Abstract: Based on seismic waveform data recorded by National Digital Seismic Network from April 20,2013 to December 2013, we calculated the source spectrum parameters of earthquakes with M_L≥3.0 of the Lushan earthquake sequence, using the ratio of P-wave to S-wave corner frequency and the ratio of P-wave to S-wave long-period spectral level to calculate the velocity ratio. The results indicate that: ① The average V_P/V_S obtained from the ratio of corner frequency is 1.27 while the average V_P/V_S obtained from the ratio of long-period spectral level is 1.67, which is closer to the value 1.73 of Poisson medium. ② Velocity ratios obtained from the two methods have a common variation tendency: after the mainshock the velocity ratio was at a low value anomalies until the last M_S5.4 aftershock , since then the velocity ratio recovered to the level above the average. ③ The velocity ratio anomaly calculated from the long-period spectral level behaved more evidently, it presents a ‘decline-recovery-occurrence’ process before the three M_S5.4 aftershocks. ④ Correction for medium attenuation has no big effect on the results getting from the two methods. ⑤ The velocity ratios obtained from single seismic station and multi-station have the same variation tendency.

Key words: Velocity ratio V_P/V_S, Source spectrum parameters, The 2013 Lushan earthquake sequence