A Sheared Sill Triggers Large Flank Displacements at Piton de la Fournaise Volcano (Réunion Island)

Abstract:

In April 2007, an eruption with an exceptional intensity occured 7 km southeast of the central cone of the Piton de la Fournaise (Réunion Island). ALOS and ENVISAT satellites recorded up to 1.4 meter of displacement towards the east, and a general subsidence up to 45 cm of the Eastern Flank of the volcano. This movement continued for more than one year after the end of the eruption. In this work, we address the Eastern Flank displacement during and after the April 2007 eruption by inverting post-eruptive InSAR data. Our purpose is twofold: assess the hazard associated to flank displacement and understand the origin of the distal location of the eruption. C-band ENVISAT interferograms from six different lines of sight were interpolated to cover a common period from May 2007 to July 2008. We combine a Mixed Boundary Element Method (MBEM) assuming linear elasticity of the edifice and a Neighborhood Algorithm to invert the surface displacement. The real topography is taken into account using a Digital Elevation Model. The fracture geometry is supposed to be a curved quadrangle. Linear parameters such as pressure and shear stress are also inverted. Inversion results show that a 5 by 8 km trapezoïdal fracture reproduces well the first order displacement of the Eastern Flank. The fracture is 500 to 1000 m deep with a low dip angle to the East. It links the summit cone to the eruptive fissure of April 2007 which corresponds to the expected magma path. The fracture is submitted to an eastward slip up to around 25 cm, and a closure up to 20 cm. To confirm the surprising result of a closure, we made an inversion with a version of MBEM where closure is forbidden and we tested a linear inversion of slip distribution on a fracture of the same geometry assuming an elastic half space. These inversions confirmed the closure and lead us to propose that the source of the April 2007 eruption is a sheared sill, which underwent closure in the post-eruptive period.