The shallow magmatic system of Fernandina Volcano, Galápagos Islands. Evidence of multiple magma reservoirs from Satellite Radar Interferometry

In the Galápagos Islands as at other ocean island volcanoes only a fraction of the magma coming from the mantle is erupted; the remaining magma is stored in crustal magma chambers, or trapped near the crust-mantle boundary. The increase of pressure generated by these magma intrusions can inflate the volcanic edifice and trigger eruptions and earthquakes. The geometry of magma reservoirs and their connections with the surface during eruptive phases, therefore, is fundamental to the development of predictive models of volcano deformation and eruption. Among the Galápagos Islands, Fernandina can be considered the most active volcano in the archipelago, having experienced 25 eruptions since 1813 and three eruptions in the last fifteen years (1995 - 2005 - 2009). In order to investigate the deformation associated with changes in pressure in the magma storage system we use Interferometric Synthetic Aperture Radar (InSAR) data acquired over Fernandina by the European Space Agency satellites ERS-1, ERS-2 and ENVISAT between 1992 and July 2010 in four different orbits. The advantage of using different viewing geometries is that the deformation signal can be independently validated and the responsible source beneath can be better constrained. We generate more than 300 interferograms and we apply the Small Baseline Subset (SBAS) method to investigate the temporal evolution of ground deformation. The analysis of single interferograms and of mean deformation velocity maps shows that the deformation at Fernandina is characterized by an almost continuous displacement of an elliptical area clearly limited by the summit caldera rim, and occasional displacement of a larger portion of the volcano edifice outside the caldera, around the summit. We then infer the presence of multiple sources of deformation below the summit caldera and the southern flank of Fernandina demonstrating that the magmatic system is composed by multiple magma reservoirs at different depths. We also determine an unusual shallow dipping geometry for the dike that fed the eruptive fissure opened on April 10, 2009 on the southwestern flank of the volcano, confirming what inferred by Jónsson et al. (1999) for the 1995 eruption.