EGU2020-19565
https://doi.org/10.5194/egusphere-egu2020-19565
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Offshore Landslides could be favored by Seismic Amplification due to Site Effects

Francoise Courboulex1, E. Diego Mercerat2, Christophe Larroque1, Sébastien Migeon1,3, Anne Deschamps1, Yann Hello1, Marion Baques1, Diane Rivet1, and David Ambrois1
Francoise Courboulex et al.
  • 1Université Côte d'Azur,CNRS, IRD, Géoazur, Valbonne, France (courboulex@geoazur.unice.fr)
  • 2CEREMA, Equipe Projet MouvGS, Valbonne, France (diego.mercerat@cerema.fr)
  • 3Sorbonne Université, Paris, France (migeon@geoazur.unice.fr)

In many seismically active areas of the world, earthquake‐induced landslides commonly account for a significant portion of the total impact of earthquakes. When landslides occur offshore, in coastal areas, they can generate proximal tsunami waves that reach the coastlines in only a few minutes, and can be very dangerous.

The triggering power of earthquakes on landslides is often estimated on seismic wave amplitude (Peak ground acceleration, Arias intensity …), which is usually computed simply from the magnitude and distance of the earthquake using ground motion prediction equations (GMPEs). In this study we show that the local amplification due to site effect can be very strong offshore, and then should not be neglected.

In order to test and quantify the potential amplification of seismic waves offshore, we installed a broadband seismometer (PRIMA station) near the transition between the continental shelf and the upper continental slope, at a water depth of 18 m, offshore Nice city airport (southeastern France).  Situated at the mouth of the Var River, this zone is unstable and prone to landslides. A catastrophic landslide and tsunami already occurred in 1979, causing 10 casualties and large damages.

We analyze the recordings of earthquakes and seismic noise at the PRIMA station by comparing them to nearby inland stations. We find that the seismic waves are strongly amplified at PRIMA at some specific frequencies (with an amplification factor greater than 10 at 0.9 Hz). Using geological and geophysical data, we show that the main amplification frequency peak (at 0.9Hz) is due to the velocity contrast between the Pliocene sedimentary layer and fine-grained sediments dated from the Holocene. This offshore site effect could have a crucial impact on the triggering of a submarine landslide by an earthquake in this region. 

It is therefore crucial to detect and quantify the seismic amplification effects caused by superficial offshore sediment, in order to take them into account in predictive model.

How to cite: Courboulex, F., Mercerat, E. D., Larroque, C., Migeon, S., Deschamps, A., Hello, Y., Baques, M., Rivet, D., and Ambrois, D.: Offshore Landslides could be favored by Seismic Amplification due to Site Effects, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19565, https://doi.org/10.5194/egusphere-egu2020-19565, 2020

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Presentation version 2 – uploaded on 03 May 2020 , no comments
Version description: As proposed by Hans-Balder Havenith, convener of the session, there is now one more slide that shows the time evolution[...]
Presentation version 1 – uploaded on 29 Apr 2020
  • CC1: Comment on EGU2020-19565, Hans-Balder Havenith, 01 May 2020

    Dear Françoise,

     

    first, I hope that you will also be able to present on May 11 .. as offshore seismic failures are interesting for many people. ..For the 1979 event - do you know on which slopes this mass movement was triggered .. 5°...10° .. more? Maybe on May 11 you could add some offshore slope map?

    yours

    Hans

    • AC1: Reply to CC1, Francoise COURBOULEX, 03 May 2020

      The average slope gradient is estimated to 13°.

      We have added a new slide to the presentation with a map of the recent slope scars in the area of interest.

      • AC1: Reply to AC1, Francoise COURBOULEX, 04 May 2020

        Concerning the 1979 landslide:

        The landslide was triggered between 15 and 60 m deep, on a slope of roughly < 2° or even less.