The August–December 2000 earthquake swarm in NW Bohemia: the first results based on automatic processing of seismograms

Main features of the August–December 2000 earthquake swarm which occurred in the major focal area of the North-West Bohemia / Vogtland swarm region are presented. Seismograms from four stations of WEBNET were automatically processed to get arrival times, first motion amplitudes and hypocentre coordinates of a representative set of events. Altogether 7017 microearthquakes in the magnitude range of ML=0–3.3 were identified. It is shown the decay of activity of individual swarm phases followed the modified Omori law, which points to a partial similarity with aftershock sequences of tectonic earthquakes. The space-time distribution of a subset of 2913 events with low location residuals shows a strong space clustering of the earthquake hypocentres and their pronounced migration between individual swarm phases. Most of the activity took place along an elliptical, nearly vertically dipping, 6 km long N-S oriented fault plane in depths ranging from 10.5 to 6.5 km. The P and T axes were estimated by FOCMEC for the 782 strong events and three groups of earthquakes with similar faulting type were distinguished. In contrast to the normal and strike-slip faulting events that created the prevailing portion of the swarm and were distributed uniformly within the focal area, the reverse events were clustered in time and space.