THE "DENSE-QUIET" CHARACTERISTIC OF SMALL EARTHQUAKES BEFORE 1966 XINGTAI STRONG EARTHQUAKES AND ITS PRELIMINARY EXPLANATION

The "dense-quiet" characteristic of small earthquakes before Xingtai strong earthquake and its spatial distribution are studied with the data recorded by Xingtai Seismic network.The results show:(1) there was "dense-quiet" phenomenon before all the over 6 magnitude earthquakes in Mar.1966,but its clear level fell with time.The phenomenon was also showed before parts 4.5 magnitude earthquakes.Because they appeared in the aftershocks of over 6 magnitude earthquake,their situation relatively complicated.Compared the quiet period,the magnitude of biggest earthquake in dense period all had some relation with that of the main earthquake.Generally speaking,the larger the magnitude of the main shock,and the longer the dense period,the bigger the magnitude of the earthquake in dense period.(2) The "dense-quiet" process before M7.2 earthquake occurred on Mar.22 was very clear in spatial.distribution.After the occurrence of M6.8 earthquake on Mar.8,a cluster of small earthquakes occurred at the southwest end of the aftershock region,which might be corresponding to the dense period before the main shock.On Mar.21~22,small earthquakes reduce obviously with a tendency to transfer from southwest to northeast,which might be the quiet period before main shock.The transfer of minor earthquakes indicated that the stress field in seismogenic zone changed rapidly,and concentrated from southwest to northeast,and with the destabilization at the northeast end on Mar.22,the M6.7 and M7.2 major shock occurred.The "dense-quiet" process before M6.2 earthquake occurred on Mar.26 was also showed clear image in space.