA Astrophysics Data System (ADS)

Li, C.; Li, Z.; Peng, Z.; Zhang, C.; Nakata, N.

2017-12-01

Oklahoma has experienced abrupt increase of induced seismicity in the last decade. An important way to fully understand seismic activities in Oklahoma is to obtain more complete earthquake catalogs and detect different types of seismic events. The IRIS Community Wavefield Demonstration Experiment was deployed near Enid, Oklahoma in Summer of 2016. The dataset from this ultra-dense array provides an excellent opportunity for detecting microseismicity in that region with wavefield approaches. Here we examine continuous waveforms recorded by 3 seismic lines using local coherence for ultra-dense arrays (Li et al., 2017), which is a measure of cross-correlation of waveform at each station with its nearby stations. So far we have detected more than 5,000 events from 06/22/2016 to 07/20/2016, and majority of them are not listed on the regional catalog of Oklahoma or global catalogs, indicating that they are local events. We also identify 15-20 long-period long-duration events, some of them lasting for more than 500 s. Such events have been found at major plate-boundary faults (also known as deep tectonic tremor), as well as during hydraulic fracturing, slow-moving landslides and glaciers. Our next step is to locate these possible tremor-like events with their relative arrival times across the array and compare their occurrence times with solid-earth tides and injection histories to better understand their driving mechanisms.

Changes in seismic velocity during the first 14 months of the 2004–2008 eruption of Mount St. Helens, Washington

USGS Publications Warehouse

Hotovec-Ellis, A.J.; Vidale, J.E.; Gomberg, Joan S.; Thelen, Weston A.; Moran, Seth C.

2015-01-01

Mount St. Helens began erupting in late 2004 following an 18 year quiescence. Swarms of repeating earthquakes accompanied the extrusion of a mostly solid dacite dome over the next 4 years. In some cases the waveforms from these earthquakes evolved slowly, likely reflecting changes in the properties of the volcano that affect seismic wave propagation. We use coda-wave interferometry to quantify small changes in seismic velocity structure (usually <1%) between two similar earthquakes and employed waveforms from several hundred families of repeating earthquakes together to create a continuous function of velocity change observed at permanent stations operated within 20 km of the volcano. The high rate of earthquakes allowed tracking of velocity changes on an hourly time scale. Changes in velocity were largest near the newly extruding dome and likely related to shallow deformation as magma first worked its way to the surface. We found strong correlation between velocity changes and the inverse of real-time seismic amplitude measurements during the first 3 weeks of activity, suggesting that fluctuations of pressure in the shallow subsurface may have driven both seismicity and velocity changes. Velocity changes during the remainder of the eruption likely result from a complex interplay of multiple effects and are not well explained by any single factor alone, highlighting the need for complementary geophysical data when interpreting velocity changes.

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

Ford, Sean R.; Walter, William R.

Seismic waveform correlation offers the prospect of greatly reducing event detection thresholds when compared with more conventional processing methods. Correlation is applicable for seismic events that in some sense repeat, that is they have very similar waveforms. A number of recent studies have shown that correlated seismic signals may form a significant fraction of seismicity at regional distances. For the particular case of multiple nuclear explosions at the same test site, regional distance correlation also allows very precise relative location measurements and could offer the potential to lower thresholds when multiple events exist. Using the Comprehensive Nuclear-Test-Ban Treaty (CTBT) Internationalmore » Monitoring System (IMS) seismic array at Matsushiro, Japan (MJAR), Gibbons and Ringdal (2012) were able to create a multichannel correlation detector with a very low false alarm rate and a threshold below magnitude 3.0. They did this using the 2006 or 2009 Democratic People’s Republic of Korea (DPRK) nuclear explosion as a template to search through a data stream from the same station to find a match via waveform correlation. In this paper, we extend the work of Gibbons and Ringdal (2012) and measure the correlation detection threshold at several other IMS arrays. We use this to address three main points. First, we show the IMS array station at Mina, Nevada (NVAR), which is closest to the Nevada National Security Site (NNSS), is able to detect a chemical explosion that is well under 1 ton with the right template. Second, we examine the two IMS arrays closest to the North Korean (DPRK) test site (at Ussuriysk, Russian Federation [USRK] and Wonju, Republic of Korea [KSRS]) to show that similarly low thresholds are possible when the right templates exist. We also extend the work of Schaff et al. (2012) and measure the correlation detection threshold at the nearest Global Seismic Network (GSN) three-component station (MDJ) at Mudanjiang, Heilongjiang Province, China, from the New China Digital Seismograph Network (IC). To conclude, we use these results to explore the recent claim by Zhang and Wen (2015) that the DPRK conducted “…a low-yield nuclear test…” on 12 May 2010.« less

Long-period seismic events with strikingly regular temporal patterns on Katla volcano's south flank (Iceland)

NASA Astrophysics Data System (ADS)

Sgattoni, Giulia; Jeddi, Zeinab; Gudmundsson, Ólafur; Einarsson, Páll; Tryggvason, Ari; Lund, Björn; Lucchi, Federico

2016-09-01

Katla is a threatening volcano in Iceland, partly covered by the Mýrdalsjökull ice cap. The volcano has a large caldera with several active geothermal areas. A peculiar cluster of long-period seismic events started on Katla's south flank in July 2011, during an unrest episode in the caldera that culminated in a glacier outburst. The seismic events were tightly clustered at shallow depth in the Gvendarfell area, 4 km south of the caldera, under a small glacier stream at the southern margin of Mýrdalsjökull. No seismic events were known to have occurred in this area before. The most striking feature of this seismic cluster is its temporal pattern, characterized by regular intervals between repeating seismic events, modulated by a seasonal variation. Remarkable is also the stability of both the time and waveform features over a long time period, around 3.5 years. We have not found any comparable examples in the literature. Both volcanic and glacial processes can produce similar waveforms and therefore have to be considered as potential seismic sources. Discerning between these two causes is critical for monitoring glacier-clad volcanoes and has been controversial at Katla. For this new seismic cluster on the south flank, we regard volcano-related processes as more likely than glacial ones for the following reasons: 1) the seismic activity started during an unrest episode involving sudden melting of the glacier and a jökulhlaup; 2) the glacier stream is small and stagnant; 3) the seismicity remains regular and stable for years; 4) there is no apparent correlation with short-term weather changes, such as rainstorms. We suggest that a small, shallow hydrothermal system was activated on Katla's south flank in 2011, either by a minor magmatic injection or by changes of permeability in a local crack system.

Estimation of source processes of the 2016 Kumamoto earthquakes from strong motion waveforms

NASA Astrophysics Data System (ADS)

Kubo, H.; Suzuki, W.; Aoi, S.; Sekiguchi, H.

2016-12-01

In this study, we estimated the source processes for two large events of the 2016 Kumamoto earthquakes (the M7.3 event at 1:25 JST on April 16, 2016 and the M6.5 event at 21:26 JST on April 14, 2016) from strong motion waveforms using multiple-time-window linear waveform inversion (Hartzell and Heaton 1983; Sekiguchi et al. 2000). Based on the observations of surface ruptures, the spatial distribution of aftershocks, and the geodetic data, a realistic curved fault model was developed for the source-process analysis of the M7.3 event. The source model obtained for the M7.3 event with a seismic moment of 5.5 × 1019 Nm (Mw 7.1) had two significant ruptures. One rupture propagated toward the northeastern shallow region at 4 s after rupture initiation, and continued with large slips to approximately 16 s. This rupture caused a large slip region with a peak slip of 3.8 m that was located 10-30 km northeast of the hypocenter and reached the caldera of Mt. Aso. The contribution of the large slip region to the seismic waveforms was large at many stations. Another rupture propagated toward the surface from the hypocenter at 2-6 s, and then propagated toward the northeast along the near surface at 6-10 s. This rupture largely contributed to the seismic waveforms at the stations south of the fault and close to the hypocenter. A comparison with the results obtained using a single fault plane model demonstrate that the use of the curved fault model led to improved waveform fit at the stations south of the fault. The extent of the large near-surface slips in this source model for the M7.3 event is roughly consistent with the extent of the observed large surface ruptures. The source model obtained for the M6.5 event with a seismic moment of 1.7 × 1018 Nm (Mw 6.1) had large slips in the region around the hypocenter and in the shallow region north-northeast of the hypocenter, both of which had a maximum slip of 0.7 m. The rupture of the M6.5 event propagated from the former region to the latter region at 1-6 s after rupture initiation, which is expected to have caused the strong ground motions due to the forward directivity effect at KMMH16 and surroundings. The occurrence of the near-surface large slips in this source model for the M6.5 event is consistent with the appearance of small surface cracks, which were observed by some residents.

Numerical simulation of seismic wave propagation from land-excited large volume air-gun source

NASA Astrophysics Data System (ADS)

Cao, W.; Zhang, W.

2017-12-01

The land-excited large volume air-gun source can be used to study regional underground structures and to detect temporal velocity changes. The air-gun source is characterized by rich low frequency energy (from bubble oscillation, 2-8Hz) and high repeatability. It can be excited in rivers, reservoirs or man-made pool. Numerical simulation of the seismic wave propagation from the air-gun source helps to understand the energy partitioning and characteristics of the waveform records at stations. However, the effective energy recorded at a distance station is from the process of bubble oscillation, which can not be approximated by a single point source. We propose a method to simulate the seismic wave propagation from the land-excited large volume air-gun source by finite difference method. The process can be divided into three parts: bubble oscillation and source coupling, solid-fluid coupling and the propagation in the solid medium. For the first part, the wavelet of the bubble oscillation can be simulated by bubble model. We use wave injection method combining the bubble wavelet with elastic wave equation to achieve the source coupling. Then, the solid-fluid boundary condition is implemented along the water bottom. And the last part is the seismic wave propagation in the solid medium, which can be readily implemented by the finite difference method. Our method can get accuracy waveform of land-excited large volume air-gun source. Based on the above forward modeling technology, we analysis the effect of the excited P wave and the energy of converted S wave due to different water shapes. We study two land-excited large volume air-gun fields, one is Binchuan in Yunnan, and the other is Hutubi in Xinjiang. The station in Binchuan, Yunnan is located in a large irregular reservoir, the waveform records have a clear S wave. Nevertheless, the station in Hutubi, Xinjiang is located in a small man-made pool, the waveform records have very weak S wave. Better understanding of the characteristics of land-excited large volume air-gun can help to better use of the air-gun source.

Convolutional neural network for earthquake detection and location

PubMed Central

Perol, Thibaut; Gharbi, Michaël; Denolle, Marine

2018-01-01

The recent evolution of induced seismicity in Central United States calls for exhaustive catalogs to improve seismic hazard assessment. Over the last decades, the volume of seismic data has increased exponentially, creating a need for efficient algorithms to reliably detect and locate earthquakes. Today’s most elaborate methods scan through the plethora of continuous seismic records, searching for repeating seismic signals. We leverage the recent advances in artificial intelligence and present ConvNetQuake, a highly scalable convolutional neural network for earthquake detection and location from a single waveform. We apply our technique to study the induced seismicity in Oklahoma, USA. We detect more than 17 times more earthquakes than previously cataloged by the Oklahoma Geological Survey. Our algorithm is orders of magnitude faster than established methods. PMID:29487899

Evaluation of accuracy of synthetic waveforms for subduction-zone earthquakes by using a land-ocean unified 3D structure model

NASA Astrophysics Data System (ADS)

Okamoto, Taro; Takenaka, Hiroshi; Nakamura, Takeshi

2018-06-01

Seismic wave propagation from shallow subduction-zone earthquakes can be strongly affected by 3D heterogeneous structures, such as oceanic water and sedimentary layers with irregular thicknesses. Synthetic waveforms must incorporate these effects so that they reproduce the characteristics of the observed waveforms properly. In this paper, we evaluate the accuracy of synthetic waveforms for small earthquakes in the source area of the 2011 Tohoku-Oki earthquake ( M JMA 9.0) at the Japan Trench. We compute the synthetic waveforms on the basis of a land-ocean unified 3D structure model using our heterogeneity, oceanic layer, and topography finite-difference method. In estimating the source parameters, we apply the first-motion augmented moment tensor (FAMT) method that we have recently proposed to minimize biases due to inappropriate source parameters. We find that, among several estimates, only the FAMT solutions are located very near the plate interface, which demonstrates the importance of using a 3D model for ensuring the self-consistency of the structure model, source position, and source mechanisms. Using several different filter passbands, we find that the full waveforms with periods longer than about 10 s can be reproduced well, while the degree of waveform fitting becomes worse for periods shorter than about 10 s. At periods around 4 s, the initial body waveforms can be modeled, but the later large-amplitude surface waves are difficult to reproduce correctly. The degree of waveform fitting depends on the source location, with better fittings for deep sources near land. We further examine the 3D sensitivity kernels: for the period of 12.8 s, the kernel shows a symmetric pattern with respect to the straight path between the source and the station, while for the period of 6.1 s, a curved pattern is obtained. Also, the range of the sensitive area becomes shallower for the latter case. Such a 3D spatial pattern cannot be predicted by 1D Earth models and indicates the strong effects of 3D heterogeneity on short-period ( ≲ 10s) waveforms. Thus, it would be necessary to consider such 3D effects when improving the structure and source models.

Three-dimensional waveform sensitivity kernels

NASA Astrophysics Data System (ADS)

Marquering, Henk; Nolet, Guust; Dahlen, F. A.

1998-03-01

The sensitivity of intermediate-period (~10-100s) seismic waveforms to the lateral heterogeneity of the Earth is computed using an efficient technique based upon surface-wave mode coupling. This formulation yields a general, fully fledged 3-D relationship between data and model without imposing smoothness constraints on the lateral heterogeneity. The calculations are based upon the Born approximation, which yields a linear relation between data and model. The linear relation ensures fast forward calculations and makes the formulation suitable for inversion schemes; however, higher-order effects such as wave-front healing are neglected. By including up to 20 surface-wave modes, we obtain Fréchet, or sensitivity, kernels for waveforms in the time frame that starts at the S arrival and which includes direct and surface-reflected body waves. These 3-D sensitivity kernels provide new insights into seismic-wave propagation, and suggest that there may be stringent limitations on the validity of ray-theoretical interpretations. Even recently developed 2-D formulations, which ignore structure out of the source-receiver plane, differ substantially from our 3-D treatment. We infer that smoothness constraints on heterogeneity, required to justify the use of ray techniques, are unlikely to hold in realistic earth models. This puts the use of ray-theoretical techniques into question for the interpretation of intermediate-period seismic data. The computed 3-D sensitivity kernels display a number of phenomena that are counter-intuitive from a ray-geometrical point of view: (1) body waves exhibit significant sensitivity to structure up to 500km away from the source-receiver minor arc; (2) significant near-surface sensitivity above the two turning points of the SS wave is observed; (3) the later part of the SS wave packet is most sensitive to structure away from the source-receiver path; (4) the sensitivity of the higher-frequency part of the fundamental surface-wave mode is wider than for its faster, lower-frequency part; (5) delayed body waves may considerably influence fundamental Rayleigh and Love waveforms. The strong sensitivity of waveforms to crustal structure due to fundamental-mode-to-body-wave scattering precludes the use of phase-velocity filters to model body-wave arrivals. Results from the 3-D formulation suggest that the use of 2-D and 1-D techniques for the interpretation of intermediate-period waveforms should seriously be reconsidered.

The Seismic component of the IBERARRAY: Placing constraints on the Lithosphere and Mantle.

NASA Astrophysics Data System (ADS)

Carbonell, R.; Diaz, J.; Villaseñor, A.; Gallart, J.; Morales, J.; Pazos, A.; Cordoba, D.; Pulgar, J.; Garcia-Lobon, J.; Harnafi, M.

2008-12-01

TOPOIBERIA, is a multidisciplinary large scale research project which aims to study the links between the deep and superficial processes within the Iberian Peninsula.One of its main experimental components is the deployment of the IBERARRAY seismic network. This is a dense array (60x60 km) of new generation dataloggers equipped with broad-band seismometers which will cover Iberia and North Morocco in three successive deployments, each lasting for about 18 months. The first leg, deployed since late 2007, covers the southern part of Iberia (35 stations) and northern Morocco (20 stations). Two data centers have been established one at the CSIC-Institute of Earth Sciences (CSIC-Barcelona) and a second at the Geologic and Mining Insititute (IGME-Madrid) the data follows a standard-conventional flow from recovery to archival. The field teams collect the recorded hard disk on the field and send data and metadata to a processing center, where raw data is collected and stored and a quality control checking is performed. This include a systematic inspection of the experimental parameters (batteries charge, thermal insulation, time adjustments, geophone leveling etc), the visual verification of the seismic waveforms and the analysis, using power density spectra (PSD), of the noise level of each station. All this information is disseminated between the research teams involved in the project using a dedicated website and the continuous seismic data is made accessible through FTP and CWQ servers. Some of the nodes of the theoretical network are covered by permanent stations of the national broad-band network (IGN) or other networks operating in the region (IAG-UGR, ROA). Data from those stations will also be integrated to the Iberarray database. This Iberarray network will provide a large database of both waveform and catalogued events, with an unprecedented resolution. Earthquake data at local, regional and teleseismic scales will be analyzed using different methodologies. The first result would be an increase in the accuracy of the location of regional seismicity and the termination of focal mechanisms. A special emphasis will be attributed to seismic tomographic techniques using travel times and waveforms of P and S arrivals at different scales as well as surface waves, using dispersion measurements as well as studies dealing with background/environmental noise. In addition, receiver function analysis for seismic imaging of deep lithospheric features and splitting analysis of shear-wave arrivals will also be developed.

Testing continuous earthquake detection and location in Alentejo (South Portugal) by waveform coherency analysis

NASA Astrophysics Data System (ADS)

Matos, Catarina; Grigoli, Francesco; Cesca, Simone; Custódio, Susana

2015-04-01

In the last decade a permanent seismic network of 30 broadband stations, complemented by dense temporary deployments, covered Portugal. This extraordinary network coverage enables now the computation of a high-resolution image of the seismicity of Portugal, which in turn will shed light on the seismotectonics of Portugal. The large data volumes available cannot be analyzed by traditional time-consuming manual location procedures. In this presentation we show first results on the automatic detection and location of earthquakes occurred in a selected region in the south of Portugal Our main goal is to implement an automatic earthquake detection and location routine in order to have a tool to quickly process large data sets, while at the same time detecting low magnitude earthquakes (i.e., lowering the detection threshold). We present a modified version of the automatic seismic event location by waveform coherency analysis developed by Grigoli et al. (2013, 2014), designed to perform earthquake detections and locations in continuous data. The event detection is performed by continuously computing the short-term-average/long-term-average of two different characteristic functions (CFs). For the P phases we used a CF based on the vertical energy trace, while for S phases we used a CF based on the maximum eigenvalue of the instantaneous covariance matrix (Vidale 1991). Seismic event detection and location is obtained by performing waveform coherence analysis scanning different hypocentral coordinates. We apply this technique to earthquakes in the Alentejo region (South Portugal), taking advantage from a small aperture seismic network installed in the south of Portugal for two years (2010 - 2011) during the DOCTAR experiment. In addition to the good network coverage, the Alentejo region was chosen for its simple tectonic setting and also because the relationship between seismicity, tectonics and local lithospheric structure is intriguing and still poorly understood. Inside the target area the seismicity clusters mainly within two clouds, oriented SE-NW and SW-NE. Should these clusters be seen as the expression of local active faults? Are they associated to lithological transitions? Or do the locations obtained from the previously sparse permanent network have large errors and generate fake clusters? We present preliminary results from this study, and compare them with manual locations. This work is supported by project QuakeLoc, reference: PTDC/GEO-FIQ/3522/2012

Source-Type Identification Analysis Using Regional Seismic Moment Tensors

NASA Astrophysics Data System (ADS)

Chiang, A.; Dreger, D. S.; Ford, S. R.; Walter, W. R.

2012-12-01

Waveform inversion to determine the seismic moment tensor is a standard approach in determining the source mechanism of natural and manmade seismicity, and may be used to identify, or discriminate different types of seismic sources. The successful applications of the regional moment tensor method at the Nevada Test Site (NTS) and the 2006 and 2009 North Korean nuclear tests (Ford et al., 2009a, 2009b, 2010) show that the method is robust and capable for source-type discrimination at regional distances. The well-separated populations of explosions, earthquakes and collapses on a Hudson et al., (1989) source-type diagram enables source-type discrimination; however the question remains whether or not the separation of events is universal in other regions, where we have limited station coverage and knowledge of Earth structure. Ford et al., (2012) have shown that combining regional waveform data and P-wave first motions removes the CLVD-isotropic tradeoff and uniquely discriminating the 2009 North Korean test as an explosion. Therefore, including additional constraints from regional and teleseismic P-wave first motions enables source-type discrimination at regions with limited station coverage. We present moment tensor analysis of earthquakes and explosions (M6) from Lop Nor and Semipalatinsk test sites for station paths crossing Kazakhstan and Western China. We also present analyses of smaller events from industrial sites. In these sparse coverage situations we combine regional long-period waveforms, and high-frequency P-wave polarity from the same stations, as well as from teleseismic arrays to constrain the source type. Discrimination capability with respect to velocity model and station coverage is examined, and additionally we investigate the velocity model dependence of vanishing free-surface traction effects on seismic moment tensor inversion of shallow sources and recovery of explosive scalar moment. Our synthetic data tests indicate that biases in scalar seismic moment and discrimination for shallow sources are small and can be understood in a systematic manner. We are presently investigating the frequency dependence of vanishing traction of a very shallow (10m depth) M2+ chemical explosion recorded at several kilometer distances, and preliminary results indicate at the typical frequency passband we employ the bias does not affect our ability to retrieve the correct source mechanism but may affect the retrieval of the correct scalar seismic moment. Finally, we assess discrimination capability in a composite P-value statistical framework.

Elastic full-waveform inversion and parameterization analysis applied to walk-away vertical seismic profile data for unconventional (heavy oil) reservoir characterization

NASA Astrophysics Data System (ADS)

Pan, Wenyong; Innanen, Kristopher A.; Geng, Yu

2018-03-01

Seismic full-waveform inversion (FWI) methods hold strong potential to recover multiple subsurface elastic properties for hydrocarbon reservoir characterization. Simultaneously updating multiple physical parameters introduces the problem of interparameter tradeoff, arising from the covariance between different physical parameters, which increases nonlinearity and uncertainty of multiparameter FWI. The coupling effects of different physical parameters are significantly influenced by model parameterization and acquisition arrangement. An appropriate choice of model parameterization is critical to successful field data applications of multiparameter FWI. The objective of this paper is to examine the performance of various model parameterizations in isotropic-elastic FWI with walk-away vertical seismic profile (W-VSP) dataset for unconventional heavy oil reservoir characterization. Six model parameterizations are considered: velocity-density (α, β and ρ΄), modulus-density (κ, μ and ρ), Lamé-density (λ, μ΄ and ρ‴), impedance-density (IP, IS and ρ″), velocity-impedance-I (α΄, β΄ and I_P^'), and velocity-impedance-II (α″, β″ and I_S^'). We begin analyzing the interparameter tradeoff by making use of scattering radiation patterns, which is a common strategy for qualitative parameter resolution analysis. In this paper, we discuss the advantages and limitations of the scattering radiation patterns and recommend that interparameter tradeoffs be evaluated using interparameter contamination kernels, which provide quantitative, second-order measurements of the interparameter contaminations and can be constructed efficiently with an adjoint-state approach. Synthetic W-VSP isotropic-elastic FWI experiments in the time domain verify our conclusions about interparameter tradeoffs for various model parameterizations. Density profiles are most strongly influenced by the interparameter contaminations; depending on model parameterization, the inverted density profile can be over-estimated, under-estimated or spatially distorted. Among the six cases, only the velocity-density parameterization provides stable and informative density features not included in the starting model. Field data applications of multicomponent W-VSP isotropic-elastic FWI in the time domain were also carried out. The heavy oil reservoir target zone, characterized by low α-to-β ratios and low Poisson's ratios, can be identified clearly with the inverted isotropic-elastic parameters.

Optimal Inversion Parameters for Full Waveform Inversion using OBS Data Set

NASA Astrophysics Data System (ADS)

Kim, S.; Chung, W.; Shin, S.; Kim, D.; Lee, D.

2017-12-01

In recent years, full Waveform Inversion (FWI) has been the most researched technique in seismic data processing. It uses the residuals between observed and modeled data as an objective function; thereafter, the final subsurface velocity model is generated through a series of iterations meant to minimize the residuals.Research on FWI has expanded from acoustic media to elastic media. In acoustic media, the subsurface property is defined by P-velocity; however, in elastic media, properties are defined by multiple parameters, such as P-velocity, S-velocity, and density. Further, the elastic media can also be defined by Lamé constants, density or impedance PI, SI; consequently, research is being carried out to ascertain the optimal parameters.From results of advanced exploration equipment and Ocean Bottom Seismic (OBS) survey, it is now possible to obtain multi-component seismic data. However, to perform FWI on these data and generate an accurate subsurface model, it is important to determine optimal inversion parameters among (Vp, Vs, ρ), (λ, μ, ρ), and (PI, SI) in elastic media. In this study, staggered grid finite difference method was applied to simulate OBS survey. As in inversion, l2-norm was set as objective function. Further, the accurate computation of gradient direction was performed using the back-propagation technique and its scaling was done using the Pseudo-hessian matrix.In acoustic media, only Vp is used as the inversion parameter. In contrast, various sets of parameters, such as (Vp, Vs, ρ) and (λ, μ, ρ) can be used to define inversion in elastic media. Therefore, it is important to ascertain the parameter that gives the most accurate result for inversion with OBS data set.In this study, we generated Vp and Vs subsurface models by using (λ, μ, ρ) and (Vp, Vs, ρ) as inversion parameters in every iteration, and compared the final two FWI results.This research was supported by the Basic Research Project(17-3312) of the Korea Institute of Geoscience and Mineral Resources(KIGAM) funded by the Ministry of Science, ICT and Future Planning of Korea.

Elastic full-waveform inversion and parametrization analysis applied to walk-away vertical seismic profile data for unconventional (heavy oil) reservoir characterization

NASA Astrophysics Data System (ADS)

Pan, Wenyong; Innanen, Kristopher A.; Geng, Yu

2018-06-01

Seismic full-waveform inversion (FWI) methods hold strong potential to recover multiple subsurface elastic properties for hydrocarbon reservoir characterization. Simultaneously updating multiple physical parameters introduces the problem of interparameter trade-off, arising from the simultaneous variations of different physical parameters, which increase the nonlinearity and uncertainty of multiparameter FWI. The coupling effects of different physical parameters are significantly influenced by model parametrization and acquisition arrangement. An appropriate choice of model parametrization is important to successful field data applications of multiparameter FWI. The objective of this paper is to examine the performance of various model parametrizations in isotropic-elastic FWI with walk-away vertical seismic profile (W-VSP) data for unconventional heavy oil reservoir characterization. Six model parametrizations are considered: velocity-density (α, β and ρ΄), modulus-density (κ, μ and ρ), Lamé-density (λ, μ΄ and ρ‴), impedance-density (IP, IS and ρ″), velocity-impedance-I (α΄, β΄ and I_P^' }) and velocity-impedance-II (α″, β″ and I_S^' }). We begin analysing the interparameter trade-off by making use of scattering radiation patterns, which is a common strategy for qualitative parameter resolution analysis. We discuss the advantages and limitations of the scattering radiation patterns and recommend that interparameter trade-offs be evaluated using interparameter contamination kernels, which provide quantitative, second-order measurements of the interparameter contaminations and can be constructed efficiently with an adjoint-state approach. Synthetic W-VSP isotropic-elastic FWI experiments in the time domain verify our conclusions about interparameter trade-offs for various model parametrizations. Density profiles are most strongly influenced by the interparameter contaminations; depending on model parametrization, the inverted density profile can be overestimated, underestimated or spatially distorted. Among the six cases, only the velocity-density parametrization provides stable and informative density features not included in the starting model. Field data applications of multicomponent W-VSP isotropic-elastic FWI in the time domain were also carried out. The heavy oil reservoir target zone, characterized by low α-to-β ratios and low Poisson's ratios, can be identified clearly with the inverted isotropic-elastic parameters.

A Joint Method of Envelope Inversion Combined with Hybrid-domain Full Waveform Inversion

NASA Astrophysics Data System (ADS)

CUI, C.; Hou, W.

2017-12-01

Full waveform inversion (FWI) aims to construct high-precision subsurface models by fully using the information in seismic records, including amplitude, travel time, phase and so on. However, high non-linearity and the absence of low frequency information in seismic data lead to the well-known cycle skipping problem and make inversion easily fall into local minima. In addition, those 3D inversion methods that are based on acoustic approximation ignore the elastic effects in real seismic field, and make inversion harder. As a result, the accuracy of final inversion results highly relies on the quality of initial model. In order to improve stability and quality of inversion results, multi-scale inversion that reconstructs subsurface model from low to high frequency are applied. But, the absence of very low frequencies (< 3Hz) in field data is still bottleneck in the FWI. By extracting ultra low-frequency data from field data, envelope inversion is able to recover low wavenumber model with a demodulation operator (envelope operator), though the low frequency data does not really exist in field data. To improve the efficiency and viability of the inversion, in this study, we proposed a joint method of envelope inversion combined with hybrid-domain FWI. First, we developed 3D elastic envelope inversion, and the misfit function and the corresponding gradient operator were derived. Then we performed hybrid-domain FWI with envelope inversion result as initial model which provides low wavenumber component of model. Here, forward modeling is implemented in the time domain and inversion in the frequency domain. To accelerate the inversion, we adopt CPU/GPU heterogeneous computing techniques. There were two levels of parallelism. In the first level, the inversion tasks are decomposed and assigned to each computation node by shot number. In the second level, GPU multithreaded programming is used for the computation tasks in each node, including forward modeling, envelope extraction, DFT (discrete Fourier transform) calculation and gradients calculation. Numerical tests demonstrated that the combined envelope inversion + hybrid-domain FWI could obtain much faithful and accurate result than conventional hybrid-domain FWI. The CPU/GPU heterogeneous parallel computation could improve the performance speed.

COST Action ES1401 TIDES: a European network on TIme DEpendent Seismology

NASA Astrophysics Data System (ADS)

Morelli, Andrea

2016-04-01

Using the full-length records of seismic events and background ambient noise, today seismology is going beyond still-life snapshots of the interior of the Earth, and look into time-dependent changes of its properties. Data availability has grown dramatically with the expansion of seismographic networks and data centers, so as to enable much more detailed and accurate analyses. COST Action ES1401 TIDES (TIme DEpendent Seismology; http://tides-cost.eu) aims at structuring the EU seismological community to enable development of data-intensive, time-dependent techniques for monitoring Earth active processes (e.g., earthquakes, volcanic eruptions, landslides, glacial earthquakes) as well as oil/gas reservoirs. The main structure of TIDES is organised around working groups on: Workflow integration of data and computing resources; Seismic interferometry and ambient noise; Forward problems and High-performance computing applications; Seismic tomography, full waveform inversion and uncertainties; Applications in the natural environment and industry. TIDES is an open network of European laboratories with complementary skills, and is organising a series of events - workshops and advanced training schools - as well as supporting short-duration scientific stays. The first advanced training school was held in Bertinoro (Italy) on June 2015, with attendance of about 100 participants from 20 European countries, was devoted to how to manage and model seismic data with modern tools. The next school, devoted to ambient noise, will be held in 2016 Portugal: the program will be announced at the time of this conference. TIDES will strengthen Europe's role in a critical field for natural hazards and natural resource management.

Improvements to a Major Digital Archive of Seismic Waveforms from Nuclear Explosions: The Borovoye Seismogram Archive

DTIC Science & Technology

2009-09-30

excitation of surface waves in the Balapan sub-region of the Soviet Semipalatinsk test site in central Asia were noted for anomalous behavior...complete recording history of Semipalatinsk Test Site (STS) explosions, waveform data from the Borovoye archive offer the opportunity to re-evaluate...Figure 2. Map of the Balapan sub-region of the Semipalatinsk Test Site showing locations of 50 tests currently understudy and the boundaries of NE

Real-time capture of seismic waves using high-rate multi-GNSS observations: Application to the 2015 Mw 7.8 Nepal earthquake

NASA Astrophysics Data System (ADS)

Geng, Tao; Xie, Xin; Fang, Rongxin; Su, Xing; Zhao, Qile; Liu, Gang; Li, Heng; Shi, Chuang; Liu, Jingnan

2016-01-01

The variometric approach is investigated to measure real-time seismic waves induced by the 2015 Mw 7.8 Nepal earthquake with high-rate multi-GNSS observations, especially with the contribution of newly available BDS. The velocity estimation using GPS + BDS shows an additional improvement of around 20% with respect to GPS-only solutions. We also reconstruct displacements by integrating GNSS-derived velocities after a linear trend removal (IGV). The displacement waveforms with accuracy of better than 5 cm are derived when postprocessed GPS precise point positioning results are used as ground truth, even if those stations have strong ground motions and static offsets of up to 1-2 m. GNSS-derived velocity and displacement waveforms with the variometric approach are in good agreement with results from strong motion data. We therefore conclude that it is feasible to capture real-time seismic waves with multi-GNSS observations using the IGV-enhanced variometric approach, which has critical implications for earthquake early warning, tsunami forecasting, and rapid hazard assessment.

A seismic data compression system using subband coding

NASA Technical Reports Server (NTRS)

Kiely, A. B.; Pollara, F.

1995-01-01

This article presents a study of seismic data compression techniques and a compression algorithm based on subband coding. The algorithm includes three stages: a decorrelation stage, a quantization stage that introduces a controlled amount of distortion to allow for high compression ratios, and a lossless entropy coding stage based on a simple but efficient arithmetic coding method. Subband coding methods are particularly suited to the decorrelation of nonstationary processes such as seismic events. Adaptivity to the nonstationary behavior of the waveform is achieved by dividing the data into separate blocks that are encoded separately with an adaptive arithmetic encoder. This is done with high efficiency due to the low overhead introduced by the arithmetic encoder in specifying its parameters. The technique could be used as a progressive transmission system, where successive refinements of the data can be requested by the user. This allows seismologists to first examine a coarse version of waveforms with minimal usage of the channel and then decide where refinements are required. Rate-distortion performance results are presented and comparisons are made with two block transform methods.

Machine Learning Seismic Wave Discrimination: Application to Earthquake Early Warning

NASA Astrophysics Data System (ADS)

Li, Zefeng; Meier, Men-Andrin; Hauksson, Egill; Zhan, Zhongwen; Andrews, Jennifer

2018-05-01

Performance of earthquake early warning systems suffers from false alerts caused by local impulsive noise from natural or anthropogenic sources. To mitigate this problem, we train a generative adversarial network (GAN) to learn the characteristics of first-arrival earthquake P waves, using 300,000 waveforms recorded in southern California and Japan. We apply the GAN critic as an automatic feature extractor and train a Random Forest classifier with about 700,000 earthquake and noise waveforms. We show that the discriminator can recognize 99.2% of the earthquake P waves and 98.4% of the noise signals. This state-of-the-art performance is expected to reduce significantly the number of false triggers from local impulsive noise. Our study demonstrates that GANs can discover a compact and effective representation of seismic waves, which has the potential for wide applications in seismology.

Adaptive Waveform Correlation Detectors for Arrays: Algorithms for Autonomous Calibration

DTIC Science & Technology

2007-09-01

March 17, 2005. The seismic signals from both master and detected events are followed by infrasound arrivals. Note the long duration of the...correlation coefficient traces with a significant array -gain. A detected event that is co-located with the master event will record the same time-difference...estimating the detection threshold reduction for a range of highly repeating seismic sources using arrays of different configurations and at different

Prolific Sources of Icequakes: The Mulock and Skelton Glaciers, Antarctica

NASA Astrophysics Data System (ADS)

Lloyd, A. J.; Wiens, D.; Lough, A. C.; Anandakrishnan, S.; Nyblade, A.; Aster, R. C.; Huerta, A. D.; Winberry, J. P.

2015-12-01

The Mulock and Skelton Glaciers are large outlet glaciers that flow through the Transantarctic Mountains and into the Ross Ice Shelf. A regional seismic deployment in the central Transantarctic Mountains (TAM) in 1999-2000 led to the identification of 63 events in the vicinity of Mulock and Skelton Glaciers [Bannister and Kennett, 2002]. A more recent study utilizing seismic data collected as part of the POLENET/A-NET and AGAP projects during 2009 again identified significant seismicity associated with these glaciers and suggested that many of these events were icequakes based on their shallow depths [Lough, 2014]. These two glaciers represent the most seismically active regions in the TAM aside from the well-studied David Glacier region [Danesi et al, 2007; Zoet et al., 2012]. In addition, many of the icequakes from this region have magnitude ML > 2.5, in contrast to most glacial events that are generally of smaller magnitude. Using the waveforms of previously identified icequakes as templates, nearby POLENET/A-NET, AGAP, and GSN seismic stations were scanned using a cross-correlation method to find similar waveforms. We then used a relative location algorithm to determine high-precision locations and depths. The use of regional velocity models derived from recent seismic studies facilitates accurate absolute locations that we interpret in the context of the local geological and glacial features. The icequakes are concentrated in heavily crevassed regions associated with steep bedrock topography, likely icefalls. Future work will focus on determining whether these events are associated with stick-slip events at the bed of the glacier and/or crevasse formation near the surface. In addition the temporal pattern of seismicity will also be examined to search for repeating icequakes, which have been identified at the base of several other glaciers.

Neo-deterministic seismic hazard scenarios for India—a preventive tool for disaster mitigation

NASA Astrophysics Data System (ADS)

Parvez, Imtiyaz A.; Magrin, Andrea; Vaccari, Franco; Ashish; Mir, Ramees R.; Peresan, Antonella; Panza, Giuliano Francesco

2017-11-01

Current computational resources and physical knowledge of the seismic wave generation and propagation processes allow for reliable numerical and analytical models of waveform generation and propagation. From the simulation of ground motion, it is easy to extract the desired earthquake hazard parameters. Accordingly, a scenario-based approach to seismic hazard assessment has been developed, namely the neo-deterministic seismic hazard assessment (NDSHA), which allows for a wide range of possible seismic sources to be used in the definition of reliable scenarios by means of realistic waveforms modelling. Such reliable and comprehensive characterization of expected earthquake ground motion is essential to improve building codes, particularly for the protection of critical infrastructures and for land use planning. Parvez et al. (Geophys J Int 155:489-508, 2003) published the first ever neo-deterministic seismic hazard map of India by computing synthetic seismograms with input data set consisting of structural models, seismogenic zones, focal mechanisms and earthquake catalogues. As described in Panza et al. (Adv Geophys 53:93-165, 2012), the NDSHA methodology evolved with respect to the original formulation used by Parvez et al. (Geophys J Int 155:489-508, 2003): the computer codes were improved to better fit the need of producing realistic ground shaking maps and ground shaking scenarios, at different scale levels, exploiting the most significant pertinent progresses in data acquisition and modelling. Accordingly, the present study supplies a revised NDSHA map for India. The seismic hazard, expressed in terms of maximum displacement (Dmax), maximum velocity (Vmax) and design ground acceleration (DGA), has been extracted from the synthetic signals and mapped on a regular grid over the studied territory.

Global Seismic Cross-Correlation Results: Characterizing Repeating Seismic Events

NASA Astrophysics Data System (ADS)

Vieceli, R.; Dodge, D. A.; Walter, W. R.

2016-12-01

Increases in seismic instrument quality and coverage have led to increased knowledge of earthquakes, but have also revealed the complex and diverse nature of earthquake ruptures. Nonetheless, some earthquakes are sufficiently similar to each other that they produce correlated waveforms. Such repeating events have been used to investigate interplate coupling of subduction zones [e.g. Igarashi, 2010; Yu, 2013], study spatio-temporal changes in slip rate at plate boundaries [e.g. Igarashi et al., 2003], observe variations in seismic wave propagation velocities in the crust [e.g. Schaff and Beroza, 2004; Sawazaki et al., 2015], and assess inner core rotation [e.g. Yu, 2016]. The characterization of repeating events on a global scale remains a very challenging problem. An initial global seismic cross-correlation study used over 310 million waveforms from nearly 3.8 million events recorded between 1970 and 2013 to determine an initial look at global correlated seismicity [Dodge and Walter, 2015]. In this work, we analyze the spatial and temporal distribution of the most highly correlated event clusters or "multiplets" from the Dodge and Walter [2015] study. We examine how the distributions and characteristics of multiplets are effected by tectonic environment, source-station separation, and frequency band. Preliminary results suggest that the distribution of multiplets does not correspond to the tectonic environment in any obvious way, nor do they always coincide with the occurrence of large earthquakes. Future work will focus on clustering correlated pairs and working to reduce the bias introduced by non-uniform seismic station coverage and data availability. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Systematic detection of seismic events at Mount St. Helens with an ultra-dense array

NASA Astrophysics Data System (ADS)

Meng, X.; Hartog, J. R.; Schmandt, B.; Hotovec-Ellis, A. J.; Hansen, S. M.; Vidale, J. E.; Vanderplas, J.

2016-12-01

During the summer of 2014, an ultra-dense array of 900 geophones was deployed around the crater of Mount St. Helens and continuously operated for 15 days. This dataset provides us an unprecedented opportunity to systematically detect seismic events around an active volcano and study their underlying mechanisms. We use a waveform-based matched filter technique to detect seismic events from this dataset. Due to the large volume of continuous data ( 1 TB), we performed the detection on the GPU cluster Stampede (https://www.tacc.utexas.edu/systems/stampede). We build a suite of template events from three catalogs: 1) the standard Pacific Northwest Seismic Network (PNSN) catalog (45 events); 2) the catalog from Hansen&Schmandt (2015) obtained with a reverse-time imaging method (212 events); and 3) the catalog identified with a matched filter technique using the PNSN permanent stations (190 events). By searching for template matches in the ultra-dense array, we find 2237 events. We then calibrate precise relative magnitudes for template and detected events, using a principal component fit to measure waveform amplitude ratios. The magnitude of completeness and b-value of the detected catalog is -0.5 and 1.1, respectively. Our detected catalog shows several intensive swarms, which are likely driven by fluid pressure transients in conduits or slip transients on faults underneath the volcano. We are currently relocating the detected catalog with HypoDD and measuring the seismic velocity changes at Mount St. Helens using the coda wave interferometry of detected repeating earthquakes. The accurate temporal-spatial migration pattern of seismicity and seismic property changes should shed light on the physical processes beneath Mount St. Helens.

The 16 August 1997 Novaya Zemlya seismic event as viewed from GSN stations KEV and KBS

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

Hartse, H.E.

1997-11-01

Using current and historic seismic records from Global Seismic Network stations KEV and KBS, the authors find that S minus P arrival time comparisons between nuclear explosions and the 16 August 1997 seismic event (m{sub b} {approx} 3.6) from near Novaya Zemlya clearly indicate that (relative to KEV) the 16 August event occurred at least 80 km east of the Russian test site. Including S minus P arrival times from KBS constrains the location to beneath the Kara Sea and in good agreement with previously reported locations, over 100 km southeast of the test site. From an analysis of P{submore » n}/S{sub n} waveform ratios at frequencies above 4 Hz, they find that the 16 August event falls within the population of regional earthquakes and is distinctly separated from Novaya Zemlya and other northern Eurasian nuclear explosion populations. Thus, given its location and waveform characteristics, they conclude the 16 August event was an earthquake. The 16 August event was not detected at teleseismic distances, and thus, this event provides a good example of the regional detection, location, and identification efforts that will be required to monitor the Comprehensive Test Ban Treaty below m{sub b} {approx} 4.« less

Modeling propagation of infrasound signals observed by a dense seismic network.

PubMed

Chunchuzov, I; Kulichkov, S; Popov, O; Hedlin, M

2014-01-01

The long-range propagation of infrasound from a surface explosion with an explosive yield of about 17.6 t TNT that occurred on June 16, 2008 at the Utah Test and Training Range (UTTR) in the western United States is simulated using an atmospheric model that includes fine-scale layered structure of the wind velocity and temperature fields. Synthetic signal parameters (waveforms, amplitudes, and travel times) are calculated using parabolic equation and ray-tracing methods for a number of ranges between 100 and 800 km from the source. The simulation shows the evolution of several branches of stratospheric and thermospheric signals with increasing range from the source. Infrasound signals calculated using a G2S (ground-to-space) atmospheric model perturbed by small-scale layered wind velocity and temperature fluctuations are shown to agree well with recordings made by the dense High Lava Plains seismic network located at an azimuth of 300° from UTTR. The waveforms of calculated infrasound arrivals are compared with those of seismic recordings. This study illustrates the utility of dense seismic networks for mapping an infrasound field with high spatial resolution. The parabolic equation calculations capture both the effect of scattering of infrasound into geometric acoustic shadow zones and significant temporal broadening of the arrivals.

The sequentially discounting autoregressive (SDAR) method for on-line automatic seismic event detecting on long term observation

NASA Astrophysics Data System (ADS)

Wang, L.; Toshioka, T.; Nakajima, T.; Narita, A.; Xue, Z.

2017-12-01

In recent years, more and more Carbon Capture and Storage (CCS) studies focus on seismicity monitoring. For the safety management of geological CO2 storage at Tomakomai, Hokkaido, Japan, an Advanced Traffic Light System (ATLS) combined different seismic messages (magnitudes, phases, distributions et al.) is proposed for injection controlling. The primary task for ATLS is the seismic events detection in a long-term sustained time series record. Considering the time-varying characteristics of Signal to Noise Ratio (SNR) of a long-term record and the uneven energy distributions of seismic event waveforms will increase the difficulty in automatic seismic detecting, in this work, an improved probability autoregressive (AR) method for automatic seismic event detecting is applied. This algorithm, called sequentially discounting AR learning (SDAR), can identify the effective seismic event in the time series through the Change Point detection (CPD) of the seismic record. In this method, an anomaly signal (seismic event) can be designed as a change point on the time series (seismic record). The statistical model of the signal in the neighborhood of event point will change, because of the seismic event occurrence. This means the SDAR aims to find the statistical irregularities of the record thought CPD. There are 3 advantages of SDAR. 1. Anti-noise ability. The SDAR does not use waveform messages (such as amplitude, energy, polarization) for signal detecting. Therefore, it is an appropriate technique for low SNR data. 2. Real-time estimation. When new data appears in the record, the probability distribution models can be automatic updated by SDAR for on-line processing. 3. Discounting property. the SDAR introduces a discounting parameter to decrease the influence of present statistic value on future data. It makes SDAR as a robust algorithm for non-stationary signal processing. Within these 3 advantages, the SDAR method can handle the non-stationary time-varying long-term series and achieve real-time monitoring. Finally, we employ the SDAR on a synthetic model and Tomakomai Ocean Bottom Cable (OBC) baseline data to prove the feasibility and advantage of our method.

Full moment tensors for small events (Mw < 3) at Uturuncu volcano, Bolivia

NASA Astrophysics Data System (ADS)

Alvizuri, Celso; Tape, Carl

2016-09-01

We present a catalogue of full seismic moment tensors for 63 events from Uturuncu volcano in Bolivia. The events were recorded during 2011-2012 in the PLUTONS seismic array of 24 broad-band stations. Most events had magnitudes between 0.5 and 2.0 and did not generate discernible surface waves; the largest event was Mw 2.8. For each event we computed the misfit between observed and synthetic waveforms, and we used first-motion polarity measurements to reduce the number of possible solutions. Each moment tensor solution was obtained using a grid search over the 6-D space of moment tensors. For each event, we show the misfit function in eigenvalue space, represented by a lune. We identify three subsets of the catalogue: (1) six isotropic events, (2) five tensional crack events, and (3) a swarm of 14 events southeast of the volcanic centre that appear to be double couples. The occurrence of positively isotropic events is consistent with other published results from volcanic and geothermal regions. Several of these previous results, as well as our results, cannot be interpreted within the context of either an oblique opening crack or a crack-plus-double-couple model. Proper characterization of uncertainties for full moment tensors is critical for distinguishing among physical models of source processes.

Homogenization of Electromagnetic and Seismic Wavefields for Joint Inverse Modeling

NASA Astrophysics Data System (ADS)

Newman, G. A.; Commer, M.; Petrov, P.; Um, E. S.

2011-12-01

A significant obstacle in developing a robust joint imaging technology exploiting seismic and electromagnetic (EM) wave fields is the resolution at which these different geophysical measurements sense the subsurface. Imaging of seismic reflection data is an order of magnitude finer in resolution and scale compared to images produced with EM data. A consistent joint image of the subsurface geophysical attributes (velocity, electrical conductivity) requires/demands the different geophysical data types be similar in their resolution of the subsurface. The superior resolution of seismic data results from the fact that the energy propagates as a wave, while propagation of EM energy is diffusive and attenuates with distance. On the other hand, the complexity of the seismic wave field can be a significant problem due to high reflectivity of the subsurface and the generation of multiple scattering events. While seismic wave fields have been very useful in mapping the subsurface for energy resources, too much scattering and too many reflections can lead to difficulties in imaging and interpreting seismic data. To overcome these obstacles a formulation for joint imaging of seismic and EM wave fields is introduced, where each data type is matched in resolution. In order to accomplish this, seismic data are first transformed into the Laplace-Fourier Domain, which changes the modeling of the seismic wave field from wave propagation to diffusion. Though high frequency information (reflectivity) is lost with this transformation, several benefits follow: (1) seismic and EM data can be easily matched in resolution, governed by the same physics of diffusion, (2) standard least squares inversion works well with diffusive type problems including both transformed seismic and EM, (3) joint imaging of seismic and EM data may produce better starting velocity models critical for successful reverse time migration or full waveform imaging of seismic data (non transformed) and (4) possibilities to image across multiple scale lengths, incorporating different types of geophysical data and attributes in the process. Important numerical details of 3D seismic wave field simulation in the Laplace-Fourier domain for both acoustic and elastic cases will also be discussed.

Seismological mechanism analysis of 2015 Luanxian swarm, Hebei province,China

NASA Astrophysics Data System (ADS)

Tan, Yipei; Liao, Xu; Ma, Hongsheng; Zhou, Longquan; Wang, Xingzhou

2017-04-01

The seismological mechanism of an earthquake swarm, a kind of seismic burst activity, means the physical and dynamic process in earthquakes triggering in the swarm. Here we focus on the seismological mechanism of 2015 Luanxian swarm in Hebei province, China. The process of digital seismic waveform data processing is divided into four steps. (1) Choose the three components waveform of earthquakes in the catalog as templates, and detect missing earthquakes by scanning the continues waveforms with matched filter technique. (2) Recalibrate P and S-wave phase arrival time using waveform cross-correlation phase detection technique to eliminate the artificial error in phase picking in the observation report made by Hebei seismic network, and then we obtain a more complete catalog and a more precise seismic phase report. (3) Relocate the earthquakes in the swarm using hypoDD based on phase arrival time we recalibrated, and analyze the characteristics of swarm epicenter migration based on the earthquake relocation result. (4) Detect whether there are repeating earthquakes activity using both waveform cross-correlation standard and whether rupture areas can overlapped. We finally detect 106 missing earthquakes in the swarm, 66 of them have the magnitude greater than ML0.0, include 2 greater than ML1.0. Relocation result shows that the epicenters of earthquakes in the swarm have a strip distribution in NE-SW direction, which indicates the seismogenic structure may be a NE-SW trending fault. The spatial-temporal distribution variation of epicenters in the swarm shows a kind of two stages linear migration characteristics, in which the first stage has appeared with a higher migration velocity as 1.2 km per day, and the velocity of the second step is 0.0024 km per day. According to the three basic models to explain the seismological mechanism of earthquake swarms: cascade model, slow slip model and fluid diffusion model, repeating earthquakes activity is difficult to explain by previous earthquakes stress triggering, however, it can be explained by continuing stress loading at the same asperity from fault slow slip. The phenomena of linear migration is more fitting slow slip model than the migration characteristics of fluid diffusion which satisfied diffusion equation. Comparing the phenomena we observed and the seismological mechanism models, we find that the Luanxian earthquake swarm may be associated with fault slow slip. Fault slow slip may play a role in Luanxian earthquake swarm triggering and sustained activity.

Geographic boundary of the “Pacific Anomaly” near the Earth’s core-mantle boundary

NASA Astrophysics Data System (ADS)

He, Y.; Wen, L.

2009-12-01

Seismic tomography have revealed a broad, seismically low velocity anomaly in the Earth’s lower mantle beneath the Pacific (we term it the “Pacific Anomaly”), surrounded by the circum-Pacific high velocity zone. Here, we determine geographical boundary and average shear velocity structure of the Pacific Anomaly near the core-mantle boundary based on travel time analysis of ScSH-SH and ScS2-SS phases. We further constrain the detailed structure of the transition from the base of the Pacific Anomaly to the northern high velocity zone along two perpendicular cross sections on the basis of forward waveform modeling of the seismic data. Two cross-sections include one great arc across the Anomaly from New Zealand to Alaska and another from Solomon Islands to North America. Our seismic data are collected from those recorded in the China National Digital Seismographic Network, and many permanent and temporal arrays from the Incorporated Research Institutions for Seismology. The observed ScS-SH and ScS2-SS differential travel time residuals allow the entire geographic boundary of the anomaly to be clearly defined. The seismic data suggest that the average shear velocity reduction inside the anomaly reaches -5% in the lowermost 300 km of the mantle. Waveform analysis of the seismic data sampling the edge of the anomaly further validates the model of the boundary previously deduced by differential-travel-time-residual data, and suggests that the northern boundary is characterized by a shear velocity model with the low-velocity region accompanied by a high velocity structure.

Waveform fitting and geometry analysis for full-waveform lidar feature extraction

NASA Astrophysics Data System (ADS)

Tsai, Fuan; Lai, Jhe-Syuan; Cheng, Yi-Hsiu

2016-10-01

This paper presents a systematic approach that integrates spline curve fitting and geometry analysis to extract full-waveform LiDAR features for land-cover classification. The cubic smoothing spline algorithm is used to fit the waveform curve of the received LiDAR signals. After that, the local peak locations of the waveform curve are detected using a second derivative method. According to the detected local peak locations, commonly used full-waveform features such as full width at half maximum (FWHM) and amplitude can then be obtained. In addition, the number of peaks, time difference between the first and last peaks, and the average amplitude are also considered as features of LiDAR waveforms with multiple returns. Based on the waveform geometry, dynamic time-warping (DTW) is applied to measure the waveform similarity. The sum of the absolute amplitude differences that remain after time-warping can be used as a similarity feature in a classification procedure. An airborne full-waveform LiDAR data set was used to test the performance of the developed feature extraction method for land-cover classification. Experimental results indicate that the developed spline curve- fitting algorithm and geometry analysis can extract helpful full-waveform LiDAR features to produce better land-cover classification than conventional LiDAR data and feature extraction methods. In particular, the multiple-return features and the dynamic time-warping index can improve the classification results significantly.

Data Quality Control of the French Permanent Broadband Network in the RESIF Framework

NASA Astrophysics Data System (ADS)

Grunberg, Marc; Lambotte, Sophie; Engels, Fabien; Dretzen, Remi; Hernandez, Alain

2014-05-01

In the framework of the RESIF (Réseau Sismologique et géodésique Français) project, a new information system is being setting up, allowing the improvement of the management and the distribution of high quality data from the different elements of RESIF and the associated networks. Within this information system, EOST (in Strasbourg) is in charge of collecting real-time permanent broadband seismic waveform, and performing Quality Control on these data. The real-time and validated data set are pushed to the French National Distribution Center (Isterre/Grenoble) in order to make them publicly available. Furthermore EOST hosts the BCSF-ReNaSS, in charge of the French metropolitan seismic bulletin. This allows to benefit from some high-end quality control based on the national and world-wide seismicity. Here we present first the real-time seismic data flow from the stations of the French National Broad Band Network to EOST, and then, the data Quality Control procedures that were recently installed, including some new developments. The data Quality Control consists in applying a variety of subprocesses to check the consistency of the whole system and process from the stations to the data center. This allows us to verify that instruments and data transmission are operating correctly. Moreover analysis of the ambient noise helps to characterize intrinsic seismic quality of the stations and to identify other kind of disturbances. The deployed Quality Control consist in a pipeline that starts with low-level procedures : check the real-time miniseed data file (file naming convention, data integrity), check for inconsistencies between waveform and meta-data (channel name, sample rate, etc.), compute waveform statistics (data availability, gap/overlap, mean, rms, time quality, spike). It is followed by some high-level procedures such as : power spectral density computation (PSD), STA/LTA computation to be correlated to the seismicity, phases picking and stations magnitudes discrepancies. The results of quality control is visualized through a web interface. This latter gathers data from different information systems to provide a global view on last events that could impact the data (like intervention on site or seismic events, etc.). This work is still an ongoing project. We intend to add more sophisticated procedures to enhanced our data Quality Control. Among them, we will deploy a seismic moment tensor inversion tool for amplitude, time and polarity control and a noise correlation procedure for time drift detections.

Super-resolution Time-Lapse Seismic Waveform Inversion

NASA Astrophysics Data System (ADS)

Ovcharenko, O.; Kazei, V.; Peter, D. B.; Alkhalifah, T.

2017-12-01

Time-lapse seismic waveform inversion is a technique, which allows tracking changes in the reservoirs over time. Such monitoring is relatively computationally extensive and therefore it is barely feasible to perform it on-the-fly. Most of the expenses are related to numerous FWI iterations at high temporal frequencies, which is inevitable since the low-frequency components can not resolve fine scale features of a velocity model. Inverted velocity changes are also blurred when there is noise in the data, so the problem of low-resolution images is widely known. One of the problems intensively tackled by computer vision research community is the recovering of high-resolution images having their low-resolution versions. Usage of artificial neural networks to reach super-resolution from a single downsampled image is one of the leading solutions for this problem. Each pixel of the upscaled image is affected by all the pixels of its low-resolution version, which enables the workflow to recover features that are likely to occur in the corresponding environment. In the present work, we adopt machine learning image enhancement technique to improve the resolution of time-lapse full-waveform inversion. We first invert the baseline model with conventional FWI. Then we run a few iterations of FWI on a set of the monitoring data to find desired model changes. These changes are blurred and we enhance their resolution by using a deep neural network. The network is trained to map low-resolution model updates predicted by FWI into the real perturbations of the baseline model. For supervised training of the network we generate a set of random perturbations in the baseline model and perform FWI on the noisy data from the perturbed models. We test the approach on a realistic perturbation of Marmousi II model and demonstrate that it outperforms conventional convolution-based deblurring techniques.

Using Ambient Seismic Noise to Monitor Post-Seismic Relaxation After the 2010 Mw 7.1 Darfield Earthquake, New Zealand

NASA Astrophysics Data System (ADS)

Savage, M. K.; Heckels, R.; Townend, J.

2015-12-01

Quantifying seismic velocity changes following large earthquakes can provide insights into the crustal response of the earth. The use of ambient seismic noise to monitor these changes is becoming increasingly widespread. Cross-correlations of long-duration ambient noise records can be used to give stable impulse response functions without the need for repeated seismic events. Temporal velocity changes were detected in the four months following the September 2010 Mw 7.1 Darfield event in South Island, New Zealand, using temporary seismic networks originally deployed to record aftershocks in the region. The arrays consisted of stations lying on and surrounding the fault, with a maximum inter-station distance of 156km. The 2010-2011 Canterbury earthquake sequence occurred largely on previously unknown and buried faults. The Darfield earthquake was the first and largest in a sequence of events that hit the region, rupturing the Greendale Fault. A surface rupture of nearly 30km was observed. The sequence also included the Mw 6.3 February 2011 Christchurch event, which caused widespread damage throughout the city and resulted in almost 200 deaths. Nine-component, day-long Green's functions were computed for frequencies between 0.1 - 1.0 Hz for full waveform seismic data from immediately after the 4th September 2010 earthquake until mid-January 2011. Using the moving window cross-spectral method, stacks of daily functions covering the study period (reference functions), were compared to consecutive 10 day stacks of cross-correlations to measure time delays between them. These were then inverted for seismic velocity changes with respect to the reference functions. Over the study period an increase in seismic velocity of 0.25% ± 0.02% was determined proximal to the Greendale fault. These results are similar to studies in other regions, and we attribute the changes to post-seismic relaxation through crack-healing of the Greendale Fault and throughout the region.

Operating the Portable Seismic Pavement Analyzer

DTIC Science & Technology

2006-12-01

8 Thickness versus modulus...amplitude versus time waveforms....................................................................... 9 Figure 10. A thickness versus Young’s modulus plot...modulus of portland cement concrete (PCC) and asphalt concrete (AC) pavements. Additional strength parameters necessary for structural pavement

Natural Gas Hydrates Estimation Using Seismic Inversion and Rock Physics

NASA Astrophysics Data System (ADS)

Dutta, N.; Dai, J.; Kleinberg, R.; Xu, H.

2005-05-01

Gas hydrate drilling worldwide indicates that the formation of gas hydrates in shallow sediments tends to increase P- and S-wave velocities of the hosting rocks. Rock physics models of gas hydrates provide the links between velocity anomalies and gas hydrate concentration. In this abstract, we evaluate the numerical predictions of some of the major rock physics models of gas hydrates and validate those with well log data from the Mallik and Blake Ridge wells. We find that a model in which the gas hydrate is a part of the rock framework produces results that are consistent with well log data. To enhance the accuracy of seismic estimation, we adopt a five-step, integrated workflow that enables us to identify and quantify gas hydrates in the deepwater Gulf of Mexico (GOM). It includes: 1) Reprocessing conventional 3D seismic data at high resolution using an amplitude-preserving flow with prestack time migration, 2) A detailed stratigraphic evaluation to identify potential hydrate zones, 3) Seismic attribute analysis to further delineate anomalous zones, 4) Full waveform prestack inversion to characterize acoustic properties of gas hydrates in 1D (Mallick, 1995; Mallick, 1999) and map in 3D using hybrid inversion techniques (Dutta, 2002; Mallick and Dutta, 2002), and 5) Quantitative estimation of gas hydrate saturation using rock property models. We illustrate the procedure using 3D seismic data, and estimate gas hydrate saturation in the study area in the GOM.

Spectral-element Seismic Wave Propagation on CUDA/OpenCL Hardware Accelerators

NASA Astrophysics Data System (ADS)

Peter, D. B.; Videau, B.; Pouget, K.; Komatitsch, D.

2015-12-01

Seismic wave propagation codes are essential tools to investigate a variety of wave phenomena in the Earth. Furthermore, they can now be used for seismic full-waveform inversions in regional- and global-scale adjoint tomography. Although these seismic wave propagation solvers are crucial ingredients to improve the resolution of tomographic images to answer important questions about the nature of Earth's internal processes and subsurface structure, their practical application is often limited due to high computational costs. They thus need high-performance computing (HPC) facilities to improving the current state of knowledge. At present, numerous large HPC systems embed many-core architectures such as graphics processing units (GPUs) to enhance numerical performance. Such hardware accelerators can be programmed using either the CUDA programming environment or the OpenCL language standard. CUDA software development targets NVIDIA graphic cards while OpenCL was adopted by additional hardware accelerators, like e.g. AMD graphic cards, ARM-based processors as well as Intel Xeon Phi coprocessors. For seismic wave propagation simulations using the open-source spectral-element code package SPECFEM3D_GLOBE, we incorporated an automatic source-to-source code generation tool (BOAST) which allows us to use meta-programming of all computational kernels for forward and adjoint runs. Using our BOAST kernels, we generate optimized source code for both CUDA and OpenCL languages within the source code package. Thus, seismic wave simulations are able now to fully utilize CUDA and OpenCL hardware accelerators. We show benchmarks of forward seismic wave propagation simulations using SPECFEM3D_GLOBE on CUDA/OpenCL GPUs, validating results and comparing performances for different simulations and hardware usages.

Refining locations of the 2005 Mukacheve, West Ukraine, earthquakes based on similarity of their waveforms

NASA Astrophysics Data System (ADS)

Gnyp, Andriy

2009-06-01

Based on the results of application of correlation analysis to records of the 2005 Mukacheve group of recurrent events and their subsequent relocation relative to the reference event of 7 July 2005, a conclusion has been drawn that all the events had most likely occurred on the same rup-ture plane. Station terms have been estimated for seismic stations of the Transcarpathians, accounting for variation of seismic velocities beneath their locations as compared to the travel time tables used in the study. In methodical aspect, potentials and usefulness of correlation analysis of seismic records for a more detailed study of seismic processes, tectonics and geodynamics of the Carpathian region have been demonstrated.

Remote Triggering of Microseismicity in Antarctica

NASA Astrophysics Data System (ADS)

Ji, M.; Li, C.; Peng, Z.; Walter, J. I.

2017-12-01

It is well known that large distant earthquakes can trigger microearthquakes/tectonic tremors during or immediately following their surface waves. Globally, triggered seismicity is mostly found in active plate boundary regions. Recent studies have shown that icequakes in Antartica can also be triggered by teleseismic events. However, it is still not clear how widespread this phenomenon is and whether there are any connections between large earthquakes and subsequent glacial movements. In this study, we conduct a systematic search for remotely triggered activity in Antarctica following recent large earthquakes, including the 2004 Mw9.1 Sumatra, 2011 Mw9.1 Tohoku, 2012 Mw8.6 Indian Ocean and 2014-2015 Chile earthquakes. We download seismic data recorded at the POLENET (YT) and the Argentina Antarctica Network (AI) from the Incorporated Research Institutions for Seismology (IRIS) Data Management Center (DMC). We apply a 2-8 Hz band-pass-filter to the continuous waveforms and visually identify local events during and immediately after the large amplitude surface waves. Spectrograms are computed as additional tools to identify triggered seismicity and are further confirmed by comparing the signals before and after the distant mainshocks. So far we have identified possible triggered seismicity in both networks' area following the 2010 Chile and 2011 Tohoku earthquakes. Our next step is to apply a waveform matching method to automatically detect possible triggered seismicity and check through all the available networks in Antarctica for the last decades, which should help to better understand the potential interaction between large earthquakes and icequakes in this region.

Spatio-temporal Variations of Characteristic Repeating Earthquake Sequences along the Middle America Trench in Mexico

NASA Astrophysics Data System (ADS)

Dominguez, L. A.; Taira, T.; Hjorleifsdottir, V.; Santoyo, M. A.

2015-12-01

Repeating earthquake sequences are sets of events that are thought to rupture the same area on the plate interface and thus provide nearly identical waveforms. We systematically analyzed seismic records from 2001 through 2014 to identify repeating earthquakes with highly correlated waveforms occurring along the subduction zone of the Cocos plate. Using the correlation coefficient (cc) and spectral coherency (coh) of the vertical components as selection criteria, we found a set of 214 sequences whose waveforms exceed cc≥95% and coh≥95%. Spatial clustering along the trench shows large variations in repeating earthquakes activity. Particularly, the rupture zone of the M8.1, 1985 earthquake shows an almost absence of characteristic repeating earthquakes, whereas the Guerrero Gap zone and the segment of the trench close to the Guerrero-Oaxaca border shows a significantly larger number of repeating earthquakes sequences. Furthermore, temporal variations associated to stress changes due to major shows episodes of unlocking and healing of the interface. Understanding the different components that control the location and recurrence time of characteristic repeating sequences is a key factor to pinpoint areas where large megathrust earthquakes may nucleate and consequently to improve the seismic hazard assessment.

Synthetic Seismogram Modeling.

DTIC Science & Technology

1982-11-15

various phases ( designated A, B, C, etc.) are indicated on the seismic record section at the top of the diagram. The observed travel times show a good...structure of the Yellowstone aperture seismic array (LAS), Moatana, U.S. region and experiment design , J. Geophys. Geol. Suwv. Open File Rep. 1671, 1972. Res...also display little For clarity in both typography and conitext, we coherence in waveform or even in the envelope of shall henceforth write -P-bar in

A Monte Carlo approach applied to ultrasonic non-destructive testing

NASA Astrophysics Data System (ADS)

Mosca, I.; Bilgili, F.; Meier, T.; Sigloch, K.

2012-04-01

Non-destructive testing based on ultrasound allows us to detect, characterize and size discrete flaws in geotechnical and architectural structures and materials. This information is needed to determine whether such flaws can be tolerated in future service. In typical ultrasonic experiments, only the first-arriving P-wave is interpreted, and the remainder of the recorded waveform is neglected. Our work aims at understanding surface waves, which are strong signals in the later wave train, with the ultimate goal of full waveform tomography. At present, even the structural estimation of layered media is still challenging because material properties of the samples can vary widely, and good initial models for inversion do not often exist. The aim of the present study is to combine non-destructive testing with a theoretical data analysis and hence to contribute to conservation strategies of archaeological and architectural structures. We analyze ultrasonic waveforms measured at the surface of a variety of samples, and define the behaviour of surface waves in structures of increasing complexity. The tremendous potential of ultrasonic surface waves becomes an advantage only if numerical forward modelling tools are available to describe the waveforms accurately. We compute synthetic full seismograms as well as group and phase velocities for the data. We invert them for the elastic properties of the sample via a global search of the parameter space, using the Neighbourhood Algorithm. Such a Monte Carlo approach allows us to perform a complete uncertainty and resolution analysis, but the computational cost is high and increases quickly with the number of model parameters. Therefore it is practical only for defining the seismic properties of media with a limited number of degrees of freedom, such as layered structures. We have applied this approach to both synthetic layered structures and real samples. The former contributed to benchmark the propagation of ultrasonic surface waves in typical materials tested with a non-destructive technique (e.g., marble, unweathered and weathered concrete and natural stone).

A Monte Carlo approach applied to ultrasonic non-destructive testing

NASA Astrophysics Data System (ADS)

Mosca, I.; Bilgili, F.; Meier, T. M.; Sigloch, K.

2011-12-01

Non-destructive testing based on ultrasound allows us to detect, characterize and size discrete flaws in geotechnical and engineering structures and materials. This information is needed to determine whether such flaws can be tolerated in future service. In typical ultrasonic experiments, only the first-arriving P-wave is interpreted, and the remainder of the recorded waveform is neglected. Our work aims at understanding surface waves, which are strong signals in the later wave train, with the ultimate goal of full waveform tomography. At present, even the structural estimation of layered media is still challenging because material properties of the samples can vary widely, and good initial models for inversion do not often exist. The aim of the present study is to analyze ultrasonic waveforms measured at the surface of Plexiglas and rock samples, and to define the behaviour of surface waves in structures of increasing complexity. The tremendous potential of ultrasonic surface waves becomes an advantage only if numerical forward modelling tools are available to describe the waveforms accurately. We compute synthetic full seismograms as well as group and phase velocities for the data. We invert them for the elastic properties of the sample via a global search of the parameter space, using the Neighbourhood Algorithm. Such a Monte Carlo approach allows us to perform a complete uncertainty and resolution analysis, but the computational cost is high and increases quickly with the number of model parameters. Therefore it is practical only for defining the seismic properties of media with a limited number of degrees of freedom, such as layered structures. We have applied this approach to both synthetic layered structures and real samples. The former contributed to benchmark the propagation of ultrasonic surface waves in typical materials tested with a non-destructive technique (e.g., marble, unweathered and weathered concrete and natural stone).

Coupling hydrodynamic and wave propagation modeling for waveform modeling of SPE.

NASA Astrophysics Data System (ADS)

Larmat, C. S.; Steedman, D. W.; Rougier, E.; Delorey, A.; Bradley, C. R.

2015-12-01

The goal of the Source Physics Experiment (SPE) is to bring empirical and theoretical advances to the problem of detection and identification of underground nuclear explosions. This paper presents effort to improve knowledge of the processes that affect seismic wave propagation from the hydrodynamic/plastic source region to the elastic/anelastic far field thanks to numerical modeling. The challenge is to couple the prompt processes that take place in the near source region to the ones taking place later in time due to wave propagation in complex 3D geologic environments. In this paper, we report on results of first-principles simulations coupling hydrodynamic simulation codes (Abaqus and CASH), with a 3D full waveform propagation code, SPECFEM3D. Abaqus and CASH model the shocked, hydrodynamic region via equations of state for the explosive, borehole stemming and jointed/weathered granite. LANL has been recently employing a Coupled Euler-Lagrange (CEL) modeling capability. This has allowed the testing of a new phenomenological model for modeling stored shear energy in jointed material. This unique modeling capability has enabled highfidelity modeling of the explosive, the weak grout-filled borehole, as well as the surrounding jointed rock. SPECFEM3D is based on the Spectral Element Method, a direct numerical method for full waveform modeling with mathematical accuracy (e.g. Komatitsch, 1998, 2002) thanks to its use of the weak formulation of the wave equation and of high-order polynomial functions. The coupling interface is a series of grid points of the SEM mesh situated at the edge of the hydrodynamic code domain. Displacement time series at these points are computed from output of CASH or Abaqus (by interpolation if needed) and fed into the time marching scheme of SPECFEM3D. We will present validation tests and waveforms modeled for several SPE tests conducted so far, with a special focus on effect of the local topography.

Density reconstruction in multiparameter elastic full-waveform inversion

NASA Astrophysics Data System (ADS)

Sun, Min'ao; Yang, Jizhong; Dong, Liangguo; Liu, Yuzhu; Huang, Chao

2017-12-01

Elastic full-waveform inversion (EFWI) is a quantitative data fitting procedure that recovers multiple subsurface parameters from multicomponent seismic data. As density is involved in addition to P- and S-wave velocities, the multiparameter EFWI suffers from more serious tradeoffs. In addition, compared with P- and S-wave velocities, the misfit function is less sensitive to density perturbation. Thus, a robust density reconstruction remains a difficult problem in multiparameter EFWI. In this paper, we develop an improved scattering-integral-based truncated Gauss-Newton method to simultaneously recover P- and S-wave velocities and density in EFWI. In this method, the inverse Gauss-Newton Hessian has been estimated by iteratively solving the Gauss-Newton equation with a matrix-free conjugate gradient algorithm. Therefore, it is able to properly handle the parameter tradeoffs. To give a detailed illustration of the tradeoffs between P- and S-wave velocities and density in EFWI, wavefield-separated sensitivity kernels and the Gauss-Newton Hessian are numerically computed, and their distribution characteristics are analyzed. Numerical experiments on a canonical inclusion model and a modified SEG/EAGE Overthrust model have demonstrated that the proposed method can effectively mitigate the tradeoff effects, and improve multiparameter gradients. Thus, a high convergence rate and an accurate density reconstruction can be achieved.

Observed Melt Season Seismicity of Taylor Glacier, Antarctica

NASA Astrophysics Data System (ADS)

Carmichael, J. D.; Pettit, E. C.; Creager, K. C.

2006-12-01

Sufficient evidence exists to suggest that interaction of crevasses and meltwater accelerates ice cliff disintegration of tidewater glaciers. It is not clear what role meltwater plays in calving characteristics from dry- based polar glaciers. We have obtained seismic data from a six-sensor seismic array deployed in October of 2004 near the terminus cliffs of Taylor Glacier, West Antarctica, to analyze near-cliff seismicity throughout a melt season. Discharge data from the adjacent Lawson stream suggests that dramatic increases in meltwater volume temporally correlate with changes in seismic character near ice cliffs. We calculated source-locations for ice-quake during hours of melting and re-freezing and found most large energy events to be located near the ice cliffs. The associated spectra and waveform characteristics are indicative of literature descriptions of crevassing events.

Surface wave tomography of the Ontong Java Plateau: Seismic probing of the largest igneous province

NASA Astrophysics Data System (ADS)

Richardson, William Philip

1998-12-01

Large igneous provinces (LIP), such as the gigantic Cretaceous oceanic plateaus, the Ontong-Java, the Manihiki and the Kerguelen, are part of a globally distributed diverse suite of massive crustal features considered to be episodic representations of mantle dynamics (Coffin and Eldholm, 1994). The Ontong Java Plateau in the central western Pacific is by far the largest (and presumably thickest) of these provinces and is believed to have been emplaced rapidly in the Aptian, ˜122 Ma (Tarduno et al., 1991). From 1994 to 1996 four PASSCAL broadband seismic stations were deployed in an array north of the OJP. Analysis was conducted on vertical component broadband seismograms from events recorded on the Micronesian Seismic Experiment array between January 1994 and March 1996. The purpose of this experiment is to investigate the crustal and upper mantle structure of the Ontong Java Plateau (OJP) employing surface wave tomographic methods. Using the partitioned waveform inversion method (Nolet, 1990) and earthquakes with published Centroid Moment Tensor (Dziewonski et al., 1981) solutions, we produce waveform fits from source-to-receiver paths that primarily sample the OJP. From these waveform fits, linearized constraints on shear velocity suggest: (1) a massively thickened crust over the center of the OJP-greater than 35km over central areas of the plateau while thinning off-center; (2) a pronounced low-velocity zone down to ˜300km depth-a robust result in agreement with recent geochemical predictions (Neal et al., 1997); (3) the probability of lateral heterogeneity across the OJP. Finally, by combining many single waveform inversions (van der Lee and Nolet, 1997b) a 3-D shear velocity model can be computed for the Ontong Java Plateau and the nearby Caroline Basin. New constraints on the crustal thickness (and hence the volume extruded) are presented, thereby adding to the understanding of the overall tectonic setting and possible emplacement mechanism of the structure.

Hydra—The National Earthquake Information Center’s 24/7 seismic monitoring, analysis, catalog production, quality analysis, and special studies tool suite

USGS Publications Warehouse

Patton, John M.; Guy, Michelle R.; Benz, Harley M.; Buland, Raymond P.; Erickson, Brian K.; Kragness, David S.

2016-08-18

This report provides an overview of the capabilities and design of Hydra, the global seismic monitoring and analysis system used for earthquake response and catalog production at the U.S. Geological Survey National Earthquake Information Center (NEIC). Hydra supports the NEIC’s worldwide earthquake monitoring mission in areas such as seismic event detection, seismic data insertion and storage, seismic data processing and analysis, and seismic data output.The Hydra system automatically identifies seismic phase arrival times and detects the occurrence of earthquakes in near-real time. The system integrates and inserts parametric and waveform seismic data into discrete events in a database for analysis. Hydra computes seismic event parameters, including locations, multiple magnitudes, moment tensors, and depth estimates. Hydra supports the NEIC’s 24/7 analyst staff with a suite of seismic analysis graphical user interfaces.In addition to the NEIC’s monitoring needs, the system supports the processing of aftershock and temporary deployment data, and supports the NEIC’s quality assurance procedures. The Hydra system continues to be developed to expand its seismic analysis and monitoring capabilities.

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

Carmichael, Joshua Daniel; Carr, Christina; Pettit, Erin C.

We apply a fully autonomous icequake detection methodology to a single day of high-sample rate (200 Hz) seismic network data recorded from the terminus of Taylor Glacier, ANT that temporally coincided with a brine release episode near Blood Falls (May 13, 2014). We demonstrate a statistically validated procedure to assemble waveforms triggered by icequakes into populations of clusters linked by intra-event waveform similarity. Our processing methodology implements a noise-adaptive power detector coupled with a complete-linkage clustering algorithm and noise-adaptive correlation detector. This detector-chain reveals a population of 20 multiplet sequences that includes ~150 icequakes and produces zero false alarms onmore » the concurrent, diurnally variable noise. Our results are very promising for identifying changes in background seismicity associated with the presence or absence of brine release episodes. We thereby suggest that our methodology could be applied to longer time periods to establish a brine-release monitoring program for Blood Falls that is based on icequake detections.« less

Analysis of induced seismicity in geothermal reservoirs – An overview

USGS Publications Warehouse

Zang, Arno; Oye, Volker; Jousset, Philippe; Deichmann, Nicholas; Gritto, Roland; McGarr, Arthur F.; Majer, Ernest; Bruhn, David

2014-01-01

In this overview we report results of analysing induced seismicity in geothermal reservoirs in various tectonic settings within the framework of the European Geothermal Engineering Integrating Mitigation of Induced Seismicity in Reservoirs (GEISER) project. In the reconnaissance phase of a field, the subsurface fault mapping, in situ stress and the seismic network are of primary interest in order to help assess the geothermal resource. The hypocentres of the observed seismic events (seismic cloud) are dependent on the design of the installed network, the used velocity model and the applied location technique. During the stimulation phase, the attention is turned to reservoir hydraulics (e.g., fluid pressure, injection volume) and its relation to larger magnitude seismic events, their source characteristics and occurrence in space and time. A change in isotropic components of the full waveform moment tensor is observed for events close to the injection well (tensile character) as compared to events further away from the injection well (shear character). Tensile events coincide with high Gutenberg-Richter b-values and low Brune stress drop values. The stress regime in the reservoir controls the direction of the fracture growth at depth, as indicated by the extent of the seismic cloud detected. Stress magnitudes are important in multiple stimulation of wells, where little or no seismicity is observed until the previous maximum stress level is exceeded (Kaiser Effect). Prior to drilling, obtaining a 3D P-wave (Vp) and S-wave velocity (Vs) model down to reservoir depth is recommended. In the stimulation phase, we recommend to monitor and to locate seismicity with high precision (decametre) in real-time and to perform local 4D tomography for velocity ratio (Vp/Vs). During exploitation, one should use observed and model induced seismicity to forward estimate seismic hazard so that field operators are in a position to adjust well hydraulics (rate and volume of the fluid injected) when induced events start to occur far away from the boundary of the seismic cloud.

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)

High resolution measurement of earthquake impacts on rock slope stability and damage using pre- and post-earthquake terrestrial laser scans

NASA Astrophysics Data System (ADS)

Hutchinson, Lauren; Stead, Doug; Rosser, Nick

2017-04-01

Understanding the behaviour of rock slopes in response to earthquake shaking is instrumental in response and relief efforts following large earthquakes as well as to ongoing risk management in earthquake affected areas. Assessment of the effects of seismic shaking on rock slope kinematics requires detailed surveys of the pre- and post-earthquake condition of the slope; however, at present, there is a lack of high resolution monitoring data from pre- and post-earthquake to facilitate characterization of seismically induced slope damage and validate models used to back-analyze rock slope behaviour during and following earthquake shaking. Therefore, there is a need for additional research where pre- and post- earthquake monitoring data is available. This paper presents the results of a direct comparison between terrestrial laser scans (TLS) collected in 2014, the year prior to the 2015 earthquake sequence, with that collected 18 months after the earthquakes and two monsoon cycles. The two datasets were collected using Riegl VZ-1000 and VZ-4000 full waveform laser scanners with high resolution (c. 0.1 m point spacing as a minimum). The scans cover the full landslide affected slope from the toe to the crest. The slope is located in Sindhupalchok District, Central Nepal which experienced some of the highest co-seismic and post-seismic landslide intensities across Nepal due to the proximity to the epicenters (<20 km) of both of the main aftershocks on April 26, 2015 (M 6.7) and May 12, 2015 (M7.3). During the 2015 earthquakes and subsequent 2015 and 2016 monsoons, the slope experienced rockfall and debris flows which are evident in satellite imagery and field photographs. Fracturing of the rock mass associated with the seismic shaking is also evident at scales not accessible through satellite and field observations. The results of change detection between the TLS datasets with an emphasis on quantification of seismically-induced slope damage is presented. Patterns in the distribution and expression of rock mass damage are also explored. The findings presented herein provide insight into the response of rock slopes to seismic shaking and highlight the application of remote sensing to understand slope behaviour.

Waveform inversion of volcano-seismic signals for an extended source

USGS Publications Warehouse

Nakano, M.; Kumagai, H.; Chouet, B.; Dawson, P.

2007-01-01

We propose a method to investigate the dimensions and oscillation characteristics of the source of volcano-seismic signals based on waveform inversion for an extended source. An extended source is realized by a set of point sources distributed on a grid surrounding the centroid of the source in accordance with the source geometry and orientation. The source-time functions for all point sources are estimated simultaneously by waveform inversion carried out in the frequency domain. We apply a smoothing constraint to suppress short-scale noisy fluctuations of source-time functions between adjacent sources. The strength of the smoothing constraint we select is that which minimizes the Akaike Bayesian Information Criterion (ABIC). We perform a series of numerical tests to investigate the capability of our method to recover the dimensions of the source and reconstruct its oscillation characteristics. First, we use synthesized waveforms radiated by a kinematic source model that mimics the radiation from an oscillating crack. Our results demonstrate almost complete recovery of the input source dimensions and source-time function of each point source, but also point to a weaker resolution of the higher modes of crack oscillation. Second, we use synthetic waveforms generated by the acoustic resonance of a fluid-filled crack, and consider two sets of waveforms dominated by the modes with wavelengths 2L/3 and 2W/3, or L and 2L/5, where W and L are the crack width and length, respectively. Results from these tests indicate that the oscillating signature of the 2L/3 and 2W/3 modes are successfully reconstructed. The oscillating signature of the L mode is also well recovered, in contrast to results obtained for a point source for which the moment tensor description is inadequate. However, the oscillating signature of the 2L/5 mode is poorly recovered owing to weaker resolution of short-scale crack wall motions. The triggering excitations of the oscillating cracks are successfully reconstructed. Copyright 2007 by the American Geophysical Union.

Swarms of repeating long-period earthquakes at Shishaldin Volcano, Alaska, 2001-2004

USGS Publications Warehouse

Petersen, Tanja

2007-01-01

During 2001–2004, a series of four periods of elevated long-period seismic activity, each lasting about 1–2 months, occurred at Shishaldin Volcano, Aleutian Islands, Alaska. The time periods are termed swarms of repeating events, reflecting an abundance of earthquakes with highly similar waveforms that indicate stable, non-destructive sources. These swarms are characterized by increased earthquake amplitudes, although the seismicity rate of one event every 0.5–5 min has remained more or less constant since Shishaldin last erupted in 1999. A method based on waveform cross-correlation is used to identify highly repetitive events, suggestive of spatially distinct source locations. The waveform analysis shows that several different families of similar events co-exist during a given swarm day, but generally only one large family dominates. A network of hydrothermal fractures may explain the events that do not belong to a dominant repeating event group, i.e. multiple sources at different locations exist next to a dominant source. The dominant waveforms exhibit systematic changes throughout each swarm, but some of these waveforms do reappear over the course of 4 years indicating repeatedly activated source locations. The choked flow model provides a plausible trigger mechanism for the repeating events observed at Shishaldin, explaining the gradual changes in waveforms over time by changes in pressure gradient across a constriction within the uppermost part of the conduit. The sustained generation of Shishaldin's long-period events may be attributed to complex dynamics of a multi-fractured hydrothermal system: the pressure gradient within the main conduit may be regulated by temporarily sealing and reopening of parallel flow pathways, by the amount of debris within the main conduit and/or by changing gas influx into the hydrothermal system. The observations suggest that Shishaldin's swarms of repeating events represent time periods during which a dominant source is activated.

Broadband seismology and the detection and verification of underground nuclear explosions

NASA Astrophysics Data System (ADS)

Tinker, Mark Andrew

1997-10-01

On September 24, 1996, President Clinton signed the Comprehensive Test Ban Treaty (CTBT), which bans the testing of all nuclear weapons thereby limiting their future development. Seismology is the primary tool used for the detection and identification of underground explosions and thus, will play a key role in monitoring a CTBT. The detection and identification of low yield explosions requires seismic stations at regional distances (<1500 km). However, because the regional wavefield propagates within the extremely heterogeneous crustal waveguide, the seismic waveforms are also very complicated. Therefore, it is necessary to have a solid understanding of how the phases used in regional discriminants develop within different tectonic regimes. Thus, the development of the seismic phases Pn and Lg, which compose the seismic discriminant Pn/Lg, within the western U.S. from the Non-Proliferation Experiment are evaluated. The most fundamental discriminant is event location as 90% of all seismic sources occur too deep within the earth to be unnatural. France resumed its nuclear testing program after a four year moratorium and conducted six tests during a five month period starting in September of 1995. Using teleseismic data, a joint hypocenter determination algorithm was used to determine the hypocenters of these six explosions. One of the most important problems in monitoring a CTBT is the detection and location of small seismic events. Although seismic arrays have become the central tool for event detection, in the context of a global monitoring treaty, there will be some dependence on sparse regional networks of three-component broadband seismic stations to detect low yield explosions. However, the full power of the data has not been utilized, namely using phases other than P and S. Therefore, the information in the surface wavetrain is used to improve the locations of small seismic events recorded on a sparse network in Bolivia. Finally, as a discrimination example in a complex region, P to S ratios are used to determine source parameters of the Msb{w} 8.3 deep Bolivia earthquake.

A Closer Look at Recent Deep Mauna Loa Seismicity

NASA Astrophysics Data System (ADS)

Okubo, P. G.; Wolfe, C. J.; Nakata, J. S.; Koyanagi, S. K.; Uribe, J. O.

2005-12-01

In 2002, Mauna Loa Volcano showed signs of reawakening, some 18 years since its last eruption in 1984. First, in April, a brief flurry of microearthquakes occurred at cataloged depths from 25 to 55 km beneath Mauna Loa's summit caldera. Then in May 2002, after the microearthquake swarm had ended, geodetic monitors across Mauna Loa's summit caldera registered a change, from line-length shortening to extension, interpreted as reinflation of a magma body approximately 4 km beneath the volcano's summit. Accordingly, the Hawaiian Volcano Observatory issued advisories related to Mauna Loa's stirring. In July 2004, HVO began to record deep long-period (LP) earthquakes beneath Mauna Loa. Historically, interpretations of such seismicity patterns have associated LP source volumes with magma chambers and magma pathways. Over a few weeks, this seismicity dramatically jumped to levels of several dozen per day. Between the months of July and December 2004, nearly 2000 Mauna Loa LPs were located between roughly 25 km and greater than 60 km depths by HVO seismic analysts. In late December, these earthquakes rather abruptly ceased, and their levels have remained low ever since. We seek a more detailed understanding of how these earthquakes may factor into Mauna Loa's eruptive framework. Given that their first arrivals are typically emergent, hypocentral estimates using only P-wave first-arrival times of LP earthquakes are often marginally constrained. With such hypocentral estimates, it is difficult to establish clear relationships among the earthquake locations themselves, or between the earthquakes and other processes like crustal extension or magma accumulation or withdrawl. Building on earlier applications to deep earthquakes in Hawaii and LP earthquakes beneath Kilauea, we are reexamining this unprecedented Mauna Loa deep seismicity with waveform correlation and precise earthquake relocation techniques. Work to date reveals that, although the waveform correlation coefficients are low, a significant subset of the deep Mauna Loa LPs can be relocated to improve our understanding of the remarkable 2004 swarm. We are currently seeking stronger resolution to determine whether the waveform data are consistent with the vertically extended, conduit-like source distributions suggested by the catalog locations or, alternatively, whether the events are consistent with one or more narrowly extended point sources.

Archiving and Distributing Seismic Data at the Southern California Earthquake Data Center (SCEDC)

NASA Astrophysics Data System (ADS)

Appel, V. L.

2002-12-01

The Southern California Earthquake Data Center (SCEDC) archives and provides public access to earthquake parametric and waveform data gathered by the Southern California Seismic Network and since January 1, 2001, the TriNet seismic network, southern California's earthquake monitoring network. The parametric data in the archive includes earthquake locations, magnitudes, moment-tensor solutions and phase picks. The SCEDC waveform archive prior to TriNet consists primarily of short-period, 100-samples-per-second waveforms from the SCSN. The addition of the TriNet array added continuous recordings of 155 broadband stations (20 samples per second or less), and triggered seismograms from 200 accelerometers and 200 short-period instruments. Since the Data Center and TriNet use the same Oracle database system, new earthquake data are available to the seismological community in near real-time. Primary access to the database and waveforms is through the Seismogram Transfer Program (STP) interface. The interface enables users to search the database for earthquake information, phase picks, and continuous and triggered waveform data. Output is available in SAC, miniSEED, and other formats. Both the raw counts format (V0) and the gain-corrected format (V1) of COSMOS (Consortium of Organizations for Strong-Motion Observation Systems) are now supported by STP. EQQuest is an interface to prepackaged waveform data sets for select earthquakes in Southern California stored at the SCEDC. Waveform data for large-magnitude events have been prepared and new data sets will be available for download in near real-time following major events. The parametric data from 1981 to present has been loaded into the Oracle 9.2.0.1 database system and the waveforms for that time period have been converted to mSEED format and are accessible through the STP interface. The DISC optical-disk system (the "jukebox") that currently serves as the mass-storage for the SCEDC is in the process of being replaced with a series of inexpensive high-capacity (1.6 Tbyte) magnetic-disk RAIDs. These systems are built with PC-technology components, using 16 120-Gbyte IDE disks, hot-swappable disk trays, two RAID controllers, dual redundant power supplies and a Linux operating system. The system is configured over a private gigabit network that connects to the two Data Center servers and spans between the Seismological Lab and the USGS. To ensure data integrity, each RAID disk system constantly checks itself against its twin and verifies file integrity using 128-bit MD5 file checksums that are stored separate from the system. The final level of data protection is a Sony AIT-3 tape backup of the files. The primary advantage of the magnetic-disk approach is faster data access because magnetic disk drives have almost no latency. This means that the SCEDC can provide better "on-demand" interactive delivery of the seismograms in the archive.

International Monitoring System Correlation Detection at the North Korean Nuclear Test Site at Punggye-ri with Insights from the Source Physics Experiment

DOE PAGES

Ford, Sean R.; Walter, William R.

2015-05-06

Seismic waveform correlation offers the prospect of greatly reducing event detection thresholds when compared with more conventional processing methods. Correlation is applicable for seismic events that in some sense repeat, that is they have very similar waveforms. A number of recent studies have shown that correlated seismic signals may form a significant fraction of seismicity at regional distances. For the particular case of multiple nuclear explosions at the same test site, regional distance correlation also allows very precise relative location measurements and could offer the potential to lower thresholds when multiple events exist. Using the Comprehensive Nuclear-Test-Ban Treaty (CTBT) Internationalmore » Monitoring System (IMS) seismic array at Matsushiro, Japan (MJAR), Gibbons and Ringdal (2012) were able to create a multichannel correlation detector with a very low false alarm rate and a threshold below magnitude 3.0. They did this using the 2006 or 2009 Democratic People’s Republic of Korea (DPRK) nuclear explosion as a template to search through a data stream from the same station to find a match via waveform correlation. In this paper, we extend the work of Gibbons and Ringdal (2012) and measure the correlation detection threshold at several other IMS arrays. We use this to address three main points. First, we show the IMS array station at Mina, Nevada (NVAR), which is closest to the Nevada National Security Site (NNSS), is able to detect a chemical explosion that is well under 1 ton with the right template. Second, we examine the two IMS arrays closest to the North Korean (DPRK) test site (at Ussuriysk, Russian Federation [USRK] and Wonju, Republic of Korea [KSRS]) to show that similarly low thresholds are possible when the right templates exist. We also extend the work of Schaff et al. (2012) and measure the correlation detection threshold at the nearest Global Seismic Network (GSN) three-component station (MDJ) at Mudanjiang, Heilongjiang Province, China, from the New China Digital Seismograph Network (IC). To conclude, we use these results to explore the recent claim by Zhang and Wen (2015) that the DPRK conducted “…a low-yield nuclear test…” on 12 May 2010.« less

Data Delivery Latency Improvements And First Steps Towards The Distributed Computing Of The Caltech/USGS Southern California Seismic Network Earthquake Early Warning System

NASA Astrophysics Data System (ADS)

Stubailo, I.; Watkins, M.; Devora, A.; Bhadha, R. J.; Hauksson, E.; Thomas, V. I.

2016-12-01

The USGS/Caltech Southern California Seismic Network (SCSN) is a modern digital ground motion seismic network. It develops and maintains Earthquake Early Warning (EEW) data collection and delivery systems in southern California as well as real-time EEW algorithms. Recently, Behr et al., SRL, 2016 analyzed data from several regional seismic networks deployed around the globe. They showed that the SCSN was the network with the smallest data communication delays or latency. Since then, we have reduced further the telemetry delays for many of the 330 current sites. The latency has been reduced on average from 2-6 sec to 0.4 seconds by tuning the datalogger parameters and/or deploying software upgrades. Recognizing the latency data as one of the crucial parameters in EEW, we have started archiving the per-packet latencies in mseed format for all the participating sites in a similar way it is traditionally done for the seismic waveform data. The archived latency values enable us to understand and document long-term changes in performance of the telemetry links. We can also retroactively investigate how latent the waveform data were during a specific event or during a specific time period. In addition the near-real time latency values are useful for monitoring and displaying the real-time station latency, in particular to compare different telemetry technologies. A future step to reduce the latency is to deploy the algorithms on the dataloggers at the seismic stations and transmit either the final solutions or intermediate parameters to a central processing center. To implement this approach, we are developing a stand-alone version of the OnSite algorithm to run on the dataloggers in the field. This will increase the resiliency of the SCSN to potential telemetry restrictions in the immediate aftermath of a large earthquake, either by allowing local alarming by the single station, or permitting transmission of lightweight parametric information rather than continuous waveform data to the central processing facility. State-of-the-art development of Internet of Things (IoT) tools and platforms, which can be used to distribute and maintain software on a large number of remote devices are making this approach to earthquake early warning more feasible.

The Northern California Earthquake Management System: A Unified System From Realtime Monitoring to Data Distribution

NASA Astrophysics Data System (ADS)

Neuhauser, D.; Dietz, L.; Lombard, P.; Klein, F.; Zuzlewski, S.; Kohler, W.; Hellweg, M.; Luetgert, J.; Oppenheimer, D.; Romanowicz, B.

2006-12-01

The longstanding cooperation between the USGS Menlo Park and UC Berkeley's Seismological Laboratory for monitoring earthquakes and providing data to the research community is achieving a new level of integration. While station support and data collection for each network (NC, BK, BP) remain the responsibilities of the host institution, picks, codas and amplitudes will be produced and shared between the data centers continuously. Thus, realtime earthquake processing from triggering and locating through magnitude and moment tensor calculation and Shakemap production will take place independently at both locations, improving the robustness of event reporting in the Northern California Earthquake Management Center. Parametric data will also be exchanged with the Southern California Earthquake Management System to allow statewide earthquake detection and processing for further redundancy within the California Integrated Seismic Network (CISN). The database plays an integral part in this system, providing the coordination for event processing as well as the repository for event, instrument (metadata) and waveform information. The same master database serves both realtime processing, data quality control and archival, and the data center which provides waveforms and earthquake data to users in the research community. Continuous waveforms from all BK, BP, and NC stations, event waveform gathers, and event information automatically become available at the Northern California Earthquake Data Center (NCEDC). Currently, the NCEDC is collecting and makes available over 4 TByes of data per year from the NCEMC stations and other seismic networks, as well as from GPS and and other geophysical instrumentation.

Next-generation seismic experiments: wide-angle, multi-azimuth, three-dimensional, full-waveform inversion

NASA Astrophysics Data System (ADS)

Bell, Rebecca; Morgan, Joanna; Warner, Michael

2016-04-01

There are many outstanding plate-tectonic scale questions that require us to know information about sub-surface physical properties, for example ascertaining the geometry and location of magma chambers and estimating the effective stress along plate boundary faults. These important scientific targets are often too deep, impractical and expensive for extensive academic drilling. Full-waveform inversion (FWI) is an advanced seismic imaging technique that has recently become feasible in three dimensions, and has been widely adopted by the oil and gas industry to image reservoir-scale targets at shallow-to-moderate depths. In this presentation we explore the potential for 3-D FWI, when combined with appropriate marine seismic acquisition, to recover high-resolution high-fidelity P-wave velocity models for sub-sedimentary targets within the crystalline crust and uppermost mantle. Using existing geological and geophysical models, we construct P-wave velocity models over three potential sub-sedimentary targets: the Soufrière Hills Volcano on Montserrat and its associated crustal magmatic system, the downgoing oceanic plate beneath the Nankai subduction margin, and the oceanic crust-uppermost mantle beneath the East Pacific Rise mid-ocean ridge. We use these models to generate realistic multi-azimuth 3-D synthetic seismic data, and attempt to invert these data to recover the original models. We explore the resolution and accuracy, sensitivity to noise and acquisition geometry, ability to invert elastic data using acoustic inversion codes, and the trade-off between low frequencies and starting velocity model accuracy. We will show that FWI applied to multi-azimuth, refracted, wide-angle, low-frequency data can resolve features in the deep crust and uppermost mantle on scales that are significantly better than can be achieved by any other geophysical technique, and that these results can be obtained using relatively small numbers (60-90) of ocean-bottom receivers combined with large numbers of air-gun shots. We demonstrate that multi-azimuth 3-D FWI is robust in the presence of noise, that acoustic FWI can invert elastic data successfully, and that the typical errors to be expected in starting models derived using travel times will not be problematic for FWI given appropriately designed acquisition. In this presentation we will also discuss a recent field-example of the use of FWI to image the Endeavour spreading centre in the northeastern Pacific. FWI is a rapidly maturing technology; its transfer from the petroleum sector to tackle a broader range of targets now appears entirely achievable.

VERCE: a productive e-Infrastructure and e-Science environment for data-intensive seismology research

NASA Astrophysics Data System (ADS)

Vilotte, Jean-Pierre; Atkinson, Malcolm; Carpené, Michele; Casarotti, Emanuele; Frank, Anton; Igel, Heiner; Rietbrock, Andreas; Schwichtenberg, Horst; Spinuso, Alessandro

2016-04-01

Seismology pioneers global and open-data access -- with internationally approved data, metadata and exchange standards facilitated worldwide by the Federation of Digital Seismic Networks (FDSN) and in Europe the European Integrated Data Archives (EIDA). The growing wealth of data generated by dense observation and monitoring systems and recent advances in seismic wave simulation capabilities induces a change in paradigm. Data-intensive seismology research requires a new holistic approach combining scalable high-performance wave simulation codes and statistical data analysis methods, and integrating distributed data and computing resources. The European E-Infrastructure project "Virtual Earthquake and seismology Research Community e-science environment in Europe" (VERCE) pioneers the federation of autonomous organisations providing data and computing resources, together with a comprehensive, integrated and operational virtual research environment (VRE) and E-infrastructure devoted to the full path of data use in a research-driven context. VERCE delivers to a broad base of seismology researchers in Europe easily used high-performance full waveform simulations and misfit calculations, together with a data-intensive framework for the collaborative development of innovative statistical data analysis methods, all of which were previously only accessible to a small number of well-resourced groups. It balances flexibility with new integrated capabilities to provide a fluent path from research innovation to production. As such, VERCE is a major contribution to the implementation phase of the ``European Plate Observatory System'' (EPOS), the ESFRI initiative of the solid-Earth community. The VRE meets a range of seismic research needs by eliminating chores and technical difficulties to allow users to focus on their research questions. It empowers researchers to harvest the new opportunities provided by well-established and mature high-performance wave simulation codes of the community. It enables active researchers to invent and refine scalable methods for innovative statistical analysis of seismic waveforms in a wide range of application contexts. The VRE paves the way towards a flexible shared framework for seismic waveform inversion, lowering the barriers to uptake for the next generation of researchers. The VRE can be accessed through the science gateway that puts together computational and data-intensive research into the same framework, integrating multiple data sources and services. It provides a context for task-oriented and data-streaming workflows, and maps user actions to the full gamut of the federated platform resources and procurement policies, activating the necessary behind-the-scene automation and transformation. The platform manages and produces domain metadata, coupling them with the provenance information describing the relationships and the dependencies, which characterise the whole workflow process. This dynamic knowledge base, can be explored for validation purposes via a graphical interface and a web API. Moreover, it fosters the assisted selection and re-use of the data within each phase of the scientific analysis. These phases can be identified as Simulation, Data Access, Preprocessing, Misfit and data processing, and are presented to the users of the gateway as dedicated and interactive workspaces. By enabling researchers to share results and provenance information, VERCE steers open-science behaviour, allowing researchers to discover and build on prior work and thereby to progress faster. A key asset is the agile strategy that VERCE deployed in a multi-organisational context, engaging seismologists, data scientists, ICT researchers, HPC and data resource providers, system administrators into short-lived tasks each with a goal that is a seismology priority, and intimately coupling research thinking with technical innovation. This changes the focus from HPC production environments and community data services to user-focused scenario, avoiding wasteful bouts of technology centricity where technologists collect requirements and develop a system that is not used because the ideas of the planned users have moved on. As such the technologies and concepts developed in VERCE are relevant to many other disciplines in computational and data driven Earth Sciences and can provide the key technologies for a European wide computational and data intensive framework in Earth Sciences.

Poroelastic Seismic Wave Propagation Modeling of CO2 Sequestration Effects

NASA Astrophysics Data System (ADS)

Aldridge, D. F.; Bartel, L. C.

2009-12-01

Long term geologic sequestration of carbon dioxide (CO2) is considered a viable approach for removing large amounts of excess carbon from the earth’s surface environment. As CO2 is injected into a subsurface porous formation, it displaces (or mixes with) in situ pore fluids. Seismic reflection and transmission responses of the formation depend on the degree of CO2 substitution. Additionally, geochemical reactions involving CO2 and mineral grains alter the bulk and shear moduli of the solid constituent and/or the matrix of the porous medium. We examine full waveform, wide-angle, amplitude vs. offset (AVO) responses of sandstone and carbonate layers. Synthetic seismic data are calculated with a 3D poroelastic wave propagation algorithm that solves Biot’s system of thirteen coupled partial differential equations via an explicit, time-domain, finite-difference method. All common seismological phases (primary and multiple reflections, mode conversions, head waves, surface and interface waves) are generated with fidelity, provided spatial and temporal gridding intervals are sufficiently fine. Initial calculations indicate that full or partial replacement of H2O by CO2 is readily detected by the AVO recording configuration, particularly with long offset events. Difference seismogram amplitudes of surface-recorded particle velocities range up to ~25%. Equivalent elastic medium responses, with elastic parameters assigned by Gassmann formulae, are inadequate at higher frequencies. Finally, these sensitivity modeling experiments are being extended to vertical seismic profiling geometries. Sandia National Laboratories is a multiprogram science and engineering facility operated by Sandia Corporation, a Lockheed-Martin company, for the US Department of Energy’s National Nuclear Security Administration, under contract DE-AC04-94AL85000.

Classifying seismic waveforms from scratch: a case study in the alpine environment

NASA Astrophysics Data System (ADS)

Hammer, C.; Ohrnberger, M.; Fäh, D.

2013-01-01

Nowadays, an increasing amount of seismic data is collected by daily observatory routines. The basic step for successfully analyzing those data is the correct detection of various event types. However, the visually scanning process is a time-consuming task. Applying standard techniques for detection like the STA/LTA trigger still requires the manual control for classification. Here, we present a useful alternative. The incoming data stream is scanned automatically for events of interest. A stochastic classifier, called hidden Markov model, is learned for each class of interest enabling the recognition of highly variable waveforms. In contrast to other automatic techniques as neural networks or support vector machines the algorithm allows to start the classification from scratch as soon as interesting events are identified. Neither the tedious process of collecting training samples nor a time-consuming configuration of the classifier is required. An approach originally introduced for the volcanic task force action allows to learn classifier properties from a single waveform example and some hours of background recording. Besides a reduction of required workload this also enables to detect very rare events. Especially the latter feature provides a milestone point for the use of seismic devices in alpine warning systems. Furthermore, the system offers the opportunity to flag new signal classes that have not been defined before. We demonstrate the application of the classification system using a data set from the Swiss Seismological Survey achieving very high recognition rates. In detail we document all refinements of the classifier providing a step-by-step guide for the fast set up of a well-working classification system.

Toward 2D Seismic Wavefield Monitoring: Seismic Gradiometry for Long-Period Seismogram and Short-Period Seismogram Envelope applied to the Hi-net Array

NASA Astrophysics Data System (ADS)

Maeda, T.; Nishida, K.; Takagi, R.; Obara, K.

2015-12-01

The high-sensitive seismograph network Japan (Hi-net) operated by National Research Institute for Earth Science and Disaster Prevention (NIED) has about 800 stations with average separation of 20 km. We can observe long-period seismic wave propagation as a 2D wavefield with station separations shorter than wavelength. In contrast, short-period waves are quite incoherent at stations, however, their envelope shapes resemble at neighbor stations. Therefore, we may be able to extract seismic wave energy propagation by seismogram envelope analysis. We attempted to characterize seismic waveform at long-period and its envelope at short-period as 2D wavefield by applying seismic gradiometry. We applied the seismic gradiometry to a synthetic long-period (20-50s) dataset prepared by numerical simulation in realistic 3D medium at the Hi-net station layout. Wave amplitude and its spatial derivatives are estimated by using data at nearby stations. The slowness vector, the radiation pattern and the geometrical spreading are extracted from estimated velocity, displacement and its spatial derivatives. For short-periods at shorter than 1 s, seismogram envelope shows temporal and spatial broadening through scattering by medium heterogeneity. It is expected that envelope shape may be coherent among nearby stations. Based on this idea, we applied the same method to the time-integration of seismogram envelope to estimate its spatial derivatives. Together with seismogram envelope, we succeeded in estimating the slowness vector from the seismogram envelope as well as long-period waveforms by synthetic test, without using phase information. Our preliminarily results show that the seismic gradiometry suits the Hi-net to extract wave propagation characteristics both at long and short periods. This method is appealing that it can estimate waves at homogeneous grid to monitor seismic wave as a wavefield. It is promising to obtain phase velocity variation from direct waves, and to grasp wave packets originating from scattering from coda, by applying the seismic gradiometry to the Hi-net.

The shallow elastic structure of the lunar crust: New insights from seismic wavefield gradient analysis

NASA Astrophysics Data System (ADS)

Sollberger, David; Schmelzbach, Cedric; Robertsson, Johan O. A.; Greenhalgh, Stewart A.; Nakamura, Yosio; Khan, Amir

2016-10-01

Enigmatic lunar seismograms recorded during the Apollo 17 mission in 1972 have so far precluded the identification of shear-wave arrivals and hence the construction of a comprehensive elastic model of the shallow lunar subsurface. Here, for the first time, we extract shear-wave information from the Apollo active seismic data using a novel waveform analysis technique based on spatial seismic wavefield gradients. The star-like recording geometry of the active seismic experiment lends itself surprisingly well to compute spatial wavefield gradients and rotational ground motion as a function of time. These observables, which are new to seismic exploration in general, allowed us to identify shear waves in the complex lunar seismograms, and to derive a new model of seismic compressional and shear-wave velocities in the shallow lunar crust, critical to understand its lithology and constitution, and its impact on other geophysical investigations of the Moon's deep interior.

Seismic, satellite, and site observations of internal solitary waves in the NE South China Sea.

PubMed

Tang, Qunshu; Wang, Caixia; Wang, Dongxiao; Pawlowicz, Rich

2014-06-20

Internal solitary waves (ISWs) in the NE South China Sea (SCS) are tidally generated at the Luzon Strait. Their propagation, evolution, and dissipation processes involve numerous issues still poorly understood. Here, a novel method of seismic oceanography capable of capturing oceanic finescale structures is used to study ISWs in the slope region of the NE SCS. Near-simultaneous observations of two ISWs were acquired using seismic and satellite imaging, and water column measurements. The vertical and horizontal length scales of the seismic observed ISWs are around 50 m and 1-2 km, respectively. Wave phase speeds calculated from seismic observations, satellite images, and water column data are consistent with each other. Observed waveforms and vertical velocities also correspond well with those estimated using KdV theory. These results suggest that the seismic method, a new option to oceanographers, can be further applied to resolve other important issues related to ISWs.

Induced seismicity in EGS reservoir : analysis of persistent multiplets at Soultz-sous-Forêts, France

NASA Astrophysics Data System (ADS)

Cauchie, Léna; Lengliné, Olivier; Schmittbuhl, Jean

2017-04-01

Abundant seismicity is generally observed during the exploitation of geothermal reservoirs, especially during phases of hydraulic stimulations. At the Enhanced Geothermal System of Soultz-Sous-Forêts in France, the induced seismicity has been thoroughly studied over the years of exploitation and the mechanism at its origin has been related to both fluid pressure increase during stimulation and aseismic creeping movements. The fluid-induced seismic events often exhibit a high degree of similarity and the mechanism at the origin of these repeated events is thought to be associated with slow slip process where asperities on the rupture zone act several times. In order to improve our knowledge on the mechanisms associated with such events and on the damaged zones involved during the hydraulic stimulations, we investigate the behaviour of the multiplets and their persistent nature, if it prevails, over several water injection intervals. For this purpose, we analysed large datasets recorded from a downhole seismic network for several water injection periods (1993, 2000, …). For each stimulation interval, thousands of events are recorded at depth. We detected the events using the continuous kurtosis-based migration method and classified them into families of comparable waveforms using an approach based on cross-correlation analysis. We obtain precise relative locations of the multiplets using differential arrival times obtained through cross-correlation of similar waveforms. Finally, the properties of the similar fluid-induced seismic events are derived (magnitude, spectral content) and examined over the several hydraulic tests. Hopefully these steps will lead to a better understanding of the repetitive nature of these events and the investigation of their persistence will outline the heterogeneities of the structures (temperatures anomalies, regional stress perturbations, fluid flow channelling) regularly involved during the different stimulations.

Bayesian Inference for Signal-Based Seismic Monitoring

NASA Astrophysics Data System (ADS)

Moore, D.

2015-12-01

Traditional seismic monitoring systems rely on discrete detections produced by station processing software, discarding significant information present in the original recorded signal. SIG-VISA (Signal-based Vertically Integrated Seismic Analysis) is a system for global seismic monitoring through Bayesian inference on seismic signals. By modeling signals directly, our forward model is able to incorporate a rich representation of the physics underlying the signal generation process, including source mechanisms, wave propagation, and station response. This allows inference in the model to recover the qualitative behavior of recent geophysical methods including waveform matching and double-differencing, all as part of a unified Bayesian monitoring system that simultaneously detects and locates events from a global network of stations. We demonstrate recent progress in scaling up SIG-VISA to efficiently process the data stream of global signals recorded by the International Monitoring System (IMS), including comparisons against existing processing methods that show increased sensitivity from our signal-based model and in particular the ability to locate events (including aftershock sequences that can tax analyst processing) precisely from waveform correlation effects. We also provide a Bayesian analysis of an alleged low-magnitude event near the DPRK test site in May 2010 [1] [2], investigating whether such an event could plausibly be detected through automated processing in a signal-based monitoring system. [1] Zhang, Miao and Wen, Lianxing. "Seismological Evidence for a Low-Yield Nuclear Test on 12 May 2010 in North Korea". Seismological Research Letters, January/February 2015. [2] Richards, Paul. "A Seismic Event in North Korea on 12 May 2010". CTBTO SnT 2015 oral presentation, video at https://video-archive.ctbto.org/index.php/kmc/preview/partner_id/103/uiconf_id/4421629/entry_id/0_ymmtpps0/delivery/http

Seismicity, arrival time delays of the seismic phases and slowness characteristics study in Abu Dabbab area, Egypt

NASA Astrophysics Data System (ADS)

Sami, Mahmoud; Hassoup, Awad; Hosny, Ahmed; Mohamed, Gadelkarem A.

2013-12-01

The temporal variations of seismicity from the Abu Dabbab area, 25 km west of the Red Sea coast, are collected from the Egyptian national seismic network (ENSN), which has magnified the detection capability in that area to ML < 1 earthquakes. These data show a sequence of the micro earthquake swarm during 2003-2011. This area has experienced larger shocks up to M = 6 during the 20th century and its seismicity is concentrated in a narrow spatial volume. We analyze the digital waveform data of about 1000 seismograms, recorded by portable network of 10 vertical component seismographs that are employed in a temporary survey experiment in the Abu Dabbab area in 2004, and the results indicate: firstly, there are similar waveform seismograms, which are classified into three groups. In each group a master event is identified. Then, the arrival time delays of the P and S phases (Δtp and Δts, respectively) are measured between the master event and its slave events. Δtp and Δts range between -0.01 and 0.02 s, respectively. These values are used to relocate the studied events. Secondly, the slowness vector (Δs) in 3-dimensional pattern, which is estimated using the genetic algorithms, is found Δsx = 0.0153, Δsy = 0.00093 and Δsz = 0.2086 s/km in the three spatial coordinates (X, Y and Z), respectively. These analyses demonstrate the inhomogeneities within the upper crust of the study area. Also, Δs shows little dependence of lateral distances and reasonably high slowness along the depth extent, which is consistent with the seismic velocity structure variations.

Study of Seismic Clusters at Bahía de Banderas Region, Mexico

NASA Astrophysics Data System (ADS)

Nunez-Cornu, F. J.; Rutz-Lopez, M.; Suarez-Plascencia, C.; Trejo-Gomez, E.

2010-12-01

Given that the coast in the states of Jalisco and south of the state of Nayarit is located within a region of high seismic potential and also because population is increasing, perhaps motivated by the development of tourism, the Civil Defense authorities of Jalisco and the Centro de Sismología y Volcanología de Occidente-SisVOc of Universidad de Guadalajara started in the year 2000 a joint project to study the seismic risk of the region, including the seismic monitoring of Colima volcano (located between the states of Jalisco and Colima). This work focuses on the study of seismicity in the area of Bahía de Banderas and northern coast of Jalisco. To this end, we perform an analysis of available seismograms to characterize active structures, their relationship to surface morphology, and possible reach of these structures into the shallow parts of the bay. The data used in this work are waveforms recorded during the year 2003 during which the seismograph network spanned the region of study. Our method is based on the identification of seismic clusters or families using cross-correlation of waveforms, earthquake relocation and modeling of fault planes. From an initial data set of 404 earthquakes located during 2003, 96 earthquakes could be related to 17 potentially active continental structures. A modeling of fault planes was possible for 11 of these structures. Subgroups of 7 structures are aligned parallel to the Middle America Trench, a possible consequence of oblique subduction. The magnitudes of earthquakes grouped into families is less than 3.6 (Ml), corresponding to fault dimensions of hundreds of meters.

Full Waveform Inversion with Multisource Frequency Selection of Marine Streamer Data

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

Huang, Yunsong; Schuster, Gerard T.

The theory and practice of multisource full waveform inversion of marine supergathers are described with a frequency-selection strategy. The key enabling property of frequency selection is that it eliminates the crosstalk among sources, thus overcoming the aperture mismatch of marine multisource inversion. Tests on multisource full waveform inversion of synthetic marine data and Gulf of Mexico data show speedups of 4× and 8×, respectively, compared to conventional full waveform inversion.

Full Waveform Inversion with Multisource Frequency Selection of Marine Streamer Data

DOE PAGES

Huang, Yunsong; Schuster, Gerard T.

2017-10-26

The theory and practice of multisource full waveform inversion of marine supergathers are described with a frequency-selection strategy. The key enabling property of frequency selection is that it eliminates the crosstalk among sources, thus overcoming the aperture mismatch of marine multisource inversion. Tests on multisource full waveform inversion of synthetic marine data and Gulf of Mexico data show speedups of 4× and 8×, respectively, compared to conventional full waveform inversion.

Ground truth seismic events and location capability at Degelen mountain, Kazakhstan

USGS Publications Warehouse

Trabant, C.; Thurber, C.; Leith, W.

2002-01-01

We utilized nuclear explosions from the Degelen Mountain sub-region of the Semipalatinsk Test Site (STS), Kazakhstan, to assess seismic location capability directly. Excellent ground truth information for these events was either known or was estimated from maps of the Degelen Mountain adit complex. Origin times were refined for events for which absolute origin time information was unknown using catalog arrival times, our ground truth location estimates, and a time baseline provided by fixing known origin times during a joint hypocenter determination (JHD). Precise arrival time picks were determined using a waveform cross-correlation process applied to the available digital data. These data were used in a JHD analysis. We found that very accurate locations were possible when high precision, waveform cross-correlation arrival times were combined with JHD. Relocation with our full digital data set resulted in a mean mislocation of 2 km and a mean 95% confidence ellipse (CE) area of 6.6 km2 (90% CE: 5.1 km2), however, only 5 of the 18 computed error ellipses actually covered the associated ground truth location estimate. To test a more realistic nuclear test monitoring scenario, we applied our JHD analysis to a set of seven events (one fixed) using data only from seismic stations within 40?? epicentral distance. Relocation with these data resulted in a mean mislocation of 7.4 km, with four of the 95% error ellipses covering less than 570 km2 (90% CE: 438 km2), and the other two covering 1730 and 8869 km2 (90% CE: 1331 and 6822 km2). Location uncertainties calculated using JHD often underestimated the true error, but a circular region with a radius equal to the mislocation covered less than 1000 km2 for all events having more than three observations. ?? 2002 Elsevier Science B.V. All rights reserved.

High-resolution surface wave tomography of the European crust and uppermost mantle from ambient seismic noise

NASA Astrophysics Data System (ADS)

Lu, Yang; Stehly, Laurent; Paul, Anne; AlpArray Working Group

2018-05-01

Taking advantage of the large number of seismic stations installed in Europe, in particular in the greater Alpine region with the AlpArray experiment, we derive a new high-resolution 3-D shear-wave velocity model of the European crust and uppermost mantle from ambient noise tomography. The correlation of up to four years of continuous vertical-component seismic recordings from 1293 broadband stations (10° W-35° E, 30° N-75° N) provides Rayleigh wave group velocity dispersion data in the period band 5-150 s at more than 0.8 million virtual source-receiver pairs. Two-dimensional Rayleigh wave group velocity maps are estimated using adaptive parameterization to accommodate the strong heterogeneity of path coverage. A probabilistic 3-D shear-wave velocity model, including probability densities for the depth of layer boundaries and S-wave velocity values, is obtained by non-linear Bayesian inversion. A weighted average of the probabilistic model is then used as starting model for the linear inversion step, providing the final Vs model. The resulting S-wave velocity model and Moho depth are validated by comparison with previous geophysical studies. Although surface-wave tomography is weakly sensitive to layer boundaries, vertical cross-sections through our Vs model and the associated probability of presence of interfaces display striking similarities with reference controlled-source (CSS) and receiver-function sections across the Alpine belt. Our model even provides new structural information such as a ˜8 km Moho jump along the CSS ECORS-CROP profile that was not imaged by reflection data due to poor penetration across a heterogeneous upper crust. Our probabilistic and final shear wave velocity models have the potential to become new reference models of the European crust, both for crustal structure probing and geophysical studies including waveform modeling or full waveform inversion.

Detailed structure of the top of the melt body beneath the East Pacific Rise at 9°40'N from waveform inversion of seismic reflection data

NASA Astrophysics Data System (ADS)

Collier, J. S.; Singh, S. C.

1997-01-01

We have applied waveform inversion to multichannel seismic reflection data collected at the East Pacific Rise at 9°40'N in order to determine the precise velocity structure of the magma body causing the axial magma chamber reflection. Our analysis supports the idea of a molten sill as previously suggested from forward modeling of seismic data from this location. Our inverted solution has a 30-m-thick sill with a P wave seismic velocity of 2.6 km s-1. Although not well constrained by the data we believe that the S wave velocity in the sill is not significantly different from 0.0 km s-1. The low P- and S wave velocities in the sill imply that it contains less than 30% crystals. The molten sill is underlain by a velocity gradient in which the P wave velocity increases from 2.6 to 3.5 km s-1 over a vertical distance of 50-m. The shape of our velocity-depth profile implies that accretion of material to the roof of the sill is minor compared to accretion to the floor. The underlying velocity gradient zone may represent crystal settling under gravity. We suggest that only material from the 30-m-thick layer can erupt.

Probing the DPRK nuclear test-site to low magnitude using seismic pattern detectors

NASA Astrophysics Data System (ADS)

Kvaerna, T.; Gibbons, S. J.; Mykkeltveit, S.

2017-12-01

Six declared nuclear explosions at North Korea's Punggye-ri test-site between October 2006 and September 2017 were detected seismically both at regional and teleseismic distances. The similarity of body-wave signals from explosion to explosion allows us to locate these events relative to each other with high accuracy. Greater uncertainty in the relative time measurements for the most recent test on 3 September 2017 results in a greater uncertainty in the relative location estimate for this event, although it appears to have taken place below optimal overburden close to the peak of Mount Mantap. A number of smaller events, detected mainly at regional distances, have been identified as being at, or very close to, the test site. Due to waveform differences and available station coverage, a simple double-difference relative location is often not possible. In addition to the apparent collapse event some 8 minutes after the declared nuclear test, small seismic events have been detected on 25 May 2014, 11 September 2016, 23 September 2017, and 12 October 2017. The signals from these events differ significantly from those from the declared nuclear tests with far weaker Pn and far stronger Lg phases. Multi-channel correlation analysis and empirical matched field processing allow us to categorize these weaker seismic events with far greater confidence than classical waveform analysis allows.

Offshore seismicity in the southeastern sea of Korea

NASA Astrophysics Data System (ADS)

Park, H.; Kang, T. S.

2017-12-01

The offshore southeastern sea area of Korea appear to have a slightly higher seismicity compared to the rest of the Korean Peninsula. According to the earthquake report by Korean Meteorological Administration (KMA), earthquakes over ML 3 has persistently occurred over once a year during the last ten years. In this study, we used 33 events in KMA catalog, which occurred in the offshore Ulsan (35.0°N-35.85°N, 129.45°E-130.75°E) from April 2007 to June 2017, as mother earthquakes. The waveform matching filter technique was used to precisely detect microearthquakes (child earthquakes) that occurred after mother earthquakes. It is the optimal linear filter for maximizing the signal-to-noise ratio in the presence of additive stochastic noise. Initially, we used the continuous seismic waveforms available from KMA and the Korea Institute of Geosciences and Mineral Resources. We added the data of F-net to increase the reliability of the results. The detected events were located by using P- and S-wave arrival times. The hypocentral depths were constrained by an iterative optimal solution technique which is proven to be effective under the poorly known structure. Focal mechanism solutions were obtained from the analysis of P-wave first-motion polarities. Seismicity patterns of microearthquakes and their focal mechanism results were analyzed to understand their seismogenic characteristics and their relationship to subsea seismotectonic structures.

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.

Seismic structures in the inner and outer core constrained by the PKP observations near the caustic distance range

NASA Astrophysics Data System (ADS)

Yu, W.; Wen, L.; Niu, F.

2002-05-01

We have extensively collected PKP waveforms around the PKP caustic distance range (141o - 147o) recorded in several dense regional arrays and the Global Seismic Network covering from 1990 to 2000. PKP observations at this distance range (141o - 147o) are usually purposely avoided in travel time analyses, because of the interference of various PKP branches. The observations there, however, will be extremely useful for constraining the seismic structures at both the top of the inner core and the bottom of the outer core. Moreover, because PKIKP phases sample a depth range of 100 km - 170 km below the inner-core boundary at this distant range, their observations fill the sampling depth gap between the PKiKP-PKIKP observations at the smaller distances and the PKPbc-PKIKP phases at the larger distances. Before the PKP caustics (141o - 145o), the diffracted PKP phases near the B caustics (PKPBdiff) and PKiKP phases are discernible in the long-period seismograms, and their differential travel times and waveforms could be used to constrain seismic structures at the bottom of the outer core and/or at the base of the mantle. The observed long-period PKiKP-PKPBdiff waveforms exhibit a hemispheric difference between those sampling the "eastern" and "western" hemispheres, with those sampling the "western" hemisphere showing larger time separations between the two phases. These observations can be explained by models with P velocity gradients of 0.0806 (km/s)/ 200 km for the "western" hemisphere and 0.114 (km/s)/200 km for the "eastern" hemisphere at the bottom of the outer core. Alternatively, these observations can also be explained by models with different velocity structures at the bottom 200 km of the mantle with P velocity variations in an order of 3 percent with respect to PREM. Broadband PKP observations after the PKP caustics (145o - 147o), on the other hand, provide high-quality constraints on the seismic structures at both the top of the inner core and the bottom of the outer core, as PKPbc phases can be used as excellent reference phases. We explore seismic models in both the bottom of the outer core and the top of the inner core, which can consistently explain the seismic observations at the caustic distance range (141o - 147o) and the PKiKP-PKIKP observations at the closer distances (Niu and Wen, Nature, 410, 1081-1084, 2001, Wen and Niu, JGR, submitted).

Development of an event search and download system for analyzing waveform data observed at seafloor seismic network, DONET

NASA Astrophysics Data System (ADS)

Takaesu, M.; Horikawa, H.; Sueki, K.; Kamiya, S.; Nakamura, T.; Nakano, M.; Takahashi, N.; Sonoda, A.; Tsuboi, S.

2014-12-01

Mega-thrust earthquakes are anticipated to occur in the Nankai Trough in southwest Japan. In the source areas, we installed seafloor seismic network, DONET (Dense Ocean-floor Network System for Earthquake and Tsunamis), in 2010 in order to monitor seismicity, crustal deformations, and tsunamis. DONET system consists of totally 20 stations, which is composed of six kinds of sensors; strong-motion and broadband seismometers, quartz and differential pressure gauges, hydrophone, and thermometer. The stations are densely distributed with an average spatial interval of 15-20 km and cover near coastal areas to the trench axis. Observed data are transferred to a land station through a fiber-optical cable and then to JAMSTEC (Japan Agency for Marine-Earth Science and Technology) data management center through a private network in real time. The data are based on WIN32 format in the private network and finally archived in SEED format in the management center to combine waveform data with related metadata. We are developing a web-based application system to easily download seismic waveform data of DONET. In this system, users can select 20 Hz broadband (BH type) and 200 Hz strong-motion (EH type) data and download them in SEED. Users can also search events from the options of time periods, magnitude, source area and depth in a GUI platform. Event data are produced referring to event catalogues from USGS and JMA (Japan Meteorological Agency). The thresholds of magnitudes for the production are M6 for far-field and M4 for local events using the USGS and JMA lists, respectively. Available data lengths depend on magnitudes and epicentral distances. In this presentation, we briefly introduce DONET stations and then show our developed application system. We open DONET data through the system and want them to be widely recognized so that many users analyze. We also discuss next plans for further developments of the system.

Improved Detection of Local Earthquakes in the Vienna Basin (Austria), using Subspace Detectors

NASA Astrophysics Data System (ADS)

Apoloner, Maria-Theresia; Caffagni, Enrico; Bokelmann, Götz

2016-04-01

The Vienna Basin in Eastern Austria is densely populated and highly-developed; it is also a region of low to moderate seismicity, yet the seismological network coverage is relatively sparse. This demands improving our capability of earthquake detection by testing new methods, enlarging the existing local earthquake catalogue. This contributes to imaging tectonic fault zones for better understanding seismic hazard, also through improved earthquake statistics (b-value, magnitude of completeness). Detection of low-magnitude earthquakes or events for which the highest amplitudes slightly exceed the signal-to-noise-ratio (SNR), may be possible by using standard methods like the short-term over long-term average (STA/LTA). However, due to sparse network coverage and high background noise, such a technique may not detect all potentially recoverable events. Yet, earthquakes originating from the same source region and relatively close to each other, should be characterized by similarity in seismic waveforms, at a given station. Therefore, waveform similarity can be exploited by using specific techniques such as correlation-template based (also known as matched filtering) or subspace detection methods (based on the subspace theory). Matching techniques basically require a reference or template event, usually characterized by high waveform coherence in the array receivers, and high SNR, which is cross-correlated with the continuous data. Instead, subspace detection methods overcome in principle the necessity of defining template events as single events, but use a subspace extracted from multiple events. This approach theoretically should be more robust in detecting signals that exhibit a strong variability (e.g. because of source or magnitude). In this study we scan the continuous data recorded in the Vienna Basin with a subspace detector to identify additional events. This will allow us to estimate the increase of the seismicity rate in the local earthquake catalogue, therefore providing an evaluation of network performance and efficiency of the method.

A Probabilistic Approach to Network Event Formation from Pre-Processed Waveform Data

NASA Astrophysics Data System (ADS)

Kohl, B. C.; Given, J.

2017-12-01

The current state of the art for seismic event detection still largely depends on signal detection at individual sensor stations, including picking accurate arrivals times and correctly identifying phases, and relying on fusion algorithms to associate individual signal detections to form event hypotheses. But increasing computational capability has enabled progress toward the objective of fully utilizing body-wave recordings in an integrated manner to detect events without the necessity of previously recorded ground truth events. In 2011-2012 Leidos (then SAIC) operated a seismic network to monitor activity associated with geothermal field operations in western Nevada. We developed a new association approach for detecting and quantifying events by probabilistically combining pre-processed waveform data to deal with noisy data and clutter at local distance ranges. The ProbDet algorithm maps continuous waveform data into continuous conditional probability traces using a source model (e.g. Brune earthquake or Mueller-Murphy explosion) to map frequency content and an attenuation model to map amplitudes. Event detection and classification is accomplished by combining the conditional probabilities from the entire network using a Bayesian formulation. This approach was successful in producing a high-Pd, low-Pfa automated bulletin for a local network and preliminary tests with regional and teleseismic data show that it has promise for global seismic and nuclear monitoring applications. The approach highlights several features that we believe are essential to achieving low-threshold automated event detection: Minimizes the utilization of individual seismic phase detections - in traditional techniques, errors in signal detection, timing, feature measurement and initial phase ID compound and propagate into errors in event formation, Has a formalized framework that utilizes information from non-detecting stations, Has a formalized framework that utilizes source information, in particular the spectral characteristics of events of interest, Is entirely model-based, i.e. does not rely on a priori's - particularly important for nuclear monitoring, Does not rely on individualized signal detection thresholds - it's the network solution that matters.

Automatic seismic waveform location using multichannel coherency migration for induced and natural earthquakes

NASA Astrophysics Data System (ADS)

Nowacki, A.; Shi, P.; Angus, D. A.; Rost, S.; Birnie, C. E.; Yuan, S.

2017-12-01

Modern, large seismic datasets place a huge burden on human analysts who traditionally have been required to manually pick distinct phase arrivals in order to locate seismic events. This burden becomes insurmountable when real-time monitoring is needed, and hence automated approaches are necessary. Whilst many methods exist, noisy data often defeat them. We propose here a novel method to migrate seismic energy back to its spatial and temporal source, based on an improved imaging condition with greater tolerance to noise. The multichannel coherency migration (MCM) method sums the correlation coefficients of traces between all available station pairs, using the predicted P- and S-wave windows for any given imaging point in the target volume. Grid searching in time and space allows the point of maximum waveform coherency and event likelihood to be found. The only adjustable parameter in the method is the cross-correlation window length, but this is determined by the dominant frequency of the signal. This is in contrast with most other methods, such as the STA-LTA imaging function, which require several parameters to be adjusted and optimised for each application. Because we use the cross-correlation between stations, incoherent noise is effectively suppressed, and even temporally coherent noise which is not located within the target volume can be minimised also. We apply the MCM to synthetic tests, and real data in geological carbon storage and volcanic settings. In comparison to migrations based on waveform envelope, STA-LTA and kurtosis imaging functions, the MCM more reliably finds the true source and better suppresses noise. Synthetic tests with real noise show that the MCM remains robust up to noise-to-signal (not a typo) ratios (NSR) of about 40. Tests with incorrect velocity models further suggest that the MCM will be a useful event detection method in the future.

An Oscillation of the Crack-like Conduit at Nevado del Ruiz Volcano, Colombia, Inferred from Multi-band Analyses of Very Long Period Seismic Events

NASA Astrophysics Data System (ADS)

Maeda, Y.; Kumagai, H.; Londono, J. M.; Lopez, C. M.; Castaño, L. M.; Beatriz, B.; García, L.

2017-12-01

Nevado del Ruiz is an active volcano in Colombia, which continues eruption activity and has been monitored by 13 broadband and 3 short-period seismic stations. In 2015-2016, a joint Japan-Colombia team installed an automatic event detection and location system based on the amplitude source location (ASL) method. Kumagai et al. (IAVCEI, 2017) indicated the existence of a magma conduit extending from the NW flank to the summit based on ASL analyses of various seismic signals including long-period (LP) and very long period (VLP) events and tremors in a 5-10 Hz frequency band. In this study, we analyzed the VLP events by waveform inversion using eight summit stations in a frequency band of 0.3-0.7 Hz. We selected 14 VLP events from May to December 2016 based on signal-to-noise ratios and simplicity of the waveforms. We assumed a homogeneous P-wave velocity of 3.5 km/s with topography in the calculation of the Green functions. We conducted frequency-domain waveform inversion assuming a tensile crack source and investigated the best location and orientation of the crack by a grid search. The inversion results pointed to a low-angle ( 30°) NW-dipping crack near the top of the conduit (approximately 1 km below the summit). The estimated source time functions displayed two or three cycles of oscillations with the seismic moment of order of 1010-1011 N m. For these 14 events, the ASLs from the 5-10 Hz frequency band were also near the top of the conduit. These results suggest the VLP and high-frequency signals are generated by an oscillation of the crack-like conduit near the summit, which may be triggered by a volume change of magma ascending in the conduit.

Development of Laboratory Seismic Exploration Experiment for Education and Demonstration

NASA Astrophysics Data System (ADS)

Kuwano, O.; Nakanishi, A.

2016-12-01

We developed a laboratory experiment to simulate a seismic refraction survey for educational purposes. The experiment is tabletop scaled experiment using the soft hydrogel as an analogue material of a layered crust. So, we can conduct the seismic exploration experiment in a laboratory or a classroom. The softness and the transparency of the gel material enable us to observe the wave propagation with our naked eyes, using the photoelastic technique. By analyzing the waveforms obtained by the image analysis of the movie of the experiment, one can estimate the velocities and the structure of the gel specimen in the same way as an actual seismic survey. We report details of the practical course and the public outreach activities using the experiment.

Pickless event detection and location: The waveform correlation event detection system (WCEDS) revisited

DOE PAGES

Arrowsmith, Stephen John; Young, Christopher J.; Ballard, Sanford; ...

2016-01-01

The standard paradigm for seismic event monitoring breaks the event detection problem down into a series of processing stages that can be categorized at the highest level into station-level processing and network-level processing algorithms (e.g., Le Bras and Wuster (2002)). At the station-level, waveforms are typically processed to detect signals and identify phases, which may subsequently be updated based on network processing. At the network-level, phase picks are associated to form events, which are subsequently located. Furthermore, waveforms are typically directly exploited only at the station-level, while network-level operations rely on earth models to associate and locate the events thatmore » generated the phase picks.« less

Efficient waveform tomography for lithospheric imaging: implications for realistic, two-dimensional acquisition geometries and low-frequency data

NASA Astrophysics Data System (ADS)

Brenders, A. J.; Pratt, R. G.

2007-01-01

We provide a series of numerical experiments designed to test waveform tomography under (i) a reduction in the number of input data frequency components (`efficient' waveform tomography), (ii) sparse spatial subsampling of the input data and (iii) an increase in the minimum data frequency used. These results extend the waveform tomography results of a companion paper, using the same third-party, 2-D, wide-angle, synthetic viscoelastic seismic data, computed in a crustal geology model 250 km long and 40 km deep, with heterogeneous P-velocity, S-velocity, density and Q-factor structure. Accurate velocity models were obtained using efficient waveform tomography and only four carefully selected frequency components of the input data: 0.8, 1.7, 3.6 and 7.0 Hz. This strategy avoids the spectral redundancy present in `full' waveform tomography, and yields results that are comparable with those in the companion paper for an 88 per cent decrease in total computational cost. Because we use acoustic waveform tomography, the results further justify the use of the acoustic wave equation in calculating P-wave velocity models from viscoelastic data. The effect of using sparse survey geometries with efficient waveform tomography were investigated for both increased receiver spacing, and increased source spacing. Sampling theory formally requires spatial sampling at maximum interval of one half-wavelength (2.5 km at 0.8 Hz): For data with receivers every 0.9 km (conforming to this criterion), artefacts in the tomographic images were still minimal when the source spacing was as large as 7.6 km (three times the theoretical maximum). Larger source spacings led to an unacceptable degradation of the results. When increasing the starting frequency, image quality was progressively degraded. Acceptable image quality within the central portion of the model was nevertheless achieved using starting frequencies up to 3.0 Hz. At 3.0 Hz the maximum theoretical sample interval is reduced to 0.67 km due to the decreased wavelengths; the available sources were spaced every 5.0 km (more than seven times the theoretical maximum), and receivers were spaced every 0.9 km (1.3 times the theoretical maximum). Higher starting frequencies than 3.0 Hz again led to unacceptable degradation of the results.

An Adaptable Seismic Data Format

NASA Astrophysics Data System (ADS)

Krischer, Lion; Smith, James; Lei, Wenjie; Lefebvre, Matthieu; Ruan, Youyi; de Andrade, Elliott Sales; Podhorszki, Norbert; Bozdağ, Ebru; Tromp, Jeroen

2016-11-01

We present ASDF, the Adaptable Seismic Data Format, a modern and practical data format for all branches of seismology and beyond. The growing volume of freely available data coupled with ever expanding computational power opens avenues to tackle larger and more complex problems. Current bottlenecks include inefficient resource usage and insufficient data organization. Properly scaling a problem requires the resolution of both these challenges, and existing data formats are no longer up to the task. ASDF stores any number of synthetic, processed or unaltered waveforms in a single file. A key improvement compared to existing formats is the inclusion of comprehensive meta information, such as event or station information, in the same file. Additionally, it is also usable for any non-waveform data, for example, cross-correlations, adjoint sources or receiver functions. Last but not least, full provenance information can be stored alongside each item of data, thereby enhancing reproducibility and accountability. Any data set in our proposed format is self-describing and can be readily exchanged with others, facilitating collaboration. The utilization of the HDF5 container format grants efficient and parallel I/O operations, integrated compression algorithms and check sums to guard against data corruption. To not reinvent the wheel and to build upon past developments, we use existing standards like QuakeML, StationXML, W3C PROV and HDF5 wherever feasible. Usability and tool support are crucial for any new format to gain acceptance. We developed mature C/Fortran and Python based APIs coupling ASDF to the widely used SPECFEM3D_GLOBE and ObsPy toolkits.

Seismic imaging of the metamorphism of young sediment into new crystalline crust in the actively rifting Imperial Valley, California

USGS Publications Warehouse

Han, Liang; Hole, John; Stock, Joann; Fuis, Gary S.; Williams, Colin F.; Delph, Jonathan; Davenport, Kathy; Livers, Amanda

2016-01-01

Plate-boundary rifting between transform faults is opening the Imperial Valley of southern California and the rift is rapidly filling with sediment from the Colorado River. Three 65–90 km long seismic refraction profiles across and along the valley, acquired as part of the 2011 Salton Seismic Imaging Project, were analyzed to constrain upper crustal structure and the transition from sediment to underlying crystalline rock. Both first arrival travel-time tomography and frequency-domain full-waveform inversion were applied to provide P-wave velocity models down to ∼7 km depth. The valley margins are fault-bounded, beyond which thinner sediment has been deposited on preexisting crystalline rocks. Within the central basin, seismic velocity increases continuously from ∼1.8 km/s sediment at the surface to >6 km/s crystalline rock with no sharp discontinuity. Borehole data show young sediment is progressively metamorphosed into crystalline rock. The seismic velocity gradient with depth decreases approximately at the 4 km/s contour, which coincides with changes in the porosity and density gradient in borehole core samples. This change occurs at ∼3 km depth in most of the valley, but at only ∼1.5 km depth in the Salton Sea geothermal field. We interpret progressive metamorphism caused by high heat flow to be creating new crystalline crust throughout the valley at a rate comparable to the ≥2 km/Myr sedimentation rate. The newly formed crystalline crust extends to at least 7–8 km depth, and it is shallower and faster where heat flow is higher. Most of the active seismicity occurs within this new crust.

Seismic imaging of the metamorphism of young sediment into new crystalline crust in the actively rifting Imperial Valley, California

NASA Astrophysics Data System (ADS)

Han, Liang; Hole, John A.; Stock, Joann M.; Fuis, Gary S.; Williams, Colin F.; Delph, Jonathan R.; Davenport, Kathy K.; Livers, Amanda J.

2016-11-01

Plate-boundary rifting between transform faults is opening the Imperial Valley of southern California and the rift is rapidly filling with sediment from the Colorado River. Three 65-90 km long seismic refraction profiles across and along the valley, acquired as part of the 2011 Salton Seismic Imaging Project, were analyzed to constrain upper crustal structure and the transition from sediment to underlying crystalline rock. Both first arrival travel-time tomography and frequency-domain full-waveform inversion were applied to provide P-wave velocity models down to ˜7 km depth. The valley margins are fault-bounded, beyond which thinner sediment has been deposited on preexisting crystalline rocks. Within the central basin, seismic velocity increases continuously from ˜1.8 km/s sediment at the surface to >6 km/s crystalline rock with no sharp discontinuity. Borehole data show young sediment is progressively metamorphosed into crystalline rock. The seismic velocity gradient with depth decreases approximately at the 4 km/s contour, which coincides with changes in the porosity and density gradient in borehole core samples. This change occurs at ˜3 km depth in most of the valley, but at only ˜1.5 km depth in the Salton Sea geothermal field. We interpret progressive metamorphism caused by high heat flow to be creating new crystalline crust throughout the valley at a rate comparable to the ≥2 km/Myr sedimentation rate. The newly formed crystalline crust extends to at least 7-8 km depth, and it is shallower and faster where heat flow is higher. Most of the active seismicity occurs within this new crust.

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

Bessel smoothing filter for spectral-element mesh

NASA Astrophysics Data System (ADS)

Trinh, P. T.; Brossier, R.; Métivier, L.; Virieux, J.; Wellington, P.

2017-06-01

Smoothing filters are extremely important tools in seismic imaging and inversion, such as for traveltime tomography, migration and waveform inversion. For efficiency, and as they can be used a number of times during inversion, it is important that these filters can easily incorporate prior information on the geological structure of the investigated medium, through variable coherent lengths and orientation. In this study, we promote the use of the Bessel filter to achieve these purposes. Instead of considering the direct application of the filter, we demonstrate that we can rely on the equation associated with its inverse filter, which amounts to the solution of an elliptic partial differential equation. This enhances the efficiency of the filter application, and also its flexibility. We apply this strategy within a spectral-element-based elastic full waveform inversion framework. Taking advantage of this formulation, we apply the Bessel filter by solving the associated partial differential equation directly on the spectral-element mesh through the standard weak formulation. This avoids cumbersome projection operators between the spectral-element mesh and a regular Cartesian grid, or expensive explicit windowed convolution on the finite-element mesh, which is often used for applying smoothing operators. The associated linear system is solved efficiently through a parallel conjugate gradient algorithm, in which the matrix vector product is factorized and highly optimized with vectorized computation. Significant scaling behaviour is obtained when comparing this strategy with the explicit convolution method. The theoretical numerical complexity of this approach increases linearly with the coherent length, whereas a sublinear relationship is observed practically. Numerical illustrations are provided here for schematic examples, and for a more realistic elastic full waveform inversion gradient smoothing on the SEAM II benchmark model. These examples illustrate well the efficiency and flexibility of the approach proposed.

Noise suppression in surface microseismic data by τ-p transform

USGS Publications Warehouse

Forghani-Arani, Farnoush; Batzle, Mike; Behura, Jyoti; Willis, Mark; Haines, Seth; Davidson, Michael

2013-01-01

Surface passive seismic methods are receiving increased attention for monitoring changes in reservoirs during the production of unconventional oil and gas. However, in passive seismic data the strong cultural and ambient noise (mainly surface-waves) decreases the effectiveness of these techniques. Hence, suppression of surface-waves is a critical step in surface microseismic monitoring. We apply a noise suppression technique, based on the τ — p transform, to a surface passive seismic dataset recorded over a Barnett Shale reservoir undergoing a hydraulic fracturing process. This technique not only improves the signal-to-noise ratios of added synthetic microseismic events, but it also preserves the event waveforms.

Dilational processes accompanying earthquakes in the Long Valley Caldera

USGS Publications Warehouse

Dreger, Douglas S.; Tkalcic, Hrvoje; Johnston, M.

2000-01-01

Regional distance seismic moment tensor determinations and broadband waveforms of moment magnitude 4.6 to 4.9 earthquakes from a November 1997 Long Valley Caldera swarm, during an inflation episode, display evidence of anomalous seismic radiation characterized by non-double couple (NDC) moment tensors with significant volumetric components. Observed coseismic dilation suggests that hydrothermal or magmatic processes are directly triggering some of the seismicity in the region. Similarity in the NDC solutions implies a common source process, and the anomalous events may have been triggered by net fault-normal stress reduction due to high-pressure fluid injection or pressurization of fluid-saturated faults due to magmatic heating.

Seismic Waveform Analysis of Underground Nuclear Explosions

DTIC Science & Technology

1979-11-15

parameters to be discussed here are Bouguer gravity (Figure 18), and station elevation (Figure 19). Tn this simple comparison of various geophysical...noted the frequent strong correlation between Bouguer gravity and elevation. Indeed, many of the geophysical parameters discussed above are interrelated

A new moonquake catalog from Apollo 17 seismic data I: Lunar Seismic Profiling Experiment: Thermal moonquakes and implications for surface processes

NASA Astrophysics Data System (ADS)

Weber, R. C.; Dimech, J. L.; Phillips, D.; Molaro, J.; Schmerr, N. C.

2017-12-01

Apollo 17's Lunar Seismic Profiling Experiment's (LSPE) primary objective was to constrain the near-surface velocity structure at the landing site using active sources detected by a 100 m-wide triangular geophone array. The experiment was later operated in "listening mode," and early studies of these data revealed the presence of thermal moonquakes - short-duration seismic events associated with terminator crossings. However, the full data set has never been systematically analyzed for natural seismic signal content. In this study, we analyze 8 months of continuous LSPE data using an automated event detection technique that has previously successfully been applied to the Apollo 16 Passive Seismic Experiment data. We detected 50,000 thermal moonquakes from three distinct event templates, representing impulsive, intermediate, and emergent onset of seismic energy, which we interpret as reflecting their relative distance from the array. Impulsive events occur largely at sunrise, possibly representing the thermal "pinging" of the nearby lunar lander, while emergent events occur at sunset, possibly representing cracking or slumping in more distant surface rocks and regolith. Preliminary application of an iterative event location algorithm to a subset of the impulsive waveforms supports this interpretation. We also perform 3D modeling of the lunar surface to explore the relative contribution of the lander, known rocks and surrounding topography to the thermal state of the regolith in the vicinity of the Apollo 17 landing site over the course of the lunar diurnal cycle. Further development of both this model and the event location algorithm may permit definitive discrimination between different types of local diurnal events e.g. lander noise, thermally-induced rock breakdown, or fault creep on the nearby Lee-Lincoln scarp. These results could place important constraints on both the contribution of seismicity to regolith production, and the age of young lobate scarps.

The SCEC Unified Community Velocity Model (UCVM) Software Framework for Distributing and Querying Seismic Velocity Models

NASA Astrophysics Data System (ADS)

Maechling, P. J.; Taborda, R.; Callaghan, S.; Shaw, J. H.; Plesch, A.; Olsen, K. B.; Jordan, T. H.; Goulet, C. A.

2017-12-01

Crustal seismic velocity models and datasets play a key role in regional three-dimensional numerical earthquake ground-motion simulation, full waveform tomography, modern physics-based probabilistic earthquake hazard analysis, as well as in other related fields including geophysics, seismology, and earthquake engineering. The standard material properties provided by a seismic velocity model are P- and S-wave velocities and density for any arbitrary point within the geographic volume for which the model is defined. Many seismic velocity models and datasets are constructed by synthesizing information from multiple sources and the resulting models are delivered to users in multiple file formats, such as text files, binary files, HDF-5 files, structured and unstructured grids, and through computer applications that allow for interactive querying of material properties. The Southern California Earthquake Center (SCEC) has developed the Unified Community Velocity Model (UCVM) software framework to facilitate the registration and distribution of existing and future seismic velocity models to the SCEC community. The UCVM software framework is designed to provide a standard query interface to multiple, alternative velocity models, even if the underlying velocity models are defined in different formats or use different geographic projections. The UCVM framework provides a comprehensive set of open-source tools for querying seismic velocity model properties, combining regional 3D models and 1D background models, visualizing 3D models, and generating computational models in the form of regular grids or unstructured meshes that can be used as inputs for ground-motion simulations. The UCVM framework helps researchers compare seismic velocity models and build equivalent simulation meshes from alternative velocity models. These capabilities enable researchers to evaluate the impact of alternative velocity models in ground-motion simulations and seismic hazard analysis applications. In this poster, we summarize the key components of the UCVM framework and describe the impact it has had in various computational geoscientific applications.

Seismicity in the block mountains between Halle and Leipzig, Central Germany: centroid moment tensors, ground motion simulation, and felt intensities of two M ≈ 3 earthquakes in 2015 and 2017

NASA Astrophysics Data System (ADS)

Dahm, Torsten; Heimann, Sebastian; Funke, Sigward; Wendt, Siegfried; Rappsilber, Ivo; Bindi, Dino; Plenefisch, Thomas; Cotton, Fabrice

2018-05-01

On April 29, 2017 at 0:56 UTC (2:56 local time), an M W = 2.8 earthquake struck the metropolitan area between Leipzig and Halle, Germany, near the small town of Markranstädt. The earthquake was felt within 50 km from the epicenter and reached a local intensity of I 0 = IV. Already in 2015 and only 15 km northwest of the epicenter, a M W = 3.2 earthquake struck the area with a similar large felt radius and I 0 = IV. More than 1.1 million people live in the region, and the unusual occurrence of the two earthquakes led to public attention, because the tectonic activity is unclear and induced earthquakes have occurred in neighboring regions. Historical earthquakes south of Leipzig had estimated magnitudes up to M W ≈ 5 and coincide with NW-SE striking crustal basement faults. We use different seismological methods to analyze the two recent earthquakes and discuss them in the context of the known tectonic structures and historical seismicity. Novel stochastic full waveform simulation and inversion approaches are adapted for the application to weak, local earthquakes, to analyze mechanisms and ground motions and their relation to observed intensities. We find NW-SE striking normal faulting mechanisms for both earthquakes and centroid depths of 26 and 29 km. The earthquakes are located where faults with large vertical offsets of several hundred meters and Hercynian strike have developed since the Mesozoic. We use a stochastic full waveform simulation to explain the local peak ground velocities and calibrate the method to simulate intensities. Since the area is densely populated and has sensitive infrastructure, we simulate scenarios assuming that a 12-km long fault segment between the two recent earthquakes is ruptured and study the impact of rupture parameters on ground motions and expected damage.

Interparameter trade-off quantification and reduction in isotropic-elastic full-waveform inversion: synthetic experiments and Hussar land data set application

NASA Astrophysics Data System (ADS)

Pan, Wenyong; Geng, Yu; Innanen, Kristopher A.

2018-05-01

The problem of inverting for multiple physical parameters in the subsurface using seismic full-waveform inversion (FWI) is complicated by interparameter trade-off arising from inherent ambiguities between different physical parameters. Parameter resolution is often characterized using scattering radiation patterns, but these neglect some important aspects of interparameter trade-off. More general analysis and mitigation of interparameter trade-off in isotropic-elastic FWI is possible through judiciously chosen multiparameter Hessian matrix-vector products. We show that products of multiparameter Hessian off-diagonal blocks with model perturbation vectors, referred to as interparameter contamination kernels, are central to the approach. We apply the multiparameter Hessian to various vectors designed to provide information regarding the strengths and characteristics of interparameter contamination, both locally and within the whole volume. With numerical experiments, we observe that S-wave velocity perturbations introduce strong contaminations into density and phase-reversed contaminations into P-wave velocity, but themselves experience only limited contaminations from other parameters. Based on these findings, we introduce a novel strategy to mitigate the influence of interparameter trade-off with approximate contamination kernels. Furthermore, we recommend that the local spatial and interparameter trade-off of the inverted models be quantified using extended multiparameter point spread functions (EMPSFs) obtained with pre-conditioned conjugate-gradient algorithm. Compared to traditional point spread functions, the EMPSFs appear to provide more accurate measurements for resolution analysis, by de-blurring the estimations, scaling magnitudes and mitigating interparameter contamination. Approximate eigenvalue volumes constructed with stochastic probing approach are proposed to evaluate the resolution of the inverted models within the whole model. With a synthetic Marmousi model example and a land seismic field data set from Hussar, Alberta, Canada, we confirm that the new inversion strategy suppresses the interparameter contamination effectively and provides more reliable density estimations in isotropic-elastic FWI as compared to standard simultaneous inversion approach.

Subband Coding Methods for Seismic Data Compression

NASA Technical Reports Server (NTRS)

Kiely, A.; Pollara, F.

1995-01-01

This paper presents a study of seismic data compression techniques and a compression algorithm based on subband coding. The compression technique described could be used as a progressive transmission system, where successive refinements of the data can be requested by the user. This allows seismologists to first examine a coarse version of waveforms with minimal usage of the channel and then decide where refinements are required. Rate-distortion performance results are presented and comparisons are made with two block transform methods.

Transdimensional inversion of scattered body waves for 1D S-wave velocity structure - Application to the Tengchong volcanic area, Southwestern China

NASA Astrophysics Data System (ADS)

Li, Mengkui; Zhang, Shuangxi; Bodin, Thomas; Lin, Xu; Wu, Tengfei

2018-06-01

Inversion of receiver functions is commonly used to recover the S-wave velocity structure beneath seismic stations. Traditional approaches are based on deconvolved waveforms, where the horizontal component of P-wave seismograms is deconvolved by the vertical component. Deconvolution of noisy seismograms is a numerically unstable process that needs to be stabilized by regularization parameters. This biases noise statistics, making it difficult to estimate uncertainties in observed receiver functions for Bayesian inference. This study proposes a method to directly invert observed radial waveforms and to better account for data noise in a Bayesian formulation. We illustrate its feasibility with two synthetic tests having different types of noises added to seismograms. Then, a real site application is performed to obtain the 1-D S-wave velocity structure beneath a seismic station located in the Tengchong volcanic area, Southwestern China. Surface wave dispersion measurements spanning periods from 8 to 65 s are jointly inverted with P waveforms. The results show a complex S-wave velocity structure, as two low velocity zones are observed in the crust and uppermost mantle, suggesting the existence of magma chambers, or zones of partial melt. The upper magma chambers may be the heart source that cause the thermal activity on the surface.

Robust automated classification of first-motion polarities for focal mechanism determination with machine learning

NASA Astrophysics Data System (ADS)

Ross, Z. E.; Meier, M. A.; Hauksson, E.

2017-12-01

Accurate first-motion polarities are essential for determining earthquake focal mechanisms, but are difficult to measure automatically because of picking errors and signal to noise issues. Here we develop an algorithm for reliable automated classification of first-motion polarities using machine learning algorithms. A classifier is designed to identify whether the first-motion polarity is up, down, or undefined by examining the waveform data directly. We first improve the accuracy of automatic P-wave onset picks by maximizing a weighted signal/noise ratio for a suite of candidate picks around the automatic pick. We then use the waveform amplitudes before and after the optimized pick as features for the classification. We demonstrate the method's potential by training and testing the classifier on tens of thousands of hand-made first-motion picks by the Southern California Seismic Network. The classifier assigned the same polarity as chosen by an analyst in more than 94% of the records. We show that the method is generalizable to a variety of learning algorithms, including neural networks and random forest classifiers. The method is suitable for automated processing of large seismic waveform datasets, and can potentially be used in real-time applications, e.g. for improving the source characterizations of earthquake early warning algorithms.

Southern Mariana OBS Experiment and Preliminary Results of Passive-Source Investigations

NASA Astrophysics Data System (ADS)

Le, B. M.; Lin, J.; Yang, T.; Shiyan 3, S. P. O. R.

2017-12-01

The Southern Mariana OBS Experiment (SMOE) was one of the first seismic experiments targeting the deepest part of Earth's surface. During the Phase I experiment in December 2016, an array of OBS instruments were deployed across the Challenger Deep that recorded both active-source and passive-source data. During the Phase II experiment in December 2016-June 2017, passive-source data were recorded. We have retrieved earthquake signals and processed the waveforms from the recorded global, regional and local events, respectively, during the Phase I experiment. Most of the waveforms recorded by the OBS array have fairly good quality with discernible main phases. Rayleigh waves from many earthquakes were analyzed using the frequency-time analysis and their group velocities at different periods were obtained. The dispersion curves from different Rayleigh wave propagating paths would be valuable for inverting the structure of the subducting Pacific and overriding Philippine Sea plates. Furthermore, we applied the ambient noise cross-correlation method and retrieved high-quality coherence surface wave waveforms. With its relatively high frequencies, the surface waves can be used to study the crustal structure of the region. Together with the Phase II data, we expect that this seismic experiment will provide unprecedented constraints on the structure and geodynamic processes of the southern Mariana trench.

Monitoring temporal variations of seismic properties of the crust induced by the 2013 Ruisui earthquake in eastern Taiwan from coda wave interferometry with ambient seismic and strain fields

NASA Astrophysics Data System (ADS)

Dai, W. P.; Hung, S. H.; Wu, S. M.; Hsu, Y. J.

2017-12-01

Owing to the rapid development in ambient noise seismology, time-lapse variations in delay time and waveform decorrelation of coda derived from noise cross correlation (NCF) have been proved very effective to monitor slight changes in seismic velocity and scattering properties of the crust induced by various loadings such as the earthquake and healing process. In this study, we employ coda wave interferometry to detect the crustal perturbations immediately preceding and following the 2013 Mw 6.2 Ruisui Earthquake which struck the northern segment of the Longitudinal Valley Fault in eastern Taiwan, a seismically very active thrust suture zone separating the Eurasian and Philippine Sea Plate. By comparing the pre- and post-event coda waves extracted from the auto- and cross-correlation functions (ACFs and CCFs) of ambient seismic and strain fields recorded by the seismometers and borehole strainmeters, respectively, in the vicinity of the source region, we present a strong case that not only coseismic velocity reduction but also preceding decorrelation of waveforms are explicitly revealed in both the seismic and strain CCFs filtered in the secondary microseism frequency band of 0.1-0.9 Hz. Such precursory signals susceptible to the scattering properties of the crust are more unequivocally identified in the coda retrieved from the strainmeter data, suggesting that the ambient strain field can act as a more sensible probe to detect tiny structural perturbations in the critically stressed fault zone at the verge of failure. In addition to coseismic velocity changes detected in both the seismic and strain NCFs, we find quasi-periodic velocity variations that only appear in the strain retrieved coda signals, with a predominant cycle of 3-4 months correlating with the groundwater fluctuations observed at Ruisui.

Repeating microseismicity in the Seoul Metropolitan Area, Korea, and its implications for the seismic hazards

NASA Astrophysics Data System (ADS)

Kim, K.; Kim, W.; Kang, S.; Ryoo, Y.; Park, Y.; Kyung, J.

2013-12-01

An earthquake with magnitude 3.0 occurred in the Seoul Metropolitan Capital Area (SNCA), Korea, on 9 February 2010. The earthquake attracted much attention and raised concerns about seismic hazards and risks in the Korea Peninsula, in particular, to the SNCA. SNCA includes the Seoul and Incheon metropolitans and most of the Gyeonggi province. It has a population of 24.5 million and is one of the largest metropolitan areas in the world. We applied waveform correlation detector to 2007-2011 continuously recorded seismic data to identify repeating earthquakes. We identify 9 micro-earthquakes during 2007-2010 periods which are not reported in the KNSN bulletin because their magnitudes are too small. Estimated magnitudes using amplitude ratios measured at the station SEO indicate the smallest event detected by the waveform cross correlation technique in the study is as low as 0.19. The number of events for our interpretation becomes 11 including 2 previously reported events and 9 newly identified ones. All of them occur in a very small area. While there are historic documents reporting earthquakes in the SNCA, repeating earthquakes or clustered seismicity from the instrumental earthquake record have not reported before. We have determined the focal mechanism solution for the representative event (9 February 2010, ML 3.0) using first motions. The preferred focal mechanism solution for the representative event is the WNW-ESE striking fault, which are consistent with the precisely determined earthquake hypocenter distribution. The orientation of P-axis is also consistent with the results in the previous studies of stress orientation in and around the Korean peninsula. The new list of earthquakes in this study is far from any complete, although we have adopted a well-established method to detect earthquakes. Considering the low seismicity, extensive efforts to monitor the micro-seismicity are definitely required to obtain comprehensive picture of the seismicity pattern in the area.

Icequake Tremors During Glacier Calving (Invited)

NASA Astrophysics Data System (ADS)

Walter, F.; O'Neel, S.; Bassis, J. N.; Fricker, H. A.; Pfeffer, W. T.

2009-12-01

Calving poses the largest uncertainty in the prediction of sea-level rise in response to global climate changes. A physically-based calving law has yet to be successfully implemented into ice-sheet models in order to adequately describe the mass loss of tidewater glaciers and ice shelves. Observations from a variety of glacial environments are needed in order to develop a theoretical framework for glacier calving. To this end, several recent investigations on glacier calving have involved the recording of seismic waves. In this context, the study of icequakes has been of high value, as it allows for detecting and monitoring of calving activity. However, there are unanswered fundamental questions concerning source aspects of calving-related seismic activity, such as focal depths of icequakes preceding and accompanying calving events, failure mechanisms and the role of fracturing and crevasse formation upstream from the glacier terminus. Icequake sources associated with opening of surface crevasses are well understood. As glacier ice is often homogeneous these waveforms are relatively simple and can be modeled using the moment tensor representation of a seismic point source. Calving-related seismicity, on the other hand, is more complex, and occurs near the terminus of a glacier, which is often highly heterogeneous due to pervasive crevassing. The signals last up to several minutes or even hours and exhibit both low-frequency (1-3Hz) as well as high-frequency (10-20Hz) energy or tremor-like waveforms. These characteristics can be explained by finite source properties, such as connecting and migrating fractures and repeated slip across contact planes between two bodies of ice. In this presentation we discuss sources of calving-related seismicity by comparing seismic calving records from several different glacial settings. We consider icequakes recorded during tidewater calving at Columbia Glacier, Alaska, during lake calving on Gornergletscher, Switzerland, and during ice shelf calving in Antarctica. The similarities and differences in seismic signatures of these different calving settings provide valuable insights and will be helpful in the theoretical treatment of glacier calving.

Relocation of recent seismicity and seismotectonic properties in the Gulf of Corinth (Greece)

NASA Astrophysics Data System (ADS)

Mesimeri, Maria; Karakostas, Vassilios; Papadimitriou, Eleftheria; Tsaklidis, George; Jacobs, Katrina

2018-02-01

Recent seismicity (2008-2014) taking place in the Gulf of Corinth and recorded, since the establishment of the Hellenic Unified Seismological Network is relocated in this study. All the available P and S manually picked phases along with the waveforms of 55 broad-band, three-component seismological stations were used. The relocation is performed using the double difference method with differential times derived from phase-picked data and waveform cross-correlation. The accuracy of the relocated catalogue, estimated using a bootstrap approach, is of the order of few hundred metres. In an attempt to define the stress regime in the area, we compute moment tensors of 72 earthquakes with ML ≥ 3.0 and use them to calculate the total seismic moment tensor. A dominant strike of 270° that found in the westernmost part, was changed to 270°-290° at the centre of the gulf, perpendicular to the almost N-S extension of the rift. Further to the east, a gradual change in fault orientation is observed. In the easternmost part, the strike becomes 240°, in agreement with the geometry of the rift. The highly accurate earthquake catalogue, consisting of ˜26 000 events, reveals two patterns of activity in the western Corinth Gulf, namely, strongly clustered seismicity in both space and time in shallow depths and below that activity a very narrow shallow north-dipping seismic zone. Earthquake clusters, mainly located in the western study area, are identified using CURATE algorithm and associated with different north or south-dipping fault segments. The seismicity in the shallow north-dipping seismic zone, defined in detail in this study, is continuous and free of earthquake clusters. This continuous activity most probably defines the boundaries between brittle and ductile layers. The central and eastern parts of the study area mainly accommodate spatiotemporal clusters.

Automated Interval velocity picking for Atlantic Multi-Channel Seismic Data

NASA Astrophysics Data System (ADS)

Singh, Vishwajit

2016-04-01

This paper described the challenge in developing and testing a fully automated routine for measuring interval velocities from multi-channel seismic data. Various approaches are employed for generating an interactive algorithm picking interval velocity for continuous 1000-5000 normal moveout (NMO) corrected gather and replacing the interpreter's effort for manual picking the coherent reflections. The detailed steps and pitfalls for picking the interval velocities from seismic reflection time measurements are describe in these approaches. Key ingredients these approaches utilized for velocity analysis stage are semblance grid and starting model of interval velocity. Basin-Hopping optimization is employed for convergence of the misfit function toward local minima. SLiding-Overlapping Window (SLOW) algorithm are designed to mitigate the non-linearity and ill- possessedness of root-mean-square velocity. Synthetic data case studies addresses the performance of the velocity picker generating models perfectly fitting the semblance peaks. A similar linear relationship between average depth and reflection time for synthetic model and estimated models proposed picked interval velocities as the starting model for the full waveform inversion to project more accurate velocity structure of the subsurface. The challenges can be categorized as (1) building accurate starting model for projecting more accurate velocity structure of the subsurface, (2) improving the computational cost of algorithm by pre-calculating semblance grid to make auto picking more feasible.

Delineation, Characterization and Assessment of Gas-hydrates: Examples from Indian Offshore

NASA Astrophysics Data System (ADS)

Sain, K.

2017-12-01

Successful test productions in McKenzie delta, Alaska, Nankai Trough and more recently in South China Sea have provided great hopes for production of gas-hydrates in near future, and boosted national programs of many countries including India. It has been imperative to map the prospective zones of gas-hydrates and evaluate their resource potential. Hence, we have adopted a systematic strategy for the delineation, characterization and quantification of gas-hydrates based on seismic traveltime tomography, full-waveform inversion, impedance inversion, attributes computation and rock-physical modeling. The bathymetry, seafloor temperature, total organic carbon content, sediment-thickness, rate of sedimentation, geothermal gradient imply that shallow sediments of Indian deep water are good hosts for occurrences of gas-hydrates. From the analysis of multi-channel seismic (MCS) data, we have identified the Krishna-Godavari (KG), Mahanadi and Andaman basins as prospective for gas-hydrates, and their presence has been validated by drilling and coring of Indian Expeditions-01 and -02. The MCS data also shows BSR-like features in the Cauvery, Kerala-Konkan and Saurashtra basins indicating that gas-hydrates cannot be ruled out from these basins also. We shall present several approaches that have been applied to field seismic and well-log data for the detection, characterization and quantification of gas-hydrates along the Indian margin.

Moment Tensor Analysis of Shallow Sources

NASA Astrophysics Data System (ADS)

Chiang, A.; Dreger, D. S.; Ford, S. R.; Walter, W. R.; Yoo, S. H.

2015-12-01

A potential issue for moment tensor inversion of shallow seismic sources is that some moment tensor components have vanishing amplitudes at the free surface, which can result in bias in the moment tensor solution. The effects of the free-surface on the stability of the moment tensor method becomes important as we continue to investigate and improve the capabilities of regional full moment tensor inversion for source-type identification and discrimination. It is important to understand these free surface effects on discriminating shallow explosive sources for nuclear monitoring purposes. It may also be important in natural systems that have shallow seismicity such as volcanoes and geothermal systems. In this study, we apply the moment tensor based discrimination method to the HUMMING ALBATROSS quarry blasts. These shallow chemical explosions at approximately 10 m depth and recorded up to several kilometers distance represent rather severe source-station geometry in terms of vanishing traction issues. We show that the method is capable of recovering a predominantly explosive source mechanism, and the combined waveform and first motion method enables the unique discrimination of these events. Recovering the correct yield using seismic moment estimates from moment tensor inversion remains challenging but we can begin to put error bounds on our moment estimates using the NSS technique.

A multi-frequency receiver function inversion approach for crustal velocity structure

NASA Astrophysics Data System (ADS)

Li, Xuelei; Li, Zhiwei; Hao, Tianyao; Wang, Sheng; Xing, Jian

2017-05-01

In order to constrain the crustal velocity structures better, we developed a new nonlinear inversion approach based on multi-frequency receiver function waveforms. With the global optimizing algorithm of Differential Evolution (DE), low-frequency receiver function waveforms can primarily constrain large-scale velocity structures, while high-frequency receiver function waveforms show the advantages in recovering small-scale velocity structures. Based on the synthetic tests with multi-frequency receiver function waveforms, the proposed approach can constrain both long- and short-wavelength characteristics of the crustal velocity structures simultaneously. Inversions with real data are also conducted for the seismic stations of KMNB in southeast China and HYB in Indian continent, where crustal structures have been well studied by former researchers. Comparisons of inverted velocity models from previous and our studies suggest good consistency, but better waveform fitness with fewer model parameters are achieved by our proposed approach. Comprehensive tests with synthetic and real data suggest that the proposed inversion approach with multi-frequency receiver function is effective and robust in inverting the crustal velocity structures.

Gas Reservoir Identification Basing on Deep Learning of Seismic-print Characteristics

NASA Astrophysics Data System (ADS)

Cao, J.; Wu, S.; He, X.

2016-12-01

Reservoir identification based on seismic data analysis is the core task in oil and gas geophysical exploration. The essence of reservoir identification is to identify the properties of rock pore fluid. We developed a novel gas reservoir identification method named seismic-print analysis by imitation of the vocal-print analysis techniques in speaker identification. The term "seismic-print" is referred to the characteristics of the seismic waveform which can identify determinedly the property of the geological objectives, for instance, a nature gas reservoir. Seismic-print can be characterized by one or a few parameters named as seismic-print parameters. It has been proven that gas reservoirs are of characteristics of negative 1-order cepstrum coefficient anomaly and Positive 2-order cepstrum coefficient anomaly, concurrently. The method is valid for sandstone gas reservoir, carbonate reservoir and shale gas reservoirs, and the accuracy rate may reach up to 90%. There are two main problems to deal with in the application of seismic-print analysis method. One is to identify the "ripple" of a reservoir on the seismogram, and another is to construct the mapping relationship between the seismic-print and the gas reservoirs. Deep learning developed in recent years is of the ability to reveal the complex non-linear relationship between the attribute and the data, and of ability to extract automatically the features of the objective from the data. Thus, deep learning could been used to deal with these two problems. There are lots of algorithms to carry out deep learning. The algorithms can be roughly divided into two categories: Belief Networks Network (DBNs) and Convolutional Neural Network (CNN). DBNs is a probabilistic generative model, which can establish a joint distribution of the observed data and tags. CNN is a feedforward neural network, which can be used to extract the 2D structure feature of the input data. Both DBNs and CNN can be used to deal with seismic data. We use an improved DBNs to identify carbonate rocks from log data, the accuracy rate can reach up to 83%. DBNs is used to deal with seismic waveform data, more information is obtained. The work was supported by NSFC under grant No. 41430323 and No. 41274128, and State Key Lab. of Oil and Gas Reservoir Geology and Exploration.

Evidence of Fluid/Gas Along the Backthrust Offshore Central Sumatra from Seismic Full Waveform Inversion

NASA Astrophysics Data System (ADS)

Singh, S. C.; Huot, G.

2014-12-01

The Sumatra subduction zone is one of the most seismically active zones on the earth and has experienced great earthquakes in the last decade, including one of the largest earthquakes of the 21st century on December 26, 2004 (M 9.3) producing a devastating tsunami. Geodetic and seismological studies suggest that a part of the subduction is still fully locked and may produce a great earthquake up to Mw=9 in the coming future. In order to understand the earthquake and subduction zone processes, we applied elastic full waveform inversion (FWI) on 50km deep seismic reflection data from Benkulu forearc basin. Data were acquired in partnership with CGG in 2009 using a 15 km ultra-long offset streamer, with shots every 50m and receivers every 12.5m. Unlike usual exploration data, the streamer was dived at 22.5m and the source was triggered at 15m. These characteristics provided wide aperture and low frequencies to the data which are required for FWI. To match our modelling with the observed data before applying FWI, we first estimated the source wavelet from simulated far-offset signature. Then, the same pass-band filter was applied to both the source and the observed data from 1 to 15 Hz frequency bandwidth. Additionally, an f-k filter was applied to the real data to remove the remaining swell noise up to 4 Hz, without losing any useful signal. Our starting 2D Vp model was found by applying traveltime tomography to a smooth 1D model. Our Vs model was linked by empirical relationship to our Vp model. By inverting first for far-offset refraction to recover the background velocity model and using a multi-scale strategy by going from 1Hz to 15Hz, we image the forearc basin down to 10km depth. We highlight the presence of a 10km wide low velocity structure at 3km depth. The sharp velocity change from 4km/s to 2km/s indicates the presence of gaz or fluid which could come from the mantle through the backthrust fault. The complexity of the sea floor makes the reflection challenging to invert but could give higher resolution. We are expecting improvement in the next months.

Improving seismic crustal models in the Corinth Gulf, Greece and estimating source depth using PL-waves

NASA Astrophysics Data System (ADS)

Vackář, Jiří; Zahradník, Jiří

2013-04-01

A recent shallow earthquake in the Corinth Gulf, Greece (Mw 5.3, January 18, 2010; Sokos et al., Tectonophysics 2012) generated unusual long-period waves (periods > 5 seconds), well recorded at several near-regional stations between the P - and S-wave arrival. The 5-second period, being significantly longer than the source duration, indicates a structural effect. The wave is similar to PL-wave or Pnl-wave, but with shorter periods and observed in much closer distances (ranging from 30 to 200 km). For theoretical description of the observed wave, structural model is required. No existing regional crustal model generates that wave, so we need to find another model, better in terms of the PL-wave existence and strength. We find such models by full waveform inversion using the subset of stations with strong PL-wave. The Discrete Wavenumber method (Bouchon, 1981; Coutant 1989) is used for forward problem and the Neighborhood Algorithm (Sambridge, 1999) for stochastic search (more details in poster by V. Plicka and J. Zahradník). We obtain a suite of models well fitting synthetic seismograms and use some of these models to evaluate dependence of the studied waves on receiver distance and azimuth as well as dependence on source depth. We compare real and synthetic dispersion curves (derived from synthetic seismograms) as an independent validation of found model and discuss limitations of using dispersion curves for these cases. We also relocated the event in the new model. Then we calculate the wavefield by two other methods: modal summation and ray theory to better understand the nature of the PL-wave. Finally, we discuss agreement of found models with published crustal models in the region. The full waveform inversion for structural parameters seems to be powerful tool for improving seismic source modeling in cases we do not have accurate structure model of studied area. We also show that the PL-wave strength has a potential to precise the earthquake depth. Acknowledgement: Seismograms of the Hellenic Unified Seismic Network were used, including the stations co-operated by the Charles University in Prague. The research was financially supported from the following grants in the Czech Republic: GACR 210/11/0854 and MSM 0021620860.

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.

New Frontiers in Characterization of Sub-Catalog Microseismicity: Utilizing Inter-Event Waveform Cross Correlation for Estimating Precise Locations, Magnitudes, and Focal Mechanisms of Tiny Earthquakes

NASA Astrophysics Data System (ADS)

Ellsworth, W. L.; Shelly, D. R.; Hardebeck, J.; Hill, D. P.

2017-12-01

Microseismicity often conveys the most direct information about active processes in the earth's subsurface. However, routine network processing typically leaves most earthquakes uncharacterized. These "sub-catalog" events can provide critical clues to ongoing processes in the source region. To address this issue, we have developed waveform-based processing that leverages the existing routine catalog of earthquakes to detect and characterize "sub-catalog" events (those absent in routine catalogs). By correlating waveforms of cataloged events with the continuous data stream, we 1) identify events with similar waveform signatures in the continuous data across multiple stations, 2) precisely measure relative time lags across these stations for both P- and S-wave time windows, and 3) estimate the relative polarity between events by the sign of the peak absolute value correlations and its height above the secondary peak. When combined, these inter-event comparisons yield robust measurements, which enable sensitive event detection, relative relocation, and relative magnitude estimation. The most recent addition, focal mechanisms derived from correlation-based relative polarities, addresses a significant shortcoming in microseismicity analyses (see Shelly et al., JGR, 2016). Depending on the application, we can characterize 2-10 times as many events as included in the initial catalog. This technique is particularly well suited for compact zones of active seismicity such as seismic swarms. Application to a 2014 swarm in Long Valley Caldera, California, illuminates complex patterns of faulting that would have otherwise remained obscured. The prevalence of such features in other environments remains an important, as yet unresolved, question.

Seismic detection and analysis of icequakes at Columbia Glacier, Alaska

USGS Publications Warehouse

O'Neel, Shad; Marshall, Hans P.; McNamara, Daniel E.; Pfeffer, William Tad

2007-01-01

Contributions to sea level rise from rapidly retreating marine-terminating glaciers are large and increasing. Strong increases in iceberg calving occur during retreat, which allows mass transfer to the ocean at a much higher rate than possible through surface melt alone. To study this process, we deployed an 11-sensor passive seismic network at Columbia Glacier, Alaska, during 2004–2005. We show that calving events generate narrow-band seismic signals, allowing frequency domain detections. Detection parameters were determined using direct observations of calving and validated using three statistical methods and hypocenter locations. The 1–3 Hz detections provide a good measure of the temporal distribution and size of calving events. Possible source mechanisms for the unique waveforms are discussed, and we analyze potential forcings for the observed seismicity.

A High-Resolution View of Global Seismicity

NASA Astrophysics Data System (ADS)

Waldhauser, F.; Schaff, D. P.

2014-12-01

We present high-precision earthquake relocation results from our global-scale re-analysis of the combined seismic archives of parametric data for the years 1964 to present from the International Seismological Centre (ISC), the USGS's Earthquake Data Report (EDR), and selected waveform data from IRIS. We employed iterative, multistep relocation procedures that initially correct for large location errors present in standard global earthquake catalogs, followed by a simultaneous inversion of delay times formed from regional and teleseismic arrival times of first and later arriving phases. An efficient multi-scale double-difference (DD) algorithm is used to solve for relative event locations to the precision of a few km or less, while incorporating information on absolute hypocenter locations from catalogs such as EHB and GEM. We run the computations on both a 40-core cluster geared towards HTC problems (data processing) and a 500-core HPC cluster for data inversion. Currently, we are incorporating waveform correlation delay time measurements available for events in selected regions, but are continuously building up a comprehensive, global correlation database for densely distributed events recorded at stations with a long history of high-quality waveforms. The current global DD catalog includes nearly one million earthquakes, equivalent to approximately 70% of the number of events in the ISC/EDR catalogs initially selected for relocation. The relocations sharpen the view of seismicity in most active regions around the world, in particular along subduction zones where event density is high, but also along mid-ocean ridges where existing hypocenters are especially poorly located. The new data offers the opportunity to investigate earthquake processes and fault structures along entire plate boundaries at the ~km scale, and provides a common framework that facilitates analysis and comparisons of findings across different plate boundary systems.

Seismic Waveform Tomography of the Iranian Region

NASA Astrophysics Data System (ADS)

Maggi, A.; Priestley, K.; Jackson, J.

2001-05-01

Surprisingly little is known about the detailed velocity structure of Iran, despite the region's importance in the tectonics of the Middle East. Previous studies have concentrated mainly on fundamental mode surface wave dispersion measurements along isolated paths (e.g.~Asudeh, 1982; Cong & Mitchell, 1998; Ritzwoller et.~al, 1998), and the propagation characteristics of crust and upper mantle body waves (e.g. Hearn & Ni 1994; Rodgers et.~al 1997). We use the partitioned waveform inversion method of Nolet (1990) on several hundred regional waveforms crossing the Iranian region to produce a 3-D seismic velocity map for the crust and upper mantle of the area. The method consists of using long period seismograms from earthquakes with well determined focal mechanisms and depths to constrain 1-D path-averaged shear wave models along regional paths. The constraints imposed on the 1-D models by the seismograms are then combined with independent constraints from other methods (e.g.~Moho depths from reciever function analysis etc.), to solve for the 3-D seismic velocity structure of the region. A dense coverage of fundamental mode rayleigh waves at a period of 100~s ensures good resolution of lithospheric scale structure. We also use 20~s period fundamental mode rayleigh waves and some Pnl wavetrains to make estimates of crustal thickness variations and average crustal velocities. A few deeper events give us some coverage of higher mode rayleigh waves and mantle S waves, which sample to the base of the upper mantle. Our crustal thickness estimates range from 45~km in the southern Zagros mountains, to 40~km in central Iran and 35~km towards the north of the region. We also find inconsistencies between the 1-D models required to fit the vertical and the tranverse seismograms, indicating the presence of anisotropy.

Crustal Stress and Strain Distribution in Sicily (Southern Italy) from Joint Analysis of Seismicity and Geodetic Data

NASA Astrophysics Data System (ADS)

Presti, D.; Neri, G.; Aloisi, M.; Cannavo, F.; Orecchio, B.; Palano, M.; Siligato, G.; Totaro, C.

2014-12-01

An updated database of earthquake focal mechanisms is compiled for the Sicilian region (southern Italy) and surrounding off-shore areas where the Nubia-Eurasia convergence coexists with the very-slow residual rollback of the Ionian subducting slab. High-quality solutions selected from literature and catalogs have been integrated with new solutions estimated in the present work using the Cut And Paste (CAP) waveform inversion method. In the CAP algorithm (Zhao and Helmberger, 1994; Zhu and Helmberger, 1996), each waveform is broken up into Pnl and surface wave segments, which are weighted differently during the inversion procedure. Integration of the new solutions with the ones selected from literature and official catalogs led us to collect a database consisting exclusively of waveform inversion data relative to earthquakes with minimum magnitude 2.6. The seismicity and focal mechanism distributions have been compared with crustal motion and strain data coming from GNSS analyses. For this purpose GNSS-based observations collected over the investigated area by episodic measurements (1994-2013) as well as continuous monitoring (since 2006) were processed by the GAMIT/GLOBK software packages (Herring et al., 2010) following the approach described in Palano et al. (2011). To adequately investigate the crustal deformation pattern, the estimated GNSS velocities were aligned to a fixed Eurasian reference frame. The good agreement found between seismic and geodetic information contributes to better define seismotectonic domains characterized by different kinematics. Moving from the available geophysical information and from an early application of FEM algorithms, we have also started to investigate stress/strain fields in the crust of the study area including depth dependence and relationships with rupture of the main seismogenic structures.

Analytical Computation of Effective Grid Parameters for the Finite-Difference Seismic Waveform Modeling With the PREM, IASP91, SP6, and AK135

NASA Astrophysics Data System (ADS)

Toyokuni, G.; Takenaka, H.

2007-12-01

We propose a method to obtain effective grid parameters for the finite-difference (FD) method with standard Earth models using analytical ways. In spite of the broad use of the heterogeneous FD formulation for seismic waveform modeling, accurate treatment of material discontinuities inside the grid cells has been a serious problem for many years. One possible way to solve this problem is to introduce effective grid elastic moduli and densities (effective parameters) calculated by the volume harmonic averaging of elastic moduli and volume arithmetic averaging of density in grid cells. This scheme enables us to put a material discontinuity into an arbitrary position in the spatial grids. Most of the methods used for synthetic seismogram calculation today receives the blessing of the standard Earth models, such as the PREM, IASP91, SP6, and AK135, represented as functions of normalized radius. For the FD computation of seismic waveform with such models, we first need accurate treatment of material discontinuities in radius. This study provides a numerical scheme for analytical calculations of the effective parameters for an arbitrary spatial grids in radial direction as to these major four standard Earth models making the best use of their functional features. This scheme can analytically obtain the integral volume averages through partial fraction decompositions (PFDs) and integral formulae. We have developed a FORTRAN subroutine to perform the computations, which is opened to utilization in a large variety of FD schemes ranging from 1-D to 3-D, with conventional- and staggered-grids. In the presentation, we show some numerical examples displaying the accuracy of the FD synthetics simulated with the analytical effective parameters.

Upper mantle velocity structure beneath southern Africa from modeling regional seismic data

NASA Astrophysics Data System (ADS)

Zhao, Ming; Langston, Charles A.; Nyblade, Andrew A.; Owens, Thomas J.

1999-03-01

The upper mantle seismic velocity structure beneath southern Africa is investigated using travel time and waveform data which come from a large mine tremor in South Africa (mb 5.6) recorded by the Tanzania broadband seismic experiment and by several stations in southern Africa. The waveform data show upper mantle triplications for both the 410- and 670-km discontinuities between distances of 2100 and 3000 km. Auxiliary travel time data along similar profiles obtained from other moderate events are also used. P wave travel times are inverted for velocity structure down to ˜800-km depth using the Wiechert-Herglotz technique, and the resulting model is evaluated by perturbing it at three depth intervals and then testing the perturbed model against the travel time and waveform data. The results indicate a typical upper mantle P wave velocity structure for a shield. P wave velocities from the top of the mantle down to 300-km depth are as much as 3% higher than the global average and are slightly slower than the global average between 300- and 420-km depth. Little evidence is found for a pronounced low-velocity zone in the upper mantle. A high-velocity gradient zone is required above the 410-km discontinuity, but both sharp and smooth 410-km discontinuities are permitted by the data. The 670-km discontinuity is characterized by high-velocity gradients over a depth range of ˜80 km around 660-km depth. Limited S wave travel time data suggest fast S wave velocities above ˜150-km depth. These results suggest that the bouyant support for the African superswell does not reside at shallow depths in the upper mantle.