Abstract: Based on the earthquakes recorded in Yibin area (28.0°N~28.5°N, 104.6°E~105.1°E), Sichuan Province, from January 2014 to August 2019, we estimated the b values in the studied area by the maximum likelihood method and obtained their temporal and spatial characteristics. The results show that, firstly, the b value decreased rapidly five months before the Changning M_S6.0 earthquake and recovered quickly after that. This phenomenon is likely to reflect the process of stress accumulation during earthquake preparation and stress release after the earthquake. Secondly, before the Changning earthquake, there was always a low b value anomaly (≤0.85) in the epicenter and its vicinity. Five months before the earthquake, a further decrease of b value occurred near the epicenter, especially in the northwest of Changning, where following three M_S5 earthquakes taking place in Gongxian were just located in this abnormal region and its edge area. Thirdly, the spatial distribution of low b value anomalies indicates the risk area of medium and strong earthquakes in the future. And, the fast decrease region in b value can be used to determine the possible location of strong aftershocks. Fourthly, when analyzing the seismic risk, we should pay more attention to the b value decrease under the background of long-term low b value, which illustrates the increasing seismic risk in high stress concentration area.

Key words: 2019 Changning M_S6.0 earthquake, b value, the Maximum Likelihood Method, Seismic risk