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.