The swarm seismic activity (august 2001) near Yambol city – Central East Bulgaria

Abstract

The paper describes the investigations of the local seismic swarm occurred near Yambol and surroundings during August 2001. The main event occurred on 13 August (origin time 14:26: 12.05 GMT, magnitude more than 4.5, coordinates 42.57 N, 26.48 E, approximate depth – about 15 km, maximum observed intensity – VI MSK). It was preceded and followed by several felt earthquakes and many (more than 100) not felt, registered only by the local seismic station 1MB earthquakes. The development of the seismic process is discussed. The recent seismic activity decrease very fast in time – no more than two weeks, which is unusual behavior of such seismic process. The comparative study about the relationships between this recent swarm and the swarm occurred during 1909 shows some similarities and some differences discussed in the paper. For the first time the full catalogue of this swarm is published. It consists of two parts. Table 3 – the earthquakes registered on more than 3 seismic stations (which allows the exact parameters determination) and very weak seismic events, registered only by the local station 1MB. For these events only the exact time and the magnitude is possible to be determined (Table 4).