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

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.