ON THE PRECURSORY SEISMIC GAPS BEFORE TWO MEDIUM-SIZE EARTHQUAKES OCCURRED IN 1974 AND 1979 IN LIYANG COUNTY, JIANGSU

Two earthquakes (ML = 5.8 and 6.3) occurred in Liyang County, Jiangsu Province, on April 22, 1974 and July 9, 1979 respectively. Seismic gaps appeared within about 23 months before each of these two earthquakes in a region of about 200 km in radius around the epicenters. During 16 months before the 1974 earthquake of magnitude 5.8, not a single small earthquake occurred in an areas of radius 240 km around the mainshock. Seven months prior to the. mainshock, the gap began to be activated and a series of small shocks broke out. Fifteen months before the 1979 earthquake of magnitude 6.3, inside an area of radius 100 km and outside an area of radius 240 km, there occurred a number of small earthquakes, thus forming a doughnut-shaped seismic gap, while during 7 months before the mainshock no small earthquake occurred in an area of radius about 200 km.Based on the distribution pattern of the isoseismal lines of both mainshocks and their aftershocks as well as the focal mechanism solutions of the two earthquakes, it could be inferred that both earthquakes are the result of the action of compressive stress in the NEE direction and tensional stress in the NNW direction. They are conjugate earthquakes of the same focal region at two different times. The processes of formation, development and disappearance of both of the seismic gaps imply the process of evolution of the heterogeneous strain of the semi-homogeneous crust under the continuous action of the ambient regional stress field.