Active faulting of the southern segment of the Rhine River Fault, southern Germany: Geomorphological and paleoseismological evidence

Abstract

Our study focuses on the southern segment of the Rhine River Fault, located in one of the most seismically active regions in intraplate Europe, at the southern end of the Upper Rhine Graben and forming part of the eastern Rhine Graben Boundary Fault. It stands out due to its impressive geomorphological expression in the landscape near the village of Tunsel, in southwestern Germany. We present details about the timing of fault activity and the contribution of earthquakes to the morphology, as this information is crucial for seismic hazard assessment, considering its location 8 km from the Fessenheim Nuclear Power Plant. Through the integration of the sedimentary sequences, morphotectonic observations, shallow geophysics, and paleoseismological trenching, our findings demonstrate that several earthquakes along the RRF have ruptured the surface (M6.7 ± 0.5). The youngest surface rupturing earthquake occurred during medieval times (E_Z) and the penultimate event (E_Y) is constrained by stratigraphic correlation with reworked Loess deposits dated to the Younger Dryas (ca. 13 kyr BP). Three older earthquake events have also been unearthed (E_X, E_W and E_V) occurring prior to the Late Glacial Maximum, where event E_X led to considerable lateral spread at the banks of the Pleistocene Rhine river. Vertical displacements reach up to 0.5 m and lateral offsets up to max. 1.5 m per earthquake event, consistently with a 15 - 30 km-long rupture of this segment of the Rhine River Fault.