The competing roles of seismicity and rainfall in slope destabilization: rockfalls triggered on a metastable volcanic edifice

The quantification of the effects of external forcings such as seismicity and rainfall on slope destabilization is an open and important question. To investigate the role of these forcings, we analyze an unprecedented 10-year long catalog of the rockfalls occurring in the Piton de la Fournaise volcano crater. Indeed, the dense seismic network operated by the Piton de la Fournaise Volcano Observatory (La Réunion Island) makes it possible to precisely locate the rockfalls and recover the volume of each event. We use statistical tools originally developed for earthquakes to study the spatio-temporal evolution of the rockfall activity and to unravel the impact of the external forcings. Our results highlight the predominant effect of low amplitude seismicity on the slope destabilization, via a progressive damaging of the slopes. Moreover, we show that the efficiency and the time delay of this dynamic triggering depends on the stability state of the slopes, i.e. the distance to failure. To better understand our observations, we compare them with laboratory experiments on granular avalanches triggered by ultrasound.