Abstract:
From July 18 to August 7 in 2021, a significant activity of
ML3.6 earthquake swarm occurred in Wuzhong-Lingwu area, Ningxia. In this paper, the earthquake swarm are relocated using the multi-step locating method. The focal mechanism and the centroid depth of Lingwu
ML3.6 earthquake on July 20, 2021 are calculated by gCAP method, and the focal mechanisms of three
ML≥3.0 earthquakes of the swarm are calculated by Snoke method. Meantime, the center solution of several focal mechanisms for the same earthquake is also determined. The results indicate: The focal mechanism solution of the
ML3.6 earthquake at 02:40 am on July 20, 2021 which is the largest earthquake in the swarm is strike 289°, dip 72°, rake −22° for nodal plane Ⅰ, strike 26°, dip 69°, rake −161° for nodal plane Ⅱ, and the centroid depth is 12 km while the initial rupture depth is 12.5 km. The focal mechanism solution of the
ML3.2 earthquake at 03:15 am on July 20, 2021 is strike 290°, dip 82°, rake −2° for nodal plane Ⅰ, strike 20°, dip 88°, rake −172° for nodal plane Ⅱ, and the initial rupture depth is 11.9 km. The focal mechanism solution of the
ML3.1 earthquake at 04:55 am on July 21, 2021 is strike 285°, dip 53°, rake 2° for nodal plane Ⅰ, strike 194°, dip 88°, rake 143° for nodal plane Ⅱ, and the initial rupture depth is 11.6 km. The principal stress axis of focal mechanism is mainly in NE direction. The focal depths of the earthquake swarm are relatively concentrated at 7—15 km, and the focal depths of
ML≥3.0 earthquakes mainly varies from 11 km to 13 km. The focal depth profiles show that the distribution direction of earthquake concentrated area is approximately upright from deep to shallow. Furthermore, it is also found that the relative shear stress on NNE nodal plane of Lingwu
ML3.6 earthquake under the regional stress field is 0.393, and the relative shear stress on NWW nodal plane is 0.945. Combing with the geological structure and known fault data, the preliminary analysis indicates that if the NNE-trending Chongxing buried fault is the seismogenic fault of Lingwu
ML3.6 earthquake, the Chongxing fault may be a weak fault zone, and the seismic rupture mode is mainly right-lateral slip. Otherwise, if the unknown NWW-trending buried fault is the seismogenic fault, the NWW-trending fault may be the releasing nodal plane with relatively maximum shear stress of the earthquake under the regional stress field, and its rupture mode is left-lateral slip.