Shear-wave Splitting Analysis Research Articles

Anisotropic structure at shallow depths across the Japan Trench

Anisotropic structures within the crust are frequently perceived to originate from stress-induced cracks, which have been mainly estimated on land through different wave speeds of orthogonally polarized S waves propagating in the anisotropic media. However, such estimations of crustal anisotropic structures in ocean areas, particularly for subduction zones around trenches, have not been investigated in detail due to the lack of long-term ocean bottom observations. In this study, we used ocean bottom seismometers of a permanent network deployed across the Japan Trench and the southern part of the Kuril Trench and applied the shear-wave splitting analysis to P-to-s converted waves extracted by receiver function analyses using teleseismic events. We estimated the anisotropic structures in marine sediments and oceanic crust for the incoming Pacific Plate and marine sediments for the overriding North American Plate. The obtained fast polarization directions for the incoming plate are mainly oriented to be parallel to the trench axis for the marine sediment and oceanic crust, which are formed by normal faults and cracks due to the upward plate bending in the outer-rise region, whereas results for marine sediments at the northern part of the Japan Trench are obliquely aligned to the trench axis. The oblique direction is consistent with the magnetic lineations of the incoming plate, indicating that ancient faults within the plate, which were formed in the shallow part of the crust during the creation of the oceanic plate at the ridge, are reactivated by the plate flexure. For the overriding plate, the fast polarization directions in the northern and southern parts of the study area are nearly normal to the trench axis. The central part shows two distinct features: the fast polarization directions parallel to the trench axis and small degrees of anisotropy. These patterns may reflect crack alignments associated with the lateral variation in postseismic crustal deformation after the 2011 Tohoku-Oki earthquake. Our results suggest substantial lateral variations in the stress field at the tip of the overriding plate along the strike direction.Graphical

Upper-mantle seismic anisotropy in the southwestern North Island, New Zealand: Implications for regional upper-mantle and slab deformation

We employed shear-wave splitting analysis on both teleseismic SKS and S waves, and S waves from deep (150–250 km) local earthquakes collected from a dense array with 43 temporary broadband seismic stations and nine long-term seismic stations centered at Mount Taranaki to characterize the upper-mantle dynamics in the southwestern North Island of New Zealand, in areas previously unexamined for shear-wave splitting. We observed dominantly trench-parallel fast polarizations and strikingly large delay times over 3 s from teleseismic analysis. In contrast, local S analysis yielded a sharp transition of fast-polarization from trench-parallel in the northeast to trench-normal in the southwest. Trench-parallel fast-polarization from teleseismic analysis may be attributed to sub-slab trench-parallel flow or to trench-parallel fractures in the subducting slab. More importantly, we attribute large delay times to deep upper-mantle (200–400 km depth) deformation, possibly associated with the dynamic interaction between the downgoing slab and the 410-km discontinuity or with the lithosphere delamination near the Taranaki-Ruapehu line. In contrast, the trench-parallel anisotropy from the local S waves in the northeast could be caused by fluid-bearing cracks in the crust of the Taupō Volcanic Zone and/or by trench-parallel fractures in the subducting slab resulting from outer rise bending. The abrupt change to trench-normal may be related to stress variations in the downgoing slab at different depths.

Mantle deformation in the highly oblique indo-burma subduction system inferred from shear wave splitting measurements

We utilized shear wave splitting analysis of teleseismic SKS, SKKS, and PKS phases to infer upper mantle deformational fabrics across a substantial area of Southeast Asia, where splitting measurements were previously limited. We used newly available permanent and temporary broadband seismic networks deployed across the Indo-Burma subduction zone and the eastern Indochina peninsula. The resulting 492 well-constrained splitting and 654 null measurements from 185 stations reveal clear large-scale patterns in the mantle deformational fabrics in response to the highly oblique active subduction and a large transform plate boundary. We identified two distinct domains of mantle deformation fabrics in the western Burma microplate and the eastern Indochina peninsula. In the former, trench parallel N-S fast polarization directions with an average lag time (δt) of 1.9 s are observed beneath the Indo-Burman Ranges. We suggest the observed splitting is partly due to anisotropy in the sub-slab region and relates to shear induced by the north moving Indian plate. The lithospheric fabric within the Indo-Burman Ranges and underlying subducting slab fabric contribute to produce the observed average δt of 1.9 s. The δt value decreases to an average of 1.0 s towards the back-arc until we reach the dextral Sagaing fault. In the second domain, starting approximately 100 km east of the Sagaing fault, we observe a consistent E-W fast direction with an average δt of 1.10 s in the eastern Shan-Thai and Indochina blocks. We interpret the E-W fabric as due to the deformation associated with the westward spreading of the Hainan mantle plume, possibly driven by overriding plate motion. Low velocities in the shallow mantle and late Cenozoic intraplate volcanism in this region support the plume-driven asthenospheric flow model in the Indochina peninsula. The sudden transition of the fast polarization direction from N-S to E-W along the eastern edge of the Burma microplate indicates the Sagaing fault acts as a mantle flow boundary between the subduction dominated trench parallel flow to the west and plume induced asthenospheric flow to the east. We also observed no net splitting beneath the Bengal basin which is most likely due to the presence of frozen vertical fabric resulting from the Kerguelen plume activity during Early Cretaceous.

Shear-wave attenuation anisotropy: a new constraint on mantle melt near the Main Ethiopian Rift

The behaviour of fluids in preferentially aligned fractures plays an important role in a range of dynamic processes within the Earth. In the near-surface, understanding systems of fluid-filled fractures is crucial for applications such as geothermal energy production, monitoring CO2 storage sites, and exploration for metalliferous sub-volcanic brines. Mantle melting is a key geodynamic process, exerting control over its composition and dynamic processes. Upper mantle melting weakens the lithosphere, facilitating rifting and other surface expressions of tectonic processes.Aligned fluid-filled fractures are an efficient mechanism for seismic velocity anisotropy, requiring very low volume fractions, but such rock physics models also predict significant shear-wave attenuation anisotropy. In comparison, the attenuation anisotropy expected for crystal preferred orietation mechanisms is negligible or would only operate outside of the seismic frequency band.Here we demonstrate a new method for measuring shear-wave attenuation anisotropy, apply it to synthetic examples, and make the first measurements of SKS attenuation anisotropy using data recorded at the station FURI, in Ethiopia. At FURI we measure attenuation anisotropy where the fast shear-wave has been more attenuated than the slow shear-wave. This can be explained by the presence of aligned fluids, most probably melts, in the upper mantle using a poroelastic squirt flow model. Modelling of this result suggests that a 1% melt fraction, hosted in aligned fractures dipping ca. 40° that strike perpendicular to the Main Ethiopian Rift, is required to explain the observed attenuation anisotropy. This agrees with previous SKS shear-wave splitting analysis which suggested a 1% melt fraction beneath FURI. The interpreted fracture strike and dip, however, disagrees with previous work in the region which interprets sub-vertical melt inclusions aligned parallel to the Main Ethiopian Rift which only produce attenuation anisotropy where the slow shear-wave is more attenuated. These results show that attenuation anisotropy could be a useful tool for detecting mantle melt, and may offer strong constraints on the extent and orientation of melt inclusions which cannot be achieved from seismic velocity anisotropy alone.

Evidence for weak azimuthal anisotropy beneath the Kumaon-Garhwal Himalaya

SUMMARY This study attempts to interrogate the upper mantle deformation pattern beneath the Kumaon-Garhwal region, located in the western Himalaya, using shear wave splitting (SWS) analysis of core-refracted (XK(K)S) phases recorded at 53 broad-band stations. The fast polarization azimuths (FPAs) revealed by 338 well constrained measurements are dominantly clustered around ENE–WSW, with a few along the NE and E–W directions. The delay times vary from 0.2 to 1.4 s, with an average of 0.6 s that is smaller than that for the Indian shield (∼0.8 s), central and eastern Himalayas. The northern part of the lesser Himalaya shows a slightly smaller delay time compared to the southern part, which is attributed to the weakening of azimuthal anisotropy caused by the dipping of the Indian lithosphere. In order to understand the crustal contribution, its anisotropy is measured by analysing the splitting of Ps conversions from the Moho (Pms), akin to that of the XK(K)S phases. However, reliable results for crustal anisotropy could be obtained only at 10 stations. The average delay time due to crustal anisotropy is 0.47 s, with a variation from 0.2 to 0.9 s. Although the dominant period of Pms is smaller than that of SK(K)S, crustal anisotropy contributing to splitting of the latter phases cannot be ruled out. The orientation of FPAs obtained from Pms phases is found to be parallel or sub-parallel to those from XK(K)S phases, suggesting a similar deformation mechanism in the mid- to lower-crust and upper mantle. On the basis of FPAs derived from XK(K)S measurements, the Kumaon-Garhwal Himalaya (KGH) region can be divided into four subregions. In the western and eastern parts, the FPAs are mostly aligned along NE and ENE–WSW, and NE, respectively. In the central and south-eastern parts, their orientation is along ENE–WSW and NW, respectively. The strong ENE–WSW orientation in the central part could result from a slightly variable anisotropy in the crust to the upper part of the lithosphere or basal topography causing deflection of mantle flow. Also, the NW orientation in the south-eastern part of KGH is associated with a shallow source within the lithosphere. Application of the spatial coherency technique to single-layered anisotropic parameters results in a depth of 220–240 km, implying that the dominant source of anisotropy could lie in the upper mantle.

Separation of fast and slow shear waves and prediction of fracture parameters based on non-orthogonal assumptions

SUMMARY Natural fractures play a significant role in oil and gas reservoirs. Accurate predictions of fracture parameters are vital in reservoir prediction and oil and gas development. The birefringent phenomenon of shear waves in fractured media makes shear wave splitting (SWS) analysis an important tool in formulating fracture predictions. The traditional SWS analysis method is based on an orthogonal assumption of fast and slow shear waves. However, in an orthotropic medium composed of a background vertical transversely isotropic medium and a set of vertical fractures, fast and slow shear waves are not necessarily orthogonal. This causes the traditional SWS analysis method to fail. To solve this problem, we proposed an SWS analysis algorithm with a non-orthogonal assumption of fast and slow shear waves in this study. First, we introduced a parameter (difference angle) to characterize the angle between slow shear waves and the normal polarization directions of the fast shear waves. Subsequently, based on the traditional two-parameter scanning algorithm, a parameter was added to facilitate three-parameter scanning. In addition, we derived an expression for the two-parameter scanning objective function using the non-orthogonal assumption. Two-parameter scanning can accurately extract fast and slow wave time delay data, but it cannot determine an accurate fast shear wave polarization direction. Therefore, we optimized the three-parameter scanning algorithm as follows: first, we used two-parameter scanning to obtain the fast and slow wave time delays and then performed further scanning to determine the polarization direction of the fast shear wave and difference angle. The optimization algorithm significantly improved the computational efficiency. Subsequently, we tested the accuracy of this method using synthetic single-trace and three-component vertical seismic profile data. We demonstrated the implementation process of the three-parameter scanning method using actual data, separated fast and slow shear waves, and predicted fracture parameters. The final fracture parameters were verified.

Feasibility of Deep Learning in Shear Wave Splitting analysis using Synthetic-Data Training and Waveform Deconvolution

Teleseismic shear-wave splitting analyses are often performed by reversing the splitting process through the application of frequency- or time-domain operations aimed at minimizing the transverse-component energy of waveforms. These operations yield two splitting parameters, ɸ (fast-axis orientation) and δt (delay time). In this study, we investigate the applicability of a baseline recurrent neural network, SWSNet, for determining the splitting parameters from pre-selected waveform windows. Due to the scarcity of sufficiently labelled real waveform data, we generate our own synthetic dataset to train the model. The model is capable of determining ɸ and δt with a root mean squared error (RMSE) of 9.7° and 0.14 s on a noisy synthetic test data. The application to real data involves a deconvolution step to homogenize the waveforms. When applied to data from the USArray dataset, the results exhibit similar patterns to those found in previous studies with mean absolute differences of 9.6° and 0.16 s in the calculation of ɸ and δt respectively.

Layered Mantle Flow Field Associated With Plate Kinematics and Slab Modulation Underneath the Horseshoe‐Shaped Banda Arc‐Islands

AbstractThe Banda arc‐continent collision zone signifies one of the most seismically active and tectonically intricate zones. The high convergence rate across the region, coupled with the exceptionally arcuate arc and subducted slab, makes it an ideal locale for investigating interactions between plate (slab) kinematics and plastic flow in the asthenosphere, which can be diagnosed by seismic anisotropy from shear wave splitting analyses. In total, 206 pairs of splitting measurements using teleseismic SKS, SKKS, and PKS, along with 43 pairs using local S phases, are obtained by utilizing broadband seismic data from five permanent seismic stations. To reduce the ambiguity in determining the origin of anisotropy leading to the teleseismic splittings, which lack vertical resolution, crustal anisotropy is constrained according to the sinusoidal moveout of converted S phases at the Moho using receiver functions. A layered anisotropic structure based on joint analyses of the anisotropy measurements characterizing different depth layers suggests the presence of trench‐parallel flow both in the mantle wedge and the sub‐slab region. The northeastward motion of the slab, entrained by the fast‐moving Australian Plate, deflects asthenospheric materials. The modulation results in trench‐parallel plastic mantle flows and leads to the steepening of the southern portion of the asymmetric spoon‐shaped Banda slab. In the shallower part of the sub‐slab region, the northeastward Australian Plate motion produces simple shear in the transitional layer between the rigid lithosphere and the viscous asthenosphere. The shear deformation induces seismic anisotropy with resulting fast orientations in accordance with the plate motion direction.

Stress Patterns and Crustal Anisotropy in the Eastern Alps: Insights From Seismological and Geological Observations

AbstractIn the Eastern Alps, the indentation of the Adriatic promontory since the Cenozoic affected the kinematics of separate crustal domains bounded by faults that accommodate lateral extrusion processes and differential shortening. Deciphering the pattern of crustal stresses in the orogen interior is challenging, due to the lack of in situ stress measurements at crustal depths. We define stress regimes and the orientations of the most‐compressive horizontal stress (SHmax) by integrating published results with new data, including stress analysis from fault plane solutions, estimation of crustal anisotropy through the shear wave splitting analysis, paleostress determination from fault slip data, and computation of cumulative seismic displacements. The retrieved regional SHmax are generally consistent with N‐S convergence. In the northern part of the study area, current stress orientations are almost parallel to paleo‐SHmax, suggesting a rather uniform compressional regime since the late Cenozoic. Conversely, sharp deflections and divergence with paleo‐SHmax appear at the western border of the Adriatic promontory across major transpressive and extensional shear zones and in the Southalpine domain, indicating a change in tectonically induced second‐order stresses. A current strike‐slip regime with subordinate orogen‐parallel seismic displacements affects a belt to north of the Periadriatic Lineament and NE‐extension characterizes the Ortles‐Engandine region. Seismic anisotropy locally exhibits fault‐parallel fast axes (Brenner‐Giudicarie fault‐systems, Dinaric and Southalpine thrusts), whereas stress‐induced anisotropy parallel to SHmax characterizes the southern part of the orogen. Cumulative seismic displacements are small compared to geodetic ones, and unravel partitioning of deformation into second‐order transpressive and extensional belts in response to indentation.

Upper mantle deformation beneath the northeastern part of Indian plate from shear-wave splitting analysis

Upper mantle deformation beneath the northeastern part of Indian plate from shear-wave splitting analysis

Toroidal mantle flow beneath the NE termination of the Kuril–Kamchatka subduction zone from seismic anisotropy

SUMMARY This study presents the findings of a splitting analysis conducted on core-refracted teleseismic shear waves (SKS, SKKS and PKS, called together as XKS) and local shear waves, obtained from a dense seismological network spanning the Kamchatka Peninsula. The objective of the study is to examine the pattern of mantle flow beneath the study area through the investigation of seismic anisotropy. The peninsula is situated at the northeastern end of the Kuril–Kamchatka subduction zone, where the Kuril trench intersects with the western boundary of the Aleutian trench. The data set utilized in this study comprises waveform data from a dense network of seismic stations (99 broad-band and short-period stations for the local shear wave splitting analysis and 69 broad-band stations for the SKS splitting analysis). The seismograms were downloaded from publicly available data repositories including the IRIS Data Management Center and the GFZ Data Services (GEOFON program). The dense station coverage allows us to investigate the lateral variations in anisotropy, providing insights into the flow patterns within the mantle. The processing of the combined data sets of local shear wave and teleseismic XKS waves allowed us to partially decipher the source of anisotropy in the mantle. Small delay (splitting) times (∼0.35 s) observed from the local-S data suggest that anisotropy in the mantle wedge is relatively weak with lateral variations. Larger splitting times (∼1.1 s) observed for the XKS waves relative to local S suggest that the main part of splitting on the XKS waves occurs in the subslab mantle. On the other hand, the rotational pattern of seismic anisotropy observed by both the local S and XKS waves suggests the presence of a toroidal flow at the NE edge of the subducting slab, which affects both the mantle wedge and subslab mantle. For the regions away from the edge of the slab, the mantle flow seems to be governed mainly by the drag of the lithospheric plate over the underlying asthenosphere.

Sensor Misorientation of CHINArray-II in Northeastern Tibetan Plateau from P- and Rayleigh-Wave Polarization Analysis

Abstract Accurate sensor orientation is important in providing reliable data used for seismological analysis such as P-wave receiver function analysis, shear-wave splitting, and ambient noise analysis. In this study, we used three distinct P-wave and Rayleigh-wave polarization analysis methods to estimate actual sensor orientation of 660 stations from CHINArray-II in northeastern margin of Tibetan plateau. We found that ∼42.12%–45.76% of the stations are well oriented with the absolute misorientation angle <3°, 42.42%–45.45% of the stations are fairly oriented with the absolute misorientation angle ranging from 3° to 10°, and 8.94%–11.82% of the stations are oriented with the absolute misorientation angle >10°. We further compared the results of some seismological analyses before and after sensor misorientation correction, such as Rayleigh-wave ellipticity (horizontal-to-vertical ratio) and P-wave receiver functions analysis. We found that when the sensor misorientation angle is large, it may lead to incorrect seismological results. With the same sensor misorientation, its influence on different seismological analysis is also different.

Automated shear-wave splitting analysis for single- and multi- layer anisotropic media

Shear-wave velocity anisotropy is present throughout the earth. The strength and orientation of anisotropy can be observed by shear-wave splitting (birefringence) accumulated between earthquake sources and receivers. Seismic deployments are getting ever larger, increasing the number of earthquakes detected and the number of source-receiver pairs. Here, we present a new Python software package, SWSPy, that fully automates shear-wave splitting analysis, useful for large datasets. The software is written in Python, so it can be easily integrated into existing workflows. Furthermore, seismic anisotropy studies typically make a single-layer approximation, but in this work we describe a new method for measuring anisotropy for multi-layered media, which is also implemented. We demonstrate the performance of SWSPy for a range of geological settings, from glaciers to Earth's mantle. We show how the package facilitates interpretation of an extensive dataset at a volcano, and how the new multi-layer method performs on synthetic and real-world data. The automated nature of SWSPy and the discrimination of multi-layer anisotropy will improve the quantification of seismic anisotropy, especially for tomographic applications. The method is also relevant for removing anisotropic effects, important for applications including full-waveform inversion and moment magnitude analysis.

Mapping of stress and structure controlled upper crustal anisotropy in Kumaon-Gharwal Himalaya

Shear wave splitting analysis of local earthquakes provides valuable insights into the structure and stress controlled upper crustal anisotropy signatures. We examine these signatures at the Kumaon-Garhwal Himalaya and provides an excellent example of anisotropic crustal structure in the ongoing continent-continent collision settings between Indian and the Eurasian plates. A total of 256 local earthquakes were selected for the analysis (1.0 ≤ M ≤ 5.4) between January 2017 and February 2021, recorded at a dense network of 51 broadband stations. The result indicates the observed crustal anisotropy is parallel to stress-aligned micro-cracks far from the major Himalayan fault zones and structure parallel near the fault zones. The dominant fast polarization directions (FPD) in the Inner Lesser Himalaya (ILH) and Higher Himalaya, are consistent with the maximum compressive horizontal stress directions (SHmax), essentially influenced by the local stress field. Whereas in the Outer Lesser Himalaya (OLH) and Sub Himalaya, fast directions are sub-parallel to the structural trends, suggesting that the anisotropy is associated with the shear fabric from recent deformation episodes related to structural induced anisotropy. The computed normalized delay times show a mean value of 1 ms/km, within the OLH and Sub-Himalaya, while in ILH and Higher Himalaya, this value increases up to 5.3 ms/km. The associated crack densities are 0.0038 and 0.0207, with shear wave velocity anisotropy of 0.38% and 2.07% respectively. Significant scatter within the depth range of 10–15 km is observed in normalized delay times, suggesting the source of anisotropy within the upper crust.

An investigation of seismic anisotropy in the crust beneath the western part of the North Anatolian Fault Zone

The North Anatolian Fault (NAF) is one of the most well-known continental strike slip faults, however, the details related to the geodynamic processes and the extent of deformation in the crust remain poorly understood. Within the context of the Dense Array for North Anatolia (DANA) project, a comprehensive data set was gathered from a dense temporary seismic network consisting of 70 stations that were deployed in early May 2012 and operated for 18 months in the Sakarya region and the surroundings. With the aim of further exploring the crustal deformation near the fault mainly caused by the strike slip motion also resulting in the alignment of minerals, the crustal seismic anisotropy beneath the western segment of NAF was investigated by local shear wave splitting analysis. Out of 1371 events, 90 well located earthquakes were extracted with magnitudes >1.4 corresponding to a total of 645 splitting measurements. This method makes use of earthquakes nearby or directly below each station benefiting from the fast polarization direction and the relevant delay time parameters as the main output. Despite the scattered patterns, the fast orientations are dominantly E-W parallel to the fault strike. Delay times between the fast and slow components of the shear wave vary between 0.01 and 0.3 s clearly revealing the existence of crustal anisotropy. In particular, measurements at each station exhibits spatial variations across the fault where it splays into two main branches beneath the study area separating different geologic units. A strong correlation could not be established between small scale faults and azimuthal anisotropy. The measurements presented here constrain crustal anisotropy above the deepest earthquakes at approximately 15 km depth.

Preliminary analysis of crustal anisotropy in the southernmost part of Simeulue Island

The study location is in the southernmost region of Simeulue Island near the Indo-Australian subduction margin. The deformation and geodynamic evolution around the island have been studied through some geophysical and geological studies. However, information regarding crustal seismic anisotropy around this region is less investigated, which may be very useful in delineating the deformation process in this area. In this work, we conducted an initial crustal anisotropy study using shear wave splitting analysis for two seismic stations from two datasets. One seismic station is from temporary deployment data recording data between December 2005 and March 2006, and the other one is from IA permanent network data. The objective of using these two different datasets is to assess the splitting analysis’s consistency and constrain the dominant cause of crustal anisotropy around the study area. The anisotropy orientations at all seismic stations are parallel to the regional stress direction. This situation may imply that anisotropy around the study area is mainly due to stress-induced anisotropy. Furthermore, the delay time distribution with depth suggests that the anisotropic source is concentrated around 15-20 km depth of the crust. This indicates a strong influence from the interaction between the lower crust and the upper part of the subducting slab.

Spatial variation in the crustal deformation pattern in the Koyna-Warna region, western India using shear wave splitting

SUMMARY This study investigates crustal anisotropy beneath the Koyna-Warna region (KWR) using waveforms of local earthquakes of magnitude ML ≥ 1.0, recorded at 24 surface and 6 borehole seismic stations during September 2005–February 2020. Waveforms of 1402 local earthquakes that fulfil the splitting criteria (ic < 45°) are used for shear wave splitting analysis, yielding 738 measurements. The study examines the spatial, temporal and depth variations in the fast polarization azimuths (FPAs) and delay times (δt). The average delay times in the Koyna and Warna regions are 0.035 and 0.050 s, respectively, which did not show any significant correlation with depth. The FPAs are predominantly oriented in the NNW–SSE direction in the KWR, commensurate with the in situ maximum horizontal stress (SHmax) obtained using data from a 3-km-deep scientific borehole and individual focal mechanism solutions. The anisotropy beneath KWR is due to the alignment of fluid-filled cracks and pore throats along the SHmax, and is attributed to stress-induced anisotropy. A considerable variation in the orientation of FPAs from NW to NNW–SSE beneath KWR could be associated with heterogeneity in the lithological properties due to variation in the fracture orientation under regional stress. The non-parallel orientation of FPAs with SHmax or faults at some stations is attributed to structure-induced anisotropy, which seems to be associated with surface topography. Measurements at stations, even near the reservoirs, did not show any significant temporal variation in the splitting parameters due to the fluctuation in the water levels.

Mantle flow underneath the South China Sea revealed by seismic anisotropy.

It has long been established that plastic flow in the asthenosphere interacts constantly with the overlying lithosphere and plays a pivotal role in controlling the occurrence of geohazards such as earthquakes and volcanic eruptions. Unfortunately, accurately characterizing the direction and lateral extents of the mantle flow field is notoriously difficult, especially in oceanic areas where deployment of ocean bottom seismometers (OBSs) is expensive and thus rare. In this study, by applying shear wave splitting analyses to a dataset recorded by an OBS array that we deployed between mid-2019 and mid-2020 in the South China Sea (SCS), we show that the dominant mantle flow field has a NNW-SSE orientation, which can be attributed to mantle flow extruded from the Tibetan Plateau by the ongoing Indian-Eurasian collision. In addition, the results suggest that E-W oriented flow fields observed in South China and the Indochina Peninsula do not extend to the central SCS.

Upper mantle anisotropy in Sri Lanka using shear-wave splitting analysis: Geodynamical implications

Upper mantle anisotropy in Sri Lanka using shear-wave splitting analysis: Geodynamical implications

Seismic anisotropy in the upper crust around the north segment of Xiaojiang faults in the SE margin of Tibetan Plateau

The Xiaojiang faults located in the SE margin of the Tibetan Plateau is a fault system of left-lateral strike-slip, striking NS, between the 2nd-order Sichuan-Yunnan block and the 1st-order South China block. The Xiaojiang faults and the surrounding areas are characterized by strong tectonic movements and intense seismic activities. Using seismic data from January 2013 to November 2020 recorded at the stations of the temporary QiaoJia seismic Array (QJ Array), deployed by the Institute of Geophysics, China Earthquake Administration, this study investigates the upper crustal anisotropy by the shear-wave splitting analysis on small local earthquakes, discusses the deformation patterns in the upper crust in the north segment of the Xiaojiang faults, evaluates the stress distribution in the study area, and analyzes its relationship with the regional tectonic structure. Adopting the data processing technique of shear-wave splitting, a total of 875 effective records were obtained at 50 stations. The mean direction of polarizations of fast shear-wave (PFS) is 162° ± 44° in the study areaand the mean normalized time-delay is 4.96 ± 2.38 ms/km. Based on the spatial distribution of the PFS and the regional geologic structure, the study area is divided into two zones: the zone N and the zone S. The PFS in the zone N is scattered, but the dominant PFS direction is in NNW, which is consistent with the direction of the regional maximum principal compressive stress. In the zone N, there are a few smaller local areas (i.e., subzones A, B, C, and D) in which the orientations of the PFS are quite different from the surrounding area. In the zone S, the dominant directions at most stations are in nearly NS, consistent with the strike of the Xiaojiang fault. It reveals the detailed spatial distribution of seismic anisotropy in the upper crust, as well as in situ principal compressive stress, indicating the influence of the regional stress, the complex tectonic environment, and maybe also the impact of the South China block. It also reveals that there also might be an upper-crust scale of tectonic line at near 26°20′N under Xiaojiang faults, which coincides with the north-south tectonic boundary in the lithospheric anisotropy.