EXCEPTIONAL GROUND MOTION DURING THE SHALLOW Mw 4.9 2019 LE TEIL EARTHQUAKE, FRANCE

Abstract

On November 11, 2019, an unusually damaging Mw4.9 earthquake occurred in the south east of France within the lower Rhône river valley, an industrial region hosting several operating nuclear power plants. This event is exceptional considering its very shallow depth (<1 km). Based on farfield seismological observations, we demonstrate that the rupture properties are consistent with the ones commonly observed for large deeper earthquakes, implying that the near-surface faulting generated strong high-frequency seismic waves. In the absence of strong motion sensors in the fault vicinity, we perform numerical predictions of the ground acceleration on a virtual array of near-fault stations, that matches with the locations of independent quantitative estimations from in-situ observations of displaced objects (natural and anthropic). Both numerical and in-situ analyses converge toward an exceptional level of ground acceleration in the fault vicinity, exceeding gravity, and at the origin of the damage. This dramatically changes the perception of the impacts of superficial moderate earthquakes on seismic hazard assessment.

Constraining the point source parameters of the 11 November 2019 Mw 4.9 Le Teil earthquake using multiple relocation approaches, first motion and full waveform inversions

“…In our approach, we simultaneously determined the aftershock hypocenter and an optimal local 1D linear gradient velocity model. A different local 1D velocity model has been determined by Cornou et al [2021] and Causse et al [2021] based on a beamforming method using the seismic noise recorded by the post-seismic stations installed in the fault vicinity. The refined model published in Causse et al [2021] exhibits an inversion of the P and S velocities, which are divided by a factor of ∼3, at 1.2 km depth.…”

“…A different local 1D velocity model has been determined by Cornou et al [2021] and Causse et al [2021] based on a beamforming method using the seismic noise recorded by the post-seismic stations installed in the fault vicinity. The refined model published in Causse et al [2021] exhibits an inversion of the P and S velocities, which are divided by a factor of ∼3, at 1.2 km depth. This model is expected to reflect the Earth structure in the epicentral area (epicentral distance <10 km).…”

“…Another reviewed location proposed by the LDG and based on a large number of regional stations also places the epicenter on the wrong side of the fault [Vallage et al, 2021]. A relocation of the mainshock hypocenter is hence required in order to determine where on the La Rouvière fault the rupture initiated, and to compare the result with analyses based on independent observations which pointed to a bilateral rupture [De Novellis et al, 2020, Causse et al, 2021, Mordret et al, 2020.…”

Seismotectonics of southeast France: from the Jura mountains to Corsica

“…1 on Figure 9) was a surprise in a place where only 39 small earthquakes (M L 1.3-2.9) have been recorded for the period 1962-2019, with focal depths in the range of 5-24 km (Figure 20A). The most surprising aspect was the very shallow depth of the 2019 earthquake focus, around 1 km [Delouis et al, 2021] which produced a strong ground motion with a maximum epicentral intensity of VIII (EMS98 macroseismic scale) and a peak ground acceleration exceeding 1g [Causse et al, 2021]. This 2019 M w 4.9 earthquake together with its shallow focus produced a co-seismic 4.5 km long surface rupture [Ritz et al, 2020].…”

Rapid response to the M w 4.9 earthquake of November 11, 2019 in Le Teil, Lower Rhône Valley, France

“…geodetic and seismological data) are extremely rich and complementary. They open routes to several research projects, including refinement of the active fault database and deep borehole drilling in the epicentral area, relocation of the mainshock, analysis of rupture mechanisms during the mainshock and nucleation process [Mordret et al, 2020], assessment of the possible role of a large limestone quarry located on the hanging wall of the La Rouvière fault (Figure 3a) on the mainshock triggering [Delouis et al, 2019], understanding the low number of aftershocks for such a magnitude earthquake, calibration and simulation of the maximum ground accelerations during the mainshock [Causse et al, 2020], evaluation of the recurrence of similar earthquakes on the fault (or, conversely, singularity of this earthquake over the last 10,000 years), impact of the current compressive tectonics on the reactivation of the Cévenne fault system branches, impact of surface rupturing on seismic hazard [Ritz et al, 2020].…”