Determination on the focal mechanism and central focal mechanism of Zhongwei M_L3.4 earthquake on June 9, 2020

On June 9, 2020, an earthquake measuring M_L3.4 occurred in Shapotou District, Zhongwei City, Ningxia. The earthquake occurred in the meizoseismal area of the 1709 M7½ earthquake of South Zhongwei, and its epicenter is located in an area where there were relatively few weak earthquakes in the past. In this paper, based on the waveform records of the Ningxia regional seismic network, the focal mechanism solution and focal centroid depth of the Zhongwei M_L3.4 earthquake on June 9, 2020 are inversed using the gCAP method. The focal mechanism solution is also calculated by the Hash method, and the central focal mechanism of these two methods is obtained. The result shows that the focal mechanism solution of gCAP method is strike 255°, dip 79°, rake −20° for nodal plane I, and strike 348°, dip 70°, rake −160° for nodal plane II, and the centroid depth is 12 km, and that the focal mechanism solution of Hash method is strike 344°, dip 89°, rake 176° for nodal plane I, and strike 74°, dip 86°, rake 1° for nodal plane II. The central focal mechanism solution of the two methods is strike 255°, dip 87°, rake −11° for nodal plane I, and strike 346°, dip 80°, rake −176° for nodal plane II, and the principal stress axis is mainly in NE direction. The minimum 3-D rotation angle between the gCAP method result and the central focal mechanism is relative small, which is 12.09°. Combing the past geologic structure data, we deduces that the main dislocation of Zhongwei M_L3.4 earthquake on June 9, 2020 is the left-lateral slip, and that the fault is more likely to belong to the nodal plane in NEE direction.