Variation of b-value before and after Yangbi four M_S≥5.0 earthquakes on May 21，2021

Four M_S≥5.0 earthquakes occurred successively in Yunnan Yangbi from 21:21 to 22:31 on May 21, 2021, which are M_S5.6, M_S6.4, M_S5.0 and M_S5.2, respectively. Based on the earthquakes recorded in focal area of Yangbi earthquake and its vicinity from January 1, 2015 to June 4, 2021, we estimated the b-value by the maximum likelihood method. Then, we analyzed the spatial distribution of b-value before and after Yangbi earthquakes and got the views below. Firstly, an earthquake swarm occurred three days before the Yangbi earthquakes, then, the b-value near the focal area of Yangbi earthquake decreased rapidly after the earthquake swarm. It may indicate that the strength of crustal medium was close to the critical state of fracture. Secondly, the four M_S≥5.0 earthquakes distribute along the edge of the abnormal area with low b-value. Thirdly, the b-value of the focal area increased obviously after the Yangbi earthquakes. It indicates that the regional stress has been released and the risk of strong earthquake is reduced in the short term. Fourthly, there are two b-value abnormal areas 40 km and 70 km northwest of the Yangbi earthquakes. It means that there is a risk of strong earthquakes in the abnormal areas in the future.