山西中强地震前波速比异常研究

A STUDY ON THE V_P/V_S RATIO ANOMALY BEFORE MODERATELY STRONG EARTHQUAKES IN SHANXI PROVINCE

本文分析了大同—阳高6.1级(1989)、5.8级(1991)、忻州5.1级(1991)、侯马5.1级(1989)以及太原、运城1989～1991年几个4.0级左右地震前后的202次小震资料。结果表明:这些主震前都存在明显的地震波速异常变化,纵横波速比V_P/V_S表现为下降—回升的异常形态,负异常持续时间与震级大小相关;主震在负异常结束后发生,滞后时间随区域地震震源深度增大而延长。同时,主震前还存在波速比低值异常区,形态和范围受震级和区域构造的制约,主震震中均落在异常区内的边界空段和波速比值最低点处,并远离异常区形成始震地点。若同一地区不同年月再发生较大地震,震前还会出现上述波速异常,异常区呈叠加、迁移或范围改变现象。

The earthquakes with magnitudes of 6.1(1989), 5.8(1991) in Datong-Yanggao, 5.1 (1991) in Xinzhou and 5.1 in Houma, along with 202 earthquakes before and after several earthquakes with ML = 4.0 or so during the period from 1989 to 1991 in Taiyuan and Yuncheng are analyzed in this paper. The results show that: the obvious variations of seismic wave velocity anomalies existed brfore these main shocks. The Vp / Vs ratio has anomaly with decreasing first and then increasing. The correlation between the continuous time of negative anomaly and the seismic magnitude is found. The main shocks occur after the negative anomaly ended ans the lag time extends with the increase of the seismic focal, depth. The low value anomalous regions of the Vp / Vs ratio exist before main shocks, their shape and range are related to the seismic magnitude and regional structure. All main shocks occur at the localities such as boundary gap segment and the locality where the Vp/ Vs ratio is lowest in the anomalous area. The epicenter of the first earthquake is far from the anomalous area. If a stronger earthquake occurs in the same area in the future, the were velocity anomaly occurs before the earthquake, the anomalous area will show some phenomena, such as lapping, migration and range change.