Seismotectonic evidence for present-day transtensional reactivation of the slowly deforming Hegau-Bodensee Graben in the northern foreland of the Central Alps

This study presents a seismotectonic analysis of the Miocene-aged Hegau-Bodensee Graben, a major tectonic element in the northern foreland of the European Central Alps. The graben is characterized by comparatively low strain rates and low-to-moderate seismicity. Our study builds on the seismological analysis of earthquakes recorded by a recently densified seismometer network. The derived high-precision absolute and relative hypocenter relocations allow to identify seismogenic structures in the pre-Mesozoic basement, which we relate to bounding faults on either side of the NW-SE striking graben. A cluster of seismicity on the SW side of the graben is associated with the previously mapped Neuhausen Fault. In contrast, the seismogenic, SW-dipping bounding faults on the opposite side of the graben, between the extinct Hegau volcanic field and the Bodanrück peninsula of Lake Constance, cannot be associated with any known fault. A set of 51 focal mechanisms allows for a high-resolution analysis of kinematics and stress regime of the graben. Our results show that the bounding faults of the graben are optimally oriented to be reactivated in transtensional mode in the present-day stress field. Slip rates across the Neuhausen and Randen Faults estimated from geodetic data are likely <0.1 mm/yr. In comparison with historical seismicity over the past 600 years and geomorphic field observations, geodetic rates of 0.1 mm/yr appear overestimated. Nevertheless, historical seismicity suggests that slip rates have the potential to generate MW 5.0 earthquakes within this slowly deforming, transtensional fault zone in the foreland of the Alpine collision zone on timescales of several hundred years.