At 2:04 on May 22, 2021, an earthquake of M7.4 occurred in Maduo County, Golog Prefecture, Qinghai Province, with the focal depth of 17 kilometers, the epicenter at 34.59°N and 98.34°E. This earthquake was the largest after the Wenchuan earthquake in China. The epicenter of the earthquake is 38km away from Maduo county seat and 385km from Xining, the provincial capital. The earthquake caused some houses to collapse and some damage to roads in the epicenter. But due to the sparse population in the epicenter area, the earthquake did not cause casualties.

Seismologist believe that the earthquake is the result of the continuous activity of the boundary fault of the Bayankala block, which is geographically located in the north of the Qinghai-Tibet Plateau and is the hub for the transformation of the direction of the crustal movement of the plateau. In recent years, many destructive earthquakes occurred inside the block. This earthquake is another strong earthquake after the M7.1 Yushu earthquake in Qinghai in 2010. According to the analysis of this earthquake briefing, the fault zone that induced this earthquake is speculated to be the Maduo-Gande fault zone or the Kunlun Mountains Pass-Jiangcuo fault zone.

In order to find out which fault is the seismogenic structure and the distribution of the seismogenic structure of this earthquake, we relocated the dense earthquakes by double-difference method based on the data of 1357 aftershocks in the Maduo M7.4 earthquake area recorded by 72 fixed stations of the digital seismic network of Gansu and its adjacent seismic network and 12 portable seismographic stations during the May 22 to May 27, and obtained the source parameters for 1289 earthquakes. The accurately located small earthquakes distribute along both sides of the Kunlun Mountains Pass-Jiangcuo Fault, which is NNW-trending obviously. It shows that the seismogenic structure of this earthquake is the Kunlun Mountains Pass-Jiangcuo Fault, rather than the Maduo Gande Fault as considered previously by some scholars. This is consistent with the research results of surface fracture zone, magnetotelluric detection, InSAR coseismic deformation and relocation of other aftershocks. Most earthquakes distribute at the depth range of 0~15km of the crust after the relocation, and the result shows that the focal depths are more concentrated. The relocation also shows that the east and west ends of the main fault have bifurcations. It may be that the complex stress distribution triggered two new branch faults during the occurrence of the great earthquake, and the overall fault shows a “tree-type” structure. The west branch trends 306°and intersects the main fault at 21°. The east branch is nearly EW trending and connected with the east section of the main fault.

Generally, the earthquakes are closely related to active tectonics, large earthquakes and its aftershocks usually occur on fault zones with obvious activity. The distribution of small earthquakes is related to the complex underground stress state and the complex structure of the fault zone. We can inverse the shapes and positions of the fault planes using spatial distribution of hypocenters of mainshock and the corresponding aftershocks, according to the principle that clustered earthquakes occur near the faults. Six rectangular regions are selected according to the distribution characteristics of relocated aftershocks and by reference to the distribution of geological faults and earthquake rupture zones. We obtained the detailed parameters of fault plane in each region by using the simulated annealing algorithm and the Gauss-Newton algorithm according to the source information after the relocation in 6 rectangular areas. On this condition, rake angle of the fault plane is further inferred from regional tectonic stress parameters. The results show that the main fault is a large, high dip angle, sinistral strike-slip fault with thrust component, striking 285°~290° and about 146km long. It extends from Tanggema Township of Maduo in the southeast(34.49°N, 98.91°E)to Gazejialong Township in the northwest(34.81°N, 97.54°E). The movement characteristics of the newly generated western segment 2 show dextral strike slip and thrust, which is diametrically opposite to that of the main fault. This shows the complexity of the earthquake rupture process, and further research is needed on the tectonic mechanics and deep structures that produce this special rupture.

Compared with the focal mechanism solutions obtained by domestic and foreign authorities, the fault plane parameters obtained in this paper are similar to them, indicating that our conclusions are reliable. Besides, the spatial distribution of inverted fault plane is basically identical to that of the rupture zone derived from post-earthquake investigation in the earthquake area.

The Laohushan fault zone is located in the northeast margin of the uplift area of the Tibetan plateau. It belongs to the eastern segment of the Laohushan-Maomaoshan-Jinqianghe Fault in the eastern segment of the North Qilian fault system. It was manifested as compressive thrust in early stage, but its mechanical properties changed into left-lateral strike-slip movement after middle Pleistocene. There occurred the Jingtai M_S6.8 earthquake in 1888, Tianzhu M_S6.2 earthquake in 1990 and Jingtai M_S5.9 earthquake in 2000 along the fault in history.
With the construction of the national important projects in earthquake industry-“Digital seismic network project of the 10th Five Year Plan” and “Chinese seismic background field detection project”, a number of modern seismological stations were built in Gansu Province and its adjacent areas. Contrast with seismographic network, the mobile broadband seismic array has the advantages of relatively dense stations, small spacing, uniform distribution, and high data integrity rate. Combining the two observational methods has gradually become the main development direction at home and abroad.
Based on the data of small earthquakes in the Laohushan zone recorded by 20 stations of the digital seismic network in Gansu and its adjacent seismic network during the years of 2008 to 2019, and 18 portable seismographic stations from the 2^nd-phase project of China Seismic Detection Array during the years of 2014—2015, we relocated the dense earthquakes by double-difference method and obtained the source parameters for 700 earthquakes. The accurately located small earthquakes distribute along both sides of the Laohushan Fault, which is NW-trending obviously. Most earthquakes distribute at the depth range of 0~10km of the earth's surface after the relocation, and the result shows that the focal depths are more concentrated.
Generally, the earthquakes are closely related to active tectonics, large earthquakes usually occur on fault zones with obvious activity, but the distribution of small earthquakes is related to the complex stress state underground and the complex structures of fault zones. We can inverse the shapes and positions of the fault planes using spatial distribution of hypocenters of small earthquakes according to the principle that clustered earthquakes occur near the faults. We obtained the parameters of the Laohushan Fault, which has a strike of 103°and a dip angle of 89°, by using the simulated annealing algorithm and the Gauss-Newton algorithm. On this condition, rake angle of the fault plane is further inferred from regional tectonic stress parameters. These inversion results of the fault parameters indicate that it's a left-lateral slip fault with a high dip angle and a length of 38km. It extends from Xijishui county town of Jingtai in the southeast to Songshan of Tianzh county town in the northwest. Comparing the inversion fault plane parameters and the focal mechanism solutions of the 1990 Tianzhu M_S6.2 and 2000 Jingtai M_S5.9 earthquakes, both of the results are identical. Besides, the spatial distribution of inverted fault plane and the location of Laohushan Fault by the previous studies are basically parallel.
In the past, the studies of active faults mainly focused on the qualitative researches of macroscopic survey. With the technological development of earthquake location and inversion method in recent years, many quantitative researches have gradually been carried out on the determination of active fault parameters. The inversion results of Laohushan fault plane and the previous studies on the geometric characteristics of the fault are verified each other. It is proved by facts that it's an important research means of active faults. It can provide more evidences for determining fault parameters by inversion.

The Gulang M8.0 great earthquake occurred in 1927, many places in Gulang and adjacent areas had suffered destruction in various degrees. So far, divergences exist in the former studies on its seismogenic structure. It is known that clustered small earthquakes often occur in vicinities of fault plane of large earthquake. In this study, the precisely relocated earthquakes which occurred near the earthquake rupture zone between 1985 and 2012 are used, and two strip-shaped zones of clustered small earthquakes are chosen according to the previous studies of the causative structure of this earthquake. Based on the simulated annealing and Gauss-Newtonian nonlinear inversion algorithms, we obtained fault plane parameters of the earthquake such as strike, dip, and location using data of densely concentrated small events. On this condition, rake angles of the fault plane are further inferred from regional tectonic stress parameter. Then we discussed briefly the seismogenic environment and causal mechanism of the earthquake, combined with the results of deep tectonics and surface investigations. The focal fault we inverted locates within the meizoseismal area (intensity Ⅺ)of the Gulang M8.0 in 1927, suggesting that the focal fault obtained by inversion is possibly the causative structure of this earthquake. Besides, we found a clustered zone of small earthquakes near the south-central part of the main fault, and a fault plane could be derived from them, which we think might be a tensional seismic fault developed on the main fault when the whole earthquake-hit region rotated counterclockwise during the big earthquake.

The southern Wudu M8.0 great earthquake occurred in 1879,many places in Wudu and adjacent areas had suffered destruction in various degrees. So far,the research results on the causative structure of this earthquake are less and inconsistent. Because it occurred in lofty mountains and the traffic is inconvenient,it is hard to make detailed field study on the earthquake site. Based on the assumption that clustered small earthquakes often occur in the vicinity of fault plane of large earthquake,and referring to the morphology of the major axis of the meizoseismal area obtained by the predecessors,we selected a strip-shaped zone from the relocated earthquake catalog which occurred near the earthquake rupture zone in the period from 1985 to 2009 to calculate fault plane parameters of the earthquake,such as strike and dip,with the simulated annealing and Gauss-Newtonian nonlinear inversion algorithms. On this condition,the rake angles of the fault plane are further inferred from regional tectonic stress parameters. We discussed the causal mechanism of the earthquake and finally identified the length and location of the seismogenic fault. In addition,clustered small earthquakes occurred frequently in the Xionghuangshan area west of the mezoseismal area,but we didn't find clear fault planes in field investigation,so,they should not have relation with the 1879 M8 southern Wudu earthquake.