Deformation associated with the Faial (Capelinhos) 1957–1958 eruption: Inferences from 1937–1997 geodetic measurements

Geodetic monuments were installed for the first time on Faial Island in 1937. Fifty years later they were reoccupied with triangulation and trilateration and, just before the 9 July 1998 earthquake, they were once again observed, now with GPS instruments. In spite of the significant volcanic and seismic events during this period, a quantitative assessment of the island's deformation has never been attempted. In this paper, we present a re-processing of geodetic data covering a time span of 60 years, to assess the long-term deformation pattern of the island. We show that displacements are large over the western part of Faial Island, and mainly limited to the 1937–1967 period; in the eastern part of Faial Island the displacement field is below the accuracy threshold of conventional geodetic techniques for the whole time span. Inter-seismic deformation of the islands that could be attributed to tele-tectonic processes is still below accuracy (based on the 1987–1997 GPS comparison), most of the island deformation probably being accommodated in a single event, the 1957 Capelinhos eruption and subsequent earthquake. Strain deduced from the geodetic dataset points to maximum horizontal strain N102° to N107°, compatible with the alignment of the volcanic cones of the western Faial fissural system. We modelled the observed deformation using a homogeneous elastic halfspace approach to conclude that it can be largely explained by a set of rectangular sources with tensile and oblique slip dislocations. These represent the bulk deformation history of that event, which comprise a dyke emplacement in depth and subsequent subsidence and accommodation by oblique normal faulting at shallower levels, after the start of Capelinhos eruption.