Abstract: On June 17, 2019, a M_S6.0 earthquake occurred in Changning, Sichuan Province. Within four months after the earthquake, the seismic activity in the focal area presented features of high frequency, high intensity and slow attenuation. During the development and evolution of this earthquake swarm, tracking and studying the characteristics of aftershock activity, including the b-value that describes the proportional relation between large and small earthquakes and the change of stress during the development of the sequence, is of great significance to monitor and analyze the seismic risk. On the basis of accurate relocation of the seismic sequences by using the seismic catalogue before and after the earthquake in the focal area, the spatial and temporal distribution of the b-value were calculated. The spatial distribution of b-value shows that, before the earthquake, the b-values near the epicenter were significantly lower than its neighboring area; after the earthquake, the b-value of the sequence began to recover from southeast to northwest, and then its distribution was relatively even. The temporal variation of b-value indicates that, after the earthquake, the b-value decreased to an extremely low level in a short time and then began to rise; during this period, b-value oscillated many times, and most strong aftershocks occurred in the process of b-value decreasing; till the end of August 2019, the seismic activity of this sequence was still active.

Key words: Changning earthquake sequence, Seismicity, Double-difference location, b-value