On the possibilities of premonitory swarms for three sequences of earthquakes of the Burma—Szechwan region

Abstract

Short term spatial and temporal variations in seismicity prior to the three sequences of earthquakes of m b ⩾ 5.8 of the Burma—Szechwan region are studied. Six years (1971–1976) of ISC seismicity data, as reported in the Regional Catalogue of Earthquakes, are considered. During the period, six earthquakes of body wave magnitude m b ⩾ 5.8 occurred in four sequences. Of these, three sequences are preceded by swarm activity in the epicentral regions. Evison (1977b) suggested that the swarm before the sequences of large shocks is a possible long-term precursor. He derived the conclusion by analyzing earthquakes in New Zealand and California. The analysis of the seismicity data for the region under investigation supports Evison's view and suggests that a relation between swarms and sequences of large events exists. The precursory time period (i.e. the time from beginning of the swarm to the main shock) for the Szechwan earthquakes of m b = 5.9 (Feb. 6, 1973) and m b = 5.8 (May 10, 1974) and the Burma earthquake of m b = 6.2 (Aug. 12, 1976) are 305, 317 and 440 days, respectively.