Shear-wave Splitting Parameters Research Articles

Discriminating Lithospheric and Asthenospheric Anisotropy Beneath Northern Oman: Sharp Contrast Observed at the Semail Gap Fault Zone

AbstractTo gain a deeper understanding of the extensive and varied lithospheric deformations beneath northern Oman, we examine seismic anisotropy in this region using splitting analysis of teleseismic shear wave data. Our study utilizes data from a dense network consisting of 13 permanent and 45 temporary seismic stations, which were operational for approximately 2.5 years starting from October 2013. By examining the azimuthal distribution of shear wave splitting (SWS) parameters, we were able to divide the study area into three sub‐regions. The stations located to the west of the Hawasina window exhibit relatively azimuthally invariant SWS parameters suggesting a single anisotropic layer. On the other hand, most of the stations located in the central and eastern regions display variations versus back‐azimuth, indicating the potential presence of depth‐dependent anisotropy. The General NW‐SE trend of the Fast Polarization Directions (FPDs) of the one‐layer anisotropy in the west and FPDs of the upper layers in the east is concordant with the strike of the structures resulting from the collision between the continental and oceanic plates. A clear contrast in SWS parameters is observed in the Semail Gap Fault Zone (SGFZ), suggesting that the SGFZ can be a lithospheric‐scale structure that hampers the intrusion of mafic magma from the southeast. Furthermore, the FPDs of the lower layer in the east exhibit an NE‐SW trend, which may be indicative of the large‐scale mantle flow resulting from the present‐day plate motion.

Subduction‐Induced Asthenospheric Flow Around the Songliao Basin in NE China Revealed by Shear Wave Splitting Measurements of Dense Seismic Arrays

AbstractThe traditional mantle plume model fails to explain late Cenozoic intraplate volcanism in Northeast China (NEC). We constrained the pattern of upper mantle deformation to study the origin of intraplate volcanism by quantifying the shear wave splitting parameters captured by three NW‐SE linear seismic arrays in NEC. The dense station spacing (10 km) allowed us to image the small‐scale variations in anisotropic structures in unprecedented detail. The WNW‐ESE oriented subduction‐parallel anisotropy is likely induced by the asthenospheric return flow in the big mantle wedge convection associated with Pacific subduction. Therein the most notable feature is a toroidal pattern of anisotropy beneath the Songliao Basin, which is distinguished by a larger splitting time (approximately 1.0 s) relative to weak anisotropy in the center. The toroidal anisotropy pattern coincides with a high‐velocity anomaly extending from uppermost mantle down to ∼300 km depth, presumably indicating early stage foundering lithosphere. The asthenospheric return flow then moved around the foundering lithosphere and induced the corresponding toroidal anisotropy. The decompression partial melting of upwelling asthenospheric materials erupted along weak zones generating late Cenozoic intraplate basalts on both flanks of the Songliao Basin. The high‐density seismic array reveals that the Pacific subduction and early stage lithospheric foundering processes controlled the genesis of late Cenozoic volcanism in NEC.

Shear wave splitting analysis of local earthquakes from dense arrays in Shimian, Sichuan

The Shimian area of Sichuan sits at the junction of the Bayan Har block, Sichuan-Yunnan rhombic block, and Yangtze block, where several faults intersect. This region features intense tectonic activity and frequent earthquakes. In this study, we used local seismic waveform data recorded using dense arrays deployed in the Shimian area to obtain the shear wave splitting parameters at 55 seismic stations and thereby determine the crustal anisotropic characteristics of the region. We then analyzed the crustal stress pattern and tectonic setting and explored their relationship in the study area. Although some stations returned a polarization direction of NNW-SSE, a dominant polarization direction of NW-SE was obtained for the fast shear wave at most seismic stations in the study area. The polarization directions of the fast shear wave were highly consistent throughout the study area. This orientation was in accordance with the direction of the regional principal compressive stress and parallel to the trend of the Xianshuihe and Daliangshan faults. The distribution of crustal anisotropy in this area was affected by the regional tectonic stress field and the fault structures. The mean delay time between fast and slow shear waves was 3.83 ms/km, slightly greater than the values obtained in other regions of Sichuan. This indicates that the crustal media in our study area had a high anisotropic strength and also reveals the influence of tectonic complexity resulting from the intersection of multiple faults on the strength of seismic anisotropy.

Observations of Mantle Seismic Anisotropy Using Array Techniques: Shear‐Wave Splitting of Beamformed SmKS Phases

AbstractShear‐wave splitting measurements are commonly used to resolve seismic anisotropy in both the upper and lowermost mantle. Typically, such techniques are applied to SmKS phases that have reflected (m‐1) times off the underside of the core‐mantle boundary before being recorded. Practical constraints for shear‐wave splitting studies include the limited number of suitable phases as well as the large fraction of available data discarded because of poor signal‐to‐noise ratios (SNRs) or large measurement uncertainties. Array techniques such as beamforming are commonly used in observational seismology to enhance SNRs, but have not been applied before to improve SmKS signal strength and coherency for shear wave splitting studies. Here, we investigate how a beamforming methodology, based on slowness and backazimuth vespagrams to determine the most coherent incoming wave direction, can improve shear‐wave splitting measurement confidence intervals. Through the analysis of real and synthetic seismograms, we show that (a) the splitting measurements obtained from the beamformed seismograms (beams) reflect an average of the single‐station splitting parameters that contribute to the beam; (b) the beams have (on average) more than twice as large SNRs than the single‐station seismograms that contribute to the beam; (c) the increased SNRs allow the reliable measurement of shear wave splitting parameters from beams down to average single‐station SNRs of 1.3. Beamforming may thus be helpful to more reliably measure splitting due to upper mantle anisotropy. Moreover, we show that beamforming holds potential to greatly improve detection of lowermost mantle anisotropy by demonstrating differential SKS–SKKS splitting analysis using beamformed USArray data.

Shear‐Wave Splitting in the Mantle Wedge: Role of Elastic Tensor Symmetry of Olivine Aggregates

AbstractUsing a 3‐D mantle wedge flow field for a generic oblique subduction system, we calculate elastic tensors of mineral aggregates in the mantle wedge for A‐, B‐, C‐, and E‐type olivine crystal preferred orientations (CPO) and apply the calculated elastic tensor in the forward calculation of shear‐wave splitting (SWS) through the mantle wedge. We find that the hexagonal approximation of the full tensor does not affect the SWS parameters (the fast direction and the delay time) significantly for all CPO types except that the delay time for C‐type CPO becomes shorter. Additionally, we find that despite the 3‐D mantle flow field that results from oblique subduction, the fast direction is margin‐normal for A‐, C‐ and E‐type CPOs and margin‐parallel for B‐type CPO.

Stress Variations in the Delaware Basin from Shear-Wave Splitting Analysis

Abstract The northwestern Delaware basin has experienced an increase in seismicity, including one M 5.0 earthquake in 2020, correlated to hydraulic fracturing and wastewater injection activities. We conduct shear-wave splitting (SWS) analysis from local earthquakes to understand the increasing and intensifying seismicity in this area. Over 840 robust SWS parameters, the fast polarization orientation and the delay time, are obtained at five Texas Seismological Network stations by analyzing seismograms of more than 4000 events from 2019 to 2021. The fast orientations from individual events vary in a broad range for all stations, indicating a complex fracture system in the upper crust, even though the averages are consistent with the local fault strikes or the maximum horizontal stress. Fast orientations with large angles from the local stress appeared after the 2020 M 5.0 earthquake, evidencing an increase of pore pressure that facilitates slips on less favorable fracture planes by the stress field. Stress change caused by this earthquake could also contribute to the increasing diversity of SWS measurements. Future data analysis and stress modeling are needed to distinguish the different mechanisms.

Varying Shear Wave Splitting Parameters Suggest Interaction Between Lithosphere and Asthenosphere in Arxan‐Chaihe Volcanic Field, NE China

Over 2 years of broadband seismic data from 27 temporary stations deployed in the Arxan-Chaihe Volcanic Field (ACVF) provides us a valued opportunity to study mantle deformation and related geodynamic processes in NE China intraplate region. We estimated 153 pairs of high-quality teleseismic shear wave splitting parameters at 22 stations and 98 non-splitting measurements at 19 stations. Most of the stations in ACVF are characterized by NNW–SSE trending fast directions with variable splitting delay times, suggesting the fossilized extensional deformation within the lithosphere is locally thermal erased by the hot mantle upwellings associated with the volcanism. In the region south of the 46.5°N, a sharp swing of fast polarization occurs, and both nearly NW-SE and W-E oriented fast directions are observed, which could be explained by small local mantle convection caused by the lithospheric delamination.

Temporal variation in crustal shear wave splitting associated with a moderate earthquake around Nias Island, Indonesia

We investigate temporal variation in shear-wave splitting parameters of local earthquakes around Nias Island, the northern part of Sumatran forearc. We analyse a total of 79 high-quality splitting measurements recorded at a GEOFON seismic station (GSI) from 2009-2015. We observe that there is no clear time variation in the fast polarization of the splitting waves. On the other hand, an increase in the delay time of the splitting waves is found prior to a medium earthquake that occurred around the island in August 2010. Therefore, we suggest that this temporal change may be related to the change in crack geometry of rocks following the accumulation of the stress in the crust before the earthquake occurred.

A Database of Teleseismic Shear-Wave Splitting Measurements for the Ordos Block and Adjacent Areas

Abstract The Ordos block of the north China craton and its surrounding regions are affected by the India–Eurasia collision to the southwest and the subduction of the Pacific beneath the Eurasian plates to the east. To provide the foundation for delineating lithospheric deformation and asthenospheric flow beneath this tectonically diverse region, we have created a database of individual shear-wave splitting (SWS) parameters by applying a uniform set of data processing procedures. After automatic data processing and manual checking, a total of 16,228 pairs of well-defined PKS, SKKS, and SKS splitting parameters (fast orientations and splitting times) are obtained from 1023 broadband seismic stations that recorded data between 2007 and 2019. Along the western and southern margins of the Ordos block, the observed seismic anisotropy is attributable to mantle flow deflected by the relatively thick lithospheric root of the Ordos block. A clear back-azimuthal dependence of the fast orientations is observed at some of the stations located in the Alxa block, Ordos block, and Sichuan basin, indicating possible existence of complex anisotropic structures. The new SWS database can be employed by researchers in various fields to study lithospheric deformation and asthenospheric flow beneath the Ordos block and surrounding regions.

Mantle Wedge Seismic Anisotropy and Shear Wave Splitting: Effects of Oblique Subduction

AbstractWe investigate the evolution of olivine crystal preferred orientation (CPO) and its effect on local shear wave splitting (SWS) in the mantle wedge of oblique subduction zones. Based on model‐predicted 3‐D mantle wedge flow fields, we compute the A‐type and E‐type olivine CPO distribution for a range of subduction obliquity. The results show that the seismically fast axis does not necessarily align with the flow direction. To model the local SWS parameter distribution for oblique subduction zones, we apply a full range of initial polarization to multilayer models that approximate the model‐predicted CPO distributions. These models result in a bimodal SWS parameter distribution, which relaxes as subduction obliquity increases. Unlike non‐oblique subduction models, these models indicate considerable variations in the SWS parameters with subduction obliquity and initial polarization and also among the forearc, arc, and backarc regions. Because of this variability, a single SWS measurement cannot constrain the CPO distribution, and shear waves with a range of initial polarization are required to interpret the SWS parameters in oblique subduction zones. Our results indicate that 3‐D mantle wedge flow due to oblique subduction cannot explain commonly observed margin‐parallel fast direction in the forearc region but can explain margin‐normal fast directions that are observed in the arc and backarc regions of oblique subduction zones.

Determination of Shear Wave Splitting Parameters in 2018 Lombok Earthquake Using Rotation Correlation Method: Preliminary Result

Shear-wave splitting (SWS), or the propagation of two independent shear waves, can be used as an indicator of seismic anisotropy. In this study, we utilize this concept using aftershock data of the 2018 Lombok earthquake which had been acquired in period of August 4 – September 9, 2018. The goal of this research is to better understand the crack distribution related to the rupture zone of the 2018 Lombok earthquake. After applying instrument correction to the data, the waveform data were then windowed in each P and S arrival time. To determine the SWS parameters, we performed rotation in each horizontal seismogram components. The horizontal components were rotated from azimuth 0° to 180° with an increment of 1°. Cross-correlation coefficient (CCC) was determined for each rotation angle. The polarization direction and the SWS delay time were chosen from the parameters shown in the highest value of CCC.

Upper mantle seismic anisotropy beneath the western and central North China Craton

We estimate over 2,000 shear wave splitting parameters from 84 teleseismic earthquakes recorded at 465 temporary broadband stations distributed over the western and central North China Craton to constrain lithospheric deformation and mantle flow. In the interior of the Ordos Block, the W-E trending fast polarization directions with small delay times are indicative of stable terrane, where a thick Archean cratonic keel has been imaged. The dominant W-E fast orientations with relatively large delay times observed in southern Trans-North China Orogen show a systematic directional deviation of ~30° from the absolute plate motion (APM) direction and are explained by a combined effect of APM-induced mantle fabric and a mantle flow around the southeast edges of the Ordos Block. The prominent rift-orthogonal fast orientations beneath rifts to the northern Ordos Block imply the existence of active mantle flow. Also, it is inferred that a mantle plume is responsible for generating the parabolic fast direction pattern beneath the Datong Volcano.

Crustal and Mantle Deformation Inherited From Obduction of the Semail Ophiolite (Oman) and Continental Collision (Zagros)

AbstractA common deviation from typical subduction models occurs when thrust sheets of oceanic crust and upper‐mantle rocks are emplaced over more buoyant continental lithosphere. The archetypal example of ophiolite obduction is the Semail ophiolite in the United Arab Emirates (UAE)‐Oman orogenic belt, formed and obducted onto the Arabian continental margin during the Late Cretaceous. The Strait of Hormuz syntaxis, the northern extent of the UAE‐Oman mountains, marks the transition from ocean‐continent convergence in the Gulf of Oman to continental collision along the Zagros Mountains. Based on new seismic data from a focused recording network, we infer continental crustal and mantle deformation in the northeastern corner of the Arabian plate (including the southern Zagros and the UAE‐Oman mountains), using observations from anisotropic tomography and shear‐wave splitting (SWS) measurements. We recover a change of ∼90° (from approximately WNW to nearly NS) in the axis of fast‐anisotropic orientations in the crust from the Zagros to the UAE‐Oman mountain belt, consistent with the dominant strike of the orogenic belts. We also find evidence in our SWS parameters for localized fossil deformation in the lithospheric mantle underlying the UAE‐Oman mountain range, possibly related to stress‐induced tectonism triggered by north‐east oriented underthrusting of the proto‐Arabian continental margin beneath the overriding Semail ophiolite. Shear‐wave‐splitting anisotropy orientations along two transects across the northern Musandam peninsula, averaging 15° anticlockwise from the north, provide the first geophysical verification of previous geological evidence that suggests a NE polarity of the Late Cretaceous Oman subduction zone system.

Upper mantle anisotropy from shear wave splitting of teleseismic earthquakes in the Kumaun-Garhwal and adjoining area of NW Himalaya

Teleseismic earthquakes data recorded by 8 broadband digital seismic stations deployed in the Kumaun-Garhwal and adjoining area of NW Himalaya are analyzed to investigate the seismic anisotropy in the upper mantle. Shear-wave splitting parameters (ɸ and ∂t) derived from the analysis of core-refracted SKS/SKKS phases provide first hand information about seismic anisotropy and deformation in the upper mantle beneath the region. A total, 209 good measurements of seismic anisotropy have been obtained from the two years of collected high-quality teleseismic data in the study region. The analysis shows considerable strength of anisotropy (delay time(∂t) 0.6–2.8 s) with average ENE–WSW oriented fast polarization orientation at most of the stations while other one either along with the absolute plate motion (APM) of the Indian plate or parallel to strike of the Himalayan mountain belt. The Fast axis direction observed at stations close to between the MBT and MCT aligns parallel to the strike of local geological faults and orthogonal to absolute plate motion (APM) direction of the Indian plate. The large variation in the splitting strength and fast polarization trends around the Kumaun-Gharwal and adjacent in NW Himalaya have invoked both lithospheric and asthenospheric anisotropy.

Anisotropic zoning in the upper crust of the Tianshan Tectonic Belt

The Tianshan Tectonic Belt is an intracontinental orogenic belt formed by continental convergence that has undergone long-term tectonic evolution. The reactivation that began during the Cenozoic Period has led to complex structural changes. The goals of this study are to review the seismic observational data obtained during 2009–2019 in the Xinjiang regional seismic network and analyze the anisotropy of the upper crust in the Tianshan area. Therefore, a shear-wave splitting system was adopted to collect and analyze shear-wave splitting parameters of 33 stations in the study area. The anisotropy of the upper crust of the Tianshan is spatially diverse, and the dominant polarization directions of fast shear-wave reflect the spatial variations of regional tectonic stress. In addition, the time delays of slow waves are proportional to the intensities of anisotropy in the upper crustal medium. The dominant polarization direction of the fast waves in the western segment of the North Tianshan Mountain, northwestern corner of the Tarim Basin, and northeastern edge of Pamir is consistent with the tectonic stress fields in the area. In the northern part of the Keping Block, the dominant polarization directions of the fast waves are consistent with the fault trends; however, they are at a high angle to the dominant directions of the regional tectonic stress field indicating that the anisotropy is affected by the faults in the area. The anisotropy of the eastern segment of the South Tianshan Mountains and the surrounding area of Urumqi are affected by the local stress field and fault structure. The polarization directions at some of the stations are sub-parallel to the directions of the regional principal stress. However, for other stations, the polarization directions are aligned with the neighboring faults. The polarization directions of the fast waves in most of the study area are consistent with the local tectonic stress fields. Thus, stress compression phenomena such as the Tarim Basin being thrusted and subducted between the Tianshan crust and the upper mantle due to the far field effects of the convergence between the Indian and Siberian plates are evident. Furthermore, the zoning of the time delays is distinct, and the time delays share an increasing trend from east to west in the North Tianshan and South Tianshan Mountain ranges. These results are consistent with the north-south convergence deformations across the Tianshan Mountains, where the deformation rate increased from east to west. The average values of time delays in northeastern Pamir are significantly higher than that in the other areas due to the occurrence of the most intensive tectonic movements suggesting that the anisotropy of the zone is significantly stronger than that of the other zones in the Tianshan Tectonic Belt. We successfully deciphered the seismic anisotropy in the upper crust and provided a comprehensive and systematic understanding of the dynamic mechanisms of the Tianshan Tectonic Belt.

Analysis of shear wave splitting parameters in los Humeros geothermal field, Puebla, Mexico

A study of anisotropic characteristics based on shear wave splitting (SWS) parameters (fast shear wave polarization and delay time) was carried out on 121 microearthquakes recorded by seismic arrays installed in Los Humeros Geothermal Field, Puebla, Mexico from 2011 to 2016. This analysis determined how both parameters seem to have been influenced by the geothermal field operation and by the spatial variations of the seismic sources with respect to the seismic array. There is a clear evidence of anisotropy since most of the SWS results show a main NW-SE polarization direction, which is not consistent with the maximum horizontal stress orientation in the region. The fast polarizations remained mostly constant for the analyzed period, keeping a preferential direction of NW-SE at the central-southern section of the study region, which in some cases follows the orientation of some proximal local faults. In the northern region, the fast polarizations appear uniform without following the orientations of the local faults. This preferential direction may be due to the shallow structures are being affected by the tectonism inhered from the basement, controlled in part by the Mexican fold and thrust belt whose main orientation is NW-SE and, to a lesser extent, by the pre-existing caldera ring faults.Observations of mean delay times mostly range between 0.025 s and 0.07 s for all seismic stations analyzed, with some individual values reaching up to 0.15 s, but with a total average of 0.041 s for the period analyzed. The maximum anisotropy percentage found was 13.54%, and the variations allowed us to characterize the geothermal field in two main areas with high fracturing. Results show Los Humeros, Antigua, Los Conejos and Las Viboras faults, and other minor faults around them as the main conduits of permeable sources. However, the inferred Antigua and Malpaís faults may also be considered important areas.Delay times δt and their respective error bars, showed an apparent decrease from 2014 to 2016, which may indicate a possible inverse correlation with an increase in fluid injection. However, the lack of data during 2013 and 2014 makes it difficult to robustly prove this hypothesis due to the change in station configuration.

Shear-wave splitting observations: Implications toward the variable anisotropic structure of the upper mantle below the Kachchh rift zone, Gujarat, India

The shear-wave splitting parameters (i.e. fast axis orientation (ψ) and delay time (δt)) are measured at twelve broadband stations in the Kachchh rift zone (KRZ), Gujarat, India, using SKS/SKKS core phases. The average splitting parameters (ψ, δt) vary from (49.41 ± 5.88, 1.63 ± 0.28) at Bhachau (i.e., 4 km SE of the 2001 Bhuj mainshock location) to (103.78 ± 12.82, 1.91 ± 0.35) at Mandvi near the coast. The present study reveals a mean ψ of (76.91o ± 4.57o) with a mean δt of 1.84 ± 0.30 s, for the KRZ. The observed large mean delay 1.84 ± 0.30 s for the region suggests a combined anisotropic contribution from both the lithospheric frozen anisotropy as well as the asthenospheric flow induced anisotropy. Inferred thicknesses of the anisotropic layers from the estimated δt vary from 160 km (in the central KRZ) to 269 km (in the surrounding unrifted zones (SURZ)). The measured deviation of ψ from the absolute plate motion (APM) direction of the Indian plate varies from 8.4o (in the central KRZ) to 62.8° (in the SURZ). The large δt and large deviation of ψ from the APM (>35°) are found to be associated with those stations (e.g. MND, KNM, VGH, NGR, VJP, TPM), which lie near to the geological boundaries in the SURZ. The main finding of the present modeling study depicts a thick highly anisotropic upper mantle underlying the KRZ, where mantle anisotropy parallel to the Kachchh rift axis direction is caused by both frozen lithospheric anisotropy and asthenospheric flow-induced anisotropy. The geographical and azimuthal variations of the measured splitting parameters detect three different anisotropic continental fragments, which might have preserved an ENE-WSW trending fossilized olivine fabric, which was formed before the assemblage of these continental fragments.

Spatio-temporal analysis of seismic anisotropy associated with the Cook Strait and Kaikōura earthquake sequences in New Zealand

SUMMARYLarge earthquakes can diminish and redistribute stress, which can change the stress field in the Earth’s crust. Seismic anisotropy, measured through shear wave splitting (SWS), is often considered to be an indicator of stress in the crust because the closure of cracks due to differential stress leads to waves polarized parallel to the cracks travelling faster than in the orthogonal direction. We examine spatial and temporal variations in SWS measurements and the Vp/Vs ratio associated with the 2013 Cook Strait (Seddon, Grassmere) and 2016 Kaikōura earthquakes in New Zealand. These earthquake sequences provide a unique data set, where clusters of closely spaced earthquakes occurred. We use an automatic, objective splitting analysis algorithm and automatic local S-phase pickers to expedite the processing and to minimize observer bias. We present SWS and Vp/Vs measurements for over 40 000 crustal earthquakes across 36 stations spanning close to $5\frac{1}{2}$ yr between 2013 and 2018. We obtain a total of 102 260 (out of 398 169) high-quality measurements. We observe significant spatial variations in the fast polarization orientation, ϕ. The orientation of gravitational stresses are consistent with most of the observed anisotropy. However, multiple mechanisms (such as structural, tectonic stresses and gravitational stresses) may control some of the observed crustal anisotropy in the study area. Systematic analysis of SWS parameters and Vp/Vs ratios revealed that apparent temporal variations are caused by variation in earthquake path through spatially varying media.

Complex Shear-Wave Anisotropy from Induced Earthquakes in West Texas

ABSTRACT We have analyzed shear-wave splitting (SWS) data from local earthquakes in the Permian basin in west Texas to understand crustal stress change and induced seismicity. Two SWS parameters, the fast polarization direction and the delay time, are computed using a semiautomatic algorithm. Most measurements are determined in the Delaware basin and the Snyder area. In both regions, SWS fast directions are mostly consistent with local SHmax at stations that are relatively far from the earthquake clusters. Varying fast directions at one station are related to different ray paths and are probably caused by heterogeneity. In the Snyder area, most northeast–southwest fast directions are from the events in the northern part of the cluster, whereas the northwest–southeast fast directions are mostly from the southern part. The northeast–southwest and northwest–southeast fast directions could be attributed to the northeast-trending normal faults and the northwest-trending strike-slip faults, respectively. SWS results in the Delaware basin have two unique features. First, most shallow earthquakes less than 4 km deep produce relatively large delay times. This observation implies that the upper crust of the Delaware basin is highly fractured, as indicated by the increasing number of induced earthquakes. Second, diverse fast directions are observed at the stations in the high-seismicity region, likely caused by the presence of multiple sets of cracks with different orientations. This situation is possible in the crust with high pore pressure, which is expected in the Delaware basin due to extensive wastewater injection and hydraulic fracturing. We propose that the diversity of SWS fast directions could be a typical phenomenon in regions with a high rate of induced seismicity.

Multiscale spatial distribution of crustal seismic anisotropy beneath the northeastern margin of the Tibetan plateau and tectonic implications of the Haiyuan fault

The Haiyuan fault is a large strike-slip fault in the northeastern (NE) margin of the Tibetan plateau. Using new data recorded by a temporary seismic array, this study obtains crustal seismic anisotropy measurements in different scales across the Haiyuan fault by analyzing splitting of shear-waves from local earthquakes. Parameters of shear-wave splitting (SWS) indicate clear zoning characteristics in seismic anisotropy in the upper crust. The study area is divided into two different anisotropic subzones by the Haiyuan fault. The dominant orientation of fast polarizations is NNE or NE north of the faults and WNW to EW south of the fault. This pattern of spatial variation indicates the controlling impact of Haiyuan fault to observed seismic anisotropy. With a zone of dozens of kilometers from the Haiyuan fault, the dominant orientations of fast polarizations are WNW, generally coincidental with the strike of the Haiyuan fault except for several stations. It indicates that the influence of the fault on stress could reach to about 10 km from the fault distance. Spatial pattern of anisotropy indicates multiple effects by stress, faults and local tectonics. It suggests different anisotropic mechanisms including stress-induced anisotropy and tectonic (or structural) anisotropy. The dominant orientation of fast polarizations can indicate in situ maximum horizontal principal compressive stress although there is always influence of tectonics.The normalized time-delays are higher around the Haiyuan fault than those away from the fault, suggesting stronger crustal seismic anisotropy in a narrow belt around the strike-slip fault zone. The SWS parameters suggest that the Haiyuan fault is the actual crustal boundary of NE Tibetan plateau, about 200 km north of the reported boundary of the Tibetan plateau block. Combining with other studies in the crust and the lithosphere, this paper infers possible two-layer crustal anisotropy beneath the Yinchuan graben, north of the Haiyuan fault. The dominant orientation of fast polarizations are NNE or NE in the upper crust, probably originated from crack-induced anisotropy, but become WNW or NW in the middle-lower crust, possibly originated from deformation anisotropy.