Active large sector collapse and architectural evolution of the Pico Island (Azores)

Volcanic ocean islands are particularly prone to flank instability. Episodes of flank destabilization can remove great amounts of volcanic material, either as creeping rotational landslides along a deep detachment, or in the form of fast running debris avalanches produced by the sudden flank rupture. Based on high-resolution DEM, fieldwork, GPS, InSAR and geochronological data, we put in evidence ongoing collapse of the southeast flank of the Pico volcanic Ridge (Azores) and its architectural evolution. OUR Data show that: (1) the collapsing sector is several cubic kilometres in dimension; (2) the collapse structure involves several curved scarps with a geometry and kinematics typical of normal faults; (3) the central part of the collapse is experiencing little horizontal displacement toward the sea (< 3 mm/yr) but significant downward movement (7-12 mm/yr); (4) motion of the island flank is not accommodated by a low-angle deep basal detachment such as observed elsewhere on typical slumps; (5) the outer part of Pico SE flank is experiencing faster subsidence than the inner parts, which likely reflects recent individualization of a steep seaward-dipping fault in the distal part of the moving mass; (6) evolution of the deformation along this accident could involve the sudden detachment of a large distal block, possibly followed by the gradual destabilization of the whole collapse area. Such scenario would have hazardous consequences for the local population and could trigger a major tsunami, which may impact the surrounding coastal environments and cause considerable damage; (7) the architectural evolution of the island has been made by short periods of volcanic construction, followed by longer periods of destruction by mass wasting episodes like the one here described.