Update of the Seismogenic Potential of the Upper Rhine Graben Southern Region

The Upper Rhine Graben (URG), located in France and Germany, is bordered by north-south trending faults, some of them considered active, posing a potential threat to dense population and infrastructures from the Alsace plain. The largest historical earthquake in the region is the 1356 Basel earthquake associated to a magnitude M6.5+/-0.5. Current seismicity (M>2.5 since 1960) is mostly diffuse and located within the graben. The seismic hazard of the URG southern region was recently assessed by Chartier et al. (2017). In this study, we build upon their evaluation by exploring uncertainties in greater detail, revisiting a number of assumptions. Based on a complex fault network from Nivière et al. (2008), we evaluate scenarios that have not been taken into account previously, exploring uncertainties on Mmax, its recurrence time, the b-value, and the moment released aseismically or through aftershocks. Uncertainties on faults’ moment deficit rates, on the observed seismic events’ magnitude-frequency distribution, and on the moment-area scaling law of earthquakes are also explored. Given the four faults considered, and the scenario in which the Black Forest fault is not active anymore but where the other faults can still rupture simultaneously, and assuming only a dip-slip mechanism, the Mmax maximum probability is estimated at Mw5.95. Considering this scenario, there would be a 99% probability that Mmax is below 7.15. In contrast, considering instead strike-slip, as suggested by paleo-seismological work from Castellnou et al. (2022), and taking the Black Forest Fault into account, Mmax maximum probability is estimated at Mw6.85. Based on this scenario, there would be a 99% probability that Mmax is less than 7.65.