High-precision relocation and focal mechanism of the 2002 rain-triggered earthquake swarms at Mt Hochstaufen, SE Germany

The 2002 seismicity of Mt Hochstaufen, SE Germany, is characterized by several swarm-type earthquake sequences that follow above-average rainfall events. First investigations have recently provided indication, that the rain events triggered this swarm-type activity. We present a detailed relocation of the 2002 seismicity, which is composed of (1) cluster analysis, (2) multievent semi-automatic phase repicking, (3) probabilistic location in a pseudo-3-D velocity model with topography and (4) master event relocation. Furthermore, focal mechanisms of selected events are derived by taking into account the first-motion polarities of P, SV and SH waves. We introduce a simple methodology to test the stability and estimate the variance of the focal mechanisms solutions. Location uncertainties of less than 100 m were achieved. The hypocentres are mainly concentrated between 1000 m above and 4000 m below sea level and aligned on two planes, which, however, cannot be associated with geological structures with reasonable certainty. Swarm-type activity follows above-average rain events and shows a depth migration of hypocentres over time. Evidence that the rain events triggered the earthquake swarms is reported by Hainzl et al. on basis of our relocation. The derived focal mechanisms seem to indicate an influence of the Saalachtal Fault Zone on the stress regime of the study area.