ON ACTIVE-QUIET PATTERN OF MICROEARTHQUAKES IN AND AROUND THE EPICENTERS PRIOR TO SOME MODERATE EARTHQUAKES IN NORTH CHINA

In this paper, the time-space distribution patterns of microearth-quakes before 11 moderate earthquakes occurred during 1970-1979 in North China have been studied. It is found that active-quiet patterns of microearthquakes, 1-2 years before most of the moderate earthquakes prevail in areas of radii 70-120km around the epicenters (earthquake precursory areas). At the same time, these microearthquakes seem to undergo a process from gradual spreading to contraction until the occurrence of the moderate earthquakes.The probability of the microearthquake frequencies for the active-quiet stages is lested by using the Method of Poisson Distribution. As a result, the probability of the majority of microearthquake frequencies for the active stage is less than 5%. Besides, in the earthquake precursory areas, the rate of change of the curve of accumulated micro-earthquake frequencies before most moderate earthquakes showa a spreading and slowing down process; but in the areas around this rate keeps basically unchanged.Finally, a model of the typical anomalous pattern before moderate earthquakes is given and preliminarily discussed.