Evidence for an active fault below the northwestern Alpine foreland of Switzerland

This study is devoted to the analysis of a prominent concentration of earthquakes whose epicenters delineate an active 20-30 km long N—S trending tectonic feature near the town of Fribourg, in the Molasse Basin of western Switzerland. This feature coincides with the possible southward continuation of the NNE—SSW trending Rhine Graben located approximately 80 km further north. In addition these epicenters are located in the vicinity of the Fribourg Syncline and the Alterswil Culmination, whose structural axes are oriented N—S in this area, instead of being aligned with the predominant regional NE—SW structural trend. Most of the earthquakes belong to one of three series of events that occurred over a time span of 2-4 months in 1987, 1995 and 1999. They include four events with magnitudes between 3 and 4 and one with a magnitude of 4.3. Focal depths, constrained by modelling sPMP-PMP traveltime differences with synthetic seismograms, are around 2 km, which places these events in the sedimentary cover. Fault plane solutions correspond to almost pure strike-slip mechanisms with nearly N—S and E—W oriented nodal planes. High-precision relative locations of individual events within the different earthquake clusters as well as of the relative locations of the clusters to each other show that these earthquakes are associated with left lateral motion along a N—S trending fault system. Deep reaching large scale flower structures in the Mesozoic and Tertiary overburden are observed on interpreted seismic profiles, close to the hypocenters. The unusual N—S trend of the Fribourg Syncline can be attributed to movements along these faults during Oligocene and Miocene times. Also magnetic data support the assumption of a N—S striking fault system in the Fribourg area, possibly related to a Permo-Carboniferous trough. Though the direct link between the fault traces in the overburden and the active fault system at depth could not be established in this study, their similar deformational style and their vicinity suggest that they are related. The total length of the inferred fault carries the potential of a magnitude 6 earthquake and thus constitutes a significant source of seismic hazar