Earthquake relocation and focal mechanism inversion can provide seismogenic structure information, especially in the source area without obvious fault trace on the surface, and further reveal the deep geometry of hidden faults. The Yangbi M_S6.4 earthquake sequence recorded by Yunnan regional seismic network from May 18 to June 4, 2021 is relocated by using the double-difference location method. A total of 3 233 events, from 4 days before and 14 days after the main shock, are relocated and the b-value in the Yangbi source region is calculated accordinly. Then, using the waveform data recorded by the Yunnan and Sichuan regional broadband seismic stations, the full moment tensor solutions of 10 earthquakes (M≥4.0), including the main earthquake, are obtained using the near-field full waveform inversion method, and further the tectonic stress field is retrieved. The high-precision relocation of earthquakes shows that there are significant differences between the foreshocks and the aftershocks in the tempo-spatial distribution. The foreshocks are primarily in a belt-like distribution along the NW-SE direction, whose epicenters are in a back-and-forth migration. The aftershocks mainly occurred on asymmetric conjugate faults along NW and NE directions, and multi-groups of aftershocks with different strikes were distributed in the south end of the NW-striking seismic zone, implying the complexity of the medium and fault geometry in the focal area. The temporal distribution of the b-value shows that the b-value has a rising trend before the main earthquake, indicating that the stress accumulation in the source area had begun to release gradually at that time, which may be related to the fact that the sequence is of the foreshock-mainshock-aftershock type. After the main shock, the variation range of b-value is large, which may reflect very strong seismicity of the aftershocks and large release of the stress. The focal mechanism solutions show that the moderate earthquakes are mainly of strike-slip with a normal component and a significant non-double-couple component, which may indicate the staggered distribution of the NW- and NE-trending faults in the source region, and the earthquake rupture is not simply the slip along the fault plane. Taking into account for the above-mentioned results as well as the compressional stress field environment in nearly NS direction and the extensional environment in nearly EW direction, the seismogenic structure of Yangbi M_S6.4 earthquake is a dextral strike-slip fault, NW striking with a high-dip angle, located in the Baoshan block, which may be a secondary fault parallel to the Weixi-Qiaohou-Weishan Fault and including multi-fault branches in NE direction in the southern segment. The tempo-spatial distribution characteristics of the earthquake sequence and the diversity of the fault plane rupture are controlled by the geometric complexity of fault system in the focal area.

A strong earthquake with magnitude M_W=7.1 occurred in the area of Molucca Sea, Indonesia on November 14, 2019(Coordinated Universal Time, UTC), and then generated a small-scale local tsunami. In order to better understand the earthquake source characteristics and seismogenic structure, as well as to assess the hazard of tsunami caused by earthquake, this paper mainly focuses on the regional tectonic background, the focal mechanism, and tsunami numerical simulation for the Molucca Sea M_W7.1 earthquake. The broadband seismic waveforms from IRIS Data Management Center are used to estimate the moment tensor solution of this earthquake by W phase method. The result shows that the Molucca Sea earthquake occurred at a shallow depth on a high dip-angle, right-lateral reverse fault, the aftershocks were distributed along the SSW-NNE direction and concentrated near the main shock. These results indicate the Molucca Sea earthquake with characteristic of compressional rupture occurred in the complex plate boundary region of eastern Indonesia, which is dominated mostly by the collision interaction of the Halmahera slab and the Sangihe slab in the east and west sides of Molucca Sea under control of current regional stress field. The coseismic displacements of Molucca Sea M_W7.1 earthquake calculated using Okada's model of rectangular dislocation in a uniform elastic half-space show that the Molucca Sea earthquake generated vertical coseismic deformation with a maximum uplift of 0.15m when the rupture occurred along the high dip-angle reverse fault. The synthetic tsunami waveforms are provided by COMCOT tsunami modelling package solving the nonlinear shallow water wave equations based on the determined fault geometry from W phase inversion. These studies indicate the vertical coseismic deformation resulting in the sudden uplift of water volume above the earthquake source, and finally inducing a small-scale local tsunami. The energy of tsunami mainly propagates to both side of the fault, and part of energy propagates to Sula Islands of Indonesia along the fault dislocation direction; and compared with the first cycle of tsunami records observed by tide gauges deployed along the coastal line of earthquake source region, the observed tsunami head wave fits well with the synthetic wave, both are consistent in amplitude and tsunami arrival time, but the follow-up waveforms are quite different. The numerical simulation of tsunami shows that, in combination with the fault geometry parameters obtained by W phase fast inversion, the tsunami numerical model can be used for tsunami early warning, and it provides sufficient accuracy for forecasting tsunami wave height, thus, having great practical significance for understanding the propagation process and disaster distribution of tsunami.

A M_S6.0 earthquake with shallow focal depth of 16km struck Changning County, Yibin City, Sichuan Province at 22:55: 43(Beijing Time)on 17 June 2019. Although the magnitude of the earthquake is moderate, it caused heavy casualties and property losses to Changning County and its surrounding areas. In the following week, a series of aftershocks with M_S≥4.0 occurred in the epicentral area successively. In order to better understand and analyze the seismotectonic structure and generation mechanism of these earthquakes, in this paper, absolute earthquake location by HYPOINVERSE 2000 method is conducted to relocate the main shock of M_S6.0 in Changning using the seismic phase observation data provided by Sichuan Earthquake Administration, and focal mechanism solutions for Changning M_S6.0 main shock and M_S≥4.0 aftershocks are inferred using the gCAP method with the local and regional broadband station waveforms recorded by the regional seismic networks of Sichuan Province, Yunnan Province, Chongqing Municipality, and Guizhou Province. The absolute relocation results show that the epicenter of the main shock is located at 28.35°N, 104.88°E, and it occurred at an unusual shallow depth about only 6.98km, which could be one of the most significant reasons for the heavier damage in the Changning and adjoining areas. The focal plane solution of the Changning M_S6.0 earthquake indicates that the main shock occurred at a thrust fault with a left-lateral strike-slip component. The full moment tensor solution provided by gCAP shows that it contains a certain percentage of non-double couple components. After the occurrence of the main shock, a series of medium and strong aftershocks with M_S≥4.0 occurred continuously along the northwestern direction, the fault plane solutions for those aftershocks show mostly strike-slip and thrust fault-type. It is found that the mode of focal mechanism has an obvious characteristic of segmentation in space, which reflects the complexity of the dislocation process of the seismogenic fault. It also shows that the Changning earthquake sequences occurred in the shallow part of the upper crust. Combining with the results from the seismic sounding profile in Changning anticline, which is the main structure in the focal area, this study finds that the existence of several steep secondary faults in the core of Changning anticline is an important reason for the diversity of focal mechanism of aftershock sequences. The characteristics of regional stress field is estimated using the STRESSINVERSE method by the information of focal mechanism solutions from our study, and the results show that the Changning area is subject to a NEE oriented maximum principal stress field with a very shallow dipping and near-vertical minimum principal stress, which is not associated with the results derived from other stress indicators. Compared with the direction of the maximum principal compressive stress axis in the whole region, the direction of the stress field in the focal area rotates from the NWW direction to the NEE direction. The Changning M_S6.0 earthquake locates in the area with complex geological structure, where there are a large number of small staggered fault zones with unstable geological structure. Combining with the direction of aftershocks distribution in Changning area, we infer that the Changning M_S6.0 earthquake is generated by rupturing of the pre-existing fault in the Changning anticline under the action of the overall large stress field, and the seismogenic fault is a high dip-angle thrust fault with left-lateral strike-slip component, trending NW.

A magnitude M_W7.0 earthquake struck north of Anchorage, Alaska, USA on 1 December 2018. This earthquake occurred in the Alaska-Aleutian subduction zone, on a fault within the subducting Pacific slab rather than on the shallower boundary between the Pacific and North American plates. In order to better understand the earthquake source characteristics and slip distribution of source rupture process as well as to explore the effect of tectonic environment on dynamic triggering of earthquake, the faulting geometry, slip distribution, seismic moment, source time function are estimated from broadband waveforms downloaded from IRIS Data Management Center. We use the regional broadband waveforms to infer the source parameters with ISOLA package and the teleseismic body wave recorded by stations of the Global Seismic Network is employed to conduct slip distribution inversion with iterative deconvolution method. The focal mechanism solution indicates that the Alaska earthquake occurred as the result of tensile-type normal faulting, the estimated centroid depth from waveform inversion shows that the earthquake occurred at the depth of 56.5km, and the centroid location is 10km far away in northeast direction relative to the location of initial epicenter. We use the aftershock distribution to constrain the fault-plane strike of a normal fault to set up the finite fault model, the finite fault inversion shows that the earthquake slip distribution is concentrated mainly on a rectangular area with 30km×20km, and the maximum slip is up to 3.6m. In addition, the slip distribution shows an asymmetrical distribution and the range of possible rupture direction, the direction of rupture extends to the northeast direction, which is same as that of aftershock distribution for a period of ten days after the mainshock. It is interesting to note that a seismic gap appears in the southwest of the seismogenic fault, we initially determined that the earthquake was a typical normal fault-type earthquake that occurred in the back-arc extensional environment of the subduction collision zone between the Pacific plate and the North American plate, this earthquake was not related to tectonic movement of faults near the Earth's surface. Due to the influence of high temperature and pressure during the subduction of the Pacific plate toward to the north, the subduction angle of the Pacific plate becomes steep, causing consequently the backward bending deformation, thus forming to a tensile environment at the trailing edge of the collision zone and generating the M_W7.0 earthquake in Alaska.

A strong earthquake with magnitude M_S6.2 hit Hutubi, Xinjiang at 13:15:03 on December 8th, 2016(Beijing Time). In order to better understand its mechanism, we performed centroid moment tensor inversion using the broadband waveform data recorded at stations from the Xinjiang regional seismic network by employing gCAP method. The best double couple solution of the M_S6.2 mainshock on December 8th, 2016 estimated from local and near-regional waveforms is strike:271°, dip:64ånd rake:90° for nodal plane I, and strike:91°, dip:26ånd rake:90°for nodal plane Ⅱ; the centroid depth is about 21km and the moment magnitude(M_W)is 5.9. ISO, CLVD and DC, the full moment tensor, of the earthquake accounted for 0.049%, 0.156% and 99.795%, respectively. The share of non-double couple component is merely 0.205%. This indicates that the earthquake is of double-couple fault mode, a typical tectonic earthquake featuring a thrust-type earthquake of squeezing property.The double difference(HypoDD)technique provided good opportunities for a comparative study of spatio-temporal properties and evolution of the aftershock sequences, and the earthquake relocation was done using HypoDD method. 486 aftershocks are relocated accurately and 327 events are obtained, whose residual of the RMS is 0.19, and the standard deviations along the direction of longitude, latitude and depth are 0.57km, 0.6km and 1.07km respectively. The result reveals that the aftershocks sequence is mainly distributed along the southern marginal fault of the Junggar Basin, extending about 35km to the NWW direction as a whole; the focal depths are above 20km for most of earthquakes, while the main shock and the biggest aftershock are deeper than others. The depth profile shows a relatively steep dip angle of the seismogenic fault plane, and the aftershocks dipping northward. Based on the spatial and temporal distribution features of the aftershocks, it is considered that the seismogenic fault plane may be the nodal plane I and the dip angle is about 271°. The structure of the Hutubi earthquake area is extremely complicated. The existing geological structure research results show that the combination zone between the northern Tianshan and the Junggar Basin presents typical intracontinental active tectonic features. There are numerous thrust fold structures, which are characterized by anticlines and reverse faults parallel to the mountains formed during the multi-stage Cenozoic period. The structural deformation shows the deformation characteristics of longitudinal zoning, lateral segmentation and vertical stratification. The ground geological survey and the tectonic interpretation of the seismic data show that the recoil faults are developed near the source area of the Hutubi earthquake, and the recoil faults related to the anticline are all blind thrust faults. The deep reflection seismic profile shows that there are several listric reverse faults dipping southward near the study area, corresponding to the active hidden reverse faults; At the leading edge of the nappe, there are complex fault and fold structures, which, in this area, are the compressional triangular zone, tilted structure and northward bedding backthrust formation. Integrating with geological survey and seismic deep soundings, the seismogenic fault of the M_S6.2 earthquake is classified as a typical blind reverse fault with the opposite direction close to the southern marginal fault of the Junggar Basin, which is caused by the fact that the main fault is reversed by a strong push to the front during the process of thrust slip. Moreover, the Manas earthquake in 1906 also occurred near the southern marginal fault in Junggar, and the seismogenic mechanism was a blind fault. This suggests that there are some hidden thrust fault systems in the piedmont area of the northern Tianshan Mountains. These faults are controlled by active faults in the deep and contain multiple sets of active faults.

In the paper,we relocate the small earthquakes occurring in Linfen region from 1981 to 2010 by using the double-difference earthquake location algorithm. Our result shows that the distribution of relocated earthquakes is characterized by cluster activities,and most of the them occurred at the depths of 5～11km and 15～27km,which suggests that the two main seismogenic layers in this region are located in the upper and mid-upper crust. Earthquake frequency is high along both sides of the Subu Fault zone in the northern Linfen Basin,and most of the earthquakes in the north of Subu Fault zone are deeper than that in the south of Subu Fault. To some extent,this result indicates that the regional tectonic setting plays a major control role in the distribution of earthquakes. Analysis on the distribution of earthquake focal depths reveals that there exist deep faults which cut through the crust into the mantle in the middle of the Linfen Basin and connect with the active faults around the basin in the upper crust. The dislocations along the active faults on both sides of the basin are a reflex on the surface of faulting of the deep faults beneath the central basin.

In this paper,we present a method which allows to calculate the mean stress field according to the total seismic moment released by earthquakes.The exact method is as follows: First,we calculate the scalar seismic moment released by each earthquake according to the statistical relationship between earthquake magnitude and its seismic moment; Second,we calculate the seismic moment tensor released by each earthquake according to the relationship between focal mechanism solution and seismic moment tensor; Then,we can get the total seismic moment tensor released in a specific time period of the study area; Finally,we calculate the eigenvector and eigenvalue of the total seismic moment tensor,the obtained eigenvector is corresponding to the mean stress field direction released by the study area. We tested the method by using the synthetic focal mechanism to which random error was added and with the focal mechanism data of Tangshan aftershock zone.The testing results show that,the released stress field of the study area obtained by our method is in consistency with the regional stress field. So our method can be applied to solve regional stress field.The more focal mechanism data used,the more stable the result would be,and closer to the real regional stress field. One of the advantages of this method is that it uses magnitude as the weight of each earthquake,so the contribution difference of the earthquake size in the stress field inversion can be better reflected. Another advantage is that it does not need to know which nodal plane of the focal mechanism is the real fault plane when we calculate stress field.