四川地区强震前弱震空区的变化特征

TEMPORAL AND SPATIAL CHARACTERISTICS OF SEISMIC GAPS FORMED BY WEAK EARTHQUAKES BEFORE STRONG EARTHQUAKES IN SICHUAN PROVINCE

  • 摘要: 本文使用四川台网地震目录全部资料,适当参考邻省地震目录,对于四川省从1970年下半年以来发生的15组5级以上地震(另有5组缺乏完整资料无法讨论),作 R-t 图、D-t 图寻找时间空区,并作相应的平面空区图.通过对比分析,发现6组6级以上和4组5级地震前震中附近出现空区现象.根据形成空区的变化过程可分为先扩(大)后缩(小)型和逐渐扩大的开放型两种类型.平面空区的位置和形状往往与构造展布有关.有5组5级震例,或因震前震中区小震不断,或因震中及其附近地区小震太少,看不出明显的空区.本文利用出现空区的10个震例,总结出空区参数与未来地震的震级的经验关系:M=5.50 lg Rmax-2.331.32 M=2.98 lgT+2.941.06 M=2.52lg(RmaxT)-0.510.96文中将上述结果与大竹政和的结果作了对比.空区不同变化形态可用金森博雄概括的突起点模式加以解释.

     

    Abstract: This work is based on the earthquake catalog prepared by the Sichuan seismological network, with reference to some earthquake catalogs of the neighbouring provinces. The earthquakes with magnitude greater than 5 that occurred during the,period from June 1970 to October 1981 are divided into 20 groups, 5 of which are excluded due to the lack of enough data. For the remaining 15 groups plots of R-t (radial distance with time) or D-t (distance along a belt with time) are made to show seismic gaps with time. Also, epicenter distribution maps are drawn to show seismic gaps in space. Through comparative analysis, it is found that in each of the 6 groups of earthquakes with maximum magnitude greater than 6 and in each of the 4 groups of'earthquakes with maximum magnitude between 5 and 6, a seismic gap appeared around the epicenter of the impending strong earthquake. According to the process of development of the seismic gaps, they may be divided into two categories: expanding and then, contracting and gradually expanding. The location and shape of the spatial gags are usually related to the regional tectonics. For the other 5 of the 15 groups also with maximum magnitudes between 5 and 6, seismic gaps did not appear. The reason may be either that earthquakes occurred incessantly in the impending epicentral area or there are too few weak earthquakes in the epicentral area and its vicinity before the main shocks.From the data of the groups of earthquakes associated with seismic gaps three empirical formulas connecting the seismic gap parameters with the magnitudes of the impending earthquakes are obtained. They are M=5.50 lg Rmax-2.331.32 M=2.98 lgT+2.941.06 M=2.52lg(RmaxT)-0.510.96 These relations are compared with that obtained by M. Ohtake. It seems that the asperity model proposed by H. Kanamori could be used to explainthe difference in the behaviour of the seismic gaps by assigning proper parameters to hismodel for different cases.

     

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