Seismic monitoring of Sechilienne rockslide (French Alps): Analysis of seismic signals and their correlation with rainfalls

In the French Alps, Sechilienne rockslide is one of the natural phenomena that presents one of the highest levels of risk in terms of socioeconomical outcomes. This rockslide has been active for a few decades and has been instrumented since 1985. The current very active volume of this rockslide is estimated to be up to 5 million m(3), located on the border of a slowly moving mass reaching 50-100 million m(3). The velocity of the most active zone reached 1.4 m/yr in 2008, about twice the value of 2000. A seismic network was installed on this rockslide in May 2007 to supplement the monitoring system. It has now recorded several thousand events, mostly rockfalls but also hundreds of local and regional earthquakes, which can be distinguished and classified from their signal characteristics. Rockfalls and microseismicity, which occur in bursts of activity, are found to be weakly, but significantly, correlated with rainfall. Rockfall occurrence increases linearly with precipitation, with however strong fluctuations of the numbers of rockfalls per day for the same rainfall intensity. No threshold was found for rainfall triggering, with even 1 mm of rain being enough to trigger rockfalls. Rockfall activity starts almost immediately during a rainfall episode and lasts for several days after the rainfall. Rain also induces strong accelerations of the rockslide movement, which also start quasi-instantaneously and last for about a month.