邢台余震区的震源参数

SOURCE PARAMETERS IN XINGTAI AFTERSHOCK ZONE

在1966年发生6.8和7.2级邢台地震的余震区里,从1976年1月至1985年11月已记录到约6000次地震,其中最大的两次地震是1981年11月9日M_L=5.8和1985年11月30日M_L=5.6的地震。本文利用P波初动半周期,测定了这一时间段里发生的69次M_L=2.7—4.4的地震的震源参数。应力降在0.01兆帕到10兆帕的范围内变化,且主要集中在0.1兆帕和1兆帕之间,因此取1兆帕以上为高应力降,用以评定该区的应力积累情况。震源半径的变化范围大,但在M_L≤3.7的范围内不随M_L而变大,而应力降则随M_L而增大。1981年的5.8级地震和1985年的5.6级地震,虽然在震前、震后的地震活动上都完全不一样,但其共同点是:震前出现的高应力降地震比较多,且其应力降的数值有随时间增高的趋势。

In the aftershock zone of the Xingtai 6.8 and 7.2 earthquakes in 1966, about 6000 earthquakes had been recorded between January 1976 and November 1985. The two largest are ML = 5.8 earthquake on November 9, 1981, and ML = 5.6 earthquake on November 30,1985. Source parameters of 69 earthquakes which occurred in the period with ML from 2.7 to 4.4 had been determined by using p-wave half period of first break. The stress drops vary in the range from 0.01 MPa to 10 MPa and are concentrated mainly between 0.1 MPa and 1 MPa, so 1 MPa and more have been taken as high stress drop for evaluating the tectonic stress accumulation in this region. The source radii vary widely, but within the range of ML≤3.7 do not increase with increasing of ML. However, stress drop increases with ML. Before the ML=5.8 in 1981 and the ML=5.6 in 1985 earthquakes more frequently occurred high stress drop events, and their values of stress drop increased with time, though the seismic activities were different both before an after the two earthquakes.