In this paper, CAP waveform inversion method and HASH method are used to calculate the new focal mechanism of the ML≥2.5 earthquake at Jinping Hydropower Station and its surrounding areas based on broadband waveforms from the Sichuan Regional Seismic Network between 2013 to 2018, and DRSSI method is applied to inversion of the stress field in the study area. Combined with the region's geological structure and hydrogeological environment background, the focal mechanism and the various characteristics of the stress field in the study area are analyzed, and the influence of reservoir loading and unloading on the seismic activity in the region is further discussed. The results are as follows: (1) The majority of the focal mechanisms represent strike-slip mechanisms, but there are some differences in focal mechanism solutions at different loading and unloading stages. The focal mechanism solutions are diversified at the early stage of reservoir impoundment and tend to be consistent with the region background at the later stage. (2) The direction of the maximum principal stress of the tectonic stress field in the study area is mainly the NNW-SSE direction, which is different from the NW-SW direction of the regional stress field. The variation results of the local stress field at different periods show that the direction of the maximum principal stress of the local stress field gradually shifts from the NNW direction to the NW direction at the later stage of impoundments. (3) Jinping hydropower station reservoir distribution around the marble, metamorphic sand plate has created favorable conditions for reservoir water infiltration, in the process of rapid water, fluid seepage, and reservoir water stress on the surface of the joint action of gravity load causing the fault, the fault enhanced dielectric strength weak, small earthquakes in this area was significantly increased at the same time, it also played a triggering role in moderate earthquakes. It is worth noting that there are certain differences in the factors affecting seismic activity in different stages.

On June 10, 2022, the MS6.0 Ma'erkang earthquake sequence occurred in the Bayan Har block. In this paper, the temporal and spatial distribution and attenuation characteristics of earthquake sequence is analyzed based on the regional structure, the temporal and spatial distribution of earthquake sequence, the focal mechanism solution, and the parameters of earthquake sequence, using the data of Sichuan Seismic Network and the temporary stations incorporated into the network. The results show that: (1) The MS6.0 Ma'erkang earthquake sequence is generally distributed in NW-SE direction, and the long axis of this series is basically consistent with the nearby Songgang fault. (2) As the earthquake sequence is formed by three large events with MS ≥5.0, the frequency of small earthquakes in the earthquake sequence area generally attenuates relatively slowly, while the activity level (magnitude) of aftershocks attenuates rapidly. (3) After the MS5.2 earthquake, the sequence parameters obtained show that the h-value is 1.09 and the p-value is stable at 1.02, indicating that the intensity and frequency attenuation of the earthquake sequence are gradually stable and tend to be normal. The b-value is 0.95, indicating that the maximum aftershock magnitude of the series is estimated to be ML5.1, and the b-value gradually tended to be stable, indicating that the stress in the region gradually tended to be balanced after the MS5.2 earthquake. The MS4.4 (ML5.0) earthquake that occurred at 4:37 on June 10 (local time) is the largest aftershock after the MS5.2 earthquake in the sequence. (4) The Songgang fault with NW-SE trend is presumed to be the main seismogenic tectonics, but the migration of three earthquakes with MS≥5.0 may also indicate that the Songgang fault is not a single seismogenic tectonics, which requires further field scientific investigation and analysis.

A series of parallel normal faults are distributed in the Helan Mountain-Yinchuan Basin tectonic belt,where a historical M8.0 earthquake occurred.It is rare that such a great earthquake occurs in a normal fault system within the continent.To deeply understand the fine structure of the normal fault system,we deployed 104 broadband temporary stations near the system,collected data from permanent stations and other temporary stations nearby,and obtained the high-precision threedimensional S-wave velocity structure beneath 206 stations via joint inversion of receiver function and surface wave.A typical graben-in-graben feature bounded by four major faults was identified in the Yinchuan Basin.We analyzed the seismicity in the normal fault system and found a seismic strip in the southern part of the basin,where there are significant changes in the sedimentary thickness,which is speculated to be the southern boundary of the normal fault system.There are significant differences in the crustal thickness and velocity structure in the crust on both sides of the boundary between the Helan Mountain and the Yinchuan Basin,and a low-velocity zone was identified in the upper mantle beneath this boundary,which could be related to the fact that the Helan Mountain-Yinchuan Basin tectonic belt is located between the Alxa Block and the Ordos Block.The M8.0 Yinchuan-Pingluo earthquake occurred at the junction of four major faults in the Yinchuan Basin,which was located in the high-velocity zone near the velocity transition zone at the basin-mountain boundary.The low-velocity zone in the upper mantle beneath this boundary may have promoted the nucleation of this earthquake.Based on evidence from geological drilling,micro seismicity,the regional stress field,and the velocity models obtained in this study,it is inferred that the eastern piedmont fault zone of the Helan Mountain was the seismogenic fault of the 1739 M8.0 Yinchuan-Pingluo earthquake.

An MS 6.8 earthquake struck the Luding county in the southeastern section of the Xianshuihe fault on 5 September 2022, causing heavy casualties and serious property losses. Using the data of 61 seismic stations surrounding the focal area, we utilize the machine learning-based LOC-FLOW for phase picking and association, and obtain a total of 13,496 aftershocks. We also collate 2,331 preshocks near the mainshock. We then conduct event relocation and three-dimensional P- and S-wave velocity structure inversion by double-difference tomography. The catalog we obtained has higher accuracy and completeness than the manual catalog. The relocation results show that the mainshock struck the Moxi section of the Xianshuihe fault at 9.2 km depth. There is a southeast-striking seismic strip of 35 km long at 5–13 km depth, and two relatively shallow seismic clusters exist to the west and northwest of the mainshock. Furthermore, there is an obvious seismic gap to the northwest of the mainshock. Based on the spatial pattern of the aftershock sequence, we speculate that the Moxi section of the Xianshuihe fault is the seismogenic fault. The earthquakes are mainly located in the regions of low velocity and low Poisson’s ratio, which is consistent with the granite that is geologically distributed along the southeast section of the Xianshuihe fault. There are two high-velocity zones located to the northwest and southeast of the mainshock, which are speculated to have controlled the rupture scale of the MS 6.8 Luding earthquake.

The Xianshuihe-Anninghe-Daliangshan Fault junction region in eastern Tibet marks the transition zone between the creeping Xianshuihe Fault and the locked Anninghe Fault. However, the role in slip partitioning of these three faults is unclear, mainly because the geodetic data lack the resolution to discriminate slip at depth. Repeating earthquakes, the seismic events rupture the same fault patch more than once, are recognized as the slip meters at depth and play an important role in the assessment of seismic hazard. In this study, repeating earth-quakes in the Xianshuihe-Anninghe-Daliangshan Fault junction region are identified and the deep fault slip characteristics are investigated. First, a complete catalog (2013-2020) for the study region is achieved by using Matched Filter Technique detecting the continuous waveforms of the dense Xichang array. Second, waveform cross-correlation and double-difference location technologies are used to identify repeating earthquakes. Finally, by checking waveforms, focal depth and satellite images, mining expositions are excluded and 9 repeating earthquake sequences (RES) are obtained. RES indicate the deep slip at a rate of 8 mm/yr on the southern Xianshuihe Fault is partitioned to the northern Anninghe Fault and the Gongyihai section of Daliangshan Fault at rates of 1 mm/yr and 2-6 mm/yr, respectively. Aseismic slip on the southern Xianshuihe Fault and northern Anninghe Fault reach to 12 km depth. A large asperity on the Anninghe Fault from Liziping to Xichang is constrained by the back ground seismicity and the location of RES, which will form a M similar to 7 potential earthquake when it ruptures.

近年来快速发展的机器学习算法显著提高了震相拾取的精度和效率.采用卷积神经网络和递归神经网络的震相识别方法对银川台阵2019年6～7月的连续波形数据进行事件检测和P、S震相拾取,并通过快速震相关联和事件定位得到了银川地区较全的地震目录.结果表明,当震相数小于10时,虽然可以检测出较多事件,但分布呈弥散状,与区域地震活动特征不符.进一步对震相数≥10的事件进行了人工复核.总体而言,随着震相数量的增加,事件的误检率逐步降低.震相数16是该地区自动检测和定位结果准确性的拐点.当震相数≥20时,全部召回了地震目录中的13个地震事件,二者平均定位差异4.27 km.经过人工复核,检测到的真实地震事件为区域内地震目录中事件数量的9倍.本文使用的基于机器学习和快速震相关联和定位方法的流程可在确保准确率的基础上降低人工检测的难度,提高地震检测的效率.