SOURCE PARAMETERS OF SMALL EARTHQUAKES BEFORE AND AFTER THE 1982 LULONG EARTHQUAKE

On 19 October 1982,an earthquake (3957'N,11904'E) with ML = 6.2 (according to Catalog of the Beijing Telemetered Seismic Network) occurred in Lulong on the northeastern edge of the aftershock zone of the Tangshan 7.8 earthquake of 1976. Earthquakes in the Tangshan-Lulong region from January 1980 to June 1984 have been examined for monitoring the variations in source parameters of small earthquakes before and after the Lulong earthquake. Source parameters of 160 earthquakes,2.7ML4.3,were determined using the P-wave first motions. In the magnitude range studied the P-wave first impulse width and the source radius do not increase with ML,and the stress drop increases with ML. The values of stress drop vary in the range from 0.1 MPa to 50 MPa,and are concentrated primarily between 1 MPa and 5 MPa; so in this paper stress drops of 5 MPa and more are taken as high stress drops and that of 1 MPa and less as low stress drops for this region. The spatial distribution of earthquakes in this region shows an apparent relation between high stress drop events and succedent ML5 earthquakes. Within one year and more before the Lulong earthquake,in the vicinity of its epicenter occurred two high-stress drop events,but in the whole reigon the number of high-stress-drop events decreased and that of low-stress-drop events increased.