The 2008 M-w 7.9 Wenchuan earthquake ruptured the middle and northeastern segments of the Longmenshan Fault Zone (LMSFZ), and the 2013 M-w 6.6 Lushan earthquake ruptured a 50-km-long fault in the southwestern segment. Subsequently, an M-w 5.8 earthquake occurred approximately 10 km distant from the M-w 6.6 Lushan earthquake. Therefore, the potential risk for larger earthquakes (>M-w 6.6) on the southwestern section must be considered. This study collects the latest seismological and GPS data to construct an integrated seismotectonic model for the two neighbouring earthquake sequences. The model integrates the fault planes involved, the main shock rupture processes, the main shock-caused Coulomb stress perturbation, the aftershock distribution and the 3-D velocity structure of the source region, providing information for seismic risk evaluation. We find that three fault planes were involved, two of which were related to the main shocks, and the third was generated by the aftershocks following the first main shock. The main shocks were caused by nearly pure thrust faulting on the two planes with dip angles of approximately 45 degrees and almost opposite dipping directions, thereby forming a conjugate angle of around 90 degrees. The third plane was located between the two main shocks, approximately parallel to the second main shock's fault plane. Each of the main shocks primarily ruptured a single asperity, displaying simple time history. The Coulomb stress change of the first main shock facilitated the generation of the second main shock and the third fault plane, and the second main shock increased the stress on the first main shock's fault plane. The aftershocks were distributed within stratified materials by spatially varying interfaces and characterized by high Vp and Vs velocity, and a low Vp/Vs ratio. The atypical dip angles of approximately 45 degrees for thrust faults and the conjugate angle of approximately 90 degrees are indicative of high stress state. The single asperity rupture implies simple stress accumulation. The main shock-caused Coulomb stress change did not reduce the seismic risk in the source region. The varying interfaces are interpreted as a consequence of long-term horizontal compression. All of these characteristics suggest that the two earthquake sequences were generated by the breakage of three immature faults under strong compression by background stress, and the high stress state remains within the southwestern LMSFZ.

The China Seismic Experimental Site (CSES) is located at the intersection of the Tibetan Plateau, South China Block, and Indian Plate and has complex geological settings and intense crustal deformation, making it one of the most seismically active areas in Chinese mainland. A high-resolution, three-dimensional (3D) crust-mantle velocity structure is crucial for understanding seismotectonic environments, lithospheric deformation mechanisms, and deep dynamic processes. We first constructed a high-vertical-resolution 3D initial velocity model using the joint inversion of receiver functions and surface waves and then obtained a 3D P- and S-wave velocity model (CSES-VM1.0) with the highest lateral resolution of 0.25° for the CSES using double-difference tomography. Owing to the limitations of the Sn observation data, the resolution of the S-wave velocity model in the lower crust and upper mantle was reduced, making it closer to the initial model provided by joint inversion. A comparison with explosive-source seismic data showed that the synthetic P-wave first-arrival travel times of the new model were closer to the observations than those of the previous velocity models. The velocity cross-sections across the source areas of the 2022 Lushan MS6.1 and Ludian MS6.8 earthquakes reveal that the former earthquake occurred near a weak contact zone between the Tibetan Plateau and Sichuan Basin, and the rupture of the latter earthquake occurred in a granitic area, with the northern end blocked by rigid high-velocity bodies. A clear high-velocity anomaly zone is distributed along the western margin of the Yangtze Block, revealing the spatial distribution of Neoproterozoic intermediate-basic intrusions. This high-velocity zone significantly controls the morphology of fault zones and influences the rupture processes of major earthquakes. Two northeast-southwest and north-south trending high-velocity anomalies were found near Panzhihua, potentially related to Neoproterozoic and Middle-Late Permian intermediate-basic intrusions. The imaging results revealed the spatial distribution of the Lincang granitoid batholith, the uplifted zone of the central axis fault in the Simao Basin, and the Ailaoshan complex belt in the southwestern CSES, demonstrating a higher spatial resolution compared to previous results. Our velocity model provides an essential foundation for deep structural studies, high-precision earthquake locations, and strong ground motion simulations in the CSES.

The lower reaches of the Jinsha River are rich in hydropower resources because of the high mountains, deep valleys, and swift currents in this area. This region also features complex tectonic structures and frequent earthquakes. After the impoundment of the reservoirs, seismic activity increased significantly. Therefore, it is necessary to study the P-wave velocity structure and earthquake locations in the lower reaches of the Jinsha River and surrounds, thus providing seismological support for subsequent earthquake prevention and disaster reduction work in reservoir areas. In this study, we selected the data of 7,670 seismic events recorded by the seismic networks in Sichuan, Yunnan, and Chongqing and the temporary seismic arrays deployed nearby. We then applied the double-difference tomography method to this data, to obtain the P-wave velocity structure and earthquake locations in the lower reaches of the Jinsha River and surrounds. The results showed that the Jinsha River basin has a complex lateral P-wave velocity structure. Seismic events are mainly distributed in the transition zones between high- and low-velocity anomalies, and seismic events are particularly intense in the Xiluodu and Baihetan reservoir areas. Vertical cross-sections through the Xiangjiaba and Xiluodu reservoir areas revealed an apparent high-velocity anomaly at approximately 6 km depth;this high-velocity anomaly plays a role in stress accumulation, with few earthquakes distributed inside the high-velocity body. After the impoundment of the Baihetan reservoir, the number of earthquakes in the reservoir area increased significantly. The seismic events in the reservoir area north of 27° N were related to the enhanced activity of nearby faults after impoundment; the earthquakes in the reservoir area south of 27° N were probably induced by additional loads (or regional stress changes), and the multiple microseismic events may have been caused by rock rupture near the main faults under high pore pressure.

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.

Based on teleseismic waveforms data of 154 earthquakes recorded by NECESSArray temporary seismic networks since 2009 to 2011, we got three-dimensional P-wave velocity perturbation and azimuthal anisotropy images of the upper mantle in Northeast China by using the travel time tomography method considering anisotropy. The results show that the P-wave velocity perturbation and azimuthal anisotropy beneath Northeast China have obvious lateral heterogeneity. There is a columnar low-velocity anomaly under the Arshan volcano extends down to the mantle transition zone, which may indicate that there is an upwelling magma channel from the deep. Below 410 km, the low-velocity anomalies under the Arshan volcano are connected with the low-velocity anomalies under the Songliao Basin. And the overall distribution of fast velocity direction (FVD) is Northwest, indicating that they may have one same heat source in a deep depth. Beneath Songliao Basin, about 100 km, FVD in the South and central part of the basin is distributed in E-W direction, and in NE-SW direction in the East. It is speculated that FVD may be affected by the E-W trending continents amalgamation zone between North China plate and Songnen block and some NE trending deep fractures. Below 410 km, FVD is mainly distributed in NW direction, similar to SKS results, which may indicate that the source depth of SKS anisotropy is deep, and its formation mechanism is related to the westward subduction of Pacific slab. The distribution of FVD within 180 km below Changbaishan volcano is consistent with the trend of continents amalgamation zone, which indicate the influence of continents amalgamation process on regional structural deformation. The NW direction is found below 300 km, which is inferred to be related to the westward subduction of Pacific slab. Within 520 similar to 660 km, there is a low-velocity anomaly area in the northwest of Changbaishan volcano. However, large azimuth anisotropy amplitude and consistent FVD trend were found in this area, which may indicate that the low-velocity anomaly is not related to the mantle plume from the deep, but related to the deep dehydration of the stagnant Pacific slab.

The Jiaodong Peninsula of eastern China is densely populated and prone to frequent moderate-strong earthquakes that are mostly distributed along a group of northeastern oriented Mesozoic-Cenozoic faults. An improved understanding of the seismic velocity structure in the area is of great significance for the roles that the faults played in the formation and evolution of the various tectonic features and for the assessment of future earthquake risks. By utilizing earthquake records of the Shandong Provincial Seismic Network for the period from January 2013 to January 2020, this study simultaneously re-locates earthquakes and determines the three-dimensional P-wave velocity structure beneath the Jiaodong Peninsula using regional P-wave travel times by applying the double-difference tomography method. After the relocation, the travel time residuals are reduced by an order of magnitude, and the epicenters are more concentrated and demonstrate a closer spatial relationship with known active faults. The resulting P-wave velocity structure at different horizon depths indicates lateral heterogeneities in the study area. Clear differences in the characteristics of velocity anomalies are observed between the Jiaobei Uplift, Jiaolai Basin and Sulu ultra-high pressure metamorphic belt. The anomalies are mostly NE oriented, which is consistent with the strike of the regional faults and may suggest structural control of the faults to the geological configurations. Earthquakes generally occurred along the edges of high or low velocity regions, and areas accommodating the intersections of the Penglai-Weihai, Penglai-Qixia and Mouping-Jimo fault zones have the greatest potential for future damaging earthquakes in the area.

The geological structural characteristics and kinetic mechanism of the eastern Altyn Tagh fault zone have been the focus of geologists for a long time. In recent years, small earthquake data has been applied to many research domains, such as active fault spatial distribution, deep and shallow structural analysis and dynamic mechanism. On the basis of previous studies, this paper collects earthquake data in the study area from 2008 to 2017, with a total 8025 earthquake events, and screens the final eligible earthquake events 6991. Using double differential positioning method repositioned, 6013 earthquake events were obtained. The focal depths of these earthquakes were generally shallow, basically within a depth of 26km, of which the earthquakes with a focal depth between 4km and 14km account for about 86% of the total. The spatial distribution of the faults and the current movement characteristics are delineated by vertical profiles of these small earthquakes. According to their spatial distribution characteristics, the dip angles of the faults of these small earthquakes were disclosed, which decreased gradually from the Altyn Tagh fault zone as represented by the near-vertical dip angle of Yemahe-Daxueshan fault, toward the direction of Qilian Mountain, which provides new evidence for the spatial distribution characteristics model of thrust faults. Based on petroleum seismic profiles, wide angle reflection/refraction profile, deep seismic reflection profile, small earthquake relocation data and research achievements of shallow active tectonics, the three-dimensional tectonic model of the study area is established. The Moho surface beneath the study area gradually deepens from north to south, and there are three faults distributed in a ladder shape. In the crust, there is a low velocity layer 10km thick, above which earthquakes occur. The fault system is generally Y-shaped, whose upper part is a series of imbricate thrust faults causing Qilian Mountain uplift and merge downward into a main fault. Finally, this paper discusses the dynamic mechanism of regional tectonic movements. The Asian plate subducted to the front of Qilian Mountains, and the upper crust was thickened by thrust tectonic model, while the middle and lower crusts thickening caused by flow under dragging of upper mantle. The upper and lower crusts are thickening as a whole.

The 8 August 2017 M-s 7.0 Jiuzhaigou earthquake was the largest event in continental China in the recent 3 yrs. It occurred at the north boundary of the Bayan Har block. The mechanism of the mainshock exhibited strike-slip faulting. Relocation of the aftershock sequence revealed a south-southeast-striking near-vertical plane, which is consistent with the south-southeast nodal plane of the focal mechanism. The earthquake was considered to rupture a branch of the eastern Kunlun fault system. The earthquake sequence was characterized by a relatively lower aftershock productivity and normal decay rate. Static Coulomb stress calculation suggested that the Jiuzhaigou mainshock was encouraged by the 2008 M-s 8.0 Wenchuan earthquake and promoted several faults in this region, such as the middle and western segment of the Tazang fault, the Minjiang fault, and the Xueshanliangzi fault. The maximum intensity was IX on the Chinese intensity scale.

Online Material: Figures of station deployment time and of spatial and temporal depiction of aftershocks, and tables of station information and aftershock catalogs. At 08:02 (local and Beijing time) on 20 April 2013, an earthquake of M s 7.0 ( M w 6.6) struck Lushan County in Sichuan Province, southwestern China (hereafter referred to as the Lushan earthquake). The Lushan earthquake resulted in casualties and severe damage to the buildings and to the economic activities of the region. The earthquake left 193 dead, up to 10,000 injured, and 25 missing. The direct economic loss hit over $1.6 billion U.S. The Lushan earthquake is the second destructive earthquake to have occurred in the southern segment of the Longmenshan fault zone since the 12 May 2008 M s 8.0 Wenchuan earthquake (Fig. 1). The distance between the epicenters of the Lushan earthquake and the Wenchuan earthquake is about 87 km (Fang, Wu, Wang, Lu, et al. , 2013). As happened following the Wenchuan earthquake, the occurrence of the Lushan earthquake also stimulated lots of discussions on the seismic risk potential and prediction in southwest China. In particular, whether it was a large aftershock of the Wenchuan earthquake has been heatedly debated (Chen et al. , 2013; Du et al. , 2013; J. Liu et al. , 2013; Wang et al. , 2013; Jia et al. , 2014). Figure 1. (a) Tectonic settings of the Longmenshan fault zone. Red and yellow stars indicate the epicenters of the Wenchuan and Lushan earthquakes, respectively. The focal mechanisms of the Lushan earthquake and Wenchuan earthquake are also shown. Red circles represent aftershocks of the Lushan and Wenchuan earthquakes. Black lines indicate the main faults. The inset map shows the location of the study region in the Chinese mainland. (b) The distribution of seismic stations in and around the epicentral region of the Lushan …

Using the seismic waveforms recorded by the seismic array deployed in the southern section of the North-South seismic belt, the focal depths of 28 M-L >= 4. 0 aftershocks of the Lushan M(s)7. 0 earthquake are determined from the arrival-time differences between sPn and Pn phase. The Pn phase waves are picked by the waveform cross-correlation method, and the sPn phases are identified by using the sliding-window correlation method. The result shows that the focal depths are 10 similar to 20 km. From the NW-SE profiles perpendicular to the seismic belt, the focal depths became shallower from west to east with a dip about 39 degrees. The aftershocks show a linear characteristic in space, implying that they may be located in or near the faults relative to the main shock, and the fault dip of the main shock may be 39 degrees. According to the spatial distribution of aftershocks, we infer that the seismogenic fault of Lushan earthquake may be the blind thrust fault to the east of the seismic zone rather than the Shuangshi-Dachuan fault.

The mainshock of April 20, 2013 Sichuan Lushan M S7.0 earthquake was relocated using a 3-D velocity model. Double difference algorithm was applied to relocate aftershock sequences of Lushan earthquake. The locations of 2405 aftershocks were determined. The location errors in E-W, N-S and U-D direction were 0.30, 0.29 and 0.59 km on average, respectively. The location of the mainshock is 102.983°E, 30.291°N and the focal depth is 17.6 km. The relocation results show that the aftershocks spread approximately 35 km in length and 16 km in width. The dominant distribution of the focal depth ranges from 10 to 20 km. A few earthquakes occurred in the shallow crust. Focal depth profiles show fault planes dip to the northwest, manifested itself as a listric thrust fault. The dip angle is steep in the shallow crust and gentle in the deep crust. Although the epicenters of aftershocks distributed mainly along both sides of the Shuangshi-Dachuan fault, the seismogenic fault may be a blind thrust fault on the eastern side of the Shuangshi-Dachuan fault. Earthquake relocation results reveal that there is a southeastward tilt aftershock belt intersecting with the seismogenic fault with y-shape. We speculate it is a back thrust fault that often appears in a thrust fault system. Lushan earthquake triggered the seismic activity of the back thrust fault.

Using travel time data from 21 temporary seismic stations and the permanent stations of Qinghai Seismological Network, we obtained precise relocation of earthquake sequence and 3D seismic velocity structure around Yushu focal area by double-difference tomography. To ensure the accuracy of the phases, the seismic events downloaded from the Data Management Center of China National Seismic Network were re-processed. The result of aftershock relocation shows that the earthquake sequence has a banding distribution along NW in both sides of the fault, which reveals horizontal lineations of hypocenters that define the narrow regions on the fault where stress is released by brittle failure. At the northwest end the aftershocks are distributed not only along the Yushu-Garze fault, but also along the direction vertical to the fault. It reveals an intersecting fault. The result of 3D seismic velocity structure showed that shallow velocity structure has good correlation with surface geology. The velocity structure of middle crust shows that Bayan Har block is characterized by high velocity, Qiangtang block is imaged as a low velocity region. The result of earthquake location and seismic velocity structure shows that the high-low velocity anomaly has certain control action to the aftershock distribution. The mainshock occurred in the transition zone between low and high velocity bodies. Most aftershocks appear to be distributed on the periphery of the high-velocity body. There are rarely earthquakes in the high-velocity body. The high-velocity body reflects the more brittle and competent parts of the crust, which are capable of sustaining greater seismogenic energy. The seismogenic energy in the high-velocity body was released into the surroundings after the mainshock, which leads to many aftershocks in the surrounding areas.

After the Yushu M S 7.1 earthquake on April 14, 2010, a large number of aftershocks were recorded by the surrounding permanent network and temporary seismic stations. Due to the distribution of stations, knowledge about velocity structure, the reliability of seismic phases, and so on, the location result from conventional method is usually of low precision, from which it is difficult to recognize the spatial and temporal distribution and the trends of aftershock activity. In this paper, by using teleseismic waveforms recorded by permanent station, the seismic velocity structure beneath the vicinity is obtained from receiver function stacking and inversion methods. And the Yushu earthquake sequences are relocated from seismic phase data by HypoDD. The results show that the Yushu M S 7.1 earthquake occurred at 13 km depth; the aftershock sequences were distributed mainly in the NWW along the Garzê-Yushu fault, and most aftershocks were concentrated in a 100 km length and 5–20 km depth. Combined with the velocity structure, it can be inferred that the earthquake mainly destroys the high-velocity layer of the upper crust. In the west of the seismic fault near (33.3°N, 96.2°E), the aftershock sequences were distributed like a straight column, suggesting there was a comminuted break from 25km depth to the ground.