THE DENSE--QUIET CHARACTERISTICS OF SMALL SHOCKS BEFORE THE MAJOR EVENTS OF THE 1966 XINGTAI EARTHQUAKES

The dense-quiet characteristics of small shocks prior to major shocks of the 1966 Xingtai earthquakes is investigated in this paper using an earthquake catalogue given by the Xingtai seismic network and so a 3-D picture of hypocentral distribution is established. Results show (1) small shocks before all major earthquakes of M6.0 that occurred in the Xingtai region in March 1966 are characterized by the dense-quiet phenomena,but is less distinguishable as time increases. Similar features are noted in small shocks before some of those Ms=4.5 strong shocks. In such cases,however,it is rather complicated as the small shocks emerge on the background of aftershocks of much stronger earthquakes of M6.0. The duration of a relative quiet period as well as the magnitude of the largest shock in a dense period is related to the magnitude of the succeeding major event. In general the higher the magnitude of the major event is,the longer the quiet period will last and the higher the magnitude of the largest shock in the dense period will be; (2) The dense-quiet process of small shocks before the Ms=7.2 mainshock of March 22 was clearly a physical process which was revealed by spatial distribution maps of the hypocenters. After the Ms=6.8 earthquake of March 8,the breaking strength of the crust was reached first at the southwest end of the aftershock zone,where the medium was weaker,causing a series of minor and moderate shocks on March 20. This corresponded to the dense period prior to the main shock of March 22. By contrast,the time period from March 21 to 22 before the main shock was the relatively quiet period during which seismicity was evidently lower and had a tendency to migrate from the SW end to the NE. This indicated a rapid change in the stress field of the earthquake area and a further transfer of stress concentration from the SW end to the NE end. It was responsible for the instability at the tip of the NE end that resulted in the Ms=6.7 and 7.2 major shocks of March 22. The same was true for the Ms=6.2 strong shock of March 26.In addition,the present paper intends to give a tentative explanation for the dense-quiet characteristics of small shocks before a strong earthquake.