Abstract
Sierra Negra Caldera, a basaltic shield volcano located in the Galapagos Islands (Ecuador), is widely known for its dramatic changes in geodetic signals, especially deformation and temporal gravity. The long time-series monitoring at Sierra Negra, with ground deformation observations available from 1992 through InSAR measurements and GNSS data, and recurrent temporal gravity surveys since 2002, has enabled investigation and modelling of its magmatic system. Our work aims to combine time-series gravity measurements with ground displacement to constrain the mass change, volume change and density of the source of unrest. Sierra Negra's most recent eruptions in 2005 and 2018 offered a unique opportunity to study, through diverse geophysical and geodetic methods, the dynamic interval of pre-eruptive magmatic recharge, eruption, and post-eruptive magmatic processes. Because of the heightened period of seismic activity at Sierra Negra detected by IG-EPN in July 2017, we reoccupied the gravity sites used before and after the 2005 Sierra Negra eruption and added additional measurement sites to improve coverage. Data was collected in a temporal gravity survey in February 2018 before the eruption, and in 3 surveys done approximately every 6 months between September 2018 and January 2020. More than 6.5 m of uplift were detected at the caldera centre of Sierra Negra between 2005 and 2018, and an accompanying 1080 mGal of gravity change was measured at sites in the centre of the caldera between 2007 and 2018. Joint inverse modelling of the gravity changes and uplift detected before the 2018 eruption is consistent with magmatic recharge in a sill-like source 1-2 km bsl, while post-eruptive deflation and a decrease in gravity detected shortly after eruption indicate mass loss from the system. Renewed uplift and gravity increase detected from 2018 onwards hint at magmatic recharge in the reservoir. Further constraining these models with observations from seismic, geochemical, and petrological methods will provide realistic scenarios of sub-surface processes at shield volcanoes, and invaluable information to forecast future eruptions.