4D gravity changes associated with the 2005 eruption of Sierra Negra volcano, Galápagos

Sierra Negra volcano, the most voluminous shield volcano in the Galapagos archipelago and one of the largest basaltic calderas in the world, erupted on October 22, 2005 after more than 25 years of quiescence. GPS and satellite radar interferometryInSAR monitoring of the deformation of the caldera floor in the months prior to the eruption documented extraordinary inflation rates 1c m/day. The total amount of uplift recorded since monitoring began in 1992 approached 5 m at the center of the caldera over the eight days of the eruption the caldera floor deflated a maximum of 5 m and subsquently renewed its inflation, but at a decelerating rate. To gain insight into the nature of the subsurface mass/density changes associated with the deformation, gravity measurements performed in 2005, 2006, and 2007 are compared to previous measurements from 2001-2002 when the volcano underwent a period of minor deflation and magma withdrawal. The residual gravity decrease between 2001-2002 and 2005 is among the largest ever recorded atan active volcano950 µGal and suggests that inflation was accompanied by a relative density decrease in the magmatic system. Forward modeling of the residual gravity data in 4D from 2002 to 2005 gives an estimate of the amount of vesiculation in the shallow sill required to explain the observed gravity variations. Geochemical constraints from melt inclusion and satellite remote-sensing data allow us to estimate the pre-eruptive gas content of the magma and place constraints on the thickness of the gas-rich sill necessary to produce the gravity anomalies observed. Results suggest that reasonable sill thicknesses 700–800 m and bubble contents 10–50 volume % can explain the large decrease in residual gravity prior to eruption. Following the eruption 2006 and 2007, the deformation and gravity patterns suggest re-equilibration of the pressure regime in the shallow magma system via a renewed influx of relatively gas-poor magma into the shallow parts of the system.