Crustal earthquake focal mechanisms are investigated in the southeastern margin of the Tibetan Plateau, where the Tibetan Plateau and stable South China Block merge. An updated database of focal mechanisms has been compiled by selecting 132 Global Centroid Moment Tensor solutions and by adding the 173 new solutions (3.5 ≤ Ms ≤ 7.4) estimated by waveform inversion in this study. A total of 305 mechanisms are included in this database. These solutions show regionally specific distributions with dominant strike-slip faulting and some normal and reverse faulting. Focal mechanism solutions have also been inverted for the stress tensor orientation to obtain the principal stress axes over the study region. Results show that the horizontal maximum principal σ 1 axes rotate clockwise with a wider range than the geodetically measured surface motion in the east, which is not limited to the Xianshuihe–Xiaojiang fault, but has some overlap with the Zhaotong–Lianfeng fault. Localized normal faulting stress regimes are observed in the Jinshajiang–Litang fault areas and the Baoshan sub-block. The minimum principal axes are oriented with a gradually changing trend from north–south to northwest–southeast, from north to south, indicating diverse compression stress patterns. Significant changes in the crustal stress field after the Wenchuan earthquake are preliminarily observed in the Baoshan sub-block where orientations of two principal axes have changed, and in the Jinggu–Ximeng sub-block areas where the strike-slip faulting stress pattern has transformed to normal faulting.

The North-South Seismic Belt is the boundary between the first-order blocks of the tectonic stress field in China. The focal mechanisms of 819 events that occurred in this belt from 1970 to 2014 are used to study its tectonic stress field, which is important for understanding the mechanism of continental earthquakes, tectonic deformation and seismic stress interaction. According to the classification criteria of World Stress Map (WSM), focal mechanism solutions are divided into six types, in which the horizontal projections of P axes mark the movement direction of the block. The stress tensors and the orientations of the maximum horizontal stress of the North-South Seismic Belt were determined by the MATLAB Package for Spatial And Temporal Stress Inversion (MSATSI) using 2 degrees X 2 degrees grids with the damping parameter 1. 6 at the 95% confidence level. The spatial distribution of different types of focal mechanisms is consistent with the nature of tectonic activity in the North-South Seismic Belt. It demonstrates that the regional tectonic stress field is controlled by tectonic movement. The horizontal projections of P axes show the segmentation patterns from north to south: NNE-NE-nearly EW-NW nearly NS-NNE direction, the horizontal projections of T axes exhibit the most prominent characteristic of spatial differences in the Sichuan-Yunnan block and neighboring regions, a clockwise rotational pattern. The spatial distribution of stress regime and the orientation of the maximum horizontal stress has obvious regional characteristics, i. e. NE direction strike-slip type stress state in the northern section, NEE-EW-NWW direction thrusting type in the middle section, SE-SSE-NS-NNE direction strike-slip and normal faulting type in the southern section, and the stress regime in the northern part and west boundary of the Sichuan-Yunnan block is EW-SE-SSE direction normal faulting. The normal fault stress regime in the western boundary of the Sichuan-Yunnan block is consistent with the scope of the low velocity zone in mid-lower crust observed by the high resolution seismic array, the ductile flow of eastern expansion of the Tibetan Plateau and the weakness of lateral boundaries (Lijiang-Xiaojinhe fault) facilitate energy release, limiting earthquake magnitude. It also localizes the transition from normal fault earthquakes with N-S extension to normal faulting with E-W extension. The results suggest that the lithospheric stress field of the North-South Seismic Belt is controlled jointly by the NE or NNE directed horizontal compressive stress from the Indian plate and eastward motion of the material of the Tibetan Plateau that slip towards SE along large strike-slip faults. Stress regime is almost consistent with the rupture mechanism of all types of earthquakes, verifying the reliability of the inversion. It can offer a reference for simulation of geodynamic processes and determination of the nature of fault slip.

Based on the digital waveforms of China National Seismic Network and Sichuan Seismic Network, The Lushan earthquake sequence (M-L >= 2.0) was relocated precisely by HypoDD, 36 best double-couple focal mechanisms (M-L >= 4.0) were determined by CAP method. The geometric fault parameters and slip direction were estimated by using distribution of the Lushan earthquake sequence and regional stress field. We analyzed the characteristics of focal depth, focal mechanisms and source rupture to determine the seismogenic structure. The main shock was relocated at 30.30 degrees N, 102.97 degrees E; the depth of the initial rupture and centriod is about 15 km and 14 km, respectively; the nodal plane parameters of the best double-couple focal mechanism, strike 209 degrees, dip 46 degrees and slip 94 degrees; strike 23 degrees, dip 44 degrees and slip 86 degrees, respectively. The focal mechanism of main shock is pure thrust type, and most of the focal mechanisms of the aftershocks show the same rupture property as the main shock. The aftershocks (M-L >= 2.0) distributed in two sides of the main shock, the length of sequence is about 30 km, the focal depth is centrally distributed in 5 similar to 27 km; the focal depth of the aftershocks (M-L >= 3.5) is centrally distributed in 9 similar to 25 km and show the focal fault dip toward northwest. We determined the focal fault parameter strike 207 degrees, dip 50 degrees and slip 92 degrees using distribution of sequences and regional stress field, most aftershocks distributed in the 10 km vicinity of the focal plane. Combine the distribution of earthquake sequence, the focal depth of the main shock with the result of rupture process, we infer that the main rupture extend along the two sides of the fault from the initial rupture point, the southern rupture is a little longer than the northern part, the slip focus on the vicinity of the initial rupture point and the rupture is not up to the surface. Based on the results of this paper, the distribution of seismic intensity and field investigation, we speculate preliminarily the focal fault is the front fault of Longmenshan, but we do not rule out the possibility that there is an unknown base fault nearby the east of main-shock.

Based on the records of CSN and regional digital seismic networks, we obtained the focal depth and focal mechanism of the M(s)5. 7 and M(s)5. 6 Yiliang earthquakes of Sep. 7,2012 with the "Cut and Paste" (CAP) method, and used the teleseismic depth phases like P, P-p, and P-s from IRIS records to determine the focal depth. Further, we analyzed the rupture characteristics and seismogenic structures of the Yiliang earthquakes combining with the earthquake epicenters, seismic intensity, and geologic settings in the region. Our result shows that the best double couple solution of the M(s)5. 7 event is 243 degrees,62 degrees, and 149 degrees for strike, dip, and rake angles respectively, the other nodal plane is 349 degrees,63 degrees, and 32 degrees; for the M(s)5. 6 event the solution is 241 degrees,37 degrees,162 degrees with the other nodal plane of 346 degrees, 79 degrees and 54 degrees. We infer that both of the M(s)5. 7 and M(s)5. 6 earthquakes occurred on the NE striking Shimen fault and the focal depths of the events are about 6 km, the released seismic energy was concentrated in shallower crust which is an important reason for the serious seismic damage in the region.

Based on digital broad-band seismic records of Qinghai,Xizang,Sichuan regional seismic network and CSN,we inversed the focal mechanisms and possible centroid depth of the M_S4.7 foreshock,M_S7.1 mainshock and M_S6.3 aftershock of the Apr 14,2010 Yushu earthquake sequences using the "Cut and Paste"(CAP) method,and analyzed the seismogenic structures combining with the distribution of active faults near the epicenter and filed investigation of surface rupture zone.The Results show that the earthquake surface rupture of the M_S7.1 mainshock is 129°,84°and 17°for strike,dip and rake angles,respectively,the centroid depth is about 6 km and the moment magnitude is 6.8;the earthquake surface rupture of the M_S4.7 foreshock is 114°,67°and-5°for strike,dip and rake angles,respectively,the centroid depth is about 11 km and the moment magnitude is 4.2;The earthquake rupture surface of the M_S6.3 strong aftershock is 123°,89°and 9°for strike,dip and rake angles,respectively,the centroid depth is about 6 km and the moment magnitude is 5.7.The seismogenic structure of the Yushu earthquake sequences is the Ganzi-Yushu-Fenghuoshan fault;the focal mechanism is mainly left lateral strike slip type.The strike of the source rupture plane is basically consistent with the general strike of surface rupture of the M7.1 mainshock.Comprehensive study on the source character of the foreshock-mainshock-strong aftershock indicates that the dip of the seismogenic structure in shallow is vertical and gradually becomes flat in the deep part

An intensive rainfall occurred from June 27 to June 30 2010 in the northwest part of Guangxi province, large areas became waterlogged. Before that, a serious drought situation continued for several months in this region. Following the large rainfall, an earthquake swarm with thousands events occurred in the boundary area of Lingyun and Fengshan, where maximum precipitation appeared. There are 2739 earthquakes being recorded since June 28 to 15 July, among them, 41 events with magnitude from M(L)2.0 to M(L)2.9 and 3 events with M(L)>= 3.0 (the maximum event is M(L)3.2) occurred on 1 July. There is an obvious spatio-temporal correlation between swarm and large rainfall, the highest seismicity is a little behind the maximum precipitation of the rainfall in time. Shallow karst structure consisting of carbonatite rock materials are prolific in a large area around the epicenter, the epicenter is located at the crossing area of NW faults and NE faults. The fault fracture zone is the preferential channels; for fluid intrusion. Earthquakes concentrate in a cluster and the focal depths are shallow. The size of earthquake distribution (radius) and focal depths are smaller than 1.6 km for 63% earthquakes, 2.5 km for 88% earthquakes and 3.4 km for 98% earthquakes. The seismic phase analysis on relatively large earthquakes shows that the fluid has been involved in the swarm and it also shows some features of karst collapse. Due to the quick migration of fluid along the fault surface, it seems that the small earthquakes are distributed along the fault, but the focal mechanism solution of relatively large earthquakes does not support the guess that the swarm is resulted from the fault movement, in another words, there is no relationship between the swarm activity and the tectonic movement. The U-D first motions of most earthquakes, recorded by the field seismic sensor in the epicenter area, are downward and the ratio of down direction is larger for smaller earthquakes, this means that most of small earthquakes may be produced by karst collapse or crack closure. The results of quantitative detection based on ETAS model show that the fluid triggering action is very strong on the swarm activity. Meanwhile, the earthquake self-generation is also very strong. According to the 1-D diffusion equation, the temporal variation of pore pressure in different depths, caused by the fluid intrusion, has been simulated, it pointed out that the increment of pore pressure caused by the fluid intrusion is the major mechanical reason of the swarm. The seismogenic mechanism of the Lingyun-Fengshan swarm has been proposed in the final.

Dozens of >M5, hundreds of >M4, and much more >M3 aftershocks occurred after the 2008/05/12 Wenchuan earthquake, which were well recorded by permanent and portable seismic stations. After relocated with P arrival, the >M3 aftershocks show two trends of distribution, with most of the aftershocks located along the north-east strike consistent with Longmenshan fault system, yet there is a north-west trend around the epicenter. It seems that substantially more aftershocks occur in regions with crystalline bedrocks. Then we collected waveform data from National Digital Seismograph Network and regional seismograph network of China, and employed “Cut and Paste” method to obtain focal mechanisms and depths of the big aftershocks (M⩾5.6). While most of those aftershocks show thrust mechanism, there are some strike slip earthquakes in the northern-most end of the rupture. Focal mechanisms show that the events located on the southern part of central Beichuan-Yingxiu Fault (BY) are mainly thrust earthquakes, which is consistent with initial mechanism of the main shock rupture. In the north part the aftershocks along the BY are also dominated by thrust slip, which is quite different from the right slip rupture of the main shock. Around Qingchuan-Pingwu Fault, the focal mechanisms are dominated by right-slip rupture with large depths (∼18 km). So we suspected that in the north part the main shock might rupture on two faults: Beichuan Fault and Qingchuan-Pingwu Fault. The complex pattern of aftershock mechanisms argues for presence of a complicated fault system in the Longmenshan area.