Multiple inflation and deflation events from 2004 to 2016 at Fogo (Água de Pau) volcano, São Miguel, Azores

Abstract

Ground deformation at São Miguel Island, Azores, has been mapped with GPS (Global Positioning System) geodetic measurements for understanding volcanic unrest. We use yearly measurements from 52 GPS stations covering the central part of the island, recorded between 2004 and 2016. The results show disturbances in the velocity field, with periods of inflation in 2004–2006 and 2011–2013, and a deflation period in 2013–2016. For each period of deformation, the velocity field is modeled by using sources of pressure change embedded within uniform elastic half-space. Point-pressure, prolate spheroid, and penny-shaped sources are considered. We compare the fits to the data using chi-square statistics and F-tests. The best fit modeled sources are located near the eastern and northeastern rim of Fogo volcano caldera, and to the east of it, at depths in the range of 3.2–3.7 km. Best fit point-pressure source models indicate volume increase in the range of 8.8-10.4×106m3 in 2004–2006, 0.7-5.4×106m3 in 2011–2013, and volume decrease in the range of 3.1-4.4×106m3 in 2013–2016 (95% confidence intervals). The geodetic results, with multiple sources of deformation located in different areas, show that the 2004–2016 activity at São Miguel Island was complex. The location of the 2004–2006 inflation, 3 km northeast of Fogo caldera, and the lack of subsidence reversal can be explained by the permanence of a newly intruded magma body. The location of the 2011–2013 inflation at the eastern rim of Fogo caldera, and the total reversal to subsidence in 2013–2016 can relate to disturbances in a magma chamber under the Fogo caldera or mixed magma-hydrothermal processes.