2021年吴忠—灵武ML3.6震群重新定位及震源机制研究

2021年7月18日—8月7日，宁夏吴忠—灵武地区发生M_L3.6显著震群活动。本文利用多阶段定位方法对该震群进行了重新定位，并根据gCAP方法反演了2021年7月20日灵武M_L3.6地震的震源机制及震源矩心深度，采用Snoke方法计算了震群中3次ML3.0以上地震的震源机制，测定了同一地震多个震源机制的中心解。结果表明，该震群中最大的地震即7月20日02时40分M_L3.6地震的震源机制为节面Ⅰ走向289°，倾角72°，滑动角-22°，节面Ⅱ走向26°，倾角69°，滑动角-161°，震源矩心深度为12 km，初始破裂深度为12.5 km;7月20日03时15分ML3.2地震的震源机制为节面Ⅰ走向290°，倾角82°，滑动角-2°，节面Ⅱ走向20°，倾角88°，滑动角-172°，初始破裂深度为11.9 km;7月21日04时55分ML3.1地震的震源机制为节面Ⅰ走向285°，倾角53°，滑动角2°，节面Ⅱ走向194°，倾角88°，滑动角143°，初始破裂深度为11.6 km，这些地震震源机制的主压应力轴主要为NE向。该震群序...

Relocation and focal mechanisms of the 2021 Wuzhong-Lingwu ML3.6 seismic swarm

From July 18 to August 7 in 2021, a significant activity of M_L3.6 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 M_L3.6 earthquake on July 20, 2021 are calculated by gCAP method, and the focal mechanisms of three M_L≥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 M_L3.6 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 M_L3.2 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 M_L3.1 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 M_L≥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 M_L3.6 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 M_L3.6 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.