SEISMIC REGIME OF THE WEST BOHEMIAN EARTHQUAKE SWARM REGION: PRELIMINARY RESULTS

The western part of the Bohemian Massif located between two tectonic units, the Moldanubian and the Saxo-Thuringian, is characterized by the re-occurrence of earthquake swarms. The focal region for these swarms includes the territory of West Bohemia and the adjacent territory of SE Saxony and NE Bavaria. During the most recent swarm in December 1985 - January 1986, more than 8000 small earthquakes were recorded: the two largest earthquakes with local magnitudes (MD of 4.6 and 4.1. This paper presents a summary of the seismic energy release in space and time for the western part of the Bohemian Massif based on seismic observations of permanent seismic stations established in West Bohemia since 1986. It was found out that micro-earthquake activity, mostly of a swarm-like character, persisted between two macroseismically observed swarms. The foci of the microearthquakes predominantly cluster in six main epicentral zones, four of which are located in West Bohemia or in its immediate vicinity in Saxony The remaining two are in Saxony and in Bavaria. The four epicentral zones in West Bohemia were studied in detail. It was found that the individual zones differ in size, in depth of hypocentres, in geometry, as well as in temporal activity. Moreover, it was found that the seismicity in the most active epicentral zone is closely related to the system of principal tectonic faults referred to as the Krusne Hory fault and the Marianske Lazne fault.