ANOMALIES OF MEAN VP/VS RATIO IN THE SERIES OF XIUYAN EARTHQUAKE

The arrival times of P and S wave for the over M2 earthquakes in the series of Xiuyan earthquake from Nov.of 1999 to May of 2000 are obtained with the data recorded by Liaoning Digital Seismic Network and some other analogue networks.The results contain 302 events and nearly 12000 arrival times, in which the arrival time difference is about 4-32s.The mean VP/VS ratio is calculated with least squares method when 5 or over 5 records are selected.In order to ensure the reliability of the results, the number of stations that used in calculation for each earthquake is at about 20.The correlation coefficient of the fitting results is over 0.99, standard error s is less than 1, and mean s is 0.48.The mean value of the total VP/VS ratios is 1.71, which is a little lower than the ratio 1.73 of linear elastic body.The depth of hypocenter for most earthquakes located with relative locating method is at about 10-12km.The path of seismic wave ray was from this depth to the surface station, and the state that mean VP/VS ratio reflected was that of the upper crust.The moving average value of VP/VS ratio showed continuously low value anomaly both before the M5.4 main shock occurred on Nov.29, 1999 and the M5.0 strong aftershock occurred on Jan.10, 2000.We believe these anomalies are credible.