ON THE SEISMIC ACTIVITY OF MEDIUM-SIZE EARTHQU- AKES BEFORE LARGE EARTHQUAKES OF MAGNIT- UDES GREATER THAN OR EQUAL TO 6.7 IN YUNNAN PROVINCE

In this paper, the seismic activity of medium-size earthquakes before five large earthquakes (Tonghai, Yongshan, Longling, Ninlang, Puer) of magnitudes M 6.7 occurred in Yunnan Province has been analysed. It is revealed that (1) one year or little longer time before the large earthquakes, seismic activity of medium-size earthquakes is distinctly enhanced, reaching 20 or more earthquakes of Ms 4 and 6 or more earthquakes of Ms 5 per year respectively; (2) at the same time, b-values within the earthquake regions decrease correspondingly; (3) few months to one year before the large earthquakes, those medium-size earthquakes scattering over very broad areas begin to move toward the epicenters of the coming large earthquakes. They gathered onto intersecting belts in the case of the Longling and Tonghai earthquakes.The phenomena may be related to the crustal structure of Yunnan and the process of development of the earthquake sources. All such medium-size earthquakes are given a term general foreshocks in this paper.