Abstract: On the basis of earthquakes observed by the Qiaojia array from March, 2012 to December, 2019, we investigated the temporal evolution of seismic strain release. We considered the slope K of cumulative seismic strain release curve as a function of time. The results show that before the Ludian M_S6.5 mainshock, the slope K increased rapidly for the deep earthquakes (focal depth h≥10 km), and remained stable for the shallow earthquakes (focal depth h<10 km), i.e., there was a significant accelerating seismic strain release in the deeper crust, but no significant accelerating seismic strain release nor significant decelerating one in the shallower crust. After the Ludian earthquake occurred, in the deeper crust k value dropped rapidly, corresponding to the decelerating seismic strain release, and approached the lowest value in early 2016, and then maintained at low values up to the end of 2019. In this period no strong earthquake occurred in and around the Qiaojia array. The power index m estimated by the seismic strain release theoretical model is 0.177 during the early stage of k value rising process, and 0.902 during the later one for deep earthquakes before the Ludian M_S6.5 mainshock, indicating that the strain release accelerated very rapidly in the deeper crust in the early stage of k value rising process. The above observations may avail to take a deeper insight about the earthquake gestation process and earthquake prediction.

Key words: The Ludian M_S6.5 mainshock, The Qiaojia seismic array, Seismic strain release, Accelerating