Multitechnology characterization of an unusual surface rupturing intraplate earthquake: the ML 5.4 2019 Le Teil event in France

Abstract

Summary Metropolitan France is a region of slow tectonic deformation with sparse seismicity. On 11 November 2019, the ML 5.4 Le Teil earthquake became the largest seismic event recorded in the last 16 years. This event was recorded by the national seismic networks and also by a wide variety of other geophysical techniques including infrasound and InSAR measurements. These complementary technologies offer the opportunity to investigate in detail the earthquake source characteristics and the associated ground motion attenuation. Both seismic waveform inversions and InSAR interferogram reveal a shallow rupture on a reverse fault with an associated moment magnitude of 4.8-4.9. Infrasound signals also provide fast evidences pointing towards the area of ground surface displacements, which coincides with La Rouvière fault, in the Cévennes fault system, known as a formerly active normal fault during the Oligocene. The very significant amount of seismic records also helps toward validating the GMPE laws available for the region. This multi-technology characterisation documents the kinematics of this rare example of shallow intraplate fault reactivation.

Insights on fault reactivation during the 2019 November 11, Mw 4.9 Le Teil earthquake in southeastern France, from a joint 3-D geological model and InSAR time-series analysis

“…The resulting slip distribution inverted for the Le Teil earthquake has an equivalent moment magnitude of 4.9, consistent with seismological estimates (Delouis et al, 2019;Cornou et al, 2020;Vallage et al, 2021).…”

“…The shallow slip deficit displayed by our model is also present in the InSAR-derived slip models from previous studies (Delouis et al, 2019;Vallage et al, 2021). In the framework of a homogeneous elastic half-space inversion, it is difficult to assess whether Surface Slip Deficit (SSD; Fialko et al, 2005) is a real feature or an artifact resulting from the lack of data coverage close to the fault, or caused by neglecting the likely complexities of the elastic medium (Xu et al, 2016;Marchandon et al, 2021).…”

“…The coseismic interferograms produced in the days following the Le Teil earthquake played a key role in guiding the early post-seismic field missions in the earthquake area (Delouis et al, 2019;Cornou et al, 2020). This dataset helped to constrain the location and spatial extent of the surface rupture, guiding further seismological, geodetic and tectonic studies of the earthquake (Mordret et al, 2020, Causse et al, 2021, Vallage et al 2021.…”

“…However, past studies of the earthquake involving InSAR data rely on the analysis of only a limited number of individual coseismic interferograms. All were computed from radar images acquired by the ESA's Sentinel-1 satellites a few days before and after the earthquake (Cornou et al, 2020, Vallage et al 2021). These interferograms were processed and unwrapped using different methodologies and remain affected by atmospheric phase delays and coherence loss.…”

Post-publication careers: ground ruptured, community united

“…In a separate project, existing data are being made available at the national level, beyond the existing synthesis 2 , and updated with regional input. Interdisciplinary cooperation has sprung up between geologists, seismologists and geophysicists from academic circles, to develop temporary instrumental networks and propose analyses of the earthquake and consequences [3][4][5][6][7] .Field geologists have continued to investigate where possible, despite the lockdowns and curfews resulting from the COVID-19 pandemic, and the motivation of our group has remained strong. Importantly, we were able to excavate a dozen paleoseismological trenches that demonstrate earlier incidences of ground rupture.…”