Research on the Three-dimensional b-value Distribution Characteristics in Tangshan Area

We collected the seismic phase reports of the M7 working groups (2002.1—2008.12) and the China Earthquake Networks Center (2009.1—2020.9) in Tangshan and its surrounding areas. Firstly, we relocate the earthquake using the double-difference tomography method. Then, we used the K-K method to remove aftershocks from relocated seismic events. Finally, the closest distance and least number of events scanning method was used to scan the plane and different depth b-value images of the Tangshan area before and after Tangshan M_L5.1 earthquake in Hebei on May 28th, 2012 and Tangshan M_L5.4 earthquake in Hebei on July 12th, 2020. We analyzed the spatial distribution and evolution of the b-value in Tangshan area before and after the occurrence of two earthquakes, and further explore the characteristics of stress evolution in the area before and after the occurrence of moderate and strong earthquakes in Tangshan area. The results show that the focal depth is more reasonable after relocation, and the distribution and evolution of b-value images of different depths were more abundant than the b-value images of plane. The Tangshan M_L5.1 earthquake and Tangshan M_L5.4 earthquake both occurred in the decreasing process of the b-value. Although the b-value in Tangshan area continued to decrease, which represented the gradual increase in stress accumulation, but the stress level in shallow is higher than in deep, the urgency of a larger earthquake is not strong at present.