Seismicity of south-western Bulgaria and border regions

• Earthquakes that occurred between 1985-1991 in SW Bulgaria and border regions with Greece and the Former Yugoslav Republic of Macedonia (bounded by 22.0°E-22.4°E and 40.8°N-42.4°N) are located to define the characteristics of the present seismicity. The hypocenters of 2657 earthquakes with magnitudes 0.3 < ML < 5.1 are determined using the program HYPOLOC (modification of the HYPO71 program) and a velocity model is obtained for the local area of SW Bulgaria. Earthquake tend to cluster in seven areas. The seismic activity in the clusters changes with time and correlates mainly with the occurrence of the strongest events in each cluster. The clusters are classified by the density of earthquake epicenters. Following this classification, those with the 'highest' seismicity are characterised as the Krupnik cluster, and those with the 'lowest' seismicity, the Butkovsko (Kerkini) Lake cluster. The location and shape of the cluster areas are related to boundaries between areas of subsidence (grabens, depressions) and areas of uplift (horsts, swells). A few areas without seismicity, which are related to horst structures are outlined. Historical data exists for each of the obtained cluster areas (from 52) for earthquakes of magnitude larger than 5.0.

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