In this paper, we relocated earthquakes occurred from April 2013 to July 2022 in Lushan seismic zone, inversed focal mechanism solution of the Lushan M_S6.1 earthquake on June 1, 2022 and discussed the seismogenic structure of the Lushan M_S6.1 earthquake and its relationship with the M_S7.0 earthquake in April 2013.

The results of the focal mechanism solution show that the Lushan M_S6.1 earthquake in 2022 is a thrust earthquake. The strike, dip and azimuth of nodal plane Ⅰ are 228°, 46° and 104° and for nodal plane Ⅱ are 28°, 46° and 76° respectively. The results of earthquake relocation show that the focal depth of the Lushan M_S6.1 earthquake sequence is shallow in the north and deep in the south, the fault length is about 10km. The focal depth is mainly concentrated between 10km to 19km. The fault dip is southeast with an angle of 60°. The initial rupture point of the main shock of the Lushan M_S6.1 earthquake is at a depth of 20km, located at the deepest part of the fault. The fault ruptured from deep to shallow. The Lushan M_S7.0 earthquake occurred on April 2013 strikes northeast and dips northwestward, but there exists a reverse fault in the aftershock sequence that has the same direction of strike but the opposite direction of dip. This reverse fault is consistent with the strike and dip of the M_S6.1 earthquake occurred in June 2022. It appears as two parallel faults in the profile. In addition to the reverse fault on the west side, the embryonic of another reverse fault seems to appear on the east side of the middle of earthquake sequence. These faults are about 10km away from the surface. The distribution of earthquakes in two northwest-oriented depth profiles shows that the dip angles of the main shock and the reverse fault of the M_S7.0 earthquake is different at different locations, and these faults are not simple straight planar sections. From one year after occurrence of the M_S7.0 earthquake to occurrence of the M_S6.1 earthquake, the seismic activity on the main fault decreased but the seismic activity on the reverse fault on the west side of the M_S7.0 earthquake sequence was more active during this period, most of the seismic activity occurred near the reverse fault that is parallel to the M_S6.1 earthquake fault.

By analyzing the seismogenic structure and seismic activity characteristics of the Lushan seismic zone, we concluded the Lushan M_S6.1 earthquake on June 1, 2022 is caused by a blind thrust fault with strike towards northeast and dip towards southeast, located 10km away from the surface. It has the opposite directions of strike and dip of the Longmenshan Fault. The epicenters of the Lushan M_S7.0 earthquake in April 2013 and the M_S6.1 earthquake in June 2022 are located near the surface exposure traces of the Shuangshi-Dachuan Fault and the Xiaoguanzi Fault, respectively. However, according to the analysis of the relocation aftershock depth in profile, the aftershock extension to the surface does not coincide with the surface exposure positions of the Shuangshi-Dachuan Fault and the Xiaoguanzi Fault. Therefore, the seismogenic faults of these two earthquakes are not the Shuangshi-Dachuan Fault and the Xiaoguanzi Fault, but two blind reverse faults. The Shuangshi-Dachuan Fault near the M_S6.1 earthquake sequence and the main shock fault of the 2013 M_S7.0 earthquake are thrust faults dipping northwest, while the Lushan M_S6.1 seismogenic fault has opposite direction of dip. The seismogenic fault of the Lushan M_S6.1 earthquake and the main thrust fault of the 2013 M_S7.0 earthquake, which strikes northeast and dips northwest with the reverse thrust fault of the hanging wall, which strikes northeast and dips southeast, together form a double layer Y-shaped structure. These faults are all blind thrust faults and belong to the Qianshan-Shanqian Fault system in the southern segment of the Longmenshan fault zone. The seismogenic structure in the Lushan seismic zone is a complex fault system composed of one main northeast strike fault with dipping northwest, and three faults dipping southeast.

From one year after occurrence of the Lushan M_S7.0 earthquake to the occurrence of the Lushan M_S6.1 earthquake, most of earthquakes in the Lushan seismic zone occurred near a reverse fault which is parallel to the Lushan M_S6.1 earthquake seismogenic fault. These earthquakes are located in the area where the coulomb stress change caused by the M_S7.0 earthquake acts as loading effect. The Lushan M_S6.1 earthquake sequence is mainly distributed in the area where the coulomb stress change plays an unloading role caused by the Lushan M_S7.0 earthquake. The research results showed that the coulomb rupture stress caused by the Lushan M_S7.0 earthquake on the seismic nodal plane of the M_S6.1 earthquake has a restraining effect on the M_S6.1 Lushan earthquake.

This study is devoted to a systematic analysis of the stress state of the eastern boundary area of Sichuan-Yunnan block based on focal mechanisms of 319 earthquakes with magnitudes between M3.0 and M6.9, occurring from January 2009 to May 2018. We firstly determined the mechanism solutions of 234 earthquakes by the CAP method, using the broadband waveforms recorded by Chinese regional permanent networks, and collected 85 centroid moment tensor solutions from the GCMT. Then we investigated the regional stress regime through a damp linear inversion. Our results show that:1)the focal mechanisms of moderate earthquakes are regionally specific with three principal types of focal mechanisms:the strike-slip faulting type, the thrust faulting type and the normal faulting type. The strike-slip faulting type is significant in the eastern boundary area of Sichuan-Yunnan block along the Xianshuihe-Xiaojiang Fault, the Daliangshan Fault, and the Zhaotong-Lianfeng Fault. The thrust faulting type and the combined thrust/strike-slip faulting type are significant along the Mabian-Yanjin Fault, Ebian-Yanfeng Fault and the eastern section of Lianfeng Fault; 2)The most robust feature of the regional stress regime is that, the azimuth of principal compressive stress axis rotates clockwise from NWW to NW along the eastern boundary of Sichuan-Yunnan Block, and the clockwise rotation angle is about 50 degrees. Meanwhile, the angels between the principal compressive axis and the trend of eastern boundary of Sichuan-Yunnan Block remain unchanged, which implies a stable coefficient of fault friction in the eastern boundary fault zone of Sichuan-Yunnan Block. The movement of the upper crust in the southeastern Tibetan plateau is a relatively rigid clockwise rotation. On the whole, the Xianshuihe-Xiaojiang Fault is a small arc on the earth, and its Euler pole axis is at(21°N, 88°E). The Daliangshan Fault is surrounded by the Anninghe-Zemuhe Fault, which formed a closed diamond shape. When the Sichuan-Yunnan block rotates clockwise, the Daliangshan Fault locates in the outer of the arc, while the Anninghe-Zemuhe Fault is in the inward of the arc, and from the mechanical point of view, left-lateral sliding movement is more likely to occur on the Daliangshan Fault. Our results can be the evidence for the study on the "cut-off" function of the Daliangshan Fault based on the stress field background; 3)The regional stress regime of the eastern boundary faults zone of the Sichuan-Yunnan Block is the same as the south section of the Dalianshan Fault, and the focal mechanism results also reveal that the Dalianshan Fault is keeping left-lateral strike-slip. There may be the same tectonic stress field that controls the earthquake activities in the southern section of Daliangshan Fault and Zhaotong-Lianfeng Fault. The regional stress regime of Zhaodong-Lianfeng Fault is also the same with the Sichuan-Yunnan Block, which implies that the control effect of the SE movement of the Sichuan-Yunnan block may extend to Weining.

On August 8, 2017, Beijing time, an earthquake of M7.0 occurred in Jiuzhaigou County, Aba Prefecture, Sichuan Province, with the epicenter located at 33.20°N 103.82°E. The earthquake caused 25 people dead, 525 people injured, 6 people missing and 170000 people affected. Many houses were damaged to various degrees. Up to October 15, 2017, a total of 7679 aftershocks were recorded, including 2099 earthquakes of M ≥ 1.0.
The M7.0 Jiuzhaigou earthquake occurred in the northeastern boundary belt of the Bayan Har block on the Qinghai-Tibet Plateau, where many active faults are developed, including the Tazhong Fault(the eastern segment of the East Kunlun Fault), the Minjiang fault zone, the Xueshan fault zone, the Huya fault zone, the Wenxian fault zone, the Guanggaishan-Daishan Fault, the Bailongjiang Fault, the Longriuba Fault and the Longmenshan Fault. As one of the important passages for the eastward extrusion movement of the Qinghai-Tibet Plateau(Tapponnier et al., 2001), the East Kunlun fault zone has a crucial influence on the tectonic activities of the northeastern boundary belt of Bayan Kala. Meanwhile, the Coulomb stress, fault strain and other research results show that the eastern boundary of the Bayan Har block still has a high risk of strong earthquakes in the future. So the study of the M7.0 Jiuzhaigou earthquake' seismogenic faults and stress fields is of great significance for scientific understanding of the seismogenic environment and geodynamics of the eastern boundary of Bayan Har block.
In this paper, the epicenter of the main shock and its aftershocks were relocated by the double-difference relocation method and the spatial distribution of the aftershock sequence was obtained. Then we determined the focal mechanism solutions of 24 aftershocks(M ≥ 3.0)by using the CAP algorithm with the waveform records of China Digital Seismic Network. After that, we applied the sliding fitting algorithm to invert the stress field of the earthquake area based on the previous results of the mechanism solutions. Combining with the previous research results of seismogeology in this area, we discussed the seismogenic fault structure and dynamic characteristics of the M7.0 Jiuzhaigou earthquake. Our research results indicated that:1)The epicenters of the M7.0 Jiuzhaigou earthquake sequence distribute along NW-SE in a stripe pattern with a long axis of about 35km and a short axis of about 8km, and with high inclination and dipping to the southwest, the focal depths are mainly concentrated in the range of 2~25km, gradually deepening from northwest to southeast along the fault, but the dip angle does not change remarkably on the whole fault. 2)The focal mechanism solution of the M7.0 Jiuzhaigou earthquake is:strike 151°, dip 69° and rake 12° for nodal plane Ⅰ, and 245°, 78° and -158° for nodal plane Ⅱ, the main shock type is pure strike-slip and the centroid depth of the earthquake is about 5km. Most of the focal mechanism of the aftershock sequence is strike-slip type, which is consistent with the main shock's focal mechanism solution; 3)In the earthquake source area, the principal compressive stress and the principal tensile stress are both near horizontal, and the principal compressive stress is near east-west direction, while the principal tensile stress is near north-south direction. The Jiuzhaigou earthquake is a strike-slip event that occurs under the horizontal compressive stress.

With the extensive application of focal mechanism solutions of micro-earthquakes,more and more attentions have been paid to the reliability of focal mechanism solution itself.The application of focal mechanism solution has been a significant progress in seismology,and it has played a non-fungible role in the recognition of seismic structure and seismogenesis.However,from the very beginning of the method development,the error has existed in the solution.In recent years,many methods and techniques for deriving focal mechanism solutions have been developed.Therefore,the analyses on the accuracy and reliability of these methods should also be made. The Capital-circle Digital Seismic Network was put into operation in 2002.In the network,107 seismographs may simultaneously record and transmit seismic data,and among them some are broadband seismograph with large dynamic range and some are short-period seismographs with greatly improved dynamic range.About half of the stations in the network had recorded clearly the two M=4 earthquakes occurred successively in Ninghe,Tangshan seismic area in April,2003,providing an opportunity to make the reliability analysis. For the M 4.1 earthquake occurred on April 23,2003,48 distinct and identifiable initial motion signals were selected from the P-wave initial motions recorded by stations with epicentral distance of less than 320km among the 107 stations.For the data recorded at 12 stations with epicentral distance of less than 43km,the emulation was made to obtain the record of displacement,and then the maximum amplitudes of the vertical-P and -S were read.The focal mechanism solution was then calculated using the grid-point trial method and the amplitude ratio method,respectively.The results indicate that this earthquake can be assigned to the strike-slip type with an upright nodal plane.The distribution of P-wave initial motion signals on the spherical surface used by the grid-point trial method is rather homogeneous and the nodal line can be better controlled,but the initial motion signals are a little bit deviated from the center of the net.The inconsistent ratio of the average focal mechanism solutions is 0.208,the minimum inconsistent ratio is 0.188 and the cut-off solutions are 60.The discrete area of the focal mechanism solutions is relatively small,so the results are reliable.The focal mechanism solution obtained by the amplitude ratio method is quite similar to the average focal mechanism solution obtained by the grid-point trial method,and the 3 stress axes are exactly located within the discrete area of the corresponding solutions.It can be seen that the solutions obtained by the two methods with different data coincide very well with each other,indicating that both methods are reliable.For further verification,the CMT solution of Harvard University for the largest earthquake occurred in this region in 1976 is used for comparison.The result shows that the difference of the solution with the solution of this moderate earthquake is very small.Therefore,it not only indicates the inheritance of the seismic activity,but also proves that the focal mechanism solutions are not randomly distributed. For the M 4.3 earthquake occurred on April 24,2003,55distinct and identifiable initial motion signals recorded at stations with epicentral distance of less than 320km were selected.And the maximum displacement amplitudes for the vertical-P and -S were obtained from the emulation of the data recorded at11stations with epicentral distance of less than 52km.The focal mechanism solution was calculated respectively by the above-mentioned two methods.The result shows that the focal mechanism solution of this event is of the dipslip type.The inconsistent ratio of focal mechanism solution obtained by the grid-point trial method is 0.182,the minimum inconsistent ratio is 0.127 and the cut-off solutions are 15.The discrete area of the focal mechanism solutions is relatively small,so the results are reliable.The focal mechanism solution obtained by the amplitude ratio method is quite similar to the average focal mechanism solutions ohtained by the grid-point trial method. For further verification, the CMT solution of Harvard University for the largest earthquake occurred in this region in 1977 was used for comparison. The result shows that it is basically consistent with the solution of this moderate earthquake.In the above-mentioned analyses, two different methods were used and two earthquakes with different rupture features were comparatively analyzed, and the results have proved the reliability of the focal mechanism solution determined by the amplitude ratio of -P and -S recorded by the regional seismic network. However,there are various factors that might cause the errors in the focal mechanism solutions,and the occurrence of moderate and small earthquakes is usually random. It is suggested, therefore, that the focal mechanism solutions of the moderate and small earthquakes inferred from the amplitude ratio should not be analyzed as a single item.The analysis should always be made by using the statistic method based on the focal mechanism solutions derived from a large amount of moderate and small earthquakes to abstract the average information and the variation of statistic characteristics. Only in this way can the obtained conclusigns be objective and reasonable.

Based upon the far-field and near-field digital seismic records, the hypocenter locations of M_L≥3 earthquakes of the Zhangbei earthquake sequence occurred during January, 1998-March, 1999 have been re-determined by using relative locating method. The results show that the process of the earthquake sequence can be divided into three stages. The first stage occurred in January 1988, which is the stage of the occurrence and adjustment of the main shocks. The seismic activity in this stage occurred mostly along NWW-SEE direction. The second stage occurred during February ～August 1998, which is the active period of moderate aftershocks occurred mostly along NNE direction. The third stage was represented by the occurrence of M 5.6 earthquake on March 11, 1999, the hypocenter of which was aligned also along NNE direction, but was out of the hypocenter area of the main shock. It seems that the two hypocenters were not connected with each other. The rupture characteristics of the Zhangbei earthquake sequence is given on the basis of the relocation of the hypocenters and the revised focal mechanisms, as well as macroscopic intensity distribution data of this earthquake sequence. It is suggested that the earthquake sequence consists of one main left-lateral strike-slip rupture plane, and 2 right-lateral strike slip secondary rupture planes. The main rupture plane is NWW-trending, NNE-dipping at an angle of 44°, and 11.5km in length. The two secondary rupture planes are NNE-trending with a high dip angle. The main rupture plane is conjugate with the NNE-trending secondary rupture planes while the two NNE-trending rupture planes are aligned in right-step en ecelon. The three rupture planes occurred sequentially, the depth of the which is 1.4～7.6 km at shallow part of the crust. The present study shows that it is a doable approach to research the focal rupture of a strong earthquake sequence from three dimensional space by using exact hypocenter 1ocation, focal mechanism and macro-intensity distribution data in "tectonically stable area" where no active fault is found.