唐山、海城、盐源-宁蒗地震强余震特征

对大地震发生后强余震的震源机制,基于主要地震断层上小余震的时空分布进行了详细研究.近期在中国境内发生的强烈板内地震,如1976年唐山、1975年海城、1976年盐源——宁蒗等地震曾被中国地震台网很好记录.主震刚发生后的小余震以及强余震前后的小余震的震源用 S——P 时间的收敛法进行测定.强余震的断层是依靠这些小余震的震中分布来描绘的.可以看出有三种情况.第一,在主震断层区内,一次强余震发生在较小余震的比较不密集的部位;第二,一次强余震沿断层发生并与主震断层是共轭的;第三,一次强余震在沿平行并离开主震断层的断层上发生.可以认为,地质条件控制着强余震的发生.作为强余震的可能前兆,则有唐山地震的一次强余震,震级为7.1.其发生前,出现一些前震以及在接近主震断层方向上的某些地震台所记录的初动方向有变化.可以设想,这里的应力场在强余震发生前是有所变化的. 

ON SOME CHARACTERISTICS OF STRONG AFTERSHOCKS OF THE 1976 TANGSHAN, 1975 HAICHENG AND 1976 YANYUAN-NINGLANG EARTHQUAKES

The source mechanisms of strong aftershocks, that occurred after large earthquakes, have been closely investigated on the basis of the space-time distribution of smaller aftershocks around the mainshock fault area.Large intra-plate earthquakes that recently occurred in China, the 1976 Tangshan, 1975 Haicheng and 1976 Yanyuan-Ninglang earthquakes were well recorded at network stations over China. The hypocenters of smaller aftershocks just after the respective mainshocks and before and after several strong aftershocks weve located by a convergence method using S-P time. The faults of the strong aftershocks were traced from epicentral distribution of these smaller aftershocks. It was found that there are three different cases. In the first case, a strong aftershock occurred in a less concentrated portion of minor aftershocks within the mainshock fault area. The second case is that a strong aftershock along a fault being conjugate to the mainshock fault, and in the third case, it took place along a fault which is nearly parallel to and apart from the main shock fault. It might be concluded that geological conditions control the location where strong aftershocks occur.As possible precursors of the strong aftershocks, it was found that there were some foreshocks prior to a Tangshan strong aftershock with a magnitude of 7.1 and the change of the first motion direction in some stations located near the directions of mainshock faults. It may be considered that the stress field around there changed to some extent before the occurrence of the strong aftershocks.