Abstract: The multi-station method and fixed-station method are applied to calculate velocity ratio of four earthquake sequences, which are Yao'an M6.5 in 2000, Shidian M5.0 in 2001, Yongsheng M6.0 in 2001 and Dayao M6.2, M6.1 earthquake sequences in 2003. The accuracy for reading arrival time of waveform is 0.02 seconds when using digital data. The confidence is at least 95% on calculating velocity ratio when considering station factor. The results show that there is a decreasing trend of velocity ratio before strong aftershocks. It is of practical value on the short-imminent term prediction of strong aftershocks. In addition, the comparison of the variation range of velocity ratio between the time when mainshock occurs and the time after mainshock can be as a reference index on studying seismic tendency of earthquake sequence.

Key words: Earthquake sequence, Multi-staiton method, Fixed-station method, Velocity ratio anomaly, Yunnan region