Analysis of a Significant Earthquake Occurred in Ordos Block Based on Focal Mechanism and Shear Wave Splitting: The Yanchi ML4.0 Earthquake on July 12, 2022
Abstract：Based on the data of Ningxia regional seismic network and earthquake warning network, we calculate the focal mechanism of the July 12, 2022 Yanchi ML4.0 earthquake by the Hash method. We obtain the focal mechanism, moment tensor solution and centroid depth of the earthquake by the gCAP method. We determine the focal mechanism central solution, and calculate the relative shear stress and normal stress on the two nodal planes of the focal mechanism solution. Finally, we carry out the polarization of fast shear wave and time delay of slow shear wave on the station that conforms to the shear wave splitting window. The results show that the Yanchi earthquake is a natural strike-slip earthquake event. The focal mechanism central solution of different methods is strike 99°, dip 71°, slide 8° for nodal plane I and strike 6°, dip 83°, slide 161° for nodal plane II ,and the P-axis azimuth is 54°. The relative shear stress generated by the stress tensor on nodal plane I and nodal plane II of the focal mechanism central solution is 0.982 and 0.942, respectively. The near-station shear wave splitting shows that the fast shear wave polarization direction is 55° and the slow shear wave time delay is 1.7 ms/km. We find that the P-axis azimuth of the focal mechanism and the polarization direction of the fast shear wave of the Yanchi ML4.0 earthquake are mainly controlled by the regional tectonic stress field, and the slides of the two nodal planes from the focal mechanism are basically consistent with the slides of the shear stress. Combined with existing research, we consider that the seismogenic fault may be a parallel associated structure near the EW-strike basement fault in the Ordos block. Under the principal compressive stress of regional tectonic stress field in the NE-SW direction, after a long period of stress accumulation, the rupture occurred on the optimal release nodal plane of shear stress, triggering the ML4.0 Yanchi earthquake.
XU Yingcai,ZENG Xianwei. Analysis of a Significant Earthquake Occurred in Ordos Block Based on Focal Mechanism and Shear Wave Splitting: The Yanchi ML4.0 Earthquake on July 12, 2022[J]. jgg, 2023, 43(9): 924-930.
