Analysis on relocation and focal mechanism of the Chengde M_L3.6 earthquake on June 17, 2021

On June 17, 2021, an M_L3.6 earthquake occurred in Shuangqiao District, Chengde City, Hebei Province. Based on the data of Hebei regional digital seismic network, the double-difference location algorithm was used to reposition the earthquake sequence and a precise epicenter distribution was obtained. The fault plane parameters of this earthquake were computed using the small event fitting method as follows, strike 63.76° and dip 70.52°. The main shock’s optimal unit source time function, moment tensor solution, and double couple solution are obtained by CAP method. The result is strike 214°, dip 41°, rake −113° for the nodal plane Ⅰ andstrike 63.4°, dip 52.8°, rake −71.2° for the nodal plane Ⅱ; the moment magnitude is M_W3.5, the depth of the moment center is 3.6 km, the maximum principal compressive stress P-axis azimuth angle is 28.4°, and the maximum recommended stress T-axis azimuth angle is 140.1°. Among them, the best fault plane solution is strike 63.4° and dip 52.8°. Based on the fault center solution calculations, the final fault plane parameters were determined to be strike 63.59° and dip 61.66°, which aligns with the region’s Shuangtashan-Shuangfengsi fault. Considering regional tectonic characteristics, it is postulated that this earthquake may have been induced by the principal stress squeezing on the fault, representing a typical normal fault activity. This study provides crucial evidence for identifying the seismogenic structure of the Chengde earthquake and the stress field status in the area.