Seismotectonic model for the earthquake swarm of 1985–1986 in the Vogtland/West Bohemia focal area

Abstract

After decades of minor seismic activity an intensive earthquake swarm occurred in 1985–1986 with a maximum activity in December 1985 and January 1986, in the well-known narrow focal zone of swarm quakes in the border region of the G.D.R. and Czechoslovakia. Local seismic networks recorded more than 8000 events during this swarm, the strongest local magnitude being 4.6. Nearly all of these swarm events were located within a relatively small focal volume 3.5 km in length, 1.5 km in width, and at depths of 6–8 km. Fault-plane solutions could be determined for 17 of the strongest events. They provided the basis for seismotectonic modelling in connection with (1) detailed tectonic data (including faults indicating conditions at the seismogenic depths, and recent active faults), (2) crustal stress data, and (3) studies on recent horizontal movements of crustal micro-blocks. According to detailed local micro-earthquake monitoring since 1962, all epicentral areas of swarms are aligned along the N153°E striking Mariánské Lázně fault (MLf), which trends approximately in the azimuth of maximum compressive stress. The MLf is intersected by a bundle of recent active, lamellar N-S to NNE-SSW fault elements. The fault-plane solutions of 14 of the events studied show strikes in this direction. The others strike along the MLf. Tectonic evidence for the conjugate planes is lacking. Both the MLf and the N-S fault elements show a tendency to dextral creep according to neotectonics and recent horizontal crustal movements, whereas a generally sinistral elastic rebound has been derived for the focal process of the swarm. This reflects alternating block movements which could also be deduced from repeated geodetic measurements. Probably, the MLf system provides a suitable setting for the foci only, whereas the faulting process itself occurs mainly on secondary faults within the system. Moreover, a splay-structure along the MLf, immediately south of the 1985–1986 epicentral area and connected with a right-stepping offset along a N-S fault element, substantiates the proneness to the occurrence of swarm-like seismic activity.