The StressTransfer Seismic Network-An Experiment to Monitor Seismically Active Fault Zones in the Northern Alpine Foreland of Southwestern Germany

Abstract

In the northern Alpine foreland of southwest Germany, continuous microseismic activity is observed, with the potential for strong earthquakes up tomagnitude 5 or even 6+. The StressTransfer network is designed to complement the existing seismic networks in three defined areas: the southern Upper Rhine graben, the Albstadt shear zone, and the central Molasse basin around the town of Bad Saulgau. The network monitors mainly the micro-seismic activity, and will allow us to study the related faults and the stress field. The network consists of 15 seismic broadband stations, five for each region that complement permanent and other temporary recording stations. The data-quality analysis proves an overall good recording quality, despite the problems due to the densely populated area. Recording stations located inside the Upper Rhine graben are characterized by a relatively high-noise level due to freeways and railways located nearby as well as sites on thick, partly unconsolidated fluvial sediments. The comparison of the I95 noise amplitude level at our recording stationswith the double P-wave onset amplitude of an exemplary M-L 1.0 earthquake in 12 km distance reveals the ability to detect even lower-magnitude events at most of the StressTransfer seismic stations. For six stations, it is difficult during daytime;at four stations, we are probably not able to detect comparable small events. From March to May 2020, we observe a reduction of the daily noise amplitude of 20%-30%, due to the severe restrictions related to the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic, giving us the chance to search for really low-magnitude events. Our study demonstrates that a careful selection of recording sites allows to monitor faulting processes even in highly populated regions with unfavorable subsurface conditions. In addition, we record teleseismicwaveforms at the newlocations, to improve the resolution of deeper structures.