Tatiana Toteva1, Blagovesta Babachkova2, Snezina Rizikova2

1 School of Earth and Atmospheric Sciences, Georgia Tech, 221 Bobby Dodd Way, Atlanta, GA 30332-0340, e-mail:

2 Department of Seismology, Geophysical Institute - BAS, Acad.G.Bonchev Bld., Block3, 1113 Sofia, Bulgaria

During the period 1997-1999, four earthquake swarms occurred in the southern part of Rila Mountain, SW BULGARIA. Rila Mountain is north of the most active part of the country, the Kresna seismic zone. The Kresna seismic zone was the site of two very strong earthquakes. They occurred in 1904, M=7.2 and M=7.8, within an interval of twenty minutes and caused much damage and human suffering. The second one is considered the strongest crustal seismic event in Europe for the last two hundred years.

The Kresna seismic zone remains the most active area in Bulgaria. Almost 2500 earthquakes of magnitude greater than 1.0, have been detected for the last ten years. An analogue earthquake detection system has operated from 1980. The seismic network consists of fourteen short-period vertical seismographs, distributed over the country. There are also two local networks in northern Bulgaria. Although Rila Mountain is in the most active part of the country, it was not known as a seismically active area. Almost no activity was detected there before 1997. The first swarm occurred in the end of June, 1997, and continued to the middle of July, with a maximum rate of twelve quakes per day. The strongest one occurred seven days after the initial activation and had a magnitude of M=3.4. Some months later, from the middle of November to the end of December, 1997, there was a larger swarm. More than ninety quakes were detected and the maximal rate of activation was twenty-two quakes per day. The strongest earthquakes (there were three with magnitudes M=3.5) were realized five to six days after the activation. The next swarm, in January 1999, consisted of about fifty earthquakes and continued only seven days, but had the highest rate of activation (twenty-two quakes) in the first day. The strongest quake (M=3.5) was realized in the last day of the swarm. The last swarm (August 1999) was the smallest one, less than forty earthquakes were detected during all the month. The strongest two had magnitudes M=3.1 and occurred in the beginning and in the end of the swarm. The hypocenters for all the swarms were above 33 km. Some of earthquakes were felt at intensity III-IV (MSK-64). The epicentral area of the swarms and the locations of recently recognized contemporary active faults seem to be associated.