On the link between external forcings and slope instabilities in the Piton de la Fournaise summit crater, Reunion Island.

A catalog of the locations and volumes of rockfalls in the Piton de la Fournaise crater has been produced using seismic records. We validate it by comparing the locations and volumes to those deduced from photogrammetric data. The analysis of 10477 rockfalls, spanning the period 2014 to 2016, reveals that renewed eruptive activity after a 41-months rest has unsettled the crater edges. No significant variation in rockfall frequency is observed over the study period. However, external forcings such as rain and seismicity are shown to potentially increase the volume of individual rockfalls. Pre-eruptive seismicity seems to be the main triggering factor for the largest volumes, with a delay of one to several days. We infer that repetitive vibrations from the many seismic events induce crack (or slip) growth in highly fractured (or granular) materials, leading to the collapse of large volumes. Regarding their spatial distribution before an eruption, the largest rockfalls seem to migrate towards the location of magma extrusion.