Characterization of the 1584 Corbeyrier and Yvorne “Ovaille” mass movement deposits by multimethod geophysical surveying and drilling

The Corbeyrier and Yvorne mass movement events that happened in the year 1584 have mainly been studied at the surface level such that the shallow subsurface structure remains largely unknown. Geophysical measurements allow for insights into the physical properties of the subsurface including the geometry of deposits after such events. We applied three methods with the specific interest of obtaining information about the spatial variation of thickness and lithology of the mass movement deposits: the horizontal to vertical spectral ratio of ambient seismic noise (54 measurements), electrical resistivity tomography (2 profiles) and ground-penetrating radar (7 profiles). These measurements were then complemented with a physical sample obtained from drilling a 10.6 m long core in the Luan forest. We found that the mass movement deposits contain angular clasts of gravel and cobbles in a clay matrix (~ 30%). We also saw a thinning of the deposits downslope such that they reached a thickness of 3.5 m in the Luan forest and a thickness greater than 10 m (estimated between 50 and 100 m) near the source region at Plan Falcon. We identified two areas which call for further investigation in terms of the possibility of sediment mobility of the deposits in the event of an earthquake: Plan Falcon and the mostly bare scree slope just below.