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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 9, issue 5 | Copyright

Special issue: Assessment of different dimensions of vulnerability to natural...

Nat. Hazards Earth Syst. Sci., 9, 1625-1641, 2009
https://doi.org/10.5194/nhess-9-1625-2009
© Author(s) 2009. This work is distributed under
the Creative Commons Attribution 3.0 License.

  02 Oct 2009

02 Oct 2009

Micro-seismic precursory cracks prior to rock-fall on coastal chalk cliffs: a case study at Mesnil-Val, Normandie, NW France

G. Senfaute1, A. Duperret2, and J. A. Lawrence3 G. Senfaute et al.
  • 1INERIS – Institut National de l'Environnement Industriel et des Risques, Parc technologique Alata BP 2, 60550 Verneuil en Halatte, France
  • 2FRE3102 – CNRS Laboratoire Ondes et Milieux Complexes, Université du Havre, 53 rue de Prony, BP540, 76053 Le Havre cedex, France
  • 3School of the Earth and Environment, University of Leeds, Leeds, Yorkshire, LS2 9JT, UK

Abstract. Erosion of rock cliffs has been considered to be relatively unpredictable. This perceived stochastic nature of the erosional processes often occurs through collapses along fractures in the rock-mass. The prediction of catastrophic cliff failures and collapses remains very difficult. For advancing in this field, it is important to understand the processes through which a crack is initiated, how it develops and propagates until the final failure. This paper examines the micro-seismic signals recorded 15 h prior to a rock-fall located at Mesnil-Val, France. The results lead to the hypothesis that several phases of failure mechanisms contribute to rock-fall occurrence. The most important phases were associated with micro-seismic event families identified by multiplet selection. Each event family contained one specific frequency spectrum showing a progressive decrease of the frequencies as the rock approached failure suggesting the following phases: 1) the micro-seismic events recorded 15 h before the rock-fall were characterised by the highest frequencies in a large spectrum-band, between ~100 and 1000 Hz (family 1), suggesting a crack initiation mechanism or the opening of existing fractures; 2) the micro-seismic events recorded several minutes before the rock-fall were associated with a clear decrease in the highest frequency components (family 2) suggesting that the mechanism was related to the growing and development (or coalesce) of existing micro-cracks into larger fractures; 3) micro-seismic events recorded just before the rock-fall were associated with a lower frequency spectrum than families 1 and 2, the highest frequency components were absent (family 3), the frequency emission source mechanism could be related to the shearing or opening of the existing large fractures permitting the complete detachment of the blocky rock-mass; 4) finally, micro-seismic events with a very low frequency spectrum (lower than 100 Hz) characterized the rock-fall impact on the ground. These encouraging results offer the possibility of using the micro-seismic system to monitor high risk sections of coastline and to advance understanding of cliff failure mechanisms.

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