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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]

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]

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]

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]