Seismogenic structure of Yongqing M_S4.3 and Langfang M_S3.0 earthquakes

On February 12, 2018 and February 3, 2019, the M_S4.3 and M_S3.0 earthquakes occurred in Yongqing county and Langfang city, Hebei Province, and the two epicenters were less than 10 km apart. In order to study the seismogenic structure of these two earthquakes, based on the wide-band data of the digital seismic network of the Capital Circle, the focal mechanism solutions of the two earthquakes were determined by the CAP method first. For the Yongqing earthquake, the strikes, dip angle and rake angle of nodal plane I are 52°, 62° and −140° separately; and the strikes, dip angle and rake angle of nodal plane II are 300°, 55° and −35° separately; For the Langfang earthquake, the strike, tilt angle and slide angle of nodal plane I are 48°, 57° and −147° separately, and the direction, inclination and sliding angle of joint II are 299°, 63° and −38° separately. The focal mechanism solutions of the two earthquakes are relatively consistent, hence it is speculated that they may have the same seismogenic fault. The source depths of the two earthquakes obtained by converted waves of near earthquakes are 19 km and 13 km, respectively. The double-difference method was used to relocate the main and aftershocks of the two earthquakes. The results show that the epicenters of the two earthquake sequences are NE-directed, and the aftershocks all occur above the main shock. The focal depths are respectively concentrated in the range of 17−20 km and 12−13 km. Besides, the short-axis profiles of the two sequences reveal that the source distributions are characterized by SE tendency and steep dip angle. By comparing the seismic sequence distribution with the focal mechanism solutions, it found that the horizontal distribution direction of thesequences ars closer to the trend of the joint surface I in the focal mechanism solutions of the two earthquakes, and the high dip angle characteristics of the depth distribution are also similar to the joint surface I. Therefore, the seismogenic fault plane is considered to be nodal plane I. By comparing the parameters of nodal I from the focal mechanism solutions, the distribution of seismic sequences and the attitude properties of regional active faults, some important knowledge about the seismogenic structure and the origin of earthquakes was obtained: ① The seismogenic structure of the Yongqing M_S4.3 earthquake and the langfang M_S3.0 earthquake are not the pre-existing normal fault of the upper crust−the Hexiwu fault, and it may relate to the nascent structure of the middle and lower crust or deep faults. ② The Yongqing and Langfang earthquakes occurred at a depth of 13−19 km. Synthesize the crustal structure, fault structure and regional rheological structure, it is speculated that this depth range may be the intracrustal brittle-ductile transition area in the Langgu depression, which is favorable structural part for earthquake preparation and occurrence.