P-wave velocity changes in the source area before and after two moderate strong earthquakes in Yingjiang， Yunnan in 2014

The time-lapse tomography method based on double-difference tomography can obtain high-precision seismic wave velocity changes. In this study, we applied this method to the seismic data set that recorded by the Yunnan seismic network to obtain the spatiotemporal changes of P-wave velocity in the source area during the M_S5.6 and M_S6.1 Yingjiang earthquakes in 2014. The results showed that the spatial and temporal characteristics of the P-wave velocity of the crust at depth around the source area suggest co-seismic velocity reductions. After the M_S5.6 earthquake occurred on May 24, the P-wave velocity of the crust dropped slightly. After the M_S6.1 earthquake occurred on May 30, the P-wave velocity dropped definitely with an amplitude of about 1%. It showed that the amplitude of the reduction is related to the magnitude of the mainshock. We also reported that P-wave velocity is also related to the the spatio-temporal changes of aftershocks distribution. It is possible that of dynamic and static stresses changes of the aftershocks caused changes of the physical properties in the source area which leads to the velocity change. Finally, about five years after the Yingjiang earthquakes, a healing process is observed. And the rising level did not reach the previous falling level. However, this does not mean that the source area is still in the healing process.