Spatiotemporal distribution of aftershocks of the 2004 December 5ML= 5.4 Waldkirch (Germany) earthquake

Abstract

SUMMARY The Waldkirch earthquake on 2004 December 5 at 1:52 UTC with ML= 5.4 was the strongest earthquake in Baden-Wurttemberg since 1978. It was followed by a large number of aftershocks. We were operational in field only 14 hr after the main shock for recording aftershocks. The aim of that field campaign was to test the performance of our newly developed system and to compare the results obtained by one small aperture seismic array to a standard aftershock network. More than 700 microearthquakes were recorded within a period of about 39 hr, the largest with magnitude ML= 3.0. Events with negative as well as positive polarities were identified as a result of alternating radiation patterns of the focal mechanisms. The analysis of the events was carried out with a statistical approach because of the high amount of events. Similarity matrices sorted by time reveal no significant event clustering, whereas matrices, rearranged by dendrograms according to the cross-correlation coefficients indicate that a part of the aftershocks are spatially clustered. An investigation of the chronological occurrence of these spatially clustered events shows a continuous seismic activity for the duration of measurement. The mean depth of the seismogenic zone could be determined to 11.1 km using tS–tP time differences. A relative location of aftershocks provides evidence for a WNW–ESE oriented fault plane direction and confirms the orientation of a nodal plane of the main shock. This Subhercynian direction is a predominant structure in the Black Forest and can be observed on many faults and mineral dykes. Our results coincide well with observations from the permanent and a temporarily installed network, which was operational 2 d after the main shock.