Insights on fault reactivation during the 2019, Mw4.9 Le Teil earthquake in southeastern France, from a joint 3D geology and InSAR study

<div> <div> <div> <p>The 2019, M_w4.9 Le Teil earthquake occurred in southeastern France, causing important damage in a slow deforming region.&#160;Field based, remote sensing and seismological studies following the event revealed its very shallow depth, a rupture length of ~5 km with surface rupture evidences and a thrusting mechanism. We further investigate this earthquake by combining geological field mapping and 3D geology, InSAR time series analysis and coseismic slip inversion.</p> <p>From structural, stratigraphic and geological data collected around the epicenter, we first produce a 3D geological model over a 70 km² and 3 km deep zone surrounding the 2019 rupture, using the GeoModeller software. This model includes the geometry of the main faults and geological layers, and especially a geometry for La Rouvi&#232;re Fault, an Oligocene normal fault likely reactivated during the earthquake.</p> <p>We also generate a time series of the surface displacement by InSAR, based on Sentinel-1 data ranging from early January 2019 to late January 2020, using the NSBAS processing chain. The spatio-temporal patterns of the surface displacement for this limited time span show neither clear pre-seismic signal nor significant postseismic slip. We extract from the InSAR time series the coseismic displacement pattern, and in particular the along-strike slip distribution that shows spatial variations. The maximum relative displacement along the Line-Of-Sight is up to ~16 cm and is located in the southwestern part of the rupture.</p> <p>We then invert for the slip distribution on the fault from the InSAR coseismic surface displacement field. We use a non-negative least square approach based on the CSI software and the fault surface trace defined in the 3D geological model, exploring the range of plausible fault dip values. Best-fitting dips range between 55&#176; and 60&#176;. Such values are slightly lower than those measured on La Rouvi&#232;re Fault planes in the field. Our model confirms the reactivation of La Rouvi&#232;re fault, with reverse slip at very shallow depth and two main slip patches reaching 30 cm and 24 cm of slip at 400-500m depth. We finally discuss how the 3D fault geometry and geological configuration could have impacted the slip distribution and propagation during the earthquake.</p> <p>This study is a step to better quantify strain accumulation and assess the seismic hazard associated with other similar faults along the C&#233;vennes fault system, in a densely populated area hosting several nuclear plants.</p> </div> </div> </div>