HR: 1340h
AN: NG33A-0883    [Abstracts]
TI: Critical behaviour of the seismic precursors of a cliff collapse
AU: Grasso, J
EM: grasso@moho.ess.ucla.edu
AF: LGIT, Grenoble Observatory, Also at USGS Menlo Park, J. Fourier University, Grenoble, 38041 France
AU: * Amitrano, D
EM: amitrano@mines.inpl-nancy.fr
AF: Laego-INPL, School of Mines, Parc de Saurupt, Nancy, 54042 France
AU: Senfaute, G
EM: senfaute@mines.inpl-nancy.fr
AF: Largo-INERIS, School of Mines, Parc de Saurupt, Nancy, 54042 France
AB: We analyse the statistical pattern of seismicity before a 1-2 103 m3 chalk cliff collapse on the Normandie ocean shore, Western France. About 500 seismic events, in both 40 Hz-1.5 kHz and 2 Hz-10kHz frequency range, have been recorded during the six months before the collapse, and more than 200 during the two hours preceding the collapse. The collapse occurred within a seismic network of five microseismic stations, in such a manner that the central station was located at few meters of the failure surface. This allows us to investigate the properties of the seismicity emitted by the damage process leading to the cliff collapse. The study focused on the statistical properties of the seismic events. The event size distribution displays a power law distribution ($N(E)\propto E^{-b}$) on more than 3 magnitude orders with an exponent of 0.55. Such a distribution can be compared to the Gutenberg-Richter law observed for crustal earthquakes. During the last hour before the collapse, we observed a continuous decrease of b-value until the collapse occurred, indicating that the proportion of large events increases toward the time to failure. We show that, contemporary to this exponent decrease, a power law acceleration of seismicity rate and energy is defined on 3 order of magnitude, within 2 hours from the collapse time. We discuss these results, in the context of brittle failure and tertiary creep models. Our analysis of this first seismic monitoring data of a cliff collapse suggests that the thermodynamic phase transition models for brittle rupture may apply for this cliff collapse. We suggest that the case study presented here is one of the first (even the first) case showing a power law acceleration before a cliff collapse. It open new routes both to monitor and to understand the physics rock slope and landslide instabilities.
DE: 5104 Fracture and flow
DE: 7209 Earthquake dynamics and mechanics
DE: 3220 Nonlinear dynamics
SC: Nonlinear Geophysics [NG]
MN: 2004 AGU Fall Meeting