岫岩地震序列的平均波速比前兆异常

主要利用辽宁数字地震台网的数据并补充部分模拟台网的资料,得到岫岩地震序列1999年11月～2000年5月2级以上地震的P、S波到时,共计包含302个事件接近12000个到时数据,纵、横波到时差约4～32s,选取同时被5个或5个以上台站所记录到的地震,由最小二乘法计算平均波速比.为了保证结果的可靠性,计算每次地震的平均波速比所使用台站的数目在20个左右;全部拟合结果的相关系数超过0.99,标准误差s小于1,s的均值为0.48.全部地震波速比的平均值是1.71,稍低于线弹性体的比值1.73.分析认为：事件通过相对定位法修订震源位置,绝大多数地震的震源深度在10～12km.那么地震波射线经过的路径是从此深度到地表台站,平均波速比所反映的是地壳上层的状态.波速比的滑动平均值在1999年11月29日的5.4级主震和2000年1月12日的5.0级强余震前出现连续低值的异常现象.讨论认为5级震前的异常变化结果可信.

Anomalies of mean VP/VS ratio in the series of Xiuyan earthquake

The arrival times of P and S wave for the over Nov. of 1999 to May of 2000 are obtained with the data M2 earthquakes in the series of Xiuyan earthquake from 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.