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Natural Hazards and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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Volume 10, issue 4
Nat. Hazards Earth Syst. Sci., 10, 819–829, 2010
https://doi.org/10.5194/nhess-10-819-2010
© Author(s) 2010. This work is distributed under
the Creative Commons Attribution 3.0 License.
Nat. Hazards Earth Syst. Sci., 10, 819–829, 2010
https://doi.org/10.5194/nhess-10-819-2010
© Author(s) 2010. This work is distributed under
the Creative Commons Attribution 3.0 License.

  15 Apr 2010

15 Apr 2010

Pre-failure behaviour of an unstable limestone cliff from displacement and seismic data

J.-L. Got1, P. Mourot2,1, and J. Grangeon1 J.-L. Got et al.
  • 1LGIT, Université de Savoie, CNRS, UMR5559, Le Bourget-du-Lac, France
  • 2MYOTIS, 20, rue du Tour de l'Eau, 38400, Saint-Martin d'Hères, France

Abstract. We monitored the displacement and seismic activity of an unstable vertical rock slice in a natural limestone cliff of the southeast Vercors massif, southeast France, during the months preceding its collapse. Displacement measurements showed an average acceleration of the movement of its top, with clear increases in the displacement velocity and in the discrete seismic event production rate during periods where temperature falls, with more activity when rainfall or frost occurs. Crises of discrete seismic events produce high amplitudes in periodograms, but do not change the high frequency base noise level rate. We infer that these crises express the critical crack growth induced by water weakening (from water vapor condensation or rain) of the rock strength rather than to a rapid change in applied stresses. Seismic noise analysis showed a steady increase in the high frequency base noise level and the emergence of spectral modes in the signal recorded by the sensor installed on the unstable rock slice during the weeks preceding the collapse. High frequency seismic noise base level seems to represent subcritical crack growth. It is a smooth and robust parameter whose variations are related to generalized changes in the rupture process. Drop of the seismic noise amplitude was concomitant with the emergence of spectral modes – that are compatible with high-order eigenmodes of the unstable rock slice – during the later stages of its instability. Seismic noise analysis, especially high frequency base noise level analysis may complement that of inverse displacement velocity in early-warning approaches when strong displacement fluctuations occur.

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