How volcanic stratigraphy constrains headscarp collapse scenarios: the Samperre cliff case study (Martinique island, Lesser Antilles)

Gravitational instabilities can be significant threats to populations and infrastructure. For hazard assessment, it is important to estimate the geometry and volume of potential unstable masses. This characterization can be particularly difficult in volcanic contexts due to the succession of deposition and erosion phases. Indeed, it results in complex layering geometries in which the interfaces between geological layers may be neither parallel nor planar. Geometry characterization is all the more complex when unstable masses are located in steep and hard to access landscapes, which limits data acquisition. In this work, we show how remote observations can be used to estimate the surface envelope of an unstable mass on a volcanic cliff. We use ortho-photographs, aerial views and topographic surveys to (i) describe the different geological units of the cliff, (ii) characterize the stability of geological units, (iii) infer the paleo-morphology of the site and (iv) estimate potential unstable volumes. We investigate the Samperre cliff in Martinique (Lesser Antilles, French West Indies) as a study site, where recurrent destabilizations since at least 1988 have produced debris flows that threaten populations and infrastructure. Our analysis suggests that the destabilizations occurring on the cliff may be associated with the re-opening of a paleo-valley filled by pyroclastic materials. We estimate that between 3.5x10(6) and 8.3x10(6) m(3) could still be mobilized by future destabilizations in the coming decades.