Polarization Direction Of Fast Wave Research Articles

Characteristics of azimuthal anisotropy in SE Tibetan plateau and its relationship with the background of block structure

The southeastern Tibetan Plateau, which includes the Tibetan Plateau, Yangtze Block, and Cathaysia Block, is geotectonically situated in the compound part of the Tethys-Himalayan tectonic domain and the Peninsular Pacific tectonic domain. It is one of the critical regions for studying the scientific problems of plateau material lateral escape, lithosphere deformation, geotectonic properties of blocks, and deep dynamics. In this study, we use ambient noise data recorded by 401 broadband stations to obtain high-resolution short-period (T = 4–32 s) Rayleigh wave azimuthally anisotropic phase velocity maps. These could provide fresh clues for an in-depth understanding of the crust-mantle velocity structure, deformation mechanism, and geotectonic evolution in the southeastern Tibetan Plateau. Within the Simao block, the strikes of the faults and the orientations of the principal compressive stress of the stress field both generally coincide with the fast-wave polarization direction (FPD). The FPD near the Lancangjiang fault zone in the west is in the NE-SW direction, near the Wuliangshan fault zone in the center is near the NS direction, and near the Red River fault zone in the east is the NW-SE direction. We estimate that the compressive stress in the southwest direction of the Tibetan Plateau material has a controlling effect on the crustal deformation of the Simao block, which is likewise blocked by the Lincang granite belt, resulting in strong tectonic deformation. The FPD of the crust in the middle Red River fault zone is NS direction, significantly different from the fault strike. Combining with the seismic activity and GPS results, the depth of 8 km below the surface of the middle Red River fault is completely locked, and we conclude that the anisotropy of the upper crust of the middle part of the Red River fault zone is related to the action of the regional tectonic stress field. Taking into account geochemical and thermochemical results, we speculate that the complex tectonic stress at the junction of the blocks leads to prominent regional characteristics of the FPDs of azimuthal anisotropy in the crust, suggesting that the Shizong-Mile fault zone may be the western boundary between the Yangtze block and the Cathaysia block.

Complex mantle flow beneath the Luxi uplift segment of the Tanlu fault zone inferred from SKS splitting measurements

To better understand the anisotropic structure and mantle dynamics in and around the Luxi uplift segment of the Tanlu fault zone in East China, we collect waveform data recorded at 47 NCISP (Northern China Interior Structure Project) portable seismic stations during November 2000 to August 2001. Using the minimum transverse energy grid-search method of SKS wave splitting measurement, effective splitting results at 35 stations are obtained. Our results show that the fast wave polarization direction (FPD) in the study area is dominated by a nearly E-W direction, with a range of 70°–116° and an average FPD of N98°E, which is generally perpendicular to the trend of the Tanlu fault zone. The splitting time varies from 0.6 to 1.5 s with an average of 0.94 s. Although there are subtle differences in FPDs of different tectonic units, it is still evident that larger splitting times amounting to ∼1.2 s are located north of the Luxi uplift, which is closely associated with the E-W horizontal flow in the big mantle wedge (BMW). In contrast, smaller splitting times are visible between the two branches of the Tanlu fault zone, indicating the existence of upwelling flow in the BMW. These complex mantle flow suggests that the BMW structure has played an important role in the deep dynamic processes beneath East Asia.

Deformation and seismic anisotropy in the SE Tibetan Plateau lithospheric mantle inferred from Maguan peridotite xenoliths

SKS wave splitting measurements revealed strong and complex seismic anisotropy in the upper mantle beneath the southeastern Tibetan Plateau. To better understand lithospheric deformation and upper mantle anisotropy observed in this region, we performed studies on microstructures, seismic anisotropy and chemical compositions of the mantle peridotite xenoliths collected from Maguan, Yunnan Province, in the SE Tibetan Plateau. The mantle xenoliths show two types of olivine crystal preferred orientation (CPO), AG–type and A–type. AG–type fabric is characterized by a concentration of [010]-axis normal to the foliation and a large circle girdle of the [100] and [001] axes in foliation. A–type fabric shows point concentrations of [100] and [010] axes subparallel to lineation and subnormal to foliation, respectively. Orthopyroxene and clinopyroxene align their [001]-axis sub-parallel to the [100]-axis of olivine. Seismic anisotropy calculated from the fabric of xenoliths displays moderate to strong polarization anisotropy (AVs) ranging from 3.1% to 7.0%, with 4.6% on average. Based on the seismic properties of xenoliths, we propose that, in SE Tibetan Plateau (south of 26°N), the SKS wave splitting is attributed to melt-enhanced anisotropy with a vertical structural frame lithosphere. Under this structural configuration, the large delay time could be expected with the fast wave polarization direction (FPD) in E–W direction. Our study also supports the model of double anisotropic layers in this area. The fast orientation of lower layer caused by lithospheric mantle anisotropy is in E–W direction, while the upper one with the NNW–SSE FPD is related to the frozen anisotropy in the middle-lower crust.

Seismic anisotropy and implications for lithospheric deformation beneath the Ordos Block and surrounding regions

SUMMARYThe Ordos Block is located at the intersection of the Tibetan Plateau, the South China Plate and the North China Craton. The deep deformation of its surrounding areas is geologically complicated. Seismic anisotropy provides information about past and present deformation of the upper mantle and helps to better understand deep deformation processes. We obtained the anisotropic pattern at high resolution within and surrounding the Ordos Block by analysing teleseismic data from ∼710 newly deployed temporary seismic stations based on SKS phases from ∼86 earthquakes. The central and eastern Ordos Block with a relatively thick and rigid lithosphere is characterized by a weak anisotropy. In the western part of the Ordos Block, the fast-wave polarization direction is dominantly NW–SE. We believe the lithosphere in the western part of the Ordos Block may have undergone significant deformation caused by expansion and compression of the Tibetan Plateau. Near the Datong Basin, the delay time is 0.92 s and the fast-wave polarization direction is mainly oriented NW–SE, perpendicular to the NE-directed compression of the Tibetan Plateau and parallel to the basin extension direction. We speculate that this anisotropy is related to the long-range effect of NE compression of the northeastern Tibetan Plateau on the low-velocity upper mantle in this area. The weak anisotropy in the central part of the Shanxi Rift indicates that the thickness and mechanical strength of the high-velocity lithosphere are higher than in the southern and northern regions.

Lithospheric dynamics in the vicinity of the Tengchong volcanic field (southeastern margin of Tibet): an investigation usingPreceiver functions

SUMMARYTengchong volcanic field (TVF) in the northern Indochina block lies in a critical area for understanding complex regional dynamics associated with continent–continent convergence between the Indian and Eurasian plates, including northeastward compression generated by subduction of the Indian Plate beneath the Burma Arc, and southeastward lateral extrusion of the crust from below central Tibet. We gathered 3408 pairs of P receiver functions with different frequencies and calculated the splitting parameters of the Moho-converted Pms phase. An anisotropic H-κ stacking algorithm was used to determine crustal thickness and Vp/Vs ratios. We also inverted for the detailed S-velocity structure of the crust and upper mantle using a two-step inversion technique. Finally, we mapped the topography of the lithosphere–asthenosphere boundary. Results show fast-wave polarization directions with a dominant NE–SW orientation and delay times varying between 0.19 and 1.22 s, with a mean of 0.48 ± 0.07 s. The crustal Vp/Vs ratio varies from 1.68 to 1.90 and shows a maximum value below the central part of the TVF, where there is relatively thin crust (∼35–39 km) and a pronounced low-velocity anomaly in the middle–lower crust. The depth of the lithosphere–asthenosphere boundary ranges from 53 to 85 km: it is relatively deep (∼70–85 km) in the vicinity of the TVF and relatively shallow in the south of the study area. In the absence of low shear wave velocity in the upper mantle below the TVF, we propose that the low-velocity anomaly in the lower crust beneath the TVF derives from the upper mantle below the neighbouring Baoshan block.

Seismic anisotropy time variations at Mt Etna

SUMMARYThe aim of this paper is to study the temporal variations in the seismic wavefield associated with the stress changes in the dynamic features of the Mt Etna volcanic activity. We used shear wave splitting analysis on a huge data set of local earthquakes, in order to identify changes of the local stress field at Mt Etna during the time interval from 2006 to 2011. This analysis allows us to obtain two parameters: the polarization direction of the fast shear wave (φ) and the time delay of the slow shear wave (Td,time delay between the split shear waves). Orientation of φ generally provides information about the anisotropic symmetry and stress direction whereas Td provide information about the average crack density along the ray path.Based on our findings it is possible to divide Etna Volcano in three different sectors, each one distinguished by typical fast wave polarization direction. We find that the western part of the volcano is controlled by the regional tectonic stress field having a NS and EW directions. Instead, the eastern part of the volcano is mainly controlled by the local volcanic stress, particularly an EW local stress field in the NE sector (Pernicana), and a quasi NS local stress field in the SE sector (Mascalucia, Timpe), where previous studies evidenced: (i) some low-Qp anomaly regions between 0 and about 6 km depth, probably associated with high pore pressure and the intense faulting and (ii) by magnetotelluric surveys, several high conductivity zones, up to 8 km depth, related to a diffuse presence of hydrothermal activity and fluid circulation. Temporal variations in time delay, mostly before the 2008–2009 lateral eruption, can be interpreted as stress accumulation increase with a consequent release of stress due to coalescing of microcracks in the conduit for the eruption of magma.

Upper‐Crustal Anisotropy of the Conjugate Strike‐Slip Fault Zone in Central Tibet Analyzed Using Local Earthquakes and Shear‐Wave Splitting

Abstract Remarkable V‐shaped conjugate strike‐slip faults extend along the Bangong–Nujiang suture in central Tibet. Motions of these faults are considered to accommodate ongoing east–west extension and north–south contraction. Fabrics within the fault zone that are anisotropic to seismic waves can provide clues as to the unusual scale and style of lithospheric deformation. With the goal of determining the upper‐crustal anisotropy pattern in central Tibet, we measured shear‐wave splitting parameters (fast wave polarization direction and delay time) using waveforms generated by 194 local earthquakes recorded by 49 stations of the SANDWICH network. Stations located in eastern and western zones of the study area show anisotropy directions that agree well with the maximum horizontal compressive stress direction. The fast polarization directions at stations near active strike‐slip faults generally run parallel to the strikes of these faults. Pervasive inactive thrust faults caused by Cretaceous–Tertiary shortening in central Tibet also clearly correlate with the anisotropy detected at nearby stations. These results demonstrate that both local structures and stress contribute to upper‐crustal anisotropy in the region. Combining the new results with previous SKS‐wave splitting results and other seismic evidence, we propose that deformation in the upper crust is mechanically decoupled from that in the upper mantle, due to eastward middle‐lower crustal flow. This crustal flow causes basal shearing required for the formation of conjugate strike‐slip faults in central Tibet.

Disaggregated anisotropy and deformation style of the upper and lower crust in the southeastern Tibetan plateau

The arrival times of the converted P-to-S phase at an intra-crustal discontinuity (Pis) or the Moho (Pms) provide a powerful diagnostic tool for detecting anisotropy with horizontal symmetry axis. In this study, we used Pis and Pms arrival times on P receiver functions to determine the anisotropy of the crust from the seismic data recorded by 285 temporary broadband stations and 3 permanent stations deployed in the southeast margin of Tibet. First we measured the splitting parameters of the upper crust by fitting the Pis phase arrival, and then we adjusted the Pms phase arrival to obtain the splitting parameters of the lower crust after correcting for the effect of the anisotropic upper crust on the Pms arrivals. So, we achieved 75 double-layer splitting measurements. In the upper crust the splitting times vary between 0.05 s and 1.34 s with an average of 0.53 s ± 0.29 s, while in the lower crust they range from 0.06 s to 1.42 s with an average of 0.62 s ± 0.33 s. The results confirm that the crust is remarkably anisotropic in the southeastern margin of Tibet. In the upper crust, the fast wave polarization directions clearly show a clockwise rotation around the Eastern Himalayan Syntax, suggesting that the extensional fluid-saturated microcracks induced by rigid extrusion from central Tibet are mostly responsible for the observed anisotropy. In contrast, the lattice preferred orientation of anisotropic minerals induced by a channel flow is the main contributor to the lower crust anisotropy.

Observation and analyses of shear wave splitting at the Larderello-Travale geothermal field, Italy

We measured shear wave splitting (SWS) parameters from a large dataset of local microearthquakes recorded at the Larderello-Travale geothermal field (LTGF; Tuscany, Italy). For that geothermal area, seismic anisotropy is distributed in the upper crust following a complex pattern. Although the overall trend reflects the strike of the normal faults dominating the region, measurements at the southern and central part of the LTGF show large (up to 90°) deviations from the dominant polarization direction. This anomalous pattern suggests that besides the extensive dilatancy anisotropy, the fast wave polarization direction is also likely affected by the presence of over-pressurized geothermal fluids, by local rearrangement of the regional stress, and by the presence of non-vertical cracks. We found large differences in normalised delay times between sparse and clustered seismicity. While the average anisotropy percentage is on the order of 1.7%, a significant amount of our measurements exceeds the 4.5%, reaching values as high as 16%. The highest anisotropy percentages are associated with earthquakes located at the center and at the SE margin of the geothermal area, at depths lower than 5 km and in the 5–10 km range, respectively. This latter occurrence may be interpreted in terms of cracks filled with fluids which, given the expected pressure and temperature conditions, are likely in supercritical conditions. Shear-wave splitting thus confirms to be a powerful tool for better constraining location and extent of those deep fractured rock portions possibly hosting supercritical fluids, that represent the next frontier of geothermal exploitation due to their enhanced heat capacity.

Comprehensive crustal structure and seismological evidence for lower crustal flow in the southeastern margin of Tibet revealed by receiver functions

The lower crustal flow model has become an interesting proposal to describe the growth and expansion of eastern Tibet. To date, however, the geophysical results are not definitive due to the non-uniqueness of the solutions found through data inversion and the resolution limitations. In this study the polarity of P-to-S converted phases at intra-crustal interfaces are analyzed in order to reveal evidence for low seismic velocity and lower-crustal flow in southeastern Tibet. We are interested in a joint study that is both a mapping of the crustal structure (Moho topography and Poisson's ratio) and an attempt to constrain an ICLVZ. We have determined the crustal thickness and Poisson's ratio from P (PRFs) and S (SRFs) receiver functions acquired in 108 permanent seismic stations deployed in the Sichuan and Yunnan areas. Also, we have obtained 98 measurements of Pms-phase splitting parameters at the array stations. The crustal thickness provided by PRFs is consistent with that given by SRFs. It varies from ~60km in the Songpan-Ganzi (SG) fold system and the northern part of the Sichuan-Yunnan diamond-shaped block (SYDSB), near the Eastern Himalayan Syntaxis, to ~33km in southern Yunnan. The Poisson's ratio varies mostly from 0.24 to 0.32; the highest values 0.28–0.32 are found along the axis formed by the Longmenshan fault, the Lijiang-Jinhe fault and in the Tengchong volcano area. The value of ~0.28 observed in SYDSB and the SG fold system is attributed to the presence of a low-velocity zone in the lower crust, and partly to the great sediment thickness in Sichuan Basin. The crust is clearly anisotropic in the southeastern margin of Tibet: the fast-wave polarization direction exhibits a NW-SE regional bias, while the time delay varies between 0.07s and 1.27s, being the average 0.54s±0.12s, implying the existence a strong shear zone in the lower crust. Based on the negative polarity observed after the direct P-wave in teleseismic receiver functions, we have detected an intra-crustal low-velocity zone (ICLVZ) in a wide region including east and southeast of Tibet and southern Yunnan, so both the shear zone and ICLVZ suggest the existence of lower crustal flow. The reverberation PpPs and PsPs+PpSs phases at the Moho are clearly weaker in SYDSB and the SG fold system than in the Indochina Block and eastern Yunnan, implying that SYDSB has a structure and tectonic history different from Indochina on its west side and eastern Yunnan on its east side. The above results provide seismological evidence for a lower crustal flow spreading from the eastern Tibet and that in its advance southward has invaded SYDSB widely, crossed the Lijiang-Jinhe fault and reached southern Yunnan.

Vertical coherence of deformation in lithosphere in the NE margin of the Tibetan plateau using GPS and shear-wave splitting data

We present 817 new SKS/SKKS shear-wave splitting observations to infer the mantle deformation field in the NE margin of the Tibetan plateau. The obvious pattern of dense splitting measurements shows that the fast wave polarization directions (φ) are oriented along a dominant NW-SE in the northeastern Tibetan plateau, Alxa block, and the western margin of the Ordos block and rotate to near E-W in the Qinling orogen, Weihe graben and Ordos block. Based on a continuous surface deformation field inferred from GPS observations and faults slip data, we model the predicted fast axis orientations (φc) at each location of averaged splitting observation. Exclusion of 32 averaged observations with larger misfits within the interior of the Ordos block, comparing splitting observations (φ) with predictions (φc) produces an average misfit of 11° indicating vertical coherent of lithospheric deformation in the NE margin of the Tibetan plateau. However, in the Qinling orogen, the thin lithosphere (<100km) could not account for the observed delay times (δt, ~1.2s), and the measured fast directions are parallel to the absolute plate motion (APM) direction, we infer that both the eastward asthenospheric flow and lithospheric left-lateral shear deformation contribute to the observed anisotropy. Within the interior of the Ordos block, where has thick lithospheric root (>300km) and weak anisotropy (δt, ~0.6s), the average misfit increases to 63°, and vertical coherence is no longer present, which suggests the rigid and stable Ordos block with little deformation still remains the lithospheric root of North China Craton.

Lattice-Preferred orientations of olivine in subducting oceanic lithosphere derived from the observed seismic anisotropies in double seismic zones

Subduction zones can generally be classified into Mariana type and Chilean type depending on plate ages, plate thicknesses, subduction angles, back-arc deformation patterns, etc. The double seismic zones (DSZs) in subduction zones are mainly divided into type I and type II which, respectively, correspond to the Mariana type and Chilean type in most cases. Seismic anisotropy is an important parameter characterizing the geophysical features of the lithosphere, including the subduction zones, and can be described by the two parameters of delay time δt and fast wave polarization direction ϕ. We totally collected 524 seismic anisotropy data records from 24 DSZs and analyzed the statistical correlations between seismic anisotropy and the related physical parameters of DSZs. Our statistical analysis demonstrated that the fast wave polarization directions are parallel to the trench strike with no more than 30° for most type I DSZs, while being nearly perpendicular to the trench strike for type II DSZs. We also calculated roughly linear correlations that the delay time δt increases with dip angles but decreases with subduction rates. A linear equation was summarized to describe the strong correlation between DSZ’s subduction angle α DSZ and seismic anisotropy in subduction zones. These results suggest that the anisotropic structure of the subducting lithosphere can be described as a possible equivalent crystal similar to the olivine crystal with three mutually orthogonal polarization axes, of which the longest and the second axes are nearly along the trench-perpendicular and trench-parallel directions, respectively.

Upper mantle anisotropy and crust-mantle deformation pattern beneath the Chinese mainland

Over the past 10 years, the number of broadband seismic stations in China has increased significantly. The broadband seismic records contain information about shear-wave splitting which plays an important role in revealing the upper mantle anisotropy in the Chinese mainland. Based on teleseismic SKS and SKKS phases recorded in the seismic stations, we used the analytical method of minimum transverse energy to determine the fast wave polarization direction and delay time of shear-wave splitting. We also collected results of shear-wave splitting in China and the surrounding regions from previously published papers. From the combined dataset we formed a shear-wave splitting dataset containing 1020 parameter pairs. These splitting parameters reveal the complexity of the upper mantle anisotropy image. Our statistical analysis indicates stronger upper mantle anisotropy in the Chinese mainland, with an average shear-wave time delay of 0.95 s; the anisotropy in the western region is slightly larger (1.01 s) than in the eastern region (0.92 s). On a larger scale, the SKS splitting and surface deformation data in the Tibetan Plateau and the Tianshan region jointly support the lithospheric deformation mode, i.e. the crust-lithospheric mantle coherent deformation. In eastern China, the average fast-wave direction is approximately parallel to the direction of the absolute plate motion; thus, the upper mantle anisotropy can be attributed to the asthenospheric flow. The area from the Ordos block to the Sichuan Basin in central China is the transition zone of deformation modes between the east and the west regions, where the anisotropy images are more complicated, exhibiting “fossil” anisotropy and/or two-layer anisotropy. The collision between the Indian Plate and the Eurasian Plate is the main factor of upper mantle anisotropy in the western region of the Chinese mainland, while the upper mantle anisotropy in the eastern region is related to the subduction of the Pacific Plate and the Philippine Sea Plate beneath the Eurasian Plate.

Seismic anisotropy of upper mantle in eastern China

Based on the polarization analysis of teleseismic SKS waveform data recorded at 65 seismic stations which respectively involved in the permanent and temporary broadband seismograph networks deployed in eastern China, the SKS fast-wave direction and the delay time between the fast and slow shear waves at each station were determined by use of SC method and the stacking analysis method, and then the image of upper mantle anisotropy in eastern China was acquired. In the study region, from south to north, the fast-wave polarization directions are basically EW in South China, gradually clockwise rotate to NWW-SEE in North China, then to NW-SE in Northeast China. The delay time falls into the interval [0.41 s, 1.52 s]. Anisotropic characteristics in eastern China indicate that the upper mantle anisotropy is possibly caused by both the collision between the Indian and Eurasian Plates and the subduction from the Pacific and Philippine Sea Plates to the Eurasian Plate. The collision between two plates made the crust of western China thickening and uplifting and the material eastwards extruding, and then caused the upper mantle flow eastwards and southeastwards. The subduction of Pacific Plate and Philippine Sea Plate has resulted in the lithosphere and the asthenosphere deformation in eastern China, and made the alignment of upper mantle peridotite lattice parallel to the deformation direction. The fast-wave polarization direction is consistent with the direction of lithosphere extension and the GPS velocity direction, implying that the crust-upper mantle deformation is possibly a vertically coherent deformation.

Shear wave splitting in the Isparta angle, southwestern Turkey: Anisotropic complexity in the mantle

This study presents shear wave splitting analysis results observed at ISP (Isparta) broadband station in the Isparta Angle, southwestern Turkey. We selected 21 good quality seismic events out of nearly 357 earthquakes and calculated splitting parameters (polarization direction of fast wave, ϕ and delay time between fast and slow waves, δt) from mainly SKS and a few SKKS phases of the selected 21 seismic events. Then, we compared calculated splitting parameters at ISP station (56° ≤ ϕ ≤ 205°; 0.37 s ≤ δt ≤ 4 s) with those previously calculated at ANTO (Ankara) and ISK (İstanbul) stations (27° ≤ ϕ ≤ 59°; 0.6 s ≤ δt ≤ 2.4 s and 26° ≤ ϕ ≤ 54°; 0.6 s ≤ δt ≤ 1.5 s) which are located at 230 and 379 km away from ISP station in central and northwestern Turkey, respectively. The backazimuthal variations of the splitting parameters at ISP station indicate a different and complex mantle polarization anisotropy for the Isparta Angle in southwestern Turkey compared to those obtained for Ankara and İstanbul stations.

Seismic anisotropy of upper mantle in Sichuan and adjacent regions

Based on the polarization analysis of teleseismic SKS waveform data recorded at 94 broadband seismic stations in Sichuan and adjacent regions, the SKS fast-wave direction and the delay time between the fast and slow shear waves were determined at each station using the grid searching method of minimum transverse energy and the stacking analysis method, and the image of upper mantle anisotropy was acquired. The fast-wave polarization directions are mainly NW-SE in the study area, NWW-SEE to its northeast and NS to its west. The delay time falls into the interval [0.47 s, 1.68 s]. The spatial variation of the fast-wave directions is similar to the variation of GPS velocity directions. The anisotropic image indicates that the regional tectonic stress field has resulted in deformation and flow of upper mantle material, and made the alignment of upper mantle peridotite lattice parallel to the direction of material deformation. The crust-upper mantle deformation in Sichuan and adjacent regions accords with the mode of vertically coherent deformation. In the eastern Tibetan Plateau, the crustal material was extruded to east or southeast due to SE traction force of the upper mantle material. The extrusion might be obstructed by a rigid block under the Sichuan Basin and the crust has been deformed. After a long-term accumulation of tectonic strain energy, the accumulative energy suddenly released in Yingxiu town of the Longmenshan region, and Wenchuan MS8.0 earthquake occurred.

SKS splitting beneath Capital area of China

Based on the polarization analysis of teleseismic SKS waveform data recorded at 49 seismic stations in Capital Area Seismograph Network, the SKS fast-wave direction and the delay time between the fast and slow shear waves at each station were determined by using the grid searching method of minimum transverse energy and the stacking analysis method, and then we acquired the image of upper mantle anisotropy in Capital area. In the study area, the fast-wave polarization direction is basically WNW-ESE, and the delay time falls into the interval from 0.56 s to 1.56 s. The results imply that the upper mantle anisotropy in Capital area is mainly caused by the subduction of the Pacific plate to Eurasian plate. The subduction has resulted in the asthenospheric material deformation in Capital area, and made the alignment of upper mantle peridotite lattice parallel to the deformation direction. And the collision between the Indian and Eurasian plates made the crust of western China thickening and uplifting and material eastwards extruding, and then caused the upper mantle flow eastwards, and made the upper mantle deformation direction parallel to the fast-wave direction. The deformation model of the crust and upper mantle is possibly vertically coherent deformation by comparing the fast-wave polarization direction with the direction of lithospheric extension and the GPS velocity direction.

Depth localized azimuthal anisotropy fromSKSandPreceiver functions: The Tien Shan

SUMMARY Shear wave splitting in the seismic SKS phase provides a unique possibility to judge on deformations at depths inaccessible for direct observations. Fast S wave polarization direction in collisional belts is often parallel to the trend of the belt, although deformations of the mantle lithosphere in low-angle thrusts would lead to the fast polarization direction normal to the trend of the belt. These considerations suggested that the upper mantle in collisional belts is decoupled from the crust. However, SKS technique is notable by a poor depth resolution, and usually it assumes that the fast polarization direction is the same at any depth, which is hard to justify. Here, to investigate depth dependent azimuthal anisotropy in the mantle, we invert jointly P receiver functions and SKS particle motions at a number of seismograph stations. The technique involves azimuthal filtering of the receiver functions and provides a criterion to discriminate between the effects of azimuthal anisotropy and lateral heterogeneity of isotropic medium. A search for the optimum models is conducted with a technique similar to simulated annealing. Testing with synthetics demonstrates that this approach is robust. The results for 10 seismograph stations in the Tien Shan, the world’s most active intracontinental collisional belt in Central Asia, reveal a pronounced change in the patterns of azimuthal anisotropy at a depth around 100 km. In the mantle lithosphere (at depths less than 100 km), anisotropy is relatively weak and fast wave polarization direction varies laterally in a broad range. This layer is not necessarily decoupled from the crust: its anisotropy can be a combined effect of present day thrusting and of deformations of the geologic past. In the lower layer (asthenosphere) the average azimuth of fast wave polarization is close to the trend of the belt, whereas magnitude of S wave anisotropy is stable and large (between 5 and 6 per cent). This anisotropy is a likely result of recent uniaxial shortening at right angle to the trend of the belt. At some stations the data require anisotropy in the crust. There is no evidence for anisotropy at depths exceeding 150‐250 km.

Study on S wave splitting in Dayao earthquake sequence with M=6.2 and M=6.1 in Yunnan in 2003

The polarization direction of fast wave and the delay time between fast and slow wave were measured for two earthquake sequences occurred continuously on 21 July (M=6.2) and 16 October (M=6.1) in Dayao, Yunnan in 2003 using cross-correlation coefficient method, after determining the high-resolution hypocentral locations of the earthquake sequences using the double-difference earthquake location algorithm. The results indicated that ① The phenomena of S wave splitting are obvious in the two earthquake sequences, and the average polarization directions of fast wave in most stations are almost consistent with regional maximum horizontal compressive stress direction except the station Santai. There are bimodal fast directions in the polarization directions at station Santai and the mean polarization direction is N80°E, indicating an inconsistent phenomenon referred to regional maximum horizontal compressive stress direction. ② There is no apparent relation between delay time and focal depth in the sequences, but the polarization direction show different character in different delay time range. ③ The comparison of S wave splitting results in the two earthquake sequences show that the polarization direction in M=6.2 earthquake sequence is more scattered and its average fast direction is 20° larger than that of M=6.1 sequence, and the delay times between two sequences show a little difference. ④ The spatial variation in S wave splitting polarization direction may be due to the stress disturbance imposed by the M=6.2 and the M=6.1 mainshocks on regional background stress field.

A Study on SKS Wave Splitting Beneath the China Mainland and Adjacent Regions

AbstractSKS wave splitting measurement is a powerful tool to characterize mantle deformation and study the dynamics and evolution of continents. We have made measurements of SKS wave splitting beneath the China mainland and adjacent regions. Our goal is to obtain the magnitude and orientation of upper mantle anisotropy and provide constraints for the evolution model of the crust and mantle. We use the technique of Silver and Chan to determine SKS wave splitting parameters at more than 80 three‐component broadband stations in China and neighboring regions, including the azimuths of fast polarization direction (ϕ) and delay times of split shear wave (δt). The fast wave polarization directions at most stations share a common preferred orientation in a same tectonic block. The fast axes show good correlation with the past and present‐day tectonic movement. Delay times range from 0.4s to 2.4s with an average about 1.2s. According to the splitting parameters of SKS wave, anisotropic characteristics in the study region are analyzed to investigate the dynamic process in the Earth.