Seismic Shear Wave Velocity Research Articles

Shear wave velocity structure beneath Bandung basin, West Java, Indonesia from ambient noise tomography

Summary We investigated the seismic shear wave velocity structure of the upper crust beneath the Bandung area in West Java, Indonesia, using ambient seismic noise tomography. We installed 60 seismographs to record ambient seismic noise continuously in the city of Bandung and its surrounding area for 8 months. After inter-station cross-correlation of recordings of ambient seismic noise, we obtained empirical Green's functions for Rayleigh waves. Group velocity dispersion curves for Rayleigh waves between periods of 1 s and 8 s were measured on each inter-station path by applying the multiple filter analysis method with phase-matched processing. The spatial variation of group velocities shows a good correlation with the geological structure of the Bandung Basin. The Rayleigh wave dispersion maps were inverted to obtain the 1D shear wave velocity profiles beneath each station, which were interpolated to infer a pseudo-3D structure under the study region. The results show that the Bandung Basin has a thick layer of sediment. Along the northern, eastern and southern mountains surrounding the Bandung Basin there is high-velocity structure, except to the west of the Tangkuban Parahu volcano, where a massive low-velocity structure extending throughout the upper crust might indicate the presence of fluids or partial melts.

Seismic attenuation model using artificial neural networks

This research aims at deriving a simple yet powerful ground motion prediction model for the Himalayas and Indo-Gangetic plains, which has a high probability of damage due to moderate or great earthquake, utilizing an entirely data-driven methodology known as an artificial neural network (ANN) from the earthquake recordings of program for excellence in strong motion studies (PESMOS) and central Indo-Gangetic plains network (CIGN) database. Apart from the widely used independent input parameters, magnitude and distance, we have introduced two new variables, focal depth and average seismic shear-wave velocity from the surface to a depth of 30 m (Vs30) to improve model predictability. Peak ground acceleration (PGA) and pseudo-spectral acceleration (PSA) at twenty-five periods are chosen as response variables.The network architecture consisting of 9 hidden nodes were found optimal for the selected database and input-output mapping. The performance of the model is ascertained based on the standard deviation of the error, and the ground motion predictability is tested using real recordings at eight stations and the corresponding widely used ground motion prediction equations (GMPEs) for the Himalayas and Indo-Gangetic Plains. The GMPEs considered for comparison are National Disaster Management Authority (2011), Raghukanth and Kavitha (2014), Anbazhagan et al. (2013), Muthuganeisan and Raghukanth 2016, and Singh et al. (2017). The total standard deviation of response variables in log10 units varies between 0.267-0.343 with period and model predictability plots shows that the current ANN model is competent to predict the response spectrum with good accuracy in both the seismically critical regions of India. Finally, the model is scripted into MATLAB and Excel and supplemented with this article for further use.

Seismic Shear Wave Velocity Fractal Dimension for Characterizing Shajara Reservoirs of the Permo-Carboniferous Shajara Formation, Saudi Arabia

The quality and assessment of a reservoir can be documented in details by the application of seismic shear wave. This research aims to calculate fractal dimension from the relationship among seismic shear wave velocity, maximum seismic shear wave velocity and wetting phase saturation and to approve it by the fractal dimension derived from the relationship among capillary pressure and wetting phase saturation. Two equations for calculating the fractal dimensions have been employed. The first one describes the functional relationship between wetting phase saturation, seismic shear wave velocity and maximum seismic shear wave velocity and fractal dimension. The second equation implies to the wetting phase saturation as a function of capillary pressure and the fractal dimension. Two procedures for obtaining the fractal dimension have been utilized. The first procedure was done by plotting the logarithm of the ratio between seismic shear wave velocity and maximum seismic shear wave velocity versus logarithm wetting phase saturation. The slope of the first procedure = 3- Df (fractal dimension). The second procedure for obtaining the fractal dimension was determined by plotting the logarithm of capillary pressure versus the logarithm of wetting phase saturation. The slope of the second procedure = Df -3. On the basis of the obtained results of the fabricated stratigraphic column and the attained values of the fractal dimension, the sandstones of the Shajara reservoirs of the Shajara Formation were divided here into three units.

Revisiting the CPT-based characterizations of a highway subgrade

ABSTRACTThis paper revisits the cone penetration test (CPT) based characterizations derived from small strain shear modulus (G0) and seismic shear wave velocity (Vs). The need for this study is demonstrated through comparisons with existing CPT-Vs/CPT – G0 models. Among the literary correlations considered in the present study, some of them showed better performance. The previous correlations show an error margin ranging between −140% and +440% in estimating the G0, whereas the predicted Vs was underestimated by 16% or overestimated by 109%. The CPT-G0 correlations utilizing the proposed compressibility index (Icc) denotes the higher regression coefficient value which designates the advantageous stress normalization. It is found that there is a good agreement between the measured and predicted Vs with expected error limits of ± 4%. Moreover, the proposed correlations in this study might be applicable to site-specific conditions due to the limitation of data points pertinent to organic silts and clays category based on the unified soil classification system.

Refraction Microtremor method for delineation of layers and lenses, and assessing liquefaction potential within an alluvial setting - Morobe Province, Papua New Guinea

Refraction Microtremor (ReMi) is a relatively new method in the geophysics industry. ReMi provides high resolution seismic shear wave velocity models up to 200m depth and as such has the potential of being an efficient method for assessing the soil liquefaction potential in seismically active regions.This paper presents a geophysical investigation carried out as part of a geotechnical feasibility study for a proposed Tailings Storage Facility (TSF) in the Morobe Province of Papua New Guinea. The primary objective of the investigation was to use geophysical methods to obtain subsurface parameters to assess the liquefaction potential within an interbedded and lensed clay/gravel alluvial setting. ReMi together with down hole and cross hole seismic methods were used to generate shear wave velocity information of multiple layers with depth, and in particular to define seismic velocity inversions.ReMi data was acquired using two array setups specifically targeting the top 100m of subsurface material and the top 50m of subsurface material at increased layer resolution. The data was inverted to produce shear wave velocity soundings which were correlated with the cross-hole and down-hole seismic methods, and with bore hole Standard Penetration Tests (SPT). The soundings were compiled to generate high resolutions shear wave velocity sections, analysis of which proved pertinent in defining the interconnectivity of the lensed clay/gravel and shear wave velocity variations for the calculation of liquefaction potential thresholds.

Stress-dependent shear wave splitting and permeability in fractured porous rock

It is well known that shear wave propagates slower across than parallel to a fracture, and as a result, a travelling shear wave splits into two directions when it encounters a fracture. Shear wave splitting and permeability of porous rock core samples having single fracture were experimentally investigated using a high-pressure triaxial cell, which can measure seismic shear wave velocities in two directions mutually perpendicular to the sample axis in addition to the longitudinal compressive wave velocity. A single fracture was created in the samples using a modified Brazilian split test device, where the cylindrical sample edges were loaded on two diametrically opposite lines by sharp guillotines along the sample length. Based on tilt tests and fracture surface profilometry, the method of artificially induced tensile fracture in the sample was found to create repeatable fracture surfaces and morphologies. Seismic velocities of the fractured samples were determined under different levels of stress confinement and fracture shear displacement or mismatch. The effective confining stress was varied from 0.5 MPa to 55 MPa, while the fractures were mismatched by 0 mm, 0.45 mm and 1 mm. The degree of matching of the fracture surfaces in the core samples was evaluated using the joint matching coefficient (JMC). Shear wave splitting, as measured by the difference in the magnitudes of shear wave velocities parallel (VS1) and perpendicular (VS2) to the fracture, is found to be insensitive to the degree of mismatching of the fracture joint surfaces at 2 MPa, and decreased and approached zero as the effective stress was increased. Simple models for the stress- and JMC-dependent shear wave splitting and fractured rock permeability were developed based on the experimental observations. The effects of the joint wall compressive strength (JCS), JMC and stress on the stress dependency of joint aperture were discussed in terms of hydro-mechanical response. Finally, a useful relationship between fractured rock permeability and shear wave splitting was found after normalization by using JMC.

Lithospheric Structure of Greenland From Ambient Noise and Earthquake Surface Wave Tomography

AbstractWe present a high‐resolution shear wave velocity model of Greenland's lithosphere from regional and teleseismic Rayleigh waves recorded by the Greenland Ice Sheet Monitoring Network supplemented with observations from several temporary seismic deployments. To construct Rayleigh wave group velocity maps, we integrated signals from regional and teleseismic earthquakes with several years of ambient seismic noise and used the dispersion to constrain crustal and upper‐mantle seismic shear wave velocity structure. Specifically, we used a Markov Chain Monte Carlo technique to estimate 3‐D shear wave velocities beneath Greenland to a depth of 200 km. Our model reveals four prominent anomalies: a deep high‐velocity feature extending from southwestern to northwestern Greenland that may be the signature of a thick cratonic keel, a corridor of relatively low upper‐mantle velocity across central Greenland that could be associated with lithospheric modification from the passage of the Iceland plume beneath Greenland or interpreted as a tectonic boundary between cratonic blocks, an upper‐crustal southwest‐northeast trending boundary separating Greenland into two regions of contrasting tectonic and crustal properties, and a midcrustal low‐velocity anomaly beneath northeastern Greenland. The nature of this midcrustal anomaly is of particular interest given that it underlies the onset of the Northeast Greenland Ice Stream and raises interesting questions regarding how deeper processes may impact the ice stream dynamics and the evolution of the Greenland Ice Sheet.

Evidence for a deep mantle source for EM and HIMU domains from integrated geochemical and geophysical constraints

Subduction of oceanic and continental crust (and associated sediments) into the mantle over geologic time generates mantle domains with geochemically distinct signatures, referred to as HIMU (high “μ”, where μ=238U/204Pb) and EM (enriched mantle) domains. Identification of EM and HIMU geochemical signatures in hotspot lavas provides evidence that subducted crustal materials are recycled into the source of hotspots. It remains uncertain where these materials are located in the mantle, and a key question is whether upwelling mantle plumes are required to transport mantle domains with EM and HIMU signatures to the shallow mantle beneath hotspots. Therefore, this study evaluates relationships between extreme EM and HIMU compositions at oceanic hotspots and the presence (or absence) of seismically-constrained mantle plumes beneath the hotspots. We draw on three existing plume catalogs based on global seismic shear-wave velocity models, and these plume catalogs indicate the presence or absence of a plume beneath each of 42 oceanic hotspots. From each hotspot, we select a lava with the highest 206Pb/204Pb composition and one with the lowest 143Nd/144Nd composition.We show that hotspots associated with seismically defined plumes show a greater likelihood of hosting lavas with either extreme EM (143Nd/144Nd ≤ 0.512630) or extreme HIMU (206Pb/204Pb ≥ 20.0) compositions than hotspots not associated with plumes, but HIMU hotspots show a stronger association with plumes than EM hotspots. The significance of the relationship between plumes and extreme geochemical signatures at hotspots improves if extreme EM and HIMU compositions are considered together instead of separately: hotspots sourced by mantle plumes are even more likely to exhibit extreme EM or extreme HIMU signatures than hotspots not sourced by plumes. The significance tests also show that hotspots with extreme EM or HIMU compositions are more likely to be associated with mantle plumes than hotspots that lack extreme geochemical signatures. A relationship between seismically detected deep mantle plumes and the presence of extreme EM or HIMU compositions at hotspots provides evidence for a deep mantle source for these geochemical domains.

Soil characterization of Tınaztepe region (İzmir/Turkey) using surface wave methods and nakamura (HVSR) technique

To determine the shear wave velocity structure and predominant period features of Tinaztepe in Izmir, Turkey, where new building sites have been planned, active–passive surface wave methods and single-station microtremor measurements are used, as well as surface acquisition techniques, including the multichannel analysis of surface waves (MASW), refraction microtremor (ReMi), and the spatial autocorrelation method (SPAC), to pinpoint shallow and deep shear wave velocity. For engineering bedrock (V s > 760 m/s) conditions at a depth of 30 m, an average seismic shear wave velocity in the upper 30 m of soil (AVs30) is not only accepted as an important parameter for defining ground behavior during earthquakes, but a primary parameter in the geotechnical analysis for areas to be classified by V s30 according to the National Earthquake Hazards Reduction Program (NEHRP). It is also determined that Z1.0, which represents a depth to V s = 1000 m/s, is used for ground motion prediction and changed from 0 to 54 m. The sediment–engineering bedrock structure for Tinaztepe that was obtained shows engineering bedrock no deeper than 30 m. When compared, the depth of engineering bedrock and dominant period map and geology are generally compatible.

Polycrystal anelasticity at near‐solidus temperatures

AbstractElasticity, anelasticity, and viscosity of polycrystalline aggregates were measured at the near‐solidus temperatures ranging from below to above the solidus temperature (Tm). The result shows that the mechanical effects of the partial melting are twofold; changes just below the solidus temperature in the absence of melt and changes at the solidus temperature due to the onset of partial melting. As homologous temperature (T/Tm) increases from about 0.92 to 1, high‐frequency part of the attenuation spectrum significantly grows. Viscosity of the grain boundary diffusion creep is also reduced in this temperature range. These changes are caused by a solid‐state mechanism and have a large amplitude even for the samples which can generate very small amounts of melt at the solidus temperature. At the onset of melting, further increases in the elastic, anelastic, and viscous compliances occur. These changes are caused by the direct effects of the melt phase and are very small for the samples with very small melt fractions. Mechanical properties of a partially molten aggregate are determined by these twofold changes, and when melt fraction is small, the former changes are dominant. We performed a parameterization of the present experimental results and applied the obtained empirical formula to the seismic tomographic data in the upper mantle. The present model explains well the steep reduction of the seismic shear wave velocity in the oceanic lithosphere just below the solidus temperature.

Enhancing the CPT correlation with the small strain shear stiffness of sands

Abstract The cone penetration test (CPT) is a valuable geotechnical insitu test. Yet, the CPT correlations with the small strain shear modulus and seismic shear wave velocity still need more research to enhance their accuracy. In this study, the stress normalizations for the net cone tip resistance and the small strain shear modulus are scrutinized. Subsequently, enhanced CPT correlations with the small strain shear modulus and the seismic shear wave velocity for sands are presented. The proposed approach utilizes published databases of CPT in sands and recent researches that quantify the small strain shear modulus using sand gradation parameters. Four case histories are analyzed using the suggested correlation and the results confirm that the presented approach is a promising enhancement to the CPT correlations with the small strain shear modulus and seismic shear wave velocity in sands.

Soil and Subsurface Sediment Microzonation Using with Seismic Refraction Tomography for Site Assessment (Case Study: IKIA Airport, Iran)

The site effects relating to the amplification of ground motion under earthquake loading are strongly influenced by both the subsurface soil condition and the geologic structure. In this study, for site characterization at the Imam Khomeini International Airport (IKIA) area in south of Tehran, in-situ seismic refraction tomography were carried out as a part of site investigations project, in addition geologic setting, borehole drilling, ground waters information and measurements. Based on seismic refraction studies, three layers are separable which with increasing in depth the S and P wave velocity is increased and this indicates increasing in compaction of soil and geologic materials. In the second and third separated layers, the zones with low and high seismic shear wave velocity is approximately equal, and northeast and southwest of the airport site has the low velocities, in addition to containing loose soils, highly weathered stones, and low depth to groundwater. In terms of Poisson’s ratio, the most important and key installations of airport site are located in suitable positions. According to Iranian Seismic Code, most of the lands around the airport are in class 2 and 3. It seems that a fault or a discontinuity is passed from northwest to the southeast of the study area. This site, according to geological, subsurface geophysical, and geotechnical boreholes studies, is high risk-earthquake prone.

Multichannel analysis of surface-waves and integration of downhole acoustic televiewer imaging, ultrasonic Vs and Vp, and vertical seismic profiling in an NEHRP-standard classification, South of Concordia, Kansas, USA

Seismic site characteristics, as pertaining to earthquake hazard reduction, are a function of the subsurface elastic moduli and the geologic structures. This study explores how multiscale (surface, downhole, and laboratory) datasets can be utilized to improve “constrained” average Vs30 (shear-wave velocity to a 30-meter depth). We integrate borehole, surface and laboratory measurements for a seismic site classification based on the standards of the National Earthquake Hazard Reduction Program (NEHRP). The seismic shear-wave velocity (Vs30) was derived from a geophysical inversion workflow that utilized multichannel analysis of surface-waves (MASW) and downhole acoustic televiewer imaging (DATI). P-wave and S-wave velocities, based on laboratory measurements of arrival times of ultrasonic-frequency signals, supported the workflow by enabling us to calculate Poisson's ratio, which was incorporated in building an initial model for the geophysical inversion of MASW. Extraction of core samples from two boreholes provided lithology and thickness calibration of the amplitudes of the acoustic televiewer imaging for each layer. The MASW inversion, for calculating Vs sections, was constrained with both ultrasonic laboratory measurements (from first arrivals of Vs and Vp waveforms at simulated in situ overburden stress conditions) and the downhole acoustic televiewer (DATV) amplitude logs. The Vs30 calculations enabled categorizing the studied site as NEHRP-class “C” — very dense soil and soft rock. Unlike shallow fractured carbonates in the studied area, S-wave and P-wave velocities at ultrasonic frequency for the deeper intact shale core-samples from two boreholes were in better agreement with the corresponding velocities from both a zero-offset vertical seismic profiling (VSP) and inversion of Rayleigh-wave velocity dispersion curves.

Seismic evidence for a chemically distinct thermochemical reservoir in Earth's deep mantle beneath Hawaii

Nearly antipodal continent-sized zones of reduced seismic shear wave velocities exist at the base of Earth's mantle, one beneath the Pacific Ocean, the other beneath the South Atlantic Ocean and Africa. Geophysicists have attributed the low velocity zones to elevated temperatures associated with large-scale mantle convection processes, specifically, hot mantle upwelling in response to cooler subduction-related downwelling currents. Hypotheses have included superplumes, isochemical heterogeneity, and stable as well as metastable basal thermochemical piles. Here we analyze waveform broadening and travel times of S waves from 11 deep focus earthquakes in the southwest Pacific recorded in North America, resulting in 8500 seismograms studied that sample the deep mantle beneath the Pacific. Waveform broadening is referenced to a mean S-wave shape constructed for each event, to define a relative “misfit”. Large misfits are consistent with multipathing that can broaden wave pulses. Misfits of deep mantle sampling S-waves infer that the structure in the northeast part of the low velocity province beneath the Pacific has a sharp side as well as a sloping sharp top to the feature. This sharp boundary morphology is consistent with geodynamic predictions for a stable thermochemical reservoir. The peak of the imaged pile is below Hawaii, supporting the hypothesis of a whole mantle plume beneath the hotspot.

A geophysical strategy for measuring the thickness of the critical zone developed over basalt lavas

Estimates of the thickness variation in lateritic weathering profiles (LWPs) are important in tropical areas underlain by young basalt lavas like those found in Hawaii. Seismic shear-wave velocity data were obtained by a new application of multichannel analysis of surface waves (MASW) to map variations in the LWP and to derive the downward rate of advance of the weathering front in basaltic lavas. The MASW technique proved highly capable of imaging the internal structure and base of the critical zone, as confirmed by borehole data and direct field measurements. Profile thickness thus obtained, rapidly and without drilling, has applications to engineering and geochemical studies. The rate of advance of the weathering front derived from MASW in Oahu ranged from 0.010 m/ka to 0.026 m/ka in mesic zones (∼1500 mm/a rainfall), whereas an area with ∼800 mm/a revealed rates from 0.005 m/ka to 0.011 m/ka. These rates are comparable to those derived from recent solute-based mass balance studies of ground and surface water. Conventional P-wave seismic reflection did not perform as well for detecting boundaries due to a gradational seismic velocity structure within the weathering profile. Shear-wave velocity models showed internal variations that may be caused by textural differences in parental lava flows. Limitations in imaging depth were overcome by innovative experiment designs. Increasing source-receiver offsets and merging surface-wave dispersion curves allowed for a more objective derivation of velocity-frequency relations. Further improvements were made from a recently developed form of the combined active and passive source technique. These advances allowed for more detailed and deeper imaging of the subsurface with greater confidence. Velocity models derived from MASW can thus describe the LWP in terms of depth and variability in stiffness.

Measurement of soil shear wave velocity using in situ and laboratory seismic methods – some methodological aspects

This article presents the results of seismic shear wave velocity (V S ) measurements using the CSWS/SAWS ( Continuous Surface Wave System/Spectral Analysis of Surface Waves ) and SDMT ( seismic flat dilatometer ) methods, as well as BET ( Bender Elements Test ), on an experimental test site (and samples taken from it). The test site, a geologically relatively uniform alluvial sand formation area, was carefully chosen and checked for uniformity by means of drillings and soundings. The research aimed to determine how results from indirect, non-invasive surface geophysical tests (SASW and CSWS) correspond with those from SDMT penetration tests as well as the BET laboratory seismic method, and how some methodological aspects can influence them. Different wave sources and frequency were examined as the main factors for interpretation. The influence of other examined factors is also discussed.

Crustal shear-wave velocity structure beneath northeast India from teleseismic receiver function analysis

We investigated the seismic shear-wave velocity structure of the crust beneath nine broadband seismological stations of the Shillong–Mikir plateau and its adjoining region using teleseismic P-wave receiver function analysis. The inverted shear wave velocity models show ∼34–38km thick crust beneath the Shillong Plateau which increases to ∼37–38km beneath the Brahmaputra valley and ∼46–48km beneath the Himalayan foredeep region. The gradual increase of crustal thickness from the Shillong Plateau to Himalayan foredeep region is consistent with the underthrusting of Indian Plate beyond the surface collision boundary. A strong azimuthal variation is observed beneath SHL station. The modeling of receiver functions of teleseismic earthquakes arriving the SHL station from NE backazimuth (BAZ) shows a high velocity zone within depth range 2–8km along with a low velocity zone within ∼8–13km. In contrast, inversion of receiver functions from SE BAZ shows high velocity zone in the upper crust within depth range ∼10–18km and low velocity zone within ∼18–36km. The critical examination of ray piercing points at the depth of Moho shows that the rays from SE BAZ pierce mostly the southeast part of the plateau near Dauki fault zone. This observation suggests the effect of underthrusting Bengal sediments and the underlying oceanic crust in the south of the plateau facilitated by the EW-NE striking Dauki fault dipping 300 toward northwest.

Localized shear in the deep lithosphere beneath the San Andreas fault system

Research Article| April 01, 2014 Localized shear in the deep lithosphere beneath the San Andreas fault system Heather A. Ford; Heather A. Ford * 1Department of Geological Sciences, Brown University, 324 Brook Street, Box 1846, Providence, Rhode Island 02912, USA *Current address: Department of Geology and Geophysics, Yale University, PO Box 208109, New Haven, Connecticut 06520-8109, USA; E-mail: heather.ford@yale.edu. Search for other works by this author on: GSW Google Scholar Karen M. Fischer; Karen M. Fischer 1Department of Geological Sciences, Brown University, 324 Brook Street, Box 1846, Providence, Rhode Island 02912, USA Search for other works by this author on: GSW Google Scholar Vedran Lekic Vedran Lekic 2Department of Geology, University of Maryland, College Park, Maryland 20742, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Heather A. Ford * 1Department of Geological Sciences, Brown University, 324 Brook Street, Box 1846, Providence, Rhode Island 02912, USA Karen M. Fischer 1Department of Geological Sciences, Brown University, 324 Brook Street, Box 1846, Providence, Rhode Island 02912, USA Vedran Lekic 2Department of Geology, University of Maryland, College Park, Maryland 20742, USA *Current address: Department of Geology and Geophysics, Yale University, PO Box 208109, New Haven, Connecticut 06520-8109, USA; E-mail: heather.ford@yale.edu. Publisher: Geological Society of America Received: 19 Sep 2013 Revision Received: 31 Dec 2013 Accepted: 10 Jan 2014 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2014 Geological Society of America Geology (2014) 42 (4): 295–298. https://doi.org/10.1130/G35128.1 Article history Received: 19 Sep 2013 Revision Received: 31 Dec 2013 Accepted: 10 Jan 2014 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Heather A. Ford, Karen M. Fischer, Vedran Lekic; Localized shear in the deep lithosphere beneath the San Andreas fault system. Geology 2014;; 42 (4): 295–298. doi: https://doi.org/10.1130/G35128.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Seismic images of the base of the lithosphere across the San Andreas fault system (California, USA) yield new constraints on the distribution of deformation in the deep lithosphere beneath this strike-slip plate boundary. We show that conversions of shear to compressional waves (Sp) across the base of the lithosphere are systematically weaker on the western side of the plate boundary, indicating that the drop in seismic shear-wave velocity from lithosphere to asthenosphere is either smaller or occurs over a larger depth range. In central and northern California, the lithosphere-asthenosphere boundary changes character across a distance of <50 km, and does so directly beneath the San Andreas fault along its simple central segment, and beneath the Calaveras–Green Valley–Bartlett Springs faults to the north. Given the absolute velocities of the North America and Pacific plates, and low viscosities inferred for the asthenosphere, these results indicate the juxtaposition of mantle lithospheres with different properties across these faults. The spatial correlation between the central San Andreas fault and the laterally abrupt change in the velocity structure of the deepest mantle lithosphere points to the accommodation of relative plate motion on a narrow shear zone (<50 km in width), and a rheology that enables strain localization throughout the thickness of the lithosphere. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Shear wave velocity prediction using seismic attributes and well log data

Formation’s properties can be estimated indirectly using joint analysis of compressional and shear wave velocities. Shear wave data is not usually acquired during well logging, which is most likely for cost saving purposes. Even if shear data is available, the logging programs provide only sparsely sampled one-dimensional measurements: this information is inadequate to estimate reservoir rock properties. Thus, if the shear wave data can be obtained using seismic methods, the results can be used across the field to estimate reservoir properties. The aim of this paper is to use seismic attributes for prediction of shear wave velocity in a field located in southern part of Iran. Independent component analysis (ICA) was used to select the most relevant attributes to shear velocity data. Considering the nonlinear relationship between seismic attributes and shear wave velocity, multi-layer feed forward neural network was used for prediction of shear wave velocity and promising results were presented.

Poisson’s Ratio of Surface Soils and Shallow Sediments Determined from Seismic Compressional and Shear Wave Velocities

This paper presents the results of seismic compressional, P- and shear, S-wave measurements carried out on the unconsolidated top-soil at the different locations of the study area to determine Poisson’s ratio. In this work, seismic refraction data is used to determine Poisson’s ratio as an aid to engineering foundation. A 12-channel seismograph with signal stacking ability was used together with high frequency (100 Hz) geophones on the top-soil. The geophone intervals were set to 5 m at the all locations. In all the locations, Vp/Vs ratio ranged from 1.0289 to 1.4185 for the top layer. Vp/Vs ratio in the second layer ranged from 1.0512 to 1.5834. The Poisson’s ratio for the first layer ranged from -8.0324 to 0.2060. For the second layer, the Poisson’s ratio ranged from -0.7567 to 0.1683. The values of Vp/Vs ratio less than in the first layer and in some locations in the second layer resulted in negative Poisson’s ratio. The low and negative values of Poisson’s ratio are symptomatic of occurrence of ripable anisotropic materials in the locations where they occur, which suggests that indicated average depth should be removed and refilled with geomaterials that may be resilient to carry engineering loads.