Retrieving 65 years of volcano summit deformation from multitemporal structure from motion: The case of Piton de la Fournaise (La Reunion Island)

The structure from motion photogrammetry technique enables use of historical airborne photography to achieve high-resolution 3-D terrain models. We apply this method on Piton de la Fournaise volcano (La Reunion), which allows a unique opportunity to retrieve high-resolution (1.5-0.11m) digital elevation models and precise deformation maps of the volcano since 1950. Our results provide evidence that the summit volume increased throughout the study period, at a stable rate of 2.2 Mm(3)/yr between 1950 and 2015, with an acceleration up to 8.0 Mm(3)/yr prior to the major 2007 eruption that was accompanied by summit caldera collapse. At the same time, summit deformation was asymmetric, with 9.2 +/- 2.5 m of eastward seaward displacement and 1.3 +/- 2.5 m to the west during 1950-2015. Our results reveal a temporal evolution in the volcano magma influx rate and deformation. Tracking these fluxes and the long-lived preferential eastern motion is crucial to mitigate risks associated to flank destabilization.