STUDY OF EARTHQUAKE SWARMS IN RELATION TO LARGE EARTHQUAKES (NORTH CHINA AREA)

From 1956 to 1976, there occurred thirty-seven earthquake swarms in North China, (i.e. east of 106 E, north of 33N, south of 42 N and west of Yellow sea). Among these, the number of earthquake swarms occurred just after the 1966 Xingtai (邢台) Earthquake to the 1976 Tangshan (唐山) Earthquake is about three times more than the number of swarms occurred in the period of 1956-1966. During the last ten years, all of the large earthquakes (M>7.0) occurred on the background of a continuously increasing activities of earthquake swarms.In view of the spatial distribution, the scope of activity of earthquake swarms) is limited in smaller areas, close to the epicenters of future large earthquakes. However, since 1974, the swarm activity has been clearly enlarged in spatial distribution. Around the epicenter of the subsequent Tangshan Earthquake, both in the NE-SW and NW-SE directions it was lengthened by more than 1000 kms, in less than half a year before the earthquake.Such a phenomenon of seismic activity spreading out from the epicenter of the impending major earthquake with the lapse of time can also be noticed before several historical earthquakes, the average spreading rate may be roughly estimated as about 100 km/year.Besides, it is also observed that precursory earthquake swarms, appear several months to one or two years before the impending large earthquakes at a not too far distance (about 100 kms).The possible differences between the precursory earthquake swarms and the ordinary ones in focal mechanism, dynamic source parameters, b value as well as the amplitude ratios of P and S waves have-been analysed. Preliminary results show that the stress drops for precursory earthquake swarms are higher. Finally, the physical and tectonic conditions under which precursory swarms might be observed are briefly discussed.