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On 21 May 2021 a magnitude Mw 6.1 earthquake occurred in Yangbi region, Yunan, China, which was widely felt and caused heavy casualties. Imaging of the source region was conducted using our improved double-difference tomography method on the huge data set recorded by 107 temporary stations of ChinArray-I and 62 permanent stations. Pronounced structural heterogeneities across the rupture source region are discovered and locations of the hypocenters of the Yangbi earthquake sequence are significantly improved as the output of the inversion. The relocated Yangbi earthquake sequence is distributed at an unmapped fault that is almost parallel and adjacent (∼15 km distance) to the Tongdian–Weishan fault (TWF) at the northern end of the Red River fault zone. Our high-resolution 3D velocity models show significant high-velocity and low-VP/VS ratios in the upper crust of the rupture zone, suggesting the existence of an asperity for the event. More importantly, low-VS and high-VP/VS anomalies below 10 km depth are imaged underlying the source region, indicating the existence of fluids and potential melts at those depths. Upward migration of the fluids and potential melts into the rupture zone could have weakened the locked asperity and triggered the occurrence of the Yangbi earthquake. The triggering effect by upflow fluids could explain why the Yangbi earthquake did not occur at the adjacent TWF where a high-stress accumulation was expected. We speculate that the fluids and potential melts in the mid-to-lower crust might have originated either from crustal channel flow from the southeast Tibet or from local upwelling related to subduction of the Indian slab to the west.

How the crust in the core of the Eastern Himalayan Syntaxis (EHS) deforms responding to the India-Asia collision remains ambiguous. Here we present the first high-resolution receiver functions image of crustal structure along a new NW-SE trending dense nodal array crossing the core of the EHS. Two sets of low velocity zones (LVZs) are clearly observed: one with a flat style beneath the western Lhasa terrane and Higher Himalaya at 18–20 km depth and the other with two west-dipping shapes below the western Yarlung-Zangbo suture within 10–30 km depth. These LVZs caused by partial melting and aqueous fluids are disconnected, impeding the formation of crustal flow. A discontinuous east-dipping intra-crustal discontinuity and a sharp Moho offset of 7 km under the Aniqiao-Motuo shear zone are identified, suggesting that the underthrusting of the Indian lower crust and pure shear mechanisms jointly dominate crustal deformation in the core of the EHS.

Various studies report on temporal changes of seismic velocities in the crust and attempt to relate the observations to changes of stress and material properties around faults. Although there are growing numbers of observations on coseismic velocity reductions, generally there is a lack of detailed observations of the healing phases. Here we report on a pronounced coseismic reduction of velocities around two locked sections (asperities) of the Longmenshan fault with a large slip during the 2008 Mw 7.9 Wenchuan earthquake and subsequent healing of the velocities. The healing phase accelerated significantly at the southern asperity right after the nearby 2013 Mw 6.6 Lushan earthquake. The results were obtained by joint inversions of travel time data at four different periods across the Wenchuan and Lushan earthquakes. The rapid acceleration of healing in response to the Lushan earthquake provides unique evidence for the high sensitivity of seismic velocities to stress changes. We suggest that stress redistribution plays an important role in rebuilding fault strength. After a fault ruptures, the recovery of its strength may be accelerated by earthquakes that redistribute stress, according to an analysis of temporal variation in crustal seismic velocity on the Longmenshan fault in China.

We present detailed lithospheric images of the NE Tibetan Plateau by applying the depthmigration technique toSreceiver functions derived from 113 broadband stations. Our migrated imagesindicate that the lithosphere‐asthenosphere boundary (LAB) lies at depths of 105–120 km beneath the Qilianterrane and reaches depths of 126–140 km below the Alxa and Ordos blocks. The most prominent variationin the LAB depth is the presence of LAB steps of no less than 20 km in the transition zone between theactive NE Tibetan Plateau and the surrounding cratonic Alxa and Ordos blocks, which conflicts with themodel of southward subduction of the Alxa and Ordos blocks. Furthermore, the marked LAB steps occur at130 ± 10 km away from the southern surficial boundary faults between the NE Tibetan Plateau and thesurrounding tectonic provinces, corresponding to the North Qilian fault and the Liupanshan fault,respectively. Therefore, we propose that these scenarios of LAB can be attributed to the delamination offragmented mantle lithosphere in the transition zone between the NE Tibetan Plateau and the surroundingAlxa and Ordos blocks, triggered by lateral asthenosphericflow. In addition, our observations of a thinlithosphere with thickness of 107–115 km beneath the Songpan‐Ganzi terrane and the west Qinlin orogengreatly facilitate the process of underlying lateral asthenosphericflow. The isostatic uplift of the plateaucaused by the delamination of fragmented mantle lithosphere, together with increased horizontalcompressive stress, may have led to the outward growth of the NE Tibetan Plateau.

Detailed crustal structure of large earthquake source regions is of great significance for understanding the earthquake generation mechanism. Numerous large earthquakes have occurred in the NE Tibetan Plateau, including the 1920 Haiyuan M8.5 and 1927 Gulang M8 earthquakes. In this paper, we obtained a high-resolution three-dimensional crustal velocity model around the source regions of these two large earthquakes using an improved double-difference seismic tomography method. High-velocity anomalies encompassing the seismogenic faults are observed to extend to depths of 15 km, suggesting the asperity (high-velocity area) plays an important role in the preparation process of large earthquakes. Asperities are strong in mechanical strength and could accumulate tectonic stress more easily in long frictional locking periods, large earthquakes are therefore prone to generate in these areas. If the close relationship between the aperity and high-velocity bodies is valid for most of the large earthquakes, it can be used to predict potential large earthquakes and estimate the seismogenic capability of faults in light of structure studies.

To explore the west‐east variations in the underthrusting Indian lithosphere, we investigate the lithospheric structure using P and S receiver functions obtained from a west‐east linear array of 19 broadband stations spanning the Tethyan Himalaya at 85.5–88.5°E. Our observations of the Main Himalayan Thrust (MHT) and Moho serving as the top and bottom boundaries at depths of 42–54 km and 60–72 km, respectively, suggest that a thinning Indian lower crust with an eastward inclined geometry is underthrusting beneath the Tethyan Himalaya. An intra‐crustal low velocity zone (LVZ) immediately above the MTH at depths of 32–42 km is imaged, likely resulting from the presence of crustal dehydration partial melting. We propose that this LVZ induces the southward extrusion of melt‐weakened crustal material with itself as a channel and facilitates the development of duplexing structures, by which Indian lower crustal materials could be accreted into the Himalayan wedge along the north‐south mid‐crustal ramp in the MHT. The eastward dipping lithosphere‐asthenosphere boundary is clearly observed ranging from 90 to 126 km, which generally coincides with the changing trends of the MHT and Moho. These observations can be attributed to the eastward steepening subduction angle of the Indian lithosphere.

In recent years, many small- to moderate-size earthquakes occurred in central northern Oklahoma, likely associated with wastewater injection. The most recent one is the M5.8 Pawnee earthquake occurred on 3 September 2016. It is still not clear what controls the locations of these injection-induced earthquakes. Here we conduct 2-D Pg wave tomography with anisotropy to image seismogenic structures in this region, using more than 10 years of Pg arrivals recorded by 81 seismic stations. Our high-resolution Pg wave tomography shows two high-velocity zones with northwest fast direction alternated by low-velocity zones with northeast fast direction. The two dominant anisotropy directions are consistent with conjugated microfractures from surface faults and two nodal planes from focal mechanisms of moderate-size earthquakes. Most moderate-size (M > 4) earthquakes occurred either close to the boundaries between high- and low-seismic velocity zones or within the high-velocity zones, suggesting that they are associated with geological boundaries of different basement rock properties or with strong material properties in the upper crust. We suggest that although these moderate-size earthquakes were induced by fluid injection, their spatial locations were likely controlled by local geologic structures.

We analyze the teleseismic waveform data recorded by 42 temporary stations from the Y2 and ANTILOPE-1 arrays using the P and S receiver function techniques to investigate the lithospheric structure beneath western Tibet. The Moho is reliably identified as a prominent feature at depths of 55-82 km in the stacked traces and in depth migrated images. It has a concave shape and reaches the deepest location at about 80 km north of the Indus-Yarlung suture (IYS). An intra-crustal discontinuity is observed at ~55 km depth below the southern Lhasa terrane, which could represent the upper border of the eclogitized underthrusting Indian lower crust. Underthrusting of the Indian crust has been widely observed beneath the Lhasa terrane and correlates well with the Bouguer gravity low, suggesting that the gravity anomalies in the Lhasa terrane are induced by topography of the Moho. At ~ 20 km depth, a mid-crustal low velocity zone (LVZ) is observed beneath the Tethyan Himalaya and southern Lhasa terrane, suggesting a layer of partial melts that decouples the thrust/fold deformation of the upper crust from the shortening and underthrusting in the lower crust. The Sp conversions at the lithosphere-asthenosphere boundary (LAB) can be recognized at depths of 130-200 km, showing that the Indian lithospheric mantle is underthrusting with a ramp-flat shape beneath southern Tibet and probably is detached from the lower crust immediately under the IYS. Our observations reconstruct the configuration of the underthrusting Indian lithosphere and indicate significant along strike variations.

High-resolution 3-D P and S wave velocity (Vp and Vs) models of the crust and uppermost mantle beneath the northeastern (NE) Tibetan Plateau and the surrounding areas were obtained by applying an improved double-difference seismic tomography method on abundant body wave travel-time data of local and regional earthquakes collected from 1985 to 2018. The improved method not only increases constraints from observed Moho depth information but also incorporates the later-arriving Pg and Sg phases to improve the ray coverage in the middle-lower crust and uppermost mantle. The results show significant low-velocity anomalies in the crust beneath the NE Tibetan Plateau and high-velocity features beneath the surrounding stable Alxa and Ordos blocks. The areas near the boundaries of the cratonic blocks show high velocity in the upper crust and uppermost mantle beneath the Hexi Corridor, and high velocity in the upper crust beneath the Liupan Shan belt. Both of those regions have significant low-velocity anomalies in the middle-lower crust similar to the Tibetan Plateau, which suggests they probably were parts of the Alxa and Ordos blocks respectively, and were cut off from the two cratonic blocks during the lateral expansion of the Tibetan Plateau because of middle-lower crustal ductile deformation and asthenospheric flow. We propose a new tectonic model controlled by middle-lower crustal ductile deformation and asthenospheric flow to illuminate the tectonic evolution and growth mechanism of the NE Tibetan Plateau.

The Mw7.8 Gorkha Earthquake struck Nepal and ruptured the boundary between the Indian and Eurasian plates. We conducted 2D Pg-wave tomography to clarify the seismogenic structure and try to understand causal mechanisms for this large earthquake, using the aftershock data recorded by 15 broadband seismic stations located near the China-Nepal border. Our high-resolution results show that coseismic slip area of the main shock is consistent with the high P-wave velocity anomaly, and the region of maximum slip has a larger area with higher velocity than the region of initial slip, possibly resulting in the dominant low-frequency radiation of energy observed after the dominant high-frequency radiation of energy in the source rupture process. The boundary between these regions of contrasting high and low seismic velocity anomalies suggests a potential crustal tearing at the southern end of the Tangra Yum Co Rift, possibly resulting from different thrust speeds in the Greater Himalaya.

In this study, we analyze available weak‐ and strong‐motion seismograms to discern the main source characteristics of the 12 August 2015 Tianjin explosions. The disaster involved two large explosions, which can be clearly identified in the near‐field seismic recordings. The first explosion (event 1) took place at 15:34:04.68 (UTC) and the second at 15:34:36.94 (UTC). The average time interval between the two events was about 32.3 s, which is consistent with the result determined by cross correlation of the whispering gallery wave ( WG phase). The two events did not occur at the same site, and the second explosion was located 65±10 m northwest (N352°W) of the first one. The local magnitudes M L of the two explosions were further measured as 2.4 and 2.9, respectively. The Fourier spectra from the two closest stations, DAG and BET, show that the predominant frequency of the waveforms ranged from 0.7 to 0.9 Hz after the long‐period wavetrains had arrived. According to the distribution of the three‐component peak ground acceleration, the area with a shaking intensity of modified Mercalli intensity VI spanned up to 131 km2. However, the peak intensity of this explosion did not reach an intensity of VII, based on the ground‐motion attenuation relation.

We have conducted a systematic shear wave splitting analysis using 1000 selected aftershocks with M > 2 from the 2013 Ms 7.0 Lushan earthquake along the Longmenshan fault system in southwest China. Polarization directions of fast shear waves show a bimodal distribution with one dominant direction approximately parallel to the fault strike and the other close to the regional maximum horizontal compressive stress direction. This indicates that in this area mechanisms causing crustal seismic anisotropy are both stress induced and fault zone structure controlled. Delay times between fast and slow shear waves do not show a clear trend of increase for deeper events, suggesting the anisotropic zone is mostly above the aftershocks, which are generally located below 8 km. We further applied a shear wave splitting tomography method to measured delay times to characterize the spatial distribution of seismic anisotropy. The three-dimensional anisotropic percentage model shows strong anisotropy above 8 km but low anisotropy below it. The mainshock slip zone and its aftershocks are associated with very low or negligible anisotropy and high velocity, indicating that the zones with high anisotropy and low velocity above 8 km are mechanically weak and it is difficult for stress to accumulate there. The main and back reverse fault zones are associated with high anisotropic anomalies above ∼8 km, likely caused by shear fabric or microfractures aligned parallel to the fault zone.

Receiver function imaging along a temporary seismic array (ANTILOPE-2) reveals detailed information of the underthrusting of the Indian crust in southern Tibet. The Moho dips northward from ~ 50 km to 80 km beneath the Himalaya terrane, and locally reaches ~ 85 km beneath the Indus–Yalung suture. It remains at ~ 80 km depth across the Lhasa terrane, and shallows to ~ 70 km depth under the Qiangtang terrane. An intra-crustal interface at ~ 60 km beneath the Lhasa terrane can be clearly followed southward through the Main Himalaya Thrust and connects the Main Boundary Thrust at the surface, which represents the border of the Indian crust that is underthrusting until south of the Bangong–Nujiang Suture. A mid-crustal low velocity zone is observed at depths of 14–30 km beneath the Lhasa and Himalaya terranes probably formed by partial melt and/or aqueous fluids.