EGU2020-8409
https://doi.org/10.5194/egusphere-egu2020-8409
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

The Mw4.9 Le Teil surface-rupturing earthquake in southern France: New insight on seismic hazard assessment in stable continental regions

Jean-François Ritz1, Stéphane Baize2, Matthieu Ferry1, Christophe Larroque3, Laurence Audin4, Bertrand Delouis3, and Emmanuel Mathot5
Jean-François Ritz et al.
  • 1CNRS, Geosciences Montpellier, Montpellier, France (jean-francois.ritz@gm.univ-montp2.fr)
  • 2Institute of Radiological protection and Nuclear Safety (IRSN), Fontenay-aux-roses, France
  • 3Géoazur, CNRS-IRD-OCA, Université de Nice Sophia Antipolis, Valbonne, France
  • 4ISTerre, IRD-CNRS-Université Grenoble Alpes, Grenoble, France
  • 5Terradue Srl, Rome, Italy

On November 11th 2019, a Mw 4.9 earthquake shook the Rhone River Valley in southern France, a rather densely populated area with many industrial facilities including several nuclear power plants. The “Le Teil” earthquake was felt as far as Montpellier and Grenoble, 120 km from the epicenter. Seismological data promptly showed that the earthquake corresponded to a reverse faulting event along a NE-SW trending fault with a focus at a very shallow depth (~1 km). In parallel, satellite-based radar observations (InSAR) showed the uplift of the SE compartment (up to 10 centimeters) along a sharp NE-SW trending ~4.5-km-long discontinuity. Field investigations conducted in the following days and weeks in the epicentral area uncovered several evidences of surface ruptures across roads and paths where the InSAR discontinuity is mapped. We also carried out airborne LiDAR surveys to map the rupture below the dense forest cover. Characteristics of surface deformations are fully consistent with InSAR and seismological data, and allow concluding to the reactivation of an Oligocene normal fault segment (i.e. La Rouvière fault) that belongs to the Cévennes fault system, a 120 km long polyphased system bounding the southern rim of the Massif Central. The absence of clear cumulative compressional deformation along the fault rupture, which on the contrary displays inherited extensional deformation (most likely Oligocene in age), suggests that the fault has not moved significantly since millions of years. These observations relaunch the question of seismic hazard assessment in stable continental regions such as continental France and most of Western Europe, where strain rates are very low.

How to cite: Ritz, J.-F., Baize, S., Ferry, M., Larroque, C., Audin, L., Delouis, B., and Mathot, E.: The Mw4.9 Le Teil surface-rupturing earthquake in southern France: New insight on seismic hazard assessment in stable continental regions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8409, https://doi.org/10.5194/egusphere-egu2020-8409, 2020

Comments on the presentation

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Presentation version 1 – uploaded on 07 May 2020
  • CC1: Comment on EGU2020-8409, Dan Clark, 07 May 2020

    Very interesting presentation! in relation to your second last slide where you ask the question of whether there has been a precedent event on the fault system, I wonder whether anyone has looked at the paleo-channels of the Rhone River on the floodplain (maybe with Lidar) to see if there are any anomalies in sinuosity? Something like Holbrook et al (2006) was able to demonstrate for the Mississippi River where it crossed the Reelfoot Fault (https://doi.org/10.1016/j.tecto.2006.04.002).

    • AC1: Reply to CC1, Stéphane Baize, 08 May 2020

      Dear Dan,

      Thanks for your comment and suggestion.

      To my knowledge, this has not been done yet. LiDAR should be available in the area. The Rhône River is a highly human-modified area but this is a topic to investigate.

      Stéphane

      • CC5: Reply to AC1, Dan Clark, 14 May 2020

        Thanks Stephane. Do we know how old are the oldest terraces of the Rhone River? If they re hundreds of thousands of years old, perhaps there is a good chance for a deformation imprint from prior events.

        • AC5: Reply to CC5, Jean-François Ritz, 14 May 2020

          Hi Dan, in the Chat, I answered about the age of the terraces still preserved along (nearby) the present Rhône River (which should be around 120 and 250 ka, but their ages are not known in terms of dating).There are also patches of much older "terraces" (paleo-fluvial deposits belonging to the paleo-Rhone river that are also preserved, and perched (uplifted) within the Ardeche margin (SE border of the Massif Central). We are starting studying them and will carry out paleoseismological investigations where there’re “crossed” by faults (so far, those deposits have been mapped as undeformed...)

  • CC2: Comment on EGU2020-8409, Götz Bokelmann, 08 May 2020

    Very interesting study (and an interesting earthquake..)! Here`s a question: can one rule out that the slip has taken place on the other (W-dipping) seismic plane?

    • AC2: Reply to CC2, Jean-François Ritz, 08 May 2020

      Hi Götz,

      yes, we can rule out that the slip has taken place on the other (W-dipping) seismic plane, because the InSAR data show that it is the southeastern compartment (hangingwall) that has been uplifted, as showns also with evidences of surface ruptures.

      Jeff

  • CC3: Comment on EGU2020-8409, Cécile Lasserre, 09 May 2020

    Hi Jeff,

    Sorry to have missed the zoom presentation. Nice observations. I have a question on the InSAR displacement map and profiles slides 11 and 12. The vertical displacement map is continuous even in the very near-fault zone. Was there any interpolation involved to produce this map ? What could be the influence of unwrapping errors due to a loss of coherence in this near-fault zone in the fault width estimate from the profile analysis (any possibility of unwrapping errors on the bottom profile showing a potentially 600-m wide fault zone for example ?) ?

    Thanks !

    • AC3: Reply to CC3, Jean-François Ritz, 09 May 2020

      Hi Cécile, Thanks for your question, which is a good one. I will let Stéphane and/or Emmanuel answer..

      Jeff

  • CC4: Comment on EGU2020-8409, Oona Scotti, 12 May 2020

    In your opinion should off-fault deformation be considered alongside on-fault deformation in PSHA-fault models where slip rates are converted into earthquake rate?

    • AC4: Reply to CC4, Jean-François Ritz, 13 May 2020

      Hi Oona,

      I am not a specialist of PSHA, but I would say yes. We probably underestimated the PSHA when considering only slip rates determined along fault (when those slip rates are estimated from morphotectonics-paleoseismology).