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During the early Cenozoic, the collision and convergence between India and Eurasia resulted in the uplift of the Tibetan Plateau and continuous northward compression, forming the Circum‐Tibetan Plateau Basin and Orogen System (CTPBOS). The Tarim Basin, located between the Tibetan Plateau and the Tianshan Mountains, plays a crucial role for studying the convergence‐driving strain propagation mechanism intra‐Asian continent during the growth processes of the Tibetan Plateau. Owing to the lack of accurate geophysical information on the deep structure of the Tarim crust，the mechanism of Cenozoic deformation in the Tarim Basin has been under debate. In this paper, the teleseismic data acquired by the broadband seismic profile across the Tarim Basin from south to north and the S‐wave receiver function method were used to obtain the depth of the Moho and the discontinuities in the lithosphere beneath the Tarim Basin. The SRF result shows that the Moho geometry has an abrupt relief under the Bachu Uplift, and Moho offset under the fault zone between the Kalashayi Fault and the Tumuxiuke Fault. The regional dip of the Moho under the Bachu area can be explained by the root of the Bachu basement‐involved uplift cutting across the whole crust and locally penetrating into the mantle lithosphere. The Bachu Uplift, located in the central Tarim terrane, has a relatively weak lithosphere. In the process of forming the Tarim large igneous province during the early Permian, the crust beneath the Bachu area was weakened and thinned by the thermo‐mechanical erosion from upwelling mantle plume. As the collision and convergence of India and Eurasia since the early Cenozoic, the convergence‐driving strain was propagated into the Tarim Basin. The pre‐existing weak Bachu Uplift was reactivated. The Tarim Basin absorbs Cenozoic compressional deformation through the crustal shortening and Moho offset of the Bachu Uplift. [ABSTRACT FROM AUTHOR]

Based on observations from a dense broadband seismic array located along the northeastern (NE) margin of the Tibetan Plateau in southeastern Gansu Province, we use receiver functions (RFs) to pick the arrival times of P-to-S converted waves and bin the traces in different grids according to the piercing points of the 410 and 660 km discontinuities in the upper mantle. The depths of the two discontinuities are estimated by the ray tracing method with the IASP91 velocity model and a 3-D tomography model. The results indicate the following: (1) The arrival times of the P410s and P660s converted phases are delayed by approximately 1 s than those predicted by the IASP91 model. The mantle transition zone (MTZ) is thicker than that in the global model. (2) The synchronous lags in the P410s and P660s arrival times are consistent with low-velocity anomalies in the upper mantle, which are believed to result mainly from the eastward migration of materials beneath the NE margin of the Tibetan Plateau. (3) Combined with previous tomography results, the depression of the '660' discontinuity and the thickened MTZ are somewhat consistent with the big mantle wedge (BMW) model. However, due to data limitations, more studies are required to explore the BMW in the future. [ABSTRACT FROM AUTHOR]

This paper aims to improve the robustness of interpretation in the S receiver function (SRF), a technique commonly used to retrieve forward scattering of S -to- P converted waves (Sdp) originated from the lithosphere–asthenosphere system (LAS) beneath the stations. Although the SRF does not suffer interferences from backward scattering waves such as the first multiples from the Moho, one major drawback in the method is that Sdp phases can interfere with P coda waves and it is conceivable that these signal-generated noise may be misinterpreted as Sdp phase from the LAS beneath seismic stations. Through systematic analysis of full-waveform synthetics and SRFs from catalogued source parameters, we find that the strong P coda waves before the S wave in the longitudinal-component waveforms result in unwanted signal-generated noise before the S wave in the synthetic SRFs. If the mean amplitude of SRFs after the S wave is large, dubious signal-generated noise before the S arrival are strong as well. In this study, we honor the level of these unwanted signal-generated noise and devise data-oriented screening criteria to minimize the interference between P coda waves and genuine S -to- P converted waves. The first criterion is LQR, a direct measure of the amplitude ratio between longitudinal P coda waves and radial S wave in the waveform data. The second criterion is AMP, the amplitude of SRFs after the S arrival. We illustrate that these criteria effectively measure the energy level of mantle waves such as the SP wave. With synthetics and real data, we demonstrate the effectiveness of LQR and AMP criteria in minimizing these unwanted signal-generated noise in the stacked SRFs down to 1–2 per cent, improving detection threshold and interpretation of Sdp phases from seismic discontinuities in the LAS. [ABSTRACT FROM AUTHOR]

Large earthquakes that occur away from plate boundaries (i.e., large intraplate earthquakes), though rare, could cause heavy damage. Understanding their causes could help with seismic hazard assessment. In this study, we image the seismogenic structure under the 1969 Ms6.4 Yangjiang intraplate earthquake within the stable South China Block with a high‐resolution three‐dimensional shear‐wave velocity model constructed from ambient noise tomography based on a dense nodal seismic array. The model images relatively low velocities at a fault intersection region from the surface to at least 13 km depth in the Yangjiang seismogenic zone. Given spatial links among the intersection zone of faults, surface seawater, and low velocities, we interpret the low velocities to reflect the presence of seawater‐filled, highly fractured rocks created by the intersection of faults. We infer that the seawater infiltration through densely‐fractures could have elevated pore pressure in the long term, as evidenced by low earthquake b values at 8–13 km depths. We hypothesize that this long‐term seawater infiltration could have lowered the stress threshold of earthquake occurrence and contributed to the generation of the 1969 Ms6.4 Yangjiang earthquake. We propose that the large intraplate earthquakes within stable plates tend to occur in regional weakening regions (e.g., fault intersections) favorable to stress buildup. Given sufficient tectonic stress accumulation, the long‐term hydrologically driven crustal stress variations could play an essential role in triggering large intraplate earthquakes. This study could potentially contribute to improving seismic hazard assessment in the Guangdong‐Hong Kong‐Macao‐Greater‐Bay‐Area, China. Plain Language Summary: Large earthquakes usually occur at boundaries between two tectonic plates (i.e., interplate earthquakes). Away from a plate boundary, some large earthquakes, though rare relative to the interplate earthquakes, could occur within the interior of plates (i.e., intraplate earthquakes), such as the most notable Mw > 7.0 New Madrid earthquakes in 1811–1812. Why and how these large intraplate earthquakes occurred remain enigmatic. The Yangjiang region (South China) that hosted the 1969 Ms6.4 Yangjiang intraplate earthquake is one of the best places to study the causes of intraplate earthquakes. This study uses ambient noise tomography to image a low‐seismic‐velocity "chimney" (in the upper crust to 13 km depth) along the seismogenic fault zone. We interpret this low‐velocity "chimney" as a path for seawater infiltration through dense fractures created by the intersection of faults. The fault intersection zone at depths of 8–13 km is a favorable place for stress buildup, as indicated by low earthquake b‐values. We further hypothesize that seawater infiltration in the long term could have lowered effective stress through elevating pore pressure, probably explaining the cause of the Ms6.4 Yangjiang earthquake. We propose that long‐term hydrologically driven crustal stress variations could be essential in triggering large intraplate earthquakes given sufficient tectonic stress accumulation. Key Points: An upper crustal seismic shear‐wave velocity model was constructed under the 1969 Ms6.4 Yangjiang earthquake in South ChinaAmbient noise tomography images low velocities under the 1969 Yangjiang earthquake seismogenic regionWater‐filled dense fractures in fault zones can probably explain the presence of low velocities [ABSTRACT FROM AUTHOR]

Abstract We used S-to-p converted waves from over a thousand seismic stations of the permanent Chinese National Seismic Network to study the large scale structure of the mantle lithosphere beneath the cratons in China. To avoid possible sidelobes of the Moho caused by the deconvolution in the S-receiver function method, we skipped the deconvolution and used the SV onset time as reference time instead of the time of the deconvolution spike. With this new method the lithosphere-asthenosphere boundary (LAB) is observed near 80 km depth below both the North and the South China cratons. However, at the north-eastern margin of the Tibetan Plateau the LAB is observed at about 160 km depth, smoothly shallowing towards the west and abruptly ending at the western end of the North China Craton. This structure is not visible in traditional S-receiver function data because it is overwhelmed by Moho sidelobes. There is no indication of a deeper (near 200 km) cratonic lithosphere-asthenosphere boundary in the other parts of the China cratons as it is observed in other cratons. We hypothesise that at the north-eastern margin of the Tibetan Plateau the original thickening of the lithosphere towards the craton is preserved whereas in most parts of the North and South China Cratons the lower part of the lithosphere is removed by some mechanism. The Moho is well observed. It deepens from 30 to 40 km at the sea shores in the east to 60–70 km below eastern Tibetan Plateau. Highlights • We corrected a problem in the S-receiver function method. • We now observe below the eastern parts of the North and the South China Cratons a lithospheric thickness of about 80 km. • At the western margin of the North China Craton we observe thickening of the lithosphere to about 160 km. [ABSTRACT FROM AUTHOR]

We provide teleseismic constraints on the continental collision zone along the SW-NE profile from the Tibetan Plateau to the Alxa block, traversing Lenglong Ling, a topographic boundary of the northeastern Tibetan Plateau. Our seismic profile crosses the Qilian Shan thrust belt, a zone of intense earthquake activity that includes a pair of M W 5.9 earthquakes (in 1986 and 2016) with a 30-year time interval occurring at the city of Menyuan near Lenglong Ling. A high degree of similarity between the two waveforms and their similar hypocenters indicate that the two events are repeating earthquakes. Both P - and S -wave receiver functions show contrasting lithospheric structure and crustal seismic properties across the repeating earthquake region, suggesting that Lenglong Ling can be the crustal boundary between the Tibetan Plateau and the Alxa block. High-frequency P receiver functions clearly show a SW-dipping underthrusted upper crust beneath the foreland of the Qilian Shan thrust belt. A strong low-velocity feature above the intracrustal underthrust beneath southwest Lenglong Ling indicates a mechanically weak zone developed coeval with the main compressional deformation during the last major tectonic event. Our observed, localized upper-crustal deformation beneath Lenglong Ling can be direct evidence of the lateral growth of the northeastern Tibetan Plateau, and can control the seismicity of the Qilian Shan thrust belt, including the Menyuan repeating earthquakes. [ABSTRACT FROM AUTHOR]

Variations of lithospheric structure across the northeastern Tibetan Plateau and its bounding Asian blocks, the Alxa block to the north and the Ordos block to the east, are crucial for understanding the rise and lateral growth of the Tibetan Plateau. Using waveforms from high-density seismic arrays in northeastern Tibetan Plateau and the surrounding regions, we investigated the lithospheric structure with S- and P-wave receiver functions. The results show strong and relatively simple negative velocity gradients in the depth range of mantle lithosphere (∼70–150 km) under the Ordos and Alxa blocks, similar to those under typical stable continental lithosphere. In contrast, under northeastern Tibetan Plateau including its marginal regions, the velocity gradients are weak and diffusive for the mantle lithosphere, which may be explained by elevated temperature and presence of partial melts. The changes of lithospheric structures are sharp between the Tibetan Plateau and the bounding Ordos and Alxa blocks, suggesting that these two blocks have restricted the lateral growth of the Tibetan Plateau as rigid boundaries. However, across the northeastern corner of the Tibetan Plateau to the Yinchuan rift, the lithospheric mantle structures are similar, suggesting a lateral mantle flow from the Tibetan Plateau to the gap between the Ordos and the Alxa blocks. The crustal structures along this transition show evidence of lateral growth of the Tibetan Plateau. In particular, the edge of thickened crust and evidence of Moho superposition are found between the Haiyuan Fault and the Tianjin-shan Fault, which may have replaced the Haiyuan Fault as the front boundary of the laterally growing Tibetan Plateau in its northeastern corner. [ABSTRACT FROM AUTHOR]

Whether or not the Asian lithosphere has underthrusted beneath the Tibetan Plateau is important for understanding the mechanisms of the plateau's growth. Using data from the permanent seismic stations in northeastern Tibetan Plateau, we studied seismic structures of the lithosphere and upper mantle across the plateau's northeastern margin using P and S receiver functions. The migrated P- and S-receiver function images reveal a thick crust and a diffuse lithosphere–asthenosphere boundary (LAB) beneath the Tibetan Plateau, contrasting sharply with the relatively thin crust and clear, sharp LAB under the bounding Asian blocks. The well-defined LAB under the Asian blocks tilts toward but does not extend significantly under the Tibetan Plateau; this is inconsistent with the model of Asian mantle lithosphere underthrusting beneath the Tibet Plateau. Instead, our results indicate limited, passive deformation of the bounding Asian lithosphere as it encounters the growing Tibetan Plateau. [ABSTRACT FROM AUTHOR]

High-resolution images of the crust were obtained along two high-density temporary seismic arrays that cross tectonic blocks in the southwest boundary of the Ordos block and its adjacent regions. Dense stations with <1-km station spacings allowed teleseismic receiver functions to unveil the unprecedented details of crustal structures from the northeast margin of the Tibetan Plateau and the Qinling orogenic belt to the southwest Ordos block. The observations from nearby broadband seismic stations and two-dimensional numerical simulation were used to verify the reliability and validity of our results from the temporary dense array data. The receiver function profiles and the migration images revealed the contrasting crustal structures beneath the southwest Ordos block and the adjacent regions. Liupanshan mountain is a boundary that resists the lateral growth of the Tibetan Plateau; however, the slight bending of the Moho indicates absence of large-scale deformation in west Liupanshan. The Moho transition zone beneath the Weihe graben and southwest corner of the Ordos block represented by the complicated P-to-S converted phases can be contributed to the eastward extrusion of hot uppermost mantle from the Tibetan Plateau driven by the mantle flow. [Display omitted] • High density seismic array revealed the crustal structures beneath the southwest Ordos block and its adjacent regions. • The results indicate the contrasting crustal structures in the southwest boundary region of Ordos block. • The mantle flow from Tibetan Plateau caused the complicated Mohobeneath Weihe Graben and Southwest corner of Ordos block. • No large-scale deformation beneath the Liupanshan imply the strong-strong convergence of the Longxi basin and Ordos block. [ABSTRACT FROM AUTHOR]

Abstract: Using the teleseismic P- and S-wave receiver functions of the dense linear temporary seismic array, the crust and uppermost mantle structures beneath the deep large faults in the northeastern margin of the Tibetan Plateau were imaged. The images of the first converted wave and the multiples indicated that the North Fault Zone of West Qinling (NWQ) Mountain and Diebu–Lueyang (DBL) faults cut the Mohorovicic (Moho) Discontinuity and cause an obvious difference feature for the Moho in the two sides of the faults. The higher Vp/Vs ratio and lower velocity layer is found beneath the west portion of the array near the Tibetan Plateau, which implies a lower crust channel flow coming from the Tibetan Plateau. The weak Moho and higher Vp/Vs ratio beneath the eastern portion of the array near the Ordos suggest the upwelling of the hot mantle material. The results also indicate an obvious deformation in the upper crust with the lower Vp/Vs ratio beneath the middle of the array. Such upper crust deformation is closely related to the topography of the surface; therefore, we deduce that the deformation of the brittle upper crust is accompanied by the formation of the local topography during the uplift of the Tibetan Plateau, which is also the primary reason for the active seismicity in the study region. The deformation of the lithosphere–asthenosphere boundary (LAB) can also be associated with the formation of the diapir caused by the upwelling hot materials in the upper mantle due to the uprising of the thrusting plate caused by the subduction of the India Plate. The existence of the lower crust channel flow, the crust shortening, and the mantle diapir in the local region simultaneously implies that the elevation and formation of the Tibetan Plateau cannot be explained with a single model. The higher resolution results for the crust and the mantle, especially beneath the block boundary region, are necessary to construct the completed geodynamic model to understand the formation of the Tibetan Plateau. [Copyright &y& Elsevier]

Clear PKIKP precursors were observed from the Lanzhou CTBTO seismic array. We measured their incident angles, arriving azimuths and differential travel times with respect to the PKIKP arrivals using array analysis techniques. These measurements allowed us to locate the scatterers that generated the observed precursors. We found that the scatterers are located in the lowermost mantle beneath eastern Tibet, which is featured by a high-velocity anomaly based on previous tomographic studies. The high velocity anomaly was interpreted as the slab remnants of the ancient Tethys subduction. We thus speculate that the observed scatterers are either related or induced by the subducted slab. [ABSTRACT FROM AUTHOR]

Based on the 3-D Earth model, the common convert points-phase weighted stacks (CCP-PWS) migration method is used to image the upper mantle discontinuities beneath Northeast China (longitude 120°-132°; latitude 38°-40°) with 802 observed receiver functions. Teleseismic records are obtained from 4 stations belonging to CCDSN and 19 stations belonging to PASSCAL. A low-velocity layer has been detected at the depth of 620 km. This low-velocity layer rises to 600 km in the east of the study region close to the subducted slab. We consider that this low-velocity layer might be the accumulated oceanic crustal material delaminated from the western Pacific subducted slab. Additionally, we detect the obvious depression of 660 km discontinuity which was attributed to the interaction between the upper mantle and subducted slab. The maximum depth of 660 km discontinuity approaches 700 km, and 660 km discontinuity splits into multiple discontinuities in the northeast of the study region. [ABSTRACT FROM AUTHOR]

As a traditional method, passive seismic exploration is used to construct the body-wave velocity structure of the upper crust, but it is cost-ineffective and depth-limited when applied to large areas. In this study, we use another more economical method to determine the S-wave velocity (SWV) of the upper crust based on the principle that the amplitude of the direct P-wave on the teleseismic receiver function is sensitive to the upper crust. Using the amplitudes of the massive receiver functions from permanent broadband seismic stations, the SWV structure of the upper crust is obtained in the coastal area of South China (CASC). A pattern of high to low SWVs is exhibited across the study area, with SWVs varying about 2.5–3.7 km/s from west to east. In the profile parallel to the coastline, lateral variations in the SWV correspond to the fault zone, indicating that the cutting depth of most coastal faults is approximately 10 km. Referring to previous studies, we deduce that the low SWV in most sub-areas can be interpreted as the joint effect of the sedimentary layer of the alluvial plain and the accumulation of underground heat flows, in addition to multistage fracturing tectonism. Moreover, the gradual change in the SWV in each profile from the surface to approximately 10 km is correlated with multiple invasions and the coverage of volcanic rocks, to a certain extent. [ABSTRACT FROM AUTHOR]

[Display omitted] • A dense seismic array deployed 18 days is used to image a 3D Vs model. • Three LVZs are revealed in the shallow crust beneath the Shiziyang Waterway area. • Three cross-cutting fault zones in the sedimentary basement are identified. • Two-stage differential fault movements dominated the opening of the Shiziyang Waterway. The Shiziyang Waterway, located in the northern continental margin of the South China Sea, has developed a complex fault system owing to the coupling action of the oceanic crust and continental crust. However, the geological structure in the shallow crust beneath the Shiziyang Waterway and the relationship between faults and waterway development are still unclear, resulting in difficulty in assessing the earthquake risk to the surrounding cities. In this study, based on a high-density short-period seismic array that was recently deployed around the Shiziyang Waterway, a high-resolution shear wave velocity model is inverted to as shallow as 4 km by the ambient noise tomography method. The results showed that at depths ranging from 0.2 to 4 km, three sets of low-velocity zones (LVZs), trending east–west, northeast, and northwest, are revealed, which are speculated to be the Shougouling fault (SGLF), Xinhui-Shiqiao fault (XSF), and Shiziyang fault bundle (SZYFb), respectively. The shear-wave velocity anomalies illustrate that the SGLF stopped extending east at the north endpoint of the Wenchong-Zhujiangkou fault (WZF). The two-stage differential vertical movements of the WZF, Nangang-Humen fault (NHF), and Hualong-Haioudao fault (HHF) dominated the opening of the Shiziyang Waterway. This study provides new seismological evidence of the opening process for the Shiziyang Waterway and reveals that internal fault activity dominated the development and evolution of the Pearl River Delta. [ABSTRACT FROM AUTHOR]

[Display omitted] • Crustal anisotropy is mainly distributed in the middle-lower crust of the South China block. • The dominant fast direction is generally near NW–SE with average delay time ∼ 0.33 s. • The crust-mantle may be coupled deformation. • A few NE–SW directions of anisotropy may be the "fossil" deformation during the Paleozoic. Exploiting the teleseismic waveform data from permanent seismic stations distributed in the South China block, we obtained the crustal anisotropy parameters at 51 stations by fitting the azimuthal variations of arrival times of the P M s converted wave of radial receiver functions. The results show the dominant direction of fast wave polarity in the South China block is NW–SE, which is basically consistent with the absolute plate motion direction. Combining the shear wave splitting and GPS results, we speculated that the tectonic extension of the Paleo-Pacific plate subduction dominates the crust–mantle anisotropy, which reflects the large-scale tectonic stress setting of the South China block. The fast wave directions of NE–SW and E–W calculated by a few stations might represent a local complex tectonic environment, and the crustal anisotropy of NE–SW is well related to the tectonic deformation in Paleozoic. The delay time of slow waves caused by crustal anisotropy ranges from 0.14 s to 0.71 s, with an average of 0.33 s. The delay time of Jiangnan orogenic belt, which located in the southeast of Yangtze block, is particularly large, which reflects the complex and intense intra-continental tectonic movements since Neoproterozoic. The crustal anisotropy along the southeast coast can be contributed to the local lithosphere underplating caused by the upwelling of asthenosphere in Cretaceous. [ABSTRACT FROM AUTHOR]

Xinfengjiang Water Reservoir (XWR), located in the northeast of Guangdong Province, South China, has hosted a large number of earthquakes since its impoundment. In order to understand potential triggering mechanisms of the reservoir‐induced seismicity that could help assess seismic hazard to the Greater Bay Area, the most populated region in China, in this study, we construct a high‐resolution shear‐wave velocity model in shallow crust at depths from surface to ∼10 km based on a high‐density short‐period seismic array recently deployed surrounding the XWR using ambient noise tomography. We image a northwest‐southeast (NW‐SE)‐trending high‐velocity belt at depths of 2–6 km and a relatively low‐velocity channel (LVC) underlying the high‐velocity belt at a depth below 6 km under the XWR. The LVC seems to indicate the existence of a NW‐SE‐trending channel for water infiltration below 6 km under the XWR. Combined with spatiotemporal variation of focal‐depth and frequency of seismicity under the XWR over the past decade, we infer that a NW‐SE‐trending preexisting fault has reactivated due to pore pressure increase via water infiltration from the dam to a recently most active region, Xichang, northwest of XWR. This scenario could possibly explain the occurrence of increasing seismicity in northwest of XWR since 2012. This study highlights the seismic evidence for pore pressure increase due to water infiltration that triggers seismicity and further offers a reference for other forms of fluid‐driven seismicity worldwide. Plain Language Summary: Human activities could induce the occurrence of earthquakes by altering the stresses and strains within the Earth's crust. The reservoir‐induced seismicity can be relatively large compared to other types of human activities. However, an understanding of underlying triggering mechanisms of induced seismicity and associated material properties for individual reservoir remains limited. In this study, we construct a high‐resolution shear‐wave velocity model in shallow crust at depths from surface to ∼10 km based on a high‐density short‐period seismic array with totally 220 seismic stations recently deployed surrounding the Xinfengjiang Water Reservoir, South China, using ambient noise tomography. Our shallow crust image, together with spatiotemporal distribution of seismicity over the past decade, shows a low‐velocity channel at depths of 6–10 km under the reservoir that is interpreted to be caused by pore pressure increase via water infiltration. The pore pressure increase could explain the recently increasing seismicity at northwestern corner of the reservoir. This study highlights seismic evidence for pore pressure increase due to water infiltration that triggers seismicity and further offers a reference for other forms of fluid‐driven seismicity worldwide. Key Points: A high‐resolution Vs model in shallow crust is constructed based on a dense seismic array under the Xinfengjiang Reservoir, South ChinaA northwest‐southeast trending preexisting fault is inferred to reactivate due to reservoir water infiltrationPore pressure increase via water infiltration is suggested to be responsible for recently increasing seismicity in the northwest of the reservoir [ABSTRACT FROM AUTHOR]

A directional borehole radar consists of one transmitting antenna in the borehole and four receiving antennas distributed at equal angles in a ring. The receiving antennas can determine the depth and orientation of targets beside the borehole. However, the problem of target orientation determination and 3D imaging algorithms remains a technological challenge. The MUSIC (multiple signal classification) algorithm requires a peak search, so the accuracy of the operation is limited by the angle interval. Based on the MUSIC algorithm, the Root-MUSIC algorithm is proposed and implemented. By replacing the spectral peak search with calculating the roots of the polynomials greatly improves the orientation recognition accuracy. Finally, the results obtained using the above algorithm are verified with synthetic data and compared with the results of the MUSIC algorithm. The results show that both the MUSIC algorithm and the Root-MUSIC algorithm can achieve very good orientation determination and 3D imaging results. In terms of accuracy, the Root-MUSIC algorithm has an obvious improvement compared with the MUSIC algorithm. [ABSTRACT FROM AUTHOR]

The Shanxi Rift located in the central part of the North China Craton (NCC) as a boundary between the Ordos block and the Huabei basin. The Shanxi graben system is a Cenozoic rift and originated from back-arc spreading related to westward subduction of the western Pacific and far field effects caused by northward subduction of the Indian plate. It has also had strong earthquake activity in China since the Quaternary. To investigate the tectonic evolution and tectonic setting of strong earthquakes in the Shanxi Rift, we apply the receiver function H - κ stacking method to determine the crustal thickness and average Vp/Vs ratio in the area. The results show that the thickness of the crust increases from approximately 30 km in the Huabei basin to approximately 47 km in the Yinshan Mountains with a close correlation between the Moho depth and topography. The Yuncheng, Linfen and Taiyuan grabens have varying degrees of crustal thinning. The crustal average Vp/Vs ratio in the Shanxi Rift has significant heterogeneity; the high Vp/Vs ratio (~ 1.85) are found in the Datong and Yuncheng grabens, and Vp/Vs ratio of the Taiyuan and Linfen grabens is approximately 1.75 which close to the global average value ~ 1.782. Combining the observations in this study with previous research, we suggest that the grabens in the Shanxi Rift experienced extensional deformation from south to north and that the possibility of strong earthquakes in the central part of the Shanxi seismic belt is greater than that on the northern and southern sides. [ABSTRACT FROM AUTHOR]

The directional borehole radar can accurately locate and image the geological target around the borehole, which overcomes the shortcomings that the conventional borehole radar can only detect the depth of the target and the distance from the borehole. The directional borehole radar under consideration consists of a transmitting antenna and four receiving antennas equally distributed on the ring in the borehole. The nonuniformity caused by the borehole and sonde, as well as the mutual coupling among the four receiving antennas, will have a serious impact on the received signal and then cause interference to the azimuth recognition for the targets. In this paper, Finite difference time domain (FDTD), including the subgrid, is applied to study these effects and interferences, and the influence of borehole, sonde, and mutual coupling among the receiving antennas is found. The results show that, without considering the sonde and the fluid in the borehole, the one transmitting and one receiving borehole radar system does not have resonance, but the wave pattern of the reflected wave will have obvious distortion. For the four receiving antennas of the borehole radar system, there is obvious resonance, which is caused by the multiple reflections between the receiving antennas. However, when the fluid in the borehole is water and the relative permittivity of the sonde is low to a certain extent, the resonance disappears; that is, the generation of resonance requires a large relative permittivity material between the receiving antennas. When the influence of the sonde is considered, the resonance disappears because the relative permittivity of the sonde is low, which makes the propagation speed of the electromagnetic wave between the antennas accelerate and lose the conditions for resonance. In addition, the diameters of the sonde and the circular array of the receiving antennas can affect the received signal: the higher the diameter of the sonde and the higher the diameter of the circular array are, the better the differentiation of the received signal. The development of the research provides scientific guidance for the design and application of borehole radar in the future. [ABSTRACT FROM AUTHOR]

Repeating earthquakes are key evidence for understanding earthquake recurrence and help improve long-term seismic risk assessment. In 1986 and 2016, two Ms6.4 earthquakes occurred in Menyuan region, near the middle segment of Qilian-Haiyuan fault zone, NE Tibetan Plateau. Long period waveforms of the two events at teleseismic stations show high degree of similarity, however, previous solutions on earthquake location and focal mechanism show remarkable discrepancies, especially for the 1986 event, as well the ruptured faults also remain ambiguous. In this study, we determined the source parameters of these two earthquakes, including the relative location, focal depth, focal mechanism, source duration and rupture directivity. The results show that two earthquakes are thrust events in shallow crust and are located within 15 km, and the 2016 earthquake ruptured down-dip for ~6 km along the southwest dipping plane. Combing the source parameters, geological data and deep seismic-reflection profiles, we infer that these two earthquakes occurred on the same secondary fault of Lenglong Ling fault (the middle segment of Qilian-Haiyuan fault), but they ruptured on different sections of the fault. Although the two events are probably not repeating earthquakes of Qilian-Haiyuan fault zone, the seismic risk of ~M6 thrust earthquake on these secondary faults and even larger strike-slip earthquake on Lenglong Ling fault are not negligible based on the source parameters and ten-year GPS observations. • Both earthquakes are thrust events in shallow crust and are located within 15 km. • Occurred on the secondary fault (SW-dipping) and ruptured on different sections • Seismic risk of ~M6 thrust earthquake on these secondary faults is not negligible. [ABSTRACT FROM AUTHOR]

A dense linear array of 38 broadband seismograph stations was deployed to traverse the northeastern margin of Tibetan plateau (NE Tibet). Shear wave splitting measurements show significant lateral variations of seismic anisotropy across NE Tibet. Combined with previous tomography studies, the SKS travel-time analysis along the array supports the inference that a cold/rigid Asian lithosphere resides beneath the Qilian and Alxa blocks while a hot/soft Tibetan lithosphere resides beneath the Songpan-Ganzi (SPGZ) and Kunlun-West Qinling (KL-WQL) blocks. The observed variations of anisotropy along the array indicate the important role the major faults have been playing in the process of lithospheric deformation in NE Tibet. The West Qinling fault (WQLF) is reckoned as the boundary between the Tibetan lithosphere and the Asian lithosphere in the study area. The Kunlun fault to West Qinling fault (KLF-to-WQLF) zone may constitute a boundary accommodating the eastward extrusion of the Tibetan lithosphere, with the rigid Asian lithosphere in the north barrier to the northeastward tectonic flow of central-eastern Tibet. A significant character of two-layer anisotropy was identified in the Qilian orogen, which was inferred to be associated with the low velocity layer (LVL) acting as a thrust decollement in the mid-to-lower crust. A thorough analysis involving crustal anisotropy and the regional XKS splitting results in NE Tibet, in association with the crustal LVL feature, indicates that decoupling deformation may dominate the lithosphere beneath the Qilian orogen while coherent deformation may dominate the lithosphere beneath the WQL and SPGZ blocks. Anisotropy beneath the Qilian orogen seems consistent with recent deformation under the boundary forces related to possible lithosphere underthrusting from its northern margin. Our shear wave splitting analysis, combined with published results, reflects a regional anisotropic regime that emphasizes the dominance of eastward extrusion of lithospheric blocks on the present-day deformation in NE Tibet. [ABSTRACT FROM AUTHOR]

Lithospheric deformation of the Tibetan plateau is caused by subduction of the Indian (northward) and Asian (southward) plates. The effects of this interaction on inner reaches of the Asian continent, between the Tibetan plateau and the North China craton (NCC) for example, remain uncertain due to poor geophysical data coverage in northeastern Tibet (NE Tibet). We provide here detailed knowledge of the lithospheric structure beneath NE Tibet as determined from a dense broadband seismic profile traversing NE Tibet to the westernmost NCC. Receiver function imaging reveals several significant features, including a north-dipping intracrustal converter (NC), Moho offset/overlap beneath major fault zones, and a low velocity layer (LVL) in the middle-lower crust. The lithosphere–asthenosphere boundary (LAB) is clearly defined and appears as a south-dipping interface that runs continuously from the Alxa interior to the Qilian orogen. Interpretation of these observations, combined with other seismic evidence, implies that the NCC lithospheric mantle has been persistently underthrust beneath the Qilian orogen. This process forms the thick-skinned crustal accretionary wedges, which develop above a middle-lower intracrustal decollement. Our results provide further deep-geophysical constraints on the Cenozoic post-collisional evolution of the convergent boundary between NE Tibet and the NCC and help clarify the mechanism of plateau growth in this boundary area. [ABSTRACT FROM AUTHOR]

Abstract: The southeastern margin of China is an ideal area to study the modern plate interactions. The previous models, however, are poorly constrained with respect to the geometry due to the sparse coverage. Here we present new observations from a temporary array with 20 stations in southeastern China. We isolated converted seismic phases in the P and S wave coda to generate receiver function from the structure of the crust and upper mantle. By using H–K stacking and common conversion point stacking, the geometry of discontinuities above was imaged along the southeastern Chinese coastline. The CCP section shows Moho dips gently northeastward with a mean depth of 30km. The 410 and 660km discontinuities are at the depth close to IASP91 model. This means that we have not seen any significant anomaly from Moho and mantle transition zone that corresponded with the geodynamics of plate subducting, instead, we found that the crust was ruptured by Min River fault to depth of the Moho. We provide a 2D map of Moho topography by combining our crustal thickness to that from permanent stations, active seismic and OBS offshore profiles. The map is consistent with previous studies and shows a crust that thins coastward and southwestward in the rate of ~1.5km per 100km. We image the lithosphere–asthenosphere boundary at a depth of 60 to 70km by using S-wave to combine with the P-wave. Our crustal structure suggests that Min River fault certainly plays an important role in adjusting regional stress field induced by plate interactions in the study area. Our lithospheric thickness is more consistent with a very strongly attenuated continental lithosphere. Given the close proximity to the coast this might suggest that the lithospheric mantle transitions to oceanic before the crust does. Alternatively, we may infer that the continental lithosphere has been thermally eroded in this region. [Copyright &y& Elsevier]