Surface rupture and shallow fault reactivation during the 2019 Mw 4.9 Le Teil earthquake, France

The Rhone River Valley in France, a densely populated area with many industrial facilities including several nuclear power plants, was shaken on November 11th 2019, by the Mw 4.9 Le Teil earthquake. Here, we report field, seismological and interferometric synthetic-aperture radar observations indicating that the earthquake occurred at a very shallow focal depth on a southeast-dipping reverse-fault. We show evidence of surface rupture and up to 15 cm uplift of the hanging wall along a northeast-southwest trending discontinuity with a length of about 5 km. Together, these lines of evidence suggest that the Oligocene La Rouviere fault was reactivated. Based on the absence of geomorphic evidence of cumulative compressional deformation along the fault, we suggest that it had not ruptured for several thousand or even tens of thousands of years. Our observations raise the question of whether displacement from surface rupture represents a hazard in regions with strong tectonic inheritance and very low strain rates. The 2019 Le Teil earthquake in southern France reactivated an existing fault and ruptured the surface, according to field, seismic and InSAR observations. The incredibly shallow hypocenter can explain the effects of the moderate Mw 4.9 event.