Abstract
Deformation and gravity monitoring at active volcanoes are routinely used to characterize sources of unrest and contribute to eruption forecasting. Despite their extensive use, there are few volcanoes in the world in which both deformation and gravity vary dramatically before eruptive activity, and even rarer where it occurs for extended periods of time. Sierra Negra volcano, a basaltic caldera in the Galapagos Islands, Ecuador, is thus remarkable both for the magnitude and timescale of its geodetic changes: more than 6.5 m of inflation and 1080 mu Gal gravity increase occurred between the 2005 and the 2018 eruption. Due to heightened activity detected by IG-EPN in 2017, we reoccupied and expanded the gravity monitoring network at Sierra Negra just ahead of and immediately after its most recent eruption in 2018. We detected a gravity decrease of -776 mu Gal in the centre of the caldera, consistent with mass loss due to the 2018 eruptive activity. Subsequent temporal gravity surveys have shown a consistent gravity increase at all measured sites, with more than 100 mu Gal positive change per year in the centre of the caldera. This was accompanied by consistent inflation of the caldera floor. Through inverse modelling of deformation and gravity change after the 2018 eruption, we infer the magmatic recharge of a sill-like source, located between 1 to 2 km below the surface, at an approximate rate of 3X10 (super 11) kg per year. Forward modelling of gravity change between 2005 and 2018 supports a similar magmatic recharge rate between the two past eruptions at Sierra Negra. By investigating past and current magma recharge, we propose an upper bound for mass increase prior to the onset of new eruptions.