Temporal variation of v_P/v_S at single seismic station before the 2013 Lushan M_S7.0 earthquake

Based on the seismic phase data recorded by Sichuan Digital Seismic Network from January of 2001 to July of 2013, this study analyzes the spatio-temporal variation of wave velocity ratio v_P/v_S before the Lushan M_S7.0 earthquake by using Wadati method of single station and multi-earthquake. To ensure the accuracy and stability of the calculated results, we carefully winnow the data. Moreover, we compare the wave velocity ratios calculated by using phase data recorded by the stations with different azimuths and different epicentral distances, so that we discuss the size of potential seismogenic body for future earthquakes. Our results show that, in the southern Longmenshan fault and its surrounding area, there was an obvious decrease in wave velocity ratio at five stations (XJI, MDS, YGD, HMS and WMP) before the Lushan M_S7.0 earthquake. In particular, for the four stations (XJI, MDS, YGD and HMS), there had been an obvious v_P/v_S decrease for 3.5 years before the Lushan earthquake; except the station XJI, at other three stations appeared a recovery of v_P/v_S ratio one year before the earthquake. The anomalous characteristics are largely different for the stations EMS and WMP, indicating some complexity. Tracing the temporal variations of v_P/v_S, it is also found that the extent of the region with anomalous wave velocity ratio tends to concentrate at the future epicenter and its surrounding regions.