Low rates of deformation of the Furnas and Fogo Volcanoes, Sao Miguel, Azores, observed with the Global Positioning System, 1993-1997

We report results of Global Positioning System (GPS) measurements at the Furnas and Fogo (Agua de Pau) Volcanoes, Sao Miguel island, Azores. GPS surveys were carried out in 1993, 1994, and 1997. The GPS network consists of 16 stations that are mainly distributed in the vicinity of the Furnas Volcano. The measurements indicate slow-progressing ground deformation of the Furnas Volcano in 1993-1997. We propose two alternative hypotheses to explain the observations. The first one assumes a source of inflation located northwest of the Furnas caldera, since there part of the GPS network seems to expand. The second hypothesis invokes a combination of two processes as being responsible for the observed deformation pattern; plate divergence between the Eurasian and African plates together with a deflation of the Furnas caldera. The inferred plate boundary deformation zone is narrow with 75% of the 4 mm/yr full plate spreading probably being accommodated over the 10-km width of Sao Miguel island. GPS stations at Fogo show displacements towards its caldera, indicating slight deflation of the volcano, possibly due to a pressure decrease in a shallow magma chamber or to extraction of water and steam from the ground by a nearby geothermal power plant. Further geodetic observations are needed, spanning a longer time interval, in order to distinguish between different hypotheses.