The April 2007 eruption of Piton de la Fournaise was the biggest eruptive crisis of the 20th
and 21st centuries for the volcano. InSAR captured a large (1.4 m) co-eruptive seaward slip
of the volcano's eastern flank, which continued for more than a year at a decreasing rate
after the end of the eruption. Co-eruptive uplift and post-eruptive subsidence were also
observed. While it is generally agreed that volcano flank displacement might be induced by
fault slip, we investigate whether this flank displacement might have been induced by a
sheared sill, as suggested by observations of sheared sills at Piton des Neiges. To test this
hypothesis, we invert a quadrangular curved source submitted to coeval pressure and shear
stress changes. Post-eruptive displacement is well explained by slip and closure of a large
fracture sub parallel to the topography (5 km by 8 km). The amount of closure is too large
and the closure time too short to be explained by a thermally compacting sill, allowing to
rule out the sill hypothesis. Co-eruptive displacement can be explained by a smaller (2 km
by 2 km) fracture at the same location, submitted to a zero overpressure and a shear stress
drop, which confirms that the determined structure is not a sill. We conclude that the
fracture is a detachment fold, shallow enough to induce the observed coeval uplift.
Observations are consistent with a rate weakening behavior for the patch which ruptured
during the April 2007 eruption, while the creeping part of the fault is consistent with a rate
strengthening behavior.