Seismic structure and segmentation of the axial valley of the Mid-Cayman Spreading Center

NASA Astrophysics Data System (ADS)

Van Avendonk, Harm J. A.; Hayman, Nicholas W.; Harding, Jennifer L.; Grevemeyer, Ingo; Peirce, Christine; Dannowski, Anke

2017-06-01

We report the results of a two-dimensional tomographic inversion of marine seismic refraction data from an array of ocean-bottom seismographs (OBSs), which produced an image of the crustal structure along the axial valley of the ultraslow spreading Mid-Cayman Spreading Center (MCSC). The seismic velocity model shows variations in the thickness and properties of the young oceanic crust that are consistent with the existence of two magmatic-tectonic segments along the 110 km long spreading center. Seismic wave speeds are consistent with exhumed mantle at the boundary between these two segments, but changes in the vertical gradient of seismic velocity suggest that volcanic crust occupies most of the axial valley seafloor along the seismic transect. The two spreading segments both have a low-velocity zone (LVZ) several kilometers beneath the seafloor, which may indicate the presence of shallow melt. However, the northern segment also has low seismic velocities (3 km/s) in a thick upper crustal layer (1.5-2.0 km), which we interpret as an extrusive volcanic section with high porosity and permeability. This segment hosts the Beebe vent field, the deepest known high-temperature black smoker hydrothermal vent system. In contrast, the southern spreading segment has seismic velocities as high as 4.0 km/s near the seafloor. We suggest that the porosity and permeability of the volcanic crust in the southern segment are much lower, thus limiting deep seawater penetration and hydrothermal recharge. This may explain why no hydrothermal vent system has been found in the southern half of the MCSC.

Merging seismic and MT in Garden Valley, Nevada

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

Telleen, K.E.

1986-04-01

In the northern part of Garden Valley, Nevada, a 1978 regional seismic program encountered a large area of poor to no-reflection data. Surface geology suggested that a large high structure might underlie the valley floor, and that shallowly buried basalts were causing the poor data. The implied strongly layered structure of electrical resistivity - resistive basalt on conductive Tertiary clastics on resistive paleozoic carbonates - formed an ideal theater for the magnetotelluric method. In 1984, Conoco acquired 48 magnetotelluric sites on about a half-mile grid. These data supported the presence of a buried high block in the Paleozoic rocks andmore » allowed confident mapping of its outlines. In addition, the magnetotelluric survey showed a thin, shallowly buried resistor coextensive with the seismic no-reflection area. In 1985, a high-effort repeat of the earlier no-reflection seismic line confirmed the high block, improved the fault interpretation, and provided weak guidance on the depth of the targeted Paleozoic rocks. Because Garden Valley's Paleozoic stratigraphy differs negligibly from that at nearby Grant Canyon field, the high block constitutes an attractive prospect - possibly the first one found in Nevada due largely to magnetotelluric surveying.« less

Salton Seismic Imaging Project Line 6: San Andreas Fault and Northern Coachella Valley Structure, Riverside and San Bernardino Counties, California

NASA Astrophysics Data System (ADS)

Catchings, R. D.; Fuis, G.; Rymer, M. J.; Goldman, M.; Tarnowski, J. M.; Hole, J. A.; Stock, J. M.; Matti, J. C.

2012-12-01

The Salton Seismic Imaging Project (SSIP) is a large-scale, active- and passive-source seismic project designed to image the San Andreas fault (SAF) and adjacent basins (Imperial and Coachella Valleys) in southernmost California. Data and preliminary results from many of the seismic profiles are reported elsewhere (including Fuis et al., Rymer et al., Goldman et al., Langenheim et al., this meeting). Here, we focus on SSIP Line 6, one of four 2-D seismic profiles that were acquired across the Coachella Valley. The 44-km-long, SSIP-Line-6 seismic profile extended from the east flank of Mt. San Jacinto northwest of Palm Springs to the Little San Bernardino Mountains and crossed the SAF (Mission Creek (MCF), Banning (BF), and Garnet Hill (GHF) strands) roughly normal to strike. Data were generated by 10 downhole explosive sources (most spaced about 3 to 5 km apart) and were recorded by approximately 347 Texan seismographs (average spacing 126 m). We used first-arrival refractions to develop a P-wave refraction tomography velocity image of the upper crust along the seismic profile. The seismic data were also stacked and migrated to develop low-fold reflection images of the crust. From the surface to about 7 km depth, P-wave velocities range from about 2.5 km/s to about 7.2 km/s, with the lowest velocities within an ~2-km-deep, ~20-km-wide basin, and the highest velocities below the transition zone from the Coachella Valley to Mt. San Jacinto and within the Little San Bernardino Mountains. The BF and GHF strands bound a shallow sub-basin on the southwestern side of the Coachella Valley, but the underlying shallow-depth (~4 km) basement rocks are P-wave high in velocity (~7.2 km/s). The lack of a low-velocity zone beneath BF and GHF suggests that both faults dip northeastward. In a similar manner, high-velocity basement rocks beneath the Little San Bernardino Mountains suggest that the MCF dips vertically or southwestward. However, there is a pronounced low-velocity zone

Bedrock mapping of buried valley networks using seismic reflection and airborne electromagnetic data

NASA Astrophysics Data System (ADS)

Oldenborger, G. A.; Logan, C. E.; Hinton, M. J.; Pugin, A. J.-M.; Sapia, V.; Sharpe, D. R.; Russell, H. A. J.

2016-05-01

In glaciated terrain, buried valleys often host aquifers that are significant groundwater resources. However, given the range of scales, spatial complexity and depth of burial, buried valleys often remain undetected or insufficiently mapped. Accurate and thorough mapping of bedrock topography is a crucial step in detecting and delineating buried valleys and understanding formative valley processes. We develop a bedrock mapping procedure supported by the combination of seismic reflection data and helicopter time-domain electromagnetic data with water well records for the Spiritwood buried valley aquifer system in Manitoba, Canada. The limited spatial density of water well bedrock observations precludes complete depiction of the buried valley bedrock topography and renders the water well records alone inadequate for accurate hydrogeological model building. Instead, we leverage the complementary strengths of seismic reflection and airborne electromagnetic data for accurate local detection of the sediment-bedrock interface and for spatially extensive coverage, respectively. Seismic reflection data are used to define buried valley morphology in cross-section beneath survey lines distributed over a regional area. A 3D model of electrical conductivity is derived from inversion of the airborne electromagnetic data and used to extrapolate buried valley morphology over the entire survey area. A spatially variable assignment of the electrical conductivity at the bedrock surface is applied to different features of the buried valley morphology identified in the seismic cross-sections. Electrical conductivity is then used to guide construction of buried valley shapes between seismic sections. The 3D locus of points defining each morphological valley feature is constructed using a path optimization routine that utilizes deviation from the assigned electrical conductivities as the cost function. Our resulting map represents a bedrock surface of unprecedented detail with more

Seismological investigation of earthquakes in the New Madrid seismic zone and the northeastern extent of the New Madrid seismic zone: Final report, September 1981-December 1986

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

Herrmann, R.B.; Taylor, K.; Nguyen, B.

1988-07-01

Earthquake activity in the Central Mississippi Valley has been monitored by an eight station seismograph network in the Wabash River Valley of southeastern Illinois and by a six station seismograph network in the New Madrid seismic zone. This network is a major component of a larger network in the region, jointly sponsored by the NRC, USGS, universities and states. During the time period of the contract, October 1981 through December 1986, 1206 earthquakes were located in the Central Mississippi Valley, of which 808 were in the New Madrid, Missouri area. Significant earthquakes studied in detail occurred in northeastern Ohio onmore » January 31, 1986 and in southeastern Illinois on June 10, 1987. Focal mechanisms have been calculated for the 10 June 1987 southern Illinois earthquake using both P-wave first motions and long-period surface-wave spectral amplitude data. The long-period surface-wave and strong ground motion accelerogram recordings of the January 3, 1986, northeastern Ohio earthquake were used to estimate the focal mechanism and source time function of the source.reverse arrow« less

Necessary Conditions for Intraplate Seismic Zones in North America

NASA Astrophysics Data System (ADS)

Thomas, William A.; Powell, Christine A.

2017-12-01

The cause of intraplate seismic zones persists as an important scientific and societal question. Most intraplate earthquakes are concentrated in specific seismic zones along or adjacent to large-scale basement structures (e.g., rifts or sutures at ancient plate boundaries) within continental crust. The major intraplate seismic zones are limited to specific segments and are not distributed along the lengths of the ancient structures. We present a new hypothesis that major intraplate seismic zones are restricted to places where concentrated crustal deformation (CCD) is overprinted on large-scale basement structures. Examples where CCD affects the stability of specific parts of large-scale structures in response to present-day stress conditions include the most active seismic zones in central and eastern North America: Charlevoix, Eastern Tennessee, and New Madrid. Our hypothesis has important implications for the assessment of seismic hazards.

40 CFR 258.14 - Seismic impact zones.

Code of Federal Regulations, 2010 CFR

2010-07-01

... 40 Protection of Environment 24 2010-07-01 2010-07-01 false Seismic impact zones. 258.14 Section 258.14 Protection of Environment ENVIRONMENTAL PROTECTION AGENCY (CONTINUED) SOLID WASTES CRITERIA FOR MUNICIPAL SOLID WASTE LANDFILLS Location Restrictions § 258.14 Seismic impact zones. (a) New MSWLF units and...

40 CFR 258.14 - Seismic impact zones.

Code of Federal Regulations, 2011 CFR

2011-07-01

... 40 Protection of Environment 25 2011-07-01 2011-07-01 false Seismic impact zones. 258.14 Section 258.14 Protection of Environment ENVIRONMENTAL PROTECTION AGENCY (CONTINUED) SOLID WASTES CRITERIA FOR MUNICIPAL SOLID WASTE LANDFILLS Location Restrictions § 258.14 Seismic impact zones. (a) New MSWLF units and...

Seismic-refraction study of suspected drift-filled bedrock valleys in Ramsey County, Minnesota

USGS Publications Warehouse

Woodward, D.G.

1985-01-01

A drift-filled bedrock valley was thought to incise the St. Peter aquifer to an altitude between 770 and 800 feet above sea level at the Koppers site. The interpretation of a seismic profile just east of the Koppers site is not conclusive, but suggests that a bedrock valley may be present near the middle of the line. The interpretation of a second seismic profile across the westward extension of the same suspected valley also is not conclusive, but suggests that a bedrock valley may be present at the north end of the line. The optimal field layout for each line at the site (longer shot offsets) could not be obtained because of limited space available in the densely developed residential neighborhoods.

GPS measurements of crustal deformation across the southern Arava Valley section of the Dead Sea Fault and implications to regional seismic hazard assessment

NASA Astrophysics Data System (ADS)

Hamiel, Yariv; Masson, Frederic; Piatibratova, Oksana; Mizrahi, Yaakov

2018-01-01

Detailed analysis of crustal deformation along the southern Arava Valley section of the Dead Sea Fault is presented. Using dense GPS measurements we obtain the velocities of new near- and far-field campaign stations across the fault. We find that this section is locked with a locking depth of 19.9 ± 7.7 km and a slip rate of 5.0 ± 0.8 mm/yr. The geodetically determined locking depth is found to be highly consistent with the thickness of the seismogenic zone in this region. Analysis of instrumental seismic record suggests that only 1% of the total seismic moment accumulated since the last large event occurred about 800 years ago, was released by small to moderate earthquakes. Historical and paleo-seismic catalogs of this region together with instrumental seismic data and calculations of Coulomb stress changes induced by the 1995 Mw 7.2 Nuweiba earthquake suggest that the southern Arava Valley section of the Dead Sea Fault is in the late stage of the current interseismic period.

Seismicity of the Indo-Australian/Solomon Sea Plate boundary in the Southeast Papua region

NASA Astrophysics Data System (ADS)

Ripper, I. D.

1982-08-01

Seismicity and earthquake focal mechanism plots of the Southeast Papua and Woodlark Basin region for the period January 1960 to May 1979 show that: (a) the West Woodlark Basin spreading centre extends from the deep West Woodlark Basin, through Dawson Strait into Goodenough Bay, Southeast Papua; (b) a southeast seismic trend in the West Woodlark Basin is associated with a left-lateral transform fault, but a gap exists between this zone and the seismic East Woodlark Basin spreading centre; (c) Southeast Papua Seismicity divides into a shallow earthquake zone in which the earthquakes occur mainly in the northeast side of the Owen Stanley Range, and an intermediate depth southwest dipping Benioff zone which extends almost from Mt. Lamington to Goroka. The Benioff zone indicates the presence of a southwest dipping slab of Solomon Sea Plate beneath the Indo-Australian Plate in the Southeast Papua and Ramu-Markham Valley region. This subduction zone has collided with the New Britain subduction zone of the Solomon Sea Plate along the Ramu-Markham Valley. The Solomon Sea Plate is now hanging suspended in the form of an arch beneath Ramu-Markham Valley, inhibiting further subduction beneath Southeast Papua.

Constraints on Shallow Crustal Structure across the San Andreas Fault Zone, Coachella Valley, Southern California: Results from the Salton Seismic Imaging Project (SSIP)

NASA Astrophysics Data System (ADS)

Hernandez, A.; Persaud, P.; Bauer, K.; Stock, J. M.; Fuis, G. S.; Hole, J. A.; Goldman, M.

2015-12-01

The strong influence of basin structure and crustal heterogeneities on seismic wave propagation suggests that these factors should be included in calculations of strong ground shaking. Knowledge of the shallow subsurface is thus essential for an accurate seismic hazard estimate for the densely populated Coachella Valley, the region north of the potential M7.8 rupture near the Salton Sea. Using SSIP data, we analyzed first arrivals from nine 65-911 kg explosive shots recorded along a profile in the Coachella Valley in order to evaluate the interpretation of our 2D tomographic results and give added details on the structural complexity of the shallow crust. The line extends 37 km from the Peninsular Ranges to the Little San Bernardino Mountains crossing the major strands of the San Andreas Fault Zone. We fit traveltime curves to our picks with forward modeling ray tracing, and determined 1D P-wave velocity models for traveltime arrivals east and west of each shot, and a 2D model for the line. We also inferred the geometry of near-vertical faults from the pre-stack line migration method of Bauer et al. (2013). In general, the 1D models east of individual shots have deeper basement contacts and lower apparent velocities, ~5 km/s at 4 km depth, whereas the models west of individual shots have shallower basement and velocities up to 6 km/s at 2 km depth. Mismatches in basement depths (assuming 5-6 km/s) between individual 1D models indicate a shallowly dipping basement, deepening eastward towards the Banning Fault and shoaling abruptly farther east. An east-dipping structure in the 2D model also gives a better fit than horizontal layers. Based on high velocity zones derived from traveltimes at 9-20 km from the western end of the line, we included an offset from ~2 km to 4 km depth near the middle of the line, which significantly improved the 2D model fit. If fault-related, this offset could represent the Garnet Hill Fault if it continues southward in the subsurface.

Constraining Subsurface Structure and Composition Using Seismic Refraction Surveys of Proglacial Valleys in the Cordillera Blanca, Peru

NASA Astrophysics Data System (ADS)

Glas, R. L.; Lautz, L.; McKenzie, J. M.; Mark, B. G.; Baker, E. A.; Aubry-Wake, C.; Somers, L. D.; Wigmore, O.

2015-12-01

As tropical glaciers rapidly recede in response to climate change, the storage and discharge of groundwater will play an increasing role in regulating river baseflow, particularly during the dry season, when stream flow is currently sustained predominantly by glacial melt. Little is understood regarding the hydrogeologic processes controlling base flow characteristics of low-gradient proglacial valleys of the Cordillera Blanca in Northwestern Peru, which has the world's highest density of tropical glaciers. To better understand the processes of groundwater storage and discharge in proglacial meadows, we completed seismic refraction surveys in three representative valleys of the Cordillera Blanca range: the Quilcayhuanca, Yanamarey, and Pachacoto valleys. The locations of survey transects were chosen based on locations of previous sediment core sampling, GPR lines, and quantification of groundwater-surface water interaction derived from dye and temperature tracing experiments. The seismic surveys consisted of 48 vertical component geophones with 2.5 m spacing. Across the three representative valleys a total of 15 surveys were conducted, covering a distance of 1800 m in cross, down, and oblique-valley directions. Preliminary interpretation of the seismic refraction data indicates a maximum imaging depth of 16 m below land surface, and a transition from glacio-lacustrine sediments to buried saturated talus at a depth of 6 m in the Quilcayhuanca valley. The organic-rich glacio-lacustrine sediments in the Yanamarey valley have seismic velocities ranging from 300 to 800 m/s and are >16 m in thickness at mid- valley. Weathered metasedimentary bedrock in the Pachacoto valley was imaged at ~5 m below the valley surface, exhibiting a p-wave velocity of 3400 m/s. The knowledge of hydrogeologic structure derived from seismic refraction surveys will provide crucial boundary conditions for future groundwater models of the valleys of the Cordillera Blanca.

The Olmsted fault zone, southernmost Illinois: A key to understanding seismic hazard in the northern new Madrid seismic zone

USGS Publications Warehouse

Bexfield, C.E.; McBride, J.H.; Pugin, Andre J.M.; Nelson, W.J.; Larson, T.H.; Sargent, S.L.

2005-01-01

Geological deformation in the northern New Madrid seismic zone, near Olmsted, Illinois (USA), is analyzed using integrated compressional-wave (P) and horizontally polarized-wave (SH) seismic reflection and regional and dedicated borehole information. Seismic hazards are of special concern because of strategic facilities (e.g., lock and dam sites and chemical plants on the Ohio River near its confluence with the Mississippi River) and because of alluvial soils subject to high amplification of earthquake shock. We use an integrated approach starting with lower resolution, but deeper penetration, P-wave reflection profiles to identify displacement of Paleozoic bedrock. Higher resolution, but shallower penetration, SH-wave images show deformation that has propagated upward from bedrock faults into Pleistocene loess. We have mapped an intricate zone more than 8 km wide of high-angle faults in Mississippi embayment sediments localized over Paleozoic bedrock faults that trend north to northeast, parallel to the Ohio River. These faults align with the pattern of epicenters in the New Madrid seismic zone. Normal and reverse offsets along with positive flower structures imply a component of strike-slip; the current stress regime favors right-lateral slip on northeast-trending faults. The largest fault, the Olmsted fault, underwent principal displacement near the end of the Cretaceous Period 65 to 70 million years ago. Strata of this age (dated via fossil pollen) thicken greatly on the downthrown side of the Olmsted fault into a locally subsiding basin. Small offsets of Tertiary and Quaternary strata are evident on high-resolution SH-wave seismic profiles. Our results imply recent reactivation and possible future seismic activity in a critical area of the New Madrid seismic zone. This integrated approach provides a strategy for evaluating shallow seismic hazard-related targets for engineering concerns. ?? 2005 Elsevier B.V. All rights reserved.

Geomorphic response to tectonically-induced ground deformation in the Wabash Valley

USGS Publications Warehouse

Fraser, G.S.; Thompson, T.A.; Olyphant, G.A.; Furer, L.; Bennett, S.W.

1997-01-01

Numerous low- to moderate-intensity earthquakes have been recorded in a zone of diffuse modern seismicity in southwest Indiana, southeast Illinois, and northernmost Kentucky. Structural elements within the zone include the Wabash Valley Fault System, the LaSalle Anticlinal Belt in western Illinois, and the Rough Creek-Shawneetown Fault System in northern Kentucky. The presence of seismically-induced liquefaction features in the near-surface alluvial sediments in the region indicates that strong ground motion has occurred in the recent geological past, but because the glacial and alluvial sediments in the Wabash Valley appear to be otherwise undisturbed, post-Paleozoic ground deformation resulting from movement on these structural elements has not yet been documented. Morphometric analysis of the land surface, detailed mapping of geomorphic elements in the valley, reconnaissance drilling of the Holocene and Pleistocene alluvium, and structural analysis of the bedrock underlying the valley were used to determine whether the geomorphology of the valley and the patterns of alluviation of the Wabash River were affected by surface deformation associated with the seismic zone during the late Pleistocene and Holocene. Among the observed features in the valley that can be attributed to deformation are: (1) tilting of the modern land surface to the west, (2) preferred channel migration toward the west side of the valley, with concomitant impact on patterns of soil development and sedimentation rate, (3) a convex longitudinal profile of the Wabash River where it crosses the LaSalle Anticlinal Belt, and (4) increased incision of the river into its floodplain downstream from the anticlinal belt.

A seismic hazard uncertainty analysis for the New Madrid seismic zone

USGS Publications Warehouse

Cramer, C.H.

2001-01-01

A review of the scientific issues relevant to characterizing earthquake sources in the New Madrid seismic zone has led to the development of a logic tree of possible alternative parameters. A variability analysis, using Monte Carlo sampling of this consensus logic tree, is presented and discussed. The analysis shows that for 2%-exceedence-in-50-year hazard, the best-estimate seismic hazard map is similar to previously published seismic hazard maps for the area. For peak ground acceleration (PGA) and spectral acceleration at 0.2 and 1.0 s (0.2 and 1.0 s Sa), the coefficient of variation (COV) representing the knowledge-based uncertainty in seismic hazard can exceed 0.6 over the New Madrid seismic zone and diminishes to about 0.1 away from areas of seismic activity. Sensitivity analyses show that the largest contributor to PGA, 0.2 and 1.0 s Sa seismic hazard variability is the uncertainty in the location of future 1811-1812 New Madrid sized earthquakes. This is followed by the variability due to the choice of ground motion attenuation relation, the magnitude for the 1811-1812 New Madrid earthquakes, and the recurrence interval for M>6.5 events. Seismic hazard is not very sensitive to the variability in seismogenic width and length. Published by Elsevier Science B.V.

A Chicken-Or-Egg Riddle: Why Do Many Large Basin and Range Ruptures Terminate in Seismically Active Geothermal Zones?

NASA Astrophysics Data System (ADS)

Stein, R. S.; Sevilgen, V.

2016-12-01

The 26 March 1872 M 7.6 Owens Valley, CA, rupture extended between two seismically active geothermal areas: Ridgecrest-Coso to the south and Mammoth-Long Valley to the north. Is this a coincidence, or is the rupture extent related to the geothermal and seismic activity? The 1872 rupture might have been confined between these two shattered, fluid-saturated zones if, because of very low friction, they do not accumulate stress. Alternatively, the 1872 earthquake could have activated these zones, as they are located where the rupture is calculated to have increased the Coulomb stress by 1-2 bars. Cause or effect, this phenomenon may be common in trans-tensional regimes: There are geothermal areas near or just beyond the ends of the 1954 M=7.1 Fairview Peak and M=6.8 Dixie Valley, NV, ruptures, with some seismicity clusters beyond the rupture tips. The 2008 Mw=6.0 South Iceland earthquake, another site of trans-tensional faulting and geothermal activity, shows intense aftershock clusters off the ends of the main rupture, resembling the 1872 earthquake. This chicken-or-egg riddle would be easy to solve if it were known whether the earthquake rate in the geothermal areas increased as a result of the 1872 shock. There is only one recorded quake before the mainshock, a July 1871 M 5.5 event at the southern tip of the 1872 rupture; this suggests the clusters might have been active before the mainshock. On the other hand, one of the two largest aftershocks of the 1872 event, a M 6.8 near Bishop two weeks after the mainshock, stuck in the center of the northern cluster, which supports the clusters were activated by the mainshock. We walk away with a tie. If the 1872 rupture did trigger the activity in the clusters, it begs a harder question: Could aftershocks or triggered seismicity have continued almost 150 years after the mainshock? The Owens Valley Fault slips 2-3 mm/yr, about a tenth of the San Andreas Fault rate. Rate/state theory and observations from the central US and

Strong Motion Network of Medellín and Aburrá Valley: technical advances, seismicity records and micro-earthquake monitoring

NASA Astrophysics Data System (ADS)

Posada, G.; Trujillo, J. C., Sr.; Hoyos, C.; Monsalve, G.

2017-12-01

The tectonics setting of Colombia is determined by the interaction of Nazca, Caribbean and South American plates, together with the Panama-Choco block collision, which makes a seismically active region. Regional seismic monitoring is carried out by the National Seismological Network of Colombia and the Accelerometer National Network of Colombia. Both networks calculate locations, magnitudes, depths and accelerations, and other seismic parameters. The Medellín - Aburra Valley is located in the Northern segment of the Central Cordillera of Colombia, and according to the Colombian technical seismic norm (NSR-10), is a region of intermediate hazard, because of the proximity to seismic sources of the Valley. Seismic monitoring in the Aburra Valley began in 1996 with an accelerometer network which consisted of 38 instruments. Currently, the network consists of 26 stations and is run by the Early Warning System of Medellin and Aburra Valley (SIATA). The technical advances have allowed the real-time communication since a year ago, currently with 10 stations; post-earthquake data is processed through operationally near-real-time, obtaining quick results in terms of location, acceleration, spectrum response and Fourier analysis; this information is displayed at the SIATA web site. The strong motion database is composed by 280 earthquakes; this information is the basis for the estimation of seismic hazards and risk for the region. A basic statistical analysis of the main information was carried out, including the total recorded events per station, natural frequency, maximum accelerations, depths and magnitudes, which allowed us to identify the main seismic sources, and some seismic site parameters. With the idea of a more complete seismic monitoring and in order to identify seismic sources beneath the Valley, we are in the process of installing 10 low-cost shake seismometers for micro-earthquake monitoring. There is no historical record of earthquakes with a magnitude

Holistic Overview of the Contribution of Tectonic, Geomorphic, and Geologic Factors to the Seismic Hazard of the Kathmandu Valley, Nepal

NASA Astrophysics Data System (ADS)

Banda, S.; Chang, A.; Sanquini, A.; Hilley, G. E.

2013-12-01

Nepal has been a seismically active region since the mid-Eocene collision of the Indian and Eurasian plates. It can be divided into four major tectonostratigraphic units. The Lesser Himalayan Zone, where Kathmandu Valley is located, is bounded to the south by the Main Boundary Thrust (MBT) and to the north by the Main Central Thrust (MCT). These faults, and the Main Frontal Thrust (MFT) traverse the NW-SE length of Nepal and sole into the Main Himalayan Thrust (MHT). Slip along these structures during the Plio-Quaternary has ponded sediment in the interior of the orogen, producing the nearly circular Kathmandu Basin, which hosts a series of radially converging rivers that exit the basin to the south. The sediment that is ponded within the basin consists of alluvial, lacustrine and debris flow deposits that are ~500 m thick. The faults in the vicinity of the Kathmandu Valley currently serve as potential earthquake sources. Sources that might plausibly be generated by these faults are constrained by structural, paleoseismic, and geodetic observations. The continued collision between India and Tibet is reflected in a convergence rate of about 20 mm/yr, as measured by Global Positioning System (GPS) geodetic networks. Strain accumulates on the MHT, and is released during large earthquakes. The epicenter of the 1934 (M8.2) earthquake, about 175 km to the east of Kathmandu, resulted in MMI VIII- IX shaking intensity in the Kathmandu Valley. Seismic waves generated from faults in proximity to Kathmandu may be amplified or attenuated at particular locations due to specific site responses that reflect the geologic framework of the Kathmandu Valley. The ponded sediments within the Kathmandu Basin may contribute to basin effects, trapping seismic waves and prolonging ground motion, as well as increasing the amplitude of the waves as they travel from crystalline outer rocks into the soft lake-bed sediments. A hazard analysis suggests that a M8.0 earthquake originating in the

Geophysical Studies Based on Gravity and Seismic Data of Tule Desert, Meadow Valley Wash, and California Wash Basins, Southern Nevada

USGS Publications Warehouse

Scheirer, Daniel S.; Page, William R.; Miller, John J.

2006-01-01

Gravity and seismic data from Tule Desert, Meadow Valley Wash, and California Wash, Nevada, provide insight into the subsurface geometry of these three basins that lie adjacent to rapidly developing areas of Clark County, Nevada. Each of the basins is the product of Tertiary extension accommodated with the general form of north-south oriented, asymmetrically-faulted half-grabens. Geophysical inversion of gravity observations indicates that Tule Desert and Meadow Valley Wash basins are segmented into subbasins by shallow, buried basement highs. In this study, basement refers to pre-Cenozoic bedrock units that underlie basins filled with Cenozoic sedimentary and volcanic units. In Tule Desert, a small, buried basement high inferred from gravity data appears to be a horst whose placement is consistent with seismic reflection and magnetotelluric observations. Meadow Valley Wash consists of three subbasins separated by basement highs at structural zones that accommodated different styles of extension of the adjacent subbasins, an interpretation consistent with geologic mapping of fault traces oblique to the predominant north-south fault orientation of Tertiary extension in this area. California Wash is a single structural basin. The three seismic reflection lines analyzed in this study image the sedimentary basin fill, and they allow identification of faults that offset basin deposits and underlying basement. The degree of faulting and folding of the basin-fill deposits increases with depth. Pre-Cenozoic units are observed in some of the seismic reflection lines, but their reflections are generally of poor quality or are absent. Factors that degrade seismic reflector quality in this area are rough land topography due to erosion, deformed sedimentary units at the land surface, rock layers that dip out of the plane of the seismic profile, and the presence of volcanic units that obscure underlying reflectors. Geophysical methods illustrate that basin geometry is more

Mapping a buried Quaternary valley and pre-Quaternary faults through seismic methods in Copenhagen, Denmark.

NASA Astrophysics Data System (ADS)

Martinez, Kerim; Alfredo Mendoza, Jose; Henrik, Olsen

2010-05-01

Limited knowledge of the subsurface geology motivates the use of geophysical techniques before large engineering projects are conducted. These applications are normally restricted to satisfy the project aims, like mapping the near surface sediments, unconsolidated rocks and/or geological structures that may affect the construction locally. However, the applications can also contribute to the general knowledge of the regional geology around the location of interest. This report highlights the mapping of a buried Quaternary valley and identification of regional faults by a reflection and refraction seismic survey performed in Copenhagen. A 13.9 Km seismic survey was carried out at Copenhagen city along six crooked lines in order to determine the velocity fields in the near subsurface segment of a planned metro line and reflection patterns in deeper levels. The aim of the survey was to collect information needed for designing the underground metro. In particular it was sought to map the interface between Quaternary sedimentary layers of clay, till and sand, and the underlying layers of Palaeogene limestone found between 7 and 40 m below the ground surface. The data acquisition was carried out using a 192 channels array, receiver groups with 5 m spacing and a Vibroseis as a source at 5 m spacing following a roll along technique to complete the survey spreads. The urban environment demanded extensive survey planning including traffic control, notifications to residents and a fluent coordination with municipal authorities in order to minimize disturbances and ensure data acquisition. The reflection data was processed under a conventional scheme and the refraction data was interpreted using a non-linear traveltime tomography algorithm. The reflection results indicate the presence of faults oriented NW-SE to NNW-SSE affecting the limestone sequences. The faults may be associated to the Sorgenfrei-Tornquist Zone at the transition between the Danish Basin and the Baltic

Seismic probing of continental subduction zones

NASA Astrophysics Data System (ADS)

Zhao, Liang; Xu, Xiaobing; Malusà, Marco G.

2017-09-01

High-resolution images of Earth's interior provide pivotal information for the understanding of a range of geodynamic processes, including continental subduction and exhumation of ultrahigh-pressure (UHP) metamorphic rocks. Here we present a synthesis of available global seismic observations on continental subduction zones, and selected examples of seismic probing from the European Alps, the Himalaya-Tibet and the Qinling-Dabie orogenic belts. Our synthesis and examples show that slabs recognized beneath exhumed continental UHP terranes generally have shallow dip angles (<45°) at depths <100 km, to become much steeper at depths >100 km. Slabs underlined by a clear high velocity anomaly from Earth's surface to the mantle are generally Cenozoic in age. Some of these slabs are continuous, whereas other continental subduction zones are located above discontinuous high velocity anomalies possibly suggesting slab breakoff. The density of seismic stations and the quality of recordings are of primary importance to get high-resolution images of the upper mantle to be used as a starting point to provide reliable geodynamic interpretations. In some cases, areas previously indicated as possible site of slab breakoff, such as the European Alps, have been later proven to be located above a continuous slab by using higher quality travel time data from denser seismic arrays. Discriminating between oceanic and continental slabs can be challenging, but valuable information can be provided by combining teleseismic tomography and receiver function analysis. The upper mantle beneath most continental UHP terranes generally shows complex seismic anisotropy patterns that are potentially preserved even in pre-Cenozoic subduction zones. These patterns can be used to provide information on continental slabs that are no longer highlighted by a clear high-velocity anomaly.

Seismic Velocity Structure across the Hayward Fault Zone Near San Leandro, California

NASA Astrophysics Data System (ADS)

Strayer, L. M.; Catchings, R.; Chan, J. H.; Richardson, I. S.; McEvilly, A.; Goldman, M.; Criley, C.; Sickler, R. R.

2017-12-01

In Fall 2016 we conducted the East Bay Seismic Investigation, a NEHRP-funded collaboration between California State University, East Bay and the United State Geological Survey. The study produced a large volume of seismic data, allowing us to examine the subsurface across the East Bay plain and hills using a variety of geophysical methods. We know of no other survey performed in the past that has imaged this area, at this scale, and with this degree of resolution. Initial models show that seismic velocities of the Hayward Fault Zone (HFZ), the East Bay plain, and the East Bay hills are illuminated to depths of 5-6 km. We used explosive sources at 1-km intervals along a 15-km-long, NE-striking ( 055°), seismic line centered on the HFZ. Vertical- and horizontal-component sensors were spaced at 100 m intervals along the entire profile, with vertical-component sensors at 20 m intervals across mapped or suspected faults. Preliminary seismic refraction tomography across the HFZ, sensu lato, (includes sub-parallel, connected, and related faults), shows that the San Leandro Block (SLB) is a low-velocity feature in the upper 1-3 km, with nearly the same Vp as the adjacent Great Valley sediments to the east, and low Vs values. In our initial analysis we can trace the SLB and its bounding faults (Hayward, Chabot) nearly vertically, to at least 2-4 km depth. Similarly, preliminary migrated reflection images suggest that many if not all of the peripheral reverse, strike-slip and oblique-slip faults of the wider HFZ dip toward the SLB, into a curtain of relocated epicenters that define the HFZ at depth, indicative of a `flower-structure'. Preliminary Vs tomography identifies another apparently weak zone at depth, located about 1.5 km east of the San Leandro shoreline, that may represent the northward continuation of the Silver Creek Fault. Centered 4 km from the Bay, there is a distinctive, 2 km-wide, uplifted, horst-like, high-velocity structure (both Vp & Vs) that bounds the

New Madrid seismic zone recurrence intervals

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

Schweig, E.S.; Ellis, M.A.

1993-03-01

Frequency-magnitude relations in the New Madrid seismic zone suggest that great earthquakes should occur every 700--1,200 yrs, implying relatively high strain rates. These estimates are supported by some geological and GPS results. Recurrence intervals of this order should have produced about 50 km of strike-slip offset since Miocene time. No subsurface evidence for such large displacements is known within the seismic zone. Moreover, the irregular fault pattern forming a compressive step that one sees today is not compatible with large displacements. There are at least three possible interpretations of the observations of short recurrence intervals and high strain rates, butmore » apparently youthful fault geometry and lack of major post-Miocene deformation. One is that the seismological and geodetic evidence are misleading. A second possibility is that activity in the region is cyclic. That is, the geological and geodetic observations that suggest relatively short recurrence intervals reflect a time of high, but geologically temporary, pore-fluid pressure. Zoback and Zoback have suggested such a model for intraplate seismicity in general. Alternatively, the New Madrid seismic zone is geologically young feature that has been active for only the last few tens of thousands of years. In support of this, observe an irregular fault geometry associated with a unstable compressive step, a series of en echelon and discontinuous lineaments that may define the position of a youthful linking fault, and the general absence of significant post-Eocene faulting or topography.« less

Double seismic zone for deep earthquakes in the izu-bonin subduction zone.

PubMed

Iidaka, T; Furukawa, Y

1994-02-25

A double seismic zone for deep earthquakes was found in the Izu-Bonin region. An analysis of SP-converted phases confirms that the deep seismic zone consists of two layers separated by approximately 20 kilometers. Numerical modeling of the thermal structure implies that the hypocenters are located along isotherms of 500 degrees to 550 degrees C, which is consistent with the hypothesis that deep earthquakes result from the phase transition of metastable olivine to a high-pressure phase in the subducting slab.

Seismic sequences in the Sombrero Seismic Zone

NASA Astrophysics Data System (ADS)

Pulliam, J.; Huerfano, V. A.; ten Brink, U.; von Hillebrandt, C.

2007-05-01

The northeastern Caribbean, in the vicinity of Puerto Rico and the Virgin Islands, has a long and well-documented history of devastating earthquakes and tsunamis, including major events in 1670, 1787, 1867, 1916, 1918, and 1943. Recently, seismicity has been concentrated to the north and west of the British Virgin Islands, in the region referred to as the Sombrero Seismic Zone by the Puerto Rico Seismic Network (PRSN). In the combined seismicity catalog maintained by the PRSN, several hundred small to moderate magnitude events can be found in this region prior to 2006. However, beginning in 2006 and continuing to the present, the rate of seismicity in the Sombrero suddenly increased, and a new locus of activity developed to the east of the previous location. Accurate estimates of seismic hazard, and the tsunamigenic potential of seismic events, depend on an accurate and comprehensive understanding of how strain is being accommodated in this corner region. Are faults locked and accumulating strain for release in a major event? Or is strain being released via slip over a diffuse system of faults? A careful analysis of seismicity patterns in the Sombrero region has the potential to both identify faults and modes of failure, provided the aggregation scheme is tuned to properly identify related events. To this end, we experimented with a scheme to identify seismic sequences based on physical and temporal proximity, under the assumptions that (a) events occur on related fault systems as stress is refocused by immediately previous events and (b) such 'stress waves' die out with time, so that two events that occur on the same system within a relatively short time window can be said to have a similar 'trigger' in ways that two nearby events that occurred years apart cannot. Patterns that emerge from the identification, temporal sequence, and refined locations of such sequences of events carry information about stress accommodation that is obscured by large clouds of

Seismic measurements of the internal properties of fault zones

USGS Publications Warehouse

Mooney, W.D.; Ginzburg, A.

1986-01-01

The internal properties within and adjacent to fault zones are reviewed, principally on the basis of laboratory, borehole, and seismic refraction and reflection data. The deformation of rocks by faulting ranges from intragrain microcracking to severe alteration. Saturated microcracked and mildly fractured rocks do not exhibit a significant reduction in velocity, but, from borehole measurements, densely fractured rocks do show significantly reduced velocities, the amount of reduction generally proportional to the fracture density. Highly fractured rock and thick fault gouge along the creeping portion of the San Andreas fault are evidenced by a pronounced seismic low-velocity zone (LVZ), which is either very thin or absent along locked portions of the fault. Thus there is a correlation between fault slip behavior and seismic velocity structure within the fault zone; high pore pressure within the pronounced LVZ may be conductive to fault creep. Deep seismic reflection data indicate that crustal faults sometimes extend through the entire crust. Models of these data and geologic evidence are consistent with a composition of deep faults consisting of highly foliated, seismically anisotropic mylonites. ?? 1986 Birkha??user Verlag, Basel.

33 CFR 3.40-65 - Sector Ohio Valley Marine Inspection Zone and Captain of the Port Zone; Marine Safety Unit...

Code of Federal Regulations, 2010 CFR

2010-07-01

... 33 Navigation and Navigable Waters 1 2010-07-01 2010-07-01 false Sector Ohio Valley Marine....40-65 Sector Ohio Valley Marine Inspection Zone and Captain of the Port Zone; Marine Safety Unit Pittsburgh. Sector Ohio Valley's office is located in Louisville, KY. A subordinate unit, Marine Safety Unit...

Seismic calibration shots conducted in 2009 in the Imperial Valley, southern California, for the Salton Seismic Imaging Project (SSIP)

USGS Publications Warehouse

Murphy, Janice; Goldman, Mark; Fuis, Gary; Rymer, Michael; Sickler, Robert; Miller, Summer; Butcher, Lesley; Ricketts, Jason; Criley, Coyn; Stock, Joann; Hole, John; Chavez, Greg

2011-01-01

Rupture of the southern section of the San Andreas Fault, from the Coachella Valley to the Mojave Desert, is believed to be the greatest natural hazard facing California in the near future. With an estimated magnitude between 7.2 and 8.1, such an event would result in violent shaking, loss of life, and disruption of lifelines (freeways, aqueducts, power, petroleum, and communication lines) that would bring much of southern California to a standstill. As part of the Nation's efforts to prevent a catastrophe of this magnitude, a number of projects are underway to increase our knowledge of Earth processes in the area and to mitigate the effects of such an event. One such project is the Salton Seismic Imaging Project (SSIP), which is a collaborative venture between the United States Geological Survey (USGS), California Institute of Technology (Caltech), and Virginia Polytechnic Institute and State University (Virginia Tech). This project will generate and record seismic waves that travel through the crust and upper mantle of the Salton Trough. With these data, we will construct seismic images of the subsurface, both reflection and tomographic images. These images will contribute to the earthquake-hazard assessment in southern California by helping to constrain fault locations, sedimentary basin thickness and geometry, and sedimentary seismic velocity distributions. Data acquisition is currently scheduled for winter and spring of 2011. The design and goals of SSIP resemble those of the Los Angeles Region Seismic Experiment (LARSE) of the 1990's. LARSE focused on examining the San Andreas Fault system and associated thrust-fault systems of the Transverse Ranges. LARSE was successful in constraining the geometry of the San Andreas Fault at depth and in relating this geometry to mid-crustal, flower-structure-like decollements in the Transverse Ranges that splay upward into the network of hazardous thrust faults that caused the 1971 M 6.7 San Fernando and 1987 M 5

Grizzly Valley fault system, Sierra Valley, CA

USGS Publications Warehouse

Gold, Ryan; Stephenson, William; Odum, Jack; Briggs, Rich; Crone, Anthony; Angster, Steve

2012-01-01

The Grizzly Valley fault system (GVFS) strikes northwestward across Sierra Valley, California and is part of a network of active, dextral strike-slip faults in the northern Walker Lane (Figure 1). To investigate Quaternary motion across the GVFS, we analyzed high-resolution (0.25 m) airborne LiDAR data (Figure 2) in combination with six, high-resolution, P-wave, seismic-reflection profiles [Gold and others, 2012]. The 0.5- to 2.0-km-long seismic-reflection profiles were sited orthogonal to suspected tectonic lineaments identified from previous mapping and our analysis of airborne LiDAR data. To image the upper 400–700 m of subsurface stratigraphy of Sierra Valley (Figure 3), we used a 230-kg accelerated weight drop source. Geophone spacing ranged from 2 to 5 m and shots were co-located with the geophones. The profiles reveal a highly reflective, deformed basal marker that we interpret to be the top of Tertiary volcanic rocks, overlain by a 120- to 300-m-thick suite of subhorizontal reflectors we interpret as Plio-Pleistocene lacustrine deposits. Three profiles image the principle active trace of the GVFS, which is a steeply dipping fault zone that offsets the volcanic rocks and the basin fill (Figures 4 & 5).

Seismic gaps and source zones of recent large earthquakes in coastal Peru

USGS Publications Warehouse

Dewey, J.W.; Spence, W.

1979-01-01

The earthquakes of central coastal Peru occur principally in two distinct zones of shallow earthquake activity that are inland of and parallel to the axis of the Peru Trench. The interface-thrust (IT) zone includes the great thrust-fault earthquakes of 17 October 1966 and 3 October 1974. The coastal-plate interior (CPI) zone includes the great earthquake of 31 May 1970, and is located about 50 km inland of and 30 km deeper than the interface thrust zone. The occurrence of a large earthquake in one zone may not relieve elastic strain in the adjoining zone, thus complicating the application of the seismic gap concept to central coastal Peru. However, recognition of two seismic zones may facilitate detection of seismicity precursory to a large earthquake in a given zone; removal of probable CPI-zone earthquakes from plots of seismicity prior to the 1974 main shock dramatically emphasizes the high seismic activity near the rupture zone of that earthquake in the five years preceding the main shock. Other conclusions on the seismicity of coastal Peru that affect the application of the seismic gap concept to this region are: (1) Aftershocks of the great earthquakes of 1966, 1970, and 1974 occurred in spatially separated clusters. Some clusters may represent distinct small source regions triggered by the main shock rather than delimiting the total extent of main-shock rupture. The uncertainty in the interpretation of aftershock clusters results in corresponding uncertainties in estimates of stress drop and estimates of the dimensions of the seismic gap that has been filled by a major earthquake. (2) Aftershocks of the great thrust-fault earthquakes of 1966 and 1974 generally did not extend seaward as far as the Peru Trench. (3) None of the three great earthquakes produced significant teleseismic activity in the following month in the source regions of the other two earthquakes. The earthquake hypocenters that form the basis of this study were relocated using station

Development of GIS Database for New Madrid Seismic Zone

NASA Astrophysics Data System (ADS)

Birhanemeskel, Y. T.; Vlahovic, G.; Arroucau, P.; Malhotra, R.; Powell, C. A.

2010-12-01

The New Madrid Seismic Zone (NMSZ) of the central Mississippi river valley is currently the most seismically active region in the central and eastern United States. A number of earthquakes occurred in NMSZ between 1811 and 1812, of which three major earthquakes with magnitudes greater than 7 destroyed the town of New Madrid, Missouri. Intraplate seismicity like the New Madrid seismicity is difficult to explain in the framework of plate tectonics and requires analyzing various geological, geophysical and seismological data to better understand its causes. ArcGIS® 9.3.1 software with license type ArcEditor was used to build a geodatabase containing multiple layers that are useful for the study of intraplate seismicity. These layers include earthquake locations, gravity and magnetic anomalies, lithology, topography, velocity anomalies as resolved by arrival time tomography and geological structures like intrusions and faults. The data for these layers were obtained from the U.S Geological Survey, from the Center for Earthquake Research and Information at the University of Memphis, TN, and from paper maps. Zipped files of various formats (.xls, .shp, .txt, .tar, etc) were downloaded and converted to a format compatible with ArcGIS. To keep compatibility of the data, editing of the attribute table of the raw data was completed before importing the data to Arc Catalog. Geo-referencing and digitizing processes were also done to import layers of contour lines and geological structures with correct vector information from papers maps. Layers were clipped in order to make sure that they fit the spatial extent of the study area (from 34°S to 40°N in latitude and from 93°W to 86°W in longitude). The New Madrid seismicity will be analyzed by looking for possible relationships that exist between the data layers using various spatial and geostatistical tools. For example the distribution of earthquakes will be analyzed with respect to the potential field and velocity

Overview of seismic potential in the central and eastern United States

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

Schweig, E.S.

1995-12-31

The seismic potential of any region can be framed in terms the locations of source zones, the frequency of earthquake occurrence for each source, and the maximum size earthquake that can be expect from each source. As delineated by modern and historical seismicity, the most important seismic source zones affecting the eastern United States include the New Madrid and Wabash Valley seismic zones of the central U.S., the southern Appalachians and Charleston, South Carolina, areas in the southeast, and the northern Appalachians and Adirondacks in the northeast. The most prominant of these in terms of current seismicity and historical seismicmore » moment release in the New Madrid seismic zone, which produced three earthquakes of moment magnitude {ge} 8 in 1811 and 1812. The frequency of earthquake recurrence can be examined using the instrumental record, the historical record, and the geological record. Each record covers a unique time period and has a different scale of temporal resolution and completeness of the data set. The Wabash Valley is an example where the long-term geological record indicates a greater potential than the instrumental and historical records. This points to the need to examine all of the evidence in any region in order to obtain a credible estimates of earthquake hazards. Although earthquake hazards may be dominated by mid-magnitude 6 earthquakes within the mapped seismic source zones, the 1994 Northridge, California, earthquake is just the most recent example of the danger of assuming future events will occur on faults known to have had past events and how destructive such an earthquake can be.« less

Potential Seismic Signatures of Megathrust Preparatory Zones

NASA Astrophysics Data System (ADS)

Parameswaran, R. M.; Maheswari, K.; Rajendran, K.

2017-12-01

The Mw 9.2, 2004 Sumatra earthquake awakened the otherwise inactive Andaman-Sumatra subduction zone (ASSZ), pushing it into an era of amplified seismicity. The subduction zone has since witnessed an array of inter- and intra-plate events along and around its trench. Several intra-plate events like the 2012 Wharton Basin earthquakes (Mw 8.6 and 8.2), are believed to be the triggered response of the plateward transmission of stresses due to the 2004 earthquake (Ishii et al., 2013). On the other hand, the Mw 7.5, 2009 33-km-deep intra-plate normal-faulting event in the northern Andaman segment is an example of outer-rise seismicity resulting from the post-seismic relaxation of the subducting slab (Andrade and Rajendran, 2011). These are aftermaths of a drastic change in the stress regime from compressional to extensional, following the 2004 megathrust event. But, pre-megathrust, aside from the inter-plate thrust mechanisms that are widely observed along the trench, how does the plate-motion-driven compression manifest in the regional seismicity? What happens to the stresses accumulating within the bending subducting slab; does it source deeper compressional events prior to a megathrust? The 2009 normal outer-rise earthquake was preceded by the 13 September 2002, Mw 6.5 Diglipur outer-rise thrust earthquake (22 km depth), both occurring at the northern terminus of the 2004-rupture, in the compressing forearc that experienced surface uplift pre-megathrust (Rajendran et al., 2003; Rajendran et al., 2007). This work, therefore, examines the slip models of such pre-event outer-rise thrust earthquakes along the stretch of the 2004 rupture zone in the ASSZ. The work is also being extended to understand the preparatory zones of other global megathrust earthquakes.

The Salton Seismic Imaging Project: Tomographic characterization of a sediment-filled rift valley and adjacent ranges, southern California

NASA Astrophysics Data System (ADS)

Davenport, K.; Hole, J. A.; Stock, J. M.; Fuis, G. S.; Carrick, E.; Tikoff, B.

2011-12-01

The Salton Trough in Southern California represents the northernmost rift of the Gulf of California extensional system. Relative motion between the Pacific and North American plates is accommodated by continental rifting in step-over zones between the San Andreas, Imperial, and Cerro Prieto transform faults. Rapid sedimentation from the Colorado River has isolated the trough from the southern portion of the Gulf of California, progressively filling the subsiding rift basin. Based on data from previous seismic surveys, the pre-existing continent has ruptured completely, and a new ~22 km thick crust has been created entirely by sedimentation overlying rift-related magmatism. The MARGINS, EarthScope, and USGS-funded Salton Seismic Imaging Project (SSIP) was designed to investigate the nature of this new crust, the ongoing process of continental rifting, and associated earthquake hazards. SSIP, acquired in March 2011, comprises 7 lines of onshore seismic refraction / wide-angle reflection data, 2 lines of refraction / reflection data in the Salton Sea, and a line of broadband stations. This presentation focuses on the refraction / wide-angle reflection line across the Imperial Valley, extending ~220 km across California from Otay Mesa, near Tijuana, to the Colorado River. The data from this line includes seventeen 100-160 kg explosive shots and receivers at 100 m spacing across the Imperial Valley to constrain the structure of the Salton Trough rift basin, including the Imperial Fault. Eight larger shots (600-920 kg) at 20-35 km spacing and receivers at 200-500 m spacing extend the line across the Peninsular Ranges and the Chocolate Mountains. These data will contrast the structure of the rift to that of the surrounding crust and provide constraints on whole-crust and uppermost mantle structure. Preliminary work has included tomographic inversion of first-arrival travel times across the Valley, emphasizing a minimum-structure approach to create a velocity model of the

A non extensive statistical physics analysis of the Hellenic subduction zone seismicity

NASA Astrophysics Data System (ADS)

Vallianatos, F.; Papadakis, G.; Michas, G.; Sammonds, P.

2012-04-01

The Hellenic subduction zone is the most seismically active region in Europe [Becker & Meier, 2010]. The spatial and temporal distribution of seismicity as well as the analysis of the magnitude distribution of earthquakes concerning the Hellenic subduction zone, has been studied using the concept of Non-Extensive Statistical Physics (NESP) [Tsallis, 1988 ; Tsallis, 2009]. Non-Extensive Statistical Physics, which is a generalization of Boltzmann-Gibbs statistical physics, seems a suitable framework for studying complex systems (Vallianatos, 2011). Using this concept, Abe & Suzuki (2003;2005) investigated the spatial and temporal properties of the seismicity in California and Japan and recently Darooneh & Dadashinia (2008) in Iran. Furthermore, Telesca (2011) calculated the thermodynamic parameter q of the magnitude distribution of earthquakes of the southern California earthquake catalogue. Using the external seismic zones of 36 seismic sources of shallow earthquakes in the Aegean and the surrounding area [Papazachos, 1990], we formed a dataset concerning the seismicity of shallow earthquakes (focal depth ≤ 60km) of the subduction zone, which is based on the instrumental data of the Geodynamic Institute of the National Observatory of Athens (http://www.gein.noa.gr/, period 1990-2011). The catalogue consists of 12800 seismic events which correspond to 15 polygons of the aforementioned external seismic zones. These polygons define the subduction zone, as they are associated with the compressional stress field which characterizes a subducting regime. For each event, moment magnitude was calculated from ML according to the suggestions of Papazachos et al. (1997). The cumulative distribution functions of the inter-event times and the inter-event distances as well as the magnitude distribution for each seismic zone have been estimated, presenting a variation in the q-triplet along the Hellenic subduction zone. The models used, fit rather well to the observed

Fault pattern at the northern end of the Death Valley - Furnace Creek fault zone, California and Nevada

NASA Technical Reports Server (NTRS)

Liggett, M. A. (Principal Investigator); Childs, J. F.

1974-01-01

The author has identified the following significant results. The pattern of faulting associated with the termination of the Death Valley-Furnace Creek Fault Zone in northern Fish Lake Valley, Nevada was studied in ERTS-1 MSS color composite imagery and color IR U-2 photography. Imagery analysis was supported by field reconnaissance and low altitude aerial photography. The northwest-trending right-lateral Death Valley-Furnace Creek Fault Zone changes northward to a complex pattern of discontinuous dip slip and strike slip faults. This fault pattern terminates to the north against an east-northeast trending zone herein called the Montgomery Fault Zone. No evidence for continuation of the Death Valley-Furnace Creek Fault Zone is recognized north of the Montgomery Fault Zone. Penecontemporaneous displacement in the Death Valley-Furnace Creek Fault Zone, the complex transitional zone, and the Montgomery Fault Zone suggests that the systems are genetically related. Mercury mineralization appears to have been localized along faults recognizable in ERTS-1 imagery within the transitional zone and the Montgomery Fault Zone.

Preliminary seismic evaluation and ranking of bridges on and over the parkways in Western Kentucky.

DOT National Transportation Integrated Search

2008-06-01

Five parkways in Western Kentucky are located in the region that is greatly influenced by the New Madrid and Wabash Valley Seismic Zones. This report executes a preliminary screening process, known also as the Seismic Rating System, for bridges on an...

Seismicity, shear failure and modes of deformation in deep subduction zones

NASA Technical Reports Server (NTRS)

Lundgren, Paul R.; Giardini, Domenico

1992-01-01

The joint hypocentral determination method is used to relocate deep seismicity reported in the International Seismological Center catalog for earthquakes deeper than 400 km in the Honshu, Bonin, Mariannas, Java, Banda, and South America subduction zones. Each deep seismic zone is found to display planar features of seismicity parallel to the Harvard centroid-moment tensor nodal planes, which are identified as planes of shear failure. The sense of displacement on these planes is one of resistance to deeper penetration.

Active tectonics of the Imperial Valley, southern California: fault damage zones, complex basins and buried faults

NASA Astrophysics Data System (ADS)

Persaud, P.; Ma, Y.; Stock, J. M.; Hole, J. A.; Fuis, G. S.; Han, L.

2016-12-01

Ongoing oblique slip at the Pacific-North America plate boundary in the Salton Trough produced the Imperial Valley. Deformation in this seismically active area is distributed across a complex network of exposed and buried faults resulting in a largely unmapped seismic hazard beneath the growing population centers of El Centro, Calexico and Mexicali. To better understand the shallow crustal structure in this region and the connectivity of faults and seismicity lineaments, we used data primarily from the Salton Seismic Imaging Project (SSIP) to construct a P-wave velocity profile to 15 km depth, and a 3-D velocity model down to 8 km depth including the Brawley Geothermal area. We obtained detailed images of a complex wedge-shaped basin at the southern end of the San Andreas Fault system. Two deep subbasins (VP <5.65 km/s) are located in the western part of the larger Imperial Valley basin, where seismicity trends and active faults play a significant role in shaping the basin edge. Our 3-D VP model reveals previously unrecognized NE-striking cross faults that are interacting with the dominant NW-striking faults to control deformation. New findings in our profile include localized regions of low VP (thickening of a 5.65-5.85 km/s layer) near faults or seismicity lineaments interpreted as possibly faulting-related. Our 3-D model and basement map reveal velocity highs associated with the geothermal areas in the eastern valley. The improved seismic velocity model from this study, and the identification of important unmapped faults or buried interfaces will help refine the seismic hazard for parts of Imperial County, California.

Subduction zone seismicity and the thermo-mechanical evolution of downgoing lithosphere

NASA Astrophysics Data System (ADS)

Wortel, M. J. R.; Vlaar, N. J.

1988-09-01

In this paper we discuss characteristic features of subduction zone seismicity at depths between about 100 km and 700 km, with emphasis on the role of temperature and rheology in controlling the deformation of, and the seismic energy release in downgoing lithosphere. This is done in two steps. After a brief review of earlier developments, we first show that the depth distribution of hypocentres at depths between 100 km and 700 km in subducted lithosphere can be explained by a model in which seismic activity is confined to those parts of the slab which have temperatures below a depth-dependent critical value T cr. Second, the variation of seismic energy release (frequency of events, magnitude) with depth is addressed by inferring a rheological evolution from the slab's thermal evolution and by combining this with models for the system of forces acting on the subducting lithosphere. It is found that considerable stress concentration occurs in a reheating slab in the depth range of 400 to 650 700 km: the slab weakens, but the stress level strongly increases. On the basis of this stress concentration a model is formulated for earthquake generation within subducting slabs. The model predicts a maximum depth of seismic activity in the depth range of 635 to 760 km and, for deep earthquake zones, a relative maximum in seismic energy release near the maximum depth of earthquakes. From our modelling it follows that, whereas such a maximum is indeed likely to develop in deep earthquake zones, zones with a maximum depth around 300 km (such as the Aleutians) are expected to exhibit a smooth decay in seismic energy release with depth. This is in excellent agreement with observational data. In conclusion, the incoroporation of both depth-dependent forces and depth-dependent rheology provides new insight into the generation of intermediate and deep earthquakes and into the variation of seismic activity with depth. Our results imply that no barrier to slab penetration at a depth of

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.

Erosional valleys in the Thaumasia region of Mars: Hydrothermal and seismic origins

USGS Publications Warehouse

Tanaka, K.L.; Dohm, J.M.; Lias, J.H.; Hare, T.M.

1998-01-01

more thick, inhibiting valley development by hydrothermal circulation. However, eruption of groundwater may have occurred from impact-induced fracturing and lateral and perhaps minor upward transport of water due to seismic pumping. The two smaller craters formed along the plateau margin where the highest potential hydraulic head would occur in aquifers beneath the plateau. In the case of the larger crater (Lowell, 200 km in diameter), potential aquifers would likely be at depths of kilometers below the cryosphere. Seismic energy generated by the Lowell impactor would have been much greater, pumping both groundwater and perhaps fluidized slurry to the surface from beneath the cryosphere to form the young valleys and flow deposit. Along the margin of Thaumasia, tectonic pressurization of groundwater also may have contributed to valley formation. Dissection of rim materials of the Argyre impact may relate to tectonic activity and the unconsolidated state of basin e??jecta.

Structure and Velocities of the Northeastern Santa Cruz Mountains and the Western Santa Clara Valley, California, from the SCSI-LR Seismic Survey

USGS Publications Warehouse

Catchings, R.D.; Goldman, M.R.; Gandhok, G.

2006-01-01

earthquakes sources. As one component of these joint studies, the U. S. Geological Survey acquired more than 28 km of combined seismic reflection/refraction data from the Santa Cruz Mountains to the central Santa Clara Valley in December 2000. The seismic investigation included both high-resolution (~5-m shot and sensor spacing) and relatively lower-resolution (~50-m sensor) seismic surveys from the central Santa Cruz Mountains to the central part of the valley. Collectively, we refer to these seismic investigations as the 2000 western Santa Clara Seismic Investigations (SCSI).

Time-reversibility in seismic sequences: Application to the seismicity of Mexican subduction zone

NASA Astrophysics Data System (ADS)

Telesca, L.; Flores-Márquez, E. L.; Ramírez-Rojas, A.

2018-02-01

In this paper we investigate the time-reversibility of series associated with the seismicity of five seismic areas of the subduction zone beneath the Southwest Pacific Mexican coast, applying the horizontal visibility graph method to the series of earthquake magnitudes, interevent times, interdistances and magnitude increments. We applied the Kullback-Leibler divergence D that is a metric for quantifying the degree of time-irreversibility in time series. Our findings suggest that among the five seismic areas, Jalisco-Colima is characterized by time-reversibility in all the four seismic series. Our results are consistent with the peculiar seismo-tectonic characteristics of Jalisco-Colima, which is the closest to the Middle American Trench and belongs to the Mexican volcanic arc.

Age constraints for the present fault configuration in the Imperial Valley, California: Evidence for northwestward propagation of the Gulf of California rift system

NASA Technical Reports Server (NTRS)

Larsen, Shawn; Reilinger, Robert

1990-01-01

Releveling and other geophysical data for the Imperial Valley of southern California suggest the northern section of the Imperial-Brawley fault system, which includes the Mesquite Basin and Brawley Seismic Zone, is much younger than the 4 to 5 million year age of the valley itself. A minimum age of 3000 years is calculated for the northern segment of the Imperial fault from correlations between surface topography and geodetically observed seismic/interseismic vertical movements. Calculations of a maximum age of 80,000 years is based upon displacements in the crystalline basement along the Imperial fault, inferred from seismic refraction surveys. This young age supports recent interpretations of heat flow measurements, which also suggest that the current patterns of seismicity and faults in the Imperial Valley are not long lived. The current fault geometry and basement morphology suggest northwestward growth of the Imperial fault and migration of the Brawley Seismic Zone. It is suggested that this migration is a manifestation of the propagation of the Gulf of California rift system into the North American continent.

An analysis of seismic hazard in the Upper Rhine Graben enlightened by the example of the New Madrid seismic zone.

NASA Astrophysics Data System (ADS)

Doubre, Cécile; Masson, Frédéric; Mazzotti, Stéphane; Meghraoui, Mustapha

2014-05-01

Seismic hazard in the "stable" continental regions and low-level deformation zones is one of the most difficult issues to address in Earth sciences. In these zones, instrumental and historical seismicity are not well known (sparse seismic networks, seismic cycle too long to be covered by the human history, episodic seismic activity) and many active structures remain poorly characterized or unknown. This is the case of the Upper Rhine Graben, the central segment of the European Cenozoic rift system (ECRIS) of Oligocene age, which extends from the North Sea through Germany and France to the Mediterranean coast over a distance of some 1100 km. Even if this region has already experienced some destructive earthquakes, its present-day seismicity is moderate and the deformation observed by geodesy is very small (below the current measurement accuracy). The strain rate does not exceed 10-10 and paleoseismic studies indicate an average return period of 2.5 to 3 103 ka for large earthquakes. The largest earthquake known for this zone is the 1356 Basel earthquake, with a magnitude generally estimated about 6.5 (Meghraoui et al., 2001) but recently re-evaluated between 6.7 and 7.1 (Fäh et al et al., 2009). A comparison of the Upper Rhine Graben with equivalent regions around the world could help improve our evaluation of seismic hazard of this region. This is the case of the New Madrid seismic zone, one of the best studied intraplate system in central USA, which experienced an M 7.0 - 7.5 earthquake in 1811-1812 and shares several characteristics with the Upper Rhine Graben, i.e. the general framework of inherited geological structures (reactivation of a failed rift / graben), seismicity patterns (spatial variability of small and large earthquakes), the null or low rate of deformation, and the location in a "stable" continental interior. Looking at the Upper Rhine Graben as an analogue of the New Madrid seismic zone, we can re-evaluate its seismic hazard and consider the

Evidence of shallow fault zone strengthening after the 1992 M7.5 Landers, California, earthquake

USGS Publications Warehouse

Li, Y.-G.; Vidale, J.E.; Aki, K.; Xu, Fei; Burdette, T.

1998-01-01

Repeated seismic surveys of the Landers, California, fault zone that ruptured in the magnitude (M) 7.5 earthquake of 1992 reveal an increase in seismic velocity with time. P, S, and fault zone trapped waves were excited by near-surface explosions in two locations in 1994 and 1996, and were recorded on two linear, three-component seismic arrays deployed across the Johnson Valley fault trace. The travel times of P and S waves for identical shot-receiver pairs decreased by 0.5 to 1.5 percent from 1994 to 1996, with the larger changes at stations located within the fault zone. These observations indicate that the shallow Johnson Valley fault is strengthening after the main shock, most likely because of closure of cracks that were opened by the 1992 earthquake. The increase in velocity is consistent with the prevalence of dry over wet cracks and with a reduction in the apparent crack density near the fault zone by approximately 1.0 percent from 1994 to 1996.

Hydrologically-driven crustal stresses and seismicity in the New Madrid Seismic Zone.

PubMed

Craig, Timothy J; Chanard, Kristel; Calais, Eric

2017-12-15

The degree to which short-term non-tectonic processes, either natural and anthropogenic, influence the occurrence of earthquakes in active tectonic settings or 'stable' plate interiors, remains a subject of debate. Recent work in plate-boundary regions demonstrates the capacity for long-wavelength changes in continental water storage to produce observable surface deformation, induce crustal stresses and modulate seismicity rates. Here we show that a significant variation in the rate of microearthquakes in the intraplate New Madrid Seismic Zone at annual and multi-annual timescales coincides with hydrological loading in the upper Mississippi embayment. We demonstrate that this loading, which results in geodetically observed surface deformation, induces stresses within the lithosphere that, although of small amplitude, modulate the ongoing seismicity of the New Madrid region. Correspondence between surface deformation, hydrological loading and seismicity rates at both annual and multi-annual timescales indicates that seismicity variations are the direct result of elastic stresses induced by the water load.

Tsunami potential assessment based on rupture zones, focal mechanisms and repeat times of strong earthquakes in the major Atlantic-Mediterranean seismic fracture zone

NASA Astrophysics Data System (ADS)

Agalos, Apostolos; Papadopoulos, Gerassimos A.; Kijko, Andrzej; Papageorgiou, Antonia; Smit, Ansie; Triantafyllou, Ioanna

2016-04-01

In the major Atlantic-Mediterranean seismic fracture zone, extended from Azores islands in the west to the easternmost Mediterranean Sea in the east, including the Marmara and Black Seas, a number of 22 tsunamigenic zones have been determined from historical and instrumental tsunami documentation. Although some tsunamis were produced by volcanic activity or landslides, the majority of them was generated by strong earthquakes. Since the generation of seismic tsunamis depends on several factors, like the earthquake size, focal depth and focal mechanism, the study of such parameters is of particular importance for the assessment of the potential for the generation of future tsunamis. However, one may not rule out the possibility for tsunami generation in areas outside of the 22 zones determined so far. For the Atlantic-Mediterranean seismic fracture zone we have compiled a catalogue of strong, potentially tsunamigenic (focal depth less than 100 km) historical earthquakes from various data bases and other sources. The lateral areas of rupture zones of these earthquakes were determined. Rupture zone is the area where the strain after the earthquake has dropped substantially with respect the strain before the earthquake. Aftershock areas were assumed to determine areas of rupture zones for instrumental earthquakes. For historical earthquakes macroseismic criteria were used such as spots of higher-degree seismic intensity and of important ground failures. For the period of instrumental seismicity, focal mechanism solutions from CMT, EMMA and other data bases were selected for strong earthquakes. From the geographical distribution of seismic rupture zones and the corresponding focal mechanisms in the entire Atlantic-Mediterranean seismic fracture zone we determined potentially tsunamigenic zones regardless they are known to have produced seismic tsunamis in the past or not. An attempt has been made to calculate in each one of such zones the repeat times of strong

Seismic Attenuation Structure and Intraplate Deformation

NASA Astrophysics Data System (ADS)

Bezada, M.; Kowalke, S.; Smale, J.

2017-12-01

It has been suggested that intraplate deformation and seismicity is localized at weak zones in the lithosphere and at rheological boundaries. Comparisons of intraplate deformation regions with mantle seismic velocity structure suggest a correlation, but are not universally accepted as compelling evidence. We present P-wave attenuation models built from records of teleseismic deep-focus earthquakes in three different regions that show significant correlation between attenuation structure and intraplate seismicity and deformation. In the eastern United States, the New Madrid, Wabash Valley, Eastern Tennessee, Central Virginia, and Carolina seismic zones all occur at or near the edges of high-Q (low attenuation) regions. In Spain, intraplate seismicity is absent from high-Q regions but relatively abundant in surrounding low-Q regions where intraplate orogeny is also observed. In Australia, where our model resolution is relatively poor owing to sparse and uneven station coverage, the Petermann and Alice Springs intraplate orogens occur near the edge of a high-Q feature roughly coinciding with the undeformed Amadeus basin. Our results suggest that lithospheric structure exerts important controls on the localization of intraplate deformation and seismicity and that seismic attenuation is a useful proxy for lithospheric strength.

Structure and Deformation in the Transpressive Zone of Southern California Inferred from Seismicity, Velocity, and Qp Models

NASA Astrophysics Data System (ADS)

Hauksson, E.; Shearer, P.

2004-12-01

We synthesize relocated regional seismicity and 3D velocity and Qp models to infer structure and deformation in the transpressive zone of southern California. These models provide a comprehensive synthesis of the tectonic fabric of the upper to middle crust, and the brittle ductile transition zone that in some cases extends into the lower crust. The regional seismicity patterns in southern California are brought into focus when the hypocenters are relocated using the double difference method. In detail, often the spatial correlation between background seismicity and late Quaternary faults is improved as the hypocenters become more clustered, and the spatial patterns are more sharply defined. Along some of the strike-slip faults the seismicity clusters decrease in width and form alignments implying that in many cases the clusters are associated with a single fault. In contrast, the Los Angeles Basin seismicity remains mostly scattered, reflecting a 3D distribution of the tectonic compression. We present the results of relocating 327,000 southern California earthquakes that occurred between 1984 and 2002. In particular, the depth distribution is improved and less affected by layer boundaries in velocity models or other similar artifacts, and thus improves the definition of the brittle ductile transition zone. The 3D VP and VP/VS models confirm existing tectonic interpretations and provide new insights into the configuration of the geological structures in southern California. The models extend from the US-Mexico border in the south to the Coast Ranges and Sierra Nevada in the north, and have 15 km horizontal grid spacing and an average vertical grid spacing of 4 km, down to 22 km depth. The heterogeneity of the crustal structure as imaged in both the VP and VP/VS models is larger within the Pacific than the North America plate, reflecting regional asymmetric variations in the crustal composition and past tectonic processes. Similarly, the relocated seismicity is

Common features and peculiarities of the seismic activity at Phlegraean Fields, Long Valley, and Vesuvius

USGS Publications Warehouse

Marzocchi, W.; Vilardo, G.; Hill, D.P.; Ricciardi, G.P.; Ricco, C.

2001-01-01

We analyzed and compared the seismic activity that has occurred in the last two to three decades in three distinct volcanic areas: Phlegraean Fields, Italy; Vesuvius, Italy; and Long Valley, California. Our main goal is to identify and discuss common features and peculiarities in the temporal evolution of earthquake sequences that may reflect similarities and differences in the generating processes between these volcanic systems. In particular, we tried to characterize the time series of the number of events and of the seismic energy release in terms of stochastic, deterministic, and chaotic components. The time sequences from each area consist of thousands of earthquakes that allow a detailed quantitative analysis and comparison. The results obtained showed no evidence for either deterministic or chaotic components in the earthquake sequences in Long Valley caldera, which appears to be dominated by stochastic behavior. In contrast, earthquake sequences at Phlegrean Fields and Mount Vesuvius show a deterministic signal mainly consisting of a 24-hour periodicity. Our analysis suggests that the modulation in seismicity is in some way related to thermal diurnal processes, rather than luni-solar tidal effects. Independently from the process that generates these periodicities on the seismicity., it is suggested that the lack (or presence) of diurnal cycles is seismic swarms of volcanic areas could be closely linked to the presence (or lack) of magma motion.

77 FR 41048 - Safety Zone; Hudson Valley Triathlon, Ulster Landing, Hudson River, NY

Federal Register 2010, 2011, 2012, 2013, 2014

2012-07-12

... Hudson Valley Triathlon swim event. This temporary safety zone is necessary to protect swimmers.... Regulatory History and Information The Hudson Valley Triathlon swim is an annual recurring event that has a... Valley Triathlon swim event will occur on July 15, 2012. On May 22, 2012, the sponsor of the event...

Oceanographer transform fault structure compared to that of surrounding oceanic crust: Results from seismic refraction data analysis

NASA Astrophysics Data System (ADS)

Ambos, E. L.; Hussong, D. M.

1986-02-01

A high quality seismic refraction data set was collected near the intersection of the tranform portion of the Oceanographer Fracture Zone (OFZ) with the adjacent northern limb of the Mid-Atlantic Ridge spreading center (MAR). One seismic line was shot down the axis of the transform valley. Another was shot parallel to the spreading center, crossing from normal oceanic crust into the transform valley, and out again. This latter line was recorded by four Ocean Bottom Seismometers (OBSs) spaced along its length, providing complete reversed coverage over the crucial transform valley zone. Findings indicate that whereas the crust of the transform valley is only slightly thinner (4.5 km) compared to normal oceanic crust (5-8 km), the structure is different. Velocities in the range of 6.9 to 7.7. km/sec, which are characteristics of seismic layer 3B, are absent, although a substantial thickness (approximately 3 km) of 6.1-6.8 km/sec material does appear to be present. The upper crust, some 2 km in thickness, is characterized by a high velocity gradient (1.5 sec -1) in which veloxity increases from 2.7 km/sec at the seafloor to 5.8 km/sec at the base of the section. A centrally-located deep of the transform valley has thinner crust (1-2 km), whereas the crust gradually thickens past the transform valley-spreading center intersection. Analysis of the seismic line crossing sub-perpendicular to the transform valley demonstrates abrupt thinning of the upper crustal section, and thickening of the lower crust outside of the trasform valley. In addition, high-velocity material seems to occur under the valley flanks, particularly the southern flanking ridge. This ridge, which is on the side of the transform opposite to the intersection of spreading ridge and transform, may be an expression of uplifted, partially serpentinized upper mantle rocks.

75 FR 29975 - Expansion of Foreign-Trade Zone 272; Lehigh Valley, Pennsylvania

Federal Register 2010, 2011, 2012, 2013, 2014

2010-05-28

... DEPARTMENT OF COMMERCE Foreign-Trade Zones Board [Order No. 1679] Expansion of Foreign-Trade Zone 272; Lehigh Valley, Pennsylvania Pursuant to its authority under the Foreign-Trade Zones Act of June... Bethlehem, Pennsylvania, adjacent to the Philadelphia Customs and Border Protection port of entry (FTZ...

Seismic reflection imaging of two megathrust shear zones in the northern Cascadia subduction zone.

PubMed

Calvert, Andrew J

2004-03-11

At convergent continental margins, the relative motion between the subducting oceanic plate and the overriding continent is usually accommodated by movement along a single, thin interface known as a megathrust. Great thrust earthquakes occur on the shallow part of this interface where the two plates are locked together. Earthquakes of lower magnitude occur within the underlying oceanic plate, and have been linked to geochemical dehydration reactions caused by the plate's descent. Here I present deep seismic reflection data from the northern Cascadia subduction zone that show that the inter-plate boundary is up to 16 km thick and comprises two megathrust shear zones that bound a >5-km-thick, approximately 110-km-wide region of imbricated crustal rocks. Earthquakes within the subducting plate occur predominantly in two geographic bands where the dip of the plate is inferred to increase as it is forced around the edges of the imbricated inter-plate boundary zone. This implies that seismicity in the subducting slab is controlled primarily by deformation in the upper part of the plate. Slip on the shallower megathrust shear zone, which may occur by aseismic slow slip, will transport crustal rocks into the upper mantle above the subducting oceanic plate and may, in part, provide an explanation for the unusually low seismic wave speeds that are observed there.

CPT site characterization for seismic hazards in the New Madrid seismic zone

USGS Publications Warehouse

Liao, T.; Mayne, P.W.; Tuttle, M.P.; Schweig, E.S.; Van Arsdale, R.B.

2002-01-01

A series of cone penetration tests (CPTs) were conducted in the vicinity of the New Madrid seismic zone in central USA for quantifying seismic hazards, obtaining geotechnical soil properties, and conducting studies at liquefaction sites related to the 1811-1812 and prehistoric New Madrid earthquakes. The seismic piezocone provides four independent measurements for delineating the stratigraphy, liquefaction potential, and site amplification parameters. At the same location, two independent assessments of soil liquefaction susceptibility can be made using both the normalized tip resistance (qc1N) and shear wave velocity (Vs1). In lieu of traditional deterministic approaches, the CPT data can be processed using probability curves to assess the level and likelihood of future liquefaction occurrence. ?? 2002 Elsevier Science Ltd. All rights reserved.

Slab temperature controls on the Tonga double seismic zone and slab mantle dehydration

PubMed Central

Wei, S. Shawn; Wiens, Douglas A.; van Keken, Peter E.; Cai, Chen

2017-01-01

Double seismic zones are two-layered distributions of intermediate-depth earthquakes that provide insight into the thermomechanical state of subducting slabs. We present new precise hypocenters of intermediate-depth earthquakes in the Tonga subduction zone obtained using data from local island–based, ocean-bottom, and global seismographs. The results show a downdip compressional upper plane and a downdip tensional lower plane with a separation of about 30 km. The double seismic zone in Tonga extends to a depth of about 300 km, deeper than in any other subduction system. This is due to the lower slab temperatures resulting from faster subduction, as indicated by a global trend toward deeper double seismic zones in colder slabs. In addition, a line of high seismicity in the upper plane is observed at a depth of 160 to 280 km, which shallows southward as the convergence rate decreases. Thermal modeling shows that the earthquakes in this “seismic belt” occur at various pressures but at a nearly constant temperature, highlighting the important role of temperature in triggering intermediate-depth earthquakes. This seismic belt may correspond to regions where the subducting mantle first reaches a temperature of ~500°C, implying that metamorphic dehydration of mantle minerals in the slab provides water to enhance faulting. PMID:28097220

Southern Appalachian Regional Seismic Network

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

Chiu, S.C.C.; Johnston, A.C.; Chiu, J.M.

1994-08-01

The seismic activity in the southern Appalachian area was monitored by the Southern Appalachian Regional Seismic Network (SARSN) since late 1979 by the Center for Earthquake Research and Information (CERI) at Memphis State University. This network provides good spatial coverage for earthquake locations especially in east Tennessee. The level of activity concentrates more heavily in the Valley and Ridge province of eastern Tennessee, as opposed to the Blue Ridge or Inner Piedmont. The large majority of these events lie between New York - Alabama lineament and the Clingman/Ocoee lineament, magnetic anomalies produced by deep-seated basement structures. Therefore SARSN, even withmore » its wide station spacing, has been able to define the essential first-order seismological characteristics of the Southern Appalachian seismic zone. The focal depths of the southeastern U.S. earthquakes concentrate between 8 and 16 km, occurring principally beneath the Appalachian overthrust. In cross-sectional views, the average seismicity is shallower to the east beneath the Blue Ridge and Piedmont provinces and deeper to the west beneath the Valley and Ridge and the North American craton. Results of recent focal mechanism studies by using the CERI digital earthquake catalog between October, 1986 and December, 1991, indicate that the basement of the Valley and Ridge province is under a horizontal, NE-SW compressive stress. Right-lateral strike-slip faulting on nearly north-south fault planes is preferred because it agrees with the trend of the regional magnetic anomaly pattern.« less

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.

Seismic Velocity Gradients Across the Transition Zone

NASA Astrophysics Data System (ADS)

Escalante, C.; Cammarano, F.; de Koker, N.; Piazzoni, A.; Wang, Y.; Marone, F.; Dalton, C.; Romanowicz, B.

2006-12-01

One-D elastic velocity models derived from mineral physics do a notoriously poor job at predicting the velocity gradients in the upper mantle transition zone, as well as some other features of models derived from seismological data. During the 2006 CIDER summer program, we computed Vs and Vp velocity profiles in the upper mantle based on three different mineral physics approaches: two approaches based on the minimization of Gibbs Free Energy (Stixrude and Lithgow-Bertelloni, 2005; Piazzoni et al., 2006) and one obtained by using experimentally determined phase diagrams (Weidner and Wang, 1998). The profiles were compared by assuming a vertical temperature profile and two end-member compositional models, the pyrolite model of Ringwood (1979) and the piclogite model of Anderson and Bass (1984). The predicted seismic profiles, which are significantly different from each other, primarily due to different choices of properties of single minerals and their extrapolation with temperature, are tested against a global dataset of P and S travel times and spheroidal and toroidal normal mode eigenfrequencies. All the models derived using a potential temperature of 1600K predict seismic velocities that are too slow in the upper mantle, suggesting the need to use a colder geotherm. The velocity gradient in the transition zone is somewhat better for piclogite than for pyrolite, possibly indicating the need to increase Ca content. The presence of stagnant slabs in the transition zone is a possible explanation for the need for 1) colder temperature and 2) increased Ca content. Future improvements in seismic profiles obtained from mineral physics will arise from better knowledge of elastic properties of upper mantle constituents and aggregates at high temperature and pressure, a better understanding of differences between thermodynamic models, and possibly the effect of water through and on Q. High resolution seismic constraints on velocity jumps at 400 and 660 km also need to be

Near-Surface Structure and Velocities of the Northeastern Santa Cruz Mountains and the Western Santa Clara Valley, California, From Seismic Imaging

USGS Publications Warehouse

Catchings, R.D.; Gandhok, G.; Goldman, M.R.; Steedman, Clare

2007-01-01

Introduction The Santa Clara Valley (SCV) is located in the southern San Francisco Bay area of California and is bounded by the Santa Cruz Mountains to the southwest, the Diablo Ranges to the northeast, and the San Francisco Bay to the north (Fig. 1). The SCV, which includes the City of San Jose, numerous smaller cities, and much of the high-technology manufacturing and research area commonly referred to as the Silicon Valley, has a population in excess of 1.7 million people (2000 U. S. Census;http://quickfacts.census.gov/qfd/states/06/06085.html The SCV is situated between major active faults of the San Andreas Fault system, including the San Andreas Fault to the southwest and the Hayward and Calaveras faults to the northeast, and other faults inferred to lie beneath the alluvium of the SCV (CWDR, 1967; Bortugno et al., 1991). The importance of the SCV as a major industrial center, its large population, and its proximity to major earthquake faults are important considerations with respect to earthquake hazards and water-resource management. The fault-bounded alluvial aquifer system beneath the valley is the source of about one-third of the water supply for the metropolitan area (Hanson et al., 2004). To better address the earthquake hazards of the SCV, the U.S. Geological Survey (USGS) has undertaken a program to evaluate potential seismic sources, the effects of strong ground shaking, and stratigraphy associated with the regional aquifer system. As part of that program and to better understand water resources of the valley, the USGS and the Santa Clara Valley Water District (SCVWD) began joint studies to characterize the faults, stratigraphy, and structures beneath the SCV in the year 2000. Such features are important to both agencies because they directly influence the availability and management of groundwater resources in the valley, and they affect the severity and distribution of strong shaking from local and regional earthquakes sources that may affect

Seismic-reflection profiles of the New Madrid seismic zone-data along the Mississippi River near Caruthersville, Missouri

USGS Publications Warehouse

Crone, A.J.; Harding, S.T.; Russ, D.P.; Shedlock, K.M.

1986-01-01

Three major seismic-reflection programs have been conducted by the USGS in the New Madrid seismic zone. The first program consisted of 32 km of conventional Vibroseis profiling designed to investigate the subsurface structure associated with scarps and lineaments in northwestern Tennessee (Zoback, 1979). A second, more extensive Vibroseis program collected about 250 km of data from all parts of the New Madrid seismic zone in Missouri, Arkansas, and Tennessee (Hamilton and Zoback, 1979, 1982; Zoback and others, 1980). The profiles presented here are part of the third program that collected about 240 km of high-resolution seismic-reflection data from a boat along the Mississippi River between Osceola, Ark., and Wickliffe, Ky. (fig. 1). The data for profiles A, B, C, and D were collected between river miles 839-1/2 and 850-1/2 from near the Interstate-155 bridge to upstream of Caruthersville, Mo. (fig. 2). Profiles on this part of the river are important for three reasons: (1) they connect many of the land-based profiles on either side of the river, (2) they are near the northeast termination of a linear, 120km-long, northeast-southwest zone of seismicity that extends from northeast Arkansas to Caruthersville, Mo. (Stauder, 1982; fig. 1), and (3) they cross the southwesterly projection of the Cottonwood Grove fault (fig. 1), a fault having a substantial amount of vertical Cenozoic offset (Zoback and others, 1980).

New constraints on mechanisms of remotely triggered seismicity at Long Valley Caldera

USGS Publications Warehouse

Brodsky, E.E.; Prejean, S.G.

2005-01-01

Regional-scale triggering of local earthquakes in the crust by seismic waves from distant main shocks has now been robustly documented for over a decade. Some of the most thoroughly recorded examples of repeated triggering of a single site from multiple, large earthquakes are measured in geothermal fields of the western United States like Long Valley Caldera. As one of the few natural cases where the causality of an earthquake sequence is apparent, triggering provides fundamental constraints on the failure processes in earthquakes. We show here that the observed triggering by seismic waves is inconsistent with any mechanism that depends on cumulative shaking as measured by integrated energy density. We also present evidence for a frequency-dependent triggering threshold. On the basis of the seismic records of 12 regional and teleseismic events recorded at Long Valley Caldera, long-period waves (>30 s) are more effective at generating local seismicity than short-period waves of comparable amplitude. If the properties of the system are stationary over time, the failure threshold for long-period waves is ~0.05 cm/s vertical shaking. Assuming a phase velocity of 3.5 km/s and an elastic modulus of 3.5 x 1010Pa, the threshold in terms of stress is 5 kPa. The frequency dependence is due in part to the attenuation of the surface waves with depth. Fluid flow through a porous medium can produce the rest of the observed frequency dependence of the threshold. If the threshold is not stationary with time, pore pressures that are >99.5% of lithostatic and vary over time by a factor of 4 could explain the observations with no frequency dependence of the triggering threshold. Copyright 2005 by the American Geophysical Union.

Prediction of subsurface fracture in mining zone of Papua using passive seismic tomography based on Fresnel zone

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

Setiadi, Herlan; Nurhandoko, Bagus Endar B.; Wely, Woen

Fracture prediction in a block cave of underground mine is very important to monitor the structure of the fracture that can be harmful to the mining activities. Many methods can be used to obtain such information, such as TDR (Time Domain Relectometry) and open hole. Both of them have limitations in range measurement. Passive seismic tomography is one of the subsurface imaging method. It has advantage in terms of measurements, cost, and rich of rock physical information. This passive seismic tomography studies using Fresnel zone to model the wavepath by using frequency parameter. Fresnel zone was developed by Nurhandoko inmore » 2000. The result of this study is tomography of P and S wave velocity which can predict position of fracture. The study also attempted to use sum of the wavefronts to obtain position and time of seismic event occurence. Fresnel zone tomography and the summation wavefront can predict location of geological structure of mine area as well.« less

High-resolution seismic profiling reveals faulting associated with the 1934 Ms 6.6 Hansel Valley earthquake (Utah, USA)

USGS Publications Warehouse

Bruno, Pier Paolo G.; Duross, Christopher; Kokkalas, Sotirios

2017-01-01

The 1934 Ms 6.6 Hansel Valley, Utah, earthquake produced an 8-km-long by 3-km-wide zone of north-south−trending surface deformation in an extensional basin within the easternmost Basin and Range Province. Less than 0.5 m of purely vertical displacement was measured at the surface, although seismologic data suggest mostly strike-slip faulting at depth. Characterization of the origin and kinematics of faulting in the Hansel Valley earthquake is important to understand how complex fault ruptures accommodate regions of continental extension and transtension. Here, we address three questions: (1) How does the 1934 surface rupture compare with faults in the subsurface? (2) Are the 1934 fault scarps tectonic or secondary features? (3) Did the 1934 earthquake have components of both strike-slip and dip-slip motion? To address these questions, we acquired a 6.6-km-long, high-resolution seismic profile across Hansel Valley, including the 1934 ruptures. We observed numerous east- and west-dipping normal faults that dip 40°−70° and offset late Quaternary strata from within a few tens of meters of the surface down to a depth of ∼1 km. Spatial correspondence between the 1934 surface ruptures and subsurface faults suggests that ruptures associated with the earthquake are of tectonic origin. Our data clearly show complex basin faulting that is most consistent with transtensional tectonics. Although the kinematics of the 1934 earthquake remain underconstrained, we interpret the disagreement between surface (normal) and subsurface (strike-slip) kinematics as due to slip partitioning during fault propagation and to the effect of preexisting structural complexities. We infer that the 1934 earthquake occurred along an ∼3-km wide, off-fault damage zone characterized by distributed deformation along small-displacement faults that may be alternatively activated during different earthquake episodes.

Rapid intraplate strain accumulation in the New Madrid seismic zone

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

Liu, L.; Zoback, M.D.; Segall, P.

1992-09-01

Remeasurement of a triangulation network in the southern part of the New Madrid seismic zone with the Global Positioning System has revealed rapid crustal strain accumulation since the 1950s. This area experienced three large (moment magnitudes greater than 8) earthquakes in 1811 to 1812. The orientation and sense of shear is consistent with right-lateral strike slip motion along a northeast-trending fault zone (as indicated by current seismicity). Detection of crustal strain accumulation may be a useful discriminant for identifying areas where potentially damaging intraplate earthquakes may occur despite the absence of large earthquakes during historic time. 34 refs.

Seismic anisotropy of the Archean crust in the Minnesota River Valley, Superior Province

NASA Astrophysics Data System (ADS)

Ferré, Eric C.; Gébelin, Aude; Conder, James A.; Christensen, Nik; Wood, Justin D.; Teyssier, Christian

2014-03-01

The Minnesota River Valley (MRV) subprovince is a well-exposed example of late Archean lithosphere. Its high-grade gneisses display a subhorizontal layering, most likely extending down to the crust-mantle boundary. The strong linear fabric of the gneisses results from high-temperature plastic flow during collage-related contraction. Seismic anisotropies measured up to 1 GPa in the laboratory, and seismic anisotropies calculated through forward-modeling indicate ΔVP ~5-6% and ΔVS ~3%. The MRV crust exhibits a strong macroscopic layering and foliation, and relatively strong seismic anisotropies at the hand specimen scale. Yet the horizontal attitude of these structures precludes any substantial contribution of the MRV crust to shear wave splitting for vertically propagating shear waves such as SKS. The origin of the regionally low seismic anisotropy must lie in the upper mantle. A horizontally layered mantle underneath the United States interior could provide an explanation for the observed low SWS.

Seismic Measurement of the Locations of the Base of Convection Zone and Helium Ionization Zone for Stars in the Kepler Seismic LEGACY Sample

NASA Astrophysics Data System (ADS)

Verma, Kuldeep; Raodeo, Keyuri; Antia, H. M.; Mazumdar, Anwesh; Basu, Sarbani; Lund, Mikkel N.; Silva Aguirre, Víctor

2017-03-01

Acoustic glitches are regions inside a star where the sound speed or its derivatives change abruptly. These leave a small characteristic oscillatory signature in the stellar oscillation frequencies. With the precision achieved by Kepler seismic data, it is now possible to extract these small amplitude oscillatory signatures, and infer the locations of the glitches. We perform glitch analysis for all the 66 stars in the Kepler seismic LEGACY sample to derive the locations of the base of the envelope convection zone (CZ) and the helium ionization zone. The signature from helium ionization zone is found to be robust for all stars in the sample, whereas the CZ signature is found to be weak and problematic, particularly for relatively massive stars with large errorbars on the oscillation frequencies. We demonstrate that the helium glitch signature can be used to constrain the properties of the helium ionization layers and the helium abundance.

Long seismic activity in the Porto dos Gaúchos Seismic Zone(PGSZ) - Amazon Craton Brazil

NASA Astrophysics Data System (ADS)

Barros, L. V.; Bowen, B. M. D.; Schmidt, K.

2017-12-01

The largest earthquake ever observed in the stable continental interior of the South American plate occurred in Serra do Tombador (ST), Mato Grosso state - Brazil, on January 31, 1955 with magnitude 6.2 m b . Since then no other earthquake has been located near the 1955 epicenter. However, in Porto dos Gaúchos (PG), 100 km northeast of ST, a recurrent seismicity has been observed since 1959. Both ST and PG are located in the Phanerozoic Parecis basin whose sediments overlies the crystalline basement of Amazon craton. Two magnitude 5 earthquakes occurred in PG, in 1998 and 2005 with intensities up to VI and V, respectively. These two main shocks were followed by aftershock sequences, studied by local seismic networks, last up today, almost 30 years later, period in which it was detected more than seven thousand of seismic events. Both sequences occurred in the same WSW-ENE oriented fault zone with right-lateral strike-slip mechanisms. The epicentral zone is near the northern border of Parecis basin, where there are buried grabens, generally trending WNW-ESE, such as the deep Mesoproterozoic Caiabis graben which lies partly beneath the Parecis basin. The seismogenic fault is located in a basement high, which is probably related with the same seismogenic feature responsible for the earthquakes in PGSZ. The 1955 earthquake, despite the uncertainty in its epicenter, does not seem to be directly related to any buried graben either. The seismicity in the PGSZ, therefore, is not directly related to rifted crust.Not considering the possibility of miss location in the ST earthquake, its isolated occurrence - from the perspective of new studies on intraplate seismicity - lead us to think that the PGSZ was activated by stresses released by the earthquake of 1955 and that the seismogenic fault of ST would have closed a cycle of activity. This would explain its seismic quiescence. However, other studies are necessary to prove this hypothesis, such as the measurement of the

New Madrid Seismic Zone: a test case for naturally induced seismicity

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

Nava, S.J.

1983-09-01

Induced seismicity caused by man-made events, such as the filling of reservoirs has been well documented. In contrast, naturally induced seismicity has received little attention. It has been shown that a fluctuation of as little as several bars can trigger reservoir induced earthquakes. Naturally occurring phenomena generate similar fluctuations and could trigger earthquakes where the faults in ambient stress field are suitably oriented and close to failure. The New Madrid Seismic Zone (NMSZ) presents an ideal test case for the study of naturally induced seismicity. The ideal data set for a study of triggering effects must contain a statistically significantmore » number of events, a constant accumulated strain, and a limited focal region. New Madrid earthquakes are well documented from 1974 to the present, down to a magnitude approx. 1.8. They lie in a distinct fault pattern and occur as a reaction to the regional stress regime. A statistical correlation was made between the earthquakes and a variety of different types of loads, to see if New Madrid seismicity could be triggered by natural fluctuations. The types of triggers investigated ranged from solid earth tides to variations in barometric pressure, rainfall, and stages of the Mississippi River. This analysis becomes complex because each factor investigated creates individual stresses, as well as having imbedded in it a reaction to other factors.« less

Seismic investigation of an ocean-continent transition zone in the northern South China Sea

NASA Astrophysics Data System (ADS)

Zhu, J.; Qiu, X.; Xu, H.; Zhan, W.; Sun, Z.

2011-12-01

Rifted continental margins and basins are mainly formed by the lithospheric extension. Thined lithosphere of passive continental margins results in decompression melt of magma and created oceanic crust and thined ocean-continent transition (OCT) zone. Two refraction profiles used ocean bottom seismometers deployed in the broad continental shelf and three multi-channel seismic reflection lines in the northern South China Sea, acquired by the ship "Shiyan 2" of the South China Sea Institute of Oceanology, Chinese Academy of Sciences in 2010, are processed and interpreted in this study. Seismic reflection lines cut through the Dongsha rise, Zhu-1 and Zhu-2 depression within a Tertiary basin, Pear River Mouth basin (called as Zhujiangkou basin). These tectonic features are clear imaged in the seismic reflection records. Numerous normal faults, cutted through the basement and related to the stretch of the northern South China Sea margin, are imaged and interpreted. Reflection characteristics of the ocean-continent transition (OCT) zone are summaried and outlined. The COT zone is mainly divided into the northern syn-rift subsidence zone, central volcano or buried volcano uplift zone and tilt faulted block near the South Chia Sea basin. Compared to the previous seismic reflection data and refraction velocity models, the segmentation range of the OCT zone is outlined, from width of about 225 km in the northeastern South China Sea , of 160 km in the central to of 110 km in the north-central South China Sea. Based on the epicenter distribution of sporadic and large than 6 magnitude earthquakes, it suggests the OCT zone in the northern South China Sea at present is still an active seismic zone.

Mid-continent earthquake zones; lessons from New Madrid, Missouri

USGS Publications Warehouse

Mitchell, B. J.

1991-01-01

Many seismically active regions occur throughout the world as concentrated zones surrounded by the relatively stable crust of shields or platforms. Examples occur in central and eastern North America, northeastern Brazil, Australia, Norway, Svalbard, Greenland, and other places. Some of these zones, such as those at New Madrid, Missouri, and in the St. Lawrence Valley on the Canadian border, extend over relatively large areas and are marked by a high level of seismicity. Others, such as that near Anna Ohio, are smaller, and the level of activity is lower. Some zones are occasinoally sites for major earthquakes which, if they are in populated regions, can cause widespread destrucion and loss of life. 

CRUSTAL REFRACTION PROFILE OF THE LONG VALLEY CALDERA, CALIFORNIA, FROM THE JANUARY 1983 MAMMOTH LAKES EARTHQUAKE SWARM.

USGS Publications Warehouse

Luetgert, James H.; Mooney, Walter D.

1985-01-01

Seismic-refraction profiles recorded north of Mammoth Lakes, California, using earthquake sources from the January 1983 swarm complement earlier explosion refraction profiles and provide velocity information from deeper in the crust in the area of the Long Valley caldera. Eight earthquakes from a depth range of 4. 9 to 8. 0 km confirm the observation of basement rocks with seismic velocities ranging from 5. 8 to 6. 4 km/sec extending at least to depths of 20 km. The data provide further evidence for the existence of a partial melt zone beneath Long Valley caldera and constrain its geometry. Refs.

Characterization of intrabasin faulting and deformation for earthquake hazards in southern Utah Valley, Utah, from high-resolution seismic imaging

USGS Publications Warehouse

Stephenson, William J.; Odum, Jack K.; Williams, Robert A.; McBride, John H.; Tomlinson, Iris

2012-01-01

We conducted active and passive seismic imaging investigations along a 5.6-km-long, east–west transect ending at the mapped trace of the Wasatch fault in southern Utah Valley. Using two-dimensional (2D) P-wave seismic reflection data, we imaged basin deformation and faulting to a depth of 1.4 km and developed a detailed interval velocity model for prestack depth migration and 2D ground-motion simulations. Passive-source microtremor data acquired at two sites along the seismic reflection transect resolve S-wave velocities of approximately 200 m/s at the surface to about 900 m/s at 160 m depth and confirm a substantial thickening of low-velocity material westward into the valley. From the P-wave reflection profile, we interpret shallow (100–600 m) bedrock deformation extending from the surface trace of the Wasatch fault to roughly 1.5 km west into the valley. The bedrock deformation is caused by multiple interpreted fault splays displacing fault blocks downward to the west of the range front. Further west in the valley, the P-wave data reveal subhorizontal horizons from approximately 90 to 900 m depth that vary in thickness and whose dip increases with depth eastward toward the Wasatch fault. Another inferred fault about 4 km west of the mapped Wasatch fault displaces horizons within the valley to as shallow as 100 m depth. The overall deformational pattern imaged in our data is consistent with the Wasatch fault migrating eastward through time and with the abandonment of earlier synextensional faults, as part of the evolution of an inferred 20-km-wide half-graben structure within Utah Valley. Finite-difference 2D modeling suggests the imaged subsurface basin geometry can cause fourfold variation in peak ground velocity over distances of 300 m.

Map of the Rinconada and Reliz Fault Zones, Salinas River Valley, California

USGS Publications Warehouse

Rosenberg, Lewis I.; Clark, Joseph C.

2009-01-01

The Rinconada Fault and its related faults constitute a major structural element of the Salinas River valley, which is known regionally, and referred to herein, as the 'Salinas Valley'. The Rinconada Fault extends 230 km from King City in the north to the Big Pine Fault in the south. At the south end of the map area near Santa Margarita, the Rinconada Fault separates granitic and metamorphic crystalline rocks of the Salinian Block to the northeast from the subduction-zone assemblage of the Franciscan Complex to the southwest. Northwestward, the Rinconada Fault lies entirely within the Salinian Block and generally divides this region into two physiographically and structurally distinct areas, the Santa Lucia Range to the west and the Salinas Valley to the east. The Reliz Fault, which continues as a right stepover from the Rinconada Fault, trends northwestward along the northeastern base of the Sierra de Salinas of the Santa Lucia Range and beyond for 60 km to the vicinity of Spreckels, where it is largely concealed. Aeromagnetic data suggest that the Reliz Fault continues northwestward another 25 km into Monterey Bay, where it aligns with a high-definition magnetic boundary. Geomorphic evidence of late Quaternary movement along the Rinconada and Reliz Fault Zones has been documented by Tinsley (1975), Dibblee (1976, 1979), Hart (1976, 1985), and Klaus (1999). Although definitive geologic evidence of Holocene surface rupture has not been found on these faults, they were regarded as an earthquake source for the California Geological Survey [formerly, California Division of Mines and Geology]/U.S. Geological Survey (CGS/USGS) Probabilistic Seismic Hazards Assessment because of their postulated slip rate of 1+-1 mm/yr and their calculated maximum magnitude of 7.3. Except for published reports by Durham (1965, 1974), Dibblee (1976), and Hart (1976), most information on these faults is unpublished or is contained in theses, field trip guides, and other types of reports

Induced seismicity constraints on subsurface geological structure, Paradox Valley, Colorado

NASA Astrophysics Data System (ADS)

Block, Lisa V.; Wood, Christopher K.; Yeck, William L.; King, Vanessa M.

2015-02-01

Precise relative hypocentres of seismic events induced by long-term fluid injection at the Paradox Valley Unit (PVU) brine disposal well provide constraints on the subsurface geological structure and compliment information available from deep seismic reflection and well data. We use the 3-D spatial distribution of the hypocentres to refine the locations, strikes, and throws of subsurface faults interpre­ted previously from geophysical surveys and to infer the existence of previously unidentified subsurface faults. From distinct epicentre lineations and focal mechanism trends, we identify a set of conjugate fracture orientations consistent with shear-slip reactivation of late-Palaeozoic fractures over a widespread area, as well as an additional fracture orientation present only near the injection well. We propose simple Mohr-Coulomb fracture models to explain these observations. The observation that induced seismicity preferentially occurs along one of the identified conjugate fracture orientations can be explained by a rotation in the direction of the regional maximum compressive stress from the time when the fractures were formed to the present. Shear slip along the third fracture orientation observed near the injection well is inconsistent with the current regional stress field and suggests a local rotation of the horizontal stresses. The detailed subsurface model produced by this analysis provides important insights for anticipating spatial patterns of future induced seismicity and for evaluation of possible additional injection well sites that are likely to be seismically and hydrologically isolated from the current well. In addition, the interpreted fault patterns provide constraints for estimating the maximum magnitude earthquake that may be induced, and for building geomechanical models to simulate pore pressure diffusion, stress changes and earthquake triggering.

Origin of the Blytheville Arch, and long-term displacement on the New Madrid seismic zone, central United States

USGS Publications Warehouse

Pratt, Thomas L.; Williams, Robert; Odum, Jackson K.; Stephenson, William J.

2013-01-01

The southern arm of the New Madrid seismic zone of the central United States coincides with the buried, ~110 km by ~20 km Blytheville Arch antiform within the Cambrian–Ordovician Reelfoot rift graben. The Blytheville Arch has been interpreted at various times as a compressive structure, an igneous intrusion, or a sediment diapir. Reprocessed industry seismic-reflection profiles presented here show a strong similarity between the Blytheville Arch and pop-up structures, or flower structures, within strike-slip fault systems. The Blytheville Arch formed in the Paleozoic, but post–Mid-Cretaceous to Quaternary strata show displacement or folding indicative of faulting. Faults within the graben structure but outside of the Blytheville Arch also appear to displace Upper Cretaceous and perhaps younger strata, indicating that past faulting was not restricted to the Blytheville Arch and New Madrid seismic zone. As much as 10–12.5 km of strike slip can be estimated from apparent shearing of the Reelfoot arm of the New Madrid seismic zone. There also appears to be ~5–5.5 km of shearing of the Reelfoot topographic scarp at the north end of the southern arm of the New Madrid seismic zone and of the southern portion of Crowley's Ridge, which is a north-trending topographic ridge just south of the seismic zone. These observations suggest that there has been substantial strike-slip displacement along the Blytheville Arch and southern arm of the New Madrid seismic zone, that strike-slip extended north and south of the modern seismic zone, and that post–Mid-Cretaceous (post-Eocene?) faulting was not restricted to the Blytheville Arch or to currently active faults within the New Madrid seismic zone.

Delineation of faulting and basin geometry along a seismic reflection transect in urbanized San Bernardino Valley, California

USGS Publications Warehouse

Stephenson, W.J.; Odum, J.K.; Williams, R.A.; Anderson, M.L.

2002-01-01

Fourteen kilometers of continuous, shallow seismic reflection data acquired through the urbanized San Bernardino Valley, California, have revealed numerous faults between the San Jacinto and San Andreas faults as well as a complex pattern of downdropped and uplifted blocks. These data also indicate that the Loma Linda fault continues northeastward at least 4.5 km beyond its last mapped location on the southern edge of the valley and to within at least 2 km of downtown San Bernardino. Previously undetected faults within the valley northeast of the San Jacinto fault are also imaged, including the inferred western extension of the Banning fault and several unnamed faults. The Rialto-Colton fault is interpreted southwest of the San Jacinto fault. The seismic data image the top of the crystalline basement complex across 70% of the profile length and show that the basement has an overall dip of roughly 10?? southwest between Perris Hill and the San Jacinto fault. Gravity and aeromagnetic data corroborate the interpreted location of the San Jacinto fault and better constrain the basin depth along the seismic profile to be as deep as 1.7 km. These data also corroborate other fault locations and the general dip of the basement surface. At least 1.2 km of apparent vertical displacement on the basement is observed across the San Jacinto fault at the profile location. The basin geometry delineated by these data was used to generate modeled ground motions that show peak horizontal amplifications of 2-3.5 above bedrock response in the 0.05- to 1.0-Hz frequency band, which is consistent with recorded earthquake data in the valley.

Seismicity and structure of Nazca Plate subduction zone in southern Peru

NASA Astrophysics Data System (ADS)

Lim, H.; Kim, Y.; Clayton, R. W.

2015-12-01

We image the Nazca plate subduction zone system by detecting and (re)locating intra-slab earthquakes in southern Peru. Dense seismic arrays (PeruSE, 2013) were deployed along four lines to target geophysical characterization of the subduction system in the transition zone between flat and normal dipping segments of the Nazca plate (2-15°S). The arc volcanism is absent near the flat slab segment, and currently, the correlation between the location of the active volcanic front and corresponding slab depth is neither clear nor consistent between previously published models from seismicity. We detect 620 local earthquakes from August 2008 to February 2013 by manually picking 6559 and 4145 arrival times for P- and S-phases, respectively. We observe that the S-phase data is helpful to reduce the trade-off between origin time and depth of deeper earthquakes (>100 km). Earthquake locations are relocated to constrain the Nazca slab-mantle interface in the slab-dip transition zone using 7322 measurements of differential times of nearby earthquake pairs by waveform cross-correlation. We also employ the double-difference tomography (Zhang and Thurber, 2003) to further improve earthquake source locations and the spatial resolution of the velocity structure simultaneously. The relocated hypocenters clearly delineate the dipping Wadati-Benioff zone in the slab-dip transition zone between the shallow- (25°) to-flat dipping slab segment in the north and the normal (40°) dipping segment in the south. The intermediate-depth seismicity in the flat slab region stops at a depth of ~100 km and a horizontal distance of ~400 km from the trench. We find a significant slab-dip difference (up to 10°) between our relocated seismicity and previously published slab models along the profile region sampling the normal-dip slab at depth (>100 km).

Reassessing the New Madrid Seismic Zone

NASA Astrophysics Data System (ADS)

Atkinson, Gail; Bakun, Bill; Bodin, Paul; Boore, David; Camer, Chris; Frankel, Art; Gasperini, Paulo; Gomberg, Joan; Hanks, Tom; Hermann, Bob; Hough, Susan; Johnston, Arch; Kenner, Shelley; Langston, Chuck; Linker, Mark; Mayne, Paul; Petersen, Mark; Powell, Christine; Prescott, Will; Schweig, Eugene; Segall, Paul; Stein, Seth; Stuart, Bill; Tuttle, Martitia; VanArsdale, Roy

The central enigma of the mid-continent region in the United States known as the New Madrid seismic zone (NMSZ; Figure 1) involves the mechanisms that give rise to recurrent great earthquakes far from plate boundaries. Given the lack of significant topographic relief that is the hallmark of tectonic activity in most actively deforming regions, most of us feel a need to “pinch ourselves to see if we're dreaming” when confronted with evidence that, at some probability levels, the earthquake hazard throughout the NMSZ is comparable to that estimated for the San Francisco Bay region.Although assessing the hazard in the NMSZ is in many ways more challenging than in the western United States, and the uncertainties are much greater, careful scientific study has led to a consensus on the issues most critical to seismic hazard assessment.

Site-specific seismic ground motion analyses for transportation infrastructure in the New Madrid seismic zone.

DOT National Transportation Integrated Search

2012-11-01

Generic, code-based design procedures cannot account for the anticipated short-period attenuation and long-period amplification of earthquake ground motions in the deep, soft sediments of the Mississippi Embayment within the New Madrid Seismic Zone (...

33 CFR 3.40-65 - Sector Ohio Valley Marine Inspection Zone and Captain of the Port Zone; Marine Safety Unit...

Code of Federal Regulations, 2012 CFR

2012-07-01

... 33 Navigation and Navigable Waters 1 2012-07-01 2012-07-01 false Sector Ohio Valley Marine Inspection Zone and Captain of the Port Zone; Marine Safety Unit Pittsburgh. 3.40-65 Section 3.40-65 Navigation and Navigable Waters COAST GUARD, DEPARTMENT OF HOMELAND SECURITY GENERAL COAST GUARD AREAS, DISTRICTS, SECTORS, MARINE INSPECTION ZONES, AND...

33 CFR 3.40-65 - Sector Ohio Valley Marine Inspection Zone and Captain of the Port Zone; Marine Safety Unit...

Code of Federal Regulations, 2014 CFR

2014-07-01

... 33 Navigation and Navigable Waters 1 2014-07-01 2014-07-01 false Sector Ohio Valley Marine Inspection Zone and Captain of the Port Zone; Marine Safety Unit Pittsburgh. 3.40-65 Section 3.40-65 Navigation and Navigable Waters COAST GUARD, DEPARTMENT OF HOMELAND SECURITY GENERAL COAST GUARD AREAS, DISTRICTS, SECTORS, MARINE INSPECTION ZONES, AND...

33 CFR 3.40-65 - Sector Ohio Valley Marine Inspection Zone and Captain of the Port Zone; Marine Safety Unit...

Code of Federal Regulations, 2011 CFR

2011-07-01

... 33 Navigation and Navigable Waters 1 2011-07-01 2011-07-01 false Sector Ohio Valley Marine Inspection Zone and Captain of the Port Zone; Marine Safety Unit Pittsburgh. 3.40-65 Section 3.40-65 Navigation and Navigable Waters COAST GUARD, DEPARTMENT OF HOMELAND SECURITY GENERAL COAST GUARD AREAS, DISTRICTS, SECTORS, MARINE INSPECTION ZONES, AND...

33 CFR 3.40-65 - Sector Ohio Valley Marine Inspection Zone and Captain of the Port Zone; Marine Safety Unit...

Code of Federal Regulations, 2013 CFR

2013-07-01

... 33 Navigation and Navigable Waters 1 2013-07-01 2013-07-01 false Sector Ohio Valley Marine Inspection Zone and Captain of the Port Zone; Marine Safety Unit Pittsburgh. 3.40-65 Section 3.40-65 Navigation and Navigable Waters COAST GUARD, DEPARTMENT OF HOMELAND SECURITY GENERAL COAST GUARD AREAS, DISTRICTS, SECTORS, MARINE INSPECTION ZONES, AND...

Reconciling short recurrence intervals with minor deformation in the New Madrid seismic zone

USGS Publications Warehouse

Schweig, E.S.; Ellis, M.A.

1994-01-01

At least three great earthquakes occurred in the New Madrid seismic zone in 1811 and 1812. Estimates of present-day strain rates suggest that such events may have a repeat time of 1000 years or less. Paleoseismological data also indicate that earthquakes large enough to cause soil liquefaction have occurred several times in the past 5000 years. However, pervasive crustal deformation expected from such a high frequency of large earthquakes is not observed. This suggests that the seismic zone is a young feature, possibly as young as several tens of thousands of years old and no more than a few million years old.At least three great earthquakes occurred in the New Madrid seismic zone in 1811 and 1812. Estimates of present-day strain rates suggest that such events may have a repeat time of 1000 years or less. Paleoseismological data also indicate that earthquakes large enough to cause soil liquefaction have occurred several times in the past 5000 years. However, pervasive crustal deformation expected from such a high frequency of large earthquakes is not observed. This suggests that the seismic zone is a young feature, possibly as young as several tens of thousands of years old and no more than a few million years old.

Seismic ground motion scenarios in Lower Tagus Valley Basin

NASA Astrophysics Data System (ADS)

Borges, José; Torres, Ricardo; Furtado, José; Silva, Hugo; Caldeira, Bento; Pinto, Carlos; Bezzeghoud, Mourad; Carvalho, João

2013-04-01

Throughout its history the Lower Tagus Valley (LTV) has been struck by several earthquakes which produced important material damage and loss of lives: The 1st of November 1755 Lisbon earthquake and the 1969 earthquake (Mw=7.3), located in the SW Iberia Margin and the 1344, 1531 and 1909 (M= 6 to 7) with epicenter located inside the LTV basin. Since this region is the most highly populated region in Portugal, it is expected that an earthquake of similar magnitude of those that have occurred in the past will cause an enormous destruction and casualties. This fact makes LTV a high priority area for earthquake research in Portugal. In order to overcome the problems related to the absence of geological outcrops, low slip-rates (<0,4 mm/year) and the processes of sedimentation and erosion, we use in this work seismic reflection profiles, potential field data, soundings, wells and geological cartography to obtain a map of the main seismogenic structures and to characterize their seismic potential [1]. Moreover, a 3D structural model has been developed for de LTV based on Seismic reflection, Seismic Noise and potential field data [2,3]. In order to improve assessment of the seismic hazard in the LTV basin, we simulate long-period (0-1 Hz) ground motion time histories for a suite of scenarios earthquakes (Mw =5.5 to 7) within the basin, using fault geometries and the 3D seismic velocity structure based on the previous mentioned works. References [1] Pinto, Carlos C. (2011). Identification of Seismogenic Structures in the Lower Tagus Basin. Master Thesis, Universidade de Évora, 128 pp. [2] Torres, R.J.G., (2012). Modelo de velocidade da Bacia do Vale do Tejo: uma abordagem baseada no estudo do ruído sísmico ambiental, Master Thesis, Universidade de Évora, 83pp. [3] Furtado, J.A (2010). Confirmação do modelo da estrutura 3D do Vale Inverior do Tejo a partir de dados de ruído sísmico ambiente, Master Thesis, Universidade de Évora, 136pp.

Disturbed zones; indicators of deep-seated subsurface faults in the Valley and Ridge and Appalachian structural front of Pennsylvania

USGS Publications Warehouse

Pohn, Howard A.; Purdy, Terri L.

1982-01-01

Field studies of geologic structures in the Valley and Ridge and adjacent parts of the Appalachian Plateau provinces in Pennsylvania have shown a new type of structure, formerly poorly understood and frequently unmapped, is a significant indicator of deep-seated subsurface faulting. These structures, herein called disturbed zones, are formed by movement between closely spaced pairs of thrust faults. Disturbed zones are characterized at the surface by long, narrow, intensely folded and faulted zones of rocks in a relatively undisturbed stratigraphic sequence. These zones are frequently kilometers to tens of kilometers long and tens to hundreds of meters wide. Although disturbed zones generally occur in sequences of alternating siltstone and shale beds, they can also occur in other lithologies including massively-bedded sandstones and carbonates. Disturbed zones are not only easily recognized in outcrop but their presence can also be inferred on geologic maps by disharmonic fold patterns, which necessitates a detachment between adjacent units that show the disharmony. A number of geologic problems can be clarified by understanding the principles of the sequence of formation and the method of location of disturbed zones, including the interpretation of some published geologic cross sections and maps. The intense folding and faulting which accompanies the formation of a typical disturbed zone produces a region of fracture porosity which, if sealed off from the surface, might well serve as a commercially-exploitable hydrocarbon trap. We believe that the careful mapping of concentrations of disturbed zones can serve as an important exploration method which is much less expensive than speculation seismic lines.

Focused seismicity triggered by flank instability on Kīlauea's Southwest Rift Zone

NASA Astrophysics Data System (ADS)

Judson, Josiah; Thelen, Weston A.; Greenfield, Tim; White, Robert S.

2018-03-01

Swarms of earthquakes at the head of the Southwest Rift Zone on Kīlauea Volcano, Hawai´i, reveal an interaction of normal and strike-slip faulting associated with movement of Kīlauea's south flank. A relocated subset of earthquakes between January 2012 and August 2014 are highly focused in space and time at depths that are coincident with the south caldera magma reservoir beneath the southern margin of Kīlauea Caldera. Newly calculated focal mechanisms are dominantly dextral shear with a north-south preferred fault orientation. Two earthquakes within this focused area of seismicity have normal faulting mechanisms, indicating two mechanisms of failure in very close proximity (10's of meters to 100 m). We suggest a model where opening along the Southwest Rift Zone caused by seaward motion of the south flank permits injection of magma and subsequent freezing of a plug, which then fails in a right-lateral strike-slip sense, consistent with the direction of movement of the south flank. The seismicity is concentrated in an area where a constriction occurs between a normal fault and the deeper magma transport system into the Southwest Rift Zone. Although in many ways the Southwest Rift Zone appears analogous to the more active East Rift Zone, the localization of the largest seismicity (>M2.5) within the swarms to a small volume necessitates a different model than has been proposed to explain the lineament outlined by earthquakes along the East Rift Zone.

Seismic fault zone trapped noise

NASA Astrophysics Data System (ADS)

Hillers, G.; Campillo, M.; Ben-Zion, Y.; Roux, P.

2014-07-01

Systematic velocity contrasts across and within fault zones can lead to head and trapped waves that provide direct information on structural units that are important for many aspects of earthquake and fault mechanics. Here we construct trapped waves from the scattered seismic wavefield recorded by a fault zone array. The frequency-dependent interaction between the ambient wavefield and the fault zone environment is studied using properties of the noise correlation field. A critical frequency fc ≈ 0.5 Hz defines a threshold above which the in-fault scattered wavefield has increased isotropy and coherency compared to the ambient noise. The increased randomization of in-fault propagation directions produces a wavefield that is trapped in a waveguide/cavity-like structure associated with the low-velocity damage zone. Dense spatial sampling allows the resolution of a near-field focal spot, which emerges from the superposition of a collapsing, time reversed wavefront. The shape of the focal spot depends on local medium properties, and a focal spot-based fault normal distribution of wave speeds indicates a ˜50% velocity reduction consistent with estimates from a far-field travel time inversion. The arrival time pattern of a synthetic correlation field can be tuned to match properties of an observed pattern, providing a noise-based imaging tool that can complement analyses of trapped ballistic waves. The results can have wide applicability for investigating the internal properties of fault damage zones, because mechanisms controlling the emergence of trapped noise have less limitations compared to trapped ballistic waves.

Seismic velocity structure in the western part of Nankai subduction zone

NASA Astrophysics Data System (ADS)

Yamamoto, Y.; Obana, K.; Takahashi, T.; Nakanishi, A.; Kodaira, S.; Kaneda, Y.

2011-12-01

In the Nankai Trough, three major seismogenic zones of megathrust earthquake exist (Tokai, Tonankai and Nankai earthquake regions). The Hyuga-nada region was distinguished from these seismogenic zones because of the lack of megathrust earthquake. However, recent studies show the possibility of simultaneous rupture of the Nankai and Hyuga-nada segments was also pointed out [e.g., Furumura et al, 2010 JGR]. Because seismic velocity structure is one of the useful and basic information for understanding the possibility of seismic linkage of Nankai and Hyuga-nada segments, Japan Agency for Marine-Earth Science and Technology has been carried out a series of wide-angle active source surveys and local seismic observations among the three major seismogenic zones and Hyuga-nada segment from 2008, as a part of "Research concerning Interaction Between the Tokai, Tonankai and Nankai Earthquakes' funded by Ministry of Education, Culture, Sports, Science and Technology, Japan". We are performing two set of three-dimensional seismic velocity tomographic inversions, one is in the Hyuga-nada region and the other is western part of the coseismic rupture area of 1946 Nankai earthquake, to discuss the relationship between the structural heterogeneities and the location of segment boundary between Hyuga-nada and Nankai segment. For the analysis of Hyuga-nada segment, we used both active and passive source data. The obtained velocity model clearly showed the subducted Kyushu-Palau ridge as thick low velocity Philippine Sea slab in the southwestern part. Our velocity image also indicates that "the thin oceanic crust zone" located between Nankai segment and Kyushu-Palau Ridge segment, founded by Nakanishi et al [2010, AGU] by analyzing of the active source survey, continuously exists from trough axis to near the coastline of Kyushu Island. The overriding plate just above the coseismic slip area of 1968 Hyuga-nada earthquake shows relatively high velocity. Although the tomographic study in

Storage of fluids and melts at subduction zones detectable by seismic tomography

NASA Astrophysics Data System (ADS)

Luehr, B. G.; Koulakov, I.; Rabbel, W.; Brotopuspito, K. S.; Surono, S.

2015-12-01

During the last decades investigations at active continental margins discovered the link between the subduction of fluid saturated oceanic plates and the process of ascent of these fluids and partial melts forming a magmatic system that leads to volcanism at the earth surface. For this purpose the geophysical structure of the mantle and crustal range above the down going slap has been imaged. Information is required about the slap, the ascent paths, as well as the reservoires of fluids and partial melts in the mantle and the crust up to the volcanoes at the surface. Statistically the distance between the volcanoes of volcanic arcs down to their Wadati Benioff zone results of approximately 100 kilometers in mean value. Surprisingly, this depth range shows pronounced seismicity at most of all subduction zones. Additionally, mineralogical laboratory investigations have shown that dehydration of the diving plate has a maximum at temperature and pressure conditions we find at around 100 km depth. The ascent of the fluids and the appearance of partial melts as well as the distribution of these materials in the crust can be resolved by seismic tomographic methods using records of local natural seismicity. With these methods these areas are corresponding to lowered seismic velocities, high Vp/Vs ratios, as well as increased attenuation of seismic shear waves. The anomalies and their time dependence are controlled by the fluids. The seismic velocity anomalies detected so far are within a range of a few per cent to more than 30% reduction. But, to explore plate boundaries large and complex amphibious experiments are required, in which active and passive seismic investigations should be combined to achieve best results. The seismic station distribution should cover an area from before the trench up to far behind the volcanic chain, to provide under favorable conditions information down to 150 km depth. Findings of different subduction zones will be compared and discussed.

Seismic Readings from the Deepest Borehole in the New Madrid Seismic Zone

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

Woolery, Edward W; Wang, Zhenming; Sturchio, Neil C

2006-03-01

Since the 1980s, the research associated with the UK network has been primarily strong-motion seismology of engineering interest. Currently the University of Kentucky operates a strong-motion network of nine stations in the New Madrid Seismic Zone. A unique feature of the network is the inclusions of vertical strong-motion arrays, each with one or two downhole accelerometers. The deepest borehole array is 260 m below the surfaces at station VASA in Fulton County, Kentucky. A preliminary surface seismic refraction survey was conducted at the site before drilling the hole at VSAS (Woolery and Wang, 2002). The depth to the Paleozoic bedrockmore » at the site was estimated to be approximately 595 m, and the depth to the first very stiff layer (i.e. Porters Creek Clay) was found to be about 260 m. These depths and stratigraphic interpretation correlated well with a proprietary seismic reflection line and the Ken-Ten Oil Exploration No. 1 Sanger hole (Schwalb, 1969), as well as our experience in the area (Street et al., 1995; Woolery et al., 1999).« less

Modeling and validation of a 3D velocity structure for the Santa Clara Valley, California, for seismic-wave simulations

USGS Publications Warehouse

Hartzell, S.; Harmsen, S.; Williams, R.A.; Carver, D.; Frankel, A.; Choy, G.; Liu, P.-C.; Jachens, R.C.; Brocher, T.M.; Wentworth, C.M.

2006-01-01

A 3D seismic velocity and attenuation model is developed for Santa Clara Valley, California, and its surrounding uplands to predict ground motions from scenario earthquakes. The model is developed using a variety of geologic and geophysical data. Our starting point is a 3D geologic model developed primarily from geologic mapping and gravity and magnetic surveys. An initial velocity model is constructed by using seismic velocities from boreholes, reflection/refraction lines, and spatial autocorrelation microtremor surveys. This model is further refined and the seismic attenuation is estimated through waveform modeling of weak motions from small local events and strong-ground motion from the 1989 Loma Prieta earthquake. Waveforms are calculated to an upper frequency of 1 Hz using a parallelized finite-difference code that utilizes two regions with a factor of 3 difference in grid spacing to reduce memory requirements. Cenozoic basins trap and strongly amplify ground motions. This effect is particularly strong in the Evergreen Basin on the northeastern side of the Santa Clara Valley, where the steeply dipping Silver Creek fault forms the southwestern boundary of the basin. In comparison, the Cupertino Basin on the southwestern side of the valley has a more moderate response, which is attributed to a greater age and velocity of the Cenozoic fill. Surface waves play a major role in the ground motion of sedimentary basins, and they are seen to strongly develop along the western margins of the Santa Clara Valley for our simulation of the Loma Prieta earthquake.

Trenching in the New Madrid seismic zone

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

Not Available

1990-01-01

Trenching studies of the San Andreas fault have been of great value to geologists in California for determining not only the prehistoric occurrences of earthquakes on the fault but also the age of these movements. In the New Madrid seismic zone, US Geological Survey scientists have been trenching across suspected faults to try to assess earthquake frequency in the Central US. The following photographs document these trenching studies.

The earthquake potential of the New Madrid seismic zone

USGS Publications Warehouse

Tuttle, Martitia P.; Schweig, Eugene S.; Sims, John D.; Lafferty, Robert H.; Wolf, Lorraine W.; Haynes, Marion L.

2002-01-01

The fault system responsible for New Madrid seismicity has generated temporally clustered very large earthquakes in A.D. 900 ± 100 years and A.D. 1450 ± 150 years as well as in 1811–1812. Given the uncertainties in dating liquefaction features, the time between the past three New Madrid events may be as short as 200 years and as long as 800 years, with an average of 500 years. This advance in understanding the Late Holocene history of the New Madrid seismic zone and thus, the contemporary tectonic behavior of the associated fault system was made through studies of hundreds of earthquake-induced liquefaction features at more than 250 sites across the New Madrid region. We have found evidence that prehistoric sand blows, like those that formed during the 1811–1812 earthquakes, are probably compound structures resulting from multiple earthquakes closely clustered in time or earthquake sequences. From the spatial distribution and size of sand blows and their sedimentary units, we infer the source zones and estimate the magnitudes of earthquakes within each sequence and thereby characterize the detailed behavior of the fault system. It appears that fault rupture was complex and that the central branch of the seismic zone produced very large earthquakes during the A.D. 900 and A.D. 1450 events as well as in 1811–1812. On the basis of a minimum recurrence rate of 200 years, we are now entering the period during which the next 1811–1812-type event could occur.

Seismic hazard and risk assessment in the intraplate environment: The New Madrid seismic zone of the central United States

USGS Publications Warehouse

Wang, Z.

2007-01-01

Although the causes of large intraplate earthquakes are still not fully understood, they pose certain hazard and risk to societies. Estimating hazard and risk in these regions is difficult because of lack of earthquake records. The New Madrid seismic zone is one such region where large and rare intraplate earthquakes (M = 7.0 or greater) pose significant hazard and risk. Many different definitions of hazard and risk have been used, and the resulting estimates differ dramatically. In this paper, seismic hazard is defined as the natural phenomenon generated by earthquakes, such as ground motion, and is quantified by two parameters: a level of hazard and its occurrence frequency or mean recurrence interval; seismic risk is defined as the probability of occurrence of a specific level of seismic hazard over a certain time and is quantified by three parameters: probability, a level of hazard, and exposure time. Probabilistic seismic hazard analysis (PSHA), a commonly used method for estimating seismic hazard and risk, derives a relationship between a ground motion parameter and its return period (hazard curve). The return period is not an independent temporal parameter but a mathematical extrapolation of the recurrence interval of earthquakes and the uncertainty of ground motion. Therefore, it is difficult to understand and use PSHA. A new method is proposed and applied here for estimating seismic hazard in the New Madrid seismic zone. This method provides hazard estimates that are consistent with the state of our knowledge and can be easily applied to other intraplate regions. ?? 2007 The Geological Society of America.

Monitoring transient changes within overpressured regions of subduction zones using ambient seismic noise.

PubMed

Chaves, Esteban J; Schwartz, Susan Y

2016-01-01

In subduction zones, elevated pore fluid pressure, generally linked to metamorphic dehydration reactions, has a profound influence on the mechanical behavior of the plate interface and forearc crust through its control on effective stress. We use seismic noise-based monitoring to characterize seismic velocity variations following the 2012 Nicoya Peninsula, Costa Rica earthquake [M w (moment magnitude) 7.6] that we attribute to the presence of pressurized pore fluids. Our study reveals a strong velocity reduction (~0.6%) in a region where previous work identified high forearc pore fluid pressure. The depth of this velocity reduction is constrained to be below 5 km and therefore not the result of near-surface damage due to strong ground motions; rather, we posit that it is caused by fracturing of the fluid-pressurized weakened crust due to dynamic stresses. Although pressurized fluids have been implicated in causing coseismic velocity reductions beneath the Japanese volcanic arc, this is the first report of a similar phenomenon in a subduction zone setting. It demonstrates the potential to identify pressurized fluids in subduction zones using temporal variations of seismic velocity inferred from ambient seismic noise correlations.

High-resolution seismic reflection/refraction imaging from Interstate 10 to Cherry Valley Boulevard, Cherry Valley, Riverside County, California: implications for water resources and earthquake hazards

USGS Publications Warehouse

Gandhok, G.; Catchings, R.D.; Goldman, M.R.; Horta, E.; Rymer, M.J.; Martin, P.; Christensen, A.

1999-01-01

This report is the second of two reports on seismic imaging investigations conducted by the U.S. Geological Survey (USGS) during the summers of 1997 and 1998 in the Cherry Valley area in California (Figure 1a). In the first report (Catchings et al., 1999), data and interpretations were presented for four seismic imaging profiles (CV-1, CV-2, CV-3, and CV-4) acquired during the summer of 1997 . In this report, we present data and interpretations for three additional profiles (CV-5, CV-6, and CV-7) acquired during the summer of 1998 and the combined seismic images for all seven profiles. This report addresses both groundwater resources and earthquake hazards in the San Gorgonio Pass area because the shallow (upper few hundred meters) subsurface stratigraphy and structure affect both issues. The cities of Cherry Valley and Beaumont are located approximately 130 km (~80 miles) east of Los Angeles, California along the southern alluvial fan of the San Bernardino Mountains (see Figure 1b). These cities are two of several small cities that are located within San Gorgonio Pass, a lower-lying area between the San Bernardino and the San Jacinto Mountains. Cherry Valley and Beaumont are desert cities with summer daytime temperatures often well above 100 o F. High water usage in the arid climate taxes the available groundwater supply in the region, increasing the need for efficient management of the groundwater resources. The USGS and the San Gorgonio Water District (SGWD) work cooperatively to evaluate the quantity and quality of groundwater supply in the San Gorgonio Pass region. To better manage the water supplies within the District during wet and dry periods, the SGWD sought to develop a groundwater recharge program, whereby, excess water would be stored in underground aquifers during wet periods (principally winter months) and retrieved during dry periods (principally summer months). The SGWD preferred a surface recharge approach because it could be less expensive than a

Seismic Evidence of A Widely Distributed West Napa Fault Zone, Hendry Winery, Napa, California

NASA Astrophysics Data System (ADS)

Goldman, M.; Catchings, R.; Chan, J. H.; Criley, C.

2015-12-01

Following the 24 August 2014 Mw 6.0 South Napa earthquake, surface rupture was mapped along the West Napa Fault Zone (WNFZ) for a distance of ~ 14 km and locally within zones up to ~ 2 km wide. Near the northern end of the surface rupture, however, several strands coalesced to form a narrow, ~100-m-wide zone of surface rupture. To determine the location, width, and shallow (upper few hundred meters) geometry of the fault zone, we acquired an active-source seismic survey across the northern surface rupture in February 2015. We acquired both P- and S-wave data, from which we developed reflection images and tomographic images of Vp, Vs, Vp/Vs, and Poisson's ratio of the upper 100 m. We also used small explosive charges within surface ruptures located ~600 m north of our seismic array to record fault-zone guided waves. Our data indicate that at the latitude of the Hendry Winery, the WNFZ is characterized by at least five fault traces that are spaced 60 to 200 m apart. Zones of low-Vs, low-Vp/Vs, and disrupted reflectors highlight the fault traces on the tomography and reflection images. On peak-ground-velocity (PGV) plots, the most pronounced high-amplitude guided-wave seismic energy coincides precisely with the mapped surface ruptures, and the guided waves also show discrete high PGV zones associated with unmapped fault traces east of the surface ruptures. Although the surface ruptures of the WNFZ were observed only over a 100-m-wide zone at the Hendry Winery, our data indicate that the fault zone is at least 400 m wide, which is probably a minimum width given the 400-m length of our seismic profile. Slip on the WNFZ is generally considered to be low relative to most other Bay Area faults, but we suggest that the West Napa Fault is a zone of widely distributed shear, and to fully account for the total slip on the WNFZ, slip on all traces of this wide fault zone must be considered.

Monitoring of Microseismicity with ArrayTechniques in the Peach Tree Valley Region

NASA Astrophysics Data System (ADS)

Garcia-Reyes, J. L.; Clayton, R. W.

2016-12-01

This study is focused on the analysis of microseismicity along the San Andreas Fault in the PeachTree Valley region. This zone is part of the transition zone between the locked portion to the south (Parkfield, CA) and the creeping section to the north (Jovilet, et al., JGR, 2014). The data for the study comes from a 2-week deployment of 116 Zland nodes in a cross-shaped configuration along (8.2 km) and across (9 km) the Fault. We analyze the distribution of microseismicity using a 3D backprojection technique, and we explore the use of Hidden Markov Models to identify different patterns of microseismicity (Hammer et al., GJI, 2013). The goal of the study is to relate the style of seismicity to the mechanical state of the Fault. The results show the evolution of seismic activity as well as at least two different patterns of seismic signals.

Co- and post-seismic deformation for the 2014 Napa Valley Earthquake from Sentinel-1A interferometry

NASA Astrophysics Data System (ADS)

Elliott, J. R.; Wright, T. J.; Elliott, A. J.; González, P. J.; Hooper, A. J.; Larsen, Y.; Marinkovic, P.; Plain, M.; Walters, R. J.

2014-12-01

Here we present analysis of co- and post-seismic deformation for the 24 August 2014 Napa Valley Earthquake derived from Sentinel-1A interferometry. We use these to derive the co-seismic slip distribution and map the evolution of post-seismic afterslip. The 24 August 2014 Napa Valley earthquake was the first earthquake for which surface deformation was measured by Sentinel-1A, a new radar satellite launched by the European Space Agency on 3 April 2014, and operated by the European Commission's Copernicus program. Sentinel-1A reached its final operational orbit on 7 August, and fortuitously acquired a pre-earthquake image of the San Francisco Bay area on that day in StripMap mode. By comparing it with an image acquired on 31 August, we formed a co-seismic interferogram, which reveals the surface deformation that occurred during the earthquake and the first 7 days of the post-seismic period. We use this to constrain a simple elastic model of the co-seismic slip distribution; preliminary inversion results show that the slip at depth reached a peak of >1.5 m at a depth of ~4 km. Following the earthquake, Sentinel-1A has acquired further acquisitions in both StripMap and Interferometric Wide Swath modes. The first 12-day post-seismic StripMap interferogram shows a sharp discontinuity along the entire fault rupture, consistent with field observations of rapid afterslip. We will use the full time series from August to December to measure the spatio-temporal behaviour of the afterslip, and discuss the implications for the frictional properties of the fault. The results from Napa point to an exciting and impactful future for the Sentinel-1 radar constellation. By mid-2014, Sentinel-1A will be acquiring data systematically over all the seismic belts, and the launch of Sentinel-1B in 2016 will increase the temporal frequency of acquisitions. The data will be available free of charge and will transform our ability to conduct tectonic geodesy, particularly in remote areas of the

Fluid-driven seismicity in a stable tectonic context: The Remiremont fault zone, Vosges, France

NASA Astrophysics Data System (ADS)

Audin, Laurence; Avouac, Jean-Philippe; Flouzat, Mireille; Plantet, Jean-Louis

2002-03-01

Some relocated seismic events, which have small magnitudes (ML < 4.8), are found to align along a 40 km-long fault zone flanking the southern Vosges Massif to the west. It joins to the south with the epicentral area of the historical 1682 earthquake (Io = VIII MSK). The Remiremont cluster was preceded by a period of seismic coalescence and triggered outward of bilateral seismic migration. The 1984 seismic crisis developed along a well defined 3 km-long vertical plane. In both cases, migration rates of the order of 5-10 km/yr over 30 km-long distances are determined. This pattern requires some mechanism of stress interaction which must act over distances of the order of 1 to 20 km within years. Given the low tectonic activity and the magnitudes of the events the stress transfer cannot result from co-seismic elastic loading or from transient strain at depth. We suggest that the seismic activity reflect rupture of asperities driven by fluid-flow in a zone of relatively high permeability.

Refraction seismic studies in the Miami River, Whitewater River, and Mill Creek valleys, Hamilton and Butler Counties, Ohio

USGS Publications Warehouse

Watkins, Joel S.

1963-01-01

Between September 17 and November 9, 1962, the U.S. Geological Survey, in cooperation with Ohio Division of Water, Miami Conservancy District, and c,ty of Cincinnati, Ohio, co.,:ducted a refraction seismic study in Hamilton and Butler Counties, southwest Ohio. The area lies between Hamilton, Ohio, and the Ohio River and includes a preglacial valley now occupied by portions of the Miami River, Whitewater River, and Mill Creek. The valley is partially filled with glacial debris which yields large quantities of good-quality water. The object of the study was to determine the thickness of these glacial deposits and the shape of the preglacial valley.

Using Seismic Refraction and Ground Penetrating Radar (GPR) to Characterize the Valley Fill in Beaver Meadows, Rocky Mountain National Park

NASA Astrophysics Data System (ADS)

Kramer, N.; Harry, D. L.; Wohl, E. E.

2010-12-01

This study is one of the first to use near surface geophysical techniques to characterize the subsurface stratigraphy in a high alpine, low gradient valley with a past glacial history and to obtain a preliminary grasp on the impact of Holocene beaver activity. Approximately 1 km of seismic refraction data and 5 km of GPR data were collected in Beaver Meadows, Rocky Mountain National Park. An asymmetric wedge of sediment ranging in depth from 0-20 m transverse to the valley profile was identified using seismic refraction. Complementary analysis of the GPR data suggests that the valley fill can be subdivided into till deposited during the Pleistocene glaciations and alluvium deposited during the Holocene. Two main facies were identified in the GPR profiles through pattern recognition. Facie Fd, which consists of chaotic discontinuous reflectors with an abundance of diffractions, is interpreted to be glacial till. Facie Fc, which is a combination of packages of complex slightly continuous reflectors interfingered with continuous horizontal to subhorizontal reflectors, is interpreted to be post-glacial alluvium and includes overbank, pond and in-channel deposits. Fc consistently overlies Fd throughout the study area and is no more than 7 m thick in the middle of the valley. The thickness of Holocene sedimentation (<7 m) is much less than the total amount of valley fill identified in the seismic refraction survey (0-20 m). A subfacie of Fc, Fch, which has reflectors with long periods was identified within Fc and is interpreted to be ponded sediments. The spatial distribution of facie Fch, along with: slight topographical features resembling buried beaver dams, a high abundance of fine sediment including silts and clays, historical records of beavers, and the name "Beaver Meadows" all suggest that Holocene beaver activity played a large role in sediment accumulation at this site, despite the lack of surficial relict beaver dams containing wood.

Fault slip rates in the modern new madrid seismic zone

PubMed

Mueller; Champion; Guccione; Kelson

1999-11-05

Structural and geomorphic analysis of late Holocene sediments in the Lake County region of the New Madrid seismic zone indicates that they are deformed by fault-related folding above the blind Reelfoot thrust fault. The widths of narrow kink bands exposed in trenches were used to model the Reelfoot scarp as a forelimb on a fault-bend fold; this, coupled with the age of folded sediment, yields a slip rate on the blind thrust of 6.1 +/- 0.7 mm/year for the past 2300 +/- 100 years. An alternative method used structural relief across the scarp and the estimated dip of the underlying blind thrust to calculate a slip rate of 4.8 +/- 0.2 mm/year. Geometric relations suggest that the right lateral slip rate on the New Madrid seismic zone is 1.8 to 2.0 mm/year.

Seismic evidence for deep fluid circulation in the overriding plate of subduction zones

NASA Astrophysics Data System (ADS)

Tauzin, B.; Reynard, B.; Bodin, T.; Perrillat, J. P.; Debayle, E.

2015-12-01

In subduction zones, non-volcanic tremors are associated with fluid circulations (Obara, 2002). Their sources are often located on the interplate boundary (Rogers and Dragert, 2003; Shelly et al, 2006; La Rocca, 2009), consistent with fluids released by the dehydration of subducted plates (Hacker et al., 2003). Reports of tremors in the overriding continental crust of several subduction zones in the world (Kao et al., 2005; Payero et al., 2008; Ide, 2012) suggest fluid circulation at shallower depths but potential fluid paths are poorly documented. Here we obtained seismic observations from receiver functions that evidence the close association between the shallow tremor zone, electrical conductivity, and tectonic features of the Cascadia overriding plate. A seismic discontinuity near 15 km depth in the crust of the overriding North American plate is attributed to the Conrad discontinuity. This interface is segmented, and its interruption is spatially correlated with conductive regions and shallow swarms of seismicity and non-volcanic tremors. These observations suggest that shallow fluid circulation, tremors and seismicity are controlled by fault zones limiting blocks of accreted terranes in the overriding plate (Brudzinski and Allen, 2007). These zones constitute fluid "escape" routes that may contribute unloading fluid pressure on the megathrust. Obara, K. (2002). Science, 296, 1679-1681. Rogers, G., & Dragert, H. (2003). Science, 300, 1942-1943. Shelly, D. R., et al. (2006). Nature, 442, 188-191. La Rocca, M., et al. (2009). Science, 323, 620-623. Kao, H., et al. (2005). Nature, 436, 841-844. Payero, J. S., et al. (2008). Geophysical Research Letters, 35. Ide, S. (2012). Journal of Geophysical Research: Solid Earth, 117. Brudzinski, M. R., & Allen, R. M. (2007). Geology, 35, 907-910.

Seismicity and Fault Zone Structure Near the Xinfengjiang Water Reservoir, Guangdong, China

NASA Astrophysics Data System (ADS)

Yang, H.; Sun, X.; He, L.; Wang, S.

2015-12-01

Xingfengjiang Water Reservoir (XWR) was built in 1958 and the first impoundment was conducted in 1959. Immediately following the reservoir impoundment, a series of earthquakes occurred in the vicinity of the XWR, including the 1962 M6.1 earthquake that occurred ~1 km next to the dam. Numerous small earthquakes take place in this region presently, making it one of the most active seismic zones in Guangdong. To investigate the present seismicity and associated fault zone structure, we deployed a temporary seismic network, including a dense linear array across the Ren-Zi-Shi fault southwest to the reservoir. The temporary network is consisted of 42 stations that are operated in the field for more than one month. Because of the mountainous terrain, it is impossible to deploy broadband sensors. Here we use DDV-5 seismometer with a central frequency of 120Hz-5s that is independent on external GPS and battery. During our deployment, numerous earthquakes were recorded. Preliminary results of travel time analysis have shown the characteristic of low velocity fault zone. More detailed analysis, including relocation of earthquakes, ambient noise cross correlation, and modeling body waves, will be presented.

Late Cenozoic structure and correlations to seismicity along the Olympic-Wallowa Lineament, northwest United States

USGS Publications Warehouse

Mann, G.M.; Meyer, C.E.

1993-01-01

Late Cenozoic fault geometry, structure, paleoseismicity, and patterns of recent seismicity at two seismic zones along the Olympic-Wallowa lineament (OWL) of western Idaho, northeast Oregon, and southeast Washington indicate limited right-oblique slip displacement along multiple northwest-striking faults that constitute the lineament. The southern end of the OWL originates in the Long Valley fault system and western Snake River Plain in western Idaho. The OWL in northeast Oregon consists of a wide zone of northwest-striking faults and is associated with several large, inferred, pull-apart basins. The OWL then emerges from the Blue Mountain uplift as a much narrower zone of faults in the Columbia Plateau known as the Wallula fault zone (WFZ). Stuctural relationships in the WFZ strongly suggest that it is a right-slip extensional duplex. -from Authors

Extension of the Cerro Prieto field and zones in the Mexicali Valley with geothermal possibilities in the future

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

Fonseca L, H.L.; de la Pena L, A.; Puente C, I.

This study concerns the possible extension of the Cerro Prieto field and identification of other zones in the Mexicali Valley with geothermal development potential by assessing the structural geologic conditions in relation to the regional tectonic framework and the integration of geologic and geophysical surveys carried out at Cerro Prieto. This study is based on data obtained from the wells drilled to date and the available geological and geophysical information. With this information, a geologic model of the field is developed as a general description of the geometry of what might be the geothermal reservoir of the Cerro Prieto field.more » In areas with geothermal potential within the Mexicali Valley, the location of irrigation wells with anomalous temperatures was taken as a point of departure for subsequent studies. Based on this initial information, gravity and magnetic surveys were made, followed by seismic reflection and refraction surveys and the drilling of 1200-m-deep multiple-use wells. Based on the results of the final integration of these studies with the geology of the region, it is suggested that the following areas should be explored further: east of Cerro Prieto, Tulecheck, Riito, Aeropuerto-Algodones, and San Luis Rio Colorado, Sonora.« less

Seismogenic zones and attenuation laws for probabilistic seismic hazard assessment in low deformation area =

NASA Astrophysics Data System (ADS)

Le Goff, Boris

Seismic Hazard Analysis (PSHA), rather than the subjective methodologies that are currently used. This study focuses particularly in the definition of the seismic sources, through the seismotectonic zoning, and the determination of historical earthquake location. An important step in the Probabilistic Seismic Hazard Analysis consists in defining the seismic source model. Such a model expresses the association of the seismicity characteristics with the tectonically-active geological structures evidenced by seismotectonic studies. Given that most of the faults, in low seismic regions, are not characterized well enough, the source models are generally defined as areal zones, delimited with finite boundary polygons, within which the seismicity and the geological features are deemed homogeneous (e.g., focal depth, seismicity rate). Besides the lack of data (short period of instrumental seismicity), such a method generates different problems for regions with low seismic activity: 1) a large sensitivity of resulting hazard maps to the location of zone boundaries, while these boundaries are set by expert decisions; 2) the zoning cannot represent any variability or structural complexity in seismic parameters; 3) the seismicity rate is distributed throughout the zone and the location of the determinant information used for its calculation is lost. We investigate an alternative approach to model the seismotectonic zoning, with three main objectives: 1) obtaining a reproducible method that 2) preserves the information on the sources and extent of the uncertainties, so as to allow to propagate them (through Ground Motion Prediction Equations on to the hazard maps), and that 3) redefines the seismic source concept to debrief our knowledge on the seismogenic structures and the clustering. To do so, the Bayesian methods are favored. First, a generative model with two zones, differentiated by two different surface activity rates, was developed, creating synthetic catalogs drawn

Probing the Detailed Seismic Velocity Structure of Subduction Zones Using Advanced Seismic Tomography Methods

NASA Astrophysics Data System (ADS)

Zhang, H.; Thurber, C. H.

2005-12-01

Subduction zones are one of the most important components of the Earth's plate tectonic system. Knowing the detailed seismic velocity structure within and around subducting slabs is vital to understand the constitution of the slab, the cause of intermediate depth earthquakes inside the slab, the fluid distribution and recycling, and tremor occurrence [Hacker et al., 2001; Obara, 2002].Thanks to the ability of double-difference tomography [Zhang and Thurber, 2003] to resolve the fine-scale structure near the source region and the favorable seismicity distribution inside many subducting slabs, it is now possible to characterize the fine details of the velocity structure and earthquake locations inside the slab, as shown in the study of the Japan subduction zone [Zhang et al., 2004]. We further develop the double-difference tomography method in two aspects: the first improvement is to use an adaptive inversion mesh rather than a regular inversion grid and the second improvement is to determine a reliable Vp/Vs structure using various strategies rather than directly from Vp and Vs [see our abstract ``Strategies to solve for a better Vp/Vs model using P and S arrival time'' at Session T29]. The adaptive mesh seismic tomography method is based on tetrahedral diagrams and can automatically adjust the inversion mesh according to the ray distribution so that the inversion mesh nodes are denser where there are more rays and vice versa [Zhang and Thurber, 2005]. As a result, the number of inversion mesh nodes is greatly reduced compared to a regular inversion grid with comparable spatial resolution, and the tomographic system is more stable and better conditioned. This improvement is quite valuable for characterizing the fine structure of the subduction zone considering the highly uneven distribution of earthquakes within and around the subducting slab. The second improvement, to determine a reliable Vp/Vs model, lies in jointly inverting Vp, Vs, and Vp/Vs using P, S, and S

A classification of morphoseismic features in the New Madrid seismic zone

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

Knox, R.; Stewart, D.

1993-03-01

The New Madrid Seismic Zone (NMSZ) contains thousands of surface features distributed over 5,000 square miles in four states. These are attributable to some combination of (1) seismically-induced liquefaction (SIL), (2) secondary deformation, and (3) seismically-induced slope failures. Most of these features were produced by the 1811--12 series of great earthquakes, but some predate and some postdate 1811--12. Subsequent non-seismic factors, such as hydrologically-induced liquefaction (HIL), mechanically-induced liquefaction (MIL), human activities, mass wasting, eolian and fluvial processes have modified all of these features. Morphoseismic features are new landforms produced by earthquakes, or are pre-existing landforms modified by them. Involved aremore » complex interrelationships among several variables, including: (1) intensity and duration of seismic ground motion, (2) surface wave harmonics, (3) depth to water table, (4) depth to basement, (5) particle size, composition, and sorting of sediment making up the liquefied (LZ) and non-liquefied zones (NLZ), (6) topographic parameters, and (7) attitudes of beds and lenses susceptible to liquefaction. Morphoseismic features are depicted as results of a time-flow sequence initiated by primary basement disturbances which produce three major categories of surface response: secondary deformation, liquefaction and slope failure. Nine subcategories incorporate features produced by or resulting in: extruded sand, intruded sand, lateral spreading, faulting, subsidence of large areas, uplift of large areas, altered streams, coherent landslides, and incoherent landslides. The total morphoseismic features identified by this classification are 34 in number.« less

3D absolute hypocentral determination - 13 years of seismicity in Ecuadorian subduction zone

NASA Astrophysics Data System (ADS)

Font, Yvonne; Segovia, Monica; Theunissen, Thomas

2010-05-01

In Ecuador, the Nazca plate is subducting beneath the North Andean Block. This subduction triggered, during the last century, 4 major earthquakes of magnitude greater than 7.7. Between 1994 and 2007, the Geophysical Institute (Escuela National Politecnica, Quito) recorded about 40 000 events in whole Ecuador ranging from Mb 1.5 to 6.9. Unfortunately, the local network shows great density discrepancy between the Coastal and Andean regions where numerous stations were installed to survey volcanic activity. Consequently, seismicity in and around the interplate seismogenic zone - producer of the most destructive earthquakes and tsunamis - is not well constrained. This study aims to improve the location of 13 years seismicity occurred during an interseismic period in order to better localize the seismic deformation and gaps. The first step consists in the construction of a 3D "georealistic" velocity model. Because local tomography cannot provide satisfactory model, we combined all local crustal/lithospheric information on the geometry and velocity properties of different geological units. Those information cover the oceanic Nazca plate and sedimentary coverture the subducting plate dip angle; the North Andean Block margin composed of accreted oceanic plateaus (the Moho depth is approximated using gravity modeling); the metamorphic volcanic chain (oceanic nature for the occidental cordillera and inter-andean valley, continental one for the oriental cordillera); The continental Guyana shield and sedimentary basins. The resulting 3D velocity model extends from 2°N to 6.5°S and 277°E to 283°E and reaches a depth of 300 km. It is discretized in constant velocity blocks of 12 x 12 x 3 km in x, y and z, respectively. The second step consists in selecting an adequate sub-set of seismic stations in order to correct the effect of station density disequilibrium between coastal and volcanic regions. Consequently, we only keep the most representative volcanic stations in terms

Anatomy of an Active Seismic Source: the Interplay between Present-Day Seismic Activity and Inherited Fault Zone Architecture (Central Apennines, Italy)

NASA Astrophysics Data System (ADS)

Fondriest, M.; Demurtas, M.; Bistacchi, A.; Fabrizio, B.; Storti, F.; Valoroso, L.; Di Toro, G.

2017-12-01

The mechanics and seismogenic behaviour of fault zones are strongly influenced by their internal structure, in terms of both fault geometry and fault rock constitutive properties. In recent years high-resolution seismological techniques yielded new constraints on the geometry and velocity structure of seismogenic faults down to 10s meters length scales. This reduced the gap between geophysical imaging of active seismic sources and field observations of exhumed fault zones. Nevertheless fundamental questions such as the origin of geometrical and kinematic complexities associated to seismic faulting remain open. We addressed these topics by characterizing the internal structure of the Vado di Corno Fault Zone, an active seismogenic normal fault cutting carbonates in the Central Apennines of Italy and comparing it with the present-day seismicity of the area. The fault footwall block, which was exhumed from < 2 km depth, was mapped with high detail (< 1 m spatial resolution) for 2 km of exposure along strike, combining field structural data and photogrammetric surveys in a three dimensional structural model. Three main structural units separated by principal fault strands were recognized: (i) cataclastic unit (20-100 m thick), (ii) damage zone (≤ 300 m thick), (iii) breccia unit ( 20 thick). The cataclastic unit lines the master fault and represents the core of the normal fault zone. In-situ shattering together with evidence of extreme (possibly coseismic) shear strain localization (e.g., mirror-like faults with truncated clasts, ultrafine-grained sheared veins) was recognized. The breccia unit is an inherited thrust zone affected by pervasive veining and secondary dolomitization. It strikes subparallel to the active normal fault and is characterized by a non-cylindrical geometry with 10-100 m long frontal and lateral ramps. The cataclastic unit cuts through thrust flats within the breccia unit, whereas normal to oblique inversion occur on frontal and lateral ramps

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

Seismicity near a Highly-Coupled Patch in the Central Ecuador Subduction Zone

NASA Astrophysics Data System (ADS)

Regnier, M. M.; Segovia, M.; Font, Y.; Charvis, P.; Galve, A.; Jarrin, P.; Hello, Y.; Ruiz, M. C.; Pazmino, A.

2017-12-01

The temporary onshore-offshore seismic network deployed during the 2-years period of the OSISEC project provides an unprecedented, detailed and well-focused image of the seismicity for magnitudes as low as 2.0 in the Central Ecuadorian subduction zone. Facing the southern border of the Carnegie Ridge, a shallow and discrete highly-coupled patch is correlated to the subduction of a large oceanic relief. No large earthquake is known in this area that is experiencing recurrent seismic swarms and slow slip events. The shallow and locked subduction interface shows no evidence of background seismicity that instead occurred down dip of the coupled patch where it is possibly controlled by structural features of the overriding plate. We show a clear spatial correlation between the background microseismicity, the down dip extension of the locked patch at 20 km depth and the geology of the upper plate. The dip angle of the interplate contact zone, defined by a smooth interpolation through the hypocenters of thrust events, is consistent with a progressive increase from 6° to 25° from the trench to 20 km depth. Offshore, a seismic swarm, concomitant with a slow slip event rupturing the locked area, highlights the reactivation of secondary active faults that developed within the thickened crust of the subducting Carnegie Ridge, at the leading edge of a large oceanic seamount. No seismicity was detected near the plate interface suggesting that stress still accumulates at small and isolated asperities

Seismic evidence for water transport out of the mantle transition zone beneath the European Alps

NASA Astrophysics Data System (ADS)

Liu, Zhen; Park, Jeffrey; Karato, Shun-ichiro

2018-01-01

The mantle transition zone has been considered a major water reservoir in the deep Earth. Mass transfer across the transition-zone boundaries may transport water-rich minerals from the transition zone into the water-poor upper or lower mantle. Water release in the mantle surrounding the transition zone could cause dehydration melting and produce seismic low-velocity anomalies if some conditions are met. Therefore, seismic observations of low-velocity layers surrounding the transition zone could provide clues of water circulation at mid-mantle depths. Below the Alpine orogen, a depressed 660-km discontinuity has been imaged clearly using seismic tomography and receiver functions, suggesting downwellings of materials from the transition zone. Multitaper-correlation receiver functions show prominent ∼0.5-1.5% velocity reductions at ∼750-800-km depths, possibly caused by partial melting in the upper part of lower mantle. The gap between the depressed 660-km discontinuity and the low-velocity layers is consistent with metallic iron as a minor phase in the topmost lower mantle reported by laboratory studies. Velocity drops atop the 410-km discontinuity are observed surrounding the Alpine orogeny, suggesting upwelling of water-rich rock from the transition zone in response to the downwelled materials below the orogeny. Our results provide evidence that convective penetration of the mantle transition zone pushes hydrated minerals both upward and downward to add hydrogen to the surrounding mantle.

ANZA Seismic Network- From Monitoring to Science

NASA Astrophysics Data System (ADS)

Vernon, F.; Eakin, J.; Martynov, V.; Newman, R.; Offield, G.; Hindley, A.; Astiz, L.

2007-05-01

The ANZA Seismic Network (http:eqinfo.ucsd.edu) utilizes broadband and strong motion sensors with 24-bit dataloggers combined with real-time telemetry to monitor local and regional seismicity in southernmost California. The ANZA network provides real-time data to the IRIS DMC, California Integrated Seismic Network (CISN), other regional networks, and the Advanced National Seismic System (ANSS), in addition to providing near real-time information and monitoring to the greater San Diego community. Twelve high dynamic range broadband and strong motion sensors adjacent to the San Jacinto Fault zone contribute data for earthquake source studies and continue the monitoring of the seismic activity of the San Jacinto fault initiated 24 years ago. Five additional stations are located in the San Diego region with one more station on San Clemente Island. The ANZA network uses the advance wireless networking capabilities of the NSF High Performance Wireless Research and Education Network (http:hpwren.ucsd.edu) to provide the communication infrastructure for the real-time telemetry of Anza seismic stations. The ANZA network uses the Antelope data acquisition software. The combination of high quality hardware, communications, and software allow for an annual network uptime in excess of 99.5% with a median annual station real-time data return rate of 99.3%. Approximately 90,000 events, dominantly local sources but including regional and teleseismic events, comprise the ANZA network waveform database. All waveform data and event data are managed using the Datascope relational database. The ANZA network data has been used in a variety of scientific research including detailed structure of the San Jacinto Fault Zone, earthquake source physics, spatial and temporal studies of aftershocks, array studies of teleseismic body waves, and array studies on the source of microseisms. To augment the location, detection, and high frequency observations of the seismic source spectrum from local

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

NASA Astrophysics Data System (ADS)

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

2017-12-01

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

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

NASA Astrophysics Data System (ADS)

Zhang, H.; Schmandt, B.

2017-12-01

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

Geophysical Investigation of Avon Valley, West-Central Montana, using Gravity and Seismic Reflection Profiling

NASA Astrophysics Data System (ADS)

Knatterud, L.; Mosolf, J.; Speece, M. A.; Zhou, X.

2014-12-01

The Avon Valley and adjacent mountains in west-central Montana lie within the Lewis and Clark Line, a major system of WNW-striking faults and folds that transect the more northerly structural grain of the northern Rockies and represent alternating episodes of transtensional and transpressional deformation. The northwest-trending valley has been previously interpreted as an extensional half graben filled with Tertiary sedimentary and volcanic deposits; however, little-to-no geophysical constraints on basin architecture or the thickness of Tertiary fill have been reported. A major northwest-striking fault with significant normal displacement clearly bounds the valley to the northeast, juxtaposing Tertiary sedimentary deposits against Proterozoic-Mesozoic units deformed by shortening structures and crosscut by Cretaceous granitic intrusions. Tertiary volcanic deposits unconformably overlying faulted and folded Phanerozoic-Proterozoic sequences in the eastern Garnet Range bound the valley to the southwest, but in the past no faults had been mapped along this margin. New mapping by the Montana Bureau of Mines and Geology (MBMG) has identified a system of high-angle, northwest- and northeast-striking, oblique-slip faults along the southwest border of the Avon calling into question if the valley is a half, full, or asymmetrical graben. Geophysical data has recently been acquired by Montana Tech to help define the structural architecture of the Avon Valley and the thickness of its Tertiary fill. Gravity data and a short seismic reflection profile have been collected and a preliminary interpretation of these data indicates a half graben with a series of normal faults bounding the western side of the valley. Ongoing gravity data collection throughout 2014 should refine this interpretation by better defining the bedrock-Tertiary interface at depth.

Integration of high-resolution seismic and aeromagnetic data for earthquake hazards evaluations: An example from the Willamette Valley, Oregon

USGS Publications Warehouse

Liberty, L.M.; Trehu, A.M.; Blakely, R.J.; Dougherty, M.E.

1999-01-01

Aeromagnetic and high-resolution seismic reflection data were integrated to place constraints on the history of seismic activity and to determine the continuity of the possibly active, yet largely concealed Mount Angel fault in the Willamette Valley, Oregon. Recent seismic activity possibly related to the 20-km-long fault includes a swarm of small earthquakes near Woodburn in 1990 and the magnitude 5.6 Scotts Mills earthquake in 1993. Newly acquired aeromagnetic data show several large northwest-trending anomalies, including one associated with the Mount Angel fault. The magnetic signature indicates that the fault may actually extend 70 km across the Willamette Valley to join the Newberg and Gales Creek faults in the Oregon Coast Range. We collected 24-fold high-resolution seismic reflection data along two transects near Woodburn, Oregon, to image the offset of the Miocene-age Columbia River Basalts (CRB) and overlying sediments at and northwest of the known mapped extent of the Mount Angel fault. The seismic data show a 100-200-m offset in the CRB reflector at depths from 300 to 700 m. Folded or offset sediments appear above the CRB with decreasing amplitude to depths as shallow as were imaged (approximately 40 m). Modeling experiments based on the magnetic data indicate, however, that the anomaly associated with the Mount Angel fault is not caused solely by an offset of the CRB and overlying sediments. Underlying magnetic sources, which we presume to be volcanic rocks of the Siletz terrane, must have vertical offsets of at least 500 m to fit the observed data. We conclude that the Mount Angel fault appears to have been active since Eocene age and that the Gales Creek, Newberg, and Mount Angel faults should be considered a single potentially active fault system. This fault, as well as other parallel northwest-trending faults in the Willamette Valley, should be considered as risks for future potentially damaging earthquakes.

Effects of Pre-existing Structures on the Seismicity of the Charlevoix Seismic Zone

NASA Astrophysics Data System (ADS)

Fadugba, O. I.; Choi, E.; Powell, C. A.

2017-12-01

The Charlevoix Seismic Zone (CSZ) occurs along the early Paleozoic St. Lawrence rift zone in southeastern Quebec at the location of a major Devonian impact crater. The crater superimposed major, steeply dipping basement faults trending N35°E. Many earthquakes are recorded each year in the CSZ and are concentrated within and beneath the impact crater. Some large-magnitude earthquakes associated with the rift faults occurred outside the crater. The primary goal of this research is to investigate combined effects of the pre-existing structures and regional stresses on earthquake activity in the CSZ. We set up some models using PyLith, an open-source finite-element code for simulations of crustal deformation. Our models will be compared with those of Baird et al. (2010), which took a different numerical approach for the same purpose of relating the regional structures, stresses, and seismicity. Adopting the results from recent hypocenter relocation study, we will modify the locations and dips of the rift faults and assess the effect of the new fault geometries on stress distributions. Finally, we will discuss whether modeled stress distributions can explain the seismicity distribution in the CSZ and published focal mechanism solutions. As a part of our efforts to enhance the reproducibility of these types of complex geodynamic models, selected models in this study will be made available in the form of sharable and reproducible packages. Such packaging is enabled by GeoTrust, an EarthCube-funded project that aims to automate the creation of a self-contained metadata package that provides a complete description of all data elements associated with a computational experiment.

Paleoseismology of the Southern Section of the Black Mountains and Southern Death Valley Fault Zones, Death Valley, United States

USGS Publications Warehouse

Sohn, Marsha S.; Knott, Jeffrey R.; Mahan, Shannon

2014-01-01

The Death Valley Fault System (DVFS) is part of the southern Walker Lane–eastern California shear zone. The normal Black Mountains Fault Zone (BMFZ) and the right-lateral Southern Death Valley Fault Zone (SDVFZ) are two components of the DVFS. Estimates of late Pleistocene-Holocene slip rates and recurrence intervals for these two fault zones are uncertain owing to poor relative age control. The BMFZ southernmost section (Section 1W) steps basinward and preserves multiple scarps in the Quaternary alluvial fans. We present optically stimulated luminescence (OSL) dates ranging from 27 to 4 ka of fluvial and eolian sand lenses interbedded with alluvial-fan deposits offset by the BMFZ. By cross-cutting relations, we infer that there were three separate ground-rupturing earthquakes on BMFZ Section 1W with vertical displacement between 5.5 m and 2.75 m. The slip-rate estimate is ∼0.2 to 1.8 mm/yr, with an earthquake recurrence interval of 4,500 to 2,000 years. Slip-per-event measurements indicate Mw 7.0 to 7.2 earthquakes. The 27–4-ka OSL-dated alluvial fans also overlie the putative Cinder Hill tephra layer. Cinder Hill is offset ∼213 m by SDVFZ, which yields a tentative slip rate of 1 to 8 mm/yr for the SDVFZ.

Reconciling short recurrence intervals with minor deformation in the new madrid seismic zone.

PubMed

Schweig, E S; Ellis, M A

1994-05-27

At least three great earthquakes occurred in the New Madrid seismic zone in 1811 and 1812. Estimates of present-day strain rates suggest that such events may have a repeat time of 1000 years or less. Paleoseismological data also indicate that earthquakes large enough to cause soil liquefaction have occurred several times in the past 5000 years. However, pervasive crustal deformation expected from such a high frequency of large earthquakes is not observed. This suggests that the seismic zone is a young feature, possibly as young as several tens of thousands of years old and no more than a few million years old.

Kinematics of the New Madrid seismic zone, central United States, based on stepover models

USGS Publications Warehouse

Pratt, Thomas L.

2012-01-01

Seismicity in the New Madrid seismic zone (NMSZ) of the central United States is generally attributed to a stepover structure in which the Reelfoot thrust fault transfers slip between parallel strike-slip faults. However, some arms of the seismic zone do not fit this simple model. Comparison of the NMSZ with an analog sandbox model of a restraining stepover structure explains all of the arms of seismicity as only part of the extensive pattern of faults that characterizes stepover structures. Computer models show that the stepover structure may form because differences in the trends of lower crustal shearing and inherited upper crustal faults make a step between en echelon fault segments the easiest path for slip in the upper crust. The models predict that the modern seismicity occurs only on a subset of the faults in the New Madrid stepover structure, that only the southern part of the stepover structure ruptured in the A.D. 1811–1812 earthquakes, and that the stepover formed because the trends of older faults are not the same as the current direction of shearing.

Western Rainier Seismic Zone Airborne Laser Swath Mapping

NASA Technical Reports Server (NTRS)

Harding, David J.; Haugerud, Ralph A.; Johnson, Samuel Y.; Scott, Kevin M.; Weaver, Craig S.; Martinez, Diana M.; Zeigler, John C.; Latypov, Damir

2003-01-01

Airborne laser swath mapping (ALSM) of the Puget Lowland conducted by TerraPoint LLC for the Purget Sound Lidar Concortium (PSLC), has been successful in revealing Holocene fault scarps and lendsliders hidden beneath the dense, temperate rain forest cover and in quantifying shoreline terrace uplift. Expanding the PSLC efforts, NASA-USGS collaboration is now focusing on topographic mapping of seismogenic zones adjacent to volcanois in the western Cascades range in order to assess the presence of active faulting and tectonic deformation, better define the extend of lahars and understand their flow processes, and characterize landslide occurrence. Mapping of the western Rainier zone (WRZ) was conducted by TerraPoint in late 2002, after leaf fall and before snow accumulation. The WRZ is a NNW-trending, approx. 30 km-long zone of seismicity west of Mount Rainier National Park. The Puget Lowland ALSM methods were modified to accommodate challenges posed by the steep, high relief terrian. The laser data, acquired with a density of approx. 2 pulses /sq m, was filtered to identify returns from the ground from which a bare Earth digital elevation model (DEM) was produced with a grid size of 1.8 m. The RMS elevation accuracy of the DEM in flat, unvegetated areas is approx. 10cm based on consistency between overlapping flight swaths and comparisons to ground control points. The resulting DEM substantially improves upon Shuttle Radar Topography Mission and USGS photogrammetric mapping. For example, the DEM defines the size and spatial distribution of flood erratics left by the Electron lahar and of megaclasts within the Round Pass lahar, important for characterizing the lahar hydraulics. A previously unknown lateral levee on the Round Pass lahar is also revealed. In addition, to illustrating geomorfic feature within the WRZ, future plans for laser mapping of the Saint Helens and Darrington seismic zones will be described.

Current Seismicity in the Vicinity of Yucca Mountain, Nevada

NASA Astrophysics Data System (ADS)

Smith, K.; von Seggern, D.; dePolo, D.

2001-12-01

The 1992 to 2000 earthquakes in the Southern Great Basin have been relocated in order to better recognize the active tectonic processes in the vicinity of Yucca Mountain. During this time period seismic monitoring in the Southern Great Basin transitioned from a primarily single-component analog network to a 3-component digital network. Through the transition analog and digital networks were run in tandem. The station density over this period is as great as any prior recording period. The analog and digital networks were administered separately during the transition, and we have merged the phase data from the two operations. We performed relocations starting in October 1992, thus creating a hypocentral list for FY1993-FY2000. Aftershocks of the June 1992 M 5.6 Little Skull Mountain earthquake, located approximately 20 km southeast of Yucca Mountain, dominate the seismicity in the Southern Great Basin from 1992-2000. After the Little Skull Mountain earthquake, there was a general increase in earthquake activity in southern NTS, principally associated with the Rock Valley fault zone. There was no corresponding increase in seismicity west of Little Skull Mountain near the potential repository site. The distribution of high-quality earthquake locations generally reflects trends in Miocene tectonism. In particular, a general north-south trending gravity low, interpreted by Carr (1984) as the Kawich-Greenwater Rift, is highlighted by the microseismicity in many areas. Locally small magnitude earthquakes tend to outline the 8-10 Ma Timber Mountain caldera in northern and central NTS. Although these structures do not generally correlate with Quaternary faults, the micro-earthquake activity may reflect zones of weakness within these older structures. A 100 km long, conspicuous, north-south trending seismic zone, which shows no correlation with know Quaternary features, aligns along the steep gravity gradient bordering the western side of the Kawich-Greenwater gravity

Contrasts in Seismicity Along the 1964 Great Alaska Earthquake Rupture Zone

NASA Astrophysics Data System (ADS)

Doser, D. I.; Veilleux, A. M.; Flores, C.; Brown, W. A.

2004-12-01

We have examined seismicity occurring over 35 years prior to and following the 1964 great Alaska earthquake. These studies indicate that the regions associated with the Prince William Sound (PWS) and Kodiak asperities have behaved very different seismically over time. Prior to 1964, the PWS region experienced an increase in crustal seismicity, especially in the region located down-dip of maximum slip in 1964. Since 1964 this same region has been aseismic at the M> 3 level. PWS intraslab seismicity has remained relatively constant, although down-dip migration of M > 4.5 events was observed following the 1964 mainshock. Low-level seismicity has occurred at the northeastern end of the Kodiak asperity throughout the past 70 years, but much of the central Kodiak asperity has been aseismic at the M > 5 level. In contrast, the southwestern edge of the Kodiak asperity has been associated with moderate to large (Mw 5.5 to 7) crustal, interplate, and intraslab events throughout the past 70 years. These changes in seismic behavior along the 1964 rupture zone are consistent with GPS/geodesy estimates of seismic coupling across the interface and with known changes in plate geometry.

Investigations into the Fish Lake Valley Fault Zone (FLVFZ) and its interactions with normal faulting within Eureka and Deep Springs Valleys

NASA Astrophysics Data System (ADS)

Lawson, M. J.; Rhodes, E.; Yin, A.

2016-12-01

In most textbooks, the San Andreas Fault is stated to be the plate boundary between the North American and the Pacific plates, as plate tectonics assumes that boundaries are essentially discrete. In the Western United States this is not the case, as up to 25% of relative plate motion is accommodated on other structures within the Walker Lane Shear Zone (WLSZ) in a diffuse 100 km margin (Faulds et al., 2005; Oldow et al., 2001). Fish Lake Valley Fault Zone (FLVFZ), situated at the northern border of Death Valley National Park, is the northern continuation of the Furnace Creek Fault Zone (FCFZ), and is an important transfer structure within the Walker Lane Shear Zone. Though the FLVFZ has a long term rate (since 10 Ma) of 5 mm/yr (Reheis and Sawyer, 1997), it has a highly variable slip rate. In the middle Pleistocene, the rate has a maximum of up to 11 mm/yr which would accommodate nearly the entirety of slip within the Walker Lane, and yet this rate decreases significantly ( 2.5 to 3 mm/yr) by the late Pleistocene due to unknown causes (Frankel et al. 2007). This variation in slip rate has been proposed by previous workers to be due to strain transience, an increase in the overall strain rate, or due to other unknown structures (Lee et al., 2009). Currently, we are investigating the cause of this variation, and the possibility of the transfer of slip to faults south of the FLVFZ on oblique normal faults within Eureka and Deep Springs Valleys. Preliminary data will be shown utilizing scarp transects, geomorphic scarp modeling, and Optically Stimulated Luminescence (OSL) dating techniques.

Long Term Seismic Observation in Mariana by OBSs : Double Seismic Zone and Upper Mantle Structure

NASA Astrophysics Data System (ADS)

Shiobara, H.; Sugioka, H.; Mochizuki, K.; Oki, S.; Kanazawa, T.; Fukao, Y.; Suyehiro, K.

2005-12-01

In order to obtain the deep arc structural image of Mariana, a large-scale seismic observation by using 58 long-term ocean bottom seismometers (LTOBS) had been performed from June 2003 until April 2004, which is a part of the MARGINS program funded by the NSF. Prior to this observation, a pilot long-term seismic array observation was conducted in the same area by using 10 LTOBSs from Oct. 2001 until Feb. 2003. At that time, 8 LTOBSs were recovered but one had no data. Recently, 2 LTOBSs, had troubles in the releasing, were recovered by the manned submersible (Shinkai 6500, Jamstec) for the research of the malfunction in July 2005. By using all 9 LTOBS's data, those are about 11 months long, hypocenter determination was performed and more than 3000 local events were found. Even with the 1D velocity structure based on the iasp91 model, double seismic zones and a systematic shift of epicenters between the PDE and this study were observed. To investigate the detail of hypocenter distribution and the 3D velocity structure, the DD inversion (tomoDD: Zhang and Thurber, 2003) was applied for this data set with the 1D structure initial model except for the crust, which has been surveyed by using a dense airgun-OBS system (Takahashi et al., 2003). The result of relocated hypocenters shows clear double seismic zones until about 200 km depth, a high activity area around the fore-arc serpentine sea-mount, the Big Blue, and a lined focuses along the current ridge axis in the back-arc basin, and the result of the tomography shows a image of subducting slab and a low-Vs region below the same sea-mount mentioned. The wedge mantle structure was not clearly resolved due to the inadequate source-receiver coverage, which will be done in the recent experiment.

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.

Mapping Inherited Fractures in the Critical Zone Using Seismic Anisotropy From Circular Surveys

NASA Astrophysics Data System (ADS)

Novitsky, Christopher G.; Holbrook, W. Steven; Carr, Bradley J.; Pasquet, Sylvain; Okaya, David; Flinchum, Brady A.

2018-04-01

Weathering and hydrological processes in Earth's shallow subsurface are influenced by inherited bedrock structures, such as bedding planes, faults, joints, and fractures. However, these structures are difficult to observe in soil-mantled landscapes. Steeply dipping structures with a dominant orientation are detectable by seismic anisotropy, with fast wave speeds along the strike of structures. We measured shallow ( 2-4 m) seismic anisotropy using "circle shots," geophones deployed in a circle around a central shot point, in a weathered granite terrain in the Laramie Range of Wyoming. The inferred remnant fracture orientations agree with brittle fracture orientations measured at tens of meters depth in boreholes, demonstrating that bedrock fractures persist through the weathering process into the shallow critical zone. Seismic anisotropy positively correlates with saprolite thickness, suggesting that inherited bedrock fractures may control saprolite thickness by providing preferential pathways for corrosive meteoric waters to access the deep critical zone.

Sedimentation in the Kane fracture zone, western North Atlantic

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

Jaroslow, G.E.

1991-03-01

The Kane fracture zone, a deep narrow trough in oceanic crust, has provided an ideal depocenter for reservation on the seismic stratigraphic record of the North Atlantic basin. The acoustic stratigraphy in single-channel and multichannel seismic reflection profiles crossing the Kane fracture zone in the western North Atlantic has been examined in order to scrutinize age processes within a fracture zone. Maps of total sediment thickness have provided insight into overall sediment distribution and the influence of topography on sedimentation. Eight reflectors have been traced and correlated with lithostratigraphy at Deep Sea Drilling Project (DSDP) sites. The Bermuda Rise, amore » prominent topographic feature, has had a profound effect on the distribution of sediments within the fracture zone. Since late Eocene, the rise has blocked transport by turbidity currents of terrigenous sediments to distal portions of the fracture valley. A 1,000-m-thick turbidite pond within the fracture zone east of the Bermuda Rise has been determined to have been derived from local sources. Within the ponded sequence a seismic discontinuity is estimated to be early Oligocene and postdates the emergence of the Bermuda Rise, adding an independent age constraint on the development of the rise. The pond terminates against a structural dam at 55{degree}20W, east of which the fracture zone is essentially sediment starved.« less

Systems for low frequency seismic and infrasound detection of geo-pressure transition zones

DOEpatents

Shook, G. Michael; LeRoy, Samuel D.; Benzing, William M.

2007-10-16

Methods for determining the existence and characteristics of a gradational pressurized zone within a subterranean formation are disclosed. One embodiment involves employing an attenuation relationship between a seismic response signal and increasing wavelet wavelength, which relationship may be used to detect a gradational pressurized zone and/or determine characteristics thereof. In another embodiment, a method for analyzing data contained within a response signal for signal characteristics that may change in relation to the distance between an input signal source and the gradational pressurized zone is disclosed. In a further embodiment, the relationship between response signal wavelet frequency and comparative amplitude may be used to estimate an optimal wavelet wavelength or range of wavelengths used for data processing or input signal selection. Systems for seismic exploration and data analysis for practicing the above-mentioned method embodiments are also disclosed.

Seismic Imaging and Characterization of Bright Spots in the West Bohemia Seismic Zone (Germany and Czech Republic)

NASA Astrophysics Data System (ADS)

Alexandrakis, C.; Schreiter, L.; Hlousek, F.; Jusri, T.; Buske, S.

2017-12-01

In crystalline environments, imaging faults, layer boundaries and small scale structures is challenging due to the complex geometry of the structures themselves and the influence of the hardrock environment on the seismic wavefield. Optimally designed active seismic surveys and careful processing can produce a clear image of the subsurface structures. However, if little is known about the local geology and tectonic state of the area, the imaged reflections can be difficult to interpret. This is the case in the West Bohemia Seismic Zone, located along the border of Germany and Czech Republic. This geodynamically active area is spotted with springs and gas vents, and frequently experiences low magnitude seismic swarms. The most active region is located in the Cheb basin and coincides with the junction of a northwest trending fault with a north-south trending shear zone, making for a structurally complex hardrock setting. In the early 1990s, two long-offset reflection seismic profiles were collected along the boundary of the Cheb basin: MVE-90 along the northern edge, and 9HR-91 in the east. These profiles were recently reprocessed using Kirchhoff PreStack Depth Migration, revealing high amplitude reflections, or bright spots, that correlate to nearby seismicity. Several studies have hypothesized that the 9HR-91 bright spots image a fluid trap, where mantle-sourced fluids accumulate, thereby facilitating slip on the faults and triggering the swarms. However, the exact nature of the bright spots remains an open question. They may be a change in lithology and/or porosity, an infilled vein or an impermeable fault. We aim to answer this question by first using Coherency-Based PreStack Depth Migration to produce detailed images of the bright spots. We then forward model the waveforms guided by the reflection coefficients in order to derive rock-physical parameters. Finally, the best-fitting models are interpreted in terms of their possible relationship to the West Bohemia

Seismic evidence for change of the tectonic regime in Messinian, northern Marmara Sea, Turkey

NASA Astrophysics Data System (ADS)

Alp, Hakan; Vardar, Denizhan; Alpar, Bedri; Ustaömer, Timur

2018-01-01

New Chirp seismic data collected from the northern margin of the Marmara Sea in June 2015 and previous Sparker seismic profiles recorded in 1999 suggest a change in tectonic regime in Messinian. New tectonic lineaments and fault segments were detected at offshore the Çekmece lagoons region that is located on one of the possible water corridors with the Paratethys. The faults only affect the older seismic unit (U1), which can be best outlined on the Chirp data. The E-W trending fault offshore Avcılar (OAF) borders the northern edge of a tightly folded sedimentary zone. The NNE-SSW trending fault, namely the Büyükçekmece Fault (BF), passing through the Büyükçekmece Bay, follows a buried valley. Its evolution must be related to the development of the Early Miocene - Early Pliocene Thrace-Eskişehir fault zone (TEFZ). BF and OAF indicate old tectonic activities in the region, which continued to the North Anatolian fault becoming the most dominant tectonic element in the region. The upper surface of the stratigraphic unit U1 and its terraces define the thickness of younger deposits (U2), which is thinner in the middle of the shelf. The morphology of the tightly folded zone controls those terraces, which correspond to the Bakırköy Formation and Kıraç member on land. The topmost parts of the terraces must have been eroded during sea level low-stands and cutting of the paleo-valleys. There is no evidence of any tectonic deformation or active fault in the younger seismic unit (U2).

Fault zone structure and seismic reflection characteristics in zones of slow slip and tsunami earthquakes

NASA Astrophysics Data System (ADS)

Bell, Rebecca; Henrys, Stuart; Sutherland, Rupert; Barker, Daniel; Wallace, Laura; Holden, Caroline; Power, William; Wang, Xiaoming; Morgan, Joanna; Warner, Michael; Downes, Gaye

2015-04-01

Over the last couple of decades we have learned that a whole spectrum of different fault slip behaviour takes place on subduction megathrust faults from stick-slip earthquakes to slow slip and stable sliding. Geophysical data, including seismic reflection data, can be used to characterise margins and fault zones that undergo different modes of slip. In this presentation we will focus on the Hikurangi margin, New Zealand, which exhibits marked along-strike changes in seismic behaviour and margin characteristics. Campaign and continuous GPS measurements reveal deep interseismic coupling and deep slow slip events (~30-60 km) at the southern Hikurangi margin. The northern margin, in contrast, experiences aseismic slip and shallow (<10-15 km) slow slip events (SSE) every 18-24 months with equivalent moment magnitudes of Mw 6.5-6.8. Updip of the SSE region two unusual megathrust earthquakes occurred in March and May 1947 with characteristics typical of tsunami earthquakes. The Hikurangi margin is therefore an excellent natural laboratory to study differential fault slip behaviour. Using 2D seismic reflection, magnetic anomaly and geodetic data we observe in the source areas of the 1947 tsunami earthquakes i) low amplitude interface reflectivity, ii) shallower interface relief, iii) bathymetric ridges, iv) magnetic anomaly highs and in the case of the March 1947 earthquake v) stronger geodetic coupling. We suggest that this is due to the subduction of seamounts, similar in dimensions to seamounts observed on the incoming Pacific plate, to depths of <10 km. We propose a source model for the 1947 tsunami earthquakes based on geophysical data and find that extremely low rupture velocities (c. 300 m/s) are required to model the observed large tsunami run-up heights (Bell et al. 2014, EPSL). Our study suggests that subducted topography can cause the nucleation of moderate earthquakes with complex, low velocity rupture scenarios that enhance tsunami waves, and the role of

Seismicity of the St. Lawrence paleorift faults overprinted by a meteorite impact crater: Implications for crustal strength based on new earthquake relocations in the Charlevoix Seismic Zone, Eastern Canada

NASA Astrophysics Data System (ADS)

Yu, H.; Harrington, R. M.; Liu, Y.; Lamontagne, M.; Pang, M.

2015-12-01

The Charlevoix Seismic Zone (CSZ), located along the St. Lawrence River (SLR) ~100 km downstream from Quebec City, is the most active seismic zone in eastern Canada with five historic earthquakes of M 6-7 and ~ 200 events/year reported by the Canadian National Seismograph Network. Cataloged earthquake epicenters outline two broad linear zones along the SLR with little shallow seismicity in between. Earthquakes form diffuse clusters between major dipping faults rather than concentrating on fault planes. Detailed fault geometry in the CSZ is uncertain and the effect on local seismicity of a meteorite impact structure that overprints the paleorift faults remains ambiguous. Here we relocate 1639 earthquakes occurring in the CSZ between 01/1988 - 10/2010 using the double-difference relocation method HypoDD and waveforms primarily from 7 local permanent stations. We use the layered SLR north shore velocity model from Lamontagne (1999), and travel time differences based on both catalog and cross-correlated P and S-phase picks. Of the 1639 relocated earthquakes, 1236 (75.4%) satisfied selection criteria of horizontal and vertical errors less than 2 km and 1 km respectively. Cross-sections of relocated seismicity show hypocenters along distinct active fault segments. Earthquakes located beneath the north shore of the SLR are likely correlated with the NW Gouffre fault, forming a ~10 km wide seismic zone parallel to the river, with dip angle changing to near vertical at the northern edge of the impact zone. In contrast, seismicity beneath the SLR forms a diffuse cloud within the impact structure, likely representing a highly fractured volume. It further implies that faults could be locally weak and subject to high pore-fluid pressures. Seismicity outside the impact structure defines linear structures aligning with the Charlevoix fault. Relocated events of M > 4 all locate outside the impact structure, indicating they nucleated on the NE-SW-oriented paleorift faults.

Fault zone characteristics and basin complexity in the southern Salton Trough, California

USGS Publications Warehouse

Persaud, Patricia; Ma, Yiran; Stock, Joann M.; Hole, John A.; Fuis, Gary S.; Han, Liang

2016-01-01

Ongoing oblique slip at the Pacific–North America plate boundary in the Salton Trough produced the Imperial Valley (California, USA), a seismically active area with deformation distributed across a complex network of exposed and buried faults. To better understand the shallow crustal structure in this region and the connectivity of faults and seismicity lineaments, we used data primarily from the Salton Seismic Imaging Project to construct a three-dimensional P-wave velocity model down to 8 km depth and a velocity profile to 15 km depth, both at 1 km grid spacing. A VP = 5.65–5.85 km/s layer of possibly metamorphosed sediments within, and crystalline basement outside, the valley is locally as thick as 5 km, but is thickest and deepest in fault zones and near seismicity lineaments, suggesting a causative relationship between the low velocities and faulting. Both seismicity lineaments and surface faults control the structural architecture of the western part of the larger wedge-shaped basin, where two deep subbasins are located. We estimate basement depths, and show that high velocities at shallow depths and possible basement highs characterize the geothermal areas.

Seismicity and Seismotectonic Properties of The Sultandağı Fault Zone (Afyonkarahisar-Konya): Western Anatolia,Turkey

NASA Astrophysics Data System (ADS)

Kalafat, D.; Gunes, Y.; Kekovali, K.; Kara, M.; Gorgun, E.

2017-12-01

n this study we investigated seismicity and source characteristics of the Sultandağı Fault Zone (SFZ). As known Western Anatolia is one of the most important seismically active region in Turkey. The relative movement of the African-Arabian plates, it causes the Anatolian Plate to movement to the west-Southwest direction 2.5 cm per year and this result provides N-S direction with extensional regime in the recent tectonic. In this study, especially with the assessment of seismic activity occurring in Afyon and around between 200-2002 years, we have been evaluated to date with seismic activity as well as fault mechanism solution. We analyzed recent seismicity and distribution of earthquakes in this region. In the last century, 3 important earthquakes occurred in the Sultandağı Fault zone (Afyon-Akşehir Graben), this result shown it was seismic active and broken fault segments caused stress balance in the region and it caused to occur with short intervals of earthquakes in 2000 and 2002, triggering each other. The scope of this tudy, we installed new BB stations in the region and we have been done of the fault plane solutions for important earthquakes. The focal mechanisms clearly exhibit the activation of a NE-SW trending normal faulting system along the SFZ region. The results of stress analysis showed that the effective current tectonic evolution of normal faulting in this region. This study is supported by Bogazici University Research Projects Commission under SRP/BAP project No. 12280. Key Words: Sultandağı fault zone, normal faulting, seismicity, fault mechanism

Dynamics of the Wulong landslide revealed by broadband seismic records

NASA Astrophysics Data System (ADS)

Li, Zhengyuan; Huang, Xinghui; Xu, Qiang; Yu, Dan; Fan, Junyi; Qiao, Xuejun

2017-02-01

The catastrophic Wulong landslide occurred at 14:51 (Beijing time, UTC+8) on 5 June 2009, in Wulong Prefecture, Southwest China. This rockslide occurred in a complex topographic environment. Seismic signals generated by this event were recorded by the seismic network deployed in the surrounding area, and long-period signals were extracted from 8 broadband seismic stations within 250 km to obtain source time functions by inversion. The location of this event was simultaneously acquired using a stepwise refined grid search approach, with an error of 2.2 km. The estimated source time functions reveal that, according to the movement parameters, this landslide could be divided into three stages with different movement directions, velocities, and increasing inertial forces. The sliding mass moved northward, northeastward and northward in the three stages, with average velocities of 6.5, 20.3, and 13.8 m/s, respectively. The maximum movement velocity of the mass reached 35 m/s before the end of the second stage. The basal friction coefficients were relatively small in the first stage and gradually increasing; large in the second stage, accompanied by the largest variability; and oscillating and gradually decreasing to a stable value, in the third stage. Analysis shows that the movement characteristics of these three stages are consistent with the topography of the sliding zone, corresponding to the northward initiation, eastward sliding after being stopped by the west wall, and northward debris flowing after collision with the east slope of the Tiejianggou valley. The maximum movement velocity of the sliding mass results from the largest height difference of the west slope of the Tiejianggou valley. The basal friction coefficients of the three stages represent the thin weak layer in the source zone, the dramatically varying topography of the west slope of the Tiejianggou valley, and characteristics of the debris flow along the Tiejianggou valley. Based on the above

Seismic reflection images of shallow faulting, northernmost Mississippi embayment, north of the New Madrid seismic zone

USGS Publications Warehouse

McBride, J.H.; Nelson, W.J.

2001-01-01

High-resolution seismic reflection surveys document tectonic faults that displace Pleistocene and older strata just beyond the northeast termination of the New Madrid seismic zone, at the northernmost extent of the Mississippi embayment. These faults, which are part of the Fluorspar Area fault complex in southeastern Illinois, are directly in line with the northeast-trending seismic zone. The reflection data were acquired using an elastic weight-drop source recorded to 500 msec by a 48-geophone array (24-fold) with a 10-ft (??3.0m) station interval. Recognizable reflections were recorded to about 200 msec (100-150 m). The effects of multiple reflections, numerous diffractions, low apparent velocity (i.e., steeply dipping) noise, and the relatively low-frequency content of the recorded signal provided challenges for data processing and interpreting subtle fault offsets. Data processing steps that were critical to the detection of faults included residual statics, post-stack migration, deconvolution, and noise-reduction filtering. Seismic migration was crucial for detecting and mitigating complex fault-related diffraction patterns, which produced an apparent 'folding' of reflectors on unmigrated sections. Detected individual offsets of shallow reflectors range from 5 to 10 m for the top of Paleozoic bedrock and younger strata. The migrated sections generally indicate vertical to steeply dipping normal and reverse faults, which in places outline small horsts and/or grabens. Tilting or folding of stratal reflectors associated with faulting is also locally observed. At one site, the observed faulting is superimposed over a prominent antiformal structure, which may itself be a product of the Quaternary deformation that produced the steep normal and reverse faults. Our results suggest that faulting of the Paleozoic bedrock and younger sediments of the northern Mississippi embayment is more pervasive and less localized than previously thought.

A comparison of seismicity in world's subduction zones: Implication by the difference of b-values

NASA Astrophysics Data System (ADS)

Nishikawa, T.; Ide, S.

2013-12-01

Since the pioneering study of Uyeda and Kanamori (1979), it has been thought that world's subduction zones can be classified into two types: Chile and Mariana types. Ruff and Kanamori (1980) suggested that the maximum earthquake size within each subduction zone correlates with convergence rate and age of subducting lithosphere. Subduction zones with younger lithosphere and larger convergence rates are associated with great earthquakes (Chile), while subduction zones with older lithosphere and smaller convergence rates have low seismicity (Mariana). However, these correlations are obscured after the 2004 Sumatra earthquake and the 2009 Tohoku earthquake. Furthermore, McCaffrey (2008) pointed out that the history of observation is much shorter than the recurrence times of very large earthquakes, suggesting a possibility that any subduction zone may produce earthquakes larger than magnitude 9. In the present study, we compare world's subduction zones in terms of b-values in the Gutenberg-Richer relation. We divided world's subduction zones into 146 regions, each of which is bordered by a trench section of about 500 km and extends for 200 km from the trench section in the direction of relative plate motion. In each region, earthquakes equal to or larger than M4.5 occurring during 1988-2009 were extracted from ISC catalog. We find a positive correlation between b-values and ages of subducting lithosphere, which is one of the two important variables discussed in Ruff and Kanamori (1980). Subduction zones with younger lithosphere are associated with high b-values and vice versa, while we cannot find a correlation between b-values and convergence rates. We used the ages determined by Müller et al. (2008) and convergence rate calculated using PB2002 (Bird, 2003) for convergence rate. We also found a negative correlation between b-values and the estimates of seismic coupling, which is defined as the ratio of the observed seismic moment release rate to the rate calculated

Seismic multiple attenuation in the northern continent-ocean transition zone of the South China Sea

NASA Astrophysics Data System (ADS)

Chen, N.; Li, C. F.

2017-12-01

In seismic exploration, especially in marine oil and gas exploration, presence of multiple reflections lowers signal-to-noise ratio of seismic data and makes it difficult to analyze seismic velocity. In northern continent-ocean transition zone of the South China Sea (SCS), low-velocity Cenozoic strata cover sets of high-velocity carbonate strata directly, and over 1000 m thick of sediments were deposited on the igneous basement in the northwest SCS. These sedimentary boundaries generate quite strong impedance interfaces and strong internal multiples. Diffractions as a result of variation of seabed topography, coupled with the vibration, free surface multiples and refraction multiples, cause a variety of strong energy disturbances and missing of frequency component. In this study, we process four recently acquired multichannel reflection seismic profiles from the northern continent-ocean transition zone of the SCS with a new combination of demultiple techniques. There is a variety of strong multiples in the raw data, and the seabed multiple occurs between 9 to 11 seconds in two-way travel time (TWTT), and we apply Surface-related Multiple Elimination (SRME) to attenuate the free surface multiples. After SRME, we use high-resolution Radon transform (RAMUR) to attenuate deep multiples concentrating below 10 seconds in TWTT. Normal moveout correction (NMO) is necessary to flatten true reflections and turn multiples into a parabola before RAMUR, and we can attenuate the deep multiples in theτ-p domain. The seabed topography varies greatly in the continent-ocean transition zone, so the diffractions are well developed. However, SRME and RAMUR are not effective in attenuating diffractions and internal multiples. We select diffracted multiple attenuation (DIMAT) after many trials and detailed analysis. The diffractions are extracted in decomposed frequency bands. The internal multiples below 11 seconds in TWTT and high-amplitude noises are successfully suppressed while

Structure and seismic activity of the Lesser Antilles subduction zone

NASA Astrophysics Data System (ADS)

Evain, M.; Galve, A.; Charvis, P.; Laigle, M.; Ruiz Fernandez, M.; Kopp, H.; Hirn, A.; Flueh, E. R.; Thales Scientific Party

2011-12-01

Several active and passive seismic experiments conducted in 2007 in the framework of the European program "Thales Was Right" and of the French ANR program "Subsismanti" provided a unique set of geophysical data highlighting the deep structure of the central part of the Lesser Antilles subduction zone, offshore Dominica and Martinique, and its seismic activity during a period of 8 months. The region is characterized by a relatively low rate of seismicity that is often attributed to the slow (2 cm/yr) subduction of the old, 90 My, Atlantic lithosphere beneath the Caribbean Plate. Based on tomographic inversion of wide-angle seismic data, the forearc can clearly be divided into an inner forearc, characterised by a high vertical velocity gradient in the igneous crust, and an outer forearc with lower crustal velocity gradient. The thick, high velocity, inner forearc is possibly the extension at depth of the Mesozoic Caribbean crust outcropping in La Désirade Island. The outer forearc, up to 70 km wide in the northern part of the study area, is getting narrower to the south and disappears offshore Martinique. Based on its seismic velocity structure with velocities higher than 6 km/s the backstop consists, at least partly, of magmatic rocks. The outer forearc is also highly deformed and faulted within the subducting trend of the Tiburon Ridge. With respect to the inner forearc velocity structure the outer forearc basement could either correspond to an accreted oceanic terrane or made of highly fractured rocks. The inner forearc is a dense, poorly deformable crustal block, tilted southward as a whole. It acts as a rigid buttress increasing the strain within both the overriding and subducting plates. This appears clearly in the current local seismicity affecting the subducting and the overriding plates that is located beneath the inner forearc. We detected earthquakes beneath the Caribbean forearc and in the Atlantic oceanic plate as well. The main seismic activity is

Angola Seismicity MAP

NASA Astrophysics Data System (ADS)

Neto, F. A. P.; Franca, G.

2014-12-01

The purpose of this job was to study and document the Angola natural seismicity, establishment of the first database seismic data to facilitate consultation and search for information on seismic activity in the country. The study was conducted based on query reports produced by National Institute of Meteorology and Geophysics (INAMET) 1968 to 2014 with emphasis to the work presented by Moreira (1968), that defined six seismogenic zones from macro seismic data, with highlighting is Zone of Sá da Bandeira (Lubango)-Chibemba-Oncócua-Iona. This is the most important of Angola seismic zone, covering the epicentral Quihita and Iona regions, geologically characterized by transcontinental structure tectono-magmatic activation of the Mesozoic with the installation of a wide variety of intrusive rocks of ultrabasic-alkaline composition, basic and alkaline, kimberlites and carbonatites, strongly marked by intense tectonism, presenting with several faults and fractures (locally called corredor de Lucapa). The earthquake of May 9, 1948 reached intensity VI on the Mercalli-Sieberg scale (MCS) in the locality of Quihita, and seismic active of Iona January 15, 1964, the main shock hit the grade VI-VII. Although not having significant seismicity rate can not be neglected, the other five zone are: Cassongue-Ganda-Massano de Amorim; Lola-Quilengues-Caluquembe; Gago Coutinho-zone; Cuima-Cachingues-Cambândua; The Upper Zambezi zone. We also analyzed technical reports on the seismicity of the middle Kwanza produced by Hidroproekt (GAMEK) region as well as international seismic bulletins of the International Seismological Centre (ISC), United States Geological Survey (USGS), and these data served for instrumental location of the epicenters. All compiled information made possible the creation of the First datbase of seismic data for Angola, preparing the map of seismicity with the reconfirmation of the main seismic zones defined by Moreira (1968) and the identification of a new seismic

The Fluid Flow Evolution During the Seismic Cycle Within Overpressured Fault Zones

NASA Astrophysics Data System (ADS)

de Paola, Nicola; Vanhunen, Jeroen; Collettini, Cristiano; Faulkner, Dan

2010-05-01

The integration of seismic reflection profiles with well-located earthquakes shows that the mainshocks of the 1997 Umbria-Marche seismic sequence (Mw < 6) nucleated at about 6 km depth, within the Triassic Evaporites, a 2 km thick sequence made of interbedded anhydrites and dolostones. Two boreholes, drilled northwest of the epicentral area, encountered CO2 fluid overpressures at about 0.8 of the lithostatic load, at about 4 km depth. It has been proposed that the time-space evolution of the 1997 aftershock sequence, was driven by the coseismic release of trapped high-pressure fluids (lv = 0.8), within the Triassic Evaporites. In order to understand whether CO2 fluid overpressure can be maintained up to the coseismic period, and trigger earthquake nucleation, we modelled fluid flow through a mature fault zone within the Triassic Evaporites. We assume that fluid flow within the fault zone occurs in accord with the Darcy's Law. Under this condition, a near lithostatic pore pressure gradient can develop, within the fault zone, when the upward transport of fluid along the fault zone exceeds the fluid loss in a horizontal direction. Our model's parameters are: a) Fault zone structure: model inputs have been obtained from large fault zone analogues derived from field observation. The architecture of large fault zones within the TE is given by a distinct fault core, up to few meters thick, of very fine-grained fault rocks (cataclasites and fault gouge), where most of the shear strain has been accommodated, surrounded by a geometrically complex and heterogeneous damage zone (up to few tens of meters wide). The damage zone is characterized by adjacent zones of heavily fractured rocks (dolostones) and foliated rocks displaying little fracturing (anhydrites). b) Fault zone permeability: field data suggests that the permeability of the fault core is relatively low due to the presence of fine grained fault rocks (k < 10E-18 m2). The permeability of the dolostones, within the

The Dandridge-Vonore Fault Zone in the Eastern Tennessee Seismic Zone (and Rejuvenation of the Smokies?)

NASA Astrophysics Data System (ADS)

Cox, R. T.; Hatcher, R. D., Jr.; Forman, S. L.; Gamble, E. D. S.; Warrell, K. F.

2017-12-01

The eastern Tennessee seismic zone (ETSZ) trends 045o from NE Alabama and NW Georgia through Tennessee to SE Kentucky, and seismicity is localized 5-26 km deep in the basement. The ETSZ is the second most seismically active region in North America east of the Rocky Mountains, although no historic earthquakes larger than Mw 4.8 have been recorded here. Late Quaternary paleoiseismic evidence suggests that the ETSZ is capable of M7+ earthquakes and that neotectonic faults may have significantly influenced the regional relief. We have identified an 80 km-long, 060o-trending corridor in eastern Tennessee that contains collinear northeast-striking thrust, strike-slip, and normal Quaternary faults with displacements of 1-2 m, herein termed the Dandridge-Vonore fault zone (DVFZ). French Broad River alluvium in the northeast DVFZ near Dandridge, TN, is displaced by a 050o-striking, SE-dipping thrust fault and by a set of related fissures that record at least two significant post 25 ka paleo-earthquakes. Southwest of Dandridge near Alcoa, TN, a 060o-striking, SE-dipping thrust fault cuts Little River alluvium and records two significant post-15 ka paleo-earthquakes. Farther southwest at Vonore, colluvium with alluvial cobbles is thrust >1 m by a 057o-striking, steeply SE-dipping fault that may also have a significant strike-slip component, and Little Tennessee River alluvium is dropped >2 m along a 070o- striking normal fault. The DVFZ partly overlaps and is collinear with a local trend of maximum seismicity that extends 30 km farther SW of the DVFZ (as currently mapped), for a total length of 110 km. The DVFZ is coincident with a steep gradient in S-wave velocities (from high velocity on the SE to low velocity on the NW) at mid-crustal depths of 20 to 24 km, consistent with a fault and source zone at hypocentral depths in the crystalline basement. Moreover, the DVFZ parallels the NW foot of Blue Ridge Mountains, and the sense of thrusting at all sites of Quaternary faulting

Trench Logs and Scarp Data from an Investigation of the Steens Fault Zone, Bog Hot Valley and Pueblo Valley, Humboldt County, Nevada

USGS Publications Warehouse

Personius, Stephen F.; Crone, Anthony J.; Machette, Michael N.; Kyung, Jai Bok; Cisneros, Hector; Lidke, David J.; Mahan, Shannon

2006-01-01

Introduction: This report contains field and laboratory data from a study of the Steens fault zone near Denio, Nev. The 200-km-long Steens fault zone forms the longest, most topographically prominent fault-bounded escarpment in the Basin and Range of southern Oregon and northern Nevada. The down-to-the-east normal fault is marked by Holocene fault scarps along nearly half its length, including the southern one-third of the fault from the vicinity of Pueblo Mountain in southern Oregon to the southern margin of Bog Hot Valley (BHV) southwest of Denio, Nev. We studied this section of the fault to better constrain late Quaternary slip rates, which we hope to compare to deformation rates derived from a recently established geodetic network in the region (Hammond and Thatcher, 2005). We excavated a trench in May 2003 across one of a series of right-stepping fault scarps that extend south from the southern end of the Pueblo Mountains and traverse the floor of Bog Hot Valley, about 4 km south of Nevada State Highway 140. This site was chosen because of the presence of well-preserved fault scarps, their development on lacustrine deposits thought to be suitable for luminescence dating, and the proximity of two geodetic stations that straddle the fault zone. We excavated a second trench in the southern BHV, but the fault zone in this trench collapsed during excavation and thus no information about fault history was documented from this site. We also excavated a soil pit on a lacustrine barrier bar in the southern Pueblo Valley (PV) to better constrain the age of lacustrine deposits exposed in the trench. The purpose of this report is to present photomosaics and trench logs, scarp profiles and slip data, soils data, luminescence and radiocarbon ages, and unit descriptions obtained during this investigation. We do not attempt to use the data presented herein to construct a paleoseismic history of this part of the Steens fault zone; that history will be the subject of a future

Structural controls on geothermal circulation in Surprise Valley, California: A re-evaluation of the Lake City fault zone

USGS Publications Warehouse

Anne E. Egger,; Glen, Jonathan; McPhee, Darcy K.

2014-01-01

Faults and fractures play an important role in the circulation of geothermal fluids in the crust, and the nature of that role varies according to structural setting and state of stress. As a result, detailed geologic and geophysical mapping that relates thermal springs to known structural features is essential to modeling geothermal systems. Published maps of Surprise Valley in northeastern California suggest that the “Lake City fault” or “Lake City fault zone” is a significant structural feature, cutting obliquely across the basin and connecting thermal springs across the valley. Newly acquired geophysical data (audio-magnetotelluric, gravity, and magnetic), combined with existing geochemical and geological data, suggest otherwise. We examine potential field profiles and resistivity models that cross the mapped Lake City fault zone. While there are numerous geophysical anomalies that suggest subsurface structures, they mostly do not coincide with the mapped traces of the Lake City fault zone, nor do they show a consistent signature in gravity, magnetics, or resistivities that would suggest a through-going fault that would promote connectivity through lateral fluid flow. Instead of a single, continuous fault, we propose the presence of a deformation zone associated with the growth of the range-front Surprise Valley fault. The implication for geothermal circulation is that this is a zone of enhanced porosity but lacks length-wise connectivity that could conduct fluids across the valley. Thermal fluid circulation is most likely controlled primarily by interactions between N-S–trending normal faults.

Two-dimensional seismic image of the San Andreas Fault in the Northern Gabilan Range, central California: Evidence for fluids in the fault zone

USGS Publications Warehouse

Thurber, C.; Roecker, S.; Ellsworth, W.; Chen, Y.; Lutter, W.; Sessions, R.

1997-01-01

A joint inversion for two-dimensional P-wave velocity (Vp), P-to-S velocity ratio (Vp/Vs), and earthquake locations along the San Andreas fault (SAF) in central California reveals a complex relationship among seismicity, fault zone structure, and the surface fault trace. A zone of low Vp and high Vp/Vs lies beneath the SAF surface trace (SAFST), extending to a depth of about 6 km. Most of the seismic activity along the SAF occurs at depths of 3 to 7 km in a southwest-dipping zone that roughly intersects the SAFST, and lies near the southwest edge of the low Vp and high Vp/Vs zones. Tests indicate that models in which this seismic zone is significantly closer to vertical can be confidently rejected. A second high Vp/Vs zone extends to the northeast, apparently dipping beneath the Diablo Range. Another zone of seismicity underlies the northeast portion of this Vp/Vs high. The high Vp/Vs zones cut across areas of very different Vp values, indicating that the high Vp/Vs values are due to the presence of fluids, not just lithology. The close association between the zones of high Vp/Vs and seismicity suggests a direct involvement of fluids in the faulting process. Copyright 1997 by the American Geophysical Union.

Seismically imaging the Afar plume

NASA Astrophysics Data System (ADS)

Hammond, J. O.; Kendall, J. M.; Bastow, I. D.; Stuart, G. W.; Keir, D.; Ayele, A.; Ogubazghi, G.; Ebinger, C. J.; Belachew, M.

2011-12-01

Plume related flood basalt volcanism in Ethiopia has long been cited to have instigated continental breakup in northeast Africa. However, to date seismic images of the mantle beneath the region have not produced conclusive evidence of a plume-like structure. As a result the nature and even existence of a plume in the region and its role in rift initiation and continental rupture are debated. Previous seismic studies using regional deployments of sensors in East-Africa show that low seismic velocities underlie northeast Africa, but their resolution is limited to the top 200-300km of the Earth. Thus, the connection between the low velocities in the uppermost mantle and those imaged in global studies in the lower mantle is unclear. We have combined new data from Afar, Ethiopia with 6 other regional experiments and global network stations across Ethiopia, Eritrea, Djibouti and Yemen, to produce high-resolution models of upper mantle P- and S- wave velocities to the base of the transition zone. Relative travel time tomographic inversions show that the top 100km is dominated by focussed low velocity zones, likely associated with melt in the lithosphere/uppermost asthenosphere. Below these depths a broad SW-NE oriented sheet like upwelling extends down to the top of the transition zone. Within the transition zone two focussed sharp-sided low velocity regions exist: one beneath the Western Ethiopian plateau outside the rift valley, and the other beneath the Afar depression. The nature of the transition zone anomalies suggests that small upwellings may rise from a broader low velocity plume-like feature in the lower mantle. This interpretation is supported by numerical and analogue experiments that suggest the 660km phase change and viscosity jump may impede flow from the lower to upper mantle creating a thermal boundary layer at the base of the transition zone. This allows smaller, secondary upwellings to initiate and rise to the surface. Our images of secondary upwellings

Steady, modest slip over multiple earthquake cycles on the Owens Valley and Little Lake fault zones

NASA Astrophysics Data System (ADS)

Amos, C. B.; Haddon, E. K.; Burgmann, R.; Zielke, O.; Jayko, A. S.

2015-12-01

A comprehensive picture of current plate-boundary deformation requires integration of short-term geodetic records with longer-term geologic strain. Comparing rates of deformation across these time intervals highlights potential time-dependencies in both geodetic and geologic records and yields critical insight into the earthquake deformation process. The southern Walker Lane Belt in eastern California represents one location where short-term strain recorded by geodesy apparently outpaces longer-term geologic fault slip measured from displaced rocks and landforms. This discrepancy persists both for individual structures and across the width of the deforming zone, where ~1 cm/yr of current dextral shear exceeds Quaternary slip rates summed across individual faults. The Owens Valley and Little Lake fault systems form the western boundary of the southern Walker Lane and host a range of published slip rate estimates from ~1 - 7 mm/yr over varying time intervals based on both geodetic and geologic measurements. New analysis of offset geomorphic piercing lines from airborne lidar and field measurements along the Owens Valley fault provides a snapshot of deformation during individual earthquakes and over many seismic cycles. Viewed in context of previously reported ages from pluvial and other landforms in Owens Valley, these offsets suggest slip rates of ~0.6 - 1.6 mm/yr over the past 103 - 105 years. Such rates agree with similar estimates immediately to the south on the Little Lake fault, where lidar measurements indicate dextral slip averaging ~0.6 - 1.3 mm/yr over comparable time intervals. Taken together, these results suggest steady, modest slip in the absence of significant variations over the Mid-to-Late Quaternary for a ~200 km span of the southwestern Walker Lane. Our findings argue against the presence of long-range fault interactions and slip-rate variations for this portion of the larger, regional fault network. This result also suggests that faster slip

Seismic hazard assessment of the cultural heritage sites: A case study in Cappadocia (Turkey)

NASA Astrophysics Data System (ADS)

Seyrek, Evren; Orhan, Ahmet; Dinçer, İsmail

2014-05-01

Turkey is one of the most seismically active regions in the world. Major earthquakes with the potential of threatening life and property occur frequently here. In the last decade, over 50,000 residents lost their lives, commonly as a result of building failures in seismic events. The Cappadocia region is one of the most important touristic sites in Turkey. At the same time, the region has been included to the Word Heritage List by UNESCO at 1985 due to its natural, historical and cultural values. The region is undesirably affected by several environmental conditions, which are subjected in many previous studies. But, there are limited studies about the seismic evaluation of the region. Some of the important historical and cultural heritage sites are: Goreme Open Air Museum, Uchisar Castle, Ortahisar Castle, Derinkuyu Underground City and Ihlara Valley. According to seismic hazard zonation map published by the Ministry of Reconstruction and Settlement these heritage sites fall in Zone III, Zone IV and Zone V. This map show peak ground acceleration or 10 percent probability of exceedance in 50 years for bedrock. In this connection, seismic hazard assessment of these heritage sites has to be evaluated. In this study, seismic hazard calculations are performed both deterministic and probabilistic approaches with local site conditions. A catalog of historical and instrumental earthquakes is prepared and used in this study. The seismic sources have been identified for seismic hazard assessment based on geological, seismological and geophysical information. Peak Ground Acceleration (PGA) at bed rock level is calculated for different seismic sources using available attenuation relationship formula applicable to Turkey. The result of the present study reveals that the seismic hazard at these sites is closely matching with the Seismic Zonation map published by the Ministry of Reconstruction and Settlement. Keywords: Seismic Hazard Assessment, Probabilistic Approach

Seismic cycle feedbacks in a mid-crustal shear zone

NASA Astrophysics Data System (ADS)

Melosh, Benjamin L.; Rowe, Christie D.; Gerbi, Christopher; Smit, Louis; Macey, Paul

2018-07-01

Mid-crustal fault rheology is controlled by alternating brittle and plastic deformation mechanisms, which cause feedback cycles that influence earthquake behavior. Detailed mapping and microstructural observations in the Pofadder Shear Zone (Namibia and South Africa) reveal a lithologically heterogeneous shear zone core with quartz-rich mylonites and ultramylonites, plastically overprinted pseudotachylyte and active shear folds. We present evidence for a positive feedback cycle in which coseismic grain size reduction facilitates active shear folding by enhancing competency contrasts and promoting crystal plastic flow. Shear folding strengthens a portion of a shear zone by limb rotation, focusing deformation and promoting plastic flow or brittle slip in resulting areas of localized high stress. Using quartz paleopiezometry, we estimate strain and slip rates consistent with other studies of exhumed shear zones and modern plate boundary faults, helping establish the Pofadder Shear Zone as an ancient analogue to modern, continental-scale, strike-slip faults. This feedback cycle influences seismicity patterns at the scale of study (10s of meters) and possibly larger scales as well, and contributes to bulk strengthening of the brittle-plastic transition on modern plate boundary faults.

Evidence of Enhanced Subrosion in a Fault Zone and Characterization of Hazard Zones with Elastic Parameters derived from SH-wave reflection Seismics and VSP

NASA Astrophysics Data System (ADS)

Wadas, S. H.; Tanner, D. C.; Tschache, S.; Polom, U.; Krawczyk, C. M.

2017-12-01

Subrosion, the dissolution of soluble rocks, e.g., sulfate, salt, or carbonate, requires unsaturated water and fluid pathways that enable the water to flow through the subsurface and generate cavities. Over time, different structures can occur that depend on, e.g., rock solubility, flow rate, and overburden type. The two main structures are sinkholes and depressions. To analyze the link between faults, groundwater flow, and soluble rocks, and to determine parameters that are useful to characterize hazard zones, several shear-wave (SH) reflection seismic profiles were surveyed in Thuringia in Germany, where Permian sulfate rocks and salt subcrop close to the surface. From the analysis of the seismic sections we conclude that areas affected by tectonic deformation phases are prone to enhanced subrosion. The deformation of fault blocks leads to the generation of a damage zone with a dense fracture network. This increases the rock permeability and thus serves as a fluid pathway for, e.g., artesian-confined groundwater. The more complex the fault geometry and the more interaction between faults, the more fractures are generated, e.g., in a strike slip-fault zone. The faults also act as barriers for horizontal groundwater flow perpendicular to the fault surfaces and as conduits for groundwater flow along the fault strike. In addition, seismic velocity anomalies and attenuation of seismic waves are observed. Low velocities <200 m/s and high attenuation may indicate areas affected by subrosion. Other parameters that characterize the underground stability are the shear modulus and the Vp/Vs ratio. The data revealed zones of low shear modulus <100 MPa and high Vp/Vs ratio >2.5, which probably indicate unstable areas due to subrosion. Structural analysis of S-wave seismics is a valuable tool to detect near-surface faults in order to determine whether or not an area is prone to subrosion. The recognition of even small fault blocks can help to better understand the hydrodynamic

Slow slip events and seismic tremor at circum-Pacific subduction zones

NASA Astrophysics Data System (ADS)

Schwartz, Susan Y.; Rokosky, Juliana M.

2007-09-01

It has been known for a long time that slip accompanying earthquakes accounts for only a fraction of plate tectonic displacements. However, only recently has a fuller spectrum of strain release processes, including normal, slow, and silent earthquakes (or slow slip events) and continuous and episodic slip, been observed and generated by numerical simulations of the earthquake cycle. Despite a profusion of observations and modeling studies the physical mechanism of slow slip events remains elusive. The concurrence of seismic tremor with slow slip episodes in Cascadia and southwestern Japan provides insight into the process of slow slip. A perceived similarity between subduction zone and volcanic tremor has led to suggestions that slow slip involves fluid migration on or near the plate interface. Alternatively, evidence is accumulating to support the notion that tremor results from shear failure during slow slip. Global observations of the location, spatial extent, magnitude, duration, slip rate, and periodicity of these aseismic slip transients indicate significant variation that may be exploited to better understand their generation. Most slow slip events occur just downdip of the seismogenic zone, consistent with rate- and state-dependent frictional modeling that requires unstable to stable transitional properties for slow slip generation. At a few convergent margins the occurrence of slow slip events within the seismogenic zone makes it highly likely that transitions in frictional properties exist there and are the loci of slow slip nucleation. Slow slip events perturb the surrounding stress field and may either increase or relieve stress on a fault, bringing it closer to or farther from earthquake failure, respectively. This paper presents a review of slow slip events and related seismic tremor observed at plate boundaries worldwide, with a focus on circum-Pacific subduction zones. Trends in global observations of slow slip events suggest that (1) slow slip is a

The double seismic zone in downgoing slabs and the viscosity of the mesosphere

NASA Technical Reports Server (NTRS)

Sleep, N. H.

1979-01-01

The seismic zone beneath several island arcs between about 100 and 200 km depth consists of an upper zone having down-dip compression and a lower zone having down-dip tension. Several numerical models of the Aleutian arc were computed to test the hypothesis that these double seismic zones are due to sagging of the slab under its own weight. This sagging occurs because the asthenosphere (between about 100 and 200 km) provides little support or resistance to the slab, which is supported from below by the more viscous mesosphere and from above by the lithosphere. The viscosity of the mesosphere was constrained to the interval between 0.25 x 10 to the 22nd and 0.5 x 10 to the 22nd P by noting that the slab would have mainly down-dip compression at higher viscosities and mainly down-dip tension at lower viscosities. The deviatoric stress in the slab and the fault plane between the slab and the island arc is about 200-300 bars (expressed as shear stress). The models were calibrated to the observed depth and gravity anomalies in the trench.

Global seismic data reveal little water in the mantle transition zone

NASA Astrophysics Data System (ADS)

Houser, C.

2016-08-01

Knowledge of the Earth's present water content is necessary to constrain the amount of water and other volatiles the Earth acquired during its formation and the amount that is cycled back into the interior from the surface. This study compares 410 and 660 km discontinuity depth with shear wave tomography within the mantle transition zone to identify regions with seismic signals consistent with water. The depth of the 410 and 660 km discontinuities is determined from a large updated dataset of SS-S410S and SS-S660S differential travel times, known as SS precursors. The discontinuity depths measured from binning and stacking the SS precursor data are then compared to the shear velocity model HMSL-S06 in the transition zone. Mapping all the possible combinations, very few locations match the predictions from mineral physics for the effects of water on discontinuity depth and shear velocity. The predictions, although not yet measured at actual transition zone temperatures and pressures, are a shallow 410 km discontinuity, a deep 660 km discontinuity, and a slow shear velocity. Only 8% of the bins with high-quality data are consistent with these predictions, and the calculated average water content within these bins is around 0.6 wt.%. A few isolated locations have patterns of velocity/topography that are consistent with water, while there are large regional-scale patterns consistent with cold/hot temperature anomalies. Combining this global analysis of long period seismic data and the current mineral physics predictions for water in transition zone minerals, I find that the mantle transition zone is generally dry, containing less than one Earth ocean of water. Although subduction zones could be locally hydrated, the combined discontinuity and velocity data show no evidence that wadsleyite or ringwoodite have been globally hydrated by subduction or initial Earth conditions.

Seismicity and structure of Nazca Plate subduction zone in southern Peru

NASA Astrophysics Data System (ADS)

Lim, H.; Kim, Y.; Clayton, R. W.; Thurber, C. H.

2016-12-01

We define subducting plate geometries in the Nazca subduction zone by (re)locating intra-slab earthquakes in southern Peru (2-18°S) and taking previously published converted phase analysis results, to clarify the slab geometry and inferred relationships to the seismicity. We also provide both P- and S-wave velocities of the subducting Nazca Plate and mantle wedge portions close to the slab using double-difference tomography (Zhang and Thurber, 2003) to understand upper plate volcanism and subduction process. A total of 492 regional earthquakes from August 2008 to February 2013 recorded from the dense seismic array (PeruSE, 2013) are selected for the relocation and tomography. The relocated seismicity shows a smooth contortion in the slab-dip transition zone for 400 km between the shallow (25°)-to-flat dipping interface in the north and 40°-dipping interface in the south. We find a significant slab-dip difference (up to 10°) between our results and previously published slab models along the profile region sampling the normal-dip slab at depth (>100 km). Robust features in both P- and S-wave tomography inversions are dipping low-velocity slabs down to 100 km transitioning to higher-velocities at 100-140 km in both flat slab and dipping slab regions. Differences in the velocities of the mantle wedge between the two regions may indicate different hydration states in the wedge.

Seismic Structure of Mantle Transition Zone beneath Northwest Pacific Subduction Zone and its Dynamic Implication

NASA Astrophysics Data System (ADS)

Li, J.; Guo, G.; WANG, X.; Chen, Q.

2017-12-01

The northwest Pacific subduction region is an ideal location to study the interaction between the subducting slab and upper mantle discontinuities. Various and complex geometry of the Pacific subducting slab can be well traced downward from the Kuril, Japan and Izu-Bonin trench using seismicity and tomography images (Fukao and Obayashi, 2013). Due to the sparse distribution of seismic stations in the sea, investigation of the deep mantle structure beneath the broad sea regions is very limited. In this study, we applied the well- developed multiple-ScS reverberations method (Wang et al., 2017) to analyze waveforms recorded by the Chinese Regional Seismic Network, the densely distributed temporary seismic array stations installed in east Asia. A map of the topography of the upper mantle discontinuities beneath the broad oceanic regions in northwest Pacific subduction zone is imaged. We also applied the receiver function analysis to waveforms recorded by stations in northeast China and obtain the detailed topography map beneath east Asia continental regions. We then combine the two kinds of topography of upper mantle discontinuities beneath oceanic and continental regions respectively, which are obtained from totally different methods. A careful image matching and spatial correlation is made in the overlapping study regions to calibrate results with different resolution. This is the first time to show systematically a complete view of the topography of the 410-km and 660-km discontinuities beneath the east Asia "Big mantle wedge" (Zhao and Ohtani, 2009) covering the broad oceanic and continental regions in the Northwestern Pacific Subduction zone. Topography pattern of the 660 and 410 is obtained and discussed. Especially we discovered a broad depression of the 410-km discontinuity covering more than 1000 km in lateral, which seems abnormal in the cold subducting tectonic environment. Based on plate tectonic reconstruction studies and HTHP mineral experiments, we

Kinematics at the Intersection of the Garlock and Death Valley Fault Zones, California: Integration of TM Data and Field Studies

NASA Technical Reports Server (NTRS)

Verosub, Kenneth L.; Brady, Roland H., III; Abrams, Michael

1989-01-01

Kinematic relationships at the intersection of the southern Death Valley and Garlock fault zones were examined to identify and delineate the eastern structural boundary between the Mojave and the Basin and Range geologic terrains, and to construct a model for the evolution of this boundary through time. In order to accomplish this, satellite imagery was combined with field investigations to study six areas in the vicinity of the intersection, or possible extensions, of the fault zones. The information gathered from these areas allows the test of various hypotheses that were proposed to explain the interaction between the Death Valley and Garlock fault zones.

Seismic evidence for flow in the hydrated mantle wedge of the Ryukyu subduction zone

PubMed Central

Nagaya, Takayoshi; Walker, Andrew M.; Wookey, James; Wallis, Simon R.; Ishii, Kazuhiko; Kendall, J. -Michael

2016-01-01

It is widely accepted that water-rich serpentinite domains are commonly present in the mantle above shallow subducting slabs and play key roles in controlling the geochemical cycling and physical properties of subduction zones. Thermal and petrological models show the dominant serpentine mineral is antigorite. However, there is no good consensus on the amount, distribution and alignment of this mineral. Seismic velocities are commonly used to identify antigorite-rich domains, but antigorite is highly-anisotropic and depending on the seismic ray path, its properties can be very difficult to distinguish from non-hydrated olivine-rich mantle. Here, we utilize this anisotropy and show how an analysis of seismic anisotropy that incorporates measured ray path geometries in the Ryukyu arc can constrain the distribution, orientation and amount of antigorite. We find more than 54% of the wedge must consist of antigorite and the alignment must change from vertically aligned to parallel to the slab. This orientation change suggests convective flow in the hydrated forearc mantle. Shear wave splitting analysis in other subduction zones indicates large-scale serpentinization and forearc mantle convection are likely to be more widespread than generally recognized. The view that the forearc mantle of cold subduction zones is dry needs to be reassessed. PMID:27436676

Seismic evidence for flow in the hydrated mantle wedge of the Ryukyu subduction zone.

PubMed

Nagaya, Takayoshi; Walker, Andrew M; Wookey, James; Wallis, Simon R; Ishii, Kazuhiko; Kendall, J-Michael

2016-07-20

It is widely accepted that water-rich serpentinite domains are commonly present in the mantle above shallow subducting slabs and play key roles in controlling the geochemical cycling and physical properties of subduction zones. Thermal and petrological models show the dominant serpentine mineral is antigorite. However, there is no good consensus on the amount, distribution and alignment of this mineral. Seismic velocities are commonly used to identify antigorite-rich domains, but antigorite is highly-anisotropic and depending on the seismic ray path, its properties can be very difficult to distinguish from non-hydrated olivine-rich mantle. Here, we utilize this anisotropy and show how an analysis of seismic anisotropy that incorporates measured ray path geometries in the Ryukyu arc can constrain the distribution, orientation and amount of antigorite. We find more than 54% of the wedge must consist of antigorite and the alignment must change from vertically aligned to parallel to the slab. This orientation change suggests convective flow in the hydrated forearc mantle. Shear wave splitting analysis in other subduction zones indicates large-scale serpentinization and forearc mantle convection are likely to be more widespread than generally recognized. The view that the forearc mantle of cold subduction zones is dry needs to be reassessed.

Mapping seismic azimuthal anisotropy of the Japan subduction zone

NASA Astrophysics Data System (ADS)

Zhao, D.; Liu, X.

2016-12-01

We present 3-D images of azimuthal anisotropy tomography of the crust and upper mantle of the Japan subduction zone, which are determined using a large number of high-quality P- and S-wave arrival-time data of local earthquakes and teleseismic events recorded by the dense seismic networks on the Japan Islands. A tomographic method for P-wave velocity azimuthal anisotropy is modified and extended to invert S-wave travel times for 3-D S-wave velocity azimuthal anisotropy. A joint inversion of the P and S wave data is conducted to constrain the 3-D azimuthal anisotropy of the Japan subduction zone. Main findings of this work are summarized as follows. (1) The high-velocity subducting Pacific and Philippine Sea (PHS) slabs exhibit trench-parallel fast-velocity directions (FVDs), which may reflect frozen-in lattice-preferred orientation of aligned anisotropic minerals formed at the mid-ocean ridge as well as shape-preferred orientation such as normal faults produced at the outer-rise area near the trench axis. (2) Significant trench-normal FVDs are revealed in the mantle wedge, which reflects corner flow in the mantle wedge due to the active subduction and dehydration of the oceanic plates. (3) Obvious toroidal FVDs and low-velocity anomalies exist in and around a window (hole) in the aseismic PHS slab beneath Southwest Japan, which may reflect a toroidal mantle flow pattern resulting from hot and wet mantle upwelling caused by the joint effects of deep dehydration of the Pacific slab and the convective circulation process in the mantle wedge above the Pacific slab. (4) Significant low-velocity anomalies with trench-normal FVDs exist in the mantle below the Pacific slab beneath Northeast Japan, which may reflect a subducting oceanic asthenosphere affected by hot mantle upwelling from the deeper mantle. ReferencesLiu, X., D. Zhao (2016) Seismic velocity azimuthal anisotropy of the Japan subduction zone: Constraints from P and S wave traveltimes. J. Geophys. Res. 121, doi

Seismic anisotropy in localized shear zones versus distributed tectonic fabrics: examples from geologic and seismic observations in western North America and the European Alps

NASA Astrophysics Data System (ADS)

Mahan, Kevin H.; Schulte-Pelkum, Vera; Condit, Cailey; Leydier, Thomas; Goncalves, Philippe; Raju, Anissha; Brownlee, Sarah; Orlandini, Omero F.

2017-04-01

Modern methods for detecting seismic anisotropy offer an array of promising tools for imaging deep crustal deformation but also present challenges, especially with respect to potential biases in both the detection methods themselves as well as in competing processes for localized versus distributed deformation. We address some of these issues from the geophysical perspective by employing azimuthally dependent amplitude and polarity variations in teleseismic receiver functions combined with a compilation of published rock elasticity tensors from middle and deep crustal rocks, and from the geological perspective through studies of shear zone deformation processes. Examples are highlighted at regional and outcrop scales from western North America and the European Alps. First, in regional patterns, strikes of seismically detected fabric from receiver functions in California show a strong alignment with current strike-slip motion between the Pacific and North American plates, with high signal strength near faults and from depths below the brittle-ductile transition suggesting these faults have deep ductile roots. In contrast, despite NE-striking shear zones being the most prominent features portrayed on Proterozoic tectonic maps of the southwestern USA, receiver function anisotropy from the central Rocky Mountain region appears to more prominently reflect broadly distributed Proterozoic fabric domains that preceded late-stage localized shear zones. Possible causes for the discrepancy fall into two categories: those that involve a) bias in seismic sampling and/or b) deformation processes that lead to either weaker anisotropy in the shear zones compared to adjacent domains or to a symmetry that is different from that conventionally assumed. Most of these explanations imply that the seismically sampled domains contain important structural information that is distinct from the shear zones. The second set of examples stem from studies of outcrop-scale shear zones in upper

Development of piggy-back basins in the Sub-Himalaya: structure of the Triyuga Valley in eastern Nepal from seismic reflection profiles

NASA Astrophysics Data System (ADS)

Lee, Y. S.; Almeida, R. V.; Hubbard, J.; Liberty, L. M.; Foster, A. E.; Sapkota, S. N.

2017-12-01

The foreland fold and thrust belt in the Nepal Himalaya has developed over the last 2 My (Mugnier et al., 2004; Van der Beek et al., 2006), and is generally referred to as the Main Frontal Thrust system (MFT; Gansser, 1964). The thrust faults there are spaced 5-30 km apart. Where the faults are furthest from each other, they create piggy-back basins, known as "dun" valleys in the Himalaya. The easternmost of these basins in Nepal is the Triyuga river valley, a 35 km wide basin where the range front abruptly steps 15 km to the south. This dun valley is thought to be the youngest of Nepal, initiating in the Late Pleistocene (Kimura, 1999). In order to understand the sub-surface structure and development of the Triyuga Valley, we analyse high resolution seismic reflection profiles across the three basin-surrounding structures, as well as a north-south profile across the basin proper, in combination with field observations. These datasets reveal that the edges of the forward step are defined by three orthogonal thrust fronts, with abrupt changes in vergence direction. Sharp geometric changes along the range front may have implications for the propagation of earthquakes along the MFT. The surface rupture of the Nepal-Bihar 1934 earthquake was inferred to go around this thrust front (Sapkota et al., 2013), however field observations suggest that although past earthquakes have likely ruptured the surface here, these faults did not slip in 1934. Further, analogue models suggest that the filling of the basin with sediments may affect the activation of out of sequence thrusts (Toscani et al., 2014). This is consistent with field evidence of Quaternary reactivation of the Main Boundary Thrust north of the Triyuga Valley. We also compare these seismic profiles to one across the Jalthal anticline, an incipient structure forming 50 km south of the range front in easternmost Nepal (a section thought to have ruptured in 125; Nakata et al., 1998). We suggest that this may be an

Seismic evidence of Quaternary faulting in the Benton Hills area, southeast Missouri

USGS Publications Warehouse

Palmer, J.R.; Shoemaker, M.; Hoffman, D.; Anderson, N.L.; Vaughn, J.D.; Harrison, R.W.

1997-01-01

Two reflection seismic profiles at English Hill, across the southern edge of the Benton Hills escarpment, southeast Missouri, establish that geologic structures at English Hill are of tectonic origin. The lowland area to the south of the escarpment is relatively undisturbed. The geology at English Hill is structurally complex, and reflection seismic and geologic data indicate extensive and episodic faulting of Paleozoic, Cretaceous, Tertiary, and Quaternary strata. The individual faults have near-vertical fault surfaces with maximum vertical separations on the order of 15 m. They appear to be clustered in north-northeast trending zones that essentially parallel one of the dominant Benton Hills structural trends. These observations suggest that previously mapped Quaternary faults at English Hill are deep-seated and tectonic in origin. This paper documents recent faulting at English Hill and is the first time late Quaternary, surface-rupture faulting has been recognized in the middle Mississippi River Valley region outside of the New Madrid seismic zone. This has important implications for earthquake assessment in the midcontinent.

High-Resolution Seismic-Reflection and Marine Magnetic Data Along the Hosgri Fault Zone, Central California

USGS Publications Warehouse

Sliter, Ray W.; Triezenberg, Peter J.; Hart, Patrick E.; Watt, Janet T.; Johnson, Samuel Y.; Scheirer, Daniel S.

2009-01-01

The U.S. Geological Survey (USGS) collected high-resolution shallow seismic-reflection and marine magnetic data in June 2008 in the offshore areas between the towns of Cayucos and Pismo Beach, Calif., from the nearshore (~6-m depth) to just west of the Hosgri Fault Zone (~200-m depth). These data are in support of the California State Waters Mapping Program and the Cooperative Research and Development Agreement (CRADA) between the Pacific Gas & Electric Co. and the U.S. Geological Survey. Seismic-reflection and marine magnetic data were acquired aboard the R/V Parke Snavely, using a SIG 2Mille minisparker seismic source and a Geometrics G882 cesium-vapor marine magnetometer. More than 550 km of seismic and marine magnetic data was collected simultaneously along shore-perpendicular transects spaced 800 m apart, with an additional 220 km of marine magnetometer data collected across the Hosgri Fault Zone, resulting in spacing locally as smallas 400 m. This report includes maps of the seismic-survey sections, linked to Google Earth software, and digital data files showing images of each transect in SEG-Y, JPEG, and TIFF formats, as well as preliminary gridded marine-magnetic-anomaly and residual-magnetic-anomaly (shallow magnetic source) maps.

Juan de Fuca slab geometry and its relation to Wadati-Benioff zone seismicity

USGS Publications Warehouse

McCrory, Patricia A.; Blair, J. Luke; Waldhause, Felix; Oppenheimer, David H.

2012-01-01

A new model of the subducted Juan de Fuca plate beneath western North America allows first-order correlations between the occurrence of Wadati-Benioff zone earthquakes and slab geometry, temperature, and hydration state. The geo-referenced 3D model, constructed from weighted control points, integrates depth information from earthquake locations and regional seismic velocity studies. We use the model to separate earthquakes that occur in the Cascadia forearc from those that occur within the underlying Juan de Fuca plate and thereby reveal previously obscured details regarding the spatial distribution of earthquakes. Seismicity within the slab is most prevalent where the slab is warped beneath northwestern California and western Washington suggesting that slab flexure, in addition to expected metamorphic dehydration processes, promotes earthquake occurrence within the subducted oceanic plate. Earthquake patterns beneath western Vancouver Island are consistent with slab dehydration processes. Conversely, the lack of slab earthquakes beneath western Oregon is consistent with an anhydrous slab. Double-differenced relocated seismicity resolves a double seismic zone within the slab beneath northwestern California that strongly constrains the location of the plate interface and delineates a cluster of seismicity 10 km above the surface that includes the 1992 M7.1 Mendocino earthquake. We infer that this earthquake ruptured a surface within the Cascadia accretionary margin above the Juan de Fuca plate. We further speculate that this earthquake is associated with a detached fragment of former Farallon plate. Other subsurface tectonic elements within the forearc may have the potential to generate similar damaging earthquakes.

Thematic Mapper and field investigations at the intersection of the Death Valley and Garlock fault zones, California

NASA Technical Reports Server (NTRS)

Brady, Roland H., III; Cregan, Alan; Clayton, Jeff; Troxel, Bennie W.; Verosub, Kenneth L.; Abrams, Michael

1989-01-01

Analysis of processed images and detailed field investigations have provided significant information concerning the late-Pliocene and Quaternary evolution of the intersection of the Garlock and Death Valley fault zones. The imagery was used to determine patterns of sedimentation and age relationships on alluvial fans and to determine the geometry, styles of deformation, and relative ages of movements on major and minor faults in the study area. The field investigation often confirmed the inferences drawn from the images and provided additional tectonic and geomorphologic data about the Quaternary deformation of the region. All the data gathered in the course of this project support the contention that the Garlock fault zone terminates in the Avawatz Mountains and that the Death Valley fault zone continues south of the intersection for at least 50 km, forming the eastern boundary of the Mojave province.

Seismic Structure of the Antarctic Upper Mantle and Transition Zone Unearthed by Full Waveform Adjoint Tomography

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

The upper mantle and transition zone beneath Antarctica and the surrounding ocean are among the poorest seismically imaged regions of the Earth's interior. Over the last 1.5 decades researchers have deployed several large temporary broadband seismic arrays focusing on major tectonic features in the Antarctic. The broader international community has also facilitated further instrumentation of the continent, often operating stations in additional regions. As of 2016, waveforms are available from almost 300 unique station locations. Using these stations along with 26 southern mid-latitude seismic stations we have imaged the seismic structure of the upper mantle and transition zone using full waveform adjoint techniques. The full waveform adjoint inversion assimilates phase observations from 3-component seismograms containing P, S, Rayleigh, and Love waves, including reflections and overtones, from 270 earthquakes (5.5 ≤ Mw ≤ 7.0) that occurred between 2001-2003 and 2007-2016. We present the major results of the full waveform adjoint inversion following 20 iterations, resulting in a continental-scale seismic model (ANT_20) with regional-scale resolution. Within East Antarctica, ANT_20 reveals internal seismic heterogeneity and differences in lithospheric thickness. For example, fast seismic velocities extending to 200-300 km depth are imaged beneath both Wilkes Land and the Gamburtsev Subglacial Mountains, whereas fast velocities only extend to 100-200 km depth beneath the Lambert Graben and Enderby Land. Furthermore, fast velocities are not found beneath portions of Dronning Maud Land, suggesting old cratonic lithosphere may be absent. Beneath West Antarctica slow upper mantle seismic velocities are imaged extending from the Balleny Island southward along the Transantarctic Mountains front, and broaden beneath the southern and northern portion of the mountain range. In addition, slow upper mantle velocities are imaged beneath the West Antarctic coast extending

Shallow seismicity patterns in the northwestern section of the Mexico Subduction Zone

NASA Astrophysics Data System (ADS)

Abbott, Elizabeth R.; Brudzinski, Michael R.

2015-11-01

This study characterizes subduction related seismicity with local deployments along the northwestern section of the Mexico Subduction Zone where 4 portions of the plate interface have ruptured in 1973, 1985, 1995, and 2003. It has been proposed that the subducted boundary between the Cocos and Rivera plates occurs beneath this region, as indicated by inland volcanic activity, a gap in tectonic tremor, and the Manzanillo Trough and Colima Graben, which are depressions thought to be associated with the splitting of the two plates after subduction. Data from 50 broadband stations that comprised the MARS seismic array, deployed from January 2006 to June 2007, were processed with the software program Antelope and its generalized source location algorithm, genloc, to detect and locate earthquakes within the network. Slab surface depth contours from the resulting catalog indicate a change in subduction trajectory between the Rivera and Cocos plates. The earthquake locations are spatially anti-correlated with tectonic tremor, supporting the idea that they represent different types of fault slip. Hypocentral patterns also reveal areas of more intense seismic activity (clusters) that appear to be associated with the 2003 and 1973 megathrust rupture regions. Seismicity concentrated inland of the 2003 rupture is consistent with slip on a shallowly dipping trajectory for the Rivera plate interface as opposed to crustal faulting in the overriding North American plate. A prominent cluster of seismicity within the suspected 1973 rupture zone appears to be a commonly active portion of the megathrust as it has been active during three previous deployments. We support these interpretations by determining focal mechanisms and detailed relocations of the largest events within the 1973 and inland 2003 clusters, which indicate primarily thrust mechanisms near the plate interface.

Monitoring deep geodynamic processes within Vrancea intermediate-depth seismic zone by geodetic means

NASA Astrophysics Data System (ADS)

Besutiu, Lucian; Zlagnean, Luminita

2015-04-01

Background Located in the bending zone of East Carpathians, the so-called Vrancea zone is one of the most active seismic regions in Europe. Despite many years of international research, its intermediate-depth seismicity within full intra-continental environment still represents a challenge of the 21st century. Infrastructure In the attempt to join the above-mentioned efforts, the Solid Earth Dynamics Department (SEDD) in the Institute of Geodynamics of the Romanian Academy has developed a special research infrastructure, mainly devoted to gravity and space geodesy observations. A geodetic network covering the epicentre area of the intermediate-depth earthquakes has been designed and implemented for monitoring deep geodynamic processes and their surface echoes. Within each base-station of the above-mentioned network, a still-reinforced concrete pillar allows for high accuracy repeated gravity and GPS determinations. Results Starting from some results of the previously run CERGOP and UNIGRACE European programmes, to which additional SEDD repeated field campaigns were added, an unusual geodynamic behaviour has been revealed in the area. 1) Crust deformation: unlike the overall uprising of East Carpathians, as a result of denudation followed by erosion, their SE bending zone, with Vrancea epicentre area exhibits a slight subsidence. 2) Gravity change: more than 200 microgals non-tidal gravity decrease over a 20 years time-span has been noticed within the subsiding area. Extended observations showed the gravity lowering as a nowadays continuing process. Interpretation This strange combination of topography subsidence and gravity lowering has been interpreted in terms of crust stretching in the Vrancea epicentre zone due to the gravity pull created by densification of the lower crust as a result of phase-transform processes taking place in the lithospheric compartment sunken into the upper mantle. The occurrence of crust earthquakes with vertical-extension focal

Imaging the Alpine Fault to depths of more than 2 km - Initial results from the 2011 WhataDUSIE seismic reflection profile, Whataroa Valley, New Zealand

NASA Astrophysics Data System (ADS)

Kovacs, A.; Gorman, A. R.; Buske, S.; Schmitt, D. R.; Eccles, J. D.; Toy, V. G.; Sutherland, R.; Townend, J.; Norris, R.; Pooley, B.; Cooper, J.; Bruce, C.; Smillie, M.; Bain, S.; Hellwig, O.; Hlousek, F.; Hellmich, J.; Riedel, M.; Schijns, H. M.

2011-12-01

The Alpine Fault is a major plate-bounding fault that is thought to fail in large earthquakes (Mw~7.9) every 200-400 years and to have last ruptured in AD 1717. It is the principal geological structure accommodating transpressional motion between the Australian and Pacific plates on the South Island, with a long-term horizontal motion over the last 1-2 million years of 21-27 mm/yr. Determining the Alpine Fault zone structure at depths of several kilometres beneath the Earth's surface is crucial for understanding not only what conditions govern earthquake rupture but also how ongoing faulting produces mountain ranges such as the Southern Alps. The valley of the Whataroa River, in the central sector of the Alpine Fault, provides rare access to the SE (hanging wall) side of the fault for the purpose of a seismic survey. During January and February 2011, a ~5-km-long seismic reflection line was collected that aimed to image the Alpine Fault at depth. The acquisition was undertaken with the use of 21 Geode seismographs and two Seistronix seismographs with a total capacity of 552 channels. Geophone spacing varied from 4 m in the north (close to the surface trace of the fault) to 8 m in the south (farther from the surface trace.) Sources were 400-g Pentex charges buried in 1.5-2.0 m deep holes of which ~100 were dug by an excavator and ~100 were dug by hand tools where heavy equipment could not access shot locations. Single shots had a nominal separation of 25 m at the north end of the line. At the south end of the line, shots were deployed in patterns of five with a nominal spacing of 125 m. Acquisition system requirements and surface morphology (meanders in the Whataroa River) required five separate acquisition systems. Timing of shots for these systems was accomplished with a radio-controlled firing system, GPS clocks linked to co-located Reftek seismographs, and overlapping traces between acquisition systems. Shot records have been merged and processed through to

Triggered deformation and seismic activity under Mammoth Mountain in long Valley caldera by the 3 November 2002 Mw 7.9 Denali fault earthquake

USGS Publications Warehouse

Johnston, M.J.S.; Prejean, S.G.; Hill, D.P.

2004-01-01

The 3 November 2002 Mw 7.9 Denali fault earthquake triggered deformational offsets and microseismicity under Mammoth Mountain (MM) on the rim of Long Valley caldera, California, some 3460 km from the earthquake. Such strain offsets and microseismicity were not recorded at other borehole strain sites along the San Andreas fault system in California. The Long Valley offsets were recorded on borehole strainmeters at three sites around the western part of the caldera that includes Mammoth Mountain - a young volcano on the southwestern rim of the caldera. The largest recorded strain offsets were -0.1 microstrain at PO on the west side of MM, 0.05 microstrain at MX to the southeast of MM, and -0.025 microstrain at BS to the northeast of MM with negative strain extensional. High sample rate strain data show initial triggering of the offsets began at 22:30 UTC during the arrival of the first Rayleigh waves from the Alaskan earthquake with peak-to-peak dynamic strain amplitudes of about 2 microstrain corresponding to a stress amplitude of about 0.06 MPa. The strain offsets grew to their final values in the next 10 min. The associated triggered seismicity occurred beneath the south flank of MM and also began at 22:30 UTC and died away over the next 15 min. This relatively weak seismicity burst included some 60 small events with magnitude all less than M = 1. While poorly constrained, these strain observations are consistent with triggered slip and intrusive opening on a north-striking normal fault centered at a depth of 8 km with a moment of l016 N m, or the equivalent of a M 4.3 earthquake. The cumulative seismic moment for the associated seismicity burst was more than three orders of magnitude smaller. These observations and this model resemble those for the triggered deformation and slip that occurred beneath the north side of MM following the 16 October 1999 M 7.1 Hector Mine, California, earthquake. However, in this case, we see little post-event slip decay reflected in

Evidences of a lithospheric fault zone in the Sicily Channel continental rift (southern Italy) from instrumental seismicity data

NASA Astrophysics Data System (ADS)

Calò, M.; Parisi, L.

2014-10-01

Sicily Channel is a portion of Mediterranean Sea, between Sicily (Southern Italy) and Tunisia, representing a part of the foreland Apennine-Maghrebian thrust belt. The seismicity of the region is commonly associated with the normal faulting related to the rifting process and volcanic activity of the region. However, certain seismic patterns suggest the existence of some mechanism coexisting with the rifting process. In this work, we present the results of a statistical analysis of the instrumental seismicity and a reliable relocalization of the events recorded in the last 30 yr in the Sicily Channel and western Sicily using the Double Difference method and 3-D Vp and Vs tomographic models. Our procedure allows us to discern the seismic regime of the Sicily sea from the Tyrrhenian one and to describe the main features of an active fault zone in the study area that could not be related to the rifting process. We report that most of the events are highly clustered in the region between 12.5°-13.5°E and 35.5°-37°N with hypocentral depth of 5-40 km, and reaching 70 km depth in the southernmost sector. The alignment of the seismic clusters, the distribution of volcanic and geothermal regions and the location of some large events occurred in the last century suggest the existence of a subvertical shear zone extending for least 250 km and oriented approximately NNE-SSW. The spatial distribution of the seismic moment suggests that this transfer fault zone is seismically discontinuous showing large seismic gaps in proximity of the Ferdinandea Island, and Graham and Nameless Bank.

Methods and systems for low frequency seismic and infrasound detection of geo-pressure transition zones

DOEpatents

Shook, G. Michael; LeRoy, Samuel D.; Benzing, William M.

2006-07-18

Methods for determining the existence and characteristics of a gradational pressurized zone within a subterranean formation are disclosed. One embodiment involves employing an attenuation relationship between a seismic response signal and increasing wavelet wavelength, which relationship may be used to detect a gradational pressurized zone and/or determine characteristics thereof. In another embodiment, a method for analyzing data contained within a response signal for signal characteristics that may change in relation to the distance between an input signal source and the gradational pressurized zone is disclosed. In a further embodiment, the relationship between response signal wavelet frequency and comparative amplitude may be used to estimate an optimal wavelet wavelength or range of wavelengths used for data processing or input signal selection. Systems for seismic exploration and data analysis for practicing the above-mentioned method embodiments are also disclosed.

Kinematics at the intersection of the Garlock and Death Valley fault zones, California: Integration of TM data and field studies

NASA Technical Reports Server (NTRS)

Abrams, Michael; Verosub, Ken; Finnerty, Tony; Brady, Roland

1987-01-01

The Garlock and Death Valley fault zones in SE California are two active strike-slip faults coming together on the east side of the Avawatz Mtns. The kinematics of this intersection, and the possible continuation of either fault zone, are being investigated using a combination of field mapping, and processing and interpretation of remotely sensed image data. Regional and local relationships are derivable from Thematic Mapper data (30 m resolution), including discrimination and relative age dating of alluvial fans, bedrock mapping, and fault mapping. Aircraft data provide higher spatial resolution over more limited areas. Hypotheses being considered are: (1) the Garlock fault extends east of the intersection; (2) the Garlock fault terminates at the intersection and the Death Valley fault continues southeastward; and (3) the Garlock fault has been offset right laterally by the Death Valley fault which continues to the southeast. Preliminary work indicates that the first hypothesis is invalid. From kinematic considerations, image analysis, and field work the third hypothesis is favored. The projected continuation of the Death Valley zone defines the boundary between the Mojave crustal block and the Basin and Range block.

Shallow seismic structure of Kunlun fault zone in northern Tibetan Plateau, China: Implications for the 2001 M s8.1 Kunlun earthquake

USGS Publications Warehouse

Wang, Chun-Yong; Mooney, W.D.; Ding, Z.; Yang, J.; Yao, Z.; Lou, H.

2009-01-01

The shallow seismic velocity structure of the Kunlun fault zone (KLFZ) was jointly deduced from seismic refraction profiling and the records of trapped waves that were excited by five explosions. The data were collected after the 2001 Kunlun M s8.1 earthquake in the northern Tibetan Plateau. Seismic phases for the in-line record sections (26 records up to a distance of 15 km) along the fault zone were analysed, and 1-D P- and S-wave velocity models of shallow crust within the fault zone were determined by using the seismic refraction method. Sixteen seismic stations were deployed along the off-line profile perpendicular to the fault zone. Fault-zone trapped waves appear clearly on the record sections, which were simulated with a 3-D finite difference algorithm. Quantitative analysis of the correlation coefficients of the synthetic and observed trapped waveforms indicates that the Kunlun fault-zone width is 300 m, and S-wave quality factor Q within the fault zone is 15. Significantly, S-wave velocities within the fault zone are reduced by 30-45 per cent from surrounding rocks to a depth of at least 1-2 km, while P-wave velocities are reduced by 7-20 per cent. A fault-zone with such P- and S-low velocities is an indication of high fluid pressure because Vs is affected more than Vp. The low-velocity and low-Q zone in the KLFZ model is the effect of multiple ruptures along the fault trace of the 2001 M s8.1 Kunlun earthquake. ?? 2009 The Authors Journal compilation ?? 2009 RAS.

Imaging Critical Zone Using High Frequency Rayleigh Wave Group Velocity Measurements Extracted from Ambient Seismic Fields Gathered With 2400 Seismic Nodes in Southeastern Wyoming.

NASA Astrophysics Data System (ADS)

Keifer, I. S.; Dueker, K. G.

2016-12-01

In an effort to characterize critical zone development in varying regions, seismologist conduct seismic surveys to assist in the realization of critical zone properties e.g. porosity and regolith thickness. A limitation of traditional critical zone seismology is that data is normally collected along lines, to generate two dimensional transects of the subsurface seismic velocity, even though the critical zone structure is 3D. Hence, we deployed six seismic 2D arrays in southeastern Wyoming to gather ambient seismic fields so that 3D shear velocity models could be produced. The arrays were made up of nominally 400 seismic stations arranged in a 200-meter square grid layout. Each array produced a half Terabyte data volume, so a premium was placed on computational efficiency throughout this study, to handle the roughly 65 billion samples recorded by each array. The ambient fields were cross-correlated on the Yellowstone Super-Computer using the pSIN code (Chen et al., 2016), which decreased correlation run times by a factor of 300 with respect to workstation computers. Group delay times extracted from cross-correlations using 8 Hz frequency bands from 10 Hz to 100 Hz show frequency dispersion at sites with shallow regolith underlain by granite bedrock. Dimensionally, the group velocity map inversion is overdetermined, even after extensive culling of spurious group delay times. Model Resolution matrices for our six arrays show values > 0.7 for most of the modal domain, approaching unity at the center of the model domain; we are then confident that we have an adequate number of rays covering our array space, and should experience minimal smearing of our resultant model due to application of inverse solution on the data. After inverting for the group velocity maps, a second inversion is performed of the group velocity maps for the 3D shear velocity model. This inversion is underdetermined and a second order Tikhonov regularization is used to obtain stable inverse images

Three-dimensional structure and seismicity beneath the Central Vanuatu subduction zone

NASA Astrophysics Data System (ADS)

Foix, O.; Crawford, W. C.; Koulakov, I.; Regnier, M. M.; Pelletier, B.; Garaebiti, E.

2017-12-01

The 1 400 km long Vanuatu subduction zone marks the subduction of the oceanic Australia plate beneath the North-Fijian microplate. Seismic and volcanic activity is high, and several morphologic features enter into subduction, affecting seismicity and probably plate coupling. The Northern d'Entrecasteaux Ridge, West-Torres plateau, and Bougainville seamount currently enter into subduction below the forearc islands of Santo and Malekula. This subduction/collision coincides with a strongly decreased local convergence velocity rate at the trench (35 mm/yr compared to 120-160 mm/yr to the north and south) and significant uplift on the overriding plate. Two large forearc islands located 20-30 km from the subduction front Santo and Malekula to the trench allow excellent coverage of the megathrust seismogenic zone for a seismological study. We use data from the 10 months, 30-station amphibious ARC-VANUATU seismology network to construct a 3D velocity model and locate 11 617 earthquakes. The 3D model reveals low P and S velocities in the uppermost tens of kilometers in front of the Northern d'Entrecasteaux Ridge and the Bougainville Guyot. These anomalies may be due to heavy faulting of related subducted features, possibly including important water infiltration. We also identify a possible seamount entered into subduction beneath a smaller uplifted island between the two main islands. The spatial distribution of earthquakes is highly variable, as is the depth limit of the seismogenic zone, suggests a complex interaction of faults and stress zones related to high and highly variable stress that may be associated with the subducted features.

The hydraulic structure of the Gole Larghe Fault Zone (Italian Southern Alps) through the seismic cycle

NASA Astrophysics Data System (ADS)

Bistacchi, A.; Mittempergher, S.; Di Toro, G.; Smith, S. A. F.; Garofalo, P. S.

2017-12-01

The 600 m-thick, strike slip Gole Larghe Fault Zone (GLFZ) experienced several hundred seismic slip events at c. 8 km depth, well-documented by numerous pseudotachylytes, was then exhumed and is now exposed in beautiful and very continuous outcrops. The fault zone was also characterized by hydrous fluid flow during the seismic cycle, demonstrated by alteration halos and precipitation of hydrothermal minerals in veins and cataclasites. We have characterized the GLFZ with > 2 km of scanlines and semi-automatic mapping of faults and fractures on several photogrammetric 3D Digital Outcrop Models (3D DOMs). This allowed us obtaining 3D Discrete Fracture Network (DFN) models, based on robust probability density functions for parameters of fault and fracture sets, and simulating the fault zone hydraulic properties. In addition, the correlation between evidences of fluid flow and the fault/fracture network parameters have been studied with a geostatistical approach, allowing generating more realistic time-varying permeability models of the fault zone. Based on this dataset, we have developed a FEM hydraulic model of the GLFZ for a period of some tens of years, covering one seismic event and a postseismic period. The higher permeability is attained in the syn- to early post-seismic period, when fractures are (re)opened by off-fault deformation, then permeability decreases in the postseismic due to fracture sealing. The flow model yields a flow pattern consistent with the observed alteration/mineralization pattern and a marked channelling of fluid flow in the inner part of the fault zone, due to permeability anisotropy related to the spatial arrangement of different fracture sets. Amongst possible seismological applications of our study, we will discuss the possibility to evaluate the coseismic fracture intensity due to off-fault damage, and the heterogeneity and evolution of mechanical parameters due to fluid-rock interaction.

Seismic attenuation structure beneath Nazca Plate subduction zone in southern Peru

NASA Astrophysics Data System (ADS)

Jang, H.; Kim, Y.; Clayton, R. W.

2017-12-01

We estimate seismic attenuation in terms of quality factors, QP and QS using P and S phases, respectively, beneath Nazca Plate subduction zone between 10°S and 18.5°S latitude in southern Peru. We first relocate 298 earthquakes with magnitude ranges of 4.0-6.5 and depth ranges of 20-280 km. We measure t*, which is an integrated attenuation through the seismic raypath between the regional earthquakes and stations. The measured t* are inverted to construct three-dimensional attenuation structures of southern Peru. Checkerboard test results for both QP and QS structures ensure good resolution in the slab-dip transition zone between flat and normal slab subduction down to a depth of 200 km. Both QP and QS results show higher attenuation continued down to a depth of 50 km beneath volcanic arc and also beneath the Quimsachata volcano, the northernmost young volcano, located far east of the main volcanic front. We also observe high attenuation in mantle wedge especially beneath the normal subduction region in both QP and QS (100-130 in QP and 100-125 in QS) and slightly higher QP and QS beneath the flat-subduction and slab-dip transition regions. We plan to relate measured attenuation in the mantle wedge to material properties such as viscosity to understand the subduction zone dynamics.

Directly imaging steeply-dipping fault zones in geothermal fields with multicomponent seismic data

DOE PAGES

Chen, Ting; Huang, Lianjie

2015-07-30

For characterizing geothermal systems, it is important to have clear images of steeply-dipping fault zones because they may confine the boundaries of geothermal reservoirs and influence hydrothermal flow. Elastic reverse-time migration (ERTM) is the most promising tool for subsurface imaging with multicomponent seismic data. However, conventional ERTM usually generates significant artifacts caused by the cross correlation of undesired wavefields and the polarity reversal of shear waves. In addition, it is difficult for conventional ERTM to directly image steeply-dipping fault zones. We develop a new ERTM imaging method in this paper to reduce these artifacts and directly image steeply-dipping fault zones.more » In our new ERTM method, forward-propagated source wavefields and backward-propagated receiver wavefields are decomposed into compressional (P) and shear (S) components. Furthermore, each component of these wavefields is separated into left- and right-going, or downgoing and upgoing waves. The cross correlation imaging condition is applied to the separated wavefields along opposite propagation directions. For converted waves (P-to-S or S-to-P), the polarity correction is applied to the separated wavefields based on the analysis of Poynting vectors. Numerical imaging examples of synthetic seismic data demonstrate that our new ERTM method produces high-resolution images of steeply-dipping fault zones.« less

Fault zone property near Xinfengjiang Reservoir using dense, across-fault seismic array

NASA Astrophysics Data System (ADS)

Lee, M. H. B.; Yang, H.; Sun, X.

2017-12-01

Properties of fault zones are important to the understanding of earthquake process. Around the fault zone is a damaged zone which is characterised by a lower seismic velocity. This is detectable as a low velocity zone and measure some physical property of the fault zone, which is otherwise difficult sample directly. A dense, across-fault array of short period seismometer is deployed on an inactive fault near Xinfengjiang Reservoir. Local events were manually picked. By computing the synthetic arrival time, we were able to constrain the parameters of the fault zone Preliminary result shows that the fault zone is around 350 m wide with a P and S velocity increase of around 10%. The fault is geologically inferred, and this result suggested that it may be a geological layer. The other possibility is that the higher velocity is caused by a combination of fault zone healing and fluid intrusion. Whilst the result was not able to tell us the nature of the fault, it demonstrated that this method is able to derive properties from a fault zone.

Towards a Detailed Seismic Structure of the Valley of Mexico's Xochimilco Lake Zone.

NASA Astrophysics Data System (ADS)

Rabade, S.; Sanchez-Sanchez, J.; Ayala Hernandez, M.; Macias, M. A.; Aguilar Calderon, L. A.; Alcántara, L.; Almora Mata, D.; Castro Parra, G.; Delgado, R.; Leonardo Suárez, M.; Molina Avila, I.; Mora, A.; Perez-Yanez, C.; Ruiz, A. L.; Sandoval, H.; Torres Noguez, M.; Vazquez Larquet, R.; Velasco Miranda, J. M.; Aguirre, J.; Ramirez-Guzmán, L.

2017-12-01

Six centuries of gradual, intentional sediment filling in the Xochimilco Lake Zone have drastically reduced the size of the lake. The basin structure and the lake's clay limits and thickness are poorly constrained, and yet, essential to explain the city's anomalous ground motion. Therefore, we conducted an experiment to define the 3D velocity model of Mexico's capital; the CDMX-E3D. The initial phase involved the deployment of a moving set of 18-broadband stations with an interstation distance of 500m over a period of 19 weeks. We collected the data and analyzed the results for the Xochimilco Lake Zone using H/V Spectral Ratios (Nakamura, 1989), which provided an improved fundamental period map of the region. Results show that periods in the former lake zone have larger variability than values previously estimated. In order to obtain group velocity maps at different periods, we estimated Green's functions from ambient noise cross-correlations following standard methodologies to invert Rayleigh wave travel times (Bensen et al., 2007). Preliminary result show very low-velocity zones (100 m/s) and thick sediment layers in most of the former Xochimilco Lake area. This Project was funded by the Secretaria de Ciencia, Tecnología e Innovación (SECITI) of Mexico City. Project SECITI/073/2016.

Seismic imaging along a 600 km transect of the Alaska Subduction zone (Invited)

NASA Astrophysics Data System (ADS)

Calkins, J. A.; Abers, G. A.; Freymueller, J. T.; Rondenay, S.; Christensen, D. H.

2010-12-01

We present earthquake locations, scattered wavefield migration images, and phase velocity maps from preliminary analysis of combined seismic data from the Broadband Experiment Across the Alaska Range (BEAAR) and Multidisciplinary Observations of Onshore Subduction (MOOS) projects. Together, these PASSCAL broadband arrays sampled a 500+ km transect across a portion of the subduction zone characterized by the Yakutat terrane/Pacific plate boundary in the downgoing plate, and the Denali volcanic gap in the overriding plate. These are the first results from the MOOS experiment, a 34-station array that was deployed from 2006-2008 to fill in the gap between the TACT offshore refraction profile (south and east of the coastline of the Kenai Peninsula), and the BEAAR array (spanning the Alaska Range between Talkeetna and Fairbanks). 2-D images of the upper 150 km of the subduction zone were produced by migrating forward- and back-scattered arrivals in the coda of P waves from large teleseismic earthquakes, highlighting S-velocity perturbations from a smoothly-varying background model. The migration images reveal a shallowly north-dipping low velocity zone that is contiguous near 20 km depth on its updip end with previously obtained images of the subducting plate offshore. The low velocity zone steepens further to the north, and terminates near 120 km beneath the Alaska Range. We interpret this low velocity zone to be the crust of the downgoing plate, and the reduced seismic velocities to be indicative of hydrated gabbroic compositions. Earthquakes located using the temporary arrays and nearby stations of the Alaska Regional Seismic Network correlate spatially with the inferred subducting crust. Cross-sections taken along nearly orthogonal strike lines through the MOOS array reveal that both the dip angle and the thickness of the subducting low velocity zone change abruptly across a roughly NNW-SSE striking line drawn through the eastern Kenai Peninsula, coincident with a

Location, structure, and seismicity of the Seattle fault zone, Washington: Evidence from aeromagnetic anomalies, geologic mapping, and seismic-reflection data

USGS Publications Warehouse

Blakely, R.J.; Wells, R.E.; Weaver, C.S.; Johnson, S.Y.

2002-01-01

A high-resolution aeromagnetic survey of the Puget Lowland shows details of the Seattle fault zone, an active but largely concealed east-trending zone of reverse faulting at the southern margin of the Seattle basin. Three elongate, east-trending magnetic anomalies are associated with north-dipping Tertiary strata exposed in the hanging wall; the magnetic anomalies indicate where these strata continue beneath glacial deposits. The northernmost anomaly, a narrow, elongate magnetic high, precisely correlates with magnetic Miocene volcanic conglomerate. The middle anomaly, a broad magnetic low, correlates with thick, nonmagnetic Eocene and Oligocene marine and fluvial strata. The southern anomaly, a broad, complex magnetic high, correlates with Eocene volcanic and sedimentary rocks. This tripartite package of anomalies is especially clear over Bainbridge Island west of Seattle and over the region east of Lake Washington. Although attenuated in the intervening region, the pattern can be correlated with the mapped strike of beds following a northwest-striking anticline beneath Seattle. The aeromagnetic and geologic data define three main strands of the Seattle fault zone identified in marine seismic-reflection profiles to be subparallel to mapped bedrock trends over a distance of >50 km. The locus of faulting coincides with a diffuse zone of shallow crustal seismicity and the region of uplift produced by the M 7 Seattle earthquake of A.D. 900-930.

Variable post-Paleozoic deformation detected by seismic reflection profiling across the northwestern "prong" of New Madrid seismic zone

USGS Publications Warehouse

McBride, J.H.; Pugin, Andre J.M.; Nelson, W.J.; Larson, T.H.; Sargent, S.L.; Devera, J.A.; Denny, F.B.; Woolery, E.W.

2003-01-01

High-resolution shallow seismic reflection profiles across the northwesternmost part of the New Madrid seismic zone (NMSZ) and northwestern margin of the Reelfoot rift, near the confluence of the Ohio and Mississippi Rivers in the northern Mississippi embayment, reveal intense structural deformation that apparently took place during the late Paleozoic and/or Mesozoic up to near the end of the Cretaceous Period. The seismic profiles were sited on both sides of the northeast-trending Olmsted fault, defined by varying elevations of the top of Mississippian (locally base of Cretaceous) bedrock. The trend of this fault is close to and parallel with an unusually straight segment of the Ohio River and is approximately on trend with the westernmost of two groups of northeast-aligned epicenters ("prongs") in the NMSZ. Initially suspected on the basis of pre-existing borehole data, the deformation along the fault has been confirmed by four seismic reflection profiles, combined with some new information from drilling. The new data reveal (1) many high-angle normal and reverse faults expressed as narrow grabens and anticlines (suggesting both extensional and compressional regimes) that involved the largest displacements during the late Cretaceous (McNairy); (2) a different style of deformation involving probably more horizontal displacements (i.e., thrusting) that occurred at the end of this phase near the end of McNairy deposition, with some fault offsets of Paleocene and younger units; (3) zones of steeply dipping faults that bound chaotic blocks similar to that observed previously from the nearby Commerce geophysical lineament (CGL); and (4) complex internal deformation stratigraphically restricted to the McNairy, suggestive of major sediment liquefaction or landsliding. Our results thus confirm the prevalence of complex Cretaceous deformations continuing up into Tertiary strata near the northern terminus of the NMSZ. ?? 2003 Elsevier Science B.V. All rights reserved.

Subduction zone locking, strain partitioning, intraplate deformation and their implications to Seismic Hazards in South America

NASA Astrophysics Data System (ADS)

Galgana, G. A.; Mahdyiar, M.; Shen-Tu, B.; Pontbriand, C. W.; Klein, E.; Wang, F.; Shabestari, K.; Yang, W.

2014-12-01

We analyze active crustal deformation in South America (SA) using published GPS observations and historic seismicity along the Nazca Trench and the active Ecuador-Colombia-Venezuela Plate boundary Zone. GPS-constrained kinematisc models that incorporate block and continuum techniques are used to assess patterns of regional tectonic deformation and its implications to seismic potential. We determine interplate coupling distributions, fault slip-rates, and intraplate crustal strain rates in combination with historic earthquakes within 40 seismic zones crust to provide moment rate constraints. Along the Nazca subduction zone, we resolve a series of highly coupled patches, interpreted as high-friction producing "asperities" beneath the coasts of Ecuador, Peru and Chile. These include areas responsible for the 2010 Mw 8.8 Maule Earthquake and the 2014 Mw 8.2 Iquique Earthquake. Predicted tectonic block motions and fault slip rates reveal that the northern part of South America deforms rapidly, with crustal fault slip rates as much as ~20 mm/a. Fault slip and locking patterns reveal that the Oca Ancón-Pilar-Boconó fault system plays a key role in absorbing most of the complex eastward and southward convergence patterns in northeastern Colombia and Venezuela, while the near-parallel system of faults in eastern Colombia and Ecuador absorb part of the transpressional motion due to the ~55 mm/a Nazca-SA plate convergence. These kinematic models, in combination with historic seismicity rates, provide moment deficit rates that reveal regions with high seismic potential, such as coastal Ecuador, Bucaramanga, Arica and Antofagasta. We eventually use the combined information from moment rates and fault coupling patterns to further constrain stochastic seismic hazard models of the region by implementing realistic trench rupture scenarios (see Mahdyiar et al., this volume).

Water, oceanic fracture zones and the lubrication of subducting plate boundaries—insights from seismicity

NASA Astrophysics Data System (ADS)

Schlaphorst, David; Kendall, J.-Michael; Collier, Jenny S.; Verdon, James P.; Blundy, Jon; Baptie, Brian; Latchman, Joan L.; Massin, Frederic; Bouin, Marie-Paule

2016-03-01

We investigate the relationship between subduction processes and related seismicity for the Lesser Antilles Arc using the Gutenberg-Richter law. This power law describes the earthquake-magnitude distribution, with the gradient of the cumulative magnitude distribution being commonly known as the b-value. The Lesser Antilles Arc was chosen because of its along-strike variability in sediment subduction and the transition from subduction to strike-slip movement towards its northern and southern ends. The data are derived from the seismicity catalogues from the Seismic Research Centre of The University of the West Indies and the Observatoires Volcanologiques et Sismologiques of the Institut de Physique du Globe de Paris and consist of subcrustal events primarily from the slab interface. The b-value is found using a Kolmogorov-Smirnov test for a maximum-likelihood straight line-fitting routine. We investigate spatial variations in b-values using a grid-search with circular cells as well as an along-arc projection. Tests with different algorithms and the two independent earthquake cataloges provide confidence in the robustness of our results. We observe a strong spatial variability of the b-value that cannot be explained by the uncertainties. Rather than obtaining a simple north-south b-value distribution suggestive of the dominant control on earthquake triggering being water released from the sedimentary cover on the incoming American Plates, or a b-value distribution that correlates with on the obliquity of subduction, we obtain a series of discrete, high b-value `bull's-eyes' along strike. These bull's-eyes, which indicate stress release through a higher fraction of small earthquakes, coincide with the locations of known incoming oceanic fracture zones on the American Plates. We interpret the results in terms of water being delivered to the Lesser Antilles subduction zone in the vicinity of fracture zones providing lubrication and thus changing the character of the

Spatio-temporal changes of seismic anisotropy in seismogenic zones

NASA Astrophysics Data System (ADS)

Saade, M.; Montagner, J.; Roux, P.; Paul, C.; Brenguier, F.; Enescu, B.; Shiomi, K.

2013-12-01

Seismic anisotropy plays a key role in the study of stress and strain fields in the earth. Potential temporal change of seismic anisotropy can be interpreted as change of the orientation of cracks in seismogenic zones and thus change of the stress field. Such temporal changes have been observed in seismogenic zones before and after earthquakes (Durand et al. , 2011) but are still not well understood. In this study, from a numerical point of view, we investigate the variations of the polarization of surface waves in anisotropic media. These variations are related to the elastic properties of the medium, in particular to anisotropy. The technique used is based on the calculation of the whole cross-correlation tensor (CCT) of ambient seismic noise. If the sources are randomly distributed in homogeneous medium, it allows us to reconstruct the Green's tensor between two stations continuously and to monitor the region through the use of its fluctuations. Therefore, the temporal change of the Green's cross-correlation tensor enables the monitoring of stress and strain fields. This technique is applied to synthetic seismograms computed in a transversally isotropic medium with horizontal symmetry axis (hereafter referred to an HTI medium) using a code RegSEM (Cupillard et al. , 2012) based on the spectral element method. We designed an experiment in order to investigate the influence of anisotropy on the CCT. In homogeneous, isotropic medium the off-diagonal terms of the Green's tensor are null. The CCT is computed between each pair of stations and then rotated in order to approximate the Green's tensor by minimizing the off-diagonal components. This procedure permits the calculation of the polarization angle of quasi-Rayleigh and quasi-Love waves, and to observe the azimuthal variation of their polarization. The results show that even a small variation of the azimuth of seismic anisotropy with respect to a certain pair of stations can induce, in some cases, a large

Seismic Reflection Transect across the Central Iberian Zone (Iberian Massif): The ALCUDIA project

NASA Astrophysics Data System (ADS)

Carbonell, R.; Simancas, F.; Martinez-Poyatos, D.; Ayarza, P.; Gonzalez, P.; Tejero, R.; Martín-Parra, L.; Matas, J.; Gonzalez-Lodeiro, F.; Pérez-Estaún, A.; García-Lobon, J.; Mansilla, L.; Palomeras, I.

2007-12-01

The lithosphere of the Central Iberian Zone (CIZ) differs from that of the southwestern Iberian Massif. They are limited by a suture zone. The seismic reflection profile IBERSEIS suggested that the activity of a Carboniferous mantle plume resulted in abundant intrusions of mafic magmas in the mid-to-lower crust which resulted in a singular crustal evolution. The current knowledge of the area based mostly in surface geological mapping suggests that basic magmatism continues further towards the north, indicating that the mantle plume may have affected a bigger area up to the Tajo depression. Furthermore, the existence of the Almadén mine, one of the largest mercury mine in the world within the CIZ, favour that the crust in this area is the result of anomalous lithospheric processes. Accordingly, the ALCUDIA project has been lauched aiming to study the structure and nature of the lithosphere of the CIZ. It includes the acquisition of a deep high resolution seismic reflection transect, detailed geological mapping, kinematic, petrologic and geochemical studies, and other geophysical studies (potential field methods). This new profile extends the previous IBERSEIS Transect towards the northeast, completing almost 600 km of deep seismic profiles, crossing the southern half of the Iberian Variscides. The transect crosses some important structures, such as the Toledo fault, Santa Elena Fault, Alcudia anticline, Almadén syncline, and some major magnetic anomalies. The preliminary results reveal that the crust is 30 km thick in average, with a horizontal Moho, a highly reflective mid-to-lower crust with a few mantle reflectors and well defined features in the upper crust with the indication of detachments zones that might link to the mid- crustal reflective zone.

Seismic activity offshore Martinique and Dominica islands (Central Lesser Antilles subduction zone) from temporary onshore and offshore seismic networks

NASA Astrophysics Data System (ADS)

Ruiz, M.; Galve, A.; Monfret, T.; Sapin, M.; Charvis, P.; Laigle, M.; Evain, M.; Hirn, A.; Flueh, E.; Gallart, J.; Diaz, J.; Lebrun, J. F.

2013-09-01

This work focuses on the analysis of a unique set of seismological data recorded by two temporary networks of seismometers deployed onshore and offshore in the Central Lesser Antilles Island Arc from Martinique to Guadeloupe islands. During the whole recording period, extending from January to the end of August 2007, more than 1300 local seismic events were detected in this area. A subset of 769 earthquakes was located precisely by using HypoEllipse. We also computed focal mechanisms using P-wave polarities of the best azimuthally constrained earthquakes. We detected earthquakes beneath the Caribbean forearc and in the Atlantic oceanic plate as well. At depth seismicity delineates the Wadati-Benioff Zone down to 170 km depth. The main seismic activity is concentrated in the lower crust and in the mantle wedge, close to the island arc beneath an inner forearc domain in comparison to an outer forearc domain where little seismicity is observed. We propose that the difference of the seismicity beneath the inner and the outer forearc is related to a difference of crustal structure between the inner forearc interpreted as a dense, thick and rigid crustal block and the lighter and more flexible outer forearc. Seismicity is enhanced beneath the inner forearc because it likely increases the vertical stress applied to the subducting plate.

Seismic properties of lawsonite eclogites from the southern Motagua fault zone, Guatemala

NASA Astrophysics Data System (ADS)

Kim, Daeyeong; Wallis, Simon; Endo, Shunsuke; Ree, Jin-Han

2016-05-01

We present new data on the crystal preferred orientation (CPO) and seismic properties of omphacite and lawsonite in extremely fresh eclogite from the southern Motagua fault zone, Guatemala, to discuss the seismic anisotropy of subducting oceanic crust. The CPO of omphacite is characterized by (010)[001], and it shows P-wave seismic anisotropies (AVP) of 1.4%-3.2% and S-wave seismic anisotropies (AVS) of 1.4%-2.7%. Lawsonite exhibits (001) planes parallel to the foliation and [010] axes parallel to the lineation, and seismic anisotropies of 1.7%-6.6% AVP and 3.4%-14.7% AVS. The seismic anisotropy of a rock mass consisting solely of omphacite and lawsonite is 1.2%-4.1% AVP and 1.8%-6.8% AVS. For events that propagate more or less parallel to the maximum extension direction, X, the fast S-wave velocity (VS) polarization is parallel to the Z in the Y-Z section (rotated from the X-Z section), causing trench-normal seismic anisotropy for orthogonal subduction. Based on the high modal abundance and strong fabric of lawsonite, the AVS of eclogites is estimated as ~ 11.7% in the case that lawsonite makes up ~ 75% of the rock mass. On this basis, we suggest that lawsonite in both blueschist and eclogite may play important roles in the formation of complex pattern of seismic anisotropy observed in NE Japan: weak trench-parallel anisotropy in the forearc basin domains and trench-normal anisotropy in the backarc region.

Deep Seismic Reflection Images of the Sumatra Seismic and Aseismic Gaps

NASA Astrophysics Data System (ADS)

Singh, S. C.; Hananto, N. D.; Chauhan, A.; Carton, H. D.; Midenet, S.; Djajadihardja, Y.

2009-12-01

The Sumatra subduction zone is seismically most active region on the Earth, and has been the site of three great earthquakes only in the last four years. The first of the series, the 2004 Boxing Day earthquake, broke 1300 km of the plate boundary and produced the devastating tsunami around the Indian Ocean. The second great earthquake occurred three months later in March 2005, about 150 km SE of the 2004 event. The Earth waited for three years, and then broke again in September 2007 at 1300 km SE of the 2004 event producing a twin earthquake of magnitudes of 8.5 and 7.9 at an interval of 12 hours, leaving a seismic gap of about 600 km between the second and third earthquake, the Sumatra Seismic Gap. Seismological and geodetic studies suggest that this gap is fully locked and may break any time. In order to study the seismic and tsunami risk in this locked region, a deep seismic reflection survey (Tsunami Investigation Deep Evaluation Seismic -TIDES) was carried out in May 2009 using the CGGVeritas vessel Geowave Champion towing a 15 long streamer, the longest ever used during a seismic survey, to image the nature of the subducting plate and associated features, including the seismogenic zone, from seafloor down to 50 km depth. A total of 1700 km of deep seismic reflection data were acquired. Three dip lines traverse the Sumatra subduction zone; one going through the Sumatra Seismic Gap, one crossing the region that broke during the 2007 great earthquake, and one going through the aseismic zone. These three dip profiles should provide insight about the locking mechanism and help us to understand why an earthquake occurs in one zone and not in aseismic zone. A strike-line was shot in the forearc basin connecting the locked zone with broken zone profiles, which should provide insight about barriers that might have stopped propagation of 2007 earthquake rupture further northward.

Excitation of high-frequency surface waves with long duration in the Valley of Mexico

NASA Astrophysics Data System (ADS)

Iida, Masahiro

1999-04-01

During the 1985 Michoacan earthquake (Ms = 8.1), large-amplitude seismograms with extremely long duration were recorded in the lake bed zone of Mexico City. We interpret high-frequency seismic wave fields in the three geotechnical zones (the hill, the transition, and the lake bed zones) in the Valley of Mexico on the basis of a systematic analysis for borehole strong motion recordings. We make identification of wave types for real seismograms. First, amplitude ratios between surface and underground seismograms indicate that predominant periods of the surface seismograms are largely controlled by the wave field incident into surficial layers in the Valley of Mexico. We interpret recorded surface waves as fundamental-mode Love waves excited in the Mexican Volcanic Belt by calculating theoretical amplification for different-scale structures. Second, according to a cross-correlation analysis, the hill and transition seismograms are mostly surface waves. In the lake bed zone, while early portions are noisy body waves, late portions are mostly surface waves. Third, using two kinds of surface arrays with different station intervals, we investigate high-frequency surface-wave propagation in the lake bed zone. The wave propagation is very complicated, depending upon the time section and the frequency band. Finally, on the basis of a statistical time series model with an information criterion, we separate S- and surface-wave portions from lake bed seismograms. Surface waves are dominant and are recognized even in the early time section. Thus high-frequency surface waves with long duration in the Valley of Mexico are excited by the Mexican Volcanic Belt.

Magnetotelluric investigation across the Agri Valley: preliminary results.

NASA Astrophysics Data System (ADS)

Balasco, Marianna; Romano, Gerardo; Siniscalchi, Agata; Alfredo Stabile, Tony

2017-04-01

The Agri Valley is an axial zone of the Southern Apennines thrust belt chain with a strong seismogenic potential where two important energy technologies responsible for inducing/triggering seismicity are active: (1) the disposal at the Costa Molina 2 injection well of the wastewater produced during the exploitation of the biggest onshore oil field in west Europe (27 wells producing more than 80,000 barrels of crude oil per day), managed by the Eni S.p.A., and (2) the water loading and unloading operations in the Pertusillo artificial reservoir. It is recognized the possibility that the fluctuation of the water level inside the reservoir, due to the hydrological cycle for example, produces pressure perturbations at the bottom of reservoir, causing induced seismicity. Furthermore it is even more known the role of fluids in the rupture processes which could cause an increase of pore pressure specially at high rate of injection fluids and/or for the presence of weakening of preexisting faults. With the aim to better characterize and understand the physical processes involved in the observed induced/triggered seismicity, in 2016 a broadband seismic network, covering an area of about 20 km x 20 km nearby the Pertusillo Dam and Costa Molina2 well has been installed in the framework of SIR-MIUR project INSIEME (INduced Seismicity in Italy: Estimation, Monitoring, and sEismic risk mitigation) and a MagnetoTelluric (MT) survey has been performed. The MT investigation consists of 25 soundings aligned along 30 km profile oriented at about N40 direction, orthogonal with the strike of the major and noticeable geological structures and crossing both of the source that may induce/trigger seismicity. In this work, we present the preliminary 2D resistivity model which provides useful deep geophysical information for understanding the geological and structural setting of the Agri Valley. Moreover, the comparison of the resistivity model with the earthquake location as inferred from

Three-dimensional structure and seismicity beneath the Central Vanuatu subduction zone

NASA Astrophysics Data System (ADS)

Foix, Oceane; Crawford, Wayne; Pelletier, Bernard; Regnier, Marc; Garaebiti, Esline; Koulakov, Ivan

2017-04-01

The 1400-km long Vanuatu subduction zone results from subduction of the oceanic Australian plate (OAP) beneath the North-Fijian microplate (NFM). Seismic and volcanic activity are both high, and several morphologic features enter into subduction, affecting seismicity and probably plate coupling. The Entrecasteaux Ridge, West-Torres plateau, and Bougainville seamount currently enter into subduction below the large forearc islands of Santo and Malekula. This collision coincides with a strongly decreased local convergence velocity rate - 35 mm/yr compared to 120-160 mm/yr to the north and south - and significant uplift on the overriding plate, indicating a high degree of deformation. The close proximity of large uplifted forearc islands to the trench provides excellent coverage of the megathrust seismogenic zone for a seismological study. We used 10 months of seismological data collected using the 30-instrument land and sea ARC-VANUATU seismology network to construct a 3D velocity model — using the LOTOS joint location/model inversion software — and locate 11655 earthquakes using the NonLinLoc software suite. The 3-D model reveals low P and S velocities in the first tens of kilometers beneath both islands, probably due to water infiltration in the heavily faulted upper plate. The model also suggests the presence of a subducted seamount beneath south Santo. The earthquake locations reveal a complex interaction of faults and stress zones related to high and highly variable deformation. Both brittle deformation and the seismogenic zone depth limits vary along-slab and earthquake clusters are identified beneath central and south Santo, at about 10-30 km of depth, and southwest of Malekula island between 10-20 km depth.

Finite-frequency wave propagation through outer rise fault zones and seismic measurements of upper mantle hydration

USGS Publications Warehouse

Miller, Nathaniel; Lizarralde, Daniel

2016-01-01

Effects of serpentine-filled fault zones on seismic wave propagation in the upper mantle at the outer rise of subduction zones are evaluated using acoustic wave propagation models. Modeled wave speeds depend on azimuth, with slowest speeds in the fault-normal direction. Propagation is fastest along faults, but, for fault widths on the order of the seismic wavelength, apparent wave speeds in this direction depend on frequency. For the 5–12 Hz Pn arrivals used in tomographic studies, joint-parallel wavefronts are slowed by joints. This delay can account for the slowing seen in tomographic images of the outer rise upper mantle. At the Middle America Trench, confining serpentine to fault zones, as opposed to a uniform distribution, reduces estimates of bulk upper mantle hydration from ~3.5 wt % to as low as 0.33 wt % H2O.

Shallow seismic reflection profiles and geological structure in the Benton Hills, southeast Missouri

USGS Publications Warehouse

Palmer, J.R.; Hoffman, D.; Stephenson, W.J.; Odum, J.K.; Williams, R.A.

1997-01-01

favorably oriented in the modern stress field to be reactivated as right-lateral strike slip faults. Currently, earthquake hazards assessments are most dependent upon historical seismicity, and there are little geological data available to evaluate the earthquake potential of fault zones outside of the NMSZ. We anticipate that future studies will provide evidence that seismicity has migrated between fault zones well beyond the middle Mississippi Valley. The potential earthquake hazards represented by faults outside the NMSZ may be significant.

Seismic Moment and Recurrence using Luminescence Dating Techniques: Characterizing brittle fault zone materials suitable for luminescence dating

NASA Astrophysics Data System (ADS)

Tsakalos, E.; Lin, A.; Bassiakos, Y.; Kazantzaki, M.; Filippaki, E.

2017-12-01

During a seismic-geodynamic process, frictional heating and pressure are generated on sediments fragments resulting in deformation and alteration of minerals contained in them. The luminescence signal enclosed in minerals crystal lattice can be affected and even zeroed during such an event. This has been breakthrough in geochronological studies as it could be utilized as a chronometer for the previous seismic activity of a tectonically active area. Although the employment of luminescence dating has in some cases been successfully described, a comprehensive study outlining and defining protocols for routine luminescence dating applied to neotectonic studies has not been forthcoming. This study is the experimental investigation, recording and parameterization of the effects of tectonic phenomena on minerals luminescence signal and the development of detailed protocols for the standardization of the luminescence methodology for directly dating deformed geological formations, so that the long-term temporal behaviour of seismically active faults could be reasonably understood and modeled. This will be achieved by: a) identifying and proposing brittle fault zone materials suitable for luminescence dating using petrological, mineralogical and chemical analyses and b) investigating the "zeroing" potential of the luminescence signal of minerals contained in fault zone materials by employing experimental simulations of tectonic processes in the laboratory, combined with luminescence measurements on samples collected from real fault zones. For this to be achieved, a number of samples collected from four faults of four different geographical regions will be used. This preliminary-first step of the study presents the microstructural, and mineralogical analyses for the characterization of brittle fault zone materials that contain suitable minerals for luminescence dating (e.g., quartz and feldspar). The results showed that the collected samples are seismically deformed fault

Imaging the Chicxulub central crater zone from large scale seismic acoustic wave propagation and gravity modeling

NASA Astrophysics Data System (ADS)

Fucugauchi, J. U.; Ortiz-Aleman, C.; Martin, R.

2017-12-01

Large complex craters are characterized by central uplifts that represent large-scale differential movement of deep basement from the transient cavity. Here we investigate the central sector of the large multiring Chicxulub crater, which has been surveyed by an array of marine, aerial and land-borne geophysical methods. Despite high contrasts in physical properties,contrasting results for the central uplift have been obtained, with seismic reflection surveys showing lack of resolution in the central zone. We develop an integrated seismic and gravity model for the main structural elements, imaging the central basement uplift and melt and breccia units. The 3-D velocity model built from interpolation of seismic data is validated using perfectly matched layer seismic acoustic wave propagation modeling, optimized at grazing incidence using shift in the frequency domain. Modeling shows significant lack of illumination in the central sector, masking presence of the central uplift. Seismic energy remains trapped in an upper low velocity zone corresponding to the sedimentary infill, melt/breccias and surrounding faulted blocks. After conversion of seismic velocities into a volume of density values, we use massive parallel forward gravity modeling to constrain the size and shape of the central uplift that lies at 4.5 km depth, providing a high-resolution image of crater structure.The Bouguer anomaly and gravity response of modeled units show asymmetries, corresponding to the crater structure and distribution of post-impact carbonates, breccias, melt and target sediments

Seismic Microzonation of the City of Cali (Western Colombia)

NASA Astrophysics Data System (ADS)

Dimate, C.; Romero, J.; Ojeda, A.; Garcia, J.; Alvarado, C.

2007-05-01

The city of Cali is located in the western margin of the Cauca Valley in the flat area between the Western and Central cordilleras of the Colombian Andes, at 70 km east of the Eastern Pacific Subduction Zone. Even though present seismic activity associated with nearest faults is low, historical records demonstrate that earthquakes have caused damage in the city going up to intensity VIII (EMS). Those earthquakes have had origin on diverse sources: the intermediate-depth Benioff zone, near and far continental crustal faults and the Pacific Subduction Zone. Taking into account the location of the city and the seismologic history of the region, neotectonic and seismological studies extending over a region of about 120000 km2 were required to compute seismic hazard. Construction of the geotechnical model of the city included detailed geological mapping, geophysical profiling, single station ambient vibration essays and the deployment of a 12-stations accelerographic network. Geotechnical properties of the soils were determined by mechanical perforations, CPTU (piezocone) and CPT (static penetration) essays, flat plate dilatometer (DMT) tests and down-hole essays which were complemented in the Lab by analysis of consolidation and static and cyclic three-axial essays. As a result, ten geotechnical zones were outlined and characterized. Finally, expected ground motions were calculated at 39 sites in the city using numerical modeling methods.

Logs and Scarp Data from a Paloseismic Investigation of the Surprise Valley Fault Zone, Modoc County, California

USGS Publications Warehouse

Personius, Stephen F.; Crone, Anthony J.; Machette, Michael N.; Lidke, David J.; Bradley, Lee-Ann; Mahan, Shannon

2007-01-01

This report contains field and laboratory data from a paleoseismic study of the Surprise Valley fault zone near Cedarville, California. The 85-km-long Surprise Valley fault zone forms the western active margin of the Basin and Range province in northeastern California. The down-to-the-east normal fault is marked by Holocene fault scarps along most of its length, from Fort Bidwell on the north to near the southern end of Surprise Valley. We studied the central section of the fault to determine ages of paleoearthquakes and to better constrain late Quaternary slip rates, which we hope to compare to deformation rates derived from a recently established geodetic network in the region (Hammond and Thatcher, 2005; 2007). We excavated a trench in June 2005 across a prominent fault scarp on pluvial Lake Surprise deltaic sediments near the mouth of Cooks Canyon, 4 km north of Cedarville. This site was chosen because of the presence of a well-preserved fault scarp and its development on lacustrine deposits thought to be suitable for luminescence dating. We also logged a natural exposure of the fault in similar deltaic sediments near the mouth of Steamboat Canyon, 11 km south of Cedarville, to better understand the along-strike extent of surface ruptures. The purpose of this report is to present photomosaics, trench, drill hole, and stream exposure logs; scarp profiles; and fault slip, tephrochronologic, radiocarbon, luminescence, and unit description data obtained during this investigation. We do not attempt to use the data presented herein to construct a paleoseismic history of this part of the Surprise Valley fault zone; that history will be the subject of a future report.

Geophysical characterization of transtensional fault systems in the Eastern California Shear Zone-Walker Lane Belt

NASA Astrophysics Data System (ADS)

McGuire, M.; Keranen, K. M.; Stockli, D. F.; Feldman, J. D.; Keller, G. R.

2011-12-01

The Eastern California Shear Zone (ECSZ) and Walker Lane belt (WL) accommodate ~25% of plate motion between the North American and Pacific plates. Faults within the Mina deflection link the ECSZ and the WL, transferring strain from the Owens Valley and Death Valley-Fish Lake Valley fault systems to the transcurrent faults of the central Walker Lane. During the mid to late Miocene the majority of strain between these systems was transferred through the Silver Peak-Lone Mountain (SPLM) extensional complex via a shallowly dipping detachment. Strain transfer has since primarily migrated north to the Mina Deflection; however, high-angle faults bounding sedimentary basins and discrepancies between geodetic and geologic models indicate that the SPLM complex may still actively transfer a portion of the strain from the ECSZ to the WL on a younger set of faults. Establishing the pattern and amount of active strain transfer within the SPLM region is required for a full accounting of strain accommodation, and provides insight into strain partitioning at the basin scale within a broader transtensional zone. To map the active structures in and near Clayton Valley, within the SPLM region, we collected seismic reflection and refraction profiles and a dense grid of gravity readings that were merged with existing gravity data. The primary goals were to determine the geometry of the high-angle fault system, the amount and sense of offset along each fault set, connectivity of the faults, and the relationship of these faults to the Miocene detachment. Seismic reflection profiles imaged the high-angle basin-bounding normal faults and the detachment in both the footwall and hanging wall. The extensional basin is ~1 km deep, with a steep southeastern boundary, a gentle slope to the northwest, and a sharp boundary on the northwest side, suggestive of another fault system. Two subparallel dip-slip faults bound the southeast (deeper) basin margin with a large lateral velocity change (from ~2

Precursory seismic quiescence along the Sumatra-Andaman subduction zone: past and present

NASA Astrophysics Data System (ADS)

Sukrungsri, Santawat; Pailoplee, Santi

2017-03-01

In this study, the seismic quiescence prior to hazardous earthquakes was analyzed along the Sumatra-Andaman subduction zone (SASZ). The seismicity data were screened statistically with mainshock earthquakes of M w ≥ 4.4 reported during 1980-2015 being defined as the completeness database. In order to examine the possibility of using the seismic quiescence stage as a marker of subsequent earthquakes, the seismicity data reported prior to the eight major earthquakes along the SASZ were analyzed for changes in their seismicity rate using the statistical Z test. Iterative tests revealed that Z factors of N = 50 events and T = 2 years were optimal for detecting sudden rate changes such as quiescence and to map these spatially. The observed quiescence periods conformed to the subsequent major earthquake occurrences both spatially and temporally. Using suitable conditions obtained from successive retrospective tests, the seismicity rate changes were then mapped from the most up-to-date seismicity data available. This revealed three areas along the SASZ that might generate a major earthquake in the future: (i) Nicobar Islands ( Z = 6.7), (ii) the western offshore side of Sumatra Island ( Z = 7.1), and (iii) western Myanmar ( Z = 6.7). The performance of a stochastic test using a number of synthetic randomized catalogues indicated these levels of anomalous Z value showed the above anomaly is unlikely due to chance or random fluctuations of the earthquake. Thus, these three areas have a high possibility of generating a strong-to-major earthquake in the future.

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

Stress development in heterogenetic lithosphere: Insights into earthquake processes in the New Madrid Seismic Zone

NASA Astrophysics Data System (ADS)

Zhan, Yan; Hou, Guiting; Kusky, Timothy; Gregg, Patricia M.

2016-03-01

The New Madrid Seismic Zone (NMSZ) in the Midwestern United States was the site of several major M 6.8-8 earthquakes in 1811-1812, and remains seismically active. Although this region has been investigated extensively, the ultimate controls on earthquake initiation and the duration of the seismicity remain unclear. In this study, we develop a finite element model for the Central United States to conduct a series of numerical experiments with the goal of determining the impact of heterogeneity in the upper crust, the lower crust, and the mantle on earthquake nucleation and rupture processes. Regional seismic tomography data (CITE) are utilized to infer the viscosity structure of the lithosphere which provide an important input to the numerical models. Results indicate that when differential stresses build in the Central United States, the stresses accumulating beneath the Reelfoot Rift in the NMSZ are highly concentrated, whereas the stresses below the geologically similar Midcontinent Rift System are comparatively low. The numerical observations coincide with the observed distribution of seismicity throughout the region. By comparing the numerical results with three reference models, we argue that an extensive mantle low velocity zone beneath the NMSZ produces differential stress localization in the layers above. Furthermore, the relatively strong crust in this region, exhibited by high seismic velocities, enables the elevated stress to extend to the base of the ancient rift system, reactivating fossil rifting faults and therefore triggering earthquakes. These results show that, if boundary displacements are significant, the NMSZ is able to localize tectonic stresses, which may be released when faults close to failure are triggered by external processes such as melting of the Laurentide ice sheet or rapid river incision.

Slab seismicity in the Western Hellenic Subduction Zone: Constraints from tomography and double-difference relocation

NASA Astrophysics Data System (ADS)

Halpaap, Felix; Rondenay, Stéphane; Ottemöller, Lars

2016-04-01

The Western Hellenic subduction zone is characterized by a transition from oceanic to continental subduction. In the southern oceanic portion of the system, abundant seismicity reaches intermediate depths of 100-120 km, while the northern continental portion rarely exhibits deep earthquakes. Our study aims to investigate how this oceanic-continental transition affects fluid release and related seismicity along strike, by focusing on the distribution of intermediate depth earthquakes. To obtain a detailed image of the seismicity, we carry out a tomographic inversion for P- and S-velocities and double-difference earthquake relocation using a dataset of unprecedented spatial coverage in this area. Here we present results of these analyses in conjunction with high-resolution profiles from migrated receiver function images obtained from the MEDUSA experiment. We generate tomographic models by inverting data from 237 manually picked, well locatable events recorded at up to 130 stations. Stations from the permanent Greek network and the EGELADOS experiment supplement the 3-D coverage of the modeled domain, which covers a large part of mainland Greece and surrounding offshore areas. Corrections for the sphericity of the Earth and our update to the SIMULR16 package, which now allows S-inversion, help improve our previous models. Flexible gridding focusses the inversion on the domains of highest gradient around the slab, and we evaluate the resolution with checker board tests. We use the resulting velocity model to relocate earthquakes via the Double-Difference method, using a large dataset of differential traveltimes obtained by crosscorrelation of seismograms. Tens of earthquakes align along two planes forming a double seismic zone in the southern, oceanic portion of the subduction zone. With increasing subduction depth, the earthquakes appear closer to the center of the slab, outlining probable deserpentinization of the slab and concomitant eclogitization of dry crustal

Some Probabilistic and Statistical Properties of the Seismic Regime of Zemmouri (Algeria) Seismoactive Zone

NASA Astrophysics Data System (ADS)

Baddari, Kamel; Bellalem, Fouzi; Baddari, Ibtihel; Makdeche, Said

2016-10-01

Statistical tests have been used to adjust the Zemmouri seismic data using a distribution function. The Pareto law has been used and the probabilities of various expected earthquakes were computed. A mathematical expression giving the quantiles was established. The extreme values limiting law confirmed the accuracy of the adjustment method. Using the moment magnitude scale, a probabilistic model was made to predict the occurrences of strong earthquakes. The seismic structure has been characterized by the slope of the recurrence plot γ, fractal dimension D, concentration parameter K sr, Hurst exponents H r and H t. The values of D, γ, K sr, H r, and H t diminished many months before the principal seismic shock ( M = 6.9) of the studied seismoactive zone has occurred. Three stages of the deformation of the geophysical medium are manifested in the variation of the coefficient G% of the clustering of minor seismic events.

3-D Velocity Model of the Coachella Valley, Southern California Based on Explosive Shots from the Salton Seismic Imaging Project

NASA Astrophysics Data System (ADS)

Persaud, P.; Stock, J. M.; Fuis, G. S.; Hole, J. A.; Goldman, M.; Scheirer, D. S.

2014-12-01

We have analyzed explosive shot data from the 2011 Salton Seismic Imaging Project (SSIP) across a 2-D seismic array and 5 profiles in the Coachella Valley to produce a 3-D P-wave velocity model that will be used in calculations of strong ground shaking. Accurate maps of seismicity and active faults rely both on detailed geological field mapping and a suitable velocity model to accurately locate earthquakes. Adjoint tomography of an older version of the SCEC 3-D velocity model shows that crustal heterogeneities strongly influence seismic wave propagation from moderate earthquakes (Tape et al., 2010). These authors improve the crustal model and subsequently simulate the details of ground motion at periods of 2 s and longer for hundreds of ray paths. Even with improvements such as the above, the current SCEC velocity model for the Salton Trough does not provide a match of the timing or waveforms of the horizontal S-wave motions, which Wei et al. (2013) interpret as caused by inaccuracies in the shallow velocity structure. They effectively demonstrate that the inclusion of shallow basin structure improves the fit in both travel times and waveforms. Our velocity model benefits from the inclusion of known location and times of a subset of 126 shots detonated over a 3-week period during the SSIP. This results in an improved velocity model particularly in the shallow crust. In addition, one of the main challenges in developing 3-D velocity models is an uneven stations-source distribution. To better overcome this challenge, we also include the first arrival times of the SSIP shots at the more widely spaced Southern California Seismic Network (SCSN) in our inversion, since the layout of the SSIP is complementary to the SCSN. References: Tape, C., et al., 2010, Seismic tomography of the Southern California crust based on spectral-element and adjoint methods: Geophysical Journal International, v. 180, no. 1, p. 433-462. Wei, S., et al., 2013, Complementary slip distributions

Stress concentration on Intraplate Seismicity: Numerical Modeling of Slab-released Fluids in the New Madrid Seismic Zone

NASA Astrophysics Data System (ADS)

Saxena, A.; Choi, E.; Powell, C. A.

2017-12-01

The mechanism behind the seismicity of the New Madrid Seismic Zone (NMSZ), the major intraplate earthquake source in the Central and Eastern US (CEUS), is still debated but new insights are being provided by recent tomographic studies involving USArray. A high-resolution tomography study by Nyamwandha et al. (2016) in the NMSZ indicates the presence of low (3 % - 5 %) upper mantle Vp and Vs anomalies in the depth range 100 to 250 km. The elevated anomaly magnitudes are difficult to explain by temperature alone. As the low-velocity anomalies beneath the northeast China are attributed to fluids released from the stagnant Pacific slab, water released from the stagnant Laramide Slab, presently located at transition zone depths beneath the CEUS might be contributing to the low velocity features in this region's upper mantle. Here, we investigate the potential impact of the slab-released fluids on the stresses at seismogenic depths using numerical modeling. We convert the tomographic results into temperature field under various assumed values of spatially uniform water content. In more realistic cases, water content is added only when the converted temperature exceeds the melting temperature of olivine. Viscosities are then computed based on the temperature and water content and given to our geodynamic models created by Pylith, an open source software for crustal dynamics. The model results show that increasing water content weakens the upper mantle more than temperature alone and thus elevates the differential stress in the upper crust. These results can better explain the tomography results and seismicity without invoking melting. We also invert the tomography results for volume fraction of orthopyroxene and temperature and compare the resultant stresses with those for pure olivine. To enhance the reproducibility, selected models in this study will be made available in the form of sharable and reproducible packages enabled by EarthCube Building block project, GeoTrust.

Gravity and magnetic anomalies used to delineate geologic features associated with earthquakes and aftershocks in the central Virginia seismic zone

NASA Astrophysics Data System (ADS)

Shah, A. K.; Horton, J.; McNamara, D. E.; Spears, D.; Burton, W. C.

2013-12-01

Estimating seismic hazard in intraplate environments can be challenging partly because events are relatively rare and associated data thus limited. Additionally, in areas such as the central Virginia seismic zone, numerous pre-existing faults may or may not be candidates for modern tectonic activity, and other faults may not have been mapped. It is thus important to determine whether or not specific geologic features are associated with seismic events. Geophysical and geologic data collected in response to the Mw5.8 August 23, 2011 central Virginia earthquake provide excellent tools for this purpose. Portable seismographs deployed within days of the main shock showed a series of aftershocks mostly occurring at depths of 3-8 km along a southeast-dipping tabular zone ~10 km long, interpreted as the causative fault or fault zone. These instruments also recorded shallow (< 4 km) aftershocks clustered in several areas at distances of ~2-15 km from the main fault zone. We use new airborne geophysical surveys (gravity, magnetics, radiometrics, and LiDAR) to delineate the distribution of various surface and subsurface geologic features of interest in areas where the earthquake and aftershocks took place. The main (causative fault) aftershock cluster coincides with a linear, NE-trending gravity gradient (~ 2 mgal/km) that extends over 20 km in either direction from the Mw5.8 epicenter. Gravity modeling incorporating seismic estimates of Moho variations suggests the presence of a shallow low-density body overlying the main aftershock cluster, placing it within the upper 2-4 km of the main-fault hanging wall. The gravity, magnetic, and radiometric data also show a bend in generally NE-SW orientation of anomalies close to the Mw5.8 epicenter. Most shallow aftershock clusters occur near weaker short-wavelength gravity gradients of one to several km length. In several cases these gradients correspond to geologic contacts mapped at the surface. Along the gravity gradients, the

Seismic Activity offshore Martinique and Dominique islands (Lesser Antilles subduction zone)

NASA Astrophysics Data System (ADS)

Ruiz Fernandez, Mario; Galve, Audrey; Monfret, Tony; Charvis, Philippe; Laigle, Mireille; Flueh, Ernst; Gallart, Josep; Hello, Yann

2010-05-01

In the framework of the European project Thales was Right, two seismic surveys (Sismantilles II and Obsantilles) were carried out to better constrain the lithospheric structure of the Lesser Antilles subduction zone, its seismic activity and to evaluate the associated seismic hazards. Sismantilles II experiment was conducted in January, 2007 onboard R/V Atalante (IFREMER). A total of 90 OBS belonging to Géoazur, INSU-CNRS and IFM-Geomar were deployed on a regular grid, offshore Antigua, Guadeloupe, Dominique and Martinique islands. During the active part of the survey, more than 2500 km of multichannel seismic profiles were shot along the grid lines. Then the OBS remained on the seafloor continuously recording for the seismic activity for approximately 4 months. On April 2007 Obsantilles experiment, carried out onboard R/V Antea (IRD), was focused on the recovery of those OBS and the redeployment of 28 instruments (Géoazur OBS) off Martinique and Dominica Islands for 4 additional months of continuous recording of the seismicity. This work focuses on the analysis of the seismological data recorded in the southern sector of the study area, offshore Martinique and Dominique. During the two recording periods, extending from January to the end of August 2007, more than 3300 seismic events were detected in this area. Approximately 1100 earthquakes had enough quality to be correctly located. Station corrections, obtained from multichannel seismic profiles, were introduced to each OBS to take in to account the sedimentary cover and better constrain the hypocentral determinations. Results show events located at shallower depths in the northern sector of the array, close to the Tiburon Ridge, where the seismic activity is mainly located between 20 to 40 km depth. In the southern sector, offshore Martinique, hypocenters become deeper, ranging to 60 km depth and dipping to the west. Focal solutions have also been obtained using the P wave polarities of the best azimuthally

Estimation of recurrence interval of large earthquakes on the central Longmen Shan fault zone based on seismic moment accumulation/release model.

PubMed

Ren, Junjie; Zhang, Shimin

2013-01-01

Recurrence interval of large earthquake on an active fault zone is an important parameter in assessing seismic hazard. The 2008 Wenchuan earthquake (Mw 7.9) occurred on the central Longmen Shan fault zone and ruptured the Yingxiu-Beichuan fault (YBF) and the Guanxian-Jiangyou fault (GJF). However, there is a considerable discrepancy among recurrence intervals of large earthquake in preseismic and postseismic estimates based on slip rate and paleoseismologic results. Post-seismic trenches showed that the central Longmen Shan fault zone probably undertakes an event similar to the 2008 quake, suggesting a characteristic earthquake model. In this paper, we use the published seismogenic model of the 2008 earthquake based on Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar (InSAR) data and construct a characteristic seismic moment accumulation/release model to estimate recurrence interval of large earthquakes on the central Longmen Shan fault zone. Our results show that the seismogenic zone accommodates a moment rate of (2.7 ± 0.3) × 10¹⁷ N m/yr, and a recurrence interval of 3900 ± 400 yrs is necessary for accumulation of strain energy equivalent to the 2008 earthquake. This study provides a preferred interval estimation of large earthquakes for seismic hazard analysis in the Longmen Shan region.

Estimation of Recurrence Interval of Large Earthquakes on the Central Longmen Shan Fault Zone Based on Seismic Moment Accumulation/Release Model

PubMed Central

Zhang, Shimin

2013-01-01

Recurrence interval of large earthquake on an active fault zone is an important parameter in assessing seismic hazard. The 2008 Wenchuan earthquake (Mw 7.9) occurred on the central Longmen Shan fault zone and ruptured the Yingxiu-Beichuan fault (YBF) and the Guanxian-Jiangyou fault (GJF). However, there is a considerable discrepancy among recurrence intervals of large earthquake in preseismic and postseismic estimates based on slip rate and paleoseismologic results. Post-seismic trenches showed that the central Longmen Shan fault zone probably undertakes an event similar to the 2008 quake, suggesting a characteristic earthquake model. In this paper, we use the published seismogenic model of the 2008 earthquake based on Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar (InSAR) data and construct a characteristic seismic moment accumulation/release model to estimate recurrence interval of large earthquakes on the central Longmen Shan fault zone. Our results show that the seismogenic zone accommodates a moment rate of (2.7 ± 0.3) × 1017 N m/yr, and a recurrence interval of 3900 ± 400 yrs is necessary for accumulation of strain energy equivalent to the 2008 earthquake. This study provides a preferred interval estimation of large earthquakes for seismic hazard analysis in the Longmen Shan region. PMID:23878524

NV PFA - Steptoe Valley

DOE Data Explorer

Jim Faulds

2015-10-29

All datasets and products specific to the Steptoe Valley model area. Includes a packed ArcMap project (.mpk), individually zipped shapefiles, and a file geodatabase for the northern Steptoe Valley area; a GeoSoft Oasis montaj project containing GM-SYS 2D gravity profiles along the trace of our seismic reflection lines; a 3D model in EarthVision; spreadsheet of links to published maps; and spreadsheets of well data.

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

NASA Astrophysics Data System (ADS)

Janiszewski, Helen Anne

A new onshore-offshore seismic dataset from the Cascadia subduction zone was used to characterize mantle lithosphere structure from the ridge to the volcanic arc, and plate interface structure offshore within the seismogenic zone. The Cascadia Initiative (CI) covered the Juan de Fuca plate offshore the northwest coast of the United States with an ocean bottom seismometer (OBS) array for four years; this was complemented by a simultaneous onshore seismic array. Teleseismic data recorded by this array allows the unprecedented imaging of an entire tectonic plate from its creation at the ridge through subduction initiation and back beyond the volcanic arc along the entire strike of the Cascadia subduction zone. Higher frequency active source seismic data also provides constraints on the crustal structure along the plate interface offshore. Two seismic datasets were used to image the plate interface structure along a line extending 100 km offshore central Washington. These are wide-angle reflections from ship-to-shore seismic data from the Ridge-To-Trench seismic cruise and receiver functions calculated from a densely spaced CI OBS focus array in a similar region. Active source seismic observations are consistent with reflections from the plate interface offshore indicating the presence of a P-wave velocity discontinuity. Until recently, there has been limited success in using the receiver function technique on OBS data. I avoid these traditional challenges by using OBS constructed with shielding deployed in shallow water on the continental shelf. These data have quieter horizontals and avoid water- and sediment-multiple contamination at the examined frequencies. The receiver functions are consistently modeled with a velocity structure that has a low velocity zone (LVZ) with elevated P to S-wave velocity ratios at the plate interface. A similar LVZ structure has been observed onshore and interpreted as a combination of elevated pore-fluid pressures or metasediments

Implications of a localized zone of seismic activity near the Inner Piedmont-Blue Ridge boundary

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

Douglas, S.; Powell, C.

1994-03-01

A small but distinct cluster of earthquake activity is located in Henderson County, NC, near the boundary of the Inner Piedmont and Blue Ridge physiographic provinces. Over twenty events have occurred within the cluster since 1776 and four had body-wave magnitudes exceeding 3.0. Average focal depth for instrumentally recorded events is 7.7 km. Epicenters plot within the Inner Piedmont, roughly 13 km from the surface expression of the Brevard fault zone. The reason for sustained earthquake activity in Henderson County is not known but the close spatial association of the events with the Brevard fault suggests a causal relationship. Themore » Brevard zone dips steeply to the SE and the events could be associated with the fault at depth. An even more intriguing possibility is that the events are associated with the intersection of the Brevard zone and the decollemont; this possibility is compatible with available information concerning the depth to the decollemont and the dip on the Brevard zone. An association of seismic activity with the Brevard zone at depth is supported by the presence of another small cluster of activity located in Rutherford County, NC. This cluster is located in the Inner Piedmont, roughly 30 km NE of the Henderson cluster and 16 km from the Brevard fault zone. Association of seismic activity with known faults is very rare in the eastern US and has implications for tectonic models and hazard evaluation. Additional research must be conducted to determine the feasibility that activity is associated with the Brevard zone.« less

Evidences of a Lithospheric Fault Zone in the Sicily Channel Continental Rift (Southern Italy) from Instrumental Seismicity Data

NASA Astrophysics Data System (ADS)

Parisi, L.; Calo, M.

2013-12-01

The Sicily Channel continental rift is located in the African Plate and is submerged by a shallow sea extending from the northern coast of Africa to the southern coast of Sicily (southern Italy). The area is affected by an extensional regime since early Pliocene, which thins the continental crust and produces NW-SE oriented Pantelleria, Linosa and Malta grabens. The rift-related volcanic activity is represented by Pantelleria and Linosa Islands and a series of magmatic manifestations roughly NNE-SSW aligned, from Linosa Island to the Nameless Bank, in proximity of the Sicilian coast. Recent rapid magmatic ascents occurred along the strip near to the Sicilian coast in a region named Graham Bank. The NNE-SSW strip has already been recognised as a separation belt between the western sector of the rift (Pantelleria graben) and the eastern one (Linosa and Malta grabens). Seismic profiles suggest the presence of near vertical structures associated with strike slip fault zones. Bathymetric data show a 15-20 km wide zone characterised by several shallow basins irregularly alternated by topographic highs. However, evidences of a N-S or NNE-SSW orientated faults have not been found. In this work we re-localised the instrumental seismicity recorded between 1981 and 2012 in the Sicily Channel and western Sicily using the Double Difference method (Waldhauser, 2001, 2012) and 3D Vp and Vs models (Calò et al., 2013). The statistical analysis of the relocated seismicity together with the study of seismic energy release distribution allows us to describe the main patterns associated with the active faults in the western Sicily Straits. Here we find that most of the events in the Sicily Channel are highly clustered between 12.5°- 13.5°E and 35.5°-37°N with hypocentral depth between 5-40 km, reaching in some cases 70 km of depth. Seismic events seem to be aligned along a sub-vertical shear zone that is long at least 250 km and oriented approximately NNE-SSW. The spatial

Angola seismicity

NASA Astrophysics Data System (ADS)

Neto, Francisco António Pereira; França, George Sand; Condori, Cristobal; Sant'Anna Marotta, Giuliano; Chimpliganond, Cristiano Naibert

2018-05-01

This work describes the development of the Angolan earthquake catalog and seismicity distribution in the Southwestern African Plate, in Angola. This region is one of the least seismically active, even for stable continental regions (SCRs) in the world. The maximum known earthquake had a magnitude of 6.0 Ms, while events with magnitudes of 4.5 have return period of about 10 years. Events with magnitude 5 and above occur with return period of about 20 years. Five seismic zones can be confirmed in Angola, within and along craton edges and in the sedimentary basins including offshore. Overall, the exposed cratonic regions tend to have more earthquakes compared to other regions such as sedimentary basins. Earthquakes tend to occur in Archaic rocks, especially inside preexisting weakness zones and in tectonic-magmatic reactivation zones of Mesozoic and Meso-Cenozoic, associated with the installation of a wide variety of intrusive rocks, strongly marked by intense tectonism. This fact can be explained by the models of preexisting weakness zones and stress concentration near intersecting structures. The Angolan passive margin is also a new region where seismic activity occurs. Although clear differences are found between different areas along the passive margin, in the middle near Porto Amboim city, seismic activity is more frequent compared with northwestern and southwestern regions.

Dense lower crust elevates long-term earthquake rates in the New Madrid seismic zone

USGS Publications Warehouse

Levandowski, William Brower; Boyd, Oliver; Ramirez-Guzman, Leonardo

2016-01-01

Knowledge of the local state of stress is critical in appraising intraplate seismic hazard. Inverting earthquake moment tensors, we demonstrate that principal stress directions in the New Madrid seismic zone (NMSZ) differ significantly from those in the surrounding region. Faults in the NMSZ that are incompatible with slip in the regional stress field are favorably oriented relative to local stress. We jointly analyze seismic velocity, gravity, and topography to develop a 3-D crustal and upper mantle density model, revealing uniquely dense lower crust beneath the NMSZ. Finite element simulations then estimate the stress tensor due to gravitational body forces, which sums with regional stress. The anomalous lower crust both elevates gravity-derived stress at seismogenic depths in the NMSZ and rotates it to interfere more constructively with far-field stress, producing a regionally maximal deviatoric stress coincident with the highest concentration of modern seismicity. Moreover, predicted principal stress directions mirror variations (observed independently in moment tensors) at the NMSZ and across the region.

Instrument Correction and Dynamic Site Profile Validation at the Central United States Seismic Observatory, New Madrid Seismic Zone

NASA Astrophysics Data System (ADS)

Brengman, C.; Woolery, E. W.; Wang, Z.; Carpenter, S.

2016-12-01

The Central United States Seismic Observatory (CUSSO) is a vertical seismic array located in southwestern Kentucky within the New Madrid seismic zone. It is intended to describe the effects of local geology, including thick sediment overburden, on seismic-wave propagation, particularly strong-motion. The three-borehole array at CUSSO is composed of seismic sensors placed on the surface, and in the bedrock at various depths within the 585 m thick sediment overburden. The array's deep borehole provided a unique opportunity in the northern Mississippi embayment for the direct geological description and geophysical measurement of the complete late Cretaceous-Quaternary sediment column. A seven layer, intra-sediment velocity model is interpreted from the complex, inhomogeneous stratigraphy. The S- and P-wave sediment velocities range between 160 and 875 m/s and between 1000 and 2300 m/s, respectively, with bedrock velocities of 1452 and 3775 m/s, respectively. Cross-correlation and direct comparisons were used to filter out the instrument response and determine the instrument orientation, making CUSSO data ready for analysis, and making CUSSO a viable calibration site for other free-field sensors in the area. The corrected bedrock motions were numerically propagated through the CUSSO soil profile (transfer function) and compared, in terms of both peak acceleration and amplitude spectra, to the recorded surface observations. Initial observations reveal a complex spectral mix of amplification and de-amplification across the array, indicating the site effect in this deep sediment setting is not simply generated by the shallowest layers.

The Eastern Tennessee Seismic Zone: Reactivation of an Ancient Continent-Continent Suture Zone

NASA Astrophysics Data System (ADS)

Powell, C. A.

2014-12-01

The eastern Tennessee seismic zone (ETSZ) may represent reactivation of an ancient shear zone that accommodated left-lateral, transpressive motion of the Amazon craton during the Grenville orogeny. Several different lines of evidence support this concept including velocity models for the crust, earthquake hypocenter alignments, focal mechanism solutions, potential field anomalies, paleomagnetic pole positions, and isotopic geochemical studies. The ETSZ trends NE-SW for about 300 km and displays remarkable correlation with the prominent New York - Alabama (NY-AL) aeromagnetic lineament. Vp and Vs models for the crust derived from a local ETSZ earthquake tomography study reveal the presence of a narrow, NE-SW trending, steeply dipping zone of low velocities that extends to a depth of at least 24 km and is associated with the vertical projection of the NY-AL aeromagnetic lineament. The low velocity zone is interpreted as a major basement fault. The recent Mw 4.2 Perry County eastern Kentucky earthquake occurred north of the ETSZ but has a focal depth and mechanism that are similar to those for ETSZ earthquakes. We investigate the possibility that the proposed ancient shear zone extends into eastern Kentucky using Bouguer and aeromagnetic maps. The southern end of the ETSZ is characterized by hypocenters that align along planes dipping at roughly 45 degrees and focal mechanisms that contain large normal faulting components. The NY-AL aeromagnetic lineament also changes trend in the southern end of the ETSZ and the exact location of the lineament is ambiguous. We suggest that the southern portion of the ETSZ involves reactivation of reverse faults (now as normal faults) that mark the ancient transition between a collisional to a more transpressive boundary between Amazonia and Laurentia during the formation of the super continent Rodinia.

Definition of Exclusion Zones Using Seismic Data

NASA Astrophysics Data System (ADS)

Bartal, Y.; Villagran, M.; Ben Horin, Y.; Leonard, G.; Joswig, M.

- In verifying compliance with the Comprehensive Nuclear-Test-Ban Treaty (CTBT), there is a motivation to be effective, efficient and economical and to prevent abuse of the right to conduct an On-site Inspection (OSI) in the territory of a challenged State Party. In particular, it is in the interest of a State Party to avoid irrelevant search in specific areas. In this study we propose several techniques to determine `exclusion zones', which are defined as areas where an event could not have possibly occurred. All techniques are based on simple ideas of arrival time differences between seismic stations and thus are less prone to modeling errors compared to standard event location methods. The techniques proposed are: angular sector exclusion based on a tripartite micro array, half-space exclusion based on a station pair, and closed area exclusion based on circumferential networks.

Seismic structure and stratigraphy of northern edge of Bahaman-Cuban collision zone

USGS Publications Warehouse

Ball, M.M.; Martin, R.G.; Bock, W.D.; Sylwester, R.E.; Bowles, R.M.; Taylor, D.; Coward, E.L.; Dodd, J.E.; Gilbert, L.

1985-01-01

Common-depth-point (CDP) seismic reflection data in the southwestern Bahamas reveal the northern edge of the tectonized zone that resulted from the late Mesozoic-early Cenozoic collision of Cuba and the Bahamas. Two seismic facies are present. A 10-km broad anticline occurs at the south end of Santaren Channel. Platform carbonates in the core of this structure overlie Early Cretaceous and older basinal carbonate deposits and are onlapped by Late Cretaceous and Cenozoic basinal facies. The structure is inferred to be a hanging-wall anticline at the northern limit of the Cuban fold-thrust belt formed in the Late Cretaceous. A deeper water embayment extended northward into the Straits of Florida, around northern Cay Sal Bank, and back into Santaren Channel during the Early Cretaceous.

Seismic investigation on the Littoral Faults Zone in the northern continental margin of South China Sea

NASA Astrophysics Data System (ADS)

Sun, J.; Xu, H.; Xia, S.; Cao, J.; Wan, K.

2017-12-01

The continental margin of the northern South China Sea (SCS) had experienced continuous evolution from an active continental margin in the late Mesozoic to a passive continental margin in the Cenozoic. The 1200km-long Littoral Faults Zone (LFZ) off the mainland South China was suggested to represent one of the sub-plate boundaries and play a key role during the evolution. Besides, four devastating earthquakes with magnitude over 7 and another 11 destructive events with M>6 were documented to have occurred along the LFZ. However, its approximity to the shoreline, the shallow water depth, and the heavy fishing activities make it hard to conduct a marine seismic investigation. As a result, understandings about the LFZ before 2000 were relatively poor and mostly descriptive. After two experiments of joint onshore-offshore wide-angle seismic surveys in the 1st decade of this century, several cruses aiming to unveil the deep structure of the LFZ were performed in the past few years, with five joint onshore-offshore wide-angle seismic survey profiles completed. Each of these profiles is perpendicular to the shoreline, with four to five seismometers of campaign mode deployed on the landside and over ten Ocean Bottom Seismometers (OBSs) spacing at 20km deployed on the seaside. Meanwhile, multi-channel seismic (MCS) data along these profiles were obtained simultaneously. Based on these data, velocity models from both forward modeling and inversion were obtained. According to these models, the LFZ was imaged to be a low-velocity fractured zone dipping to the SSE-SE at a high-angle and cutting through the thinned continental crust at some locations. Width of the fractured zone varies from 6km to more than 10km from site to site. With these results, it is suggested that the LFZ accommodates the stresses from both the east side, where the Eurasia/Philippine Sea plate converging and mountain building is ongoing, and the west side, where a strike-slip between the Indochina

In situ stress and fracture permeability along the Stillwater fault zone, Dixie Valley Nevada

USGS Publications Warehouse

Hickman, S.H.; Barton, C.A.; Zoback, M.D.; Morin, R.; Sass, J.; Benoit, R.

1997-01-01

Borehole televiewer and hydrologic logging and hydraulic fracturing stress measurements were carried out in a 2.7-km-deep geothermal production well (73B-7) drilled into the Stillwater fault zone. Precision temperature and spinner flowmeter logs were also acquired in well 73B-7, with and without simultaneously injecting water into the well. Localized perturbations to well-bore temperature and flow were used to identify hydraulically conductive fractures. Comparison of these data with fracture orientations from the televiewer log indicates that permeable fractures within and adjacent to the Stillwater fault zone are critically stressed, potentially active shear planes in the current west-northwest extensional stress regime at Dixie Valley.

Structure of pseudotachylyte vein systems as a key to co-seismic rupture dynamics: the case of Gavilgarh-Tan Shear Zone, central India

NASA Astrophysics Data System (ADS)

Chattopadhyay, A.; Bhattacharjee, D.; Mukherjee, S.

2014-04-01

The secondary fractures associated with a major pseudotachylyte-bearing fault vein in the sheared aplitic granitoid of the Proterozoic Gavilgarh-Tan Shear Zone in central India are mapped at the outcrop scale. The fracture maps help to identify at least three different types of co-seismic ruptures, e.g., X-X', T1 and T2, which characterize sinistral-sense shearing of rocks, confined between two sinistral strike-slip faults slipping at seismic rate. From the asymmetric distribution of tensile fractures around the sinistral-sense fault vein, the direction of seismic rupture propagation is predicted to have occurred from west-southwest to east-northeast, during an ancient (Ordovician?) earthquake. Calculations of approximate co-seismic displacement on the faults and seismic moment ( M 0) of the earthquake are attempted, following the methods proposed by earlier workers. These estimates broadly agree to the findings from other studied fault zones (e.g., Gole Larghe Fault zone, Italian Alps). This study supports the proposition by some researchers that important seismological information can be extracted from tectonic pseudotachylytes of all ages, provided they are not reworked by subsequent tectonic activity.

Seismic zoning (first approximation) using data of the main geomagnetic field

NASA Astrophysics Data System (ADS)

Khachikyan, Galina; Zhumabayev, Beibit; Toyshiev, Nursultan; Kairatkyzy, Dina; Seraliyev, Alibek; Khassanov, Eldar

2017-04-01

Seismic zoning is among the most complicated and extremely important problems of modern seismology. In solving this problem, a very important parameter is maximal possible earthquake magnitude (Mmax) which is believed at present depends on horizontal size of geoblocks. At the same time, it was found by Khachikyan et al. [2012, IJG, doi: 10.4236/ijg.2012.35109] that Mmax value in any seismic region may be determined using Z_GSM value that is geomagnetic Z-component in this region estimated in geocentric solar-magnetosphere coordinate system (GSM). On the base of the global seismological catalog NEIC with M≥4.5 for 1973-2010 years, and the International Geomagnetic Reference Field (IGRF) model, an empirical relation was obtained as follows: Mmax= a + b {log[abs(Z_GSM)]}. For the case of the whole planet, obtained empirical coefficients are as follows: a = (5,22 ± 0,17), and b = (0,78 ± 0,06) with correlation coefficient R=0.91, standard deviation SD=0.56, and probability 95%. Further investigations showed that the coefficients of the regression equation are different for different seismically active regions of the planet. For example, to the territory of the San Andreas Fault, defined by the coordinates 30-45N, 105-135W obtained values are as follows: a = (4,04 ± 0.38) and b = (0.7 ± 0.13) with correlation coefficient R = 0.91, standard deviation SD = 0.34, and probability of 95%. For territory of inland seismicity in Eurasia defined by the coordinates 30-45N, 0-110E, a = (12.44 ± 0.48) and b = (1,15 ± 0.2) with correlation coefficient R = 0.87, standard deviation SD = 0.98, and probability of 95%, and for the territory of the strongest seismicity in the world defined by the coordinates 20S-20N, 90-150E, obtained values of a = (- 17.5 ± 1,5) and b = (5,7 ± 0.4) with correlation coefficient R = 0.97, standard deviation SD = 0.4, and probability of 95%. The relationship between the intensity of the main geomagnetic field and released seismic energy is

Earthquake Source Parameter Estimates for the Charlevoix and Western Quebec Seismic Zones in Eastern Canada

NASA Astrophysics Data System (ADS)

Onwuemeka, J.; Liu, Y.; Harrington, R. M.; Peña-Castro, A. F.; Rodriguez Padilla, A. M.; Darbyshire, F. A.

2017-12-01

The Charlevoix Seismic Zone (CSZ), located in eastern Canada, experiences a high rate of intraplate earthquakes, hosting more than six M >6 events since the 17th century. The seismicity rate is similarly high in the Western Quebec seismic zone (WQSZ) where an MN 5.2 event was reported on May 17, 2013. A good understanding of seismicity and its relation to the St-Lawrence paleorift system requires information about event source properties, such as static stress drop and fault orientation (via focal mechanism solutions). In this study, we conduct a systematic estimate of event source parameters using 1) hypoDD to relocate event hypocenters, 2) spectral analysis to derive corner frequency, magnitude, and hence static stress drops, and 3) first arrival polarities to derive focal mechanism solutions of selected events. We use a combined dataset for 817 earthquakes cataloged between June 2012 and May 2017 from the Canadian National Seismograph Network (CNSN), and temporary deployments from the QM-III Earthscope FlexArray and McGill seismic networks. We first relocate 450 events using P and S-wave differential travel-times refined with waveform cross-correlation, and compute focal mechanism solutions for all events with impulsive P-wave arrivals at a minimum of 8 stations using the hybridMT moment tensor inversion algorithm. We then determine corner frequency and seismic moment values by fitting S-wave spectra on transverse components at all stations for all events. We choose the final corner frequency and moment values for each event using the median estimate at all stations. We use the corner frequency and moment estimates to calculate moment magnitudes, static stress-drop values and rupture radii, assuming a circular rupture model. We also investigate scaling relationships between parameters, directivity, and compute apparent source dimensions and source time functions of 15 M 2.4+ events from second-degree moment estimates. To the first-order, source dimension

Seismic constraints on a large dyking event and initiation of a transform fault zone in Western Gulf of Aden

NASA Astrophysics Data System (ADS)

Ahmed, AbdulHakim; Doubre, Cecile; Leroy, Sylvie; Perrot, Julie; Audin, Laurence; Rolandone, Frederique; Keir, Derek; Al-Ganad, Ismael; Sholan, Jamal; Khanbari, Khaled; Mohamed, Kassim; Vergne, Jerome; Jacques, Eric; Nercessian, Alex

2013-04-01

In November 2010, a large number of events were recorded by the world seismic networks showing important activity occurring along the western part of the Aden Ridge. West of the Shulka El Sheik fracture zone, events in this large seismic swarm (magnitudes above 5) occurred in a complex area, where the change of both the ridge direction and the bathymetry suggest the propagation of the ridge into a continental lithosphere and the influence of the Afar plume. We combine several sets of data from permanent networks and temporary 3C broad stations installed after the beginning of the event along the southern and eastern coasts of Yemen and Djibouti respectively, we located more than 600 earthquakes with magnitudes ranging from 2.5 to 5.6 that occurred during the first months following the first event. The spatial distribution of the main seismicity reveals a very clear N115° -trending alignment, parallel to the mean direction of the en-echelon spreading segments that form the ridge at this longitude. Half of the events, which represent half of the total seismic energy released during the first months, are located in the central third section of the segment. Here several volcanic cones and recent lava flows observed from bathymetric and acoustic reflectivity data during the Tadjouraden cruise (Audin, 1999, Dauteuil et al., 2001) constitute the sea floor. In addition to this main activity, two small groups of events suggest the activiation of landslides into a large fan and the activity in a volcanic area 50 km due east from the main active zone. The time evolution of the seismicity shows several bursts of activity. Some of them are clearly related to sudden activities within the volcanic areas, when others exhibit horizontal migration of the events, with velocity around ˜ 1 km/h. The time-space evolution of the seismicity clearly reveals the intrusion of dykes associated with magma propagation from the crustal magmatic centres into the rift zone. Taking into account

Complex Modeling of the Seismic Structure of the Trans-European Suture Zone's Margin from Receiver Function Analysis

NASA Astrophysics Data System (ADS)

Wilde-Piorko, M.; Chrapkiewicz, K.; Lepore, S.; Polkowski, M.; Grad, M.

2016-12-01

The Trans-European Suture Zone (TESZ) is one of the most prominent suture zones in Europe separating the young Paleozoic Platform from the much older Precambrian East European Craton. The data recorded by "13 BB Star" broadband seismic stations (Grad et al., 2015) are analyzed to investigate the crustal and upper mantle structure of the margin of the Trans-European Suture Zone (TESZ) in northern Poland. Receiver function (RF) locally provides the signature of sharp seismic discontinuities and information about the shear wave (S-wave) velocity distribution beneath the seismic station. Recorded seismograms are rotated from ZNE to LQT system with method using the properties of RF (Wilde-Piórko, 2015). Different techniques of receiver function interpretation are applied, including 1-D inversion of RF, 1-D forward modeling of RF, 2.5D forward modeling of RF, 1-D join inversion of RF and dispersion curves of surface wave, to find the best S-wave velocity model of the TESZ margin. A high-resolution 3D P-wave velocity model in the area of Poland (Grad et al. 2016) are used as a starting model. National Science Centre Poland provided financial support for this work by NCN grant DEC-2011/02/A/ST10/00284.

Seismic structure of southern margin of the 2011 Tohoku-Oki Earthquake aftershocks area: slab-slab contact zone beneath northeastern Kanto, central Japan

NASA Astrophysics Data System (ADS)

Kurashimo, E.; Sato, H.; Abe, S.; Mizohata, S.; Hirata, N.

2011-12-01

The 2011 Tohoku-Oki Earthquake (Mw9.0) occurred on the Japan Trench off the eastern shore of northern Honshu, Japan. The southward expansion of the afterslip area has reached the Kanto region, central Japan (Ozawa et al., 2011). The Philippine Sea Plate (PHS) subducts beneath the Kanto region. The bottom of the PHS is in contact with the upper surface of the Pacific Plate (PAC) beneath northeastern Kanto. Detailed structure of the PHS-PAC contact zone is important to constrain the southward rupture process of the Tohoku-Oki Earthquake and provide new insight into the process of future earthquake occurrence beneath the Kanto region. Active and passive seismic experiments were conducted to obtain a structural image beneath northeastern Kanto in 2010 (Sato et al., 2010). The geometry of upper surface of the PHS has been revealed by seismic reflection profiling (Sato et al., 2010). Passive seismic data set is useful to obtain a deep structural image. Two passive seismic array observations were conducted to obtain a detailed structure image of the PHS-PAC contact zone beneath northeastern Kanto. One was carried out along a 50-km-long seismic line trending NE-SW (KT-line) and the other was carried out along a 65-km-long seismic line trending NW-SE (TM-line). Sixty-five 3-component portable seismographs were deployed on KT-line with 500 to 700 m interval and waveforms were continuously recorded during a four-month period from June, 2010. Forty-five 3-component portable seismographs were deployed on TM-line with about 1-2 km spacing and waveforms were continuously recorded during the seven-month period from June, 2010. Arrival times of earthquakes were used in a joint inversion for earthquake locations and velocity structure, using the iterative damped least-squares algorithm, simul2000 (Thurber and Eberhart-Phillips, 1999). The relocated hypocenter distribution shows that the seismicity along the upper surface of the PAC is located at depths of 45-75 km beneath

Miscellaneous High-Resolution Seismic Imaging Investigations in Salt Lake and Utah Valleys for Earthquake Hazards

USGS Publications Warehouse

Stephenson, W.J.; Williams, R.A.; Odum, J.K.; Worley, D.M.

2007-01-01

Introduction In support of earthquake hazards and ground motion studies by researchers at the Utah Geological Survey, University of Utah, Utah State University, Brigham Young University, and San Diego State University, the U.S. Geological Survey Geologic Hazards Team Intermountain West Project conducted three high-resolution seismic imaging investigations along the Wasatch Front between September 2003 and September 2005. These three investigations include: (1) a proof-of-concept P-wave minivib reflection imaging profile in south-central Salt Lake Valley, (2) a series of seven deep (as deep as 400 m) S-wave reflection/refraction soundings using an S-wave minivib in both Salt Lake and Utah Valleys, and (3) an S-wave (and P-wave) investigation to 30 m at four sites in Utah Valley and at two previously investigated S-wave (Vs) minivib sites. In addition, we present results from a previously unpublished downhole S-wave investigation conducted at four sites in Utah Valley. The locations for each of these investigations are shown in figure 1. Coordinates for the investigation sites are listed in Table 1. With the exception of the P-wave common mid-point (CMP) reflection profile, whose end points are listed, these coordinates are for the midpoint of each velocity sounding. Vs30 and Vs100, also shown in Table 1, are defined as the average shear-wave velocities to depths of 30 and 100 m, respectively, and details of their calculation can be found in Stephenson and others (2005). The information from these studies will be incorporated into components of the urban hazards maps along the Wasatch Front being developed by the U.S. Geological Survey, Utah Geological Survey, and numerous collaborating research institutions.

Seismicity, Deformation, and Metamorphism in the Western Hellenic Subduction Zone: New Constraints From Tomography

NASA Astrophysics Data System (ADS)

Halpaap, Felix; Rondenay, Stéphane; Ottemöller, Lars

2018-04-01

The Western Hellenic Subduction Zone is characterized by a transition from oceanic to continental subduction. In the southern oceanic portion of the system, abundant seismicity reaches depths of 100 km to 190 km, while the northern continental portion rarely exhibits deep earthquakes. Our study investigates how this oceanic-continental transition affects fluid release and related seismicity along strike. We present results from local earthquake tomography and double-difference relocation in conjunction with published images based on scattered teleseismic waves. Our tomographic images recover both subducting oceanic and continental crusts as low-velocity layers on top of high-velocity mantle. Although the northern and southern trenches are offset along the Kephalonia Transform Fault, continental and oceanic subducting crusts appear to align at depth. This suggests a smooth transition between slab retreat in the south and slab convergence in the north. Relocated hypocenters outline a single-planed Wadati-Benioff Zone with significant along-strike variability in the south. Seismicity terminates abruptly north of the Kephalonia Transform Fault, likely reflecting the transition from oceanic to continental subducted crust. Near 90 km depth, the low-velocity signature of the subducting crust fades out and the Wadati-Benioff Zone thins and steepens, marking the outline of the basalt-eclogite transition. Subarc melting of the mantle is only observed in the southernmost sector of the oceanic subduction, below the volcanic part of the arc. Beneath the nonvolcanic part, the overriding crust appears to have undergone large-scale silica enrichment. This enrichment is observed as an anomalously low Vp/Vs ratio and requires massive transport of dehydration-derived fluids updip through the subducting crust.

The Parkfield-Cholame, California, earthquakes of June-August, 1966; instrumental seismic studies

USGS Publications Warehouse

Eaton, Jerry P.

1967-01-01

U.S. Geological Survey instrumental seismic studies in the Parkfield-Cholame area consist of three related parts that were undertaken as pilot studies in a program designed to develop improved tools and concepts for investigating the properties and behavior of the San Andreas fault. These studies include: 1. The long=term monitoring of the seismic background on the San Andreas fault in Cholame Valley by means of a short-period Benioff seismograph station at Gold Hill. 2. The investigation of the geometry of the zone of aftershocks of the June 27 earthquakes by means of a small portable cluster of short-period, primarily vertical-component seismographs. 3. The seismic-refraction calibration of the region enclosing the aftershock source by means of three short reversed refraction profiles and a "calibration shot" near the epicenter of the main June 27 earthquake. This brief report outlines the work that has been completed and presents some preliminary results obtained from analysis of records from Gold Hill and the portable cluster.

Syntectonic Mississippi River Channel Response: Integrating River Morphology and Seismic Imaging to Detect Active Faults

NASA Astrophysics Data System (ADS)

Magnani, M. B.

2017-12-01

Alluvial rivers, even great rivers such as the Mississippi, respond to hydrologic and geologic controls. Temporal variations of valley gradient can significantly alter channel morphology, as the river responds syntectonically to attain equilibrium. The river will alter its sinuosity, in an attempt to maintain a constant gradient on a surface that changes slope through time. Therefore, changes of river pattern can be the first clue that active tectonics is affecting an area of pattern change. Here I present geomorphological and seismic imaging evidence of a previously unknown fault crossing the Mississippi river south of the New Madrid seismic zone, between Caruthersville, Missouri and Osceola, Arkansas, and show that both datasets support Holocene fault movement, with the latest slip occurring in the last 200 years. High resolution marine seismic reflection data acquired along the Mississippi river imaged a NW-SE striking north-dipping fault displacing the base of the Quaternary alluvium by 15 m with reverse sense of movement. The fault consistently deforms the Tertiary, Cretaceous and Paleozoic formations. Historical river channel planforms dating back to 1765 reveal that the section of the river channel across the fault has been characterized by high sinuosity and steep projected-channel slope compared to adjacent river reaches. In particular, the reach across the fault experienced a cutoff in 1821, resulting in a temporary lowering of sinuosity followed by an increase between the survey of 1880 and 1915. Under the assumption that the change in sinuosity reflects river response to a valley slope change to maintain constant gradient, I use sinuosity through time to calculate the change in valley slope since 1880 and therefore to estimate the vertical displacement of the imaged fault in the past 200 years. Based on calculations so performed, the vertical offset of the fault is estimated to be 0.4 m, accrued since at least 1880. If the base of the river alluvium

The May 29 2008 earthquake aftershock sequence within the South Iceland Seismic Zone: Fault locations and source parameters of aftershocks

NASA Astrophysics Data System (ADS)

Brandsdottir, B.; Parsons, M.; White, R. S.; Gudmundsson, O.; Drew, J.

2010-12-01

The mid-Atlantic plate boundary breaks up into a series of segments across Iceland. The South Iceland Seismic Zone (SISZ) is a complex transform zone where left-lateral E-W shear between the Reykjanes Peninsula Rift Zone and the Eastern Volcanic Zone is accommodated by bookshelf faulting along N-S lateral strike-slip faults. The SISZ is also a transient feature, migrating sideways in response to the southward propagation of the Eastern Volcanic Zone. Sequences of large earthquakes (M > 6) lasting from days to years and affecting most of the seismic zone have occurred repeatedly in historical time (last 1100 years), separated by intervals of relative quiescence lasting decades to more than a century. On May 29 2008, a Mw 6.1 earthquake struck the western part of the South Iceland Seismic Zone, followed within seconds by a slightly smaller event on a second fault ~5 km further west. Aftershocks, detected by a temporal array of 11 seismometers and three permanent Icelandic Meteorological Office stations were located using an automated Coalescence Microseismic Mapping technique. The epicenters delineate two major and several smaller N-S faults as well as an E-W zone of activity stretching further west into the Reykjanes Peninsula Rift Zone. Fault plane solutions show both right lateral and oblique strike slip mechanisms along the two major N-S faults. The aftershocks deepen from 3-5 km in the north to 8-9 km in the south, suggesting that the main faults dip southwards. The faulting is interpreted to be driven by the local stress due to transform motion between two parallel segments of the divergent plate boundary crossing Iceland.

CAFE: a seismic investigation of water percolation in the Cascadia subduction zone

NASA Astrophysics Data System (ADS)

Rondenay, S.; Abers, G. A.; Creager, K. C.; Malone, S. D.; MacKenzie, L.; Zhang, Z.; van Keken, P. E.; Wech, A. G.; Sweet, J. R.; Melbourne, T. I.; Hacker, B. R.

2008-12-01

Subduction zones transport water into the Earth's interior. The subsequent release of this water through dehydration reactions may trigger intraslab earthquakes and arc volcanism, regulate slip on the plate interface, control plate buoyancy, and regulate the long-term budget of water on the planet's surface. As part of Earthscope, we have undertaken an experiment named CAFE (Cascadia Arrays for Earthscope) seeking to better constrain these effects in the Cascadia subduction zone. The basic experiment has four components: (1) a 47-element broadband imaging array of Flexible Array instruments integrated with Bigfoot; (2) three small-aperture seismic arrays with 15 additional short-period instruments near known sources of Episodic Tremor and Slip (ETS) events; (3) analysis of the PBO and PANGA GPS data sets to define the details of episodic slip events; and (4) integrative modeling with complementary constraints from petrology and geodynamics. Here, we present a summary of the results that have been obtained to date by CAFE, with a focus on high-resolution seismic imaging. A 250 km-long by 120 km-deep seismic profile extending eastward from the Washington coast was generated by 2-D Generalized Radon Transform Inversion of the broadband data. It images the subducted crust as a shallow-dipping, low-velocity layer from 20km depth beneath the coast to 40km depth beneath the forearc. The termination of the low-velocity layer is consistent with the depth at which hydrated metabasalts of the subducted crust are expected to undergo eclogitization, a reaction that is accompanied by the release of water and an increase in seismic velocities. Slab earthquakes are located in both the oceanic crust and mantle at depths <40 km, and exclusively in the oceanic mantle at greater depth, as would be expected if they are related to slab dehydration. Two ETS events have occurred during the course of the deployment. They were precisely located and are confined to the region above which the

Carbon Dioxide Emissions From Kill Zones Around the Resurgent Dome, Long Valley Caldera, CA

NASA Astrophysics Data System (ADS)

Bergfeld, D.; Evans, W. C.; Farrar, C. D.; Howle, J. F.

2004-12-01

An episode of seismic unrest beneath the resurgent dome at Long Valley caldera (LVC) in eastern California began in 1980 and is associated with approximately 80 cm of cumulative uplift on parts of the dome since that time. Studies of hydrologic and geochemical parameters can be useful in determining the source of uplift; and of particular relevance here, studies of diffuse soil degassing and temperature have been used to examine relations between gas emissions, uplift, and energy release. We present results from an eighteen-month investigation of soil temperature, soil-gas chemistry and CO2 efflux from fourteen discrete areas of vegetation kill that have appeared inside the caldera over the past two decades. Compared with the tree-kill around Mammoth Mountain on the southwest rim of the caldera, dead zones we studied around the resurgent dome are small. Individually the areas cover between 800 and 36,000 m2. All of the areas have some sites with elevated CO2 flux and elevated soil temperature. \\delta 13C values of CO2 from sites in eight of the studied areas are between -5.7 and -3.9\\permil, and are within the range of magmatic CO2. Results from the flux measurements indicate that on average total CO2 emissions from four of the areas sum about 10 tonnes per day. The other vegetation kill areas currently have only a few sites that exhibit anomalous soil temperatures and CO2 flux, and CO2 emissions from these areas are typically less than 0.3 of a tonne per day. The chemical composition of gas emissions from thermal ground in kill zones located 1.5 to 2 km northwest of the Casa Diablo geothermal power plant demonstrate a connection between some of the dead areas and perturbations related to geothermal fluid production. These results and estimates of thermal output from two of the high flux grids are used to evaluate the premise that the gaseous and thermal anomalies are related to magmatic intrusion beneath the resurgent dome.

Reassessment of the Seismicity and seismic hazards of Libya

NASA Astrophysics Data System (ADS)

Ben Suleman, A.; Elmeladi, A.

2009-04-01

The tectonic evolution of Libya, located at the northern extreme of the African continent, has yielded a complex crustal structure that is composed of a series of basins and uplifts. The present day deformation of Libya is the result of the Eurasia-Africa continental collision. At the end of the year 2005, The Libyan National Seismological Network was established to monitor local, regional and teleseismic activities, as well as to provide high quality data for research projects both locally and on the regional and global scale. This study aims to discuss the seismicity of Libya by using the new data from the Libyan national seismological network and to focus on the seismic hazards. At first glance the seismic activity map shows dominant trends of seismicity with most of the seismic activity concentrated along the northern coastal areas. Four major seismic trends were quite noticeable. A first trend is a NW-SE direction coinciding with the eastern boarder of the Hun Graben. A second trend is also a NW-SE direction in the offshore area and might be a continuation of this trend. The other two trends were located in the western Gulf of Sirt and Cyrenaica platform. The rest of seismicity is diffuse either offshore or in land, with no good correlation with well-mapped faults. Detailed investigations of the Libyan seismicity indicates that the Libya has experienced earthquakes of varying magnitudes and that there is definitely a certain amount of seismic risk involved in engineering projects, particularly in the northern regions. Detailed investigation of the distribution of the Libyan earthquakes in space and time along with all other geological considerations suggested the classification of the country into four seismic zones with the Hun graben zone being the most seismically active zone.

Passive seismic imaging based on seismic interferometry: method and its application to image the structure around the 2013 Mw6.6 Lushan earthquake

NASA Astrophysics Data System (ADS)

Gu, N.; Zhang, H.

2017-12-01

Seismic imaging of fault zones generally involves seismic velocity tomography using first arrival times or full waveforms from earthquakes occurring around the fault zones. However, in most cases seismic velocity tomography only gives smooth image of the fault zone structure. To get high-resolution structure of the fault zones, seismic migration using active seismic data needs to be used. But it is generally too expensive to conduct active seismic surveys, even for 2D. Here we propose to apply the passive seismic imaging method based on seismic interferometry to image fault zone detailed structures. Seismic interferometry generally refers to the construction of new seismic records for virtual sources and receivers by cross correlating and stacking the seismic records on physical receivers from physical sources. In this study, we utilize seismic waveforms recorded on surface seismic stations for each earthquake to construct zero-offset seismic record at each earthquake location as if there was a virtual receiver at each earthquake location. We have applied this method to image the fault zone structure around the 2013 Mw6.6 Lushan earthquake. After the occurrence of the mainshock, a 29-station temporary array is installed to monitor aftershocks. In this study, we first select aftershocks along several vertical cross sections approximately normal to the fault strike. Then we create several zero-offset seismic reflection sections by seismic interferometry with seismic waveforms from aftershocks around each section. Finally we migrate these zero-offset sections to create seismic structures around the fault zones. From these migration images, we can clearly identify strong reflectors, which correspond to major reverse fault where the mainshock occurs. This application shows that it is possible to image detailed fault zone structures with passive seismic sources.

Dietary attitudes and behaviours of women in China after the 2008 Wenchuan earthquake in three seismically different zones.

PubMed

Hu, Ping; Han, Ling-Li; Hou, Feng-Gang; Xu, Xiang-Long; Sharma, Manoj; Zhao, Yong

2016-12-01

The sudden occurrence of the 2008 Wenchuan Earthquake not only devastated people's health, but also may have impacted on the psychological and dietary attitudes and behaviours of the survivors. Although the influence of natural disaster on people's health has been extensively investigated, there is a lack of information about the effects on people's dietary attitudes and behaviours. Our aim was to evaluate the influence of the Wenchuan earthquake on the dietary attitudes and behaviours of adult women from different zones of China in July 2008. 736 women, aged 18-55 years old, were randomly selected and interviewed after the earthquake. Women were selected from three zones: the earthquake zone (n=206), the shaking zone (n=326), and the non-seismic zone (n=204). Although nutrition knowledge mean scores of women in the three zones were relatively low, the women in the earthquake zone became more vigilant about the nutritional value and acceptability of food than women in the other two zones. Nevertheless, women in the earthquake zone also developed some arguably untoward, if understandable, behaviour after the disaster. They increased their consumption and tendency to stock instant food and snack items. That said, these findings were modulated by other factors such as age, residence, Body Mass Index (BMI), and nutrition knowledge itself, as were post-earthquake eating behaviours, as judged by the ordinal logistic regression analyses performed. The major Wenchuan earthquake was associated with differentials in dietary attitudes and behaviours among women by seismic zone.

Preliminary results of characteristic seismic anisotropy beneath Sunda-Banda subduction-collision zone

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

Wiyono, Samsul H., E-mail: samsul.wiyono@bmkg.go.id; Indonesia’s Agency for Meteorology Climatology and Geophysics, Jakarta 10610; Nugraha, Andri Dian, E-mail: nugraha@gf.itb.ac.id

2015-04-24

Determining of seismic anisotropy allowed us for understanding the deformation processes that occured in the past and present. In this study, we performed shear wave splitting to characterize seismic anisotropy beneath Sunda-Banda subduction-collision zone. For about 1,610 XKS waveforms from INATEWS-BMKG networks have been analyzed. From its measurements showed that fast polarization direction is consistent with trench-perpendicular orientation but several stations presented different orientation. We also compared between fast polarization direction with absolute plate motion in the no net rotation and hotspot frame. Its result showed that both absolute plate motion frame had strong correlation with fast polarization direction. Strongmore » correlation between the fast polarization direction and the absolute plate motion can be interpreted as the possibility of dominant anisotropy is in the asthenosphere.« less

Summary of workshops concerning regional seismic source zones of parts of the conterminous United States, convened by the U.S. Geological Survey, 1979-1980, Golden, Colorado

USGS Publications Warehouse

Thenhaus, P.C.; McKeown, F.A.; Bucknam, R.C.; Ross, D.C.; Anderson, R.E.; Irwin, W.P.; Russ, D.P.; Diment, W.H.; Thenhaus, Paul C.

1983-01-01

Workshops were convened by the U.S. Geological Survey to obtain the latest information and concepts relative to defining seismic source zones for five regions of the United States. The zones, with some modifications, have been used in preparation of new national probabilistic ground motion hazard maps by the U.S. Geological Survey. The five regions addressed are the Great Basin, the Northern Rocky Mountains, the Southern Rocky Mountains, the Central Interior, and the northeastern United States. Discussions at the workshops focussed on possible temporal and spatial variations of seismicity within the regions, latest ages of surface-fault displacements, most recent uplift or subsidence, geologic structural provinces as they relate to seismicity, and speculation on earthquake causes. Within the Great Basin region, the zones conform to areas characterized by a predominance of faults that have certain ages of latest surface displacements. In the Northern and Southern Rocky Mountain regions, zones primarily conform to distinctive structural terrane. In the Central Interior, primary emphasis was placed on an interpretation of the areal distribution of historic seismicity, although geophysical studies in the Reelfoot rift area provided data for defining zones in the New Madrid earthquake area. An interpretation of the historic seismicity also provided the basis for drawing the zones of the New England region. Estimates of earthquake maximum magnitudes and of recurrence times for these earthquakes are given for most of the zones and are based on either geologic data or opinion.

Evidences of Attenuation Zones Under Vesuvius Volcano By Local and Regional Seismicity

NASA Astrophysics Data System (ADS)

Cubellis, E.; Marturano, A.

The seismicity at Vesuvius is characterised by events of moderate-energy concentrated in the caldera area. The foci of events are shallow, with depths less than 6 km under sea level. Periods of greater actvity were recorded in 1989, 1990, and, more recently, in 1995 and 1996. On October, 9, 1999 an earthquake (Ml=3.6) felt outside vesuvian area took place at Vesuvius-crater. It was not only the most energetic one since the last eruption of 1944 but also one of the most energetic among those occurring in the Vesuvian area since Roman times, as shown by an analysis of historical seismicity. Following the 9 october 1999 event, questionnaires were sent to all middle schools in the Vesuvian area and surrounding towns in order to define the extent to which the earthquake had been felt. The felt index was thus obtained, which represent the per- centage response to the question: Did you feel the earthquake? and used in later data processing. The felt index is a continuous parameter and this feature makes it possible, among other things, to relate it to ground motion parameters and overcome the prob- lem of the limits involved in using integer values of intensity. In particular, Q quality factor was determined by assuming direct proportionality between energy and felt in- dex. The values obtained were Q=60-90 and, Qa=100-150, in reasonable agreement with the P-wave quality factor of 70 to 100 reported below active volcanoes, consis- tent with high temperatures and generally associated with the presence of magmatic bodies. The near Southern Apennine seismogenetic zone, 50-100 km from Vesuvius, is characterised by prevalent normal faulting and large historical earthquakes. The last, the Irpinia earthquake of November 23, 1980 (Ms=6.9), developed on three fault sources at least, with apenninic trend (NW-SE), was characterised by elevated atten- uation zones in epicentral and external areas too. In particular, the macroseismic field showed a 25 km wide circular attenuation zone

Seismic Moment, Seismic Energy, and Source Duration of Slow Earthquakes: Application of Brownian slow earthquake model to three major subduction zones

NASA Astrophysics Data System (ADS)

Ide, Satoshi; Maury, Julie

2018-04-01

Tectonic tremors, low-frequency earthquakes, very low-frequency earthquakes, and slow slip events are all regarded as components of broadband slow earthquakes, which can be modeled as a stochastic process using Brownian motion. Here we show that the Brownian slow earthquake model provides theoretical relationships among the seismic moment, seismic energy, and source duration of slow earthquakes and that this model explains various estimates of these quantities in three major subduction zones: Japan, Cascadia, and Mexico. While the estimates for these three regions are similar at the seismological frequencies, the seismic moment rates are significantly different in the geodetic observation. This difference is ascribed to the difference in the characteristic times of the Brownian slow earthquake model, which is controlled by the width of the source area. We also show that the model can include non-Gaussian fluctuations, which better explains recent findings of a near-constant source duration for low-frequency earthquake families.

Seismicity and Tectonics of the West Kaibab Fault Zone, AZ

NASA Astrophysics Data System (ADS)

Wilgus, J. T.; Brumbaugh, D. S.

2014-12-01

The West Kaibab Fault Zone (WKFZ) is the westernmost bounding structure of the Kaibab Plateau of northern Arizona. The WKFZ is a branching complex of high angle, normal faults downthrown to the west. There are three main faults within the WKFZ, the Big Springs fault with a maximum of 165 m offset, the Muav fault with 350 m of displacement, and the North Road fault having a maximum throw of approximately 90 m. Mapping of geologically recent surface deposits at or crossing the fault contacts indicates that the faults are likely Quaternary with the most recent offsets occurring <1.6 Ma. Slip rates are estimated to be less than 0.2 mm/yr. No historic fault slip has been documented. The WKFZ is one of the most seismically active areas in Arizona and lies within the Northern Arizona Seismic Belt (NASB), which stretches across northern Arizona trending NW-SE. The data set for this study includes 156 well documented events with the largest being a M5.75 in 1959 and including a swarm of seven earthquakes in 2012. The seismic data set (1934-2014) reveals that seismic activity clusters in two regions within the study area, the Fredonia cluster located in the NW corner of the study area and the Kaibab cluster located in the south central portion of the study area. The fault plane solutions to date indicate NE-SW to EW extension is occurring in the study area. Source relationships between earthquakes and faults within the WKFZ have not previously been studied in detail. The goal of this study is to use the seismic data set, the available data on faults, and the regional physiography to search for source relationships for the seismicity. Analysis includes source parameters of the earthquake data (location, depth, and fault plane solutions), and comparison of this output to the known faults and areal physiographic framework to indicate any active faults of the WKFZ, or suggested active unmapped faults. This research contributes to a better understanding of the present nature of

Seismic refraction survey in the Great Miami River Valley and vicinity, Montgomery, Warren, and Butler Counties, Ohio

USGS Publications Warehouse

Watkins, Joel S.; Spieker, Andrew M.

1964-01-01

As part of a continuing program to define the thickness and extent of water-bearing sand and gravel deposits in southwestern Ohio, the U.S. Geological Survey, in cooperation with the Ohio Division of Water and The Miami Conservancy District, completed a seismic refraction survey of the Great Miami River valley and adjacent areas between Dayton and Hamilton, Ohio, in the fall of 1963. A similar survey of the adjoining lower Great Miami River and Whitewater River valleys was completed in 1962 (Watkins, 1963; Spieker and Watkins, unpublished data).The area of the survey includes known or inferred portions of an interglacial drainage system which is deeply entrenched into bedrock. Ohio was covered by glaciers at least three times during the Pleistocene epoch. As each glacier melted, rock fragments absorbed by the glacier were transported and deposited in these buried valleys by torrents of meltwater. The total thickness of glacial drift is over 300 feet in some places. Much of the glacial material is highly permeable and saturated with large quantities of water of good quality. The underlying bedrock is virtually impermeable and yields only meager quantities of water. The cities of Dayton, Middletown, Hamilton, and many industries in the Miami River valley rely on wells in the glacial deposits as their principal source of water. The purpose of the present survey is to define the thickness and extent of these important water-bearing formations. Such information will make possible a more accurate evaluation of the area's water resources than has previously have been possible.

Maximum magnitude estimations of induced earthquakes at Paradox Valley, Colorado, from cumulative injection volume and geometry of seismicity clusters

NASA Astrophysics Data System (ADS)

Yeck, William L.; Block, Lisa V.; Wood, Christopher K.; King, Vanessa M.

2015-01-01

The Paradox Valley Unit (PVU), a salinity control project in southwest Colorado, disposes of brine in a single deep injection well. Since the initiation of injection at the PVU in 1991, earthquakes have been repeatedly induced. PVU closely monitors all seismicity in the Paradox Valley region with a dense surface seismic network. A key factor for understanding the seismic hazard from PVU injection is the maximum magnitude earthquake that can be induced. The estimate of maximum magnitude of induced earthquakes is difficult to constrain as, unlike naturally occurring earthquakes, the maximum magnitude of induced earthquakes changes over time and is affected by injection parameters. We investigate temporal variations in maximum magnitudes of induced earthquakes at the PVU using two methods. First, we consider the relationship between the total cumulative injected volume and the history of observed largest earthquakes at the PVU. Second, we explore the relationship between maximum magnitude and the geometry of individual seismicity clusters. Under the assumptions that: (i) elevated pore pressures must be distributed over an entire fault surface to initiate rupture and (ii) the location of induced events delineates volumes of sufficiently high pore-pressure to induce rupture, we calculate the largest allowable vertical penny-shaped faults, and investigate the potential earthquake magnitudes represented by their rupture. Results from both the injection volume and geometrical methods suggest that the PVU has the potential to induce events up to roughly MW 5 in the region directly surrounding the well; however, the largest observed earthquake to date has been about a magnitude unit smaller than this predicted maximum. In the seismicity cluster surrounding the injection well, the maximum potential earthquake size estimated by these methods and the observed maximum magnitudes have remained steady since the mid-2000s. These observations suggest that either these methods

GPS Imaging suggests links between climate, magmatism, seismicity, and tectonics in the Sierra Nevada-Long Valley Caldera-Walker Lane system, western United States

NASA Astrophysics Data System (ADS)

Hammond, W. C.; Blewitt, G.; Kreemer, C.; Smith, K.

2017-12-01

The Walker Lane is a region of complex active crustal transtension in the western Great Basin of the western United States, accommodating about 20% of the 50 mm/yr relative motion between the Pacific and North American plates. The Long Valley caldera lies in the central Walker Lane in eastern California, adjacent to the eastern boundary of the Sierra Nevada/Great Valley microplate, and experiences intermittent inflation, uplift, and volcanic unrest from the magma chamber that resides at middle crustal depths. Normal and transform faults accommodating regional tectonic transtension pass by and through the caldera, complicating the interpretation of the GPS-measured strain rate field, estimates of fault slip rates, and seismic hazard. Several dozen continuously recording GPS stations measure strain and uplift in the area with mm precision. They observe that the most recent episode of uplift at Long Valley began in mid-2011, continuing until late 2016, raising the surface by 100 mm in 6 years. The timing of the initiation of uplift coincides with the beginning of severe drought in California. Furthermore, the timing of a recent pause in uplift coincides with the very wet 2016-2017 winter, which saw approximately double normal snow pack. In prior studies, we showed that the timing of changes in geodetically measured uplift rate of the Sierra Nevada coincides with the timing of drought conditions in California, suggesting a link between hydrological loading and Sierra Nevada elevation. Here we take the analysis three steps further to show that changes in Sierra Nevada uplift rate coincide in time with 1) enhanced inflation at the Long Valley caldera, 2) shifts in the patterns and rates of horizontal tensor strain rate, and 3) seismicity patterns in the central Walker Lane. We use GPS solutions from the Nevada Geodetic Laboratory and the new GPS Imaging technique to produce robust animations of the time variable strain and uplift fields. The goals of this work are to

Discrimination and Assessment of Induced Seismicity in Active Tectonic Zones: A Case Study from Southern California

NASA Astrophysics Data System (ADS)

Bachmann, C. E.; Lindsey, N.; Foxall, W.; Robertson, M.

2014-12-01

Earthquakes induced by human activity have become a matter of heightened public concern during recent years. Of particular concern is seismicity associated with wastewater injection, which has included events having magnitudes greater than 5. The causes of the induced events are primarily changes in pore-pressure, fluid volume and perhaps temperature due to injection. Recent research in the US has focused on mid-continental regions having low rates of naturally-occurring seismicity, where induced events can be identified by relatively straightforward spatial and temporal correlation of seismicity with high-volume injection activities. Recent examples include events correlated with injection of wastewater in Oklahoma, Arkansas, Texas and Ohio, and long-term brine injection in the Paradox Valley in Colorado. Even in some of the cases where there appears at first sight to be a clear spatial correlation between seismicity and injection, it has been difficult to establish causality definitively. Here, we discuss methods to identify induced seismicity in active tectonic regions. We concentrate our study on Southern California, where large numbers of wastewater injection wells are located in oil-producing basins that experience moderate to high rates of naturally-occurring seismicity. Using the catalog of high-precision CISN relocations produced by Hauksson et al. (BSSA, 2012), we aim to discriminate induced from natural events based on spatio-temporal patterns of seismicity occurrence characteristics and their relationships to injection activities, known active faults and other faults favorably oriented for slip under the tectonic stress field. Since the vast majority of induced earthquakes are very small, it is crucial to include all events above the detection threshold of the CISN in each area studied. In addition to exploring the correlation of seismicity to injection activities in time and space, we analyze variations in frequency-magnitude distributions, which can

Study of time dynamics of seismicity for the Mexican subduction zone by means of the visibility graph method.

NASA Astrophysics Data System (ADS)

Ramírez-Rojas, Alejandro; Telesca, Luciano; Lovallo, Michele; Flores, Leticia

2015-04-01

By using the method of the visibility graph (VG), five magnitude time series extracted from the seismic catalog of the Mexican subduction zone were investigated. The five seismic sequences represent the seismicity which occurred between 2005 and 2012 in five seismic areas: Guerrero, Chiapas, Oaxaca, Jalisco and Michoacan. Among the five seismic sequences, the Jalisco sequence shows VG properties significantly different from those shown by the other four. Such a difference could be inherent in the different tectonic settings of Jalisco with respect to those characterizing the other four areas. The VG properties of the seismic sequences have been put in relationship with the more typical seismological characteristics (b-value and a-value of the Gutenberg-Richter law). The present study was supported by the Bilateral Project Italy-Mexico "Experimental Stick-slip models of tectonic faults: innovative statistical approaches applied to synthetic seismic sequences", jointly funded by MAECI (Italy) and AMEXCID (Mexico) in the framework of the Bilateral Agreement for Scientific and Technological Cooperation PE 2014-2016

Multiple mantle upwellings through the transition zone beneath the Afar Depression?

NASA Astrophysics Data System (ADS)

Hammond, J. O.; Kendall, J. M.; Stuart, G. W.; Thompson, D. A.; Ebinger, C. J.; Keir, D.; Ayele, A.; Goitom, B.; Ogubazghi, G.

2012-12-01

Previous seismic studies using regional deployments of sensors in East-Africa show that low seismic velocities underlie Africa, but their resolution is limited to the top 200-300km of the Earth. Thus, the connection between the low velocities in the uppermost mantle and those imaged in global studies in the lower mantle is unclear. We have combined new data from Afar, Ethiopia with 7 other regional experiments and global network stations across Kenya, Ethiopia, Eritrea, Djibouti and Yemen, to produce high-resolution models of upper mantle P- and S-wave velocities to the base of the transition zone. Relative travel time tomographic inversions show that within the transition zone two focussed sharp-sided low velocity regions exist: one beneath the Western Ethiopian plateau outside the rift valley, and the other beneath the Afar depression. Estimates of transition zone thickness suggest that this is unlikely to be an artefact of mantle discontinuity topography as a transition zone of normal thickness underlies the majority of Afar and surrounding regions. However, a low velocity layer is evident directly above the 410 discontinuity, co-incident with some of the lowest seismic velocities suggesting that smearing of a strong low velocity layer of limited depth extent may contribute to the tomographic models in north-east Afar. The combination of seismic constraints suggests that small low temperature (<50K) upwellings may rise from a broader low velocity plume-like feature in the lower mantle. This interpretation is supported by numerical and analogue experiments that suggest the 660km phase change and viscosity jump may impede flow from the lower to upper mantle creating a thermal boundary layer at the base of the transition zone. This allows smaller, secondary upwellings to initiate and rise to the surface. These, combined with possible evidence of melt above the 410 discontinuity can explain the seismic velocity models. Our images of secondary upwellings suggest that

High resolution seismics methods in application to fault zone detection

NASA Astrophysics Data System (ADS)

Matula, Rafal; Czaja, Klaudia; Mahmod, Adam Ahmed

2014-05-01

Surveys were carried out along border line between Outer Carpathians, Inner Carpathians and Pieniny Klippen Belt. Main point of interest was imaging transition zone structured by para-conglomerates, sandstone and clays lenses, crossing in near neighbourhood of Stare Bystre, village in the southern part of Poland. Actually geological works states existence of two hypothetical faults, first at the direction NE-SW and second NNW-SSE. Main aim of geological and geophysical investigation was to prove that mentioned fault has a system of smaller discontinuities connected with previous main fault activity. Para-conglomerate exposures, which is localized close to discussed fault is cut by visible system of cracks. That fact provide geological evidences that this system could be the effect of previous fault activity so in other words, it has a continuation up to main discontinuities. What is more part of the same formation para-conglomerates is covered by Neogen river sediments, so non-direct detection methods of cracks azimuth must be applied. Geophysical investigation was located near mentioned exposure and conducted in 3-D variant. Measurements were extremely focused on determining any changes of elevation buried para-conglomerates and velocity variation inside studied sediments. Seismic methods such as refraction and refraction tomography were used to imaging bedrock. Surveys were carried out in non typical acquisition, azimuthal schema. During field works 24- channels seismograph and 4 Hz, 10 Hz and 100 Hz geophones were used. Hypothetical discontinuities were estimated after analysing seismic records and expressed by velocity variation in bedding rocks and additionally evaluated changes in its elevation. Furthermore, in this study attempt of use refraction wave attributes related to loosing rock - para-conglomerates continuity were exposed. The presentation of geophysical data had a volumetric character what was easier to interpret and better related to assumptions

Characterization of a 21-Story Reinforced Building in the Valley of Mexico Using MEMS Accelerometers.

NASA Astrophysics Data System (ADS)

Husker, A. L.; Dominguez, L. A.; Becerril, A.; Espejo, L.; Cochran, E. S.

2014-12-01

Low cost MEMS accelerometers are becoming increasingly higher resolution making them useful in strong motion studies. Here we present a building response analysis in the lakebed zone of the Valley of Mexico. The Valley of Mexico represents one of the highest seismic risk locations in the world and incorporates Mexico City and part of Mexico State. More than 20 million people live there and it is the political and economic center of Mexico. In addition the valley has very high site effects with amplifications 100 - 500 times that of sites outside of the basin (Singh et al., 1988; Singh et al., 1995). We instrumented a 21-story building with MEMS accelerometers as part of the Quake Catcher Network or Red Atrapa Sismos as it is called in Mexico. The building known as the Centro Cultural de Tlateloco is located in an important historical and political area as well as a zone with some of the highest amplifications in the Valley of Mexico that had some of the worst destruction after the 1985 M8.1 Michoacan earthquake. During the earthquake most of the buildings that failed were between 7 - 18 stories tall. The peak accelerations near Tlateloco were at periods of 2 seconds. Since the earthquake the building has been retrofitted with N-S crossing supports to help withstand another earthquake. We present the measurements of frequencies and amplifications between floors for the length of the building.

Seismic risk assessment of Navarre (Northern Spain)

NASA Astrophysics Data System (ADS)

Gaspar-Escribano, J. M.; Rivas-Medina, A.; García Rodríguez, M. J.; Benito, B.; Tsige, M.; Martínez-Díaz, J. J.; Murphy, P.

2009-04-01

The RISNA project, financed by the Emergency Agency of Navarre (Northern Spain), aims at assessing the seismic risk of the entire region. The final goal of the project is the definition of emergency plans for future earthquakes. With this purpose, four main topics are covered: seismic hazard characterization, geotechnical classification, vulnerability assessment and damage estimation to structures and exposed population. A geographic information system is used to integrate, analyze and represent all information colleted in the different phases of the study. Expected ground motions on rock conditions with a 90% probability of non-exceedance in an exposure time of 50 years are determined following a Probabilistic Seismic Hazard Assessment (PSHA) methodology that includes a logic tree with different ground motion and source zoning models. As the region under study is located in the boundary between Spain and France, an effort is required to collect and homogenise seismological data from different national and regional agencies. A new homogenised seismic catalogue, merging data from Spanish, French, Catalonian and international agencies and establishing correlations between different magnitude scales, is developed. In addition, a new seismic zoning model focused on the study area is proposed. Results show that the highest ground motions on rock conditions are expected in the northeastern part of the region, decreasing southwards. Seismic hazard can be expressed as low-to-moderate. A geotechnical classification of the entire region is developed based on surface geology, available borehole data and morphotectonic constraints. Frequency-dependent amplification factors, consistent with code values, are proposed. The northern and southern parts of the region are characterized by stiff and soft soils respectively, being the softest soils located along river valleys. Seismic hazard maps including soil effects are obtained by applying these factors to the seismic hazard maps

First results on the crustal structure of the Natal Valley from combined wide-angle and reflection seismic data (MOZ3/5 cruise), South Mozambique Margin.

NASA Astrophysics Data System (ADS)

Leprêtre, Angélique; Verrier, Fanny; Evain, Mikael; Schnurle, Philippe; Watremez, Louise; Aslanian, Daniel; de Clarens, Philippe; Dias, Nuno; Afilhado, Alexandra; Leroy, Sylvie; d'Acremont, Elia; Castilla, Raymi; Moulin, Maryline

2017-04-01

The Natal valley (South Mozambique margin) is a key area for the understanding of the SW Indian Ocean history since the Gondwana break-up, and widely, the structure of a margin system at the transition between divergent and strike-slip segments. As one part of the PAMELA project (PAssive Margins Exploration Laboratories), conducted by TOTAL, IFREMER, in collaboration with Université de Bretagne Occidentale, Université Rennes 1, Université Pierre and Marie Curie, CNRS et IFPEN, the Natal Valley and the East Limpopo margin have been explored during the MOZ3/5 cruise (2016), conducted onboard the R/V Pourquoi Pas?, through the acquisition of 7 wide-angle profiles and coincident marine multichannel (720 traces) seismic as well as potential field data. Simultaneously, land seismometers were deployed in the Mozambique coastal plains, extending six of those profiles on land for about 100 km in order to provide information on the onshore-offshore transition. Wide-angle seismic data are of major importance as they can provide constrains on the crustal structure of the margin and the position of the continent-ocean boundary in an area where the crustal nature is poorly known and largely controversial. The aim of this work is to present the first results on the crustal structure from P-waves velocity modeling along two perpendicular MZ1 & MZ7 wide-angle profiles crossing the Natal Valley in an E-W and NNW-SSE direction respectively, which reveal a crust up to 30 km thick below the Natal Valley and thus raises questions of a purely oceanic origin of the Valley. The post-doc of Angélique Leprêtre is co-funded by TOTAL and IFREMER as part of the PAMELA (Passive Margin Exploration Laboratories) scientific project.

Earthquakes in the Orozco transform zone: seismicity, source mechanisms, and tectonics

USGS Publications Warehouse

Tréhu, Anne M.; Solomon, Sean C.

1983-01-01

As part of the Rivera Ocean Seismic Experiment, a network of ocean bottom seismometers and hydrophones was deployed in order to determine the seismic characteristics of the Orozco transform fault in the central eastern Pacific. We present hypocentral locations and source mechanisms for 70 earthquakes recorded by this network. All epicenters are within the transform region of the Orozco Fracture Zone and clearly delineate the active plate boundary. About half of the epicenters define a narrow line of activity parallel to the spreading direction and situated along a deep topographic trough that forms the northern boundary of the transform zone (region 1). Most focal depths for these events are very shallow, within 4 km of the seafloor; several well-determined focal depths, however, are as great as 7 km. No shallowing of seismic activity is observed as the rise-transform intersection is approached; to the contrary, the deepest events are within 10 km of the intersection. First motion polarities for most of the earthquakes in region 1 are compatible with right-lateral strike slip faulting along a nearly vertical plane, striking parallel to the spreading direction. Another zone of activity is observed in the central part of the transform (region 2). The apparent horizontal and vertical distribution of activity in this region is more scattered than in the first, and the first motion radiation patterns of these events do not appear to be compatible with any known fault mechanism. Pronounced lateral variations in crustal velocity structure are indicated for the transform region from refraction data and measurements of wave propagation directions. The effect of this lateral heterogeneity on hypocenters and fault plane solutions is evaluated by tracing rays through a three-dimensional velocity grid. While findings for events in region 1 are not significantly affected, in region 2, epicentral mislocations of up to 10 km and azimuthal deflections of up to 45° may result from

Deep seismic reflection evidence for ancient subduction and collision zones within the continental lithosphere of northwestern Europe

NASA Astrophysics Data System (ADS)

Balling, N.

2000-12-01

Deep seismic profiling experiments in the region of NW Europe (including BABEL in the Gulf of Bothnia and the Baltic Sea, Mobil Search in the Skagerrak and MONA LISA in the North Sea) have demonstrated the existence of seismic reflectors in the mantle lithosphere beneath the Baltic Shield, the Tornquist Zone and the North Sea basins. Different sets of reflectors are observed, notably dipping and sub-horizontal. Dipping, distinct reflectivity, which may be followed from Moho/Moho offsets into the deeper parts of the continental lithosphere, is of special interest because of its tectonic and geodynamic significance. Such reflectivity, observed in several places, dipping 15-35° and covering a depth range of 30-90 km, constrained by surface geological information and radiometric age data, is interpreted to represent fossil, ancient subduction and collison zones. Subduction slabs with remnant oceanic basaltic crust transformed into eclogite is assumed, in particular, to generate deep seismic reflectivity. Deep seismic evidence is presented for subduction, crustal accretion and collision processes with inferred ages from 1.9 to 1.1 Ga from the main structural provinces within the Baltic Shield including Svecofennian, Transscandinavian Igneous Belt, Gothian and Sveconorwegian. Along the southwestern border of Baltica (in the southeastern North Sea) south-dipping crustal and sub-crustal reflectivity is observed down to a depth of about 90 km, close to the lithosphere-asthenosphere boundary. These structures are interpreted to reveal a lithosphere-scale Caledonian (ca. 440 Ma) suture zone resulting from the closure of the Tornquist Sea/Thor Ocean and the amalgamation of Baltica and Eastern Avalonia. These results demonstrate that deep structures within the continental lithosphere, originating from early crust-forming plate tectonic processes, may survive for a very long time and form seismic marker reflectivity of great value in geotectonic interpretation and

Seismic interpretation of the deep structure of the Wabash Valley Fault System

USGS Publications Warehouse

Bear, G.W.; Rupp, J.A.; Rudman, A.J.

1997-01-01

Interpretations of newly available seismic reflection profiles near the center of the Illinois Basin indicate that the Wabash Valley Fault System is rooted in a series of basement-penetrating faults. The fault system is composed predominantly of north-northeast-trending high-angle normal faults. The largest faults in the system bound the 22-km wide 40-km long Grayville Graben. Structure contour maps drawn on the base of the Mount Simon Sandstone (Cambrian System) and a deeper pre-Mount Simon horizon show dip-slip displacements totaling at least 600 meters across the New Harmony fault. In contrast to previous interpretations, the N-S extent of significant fault offsets is restricted to a region north of 38?? latitude and south of 38.35?? latitude. This suggests that the graben is not a NE extension of the structural complex composed of the Rough Creek Fault System and the Reelfoot Rift as previously interpreted. Structural complexity on the graben floor also decreases to the south. Structural trends north of 38?? latitude are offset laterally across several large faults, indicating strike-slip motions of 2 to 4 km. Some of the major faults are interpreted to penetrate to depths of 7 km or more. Correlation of these faults with steep potential field gradients suggests that the fault positions are controlled by major lithologic contacts within the basement and that the faults may extend into the depth range where earthquakes are generated, revealing a potential link between specific faults and recently observed low-level seismicity in the area.

Sublacustrine river valley in the shelf zone of the Black Sea parallel to the Bulgarian coast

NASA Astrophysics Data System (ADS)

Preisinger, A.; Aslanian, S.; Beigelbeck, R.; Heinitz, W.-D.

2009-04-01

The considered sublacustrine river valley is situated in the shelf zone of the Black Sea. It runs in parallel to the Bulgarian coast, was formed in the time period of the Younger Dryas (Preisinger et al., 2005), and features an inclination of about 0.5 m/km. An about 200 km long sediment wall separates the approximately 10 km broad river valley from the outside shelf zone. This wall was generated during the Older Dryas until the beginning of the Younger Dryas. Its shape was formed by transportation of water and sediment from the Strait of Kerch by a circulating rim current in the Black Sea and water as well as sediment flow of the Danube in direction to the Bosporus. New investigations of the sediments of this river valley were performed by utilizing a Sediment Echo Sounder (SES 2000). This Echo Sounder is a parametric sub-bottom profiler enabling a high resolution sub-bottom analyses. It is capable of penetrating sea beds up to more than 50 m of water depth. The received echo data are real-time processed. The signal amplitudes are valuated in context to a logarithmic scale and graphically visualized by means of a colorized echogram utilizing false colours ranging from red for a high to blue representing a low signal (W.-D. Heinitz et al., 1998). The highest signal (red) is given by the acoustic impedance of the boundary between sea water and river sediment. The echograms of the river valley depict spatially isolated (red) high-signal peaks, which are periodically repeated in vertical direction between the sediment surface and the bottom of the valley. The number of these high-signal parts increase with an increasing valley depth. Studying of the distribution of these peaks allows to draw conclusions regarding the content and composition of the sediment. This prediction of the sediment composition obtained by means of the SES 2000 was successfully verified by analyzing a gravity core taken near Nos Maslen (at 44 m water depth) with a particular focus on the water

Zircon oxygen isotopes reveal Ivrea-Verbano Zone source characteristics of the Sesia Valley Caldera

NASA Astrophysics Data System (ADS)

Economos, R. C.; Quick, J. E.; Sinigoi, S.; de Silva, S. L.

2013-12-01

The Sesia Valley, in the Italian Alpine foothills, contains >14 km diameter caldera adjacent to and structurally shallower than the famous Ivrea-Verbano Zone deep crustal section. The caldera and its associated eruptive sequence presents opportunity to explore volcanic magmatism in light of exposed and well characterized source candidates, namely lower crustal gabbros and the mid-crustal metasedimentary Kinzigite formation. Original geochemical characteristics of volcanic units have been obscured by the effects of subsequent hydrothermal alteration. The resistance of the mineral zircon to fluid alteration makes it a prime candidate for the preservation and exploration of these geochemical signals, such as O isotopes. Lower crustal gabbros in the Ivrea-Verbano Zone have broadly monotonic whole-rock δ18O values between +8 and +9‰VSMOW (Sinigoi et al., 1994). Kinzigites preserve a much higher and more heterogeneous δ18O values, typically ranging from +10‰ up to +15‰ (Baker, 1990). Zircons from the caldera-forming rhyolitic eruption units and a pre-caldera rhyodacitic unit were analyzed by ion microprobe at UCLA for in-situ oxygen isotope ratios. External reproducibility of within-mount standard R33 grains range from 0.27 to 0.36‰. Rhyolites from the caldera-forming eruption yield a range of δ18O(zircon) values from 6.3‰ to 8.3‰. This range displays rough correlation with CL activity - CL active grains have lower δ18O(zircon) values while CL dark grains have higher δ18O(zircon) values. This variation may correlate with U contents, which are notoriously low in zircons from Ivrea-Verbano Zone gabbros. We argue that the range in O isotope values suggests zircons are a good fit for magmas influenced by gabbro and Kinzigite sources. However, these zircons do not appear to be inherited directly from either the gabbro or Kinzigite sources as their O isotope signatures are typically intermediate between the two. The pre-caldera rhyodacite sample displays a

Infrasound and seismic array analysis of snow avalanches: results from the 2015-2017 experiment in Dischma valley above Davos, Switzerland

NASA Astrophysics Data System (ADS)

Marchetti, Emanuele; van Herwijnen, Alec; Ripepe, Maurizio

2017-04-01

While flowing downhill a snow avalanche radiates seismic and infrasonic waves being coupled both with the ground and the atmosphere. Infrasound waves are mostly generated by the powder cloud of the avalanche, while seismic waves are mostly generated by the dense flowing snow mass on the ground, resulting in different energy partitioning between seismic and infrasound for different kinds of avalanches. This results into a general uncertainty on the efficiency of seismic and infrasound monitoring, in terms of the size and source-to-receiver distance of detectable events. Nevertheless, both seismic and infrasound have been used as monitoring systems for the remote detection of snow avalanches, being the reliable detection of snow avalanches of crucial importance to better understand triggering mechanisms, identify possible precursors, or improve avalanche forecasting. We present infrasonic and seismic array data collected during the winters of 2015- 2016 and 2016-2017 in the Dischma valley above Davos, Switzerland, where a five element infrasound array and a 7 element seismic array had been deployed at short distance from each other and with several avalanche paths nearby. Avalanche observation in the area is performed through automatic cameras providing additional information on the location, type (dry or wet), size and occurrence time of the avalanches released. The use of arrays instead of single sensors allows increasing the signal-to-noise ratio and identifying events in terms of back-azimuth and apparent velocity of the wave-field, thus providing indication on the source position of the recorded signal. For selected snow avalanches captured with automatic cameras, we therefore perform seismic and infrasound array processing to constrain the avalanche path and dynamics and investigate the partitioning of seismic and infrasound energy for the different portions of the avalanche path. Moreover we compare results of seismic and infrasound array processing for the

Heterogeneous distribution of water in the mantle transition zone beneath United States inferred from seismic observations

NASA Astrophysics Data System (ADS)

Wang, Y.; Pavlis, G. L.; Li, M.

2017-12-01

The amount of water in the Earth's deep mantle is critical for the evolution of the solid Earth and the atmosphere. Mineral physics studies have revealed that Wadsleyite and Ringwoodite in the mantle transition zone could store several times the volume of water in the ocean. However, the water content and its distribution in the transition zone remain enigmatic due to lack of direct observations. Here we use seismic data from the full deployment of the Earthscope Transportable Array to produce 3D image of P to S scattering of the mantle transition zone beneath the United States. We compute the image volume from 141,080 pairs of high quality receiver functions defined by the Earthscope Automated Receiver Survey, reprocessed by the generalized iterative deconvolution method and imaged by the plane wave migration method. We find that the transition zone is filled with previously unrecognized small-scale heterogeneities that produce pervasive, negative polarity P to S conversions. Seismic synthetic modeling using a point source simulation method suggests two possible structures for these objects: 1) a set of randomly distributed blobs of slight difference in size, and 2) near vertical diapir structures from small scale convections. Combining with geodynamic simulations, we interpret the observation as compositional heterogeneity from small-scale, low-velocity bodies that are water enriched. Our results indicate there is a heterogeneous distribution of water through the entire mantle transition zone beneath the contiguous United States.

The energy release in earthquakes, and subduction zone seismicity and stress in slabs. Ph.D. Thesis

NASA Technical Reports Server (NTRS)

Vassiliou, M. S.

1983-01-01

Energy release in earthquakes is discussed. Dynamic energy from source time function, a simplified procedure for modeling deep focus events, static energy estimates, near source energy studies, and energy and magnitude are addressed. Subduction zone seismicity and stress in slabs are also discussed.

Seismic hazard estimation of northern Iran using smoothed seismicity

NASA Astrophysics Data System (ADS)

Khoshnevis, Naeem; Taborda, Ricardo; Azizzadeh-Roodpish, Shima; Cramer, Chris H.

2017-07-01

This article presents a seismic hazard assessment for northern Iran, where a smoothed seismicity approach has been used in combination with an updated seismic catalog and a ground motion prediction equation recently found to yield good fit with data. We evaluate the hazard over a geographical area including the seismic zones of Azerbaijan, the Alborz Mountain Range, and Kopeh-Dagh, as well as parts of other neighboring seismic zones that fall within our region of interest. In the chosen approach, seismic events are not assigned to specific faults but assumed to be potential seismogenic sources distributed within regular grid cells. After performing the corresponding magnitude conversions, we decluster both historical and instrumental seismicity catalogs to obtain earthquake rates based on the number of events within each cell, and smooth the results to account for the uncertainty in the spatial distribution of future earthquakes. Seismicity parameters are computed for each seismic zone separately, and for the entire region of interest as a single uniform seismotectonic region. In the analysis, we consider uncertainties in the ground motion prediction equation, the seismicity parameters, and combine the resulting models using a logic tree. The results are presented in terms of expected peak ground acceleration (PGA) maps and hazard curves at selected locations, considering exceedance probabilities of 2 and 10% in 50 years for rock site conditions. According to our results, the highest levels of hazard are observed west of the North Tabriz and east of the North Alborz faults, where expected PGA values are between about 0.5 and 1 g for 10 and 2% probability of exceedance in 50 years, respectively. We analyze our results in light of similar estimates available in the literature and offer our perspective on the differences observed. We find our results to be helpful in understanding seismic hazard for northern Iran, but recognize that additional efforts are necessary to

Implications of Preliminary Gravity and Magnetic Surveys to the Understanding of the Bartlett Springs Fault Zone, Northern California Coast Ranges

NASA Astrophysics Data System (ADS)

Langenheim, V. E.; Jachens, R. C.; Morin, R. L.; McCabe, C. M.; Page, W. D.

2007-12-01

of this anomaly, caused by ultramafic rocks, coincides with the Bartlett Springs fault zone for nearly 15 km. Lake magnetic data indicate as many as three right-stepping strands of the Bartlett Springs fault zone within the gravity- defined pull-apart basin. Two pairs of magnetic anomalies appear to be dextrally offset along the fault, arguing for about 8-9 km of cumulative offset on the fault since the passage of the triple junction at about 3.5 Ma. This estimate is similar to proposed offsets of the Eel River (8.6-10.9 km) at Lake Pillsbury. The minimum long-term slip rate is thus 2.3-3.1 mm/yr, considerably slower than geodetic rates of 5-8 mm/yr. Seismicity forms a 5-km-wide diffuse zone along the Bartlett Springs fault zone in the Lake Pillsbury area, with fewer earthquakes about 5 km northwest of the lake and its associated magnetic anomaly. The McCreary Glade seismicity lineament, located between Potter Valley and Lake Pillsbury, has been attributed to a dike intrusion at depth or reactivation of an older structure. These earthquakes coincide with the northeast edge of a 100-km-long belt of aeromagnetic anomalies and thus appear to have reactivated an older basement feature. The coincidence of the Bartlett Springs fault zone and significant gravity gradients also argues that the much younger fault zone has reactivated older basement features. Our analysis shows that a modern, high-resolution aeromagnetic survey is needed to confirm these preliminary interpretations.

Compressive sensing of frequency-dependent seismic radiation from subduction zone megathrust ruptures

PubMed Central

Yao, Huajian; Shearer, Peter M.; Gerstoft, Peter

2013-01-01

Megathrust earthquakes rupture a broad zone of the subducting plate interface in both along-strike and along-dip directions. The along-dip rupture characteristics of megathrust events, e.g., their slip and energy radiation distribution, reflect depth-varying frictional properties of the slab interface. Here, we report high-resolution frequency-dependent seismic radiation of the four largest megathrust earthquakes in the past 10 y using a compressive-sensing (sparse source recovery) technique, resolving generally low-frequency radiation closer to the trench at shallower depths and high-frequency radiation farther from the trench at greater depths. Together with coseismic slip models and early aftershock locations, our results suggest depth-varying frictional properties at the subducting plate interfaces. The shallower portion of the slab interface (above ∼15 km) is frictionally stable or conditionally stable and is the source region for tsunami earthquakes with large coseismic slip, deficient high-frequency radiation, and few early aftershocks. The slab interface at intermediate depths (∼15–35 km) is the main unstable seismogenic zone for the nucleation of megathrust quakes, typically with large coseismic slip, abundant early aftershocks, and intermediate- to high-frequency radiation. The deeper portion of the slab interface (∼35–45 km) is seismically unstable, however with small coseismic slip, dominant high-frequency radiation, and relatively fewer aftershocks.

Scaling of the critical slip distance for seismic faulting with shear strain in fault zones

USGS Publications Warehouse

Marone, Chris; Kilgore, Brian D.

1993-01-01

THEORETICAL and experimentally based laws for seismic faulting contain a critical slip distance1-5, Dc, which is the slip over which strength breaks down during earthquake nucleation. On an earthquake-generating fault, this distance plays a key role in determining the rupture nucleation dimension6, the amount of premonitory and post-seismic slip7-10, and the maximum seismic ground acceleration1,11. In laboratory friction experiments, Dc has been related to the size of surface contact junctions2,5,12; thus, the discrepancy between laboratory measurements of Dc (??? 10-5 m) and values obtained from modelling earthquakes (??? 10-2 m) has been attributed to differences in roughness between laboratory surfaces and natural faults5. This interpretation predicts a dependence of Dc on the particle size of fault gouge 2 (breccia and wear material) but not on shear strain. Here we present experimental results showing that Dc scales with shear strain in simulated fault gouge. Our data suggest a new physical interpretation for the critical slip distance, in which Dc is controlled by the thickness of the zone of localized shear strain. As gouge zones of mature faults are commonly 102-103 m thick13-17, whereas laboratory gouge layers are 1-10 mm thick, our data offer an alternative interpretation of the discrepancy between laboratory and field-based estimates of Dc.

Analysis of seismic waves crossing the Santa Clara Valley using the three-component MUSIQUE array algorithm

NASA Astrophysics Data System (ADS)

Hobiger, Manuel; Cornou, Cécile; Bard, Pierre-Yves; Le Bihan, Nicolas; Imperatori, Walter

2016-10-01

We introduce the MUSIQUE algorithm and apply it to seismic wavefield recordings in California. The algorithm is designed to analyse seismic signals recorded by arrays of three-component seismic sensors. It is based on the MUSIC and the quaternion-MUSIC algorithms. In a first step, the MUSIC algorithm is applied in order to estimate the backazimuth and velocity of incident seismic waves and to discriminate between Love and possible Rayleigh waves. In a second step, the polarization parameters of possible Rayleigh waves are analysed using quaternion-MUSIC, distinguishing retrograde and prograde Rayleigh waves and determining their ellipticity. In this study, we apply the MUSIQUE algorithm to seismic wavefield recordings of the San Jose Dense Seismic Array. This array has been installed in 1999 in the Evergreen Basin, a sedimentary basin in the Eastern Santa Clara Valley. The analysis includes 22 regional earthquakes with epicentres between 40 and 600 km distant from the array and covering different backazimuths with respect to the array. The azimuthal distribution and the energy partition of the different surface wave types are analysed. Love waves dominate the wavefield for the vast majority of the events. For close events in the north, the wavefield is dominated by the first harmonic mode of Love waves, for farther events, the fundamental mode dominates. The energy distribution is different for earthquakes occurring northwest and southeast of the array. In both cases, the waves crossing the array are mostly arriving from the respective hemicycle. However, scattered Love waves arriving from the south can be seen for all earthquakes. Combining the information of all events, it is possible to retrieve the Love wave dispersion curves of the fundamental and the first harmonic mode. The particle motion of the fundamental mode of Rayleigh waves is retrograde and for the first harmonic mode, it is prograde. For both modes, we can also retrieve dispersion and ellipticity

Spatial relationships between crustal structures and mantle seismicity in the Vrancea Seismogenic Zone of Romania: Implications for geodynamic evolution

NASA Astrophysics Data System (ADS)

Enciu, Dana-Mihaela

Integration of active and passive-source seismic data is employed to study the relationships between crustal structures and seismicity in the SE Carpathian foreland of Romania, and the connection with the Vrancea Seismogenic Zone. Relocated crustal epicenters and focal mechanisms are correlated with industry seismic profiles Comanesti, Ramnicu Sarat, Braila and Buzau, the reprocessed DACIA PLAN profile and the DRACULA (Deep Reflection Acquisition Constraining Unusual Lithospheric Activity) II and III profiles in order to understand the link between neo-tectonic foreland deformation and Vrancea mantle seismicity. Projection of crustal foreland hypocenters onto deep seismic profiles identified active crustal faults suggesting a mechanical coupling between sedimentary, crustal and upper mantle structures on the Trotus, Sinaia and newly observed Ialomita Faults. Seismic reflection imaging revealed the absence of west dipping reflectors in the crust and an east dipping to horizontal Moho in the proximity of the Vrancea area. These findings argue against both 'subduction-in-place' and 'slab break-off' as viable mechanisms for generating Vrancea mantle seismicity.

Characterizing a large shear-zone with seismic and magnetotelluric methods: The case of the Dead Sea Transform

USGS Publications Warehouse

Maercklin, N.; Bedrosian, P.A.; Haberland, C.; Ritter, O.; Ryberg, T.; Weber, M.; Weckmann, U.

2005-01-01

Seismic tomography, imaging of seismic scatterers, and magnetotelluric soundings reveal a sharp lithologic contrast along a ???10 km long segment of the Arava Fault (AF), a prominent fault of the southern Dead Sea Transform (DST) in the Middle East. Low seismic velocities and resistivities occur on its western side and higher values east of it, and the boundary between the two units coincides partly with a seismic scattering image. At 1-4 km depth the boundary is offset to the east of the AF surface trace, suggesting that at least two fault strands exist, and that slip occurred on multiple strands throughout the margin's history. A westward fault jump, possibly associated with straightening of a fault bend, explains both our observations and the narrow fault zone observed by others. Copyright 2005 by the American Geophysical Union.

Strain distribution across magmatic margins during the breakup stage: Seismicity patterns in the Afar rift zone

NASA Astrophysics Data System (ADS)

Brown, C.; Ebinger, C. J.; Belachew, M.; Gregg, T.; Keir, D.; Ayele, A.; Aronovitz, A.; Campbell, E.

2008-12-01

Fault patterns record the strain history along passive continental margins, but geochronological constraints are, in general, too sparse to evaluate these patterns in 3D. The Afar depression in Ethiopia provides a unique setting to evaluate the time and space relations between faulting and magmatism across an incipient passive margin that formed above a mantle plume. The margin comprises a high elevation flood basalt province with thick, underplated continental crust, a narrow fault-line escarpment underlain by stretched and intruded crust, and a broad zone of highly intruded, mafic crust lying near sealevel. We analyze fault and seismicity patterns across and along the length of the Afar rift zone to determine the spatial distribution of strain during the final stages of continental breakup, and its relation to active magmatism and dike intrusions. Seismicity data include historic data and 2005-2007 data from the collaborative US-UK-Ethiopia Afar Geodynamics Project that includes the 2005-present Dabbahu rift episode. Earthquake epicenters cluster within discrete, 50 km-long magmatic segments that lack any fault linkage. Swarms also cluster along the fault-line scarp between the unstretched and highly stretched Afar rift zone; these earthquakes may signal release of stresses generated by large lateral density contrasts. We compare Coulomb static stress models with focal mechanisms and fault kinematics to discriminate between segmented magma intrusion and crank- arm models for the central Afar rift zone.

Deformation fabrics of blueschist facies phengite-rich, epidote-glaucophane schists from Ring Mountain, California and implications for seismic anisotropy in subduction zone

NASA Astrophysics Data System (ADS)

Jung, H.; HA, Y.; Raymond, L. A.

2016-12-01

In many subduction zones, strong seismic anisotropy is observed. A part of the seismic anisotropy can be attributed to the subducting oceanic crust, which is transformed to blueschist facies rocks under high-pressure, high-temperature conditions. Because glaucophane, epidote, and phengite constituting the glaucophane schists are very anisotropic elastically, seismic anisotropy in the oceanic crust in hot subduction zones can be attributed to the lattice preferred orientation (LPO) of these minerals. We studied deformation fabrics and seismic properties of phengite-rich, epidote-glaucophane schists from the Franciscan Complex of Ring Mountain, California. The blueschist samples are mainly composed of glaucophane, epidote, and phengite, with minor garnet, titanite, and chlorite. Some samples contain abundant phengite (up to 40 %). We determined LPOs of minerals using SEM/EBSD and calculated seismic anisotropy of minerals and whole rocks. LPOs of glaucophane have [001] axes aligned subparallel to lineation, and both (110) poles and [100] axes subnormal to foliation. Epidote [001] axes are aligned subnormal to foliation, with both (110) and (010) poles subparallel to lineation. LPOs of phengite are characterized by maxima of [001] axes subnormal to foliation, and both (110) and (010) poles and [100] axes aligned in a girdle subparallel to foliation. Phengite showed much stronger seismic anisotropy (AVP = 42%, max.AVS = 37%) than glaucophane or epidote. Glaucophane schist with abundant phengite showed much stronger seismic anisotropy (AVP = 30%, max.AVS = 23%) than epidote-glaucophane schist without phengite (AVP = 13%, max.AVS = 9%). Therefore, phengite clearly can significantly affect seismic anisotropy of whole rocks. When the subduction angle of phengite-rich blueschist facies rocks is considered for a 2-D corner flow model, the polarization direction of fast S-waves for vertically propagating S-waves changed to a nearly trench-parallel direction for the subduction

Detailed Image of the Subducting Plate and Upper mantle Seismic Discontinuities in the Mariana Subduction Zone

NASA Astrophysics Data System (ADS)

Tibi, R.; Wiens, D. A.; Shiobara, H.; Sugioka, H.; Yuan, X.

2006-12-01

We use P-to-S converted teleseismic phases recorded at island and ocean bottom stations in Mariana to image the subducting plate and the upper mantle seismic discontinuities in the Mariana subduction zone. The land and seafloor stations which operated from June 2003 to May 2004, were deployed within the framework of the MARGINS Subduction Factory experiment of the Mariana system. The crust in the sudducting plate is observed at about 80--90 km depth beneath the islands of Saipan, Tinian and Rota. For most of the island stations, a low velocity layer is imaged in the forearc at depth between about 20 and 60 km, with decreasing depths toward the arc. The nature of this feature is not yet clear. We found evidence for double seismic discontinuities at the base of the transition zone near the Mariana slab. A shallower discontinuity is imaged at depths of ~650--715 km, and a deeper interface lies at ~740-- 770 km depth. The amplitudes of the seismic signals suggest that the shear velocity contrasts across the two features are comparable. These characteristics support the interpretation that the discontinuities are the results of the phase transformations in olivine (ringwoodite to post-spinel) and garnet (ilminite to perovskite), respectively, for the pyrolite model of mantle composition.

Evidence of a possible NNE-trending fault zone in the Summerville, South Carolina, area from shallow seismic reflection surveys

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

Marple, R.T.; Talwani, P.

1994-03-01

Five high-resolution seismic-reflection surveys trending approximately WNW-ESE and totaling about 31 km were acquired in the Summerville, South Carolina, area. The surveys trend across the postulated Woodstock fault zone. These newly acquired data together with earlier data revealed the existence of an [approximately]50-km-long feature associated with gentle warping of the shallow sediments that lies along a recently described zone of river anomalies (ZRA). The first ([approximately]5.9-km-long) seismic reflection profile located about 14 km NNE of Summerville revealed that the J reflector (basalt) at about 670 m depth is offset about 30--40 m with the west side up. The overlying sedimentsmore » displayed upwarping rather than brittle offset. A second ([approximately]6.7-km-long) survey located along interstate Highway 26 revealed as much as 30--40 m of upwarping of the sediments above about 450 m depth. A third ([approximately]7.3-km-long) profile acquired through the town of Summerville revealed four, [approximately]200--300 m wide, nearly vertical zones in which the reflectors are noncoherent. Away from these zones the reflectors are relatively flat and are slightly higher on the west side of each zone. The fourth (3-km-long) survey was located about 5 km SW of Middleton Gardens and indicated minor faulting at about 500 m depth. The fifth ([approximately]6.4-km-long) seismic survey acquired just north of Ravenel revealed an [approximately]0.5-km-wide zone in which the reflectors in the top 350 m displayed as much as 20 m of upwarping. On all the surveys, except for the first, the basalt was at too great a depth to be resolved.« less

Neogene Rift Propagation of the East African Rift System (EARS) into Central Africa and its Implications: Tectonic, Topographic and Geomorphic Impacts of the Luangwa and Luapula Rift Valleys on the Upper Congo Drainage Basin, Lake Bangweulu Wetlands and the Development of the Diffuse Southwestern Tip of the EARS.

NASA Astrophysics Data System (ADS)

Daly, M. C.; Watts, A. B.

2017-12-01

Integration of geomorphology, seismic reflection and gravity data, seismicity, DEM analysis and modelling defines a zone of NE/SW trending rifts extending into Central and SW Africa, orthogonal to the conventionally defined East African Rift System (EARS). These large-scale tectonic features have a relatively low level of seismicity and volcanism compared to the EARS, yet they generate significant topography and control the upper Congo drainage basin. They may also represent the beginning of an active but diffuse plate boundary developing to the southwest across Central Africa. The dominant feature of this broad zone is the Luangwa Rift Valley of eastern Zambia. Seismic reflection data show the Luangwa Rift developed as a thick ( 5km) Permo-Triassic basin. Inverted in the Mesozoic, it then experienced major Neogene extensional reactivation. The latter resulted in today's major border faults of varying polarity, with fault plane escarpments of up to 1000m, and associated rift flank uplifts that elevate the Central African plateau surface by 200 m. Late Miocene alluvial fans indicate a minimum age for the initiation of reactivation. Although having similar structural features to the EARS, the Luangwa Rift has a lower level of active seismicity and volcanism. 400 km northwest of the Luangwa, the north/south Luapula rift valley passes into the NE trending Mweru and Mweru Wantipa rift lakes. Pronounced border faults and fault terraces mark the NW and SE margins of these shallow lakes. Between the Luangwa and Luapula rift valleys lies the extensive upper Congo drainage basin of the Chambeshi river and the Lake Bangweulu wetlands. DEM mapping of topography from the Luangwa rift to the Luapula-Mweru Wantipa rift shows a low amplitude, large wavelength flexure of the Central African plateau surface compatible with an effective elastic thickness of 35 km. This regional warping controls the location and shape of the Chambeshi drainage basin and the Lake Bangweulu Wetlands

Estimation of 1-D velocity models beneath strong-motion observation sites in the Kathmandu Valley using strong-motion records from moderate-sized earthquakes

NASA Astrophysics Data System (ADS)

Bijukchhen, Subeg M.; Takai, Nobuo; Shigefuji, Michiko; Ichiyanagi, Masayoshi; Sasatani, Tsutomu; Sugimura, Yokito

2017-07-01

The Himalayan collision zone experiences many seismic activities with large earthquakes occurring at certain time intervals. The damming of the proto-Bagmati River as a result of rapid mountain-building processes created a lake in the Kathmandu Valley that eventually dried out, leaving thick unconsolidated lacustrine deposits. Previous studies have shown that the sediments are 600 m thick in the center. A location in a seismically active region, and the possible amplification of seismic waves due to thick sediments, have made Kathmandu Valley seismically vulnerable. It has suffered devastation due to earthquakes several times in the past. The development of the Kathmandu Valley into the largest urban agglomerate in Nepal has exposed a large population to seismic hazards. This vulnerability was apparent during the Gorkha Earthquake (Mw7.8) on April 25, 2015, when the main shock and ensuing aftershocks claimed more than 1700 lives and nearly 13% of buildings inside the valley were completely damaged. Preparing safe and up-to-date building codes to reduce seismic risk requires a thorough study of ground motion amplification. Characterizing subsurface velocity structure is a step toward achieving that goal. We used the records from an array of strong-motion accelerometers installed by Hokkaido University and Tribhuvan University to construct 1-D velocity models of station sites by forward modeling of low-frequency S-waves. Filtered records (0.1-0.5 Hz) from one of the accelerometers installed at a rock site during a moderate-sized (mb4.9) earthquake on August 30, 2013, and three moderate-sized (Mw5.1, Mw5.1, and Mw5.5) aftershocks of the 2015 Gorkha Earthquake were used as input motion for modeling of low-frequency S-waves. We consulted available geological maps, cross-sections, and borehole data as the basis for initial models for the sediment sites. This study shows that the basin has an undulating topography and sediment sites have deposits of varying thicknesses

Relations between seismicity and deformation during unrest in Long Valley Caldera, California, from 1995 through 1999

USGS Publications Warehouse

Hill, D.P.; Langbein, J.O.; Prejean, S.

2003-01-01

Unrest in Long Valley Caldera and the adjacent Sierra Nevada from 1995 through 2000 was dominated by three major episodes: (1) the March-April 1996 earthquake swarm in the east lobe of the south moat; (2) the July 1997-January 1998 caldera-wide unrest; and (3) a sequence of three M>5 earthquakes (9 June 1998, 13 July 1998, and 15 May 1999 UT) located in the Sierra Nevada block immediately south of the caldera. These three unrest episodes each had distinct characteristics with distinct implications for associated hazards. Seismicity developed as earthquake swarms for the 1996 and 1997-98 episodes, both of which were within the caldera. In contrast, the series of three M>5 earthquakes south of the caldera in 1998-99 each developed as a mainshock-aftershock sequence. Marginal deformation within the caldera associated with the 1996 swarm and the 1998-99 M>5 earthquakes is consistent with the cumulative seismic moments for the respective sequences. Deformation associated with the 1997-98 episode, however, was roughly five times larger than can be accounted for by the cumulative seismic moment of the associated earthquake swarm. We conclude that the 1997-98 episode was associated with mass transport (local intrusion of magma or magmatic brine) and that the associated earthquake swarm activity, which had a relatively high b -value of 1.2, was largely driven by the intrusive process. In contrast, the 1996 earthquake swarm and the 1998-99 M>5 mainshock-aftershock sequences, both with 'normal' b -values of ???0.9, represent brittle relaxation to previously accumulated stresses associated with little or no mass transport. These relations emphasize the importance of simultaneous, real-time monitoring of both seismicity and deformation as a basis for judging whether an evolving unrest episode has the potential for culminating in a volcanic eruption. ?? 2003 Elsevier B.V. All rights reserved.

Structural controls on the spatial distribution and geochemical composition of volcanism in a continental rift zone; an example from Owens Valley, eastern California

NASA Astrophysics Data System (ADS)

Haproff, P. J.; Yin, A.

2014-12-01

Bimodal volcanism is common in continental rift zones. Structural controls to the emplacement and compositions of magmas, however, are not well understood. To address this issue, we examine the location, age, and geochemistry of active volcanic centers, and geometry and kinematics of rift-related faults across the active transtensional Owens Valley rift zone. Building on existing studies, we postulate that the spatial distribution and geochemical composition of volcanism are controlled by motion along rift-bounding fault systems. Along-strike variation in fault geometry and characteristics of active volcanism allow us to divide Owens Valley into three segments: southern, northern, and central. The southern segment of Owens Valley is a simple shear, asymmetric rift bounded to the west by the east-dipping Sierra Nevada frontal fault (SNFF). Active vents of Coso volcanic field are distributed along the eastern rift shoulder and characterized by the eruption of bimodal lavas. The SNFF within this segment is low-angle and penetrates through the lithosphere and into the ductile asthenosphere, allowing for mantle-derived magma to migrate across the weakest part of the fault zone beneath the eastern rift shoulder. Magma thermally weakens wall rocks and eventually stalls in the crust where the melt develops a greater felsic component prior to eruption. The northern segment of Owens Valley displays similar structural geometry, as the west-dipping White Mountains fault (WMF) is listric at depth and offsets the crust and mantle lithosphere, allowing for vertical transport of magma and reservoir emplacement within the crust. Bimodal lavas periodically erupted in the Long Valley Caldera region along the western rift shoulder. The central segment of Owens Valley is a pure shear, symmetric graben generated by motion along the SNFF and WMF. The subvertical, right-slip Owens Valley fault (OVF) strikes along the axis of the valley and penetrates through the lithosphere into the

Identifying the Transition Zone Between East and West Dharwar Craton by Seismic Imaging

NASA Astrophysics Data System (ADS)

Ashish; Parvez, Imtiyaz A.

2018-01-01

The data from 12 temporary broadband seismic stations operated across east-west corridor in Dharwar region of Indian Peninsula along with ten other seismic stations operated by CSIR National Geophysical Research Institute (NGRI) in the region have been analysed that provide high-resolution image of southern Dharwar crust. Crust along the corridor is imaged by receiver function H-k stacking, common conversion point stacking using data from 22 sites in combination with joint inversion modeling of receiver functions and Rayleigh wave group velocity dispersion curves. The velocity image reveals thinner crust (36-38 km) except one site (coinciding with Cuddapah basin on the surface) in East Dharwar Craton (EDC), while crust beneath the West Dharwar Craton (WDC) is thicker (46-50 km). This study also observed a transition zone between EDC and WDC starting west of Closepet granite to the east of Chitradurga Schist Belt (CSB), which shows diffused Moho with a thickness of 40-44 km. Chitradurga Schist Belt is identified as the contact between Mesoarchean (WDC) and Neoarchean (EDC) crustal blocks. The lowermost part of the crust (V_s > 4.0) is thin (2-6 km) beneath EDC, intermediate (6-8 km) beneath transition zone and thicker (14-30 km) beneath WDC across the profile.

Integration of P- and SH-wave high-resolution seismic reflection and micro-gravity techniques to improve interpretation of shallow subsurface structure: New Madrid seismic zone

USGS Publications Warehouse

Bexfield, C.E.; McBride, J.H.; Pugin, Andre J.M.; Ravat, D.; Biswas, S.; Nelson, W.J.; Larson, T.H.; Sargent, S.L.; Fillerup, M.A.; Tingey, B.E.; Wald, L.; Northcott, M.L.; South, J.V.; Okure, M.S.; Chandler, M.R.

2006-01-01

Shallow high-resolution seismic reflection surveys have traditionally been restricted to either compressional (P) or horizontally polarized shear (SH) waves in order to produce 2-D images of subsurface structure. The northernmost Mississippi embayment and coincident New Madrid seismic zone (NMSZ) provide an ideal laboratory to study the experimental use of integrating P- and SH-wave seismic profiles, integrated, where practicable, with micro-gravity data. In this area, the relation between "deeper" deformation of Paleozoic bedrock associated with the formation of the Reelfoot rift and NMSZ seismicity and "shallower" deformation of overlying sediments has remained elusive, but could be revealed using integrated P- and SH-wave reflection. Surface expressions of deformation are almost non-existent in this region, which makes seismic reflection surveying the only means of detecting structures that are possibly pertinent to seismic hazard assessment. Since P- and SH-waves respond differently to the rock and fluid properties and travel at dissimilar speeds, the resulting seismic profiles provide complementary views of the subsurface based on different levels of resolution and imaging capability. P-wave profiles acquired in southwestern Illinois and western Kentucky (USA) detect faulting of deep, Paleozoic bedrock and Cretaceous reflectors while coincident SH-wave surveys show that this deformation propagates higher into overlying Tertiary and Quaternary strata. Forward modeling of micro-gravity data acquired along one of the seismic profiles further supports an interpretation of faulting of bedrock and Cretaceous strata. The integration of the two seismic and the micro-gravity methods therefore increases the scope for investigating the relation between the older and younger deformation in an area of critical seismic hazard. ?? 2006 Elsevier B.V. All rights reserved.

Seismic-sequence stratigraphy and geologic structure of the Floridan aquifer system near "Boulder Zone" deep wells in Miami-Dade County, Florida

USGS Publications Warehouse

Cunningham, Kevin J.

2015-01-01

In addition to the preceding seismic-reflection analysis, interpretation of geophysical well log data from four effluent injection wells at the North District “Boulder Zone” Well Field delineated a narrow karst collapse structure beneath the injection facility that extends upward about 900 ft from the top of the Boulder Zone to about 125 ft above the top of the uppermost major permeable zone of the Lower Floridan aquifer. No karst collapse structures were identified in the seismic-reflection profiles acquired near the North District “Boulder Zone” Well Field. However, karst collapse structures at the level of the lowermost major permeable zone of the Lower Floridan aquifer at the South District “Boulder Zone” Well Field are present at three locations, as indicated by seismic-reflection data acquired in the C–1 Canal bordering the south side of the injection facility. Results from the North District “Boulder Zone” Well Field well data indicate that a plausible hydraulic connection between faults and stratiform permeability zones may contribute to the upward transport of effluent, terminating above the base of the deepest U.S. Environmental Protection Agency designated underground source of drinking water at the North District “Boulder Zone” Well Field.

Near-vertical seismic reflection image using a novel acquisition technique across the Vrancea Zone and Foscani Basin, south-eastern Carpathians (Romania)

NASA Astrophysics Data System (ADS)

Panea, I.; Stephenson, R.; Knapp, C.; Mocanu, V.; Drijkoningen, G.; Matenco, L.; Knapp, J.; Prodehl, K.

2005-12-01

The DACIA PLAN (Danube and Carpathian Integrated Action on Process in the Lithosphere and Neotectonics) deep seismic sounding survey was performed in August-September 2001 in south-eastern Romania, at the same time as the regional deep refraction seismic survey VRANCEA 2001. The main goal of the experiment was to obtain new information on the deep structure of the external Carpathians nappes and the architecture of Tertiary/Quaternary basins developed within and adjacent to the seismically-active Vrancea zone, including the Focsani Basin. The seismic reflection line had a WNW-ESE orientation, running from internal East Carpathians units, across the mountainous south-eastern Carpathians, and the foreland Focsani Basin towards the Danube Delta. There were 131 shot points along the profile, with about 1 km spacing, and data were recorded with stand-alone RefTek-125s (also known as "Texans"), supplied by the University Texas at El Paso and the PASSCAL Institute. The entire line was recorded in three deployments, using about 340 receivers in the first deployment and 640 receivers in each of the other two deployments. The resulting deep seismic reflection stacks, processed to 20 s along the entire profile and to 10 s in the eastern Focsani Basin, are presented here. The regional architecture of the latter, interpreted in the context of abundant independent constraint from exploration seismic and subsurface data, is well imaged. Image quality within and beneath the thrust belt is of much poorer quality. Nevertheless, there is good evidence to suggest that a thick (˜10 km) sedimentary basin having the structure of a graben and of indeterminate age underlies the westernmost part of the Focsani Basin, in the depth range 10-25 km. Most of the crustal depth seismicity observed in the Vrancea zone (as opposed to the more intense upper mantle seismicity) appears to be associated with this sedimentary basin. The sedimentary successions within this basin and other horizons

Historical seismicity, palaeoseismicity and seismic risk in Western Macedonia, Northern Greece

NASA Astrophysics Data System (ADS)

Stiros, Stathis C.

Western Macedonia, Northern Greece, was a seismically quiescent region for one or more centuries, and was regarded as a nearly aseismic, rigid block inside a broad zone of distributed continental deformation and faulting, and a region of minimum seismic risk. Consequently, the May 13, 1995 destructive earthquake ( M = 6.6) which hit this assumed aseismic zone was a surprise for scientists, government and population. However, historical and archaeoseismic evidence, as well as coastal change data indicate that the assumed aseismic region of Western Macedonia has been affected in the last 2,000 years by at least seven, and possibly nine destructive earthquakes. One of these earthquakes occurred in circa 1700, and probably had the same epicentre with, but higher magnitude than the 1995 shock. The earthquake in circa 1700 is deduced from historical data and is modelled on the base of a swarm of church repairs which is explained as post-seismic recovery of the broader Kozani area: except for certain well known cases of towns or areas in which religious privileges were granted, large scale repairs or reconstruction of churches during the Ottoman period were possible only after Sultan's permissions, usually following earthquakes and other calamities. It can hence be concluded that some, at least, of the apparently aseismic regions inside broad zones of distributed seismicity are hit by stronger shocks, but with longer (200 years or more) recurrence intervals than their adjacent zones. Consequently, the seismic risk of the apparently aseismic regions is certainly not low, especially since relatively long periods of seismic quiescence lead to constructions vulnerable to earthquakes.

Optimal distribution of borehole geophones for monitoring CO2-injection-induced seismicity

NASA Astrophysics Data System (ADS)

Huang, L.; Chen, T.; Foxall, W.; Wagoner, J. L.

2016-12-01

The U.S. DOE initiative, National Risk Assessment Partnership (NRAP), aims to develop quantitative risk assessment methodologies for carbon capture, utilization and storage (CCUS). As part of tasks of the Strategic Monitoring Group of NRAP, we develop a tool for optimal design of a borehole geophones distribution for monitoring CO2-injection-induced seismicity. The tool consists of a number of steps, including building a geophysical model for a given CO2 injection site, defining target monitoring regions within CO2-injection/migration zones, generating synthetic seismic data, giving acceptable uncertainties in input data, and determining the optimal distribution of borehole geophones. We use a synthetic geophysical model as an example to demonstrate the capability our new tool to design an optimal/cost-effective passive seismic monitoring network using borehole geophones. The model is built based on the geologic features found at the Kimberlina CCUS pilot site located in southern San Joaquin Valley, California. This tool can provide CCUS operators with a guideline for cost-effective microseismic monitoring of geologic carbon storage and utilization.

Paleoearthquakes and long-term seismic regime in the Longmenshan fault zone, Southwest China

NASA Astrophysics Data System (ADS)

Liu, J.; Rodina, S. N.; Rogozhin, E. A.

2017-11-01

The collected paleoseismological data about the ancient earthquakes are analyzed for the zone of the Wenchuan earthquake. Four earthquakes with magnitude M = 8.0 occurred over a period of 10000 years and two earthquakes with magnitude 7.5 occurred over 1200 years. The obtained data allowed us to reconstruct the long-term seismic regime within the studied territory by constructing the frequency-magnitude relationship based on the instrumental, historical, and paleoseismological data.

The Salzach Valley overdeeping: A most precise bedrock model of a major alpine glacial basin

NASA Astrophysics Data System (ADS)

Pomper, Johannes; Salcher, Bernhard; Eichkitz, Christoph

2016-04-01

Overdeepenings are impressive phenomena related to the erosion in the ablation zone of major glaciers. They are common features in glaciated and deglaciated regions worldwide and their sedimentary fillings may act as important archives for regional environmental change and glaciation history. Sedimentary fillings are also important targets of geotechnical exploration and construction including groundwater resource management, shallow geothermal exploitation, tunneling and the foundation of buildings. This is especially true in densely populated areas such as the European Alps and their foreland areas, regions which have been multiply glaciated during the last million years. However, due depths often exceeding some hundreds of meters, the overall knowledge on their geometry, formation and sedimentary content is still poor and commonly tied to some local spots. Here we present a bedrock model of the overall lower Salzach Valley, one of the largest glacial overdeepings in the European Alps. We utilized seismic sections from hydrocarbon exploration surveys and deep drillings together with topographic and modelling data to construct a 3D bedrock model. Through the existence of seismic inline and crossline valley sections, multiple drillings reaching the bedrock surface, log and abundant outcrop data we were, as far to our knowledge, able to create the most accurate digital bedrock topography of an alpine major overdeepening. We furthermore analyzed the sedimentary content of the valley as recorded by driller's lithologic logs. Our results suggest that the valley is far from being a regular U-shaped trough with constant depth, rather highlighting highs and lows of different magnitude and underground valley widths of variable extent. Data also indicates that the largest overdeepening of bedrock, reaching around 450 m below the alluvial fill, is not situated after a major glacial confluence following a prominent bedrock gorge but shifted several km down the valley. The

Seismic-refraction survey to the top of salt in the north end of the Salt Valley Anticline, Grand County, Utah

USGS Publications Warehouse

Ackermann, Hans D.

1979-01-01

A seismic-refraction survey, consisting of three lines about 2700, 2760, and 5460 meters long, was made at the north end of the Salt Valley anticline of the Paradox Basin in eastern Utah. The target was the crest of a diapiric salt mass and the overlying, deformed caprock. The interpretations reveal an undulating salt surface with as much as 80 meters of relief. The minimum depth of about 165 meters is near the location of three holes drilled by the U.S. Department of Energy for the purpose of evaluating the Salt Valley anticline as a potential site for radioactive waste storages Caprock properties were difficult to estimate because the contorted nature of these beds invalidated a geologic interpretation in terms of velocity layers. However, laterally varying velocities of the critically refracted rays throughout the area suggest differences in the gross physical properties of the caprock.

Seismic structure and activity of the north-central Lesser Antilles subduction zone from an integrated approach: Similarities with the Tohoku forearc

NASA Astrophysics Data System (ADS)

Laigle, M.; Hirn, A.; Sapin, M.; Bécel, A.; Charvis, P.; Flueh, E.; Diaz, J.; Lebrun, J.-F.; Gesret, A.; Raffaele, R.; Galvé, A.; Evain, M.; Ruiz, M.; Kopp, H.; Bayrakci, G.; Weinzierl, W.; Hello, Y.; Lépine, J.-C.; Viodé, J.-P.; Sachpazi, M.; Gallart, J.; Kissling, E.; Nicolich, R.

2013-09-01

The 300-km-long north-central segment of the Lesser Antilles subduction zone, including Martinique and Guadeloupe islands has been the target of a specific approach to the seismic structure and activity by a cluster of active and passive offshore-onshore seismic experiments. The top of the subducting plate can be followed under the wide accretionary wedge by multichannel reflection seismics. This reveals the hidden updip limit of the contact of the upper plate crustal backstop onto the slab. Two OBS refraction seismic profiles from the volcanic arc throughout the forearc domain constrain a 26-km-large crustal thickness all along. In the common assumption that the upper plate Moho contact on the slab is a proxy of its downdip limit these new observations imply a three times larger width of the potential interplate seismogenic zone under the marine domain of the Caribbean plate with respect to a regular intra-oceanic subduction zone. Towards larger depth under the mantle corner, the top of the slab imaged from the conversions of teleseismic body-waves and the locations of earthquakes appears with kinks which increase the dip to 10-20° under the forearc domain, and then to 60° from 70 km depth. At 145 km depth under the volcanic arc just north of Martinique, the 2007 M 7.4 earthquake, largest for half a century in the region, allows to document a deep slab deformation consistent with segmentation into slab panels. In relation with this occurrence, an increased seismic activity over the whole depth range provides a new focussed image thanks to the OBS and land deployments. A double-planed dipping slab seismicity is thus now resolved, as originally discovered in Tohoku (NE Japan) and since in other subduction zones. Two other types of seismic activity uniquely observed in Tohoku, are now resolved here: "supraslab" earthquakes with normal-faulting focal mechanisms reliably located in the mantle corner and "deep flat-thrust" earthquakes at 45 km depth on the interplate

Crustal shortening followed by extensional collapse of the Cordilleran orogenic belt in northwestern Montana: Evidence from vintage seismic reflection profiles acquired in the Swan Range and Swan Valley

NASA Astrophysics Data System (ADS)

Rutherford, B. S.; Speece, M. A.; Stickney, M. C.; Mosolf, J. G.

2013-12-01

Reprocessing of one 24-fold (96 channel) and four 30-fold (120 channel) 2D seismic reflection profiles have revealed crustal scale reflections in the Swan Range and adjacent Swan River Valley of northwestern Montana. The five reprocessed profiles constitute 142.6 of the 303.3 linear km acquired in 1983-84 by Techo of Denver, Colorado. The four 30-fold profiles used helicopter-assisted dynamite shooting (Poulter method) and the 24-fold profile used the Vibroseis method. Acquisition parameters were state of the art for the time. The Swan Range lies east of the Rocky Mountain Trench and is part of the Cordilleran foreland thrust belt where the Lewis thrust system emplaced a thick slab of Proterozoic Belt Supergroup strata eastward and over Paleozoic and Mesozoic rocks during the Late Cretaceous to early Paleocene Laramide orogeny. Deeply drilled borehole data are absent within the study area; however, we generated a synthetic seismogram from the Arco-Marathon 1 Paul Gibbs well (total depth=5418 m), located approximately 70 km west of the reprocessed profiles, and correlated the well data to surface seismic profiles. Large impedance contrasts in the log data are interpreted to be tholeiitic Moyie sills within the Prichard Formation argillite (Lower Belt), which produce strong reflection events in regional seismic sections and result in highly reflective, east-dipping events in the reprocessed profiles. We estimate a depth of 10 km (3 to 3.5 seconds) to the basal detachment of the Lewis thrust sheet. The décollement lies within Belt Supergroup strata to the west of the Swan River Valley before contacting unreflective, west-dipping crystalline basement beneath the Swan Range--a geometry that results in a wedge of eastward-thinning, autochthonous Belt rocks. Distinct fault-plane signatures from the west-dipping, range-bounding Swan fault--produced by extensional collapse of the over-thickened Cordillera--are not successfully imaged. However, reflections from Cenozoic

Structure of the Koyna-Warna Seismic Zone, Maharashtra, India: A possible model for large induced earthquakes elsewhere

USGS Publications Warehouse

Catchings, Rufus D.; Dixit, M.M.; Goldman, Mark R.; Kumar, S.

2015-01-01

The Koyna-Warna area of India is one of the best worldwide examples of reservoir-induced seismicity, with the distinction of having generated the largest known induced earthquake (M6.3 on 10 December 1967) and persistent moderate-magnitude (>M5) events for nearly 50 years. Yet, the fault structure and tectonic setting that has accommodated the induced seismicity is poorly known, in part because the seismic events occur beneath a thick sequence of basalt layers. On the basis of the alignment of earthquake epicenters over an ~50 year period, lateral variations in focal mechanisms, upper-crustal tomographic velocity images, geophysical data (aeromagnetic, gravity, and magnetotelluric), geomorphic data, and correlation with similar structures elsewhere, we suggest that the Koyna-Warna area lies within a right step between northwest trending, right-lateral faults. The sub-basalt basement may form a local structural depression (pull-apart basin) caused by extension within the step-over zone between the right-lateral faults. Our postulated model accounts for the observed pattern of normal faulting in a region that is dominated by north-south directed compression. The right-lateral faults extend well beyond the immediate Koyna-Warna area, possibly suggesting a more extensive zone of seismic hazards for the central India area. Induced seismic events have been observed many places worldwide, but relatively large-magnitude induced events are less common because critically stressed, preexisting structures are a necessary component. We suggest that releasing bends and fault step-overs like those we postulate for the Koyna-Warna area may serve as an ideal tectonic environment for generating moderate- to large- magnitude induced (reservoir, injection, etc.) earthquakes.

Imaging the Seismic Cycle in the Central Andean Subduction Zone from Geodetic Observations

NASA Astrophysics Data System (ADS)

Ortega-Culaciati, F.; Becerra-Carreño, V. C.; Socquet, A.; Jara, J.; Carrizo, D.; Norabuena, E. O.; Simons, M.; Vigny, C.; Bataille, K. D.; Moreno, M.; Baez, J. C.; Comte, D.; Contreras-Reyes, E.; Delorme, A.; Genrich, J. F.; Klein, E.; Ortega, I.; Valderas, M. C.

2015-12-01

We aim to quantify spatial and temporal evolution of fault slip behavior during all stages of the seismic cycle in subduction megathrusts, with the eventual goal of improving our understanding of the mechanical behavior of the subduction system and its implications for earthquake and tsunami hazards. In this work, we analyze the portion of the Nazca-SouthAmerican plates subduction zone affected by the 1868 southern Peru and 1877 northern Chile mega-earthquakes. The 1868 and 1878 events defined a seismic gap that did not experience a large earthquake for over 124 years. Only recently, the 1995 Mw 8.1 Antofagasta, 2001 Mw 8.4 Arequipa, 2007 Mw 7.7 Tocopilla, and 2014 Mw 8.2 Pisagua earthquakes released only a small fraction of the potential slip budget, thereby raising concerns about continued seismic and tsunami hazard. We use over a decade of observations from continuous and campaign GPS networks to analyze inter-seismic strain accumulation, as well as co-seimic deformation associated to the more recent earthquakes in the in the Central Andean region. We obtain inferences of slip (and back-slip) behavior using a consistent and robust inversion framework that accounts for the spatial variability of the constraint provided by the observations on slip across the subduction megathrust. We present an updated inter-seismic coupling model and estimates of pre-, co- and post- seismic slip behavior associated with the most recent 2014 Mw 8.2 Pisagua earthquake. We analyze our results, along with published information on the recent and historical large earthquakes, to characterize the regions of the megathrust that tend to behave aseismically, and those that are capable to accumulate a slip budget (ultimately leading to the generation of large earthquakes), to what extent such regions may overlap, and discuss the potential for large earthquakes in the region.

Long Duration of Ground Motion in the Paradigmatic Valley of Mexico

PubMed Central

Cruz-Atienza, V. M.; Tago, J.; Sanabria-Gómez, J. D.; Chaljub, E.; Etienne, V.; Virieux, J.; Quintanar, L.

2016-01-01

Built-up on top of ancient lake deposits, Mexico City experiences some of the largest seismic site effects worldwide. Besides the extreme amplification of seismic waves, duration of intense ground motion from large subduction earthquakes exceeds three minutes in the lake-bed zone of the basin, where hundreds of buildings collapsed or were seriously damaged during the magnitude 8.0 Michoacán earthquake in 1985. Different mechanisms contribute to the long lasting motions, such as the regional dispersion and multiple-scattering of the incoming wavefield from the coast, more than 300 km away the city. By means of high performance computational modeling we show that, despite the highly dissipative basin deposits, seismic energy can propagate long distances in the deep structure of the valley, promoting also a large elongation of motion. Our simulations reveal that the seismic response of the basin is dominated by surface-waves overtones, and that this mechanism increases the duration of ground motion by more than 170% and 290% of the incoming wavefield duration at 0.5 and 0.3 Hz, respectively, which are two frequencies with the largest observed amplification. This conclusion contradicts what has been previously stated from observational and modeling investigations, where the basin itself has been discarded as a preponderant factor promoting long and devastating shaking in Mexico City. PMID:27934934

Long Duration of Ground Motion in the Paradigmatic Valley of Mexico

NASA Astrophysics Data System (ADS)

Cruz-Atienza, V. M.; Tago, J.; Sanabria-Gómez, J. D.; Chaljub, E.; Etienne, V.; Virieux, J.; Quintanar, L.

2016-12-01

Built-up on top of ancient lake deposits, Mexico City experiences some of the largest seismic site effects worldwide. Besides the extreme amplification of seismic waves, duration of intense ground motion from large subduction earthquakes exceeds three minutes in the lake-bed zone of the basin, where hundreds of buildings collapsed or were seriously damaged during the magnitude 8.0 Michoacán earthquake in 1985. Different mechanisms contribute to the long lasting motions, such as the regional dispersion and multiple-scattering of the incoming wavefield from the coast, more than 300 km away the city. By means of high performance computational modeling we show that, despite the highly dissipative basin deposits, seismic energy can propagate long distances in the deep structure of the valley, promoting also a large elongation of motion. Our simulations reveal that the seismic response of the basin is dominated by surface-waves overtones, and that this mechanism increases the duration of ground motion by more than 170% and 290% of the incoming wavefield duration at 0.5 and 0.3 Hz, respectively, which are two frequencies with the largest observed amplification. This conclusion contradicts what has been previously stated from observational and modeling investigations, where the basin itself has been discarded as a preponderant factor promoting long and devastating shaking in Mexico City.

Long Duration of Ground Motion in the Paradigmatic Valley of Mexico.

PubMed

Cruz-Atienza, V M; Tago, J; Sanabria-Gómez, J D; Chaljub, E; Etienne, V; Virieux, J; Quintanar, L

2016-12-09

Built-up on top of ancient lake deposits, Mexico City experiences some of the largest seismic site effects worldwide. Besides the extreme amplification of seismic waves, duration of intense ground motion from large subduction earthquakes exceeds three minutes in the lake-bed zone of the basin, where hundreds of buildings collapsed or were seriously damaged during the magnitude 8.0 Michoacán earthquake in 1985. Different mechanisms contribute to the long lasting motions, such as the regional dispersion and multiple-scattering of the incoming wavefield from the coast, more than 300 km away the city. By means of high performance computational modeling we show that, despite the highly dissipative basin deposits, seismic energy can propagate long distances in the deep structure of the valley, promoting also a large elongation of motion. Our simulations reveal that the seismic response of the basin is dominated by surface-waves overtones, and that this mechanism increases the duration of ground motion by more than 170% and 290% of the incoming wavefield duration at 0.5 and 0.3 Hz, respectively, which are two frequencies with the largest observed amplification. This conclusion contradicts what has been previously stated from observational and modeling investigations, where the basin itself has been discarded as a preponderant factor promoting long and devastating shaking in Mexico City.

Optimizing the design of vertical seismic profiling (VSP) for imaging fracture zones over hardrock basement geothermal environments

NASA Astrophysics Data System (ADS)

Reiser, Fabienne; Schmelzbach, Cedric; Maurer, Hansruedi; Greenhalgh, Stewart; Hellwig, Olaf

2017-04-01

A primary focus of geothermal seismic imaging is to map dipping faults and fracture zones that control rock permeability and fluid flow. Vertical seismic profiling (VSP) is therefore a most valuable means to image the immediate surroundings of an existing borehole to guide, for example, the placing of new boreholes to optimize production from known faults and fractures. We simulated 2D and 3D acoustic synthetic seismic data and processed it through to pre-stack depth migration to optimize VSP survey layouts for mapping moderately to steeply dipping fracture zones within possible basement geothermal reservoirs. Our VSP survey optimization procedure for sequentially selecting source locations to define the area where source points are best located for optimal imaging makes use of a cross-correlation statistic, by which a subset of migrated shot gathers is compared with a target or reference image from a comprehensive set of source gathers. In geothermal exploration at established sites, it is reasonable to assume that sufficient à priori information is available to construct such a target image. We generally obtained good results with a relatively small number of optimally chosen source positions distributed over an ideal source location area for different fracture zone scenarios (different dips, azimuths, and distances from the surveying borehole). Adding further sources outside the optimal source area did not necessarily improve the results, but rather resulted in image distortions. It was found that fracture zones located at borehole-receiver depths and laterally offset from the borehole by 300 m can be imaged reliably for a range of the different dips, but more source positions and large offsets between sources and the borehole are required for imaging steeply dipping interfaces. When such features cross-cut the borehole, they are particularly difficult to image. For fracture zones with different azimuths, 3D effects are observed. Far offset source positions

Estimation of earthquake effects associated with a great earthquake in the New Madrid seismic zone

USGS Publications Warehouse

Hopper, Margaret G.; Algermissen, Sylvester Theodore; Dobrovolny, Ernest E.

1983-01-01

Estimates have been made of the effects of a large Ms = 8.6, Io = XI earthquake hypothesed to occur anywhere in the New Madrid seismic zone. The estimates are based on the distributions of intensities associated with the earthquakes of 1811-12, 1843 and 1895 although the effects of other historical shocks are also considered. The resulting composite type intensity map for a maximum intensity XI is believed to represent the upper level of shaking likely to occur. Specific intensity maps have been developed for six cities near the epicentral region taking into account the most likely distribution of site response in each city. Intensities found are: IX for Carbondale, IL; VIII and IX for Evansville, IN; VI and VIII for Little Rock, AR; IX and X for Memphis, TN; VIII, IX, and X for Paducah, KY; and VIII and X for Poplar Bluff, MO. On a regional scale, intensities are found to attenuate from the New Madrid seismic zone most rapidly to the west and southwest sides of the zone, most slowly to the northwest along the Mississippi River, on the northeast along the Ohio River, and on the southeast toward Georgia and South Carolina. Intensities attenuate toward the north, east, and south in a more normal fashion. Known liquefaction effects are documented but much more research is needed to define the liquefaction potential.

Earthquake-induced liquefaction features in the coastal setting of South Carolina and in the fluvial setting of the New Madrid seismic zone

USGS Publications Warehouse

Obermeier, S.F.; Jacobson, R.B.; Smoot, J.P.; Weems, R.E.; Gohn, G.S.; Monroe, J.E.; Powars, D.S.

1990-01-01

Many types of liquefaction-related features (sand blows, fissures, lateral spreads, dikes, and sills) have been induced by earthquakes in coastal South Carolina and in the New Madrid seismic zone in the Central United States. In addition, abundant features of unknown and nonseismic origin are present. Geologic criteria for interpreting an earthquake origin in these areas are illustrated in practical applications; these criteria can be used to determine the origin of liquefaction features in many other geographic and geologic settings. In both coastal South Carolina and the New Madrid seismic zone, the earthquake-induced liquefaction features generally originated in clean sand deposits that contain no or few intercalated silt or clay-rich strata. The local geologic setting is a major influence on both development and surface expression of sand blows. Major factors controlling sand-blow formation include the thickness and physical properties of the deposits above the source sands, and these relationships are illustrated by comparing sand blows found in coastal South Carolina (in marine deposits) with sand blows found in the New Madrid seismic zone (in fluvial deposits). In coastal South Carolina, the surface stratum is typically a thin (about 1 m) soil that is weakly cemented with humate, and the sand blows are expressed as craters surrounded by a thin sheet of sand; in the New Madrid seismic zone the surface stratum generally is a clay-rich deposit ranging in thickness from 2 to 10 m, in which case sand blows characteristically are expressed as sand mounded above the original ground surface. Recognition of the various features described in this paper, and identification of the most probable origin for each, provides a set of important tools for understanding paleoseismicity in areas such as the Central and Eastern United States where faults are not exposed for study and strong seismic activity is infrequent.

Shallow velocity structure of the Alaska Peninsula subduction zone and implications for controls on seismic behavior

NASA Astrophysics Data System (ADS)

Li, J.; Shillington, D. J.; Becel, A.; Nedimovic, M. R.; Kuehn, H.; Webb, S. C.; Abers, G. A.; Keranen, K. M.; Saffer, D. M.

2014-12-01

Downdip and along-strike variations in the seismic behavior of subduction zone megathrust faults are thought to be strongly controlled by changes in the material properties along the plate boundary. Roughness and hydration of the incoming plate, fluid pressure and lithology in the subducting sediment channel are likely to control the distribution of shallower rupture. Here, we focus on the subduction zone offshore of the Alaska Peninsula. In 2011, the ALEUT program acquired deep penetration multichannel seismic (MCS) reflection and ocean bottom seismometer (OBS) data across the apparently freely sliding Shumagin Gap, the locked Semidi segment that last ruptured in 1938 M8.2 earthquake, and the locked western Kodiak asperity, which ruptured in the 1964 M9.2 earthquake. Seismic reflection data from the ALEUT cruise reveal significant variability in the thickness of sediment on the incoming plate and entering the trench, and the roughness and degree of hydration of the incoming plate. Oceanic crust entering the trench in the Shumagin gap is rugged with extensive faults and only a thin layer of sediment (<0.5 km thick). Farther east in the Semidi segment, the subducting plate has a smoother surface with thicker sediments (~1 km thick) and less faulting/hydration. To better constrain the properties of the accretionary prism and shallow part of the plate boundary, we are undertaking travel time tomography using reflection/refraction phases in OBS and MCS data, and constraints on the interface geometry from MCS images to estimate the detailed shallow velocity structure, with particular focus on properties within the shallow subduction channel. We observe refractions and reflections in OBS data from the shallow part of the subduction zone in both the Shumagin Gap and Semidi segment, including reflections off the top and base of what appears to be a layer of subducting sediment, which can be used for this work. We plan to present initial models of the shallow part of the

The relationship between Anopheles gambiae density and rice cultivation in the savannah zone and forest zone of Côte d'Ivoire.

PubMed

Briët, Olivier J T; Dossou-Yovo, Joel; Akodo, Elena; van de Giesen, Nick; Teuscher, Thomas M

2003-05-01

In 13 villages in the savannah zone and 21 villages in the forest zone of Côte d'Ivoire, the biting density of the principal malaria vector, Anopheles gambiae, was studied as a function of rice cultivation in the inland valleys in a 2-km radius around each village. In the savannah villages, during the main season cropping period, surface water on rice-cultivated and to a lesser extent on uncultivated inland valleys seems to contribute strongly to the A. gambiae population density. For the off-season cropping period (which starts after the first light rains in the savannah zone), correlations were weaker. Breeding sites other than in inland valleys may play an important role in the savannah zone. In the forest zone, however, the A. gambiae population density was strongly correlated with the surface water availability (SWA) in the rice-cultivated inland valleys, whereas the correlation with the SWA in other (uncultivated) inland valleys was weak. The requirement of sunlit breeding sites for A. gambiae might explain this difference between zones. In the forest zone, only inland valleys cleared for rice cultivation meet this requirement, whereas all other inland valleys are covered with dense vegetation. In the savannah zone, however, most undergrowth is burnt during the dry season, which permits sunlight to reach puddles resulting from the first rains.

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.

The assessment of seismic hazard for Gori, (Georgia) and preliminary studies of seismic microzonation

NASA Astrophysics Data System (ADS)

Gogoladze, Z.; Moscatelli, M.; Giallini, S.; Avalle, A.; Gventsadze, A.; Kvavadze, N.; Tsereteli, N.

2016-12-01

Seismic risk is a crucial issue for South Caucasus, which is the main gateway between Asia and Europe. The goal of this work is to propose new methods and criteria for defining an overall approach aimed at assessing and mitigating seismic risk in Georgia. In this reguard seismic microzonation represents a highly useful tool for seismic risk assessmentin land management, for design of buildings or structures and for emergency planning.Seismic microzonation assessment of local seismic hazard,which is a component of seismicity resulting from specific local characteristics which cause local amplification and soil instability, through identification of zones with seismically homogeneous behavior. This paper presents the results of preliminary study of seismic microzonation of Gori, Georgia. Gori is and is located in the Shida Kartli region and on both sides of Liachvi and Mtkvari rivers, with area of about 135 km2around the Gori fortress. Gori is located in Achara-Trialeti fold-thrust belt, that is tectonically unstable. Half of all earthquakes in Gori area with magnitude M≥3.5 have happened along this fault zone and on basis of damage caused by previous earthquakes, this territory show the highest level of risk (the maximum value of direct losses) in central part of the town. The seismic microzonation map of level 1 for Gori was carried out using: 1) Already available data (i.e., topographic map and boreholes data), 2) Results of new geological surveys and 3) Geophysical measurements (i.e., MASW and noise measurements processed with HVSR technique). Our preliminary results highlight the presence of both stable zones susceptible to local amplifications and unstable zones susceptible to geological instability. Our results are directed to establish set of actions aimed at risk mitigation before initial onset of emergency, and to management of the emergency once the seismic event has occurred. The products obtained, will contain the basic elements of an integrated system

Core-Log-Seismic Integrative Study of a Subduction Zone Megasplay Fault -An Example from the Nobeoka Thrust, Shimanto Belt, Southwest Japan

NASA Astrophysics Data System (ADS)

Hamahashi, M.; Tsuji, T.; Saito, S.; Tanikawa, W.; Hamada, Y.; Hashimoto, Y.; Kimura, G.

2016-12-01

Investigating the mechanical properties and deformation patterns of megathrusts in subduction zones is important to understand the generation of large earthquakes. The Nobeoka Thrust, a fossilized megasplay fault in Kyushu Shimanto Belt, southwest Japan, exposes foliated fault rocks that were formed under the temperature range of 180-350° (Kondo et al., 2005). During the Nobeoka Thrust Drilling Project (2011), core samples and geophysical logging data were obtained recovering a continuous distribution of multiple fault zones, which provide the opportunity to examine their structure and physical properties in various scales (Hamahashi et al., 2013; 2015). By performing logging data analysis, discrete sample physical property measurements, and synthetic modeling of seismic reflections along the Nobeoka Thrust, we conducted core-log-seismic integrative study to characterize the effects of damage zone architecture and structural anisotropy towards the physical properties of the megasplay. A clear contrast in physical properties across the main fault core and surrounding damage zones were identified, where the fault rocks preserve the porosity of 4.8% in the hanging wall and 7.6% in the footwall, and P-wave velocity of 4.8 km/s and 4.2 km/s, respectively. Multiple sandstone-rich- and shale-rich damage zones were found from the drilled cores, in which velocity decreases significantly in the brecciated zones. The internal structure of these foliated fault rocks consist of heterogeneous lithology and texture, and velocity anisotropy ranges 1-18% (P-wave) and 1.5-80% (S-wave), affected by structural dip angle, foliation density, and sandstone/mudstone ratio. To evaluate the fault properties at the seismogenic depth, we developed velocity/earth models and synthetic modeling of seismic reflection using acoustic logs across the thrust and parameterized lithological and structural elements in the identified multiple damage zones.

The seismic velocity structure of a foreshock zone on an oceanic transform fault: Imaging a rupture barrier to the 2008 Mw 6.0 earthquake on the Gofar fault, EPR

NASA Astrophysics Data System (ADS)

Roland, E. C.; McGuire, J. J.; Lizarralde, D.; Collins, J. A.

2010-12-01

East Pacific Rise (EPR) oceanic transform faults are known to exhibit a number of unique seismicity characteristics, including abundant seismic swarms, a prevalence of aseismic slip, and high rates of foreshock activity. Until recently the details of how this behavior fits into the seismic cycle of large events that occur periodically on transforms have remained poorly understood. In 2008 the most recent seismic cycle of the western segment (G3) of the Gofar fault (4 degrees South on the EPR) ended with a Mw 6.0 earthquake. Seismicity associated with this event was recorded by a local array of ocean bottom seismometers, and earthquake locations reveal several distinct segments with unique slip behavior on the G3 fault. Preceding the Mw 6.0 event, a significant foreshock sequence was recorded just to the east of the mainshock rupture zone that included more than 20,000 detected earthquakes. This foreshock zone formed the eastern barrier to the mainshock rupture, and following the mainshock, seismicity rates within the foreshock zone remained unchanged. Based on aftershock locations of events following the 2007 Mw 6.0 event that completed the seismic cycle on the eastern end of the G3 fault, it appears that the same foreshock zone may have served as the western rupture barrier for that prior earthquake. Moreover, mainshock rupture associated with each of the last 8 large (~ Mw 6.0) events on the G3 fault seems to terminate at the same foreshock zone. In order to elucidate some of the structural controls on fault slip and earthquake rupture along transform faults, we present a seismic P-wave velocity profile crossing the center of the foreshock zone of the Gofar fault, as well as a profile for comparison across the neighboring Quebrada fault. Although tectonically similar, Quebrada does not sustain large earthquakes and is thought to accommodate slip primarily aseismically and with small magnitude earthquake swarms. Velocity profiles were obtained using data collected

Structure of the North Anatolian Fault Zone from the Autocorrelation of Ambient Seismic Noise

NASA Astrophysics Data System (ADS)

Taylor, George; Rost, Sebastian; Houseman, Gregory

2016-04-01

In recent years the technique of cross-correlating the ambient seismic noise wavefield at two seismometers to reconstruct empirical Green's Functions for the determination of Earth structure has been a powerful tool to study the Earth's interior without earthquakes or man-made sources. However, far less attention has been paid to using auto-correlations of seismic noise to reveal body wave reflections from interfaces in the subsurface. In principle, the Green's functions thus derived should be comparable to the Earth's impulse response to a co-located source and receiver. We use data from a dense seismic array (Dense Array for Northern Anatolia - DANA) deployed across the northern branch of the North Anatolian Fault Zone (NAFZ) in the region of the 1999 magnitude 7.6 Izmit earthquake in western Turkey. The NAFZ is a major strike-slip system that extends ~1200 km across northern Turkey and continues to pose a high level of seismic hazard, in particular to the mega-city of Istanbul. We construct body wave images for the entire crust and the shallow upper mantle over the ~35 km by 70 km footprint of the 70-station DANA array. Using autocorrelations of the vertical component of ground motion, P-wave reflections can be retrieved from the wavefield to constrain crustal structure. We show that clear P-wave reflections from the crust-mantle boundary (Moho) can be retrieved using the autocorrelation technique, indicating topography on the Moho on horizontal scales of less than 10 km. Offsets in crustal structure can be identified that seem to be correlated with the surface expression of the northern branch of the fault zone, indicating that the NAFZ reaches the upper mantle as a narrow structure. The southern branch has a less clear effect on crustal structure. We also see evidence of several discontinuities in the mid-crust in addition to an upper mantle reflector that we interpret to represent the Hales discontinuity.

Variability of High Resolution Vp/Vs and Seismic Velocity Structure Along the Nicaragua/Costa Rica Segment of the Middle America Subduction Zone

NASA Astrophysics Data System (ADS)

Moore-Driskell, M. M.; DeShon, H. R.

2012-12-01

Previous studies of subduction zone earthquakes have shown that fault conditions control earthquake rupture and behavior. There are many potential properties that may vary along the subduction margin that could cause fault zone variability, including plate age, temperature, and/or geometry, convergence rate, state of hydration, overriding geology, subducting sediment packages, or subducting seamounts/ridges. The Nicaragua/Costa Rica segment of the Middle America subduction zone is highly variable along strike and down dip. We use this margin to examine how these variable conditions affect earthquake behavior by determining local ratios of compressional to shear wave velocities (Vp/Vs) and detailed seismic velocity structure. Vp/Vs is one of the best tools available to reliably define fault conditions because it is directly related to the Poisson's ratio of the fault material, and it is sensitive to the presence of fluids and changing permeability. Thus with well-resolved near source Vp/Vs measurements we can infer composition and/or high fluid pressures. Here, we use a technique developed by Lin and Shearer (2007) to determine local Vp/Vs in small areas (~2 x 2 x 2 km) with high seismicity. Within the seismogenic zone, we find the margin to be highly variable along strike in Vp/Vs and seismic velocity. These changes correlate to documented variability in incoming plate properties. Increased Vp/Vs is associated with intraplate earthquakes along Nicaragua and northern Costa Rica. We compare our results with other geophysical studies including new high-resolution images of seismic velocity structure, an extensive catalog of high quality relocated events, apparent stress calculations, coupling, and SSE/NVT occurrence. A better understanding of the connection between fault properties and earthquake behavior gives insight into the role of fluids in seismogenesis, the spectrum of earthquake rupture, and possible hazard at subduction zones.

Ophiolitic basement to the Great Valley forearc basin, California, from seismic and gravity data: Implications for crustal growth at the North American continental margin

USGS Publications Warehouse

Godfrey, N.J.; Beaudoin, B.C.; Klemperer, S.L.; Levander, A.; Luetgert, J.; Meltzer, A.; Mooney, W.; Tréhu, A.

1997-01-01

The nature of the Great Valley basement, whether oceanic or continental, has long been a source of controversy. A velocity model (derived from a 200-km-long east-west reflection-refraction profile collected south of the Mendocino triple junction, northern California, in 1993), further constrained by density and magnetic models, reveals an ophiolite underlying the Great Valley (Great Valley ophiolite), which in turn is underlain by a westward extension of lower-density continental crust (Sierran affinity material). We used an integrated modeling philosophy, first modeling the seismic-refraction data to obtain a final velocity model, and then modeling the long-wavelength features of the gravity data to obtain a final density model that is constrained in the upper crust by our velocity model. The crustal section of Great Valley ophiolite is 7-8 km thick, and the Great Valley ophiolite relict oceanic Moho is at 11-16 km depth. The Great Valley ophiolite does not extend west beneath the Coast Ranges, but only as far as the western margin of the Great Valley, where the 5-7-km-thick Great Valley ophiolite mantle section dips west into the present-day mantle. There are 16-18 km of lower-density Sierran affinity material beneath the Great Valley ophiolite mantle section, such that a second, deeper, "present-day" continental Moho is at about 34 km depth. At mid-crustal depths, the boundary between the eastern extent of the Great Valley ophiolite and the western extent of Sierran affinity material is a near-vertical velocity and density discontinuity about 80 km east of the western margin of the Great Valley. Our model has important implications for crustal growth at the North American continental margin. We suggest that a thick ophiolite sequence was obducted onto continental material, probably during the Jurassic Nevadan orogeny, so that the Great Valley basement is oceanic crust above oceanic mantle vertically stacked above continental crust and continental mantle.

Water-table and discharge changes associated with the 2016-2017 seismic sequence in central Italy: hydrogeological data and a conceptual model for fractured carbonate aquifers

NASA Astrophysics Data System (ADS)

Petitta, Marco; Mastrorillo, Lucia; Preziosi, Elisabetta; Banzato, Francesca; Barberio, Marino Domenico; Billi, Andrea; Cambi, Costanza; De Luca, Gaetano; Di Carlo, Giuseppe; Di Curzio, Diego; Di Salvo, Cristina; Nanni, Torquato; Palpacelli, Stefano; Rusi, Sergio; Saroli, Michele; Tallini, Marco; Tazioli, Alberto; Valigi, Daniela; Vivalda, Paola; Doglioni, Carlo

2018-01-01

A seismic sequence in central Italy from August 2016 to January 2017 affected groundwater dynamics in fractured carbonate aquifers. Changes in spring discharge, water-table position, and streamflow were recorded for several months following nine Mw 5.0-6.5 seismic events. Data from 22 measurement sites, located within 100 km of the epicentral zones, were analyzed. The intensity of the induced changes were correlated with seismic magnitude and distance to epicenters. The additional post-seismic discharge from rivers and springs was found to be higher than 9 m3/s, totaling more than 0.1 km3 of groundwater release over 6 months. This huge and unexpected contribution increased streamflow in narrow mountainous valleys to previously unmeasured peak values. Analogously to the L'Aquila 2009 post-earthquake phenomenon, these hydrogeological changes might reflect an increase of bulk hydraulic conductivity at the aquifer scale, which would increase hydraulic heads in the discharge zones and lower them in some recharge areas. The observed changes may also be partly due to other mechanisms, such as shaking and/or squeezing effects related to intense subsidence in the core of the affected area, where effects had maximum extent, or breaching of hydraulic barriers.

Water-table and discharge changes associated with the 2016-2017 seismic sequence in central Italy: hydrogeological data and a conceptual model for fractured carbonate aquifers

NASA Astrophysics Data System (ADS)

Petitta, Marco; Mastrorillo, Lucia; Preziosi, Elisabetta; Banzato, Francesca; Barberio, Marino Domenico; Billi, Andrea; Cambi, Costanza; De Luca, Gaetano; Di Carlo, Giuseppe; Di Curzio, Diego; Di Salvo, Cristina; Nanni, Torquato; Palpacelli, Stefano; Rusi, Sergio; Saroli, Michele; Tallini, Marco; Tazioli, Alberto; Valigi, Daniela; Vivalda, Paola; Doglioni, Carlo

2018-06-01

A seismic sequence in central Italy from August 2016 to January 2017 affected groundwater dynamics in fractured carbonate aquifers. Changes in spring discharge, water-table position, and streamflow were recorded for several months following nine Mw 5.0-6.5 seismic events. Data from 22 measurement sites, located within 100 km of the epicentral zones, were analyzed. The intensity of the induced changes were correlated with seismic magnitude and distance to epicenters. The additional post-seismic discharge from rivers and springs was found to be higher than 9 m3/s, totaling more than 0.1 km3 of groundwater release over 6 months. This huge and unexpected contribution increased streamflow in narrow mountainous valleys to previously unmeasured peak values. Analogously to the L'Aquila 2009 post-earthquake phenomenon, these hydrogeological changes might reflect an increase of bulk hydraulic conductivity at the aquifer scale, which would increase hydraulic heads in the discharge zones and lower them in some recharge areas. The observed changes may also be partly due to other mechanisms, such as shaking and/or squeezing effects related to intense subsidence in the core of the affected area, where effects had maximum extent, or breaching of hydraulic barriers.

Fault zones ruptured during the early 2014 Cephalonia Island (Ionian Sea, Western Greece) earthquakes (January 26 and February 3, Mw 6.0) based on the associated co-seismic surface ruptures

NASA Astrophysics Data System (ADS)

Lekkas, Efthymios L.; Mavroulis, Spyridon D.

2016-01-01

The early 2014 Cephalonia Island (Ionian Sea, Western Greece) earthquake sequence comprised two main shocks with almost the same magnitude (moment magnitude (Mw) 6.0) occurring successively within a short time (January 26 and February 3) and space (Paliki peninsula in Western Cephalonia) interval. Εach earthquake was induced by the rupture of a different pre-existing onshore active fault zone and produced different co-seismic surface rupture zones. Co-seismic surface rupture structures were predominantly strike-slip-related structures including V-shaped conjugate surface ruptures, dextral and sinistral strike-slip surface ruptures, restraining and releasing bends, Riedel structures ( R, R', P, T), small-scale bookshelf faulting, and flower structures. An extensional component was present across surface rupture zones resulting in ground openings (sinkholes), small-scale grabens, and co-seismic dip-slip (normal) displacements. A compressional component was also present across surface rupture zones resulting in co-seismic dip-slip (reverse) displacements. From the comparison of our field geological observations with already published surface deformation measurements by DInSAR Interferometry, it is concluded that there is a strong correlation among the surface rupture zones, the ruptured active fault zones, and the detected displacement discontinuities in Paliki peninsula.

Planar seismic source characterization models developed for probabilistic seismic hazard assessment of Istanbul

NASA Astrophysics Data System (ADS)

Gülerce, Zeynep; Buğra Soyman, Kadir; Güner, Barış; Kaymakci, Nuretdin

2017-12-01

This contribution provides an updated planar seismic source characterization (SSC) model to be used in the probabilistic seismic hazard assessment (PSHA) for Istanbul. It defines planar rupture systems for the four main segments of the North Anatolian fault zone (NAFZ) that are critical for the PSHA of Istanbul: segments covering the rupture zones of the 1999 Kocaeli and Düzce earthquakes, central Marmara, and Ganos/Saros segments. In each rupture system, the source geometry is defined in terms of fault length, fault width, fault plane attitude, and segmentation points. Activity rates and the magnitude recurrence models for each rupture system are established by considering geological and geodetic constraints and are tested based on the observed seismicity that is associated with the rupture system. Uncertainty in the SSC model parameters (e.g., b value, maximum magnitude, slip rate, weights of the rupture scenarios) is considered, whereas the uncertainty in the fault geometry is not included in the logic tree. To acknowledge the effect of earthquakes that are not associated with the defined rupture systems on the hazard, a background zone is introduced and the seismicity rates in the background zone are calculated using smoothed-seismicity approach. The state-of-the-art SSC model presented here is the first fully documented and ready-to-use fault-based SSC model developed for the PSHA of Istanbul.

Earthquake-induced liquefaction features in the coastal setting of South Carolina and in the fluvial setting of the New Madrid Seismic Zone

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

Obermeier, S.F.; Jacobson, R.B.; Smoot, J.P.

1990-01-01

In both coastal South Carolina and the New Madrid seismic zone, the earthquake-induced liquefaction features generally originated in clean sand deposits that contain no or few intercalated silt- or clay-rich strata. The local geologic setting is a major influence on both development and surface expression of sand blows. Major factors controlling sand-blow formation include the thickness and physical properties of the deposits above the source sands, and these relationships are illustrated by comparing sand blows found in coastal South Carolina (in marine deposits) with sand blows found in the New Madrid seismic zone (in fluvial deposits). In coastal South Carolina,more » the surface stratum is typically a thin (about 1 m) soil that is weakly cemented with humate, and the sand blows are expressed as craters surrounded by a thin sheet of sand; in the New Madrid seismic zone the surface stratum generally is a clay-rich deposit ranging in thickness from 2 to 10 m, in which case sand blows characteristically are expressed as sand mounded above the original ground surface. Recognition of the various features described in this paper, and identification of the most probable origin for each, provides a set of important tools for understanding paleoseismicity in areas such as the Central and Eastern US where faults are not exposed for study and strong seismic activity is infrequent.« less

Crustal structure of the Izu Collision zone, central Japan, revealed by dense seismic array observations

NASA Astrophysics Data System (ADS)

Kurashimo, E.; Sato, H.; Abe, S.; Kato, N.; Ishikawa, M.; Obara, K.

2009-12-01

In central Japan, the Philippine Sea Plate (PSP) subducts beneath the Tokyo Metropolitan area, the Kanto region. In western Kanto region, the Izu-Bonin arc (IBA) within the PSP has been colliding from the south with the Honshu arc, forming a complex structure called the Izu-Collision zone (ICZ). Several active faults were formed in and around the ICZ. The geometry of the subducting PSP and the overlying crustal structure of the ICZ are important to constrain the process of earthquake occurrence and the crustal evolution process associated with arc-arc collision. Recent seismic experiments reveal the geometry of the subducting PSP beneath the Kanto region (Sato et al., 2005). The Japanese islands, including the ICZ, are covered with dense arrays of permanent seismic stations, which provide good constraints on velocity structures by a tomographic method. Such studies reveal a general picture of the lithospheric structure such as a descending plate configuration (e.g. Matsubara et al., 2008). However, since an average spacing of the permanent station is typically 20 km, a detailed structure in the upper crust, which is imperative for an understanding of the active tectonics, cannot be well constrained by permanent array alone. Two dense seismic array observations were conducted to obtain a structural image beneath the ICZ. One is a 40-km-long line (EW-line) located in the northern part of the ICZ and the other is a 55-km-long line (NS-line) located in the central part of the ICZ. Seventy-five 3-component portable seismographs were deployed on EW-line with 500 to 700 m interval and waveforms were recorded during a four-month period from October, 2008. Forty 3-component portable seismographs were deployed on NS-line with about 1 km spacing and waveforms were recorded during the three month period from January, 2006. In order to obtain a high-resolution velocity model, a well-controlled hypocenter is essential. Due to this, we combined the seismic array data with

Permeability and seismic velocity anisotropy across a ductile-brittle fault zone in crystalline rock

NASA Astrophysics Data System (ADS)

Wenning, Quinn C.; Madonna, Claudio; de Haller, Antoine; Burg, Jean-Pierre

2018-05-01

This study characterizes the elastic and fluid flow properties systematically across a ductile-brittle fault zone in crystalline rock at the Grimsel Test Site underground research laboratory. Anisotropic seismic velocities and permeability measured every 0.1 m in the 0.7 m across the transition zone from the host Grimsel granodiorite to the mylonitic core show that foliation-parallel P- and S-wave velocities systematically increase from the host rock towards the mylonitic core, while permeability is reduced nearest to the mylonitic core. The results suggest that although brittle deformation has persisted in the recent evolution, antecedent ductile fabric continues to control the matrix elastic and fluid flow properties outside the mylonitic core. The juxtaposition of the ductile strain zone next to the brittle zone, which is bounded inside the two mylonitic cores, causes a significant elastic, mechanical, and fluid flow heterogeneity, which has important implications for crustal deformation and fluid flow and for the exploitation and use of geothermal energy and geologic waste storage. The results illustrate how physical characteristics of faults in crystalline rocks change in fault zones during the ductile to brittle transitions.

Dominant seismic sources for the cities in South Sumatra

NASA Astrophysics Data System (ADS)

Sunardi, Bambang; Sakya, Andi Eka; Masturyono, Murjaya, Jaya; Rohadi, Supriyanto; Sulastri, Putra, Ade Surya

2017-07-01

Subduction zone along west of Sumatra and Sumatran fault zone are active seismic sources. Seismotectonically, South Sumatra could be affected by earthquakes triggered by these seismic sources. This paper discussed contribution of each seismic source to earthquake hazards for cities of Palembang, Prabumulih, Banyuasin, OganIlir, Ogan Komering Ilir, South Oku, Musi Rawas and Empat Lawang. These hazards are presented in form of seismic hazard curves. The study was conducted by using Probabilistic Seismic Hazard Analysis (PSHA) of 2% probability of exceedance in 50 years. Seismic sources used in analysis included megathrust zone M2 of Sumatra and South Sumatra, background seismic sources and shallow crustal seismic sources consist of Ketaun, Musi, Manna and Kumering faults. The results of the study showed that for cities relatively far from the seismic sources, subduction / megathrust seismic source with a depth ≤ 50 km greatly contributed to the seismic hazard and the other areas showed deep background seismic sources with a depth of more than 100 km dominate to seismic hazard respectively.

Scenario earthquake hazards for the Long Valley Caldera-Mono Lake area, east-central California (ver. 2.0, January 2018)

USGS Publications Warehouse

Chen, Rui; Branum, David M.; Wills, Chris J.; Hill, David P.

2014-06-30

As part of the U.S. Geological Survey’s (USGS) multi-hazards project in the Long Valley Caldera-Mono Lake area, the California Geological Survey (CGS) developed several earthquake scenarios and evaluated potential seismic hazards, including ground shaking, surface fault rupture, liquefaction, and landslide hazards associated with these earthquake scenarios. The results of these analyses can be useful in estimating the extent of potential damage and economic losses because of potential earthquakes and also for preparing emergency response plans.The Long Valley Caldera-Mono Lake area has numerous active faults. Five of these faults or fault zones are considered capable of producing magnitude ≥6.7 earthquakes according to the Uniform California Earthquake Rupture Forecast, Version 2 (UCERF 2) developed by the 2007 Working Group on California Earthquake Probabilities (WGCEP) and the USGS National Seismic Hazard Mapping Program. These five faults are the Fish Slough, Hartley Springs, Hilton Creek, Mono Lake, and Round Valley Faults. CGS developed earthquake scenarios for these five faults in the study area and for the White Mountains Fault Zone to the east of the study area.In this report, an earthquake scenario is intended to depict the potential consequences of significant earthquakes. A scenario earthquake is not necessarily the largest or most damaging earthquake possible on a recognized fault. Rather it is both large enough and likely enough that emergency planners should consider it in regional emergency response plans. In particular, the ground motion predicted for a given scenario earthquake does not represent a full probabilistic hazard assessment, and thus it does not provide the basis for hazard zoning and earthquake-resistant building design.Earthquake scenarios presented here are based on fault geometry and activity data developed by the WGCEP, and are consistent with the 2008 Update of the United States National Seismic Hazard Maps (NSHM). Alternatives

Interseismic Coupling, Co- and Post-seismic Slip: a Stochastic View on the Northern Chilean Subduction Zone

NASA Astrophysics Data System (ADS)

Jolivet, R.; Duputel, Z.; Simons, M.; Jiang, J.; Riel, B. V.; Moore, A. W.; Owen, S. E.

2017-12-01

Mapping subsurface fault slip during the different phases of the seismic cycle provides a probe of the mechanical properties and the state of stress along these faults. We focus on the northern Chile megathrust where first order estimates of interseismic fault locking suggests little to no overlap between regions slipping seismically versus those that are dominantly aseismic. However, published distributions of slip, be they during seismic or aseismic phases, rely on unphysical regularization of the inverse problem, thereby cluttering attempts to quantify the degree of overlap between seismic and aseismic slip. Considering all the implications of aseismic slip on our understanding of the nucleation, propagation and arrest of seismic ruptures, it is of utmost importance to quantify our confidence in the current description of fault coupling. Here, we take advantage of 20 years of InSAR observations and more than a decade of GPS measurements to derive probabilistic maps of inter-seismic coupling, as well as co-seismic and post-seismic slip along the northern Chile subduction megathrust. A wide InSAR velocity map is derived using a novel multi-pixel time series analysis method accounting for orbital errors, atmospheric noise and ground deformation. We use AlTar, a massively parallel Monte Carlo Markov Chain algorithm exploiting the acceleration capabilities of Graphic Processing Units, to derive the probability density functions (PDF) of slip. In northern Chile, we find high probabilities for a complete release of the elastic strain accumulated since the 1877 earthquake by the 2014, Iquique earthquake and for the presence of a large, independent, locked asperity left untapped by recent events, north of the Mejillones peninsula. We evaluate the probability of overlap between the co-, inter- and post-seismic slip and consider the potential occurrence of slow, aseismic slip events along this portion of the subduction zone.

A tree fell in the forest, and SPREE heard it: seismic recording of the 2011 St. Croix Valley Blowdown

NASA Astrophysics Data System (ADS)

Wolin, E.; van der Lee, S.

2016-12-01

As part of the Superior Province Rifting Earthscope Experiment (SPREE), 82 broadband seismic stations from the EarthScope Flexible Array pool were deployed by the SPREE team from April 2011 through October 2013, to explore the deep structure of the Mid-Continent Rift System (Stein et al., 2011). The deployment included two crosslines with approximate station spacing of 10 km centered near the northern Minnesota-Wisconsin border. Analysis of long-period noise reveals strong seasonal and diurnal variations (Wolin et al., 2015). On 1 July 2011, a severe thunderstorm system swept over the St. Croix Valley, passing directly over the dense SPREE array. This storm system was accompanied by a series of downbursts that generated straight-line winds in excess of 100 km/hr, resulting in extensive damage to hundreds of thousands of acres of forest. Seven SPREE stations were located in the path of the storm, with two stations in the center of areas that were heavily damaged by downbursts. The stations remained in operation throughout this extreme weather event, capturing a unique record of ground noise generated by the storm system. We compare available radar reflectivity data with seismic noise power spectra throughout the event and show that storm cells generated significant broadband seismic signals as they passed over the region. Relative to typical background seismic noise levels, power between 0.05-10 Hz increased by 5-20 dB during the storm. Seismic noise levels can be compared to available wind speed data to provide a detailed record of wind speeds during the weather event. We also explore the long-period coherence of energy across the array, which is potentially useful to help constrain near-surface velocity structure at the array sites as well as to better characterize how atmospheric processes couple into the solid earth during severe weather events.

Seismic Monitoring of Rock Falls in Yosemite National Park

NASA Astrophysics Data System (ADS)

Zimmer, V. L.; Stock, G. M.; Sitar, N.

2008-12-01

Between 1857 and 2007, more than 600 landslide events have been documented in Yosemite National Park, with the vast majority of events occurring as rock falls in Yosemite Valley. The conditions leading to and triggering rock fall are understood in approximately 50 percent of cases, but in the other 50 percent, there were no apparent triggers. Occasionally, large rock falls have been preceded by smaller events that, in retrospect, may have been precursors. Close range seismic monitoring presents an opportunity to study the conditions leading up to rock fall, as well as the mechanics of the actual rock fall as recorded seismically. During the winter of 2007-08, we conducted a rock fall seismic monitoring feasibility study in Yosemite Valley. A station consisting of an 8 Hz geophone and an accelerometer was placed on a ledge 1000 feet above the valley floor, in a historically active rock fall area known as the Three Brothers. At least two rock falls in this area were recorded by the instrumentation and witnessed by visitors, representing the first time rock falls have been recorded with seismic instrumentation in Yosemite Valley. Significant energy was recorded in a wide frequency range, from a few Hz to approximately 150 Hz, limited by the geophone response and attenuation of the signal due to distance to the source (400 m). Furthermore, there exists a weak signal approximately 5-10 seconds before the obvious rock fall signature. We hypothesize that the weak signal represents rock fall initiation manifesting as the first blocks sliding down the cliff face, while the stronger impulses represent these blocks impacting ledges and the bottom talus field. This study demonstrated that rock fall monitoring is feasible with seismic instrumentation, and serves as the catalyst for future studies using a network of sensors for more advanced analysis.

Kinematic evolution of the Maacama Fault Zone, Northern California Coast Ranges

NASA Astrophysics Data System (ADS)

Schroeder, Rick D.

The Maacama Fault Zone (MFZ) is a major component of the Pacific-North American transform boundary in northern California, and its distribution of deformation and kinematic evolution defines that of a young continental transform boundary. The USGS Quaternary database (2010) currently defines the MFZ as a relatively narrow fault zone; however, a cluster analysis of microearthquakes beneath the MFZ defines a wider fault zone, composed of multiple seismogenically active faults. The surface projection of best-fit tabular zones through foci clusters correlates with previously interpreted faults that were assumed inactive. New investigations further delineate faults within the MFZ based on geomorphic features and shallow resistivity surveys, and these faults are interpreted to be part of several active pull-apart fault systems. The location of faults and changes in their geometry in relation to geomorphic features, indicate >8 km of cumulative dextral displacement across the eastern portion of the MFZ at Little Lake Valley, which includes other smaller offsets on fault strands in the valley. Some faults within the MFZ have geometries consistent with reactivated subduction-related reverse faults, and project near outcrops of pre-existing faults, filled with mechanically weak minerals. The mechanical behavior of fault zones is influenced by the spatial distribution and abundance of mechanically weak lithologies and mineralogies within the heterogeneous Franciscan melange that the MFZ displaces. This heterogeneity is characterized near Little Lake Valley (LLV) using remotely sensed data, field mapping, and wellbore data, and is composed of 2--5 km diameter disk-shaped coherent blocks that can be competent and resist deformation. Coherent blocks and the melange that surrounds them are the source for altered minerals that fill portions of fault zones. Mechanically weak minerals in pre-existing fault zones, identified by X-ray diffraction and electron microprobe analyses, are

The CoRoT target HD 49933: a possible seismic signature of heavy elements ionization in the deep convective zone

NASA Astrophysics Data System (ADS)

Brito, Ana; Lopes, Ilídio

2017-04-01

We use a seismic diagnostic, based on the derivative of the phase shift of the acoustic waves reflected by the surface, to probe the outer layers of the star HD 49933. This diagnostic is particularly sensitive to partial ionization processes occurring above the base of the convective zone. The regions of partial ionization of light elements, hydrogen and helium, have well-known seismological signatures. In this work, we detect a different seismic signature in the acoustic frequencies, which we showed to correspond to the location where the partial ionization of heavy elements occurs. The location of the corresponding acoustic glitch lies between the region of the second ionization of helium and the base of the convective zone, approximately 5 per cent below the surface of the stars.

Driving Processes of Earthquake Swarms: Evidence from High Resolution Seismicity

NASA Astrophysics Data System (ADS)

Ellsworth, W. L.; Shelly, D. R.; Hill, D. P.; Hardebeck, J.; Hsieh, P. A.

2017-12-01

Earthquake swarms are transient increases in seismicity deviating from a typical mainshock-aftershock pattern. Swarms are most prevalent in volcanic and hydrothermal areas, yet also occur in other environments, such as extensional fault stepovers. Swarms provide a valuable opportunity to investigate source zone physics, including the causes of their swarm-like behavior. To gain insight into this behavior, we have used waveform-based methods to greatly enhance standard seismic catalogs. Depending on the application, we detect and precisely relocate 2-10x as many events as included in the initial catalog. Recently, we have added characterization of focal mechanisms (applied to a 2014 swarm in Long Valley Caldera, California), addressing a common shortcoming in microseismicity analyses (Shelly et al., JGR, 2016). In analysis of multiple swarms (both within and outside volcanic areas), several features stand out, including: (1) dramatic expansion of the active source region with time, (2) tendency for events to occur on the immediate fringe of prior activity, (3) overall upward migration, and (4) complex faulting structure. Some swarms also show an apparent mismatch between seismicity orientations (as defined by patterns in hypocentral locations) and slip orientations (as inferred from focal mechanisms). These features are largely distinct from those observed in mainshock-aftershock sequences. In combination, these swarm behaviors point to an important role for fluid pressure diffusion. Swarms may in fact be generated by a cascade of fluid pressure diffusion and stress transfer: in cases where faults are critically stressed, an increase in fluid pressure will trigger faulting. Faulting will in turn dramatically increase permeability in the faulted area, allowing rapid equilibration of fluid pressure to the fringe of the rupture zone. This process may perpetuate until fluid pressure perturbations drop and/or stresses become further from failure, such that any

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.

Geophysical Surveys of the Hydrologic Basin Underlying Yosemite Valley, California.

NASA Astrophysics Data System (ADS)

Maher, E. L.; Shaw, K. A.; Carey, C.; Dunn, M. E.; Whitman, S.; Bourdeau, J.; Eckert, E.; Louie, J. N.; Stock, G. M.

2017-12-01

UNR students in an Applied Geophysics course conducted geophysical investigations in Yosemite Valley during the months of March and August 2017. The goal of the study is to understand better the depth to bedrock, the geometry of the bedrock basin, and the properties of stratigraphy- below the valley floor. Gutenberg and others published the only prior geophysical investigation in 1956, to constrain the depth to bedrock. We employed gravity, resistivity, and refraction microtremor(ReMi) methods to investigate the interface between valley fill and bedrock, as well as shallow contrasts. Resistivity and ReMi arrays along three north-south transects investigated the top 50-60m of the basin fill. Gravity results constrained by shallow measurements suggest a maximum depth of 1000 m to bedrock. ReMi and resistivity techniques identified shallow contrasts in shear velocity and electrical resistivity that yielded information about the location of the unconfined water table, the thickness of the soil zone, and spatial variation in shallow sediment composition. The upper several meters of sediment commonly showed shear velocities below 200 m/s, while biomass-rich areas and sandy river banks could be below 150 m/s. Vs30 values consistently increased towards the edge of the basin. The general pattern for resistivity profiles was a zone of relatively high resistivity, >100 ohm-m, in the top 4 meters, followed by one or more layers with decreased resistivity. According to gravity measurements, assuming either -0.5 g/cc or -0.7 g/cc density contrast between bedrock and basin sediments, a maximum depth to bedrock is found south of El Capitan at respectively, 1145 ± 215 m or 818 ± 150 m. Longitudinal basin geometry coincides with the basin depth geometry discussed by Gutenberg in 1956. Their results describe a "double camel" shape where the deepest points are near El Capitan and the Ahwahnee Hotel and is shallowest near Yosemite Falls, in a wider part of the valley. An August Deep

Evolution of Late Miocene to Contemporary Displacement Transfer Between the Northern Furnace Creek and Southern Fish Lake Valley Fault Zones and the Central Walker Lane, Western Great Basin, Nevada

NASA Astrophysics Data System (ADS)

Oldow, J. S.; Geissman, J. W.

2013-12-01

Late Miocene to contemporary displacement transfer from the north Furnace Creek (FCF) and southern Fish Lake Valley (FLVF) faults to structures in the central Walker Lane was and continues to be accommodated by a belt of WNW-striking left-oblique fault zones in the northern part of the southern Walker Lane. The WNW fault zones are 2-9 km wide belts of anastomosing fault strands that intersect the NNW-striking FCF and southern FLVF in northern Death Valley and southern Fish Lake Valley, respectively. The WNW fault zones extend east for over 60 km where they merge with a 5-10 km wide belt of N10W striking faults that marks the eastern boundary of the southern Walker Lane. Left-oblique displacement on WNW faults progressively decreases to the east, as motion is successively transferred northeast on NNE-striking faults. NNE faults localize and internally deform extensional basins that each record cumulative net vertical displacements of between 3.0 and 5.2 km. The transcurrent faults and associated basins decrease in age from south to north. In the south, the WNW Sylvania Mountain fault system initiated left-oblique motion after 7 Ma but does not have evidence of contemporary displacement. Farther north, the left-oblique motion on the Palmetto Mountain fault system initiated after 6.0 to 4.0 Ma and has well-developed scarps in Quaternary deposits. Cumulative left-lateral displacement for the Sylvania Mountain fault system is 10-15 km, and is 8-12 km for the Palmetto fault system. The NNE-striking faults that emanate from the left-oblique faults merge with NNW transcurrent faults farther north in the eastern part of the Mina deflection, which links the Owens Valley fault of eastern California to the central Walker Lane. Left-oblique displacement on the Sylvania Mountain and Palmetto Mountain fault zones deformed the Furnace Creek and Fish Lake Valley faults. Left-oblique motion on Sylvania Mountain fault deflected the FCF into the 15 km wide Cucomungo Canyon restraining

Geochemistry of soil gas in the seismic fault zone produced by the Wenchuan Ms 8.0 earthquake, southwestern China

PubMed Central

2010-01-01

The spatio-temporal variations of soil gas in the seismic fault zone produced by the 12 May 2008 Wenchuan Ms 8.0 earthquake were investigated based on the field measurements of soil gas concentrations after the main shock. Concentrations of He, H2, CO2, CH4, O2, N2, Rn, and Hg in soil gas were measured in the field at eight short profiles across the seismic rupture zone in June and December 2008 and July 2009. Soil-gas concentrations of more than 800 sampling sites were obtained. The data showed that the magnitudes of the He and H2 anomalies of three surveys declined significantly with decreasing strength of the aftershocks with time. The maximum concentrations of He and H2 (40 and 279.4 ppm, respectively) were found in three replicates at the south part of the rupture zone close to the epicenter. The spatio-temporal variations of CO2, Rn, and Hg concentrations differed obviously between the north and south parts of the fault zone. The maximum He and H2 concentrations in Jun 2008 occurred near the parts of the rupture zone where vertical displacements were larger. The anomalies of He, H2, CO2, Rn, and Hg concentrations could be related to the variation in the regional stress field and the aftershock activity. PMID:21134257

Seismic Hazard Maps for Seattle, Washington, Incorporating 3D Sedimentary Basin Effects, Nonlinear Site Response, and Rupture Directivity

USGS Publications Warehouse

Frankel, Arthur D.; Stephenson, William J.; Carver, David L.; Williams, Robert A.; Odum, Jack K.; Rhea, Susan

2007-01-01

This report presents probabilistic seismic hazard maps for Seattle, Washington, based on over 500 3D simulations of ground motions from scenario earthquakes. These maps include 3D sedimentary basin effects and rupture directivity. Nonlinear site response for soft-soil sites of fill and alluvium was also applied in the maps. The report describes the methodology for incorporating source and site dependent amplification factors into a probabilistic seismic hazard calculation. 3D simulations were conducted for the various earthquake sources that can affect Seattle: Seattle fault zone, Cascadia subduction zone, South Whidbey Island fault, and background shallow and deep earthquakes. The maps presented in this document used essentially the same set of faults and distributed-earthquake sources as in the 2002 national seismic hazard maps. The 3D velocity model utilized in the simulations was validated by modeling the amplitudes and waveforms of observed seismograms from five earthquakes in the region, including the 2001 M6.8 Nisqually earthquake. The probabilistic seismic hazard maps presented here depict 1 Hz response spectral accelerations with 10%, 5%, and 2% probabilities of exceedance in 50 years. The maps are based on determinations of seismic hazard for 7236 sites with a spacing of 280 m. The maps show that the most hazardous locations for this frequency band (around 1 Hz) are soft-soil sites (fill and alluvium) within the Seattle basin and along the inferred trace of the frontal fault of the Seattle fault zone. The next highest hazard is typically found for soft-soil sites in the Duwamish Valley south of the Seattle basin. In general, stiff-soil sites in the Seattle basin exhibit higher hazard than stiff-soil sites outside the basin. Sites with shallow bedrock outside the Seattle basin have the lowest estimated hazard for this frequency band.

Tectonics and seismicity of the southern Washington Cascade range

USGS Publications Warehouse

Stanley, W.D.; Johnson, S.Y.; Qamar, A.I.; Weaver, C.S.; Williams, J.M.

1996-01-01

Geophysical, geological, and seismicity data are combined to develop a transpressional strain model for the southern Washington Cascades region. We use this model to explain oblique fold and fault systems, transverse faults, and a linear seismic zone just west of Mt. Rainier known as the western Rainier zone. We also attempt to explain a concentration of earthquakes that connects the northwest-trending Mount St. Helens seismic zone to the north-trending western Rainier zone. Our tectonic model illustrates the pervasive effects of accretionary processes, combined with subsequent transpressive forces generated by oblique subduction, on Eocene to present crustal processes, such as seismicity and volcanism.

Geophysical Investigation of the Lake City Fault Zone, Surprise Valley, California, and Implications for Geothermal Circulation

NASA Astrophysics Data System (ADS)

McPhee, D. K.; Glen, J. M.; Egger, A. E.; Chuchel, B. A.

2009-12-01

New audiomagnetotelluric (AMT), gravity, and magnetic data were collected in Surprise Valley, northwestern Basin and Range, in order to investigate the role that the Lake City Fault Zone (LCFZ) may play in controlling geothermal circulation in the area. Surprise Valley hosts an extensional geothermal system currently undergoing exploration for development on several scales. The focus of much of that exploration has been the LCFZ, a set of NW-SE-trending structures that has been suggested on the basis of (1) low-relief scarps in the NW portion of the zone, (2) dissolved mineral-rich groundwater chemistry along its length, and (3) parallelism with a strong regional fabric that includes the Brothers Fault Zone. The LCFZ extends across the valley at a topographic high, intersecting the N-S-trending basin-bounding faults where major hot springs occur. This relationship suggests that the LCFZ may be a zone of permeability for flow of hydrothermal fluids. Previous potential field data indicate that there is no vertical offset along this fault zone, and little signature at all in either the gravity or magnetic data; along with the lack of surface expression along most of its length, the subsurface geometry of the LCFZ and its influence on geothermal fluid circulation remains enigmatic. The LCFZ therefore provides an ideal opportunity to utilize AMT data, which measures subsurface resistivity and therefore - unlike potential field data - is highly sensitive to the presence of saline fluids. AMT data and additional gravity and magnetic data were collected in 2009 along 3 profiles perpendicular to the LCFZ in order to define the subsurface geometry and conductivity of the fault zone down to depths of ~ 500 m. AMT soundings were collected using the Geometrics Stratagem EH4 system, a four channel, natural and controlled-source tensor system recording in the range of 10 to 92,000 Hz. To augment the low signal in the natural field a transmitter of two horizontal-magnetic dipoles

Subduction Zone Dewatering at the Southern End of New Zealand's Hikurangi Margin - Insights from 2D Seismic Tomography

NASA Astrophysics Data System (ADS)

Crutchley, G. J.; Klaeschen, D.

2016-12-01

The southern end of New Zealand's Hikurangi subduction margin is characterised by highly-oblique convergence as it makes a southward transition into a right-lateral transform plate boundary. Long-offset seismic data that cross part of the offshore portion of this transition zone give new insight into the nature of the margin. We have carried out two-dimensional pre-stack depth migrations with an iterative reflection tomography to update the velocity field on two seismic lines in this area. The depth-migrated sections show much-improved imaging of faulting within the wedge, and the seismic velocities themselves give clues about the distribution of gas and/or overpressured regions at the plate boundary and within the overlying wedge. A fascinating observation is a major splay fault that has been (or continues to be) a preferred dewatering pathway through the wedge, evidenced by a thermal anomaly that has left its mark on the overlying gas hydrate layer. Another interesting observation is a thick and laterally extensive low velocity zone beneath the subduction interface, which might have important implications for the long-term mechanical stability of the interface. Our on-going work on these data is focused on amplitude versus offset analysis in an attempt to better understand the nature of the subduction interface and also the shallower gas hydrate system. This study is an example of how distinct disturbances of the gas hydrate system can provide insight into subduction zone fluid flow processes that are important for understanding wedge stability and ultimately earthquake hazard.

Seismic hazard assessment of the Kivu rift segment based on a new seismotectonic zonation model (western branch, East African Rift system)

NASA Astrophysics Data System (ADS)

Delvaux, Damien; Mulumba, Jean-Luc; Sebagenzi, Mwene Ntabwoba Stanislas; Bondo, Silvanos Fiama; Kervyn, François; Havenith, Hans-Balder

2017-10-01

In the frame of the Belgian GeoRisCA multi-risk assessment project focusing on the Kivu and northern Tanganyika rift region in Central Africa, a new probabilistic seismic hazard assessment has been performed for the Kivu rift segment in the central part of the western branch of the East African rift system. As the geological and tectonic setting of this region is incompletely known, especially the part lying in the Democratic Republic of the Congo, we compiled homogeneous cross-border tectonic and neotectonic maps. The seismic risk assessment is based on a new earthquake catalogue based on the ISC reviewed earthquake catalogue and supplemented by other local catalogues and new macroseismic epicenter data spanning 126 years, with 1068 events. The magnitudes have been homogenized to Mw and aftershocks removed. The final catalogue used for the seismic hazard assessment spans 60 years, from 1955 to 2015, with 359 events and a magnitude of completeness of 4.4. The seismotectonic zonation into 7 seismic source areas was done on the basis of the regional geological structure, neotectonic fault systems, basin architecture and distribution of thermal springs and earthquake epicenters. The Gutenberg-Richter seismic hazard parameters were determined by the least square linear fit and the maximum likelihood method. Seismic hazard maps have been computed using existing attenuation laws with the Crisis 2012 software. We obtained higher PGA values (475 years return period) for the Kivu rift region than the previous estimates. They also vary laterally in function of the tectonic setting, with the lowest value in the volcanically active Virunga - Rutshuru zone, highest in the currently non-volcanic parts of Lake Kivu, Rusizi valley and North Tanganyika rift zone, and intermediate in the regions flanking the axial rift zone.

Comparison of Earthquake Damage Patterns and Shallow-Depth Vs Structure Across the Napa Valley, Inferred From Multichannel Analysis of Surface Waves (MASW) and Multichannel Analysis of Love Waves (MALW) Modeling of Basin-Wide Seismic Profiles

NASA Astrophysics Data System (ADS)

Chan, J. H.; Catchings, R.; Strayer, L. M.; Goldman, M.; Criley, C.; Sickler, R. R.; Boatwright, J.

2017-12-01

We conducted an active-source seismic investigation across the Napa Valley (Napa Valley Seismic Investigation-16) in September of 2016 consisting of two basin-wide seismic profiles; one profile was 20 km long and N-S-trending (338°), and the other 15 km long and E-W-trending (80°) (see Catchings et al., 2017). Data from the NVSI-16 seismic investigation were recorded using a total of 666 vertical- and horizontal-component seismographs, spaced 100 m apart on both seismic profiles. Seismic sources were generated by a total of 36 buried explosions spaced 1 km apart. The two seismic profiles intersected in downtown Napa, where a large number of buildings were red-tagged by the City following the 24 August 2014 Mw 6.0 South Napa earthquake. From the recorded Rayleigh and Love waves, we developed 2-Dimensional S-wave velocity models to depths of about 0.5 km using the multichannel analysis of surface waves (MASW) method. Our MASW (Rayleigh) and MALW (Love) models show two prominent low-velocity (Vs = 350 to 1300 m/s) sub-basins that were also previously identified from gravity studies (Langenheim et al., 2010). These basins trend N-W and also coincide with the locations of more than 1500 red- and yellow-tagged buildings within the City of Napa that were tagged after the 2014 South Napa earthquake. The observed correlation between low-Vs, deep basins, and the red-and yellow-tagged buildings in Napa suggests similar large-scale seismic investigations can be performed. These correlations provide insights into the likely locations of significant structural damage resulting from future earthquakes that occur adjacent to or within sedimentary basins.

Shallow Vs Structure Accross Hayward Fault Zone Inferred from Multichannel Analysis of Surface Waves (MASW)

NASA Astrophysics Data System (ADS)

Chan, J. H.; Richardson, I. S.; Strayer, L. M.; Catchings, R.; McEvilly, A.; Goldman, M.; Criley, C.; Sickler, R. R.

2017-12-01

The Hayward Fault Zone (HFZ) includes the Hayward fault (HF), as well as several named and unnamed subparallel, subsidiary faults to the east, among them the Quaternary-active Chabot Fault (CF), the Miller Creek Fault (MCF), and a heretofore unnamed fault, the Redwood Thrust Fault (RTF). With an ≥M6.0 recurrence interval of 130 y for the HF and the last major earthquake in 1868, the HFZ is a major seismic hazard in the San Francisco Bay Area, exacerbated by the many unknown and potentially active secondary faults of the HFZ. In 2016, researchers from California State University, East Bay, working in concert with the United States Geological Survey conducted the East Bay Seismic Investigation (EBSI). We deployed 296 RefTek RT125 (Texan) seismographs along a 15-km-long linear seismic profile across the HF, extending from the bay in San Leandro to the hills in Castro Valley. Two-channel seismographs were deployed at 100 m intervals to record P- and S-waves, and additional single-channel seismographs were deployed at 20 m intervals where the seismic line crossed mapped faults. The active-source survey consisted of 16 buried explosive shots located at approximately 1-km intervals along the seismic line. We used the Multichannel Analysis of Surfaces Waves (MASW) method to develop 2-D shear-wave velocity models across the CF, MCF, and RTF. Preliminary MASW analysis show areas of anomalously low S-wave velocities , indicating zones of reduced shear modulus, coincident with these three mapped faults; additional velocity anomalies coincide with unmapped faults within the HFZ. Such compliant zones likely correspond to heavily fractured rock surrounding the faults, where the shear modulus is expected to be low compared to the undeformed host rock.

Using thermodynamic data to reproduce main seismic features of transition zone

NASA Astrophysics Data System (ADS)

Fomin, Ilya; Saukko, Anna; Edwards, Paul; Schiffer, Christian

2016-04-01

Most of the seismic tomography studies nowadays are based on comprehensive models with optimization of lots of parameters. These models are able to resolve very subtle features of the Earth's mantle, but the influence of each specific parameter is not seen directly. In our research we try to minimize the number of processed parameters to produce simple synthetic cases. The main goals of our model are to see how water content influences the depth of the transition zone, and if melting at the transition zone is plausible. We also attempt to see how water content and the presence of melts influence the signal strength of the transition zone in receiver functions. Our MATLAB-code calculates phase assemblage according to specific temperature and pressure within 2D numerical domain (e.g. 300x700 km). Phase properties are calculated with database of Stixrude and Lithgow-Bertelloni [2011], with corrections for water impact on elastic constants according to Liu et al., [2012]. We use the mantle phase composition 55% garnet and 45% olivine-polymorph, soliduses by Ohtani et al. [2004] and melt properties by Sakamaki et al. [2006]. These data are used to calculate seismic velocities and, furthermore, receiver functions with standard routines (e.g.[Schiffer et al., 2012]). Model predicts Vs within 5 to 5.5 km/s and Vp around 9.5-10 km/s within transition zone (Vp/Vs = 1.84-1.87), which is close to standard values. The presence of water enlarges the wadsleyite region, but also dampens the peak of receiver functions down to background level. Increase in water content causes melting at much shallower depths. Using a normal thermal gradient, we can get up to 10% of melt at depths around 390 km with 80% of water saturation, shown by a negative anomaly on receiver functions. This result is similar to data obtained for Afar Plateau [Thompson et al., 2015]. With cratonic thermal gradient, the olivine-wadsleyite transition and corresponding melt layer appear at depths around 350 km

Characterizing potentially induced earthquake rate changes in the Brawley Seismic Zone, southern California

USGS Publications Warehouse

Llenos, Andrea L.; Michael, Andrew J.

2016-01-01

The Brawley seismic zone (BSZ), in the Salton trough of southern California, has a history of earthquake swarms and geothermal energy exploitation. Some earthquake rate changes may have been induced by fluid extraction and injection activity at local geothermal fields, particularly at the North Brawley Geothermal Field (NBGF) and at the Salton Sea Geothermal Field (SSGF). We explore this issue by examining earthquake rate changes and interevent distance distributions in these fields. In Oklahoma and Arkansas, where considerable wastewater injection occurs, increases in background seismicity rate and aftershock productivity and decreases in interevent distance were indicative of fluid‐injection‐induced seismicity. Here, we test if similar changes occur that may be associated with fluid injection and extraction in geothermal areas. We use stochastic epidemic‐type aftershock sequence models to detect changes in the underlying seismogenic processes, shown by statistically significant changes in the model parameters. The most robust model changes in the SSGF roughly occur when large changes in net fluid production occur, but a similar correlation is not seen in the NBGF. Also, although both background seismicity rate and aftershock productivity increased for fluid‐injection‐induced earthquake rate changes in Oklahoma and Arkansas, the background rate increases significantly in the BSZ only, roughly corresponding with net fluid production rate increases. Moreover, in both fields the interevent spacing does not change significantly during active energy projects. This suggests that, although geothermal field activities in a tectonically active region may not significantly change the physics of earthquake interactions, earthquake rates may still be driven by fluid injection or extraction rates, particularly in the SSGF.

Midplate seismicity exterior to former rift-basins

USGS Publications Warehouse

Dewey, J.W.

1988-01-01

Midplate seismicity associated with some former rift-zones is distributed diffusely near, but exterior to, the rift basins. This "basin-exterior' seismicity cannot be attributed to reactivation of major basin-border faults on which uppercrustal extension was concentrated at the time of rifting, because the border faults dip beneath the basins. The seismicity may nonetheless represent reactivation of minor faults that were active at the time of rifting but that were located outside of the principal zones of upper-crustal extension; the occurrence of basin-exterior seismicity in some present-day rift-zones supports the existence of such minor basin-exterior faults. Other hypotheses for seismicity exterior to former rift-basins are that the seismicity reflects lobes of high stress due to lithospheric-bending that is centered on the axis of the rift, that the seismicity is localized on the exteriors of rift-basins by basin-interiors that are less deformable in the current epoch than the basin exteriors, and that seismicity is localized on the basin-exteriors by the concentration of tectonic stress in the highly elastic basin-exterior upper-crust. -from Author

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

NASA Astrophysics Data System (ADS)

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

2018-04-01

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

Internal structure, fault rocks, and inferences regarding deformation, fluid flow, and mineralization in the seismogenic Stillwater normal fault, Dixie Valley, Nevada

USGS Publications Warehouse

Caine, Jonathan S.; Bruhn, R.L.; Forster, C.B.

2010-01-01

Outcrop mapping and fault-rock characterization of the Stillwater normal fault zone in Dixie Valley, Nevada are used to document and interpret ancient hydrothermal fluid flow and its possible relationship to seismic deformation. The fault zone is composed of distinct structural and hydrogeological components. Previous work on the fault rocks is extended to the map scale where a distinctive fault core shows a spectrum of different fault-related breccias. These include predominantly clast-supported breccias with angular clasts that are cut by zones containing breccias with rounded clasts that are also clast supported. These are further cut by breccias that are predominantly matrix supported with angular and rounded clasts. The fault-core breccias are surrounded by a heterogeneously fractured damage zone. Breccias are bounded between major, silicified slip surfaces, forming large pod-like structures, systematically oriented with long axes parallel to slip. Matrix-supported breccias have multiply brecciated, angular and rounded clasts revealing episodic deformation and fluid flow. These breccias have a quartz-rich matrix with microcrystalline anhedral, equant, and pervasively conformable mosaic texture. The breccia pods are interpreted to have formed by decompression boiling and rapid precipitation of hydrothermal fluids whose flow was induced by coseismic, hybrid dilatant-shear deformation and hydraulic connection to a geothermal reservoir. The addition of hydrothermal silica cement localized in the core at the map scale causes fault-zone widening, local sealing, and mechanical heterogeneities that impact the evolution of the fault zone throughout the seismic cycle. ?? 2010.

Seismic and Tectonic Regionalization of the State of Michoacan.

NASA Astrophysics Data System (ADS)

Vazquez Rosas, R.; Aguirre, J.; Garduño-Monroy, V. H.; Ramirez-Guzman, L.

2017-12-01

In Mexico it is a country with seismically active regions, mainly the zones that are next to the pacific where the zone of subduction is located, in this work we focus on the state of Michoacán, since this has not been completely studied in the last 30 years after the earthquake in Michoacán in 1985. The first most important step is to know the region which are the most seismic zones within the state and one way is to carry out the regionalization of Michoacán identifying the sources of earthquakes as well as where occur more frequently.If we could know each of the factors that influence seismicity and describe every point of the terrain, every rupture, every rock, etc., then we could describe in an analytical way the seismic process and predict the occurrence of earthquakes such as eclipses. Unfortunately the number of parameters is so enormous that we cannot arrive at an exact description; however, we can take advantage of statistical properties to evaluate probabilities, even in the case of small systems such as a particular seismic zone.In this paper, epicenter data were collected from 1970 to 2014, and with them a statistical study was carried out and the epicenter data plotted using data reported by the National Seismological Service and the IRIS catalog as well as some data from the Institute of engineering UNAM. Where earthquakes of equal and greater than M = 4 were used. Graphing these in function with the depth and with that it was graficaron and was made an overlapping the faults of the state and with that it was divided in 4 seismic zones in function of the faults and the localized seismicity.Zone A. is located within the Michoacán Block set of faults, as well as part of the subduction zone on the coast of the state. Seismicity in this area is high. Zone B-1. This is located between the limits of Jalisco and Michoacán in the set of faults called Tepalcatepec depression and limits with the Jorullo-Tacámbaro fracture. At this site seismicity is

Seismic facies analysis of shallowly buried channels, New Jersey continental shelf: understanding late Quaternary paleoenvironments during the last transgression

NASA Astrophysics Data System (ADS)

Nordfjord, S.; Goff, J. A.; Austin, J. A.; Gulick, S. P.; Sommerfield, C.; Alexander, C.; Schock, S.

2004-12-01

We are investigating the late Quaternary sedimentary record of the New Jersey mid-outer continental shelf using deep-towed chirp sonar (1-4 kHz and 1-15 kHz) profiles, coupled with lithologic and chronostratigraphic control from long sediment cores collected using the DOSECC AHC-800 drilling system. We have seismically mapped extensive, shallowly buried, dendritic drainage systems. Observed seismic facies distributions suggest the complex nature of channel fills, and synthetic seismograms derived from MST logs enable us to correlate the chirp data to changes in lithology and physical properties of the cored samples, including channel fills, confirming that fine-grained material is transparent seismically, while interbedded sand and mud produce laminated reflections. We suggest that these channels probably formed during shelfal exposure coincident with the last glacial lowstand along this margin. Observed seismic facies superposition within valley fills is in part consistent with a tripartite zonation derived from wave-dominated estuary models. We have mapped four main facies within these dendritic incised valleys: (1) The lower facies, SF1, consists of a high-amplitude chaotic configuration. We interpret this facies as lowstand fluvial fill; (2) Overlying facies SF2 is generally a thin layer (<1-2m) of stratified, high amplitude reflectors in valley axes. This facies is characterized by small wedges along channel flanks, with a generally transparent acoustic response, but occasionally also by internal clinoforms. This facies could have been deposited as transgression began, by backfilling of valleys (bayhead delta? aggradational alluvial deposits?); (3) SF3 is generally transparent; subtle horizontal and parallel reflectors onlap channel flanks. We interpret this facies as representing central basin/bay deposits, a low-energy zones during the transgression, perhaps related to turbidity maxima; (4) SF4 is observed only in the seaward end of the valley. This facies

Deformation of ``Villafranchian'' lacustrine sediments in the Chisone Valley (Western Alps, Italy)

NASA Astrophysics Data System (ADS)

Collo, Giovanni; Giardino, Marco

1997-09-01

The Chisone Valley is located in the internal NW Alps, in the Pinerolese District, an area characterized by present low to medium seismicity. Fine-grained sediments (sand, silt and clay with interbedded gravel) crop out in the lower Chisone Valley: they were first interpreted as glaciolacustrine deposits, and then as a lacustrine infilling of the valley floor probably due to differential uplifting of the valley mouth. Review of this data, together with new field and palynological observations, lead us to refer the lacustrine deposits to approximately the Lower Pleistocene (Villafranchian). In many outcrops, the lacustrine deposits show strong soft-sediment deformation such as convolute laminations, water-escape structures and disrupted beds, some of them associated with folds and faults (cm to dm in size); only two sites show metric to decametric folds and faults trending E-W and N-S. Detailed structural analysis conducted along a recently exposed section (Rio Gran Dubbione site) shows several soft-sediment deformation features on the limbs of mesoscale folds. Because of their intimate structural association, the origin of these minor structures seems to be connected to synsedimentary activity on reverse and normal faults (m to dm in size) affecting the lacustrine deposits in the same locality. Soft-sediment deformation features can be interpreted as possible paleoseismites. If so, the present seismicity of the Pinerolese District, which is the major area of such activity in NW Italy, cannot be considered an isolated episode in the geological evolution of the region; even if there is no supporting evidence for continuous seismicity, the deformations in the lacustrine sediments of the Chisone Valley testify to Early Pleistocene seismic activity, probably related to the recent tectonic evolution of the internal side of the NW Alps.

Integrated geophysical and geological study of the tectonic framework of the 38th Parallel Lineament in the vicinity of its intersection with the extension of the New Madrid Fault Zone

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

Braile, L.W.; Hinze, W.J.; Keller, G.R.

1978-06-01

Extensive gravity and aeromagnetic surveys have been conducted in critical areas of Kentucky, Illinois, and Indiana centering around the intersection of the 38th Parallel Lineament and the extension of the New Madrid Fault Zone. Available aeromagnetic maps have been digitized and these data have been processed by a suite of computer programs developed for this purpose. Seismic equipment has been prepared for crustal seismic studies and a 150 km long seismic refraction line has been observed along the Wabash River Valley Fault System. Preliminary basement rock and configuration maps have been prepared based on studies of the samples derived frommore » basement drill holes. Interpretation of these data are at a preliminary stage, but studies to this date indicate that the 38th Parallel Lineament features extend as far north as 39/sup 0/N and a subtle northeasterly striking magnetic and gravity anomaly cuts across Indiana from the southwest corner of the state, roughly on strike with the New Madrid Seismic Zone.« less

Structural interpretation and physical property estimates based on COAST 2012 seismic reflection profiles offshore central Washington, Cascadia subduction zone

NASA Astrophysics Data System (ADS)

Webb, S. I.; Tobin, H. J.; Everson, E. D.; Fortin, W.; Holbrook, W. S.; Kent, G.; Keranen, K. M.

2014-12-01

The Cascadia subduction zone has a history of large magnitude earthquakes, but a near-total lack of plate interface seismicity, making the updip limit of the seismogenic zone difficult to locate. In addition, the central Cascadia accretionary prism is characterized by an extremely low wedge taper angle, landward vergent initial thrusting, and a flat midslope terrace between the inner and outer wedges, unlike most other accretionary prisms (e.g. the Nankai Trough, Japan). The Cascadia Open Access Seismic Transect (COAST) lines were shot by R/V Marcus Langseth in July of 2012 off central Washington to image this subduction zone. Two trench-parallel and nine trench-perpendicular lines were collected. In this study, we present detailed seismic interpretation of both time- and depth-migrated stacked profiles, focused on elucidating the deposition and deformation of both pre- and syn-tectonic sediment in the trench and slope. Distribution and timing of sediments and their deformation is used to unravel the evolution of the wedge through time. Initially, interpretation of the time-sections is carried out to support the building of tomographic velocity models to aid in the pre-stack depth migration (PSDM) of selected lines. In turn, we use PSDM velocity models to estimate porosity and pore pressure conditions at the base of the wedge and across the basal plate interface décollement where possible, using established velocity-porosity transforms. Interpretation in this way incorporates both accurate structural relationships and robust porosity models to document wedge development and present-day stress state, in particular regions of potential overpressure. Results shed light on the origin and evolution of the mid-slope terrace and the low taper angle for the forearc wedge. This work may shed light ultimately on the position of the potential updip limit of the seismogenic zone beneath the wedge.

High-resolution seismic survey for the characterization of planned PIER-ICDP fluid-monitoring sites in the Eger Rift zone

NASA Astrophysics Data System (ADS)

Simon, H.; Buske, S.

2017-12-01

The Eger Rift zone (Czech Republic) is a intra-continental non-volcanic region and is characterized by outstanding geodynamic activities, which result in earthquake swarms and significant CO2 emanations. Because fluid-induced stress can trigger earthquake swarms, both natural phenomena are probably related to each other. The epicentres of the earthquake swarms cluster at the northern edge of the Cheb Basin. Although the location of the cluster coincides with the major Mariánské-Lázně Fault Zone (MLFZ) the strike of the focal plane indicates another fault zone, the N-S trending Počátky-Plesná Zone (PPZ). Isotopic analysis of the CO2-rich fluids revealed a significant portion of upper mantle derived components, hence a magmatic fluid source in the upper mantle was postulated. Because of these phenomena, the Eger Rift area is a unique site for interdisciplinary drilling programs to study the fluid-earthquake interaction. The ICDP project PIER (Probing of Intra-continental magmatic activity: drilling the Eger Rift) will set up an observatory, consisting of five monitoring boreholes. In preparation for the drilling, the goal of the seismic survey is the characterization of the projected fluid-monitoring drill site at the CO2 degassing mofette field near Hartoušov. This will be achieved by a 6 km long profile with dense source and receiver spacing. The W-E trending profile will cross the proposed drill site and the surface traces of MLFZ and PPZ. The outcome of the seismic survey will be a high-resolution structural image of potential reflectors related to these fault zones. This will be achieved by the application of advanced pre-stack depth migration methods and a detailed P-wave velocity distribution of the area obtained from first arrival tomography. During interpretation of the seismic data, a geoelectrical resistivity model, acquired along the same profile line, will provide important constraints, especially with respect to fluid pathways.

Late Quaternary history of the Owens Valley fault zone, eastern California, and surface rupture associated with the 1872 earthquake

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

Beanland, S.; Clark, M.M.

1993-04-01

The right-lateral Owens Valley fault zone (OVFZ) in eastern California extends north about 100 km from near the northwest shore of Owens Lake to beyond Big Pine. It passes through Lone Pine near the eastern base of the Alabama Hills and follows the floor of Owens Valley northward to the Poverty Hills, where it steps 3 km to the left and continues northwest across Crater Mountain and through Big Pine. Data from one site suggest an average net slip rate for the OVFZ of 1.5 [+-] 1 mm/yr for the past 300 ky. Several other sites yield an average Holocenemore » net slip rate of 2 [+-] 1 mm/yr. The OVFZ apparently has experienced three major Holocene earthquakes. The minimum average recurrence interval is 5,000 years at the subsidiary Lone Pine fault, whereas it is 3,300 to 5,000 years elsewhere along the OVFZ. The prehistoric earthquakes are not dated, so an average recurrence interval need not apply. However, roughly equal (characteristic) displacement apparently happened during each Holocene earthquake. The Owens Valley fault zone accommodates some of the relative motion (dextral shear) between the North American and Pacific plates along a discrete structure. This shear occurs in the Walker Lane belt of normal and strike-slip faults within the mainly extensional Basin and Range Province. In Owens Valley displacement is partitioned between the OVFZ and the nearby, subparallel, and purely normal range-front faults of the Sierra Nevada. Compared to the OVFZ, these range-front normal faults are very discontinuous and have smaller Holocene slip rates of 0.1 to 0.8 mm/yr, dip slip. Contemporary activity on adjacent faults of such contrasting styles suggests large temporal fluctuations in the relative magnitudes of the maximum and intermediate principal stresses while the extension direction remains consistently east-west.« less

Seismic risk perception in Italy

NASA Astrophysics Data System (ADS)

Crescimbene, Massimo; La Longa, Federica; Camassi, Romano; Pino, Nicola Alessandro; Peruzza, Laura

2014-05-01

Risk perception is a fundamental element in the definition and the adoption of preventive counter-measures. In order to develop effective information and risk communication strategies, the perception of risks and the influencing factors should be known. This paper presents results of a survey on seismic risk perception in Italy conducted from January 2013 to present . The research design combines a psychometric and a cultural theoretic approach. More than 7,000 on-line tests have been compiled. The data collected show that in Italy seismic risk perception is strongly underestimated; 86 on 100 Italian citizens, living in the most dangerous zone (namely Zone 1), do not have a correct perception of seismic hazard. From these observations we deem that extremely urgent measures are required in Italy to reach an effective way to communicate seismic risk. Finally, the research presents a comparison between groups on seismic risk perception: a group involved in campaigns of information and education on seismic risk and a control group.

Rheology and stress in subduction zones around the aseismic/seismic transition

NASA Astrophysics Data System (ADS)

Platt, John P.; Xia, Haoran; Schmidt, William Lamborn

2018-12-01

Subduction channels are commonly occupied by deformed and metamorphosed basaltic rocks, together with clastic and pelagic sediments, which form a zone up to several kilometers thick to depths of at least 40 km. At temperatures above 350 °C (corresponding to depths of > 25-35 km), the subduction zone undergoes a transition to aseismic behavior, and much of the relative motion is accommodated by ductile deformation in the subduction channel. Microstructures in metagreywacke suggest deformation occurs mainly by solution-redeposition creep in quartz. Interlayered metachert shows evidence for dislocation creep at relatively low stresses (8-13 MPa shear stress). Metachert is likely to be somewhat stronger than metagreywacke, so this value may be an upper limit for the shear stress in the channel as a whole. Metabasaltic rocks deform mainly by transformation-assisted diffusional creep during low-temperature metamorphism and, when dry, are somewhat stronger than metachert. Quartz flow laws for dislocation and solution-redeposition creep suggest strain rates of 10-12 s-1 at 500 °C and 10 MPa shear stress: this is sufficient to accommodate a 100 mm/yr. convergence rate within a 1 km wide ductile shear zone. The up-dip transition into the seismic zone occurs through a region where deformation is still distributed over a thickness of several kilometers, but occurs by a combination of microfolding, dilational microcracking, and solution-redeposition creep. This process requires a high fluid flux, released by dehydration reactions down-dip, and produces a highly differentiated deformational fabric with alternating millimeter-scale quartz and phyllosilicate-rich bands, and very abundant quartz veins. Bursts of dilational microcracking in zones 100-200 m thick may cause cyclic fluctuations in fluid pressure and may be associated with episodic tremor and slow slip events. Shear stress estimates from dislocation creep microstructures in dynamically recrystallized metachert are 10

Stress sensitivity of fault seismicity: A comparison between limited-offset oblique and major strike-slip faults

USGS Publications Warehouse

Parsons, T.; Stein, R.S.; Simpson, R.W.; Reasenberg, P.A.

1999-01-01

We present a new three-dimensional inventory of the southern San Francisco Bay area faults and use it to calculate stress applied principally by the 1989 M = 7.1 Loma Prieta earthquake and to compare fault seismicity rates before and after 1989. The major high-angle right-lateral faults exhibit a different response to the stress change than do minor oblique (right-lateral/thrust) faults. Seismicity on oblique-slip faults in the southern Santa Clara Valley thrust belt increased where the faults were unclamped. The strong dependence of seismicity change on normal stress change implies a high coefficient of static friction. In contrast, we observe that faults with significant offset (>50-100 km) behave differently; microseismicity on the Hayward fault diminished where right-lateral shear stress was reduced and where it was unclamped by the Loma Prieta earthquake. We observe a similar response on the San Andreas fault zone in southern California after the Landers earthquake sequence. Additionally, the offshore San Gregorio fault shows a seismicity rate increase where right-lateral/oblique shear stress was increased by the Loma Prieta earthquake despite also being clamped by it. These responses are consistent with either a low coefficient of static friction or high pore fluid pressures within the fault zones. We can explain the different behavior of the two styles of faults if those with large cumulative offset become impermeable through gouge buildup; coseismically pressurized pore fluids could be trapped and negate imposed normal stress changes, whereas in more limited offset faults, fluids could rapidly escape. The difference in behavior between minor and major faults may explain why frictional failure criteria that apply intermediate coefficients of static friction can be effective in describing the broad distributions of aftershocks that follow large earthquakes, since many of these events occur both inside and outside major fault zones.

Seismic refraction studies of volcanic crust in Costa Rica and of critical zones in the southern Sierra Nevada, California and Laramie Range, Wyoming

NASA Astrophysics Data System (ADS)

Hayes, Jorden L.

This work demonstrates the utility of seismic refraction surveys to understanding geologic processes at a range of scales. Each chapter presents subsurface maps of seismic p-wave velocities, which vary due to contrasts in elastic material properties. In the following chapters we examine seismic p-wave velocity variations that result from volcanic and tectonic processes within Earth's crust and chemical and physical weathering processes within Earth's near-surface environment. Chapter one presents results from an across-arc wide-angle seismic refraction survey of the Costa Rican volcanic front. These results support the hypothesis that juvenile continental crust may form along volcanic island arcs if built upon relatively thick substrates (i.e., large igneous provinces). Comparisons of velocity-depth functions show that velocities within the active arc of Costa Rica are lower than other modern island arcs (i.e., volcanic arcs built upon oceanic crust) and within the high-velocity extreme of bulk continental crust. Chapter two shows that physical processes can dominate over chemical processes in generating porosity in the deep critical zone and outlines a new framework for interpreting subsurface chemical and physical weathering at the landscape scale. Direct measurements of saprolite from boreholes at the Southern Sierra Nevada Critical Zone Observatory show that, contrary to convention, saprolite may experience high levels of volumetric strain (>35%) and uniform mass loss in the upper 11 m. By combining observations from boreholes and seismic refraction surveys we create a map of volumetric strain across the landscape. Variations in inferred volumetric strain are consistent with opening-mode fracture patterns predicted by topographic and tectonic stress models. Chapter three is a characterization of fracture distribution in the deep critical zone from geophysical and borehole observations in the Laramie Mountains, Wyoming. Data from core and down-hole acoustic

Slow Earthquakes in the Alaska-Aleutian Subduction Zone Detected by Multiple Mini Seismic Arrays

NASA Astrophysics Data System (ADS)

LI, B.; Ghosh, A.; Thurber, C. H.; Lanza, F.

2017-12-01

The Alaska-Aleutian subduction zone is one of the most seismically and volcanically active plate boundaries on earth. Compared to other subduction zones, the slow earthquakes, such as tectonic tremors (TTs) and low frequency earthquakes (LFEs), are relatively poorly studied due to the limited data availability and difficult logistics. The analysis of two-months of continuous data from a mini array deployed in 2012 shows abundant tremor and LFE activities under Unalaska Island that is heterogeneously distributed [Li & Ghosh, 2017]. To better study slow earthquakes and understand their physical characteristics in the study region, we deployed a hybrid array of arrays, consisting of three well-designed mini seismic arrays and five stand alone stations, in the Unalaska Island in 2014. They were operational for between one and two years. Using the beam back-projection method [Ghosh et al., 2009, 2012], we detect continuous tremor activities for over a year when all three arrays are running. The sources of tremors are located south of the Unalaska and Akutan Islands, at the eastern and down-dip edge of the rupture zone of the 1957 Mw 8.6 earthquake, and they are clustered in several patches, with a gap between the two major clusters. Tremors show multiple migration patterns with propagation in both along-strike and dip directions and a wide range of velocities. We also identify tens of LFE families and use them as templates to search for repeating LFE events with the matched-filter method. Hundreds to thousands of LFEs for each family are detected and their activities are spatiotemporally consistent with tremor activities. The array techniques are revealing a near-continuous tremor activity in this area with remarkable spatiotemporal details. It helps us to better recognize the physical properties of the transition zone, provides new insights into the slow earthquake activities in this area, and explores their relation with the local earthquakes and the potential slow