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.