Ground Deformation Prior to Phreatic Eruptions at Rincón de la Vieja Volcano, Costa Rica Detected Using GNSS

Studies of deformation in volcanic settings are typically focused on active magmatic systems where inflation due to the injection of magma is an important indicator of potential eruptive activity. Phreatic eruptions are the result of the sudden flashing of trapped water to steam without the eruption of magma, and the resulting deformation is at a much smaller scale that is more difficult to detect. Rincon de la Vieja volcano is highly active system in northern Costa Rica that experiences frequent, predominantly phreatic and phreatomagmatic, eruptive activity. Multiple GNSS stations are set up and maintained by OVSICORI as part of their nationwide network including stations located 1 km south-west (summit station) and 5 km northwest (flank station) of the active vent. The summit station shows repeated movement towards the vent (northeast) 1-2 months prior to significant eruptions (infrasound magnitude > 4.5) on January 30^th, 2020, April 15^th, 2020, May 25^th, 2020 and January 6^th, 2022. Using a 30-day moving average, the north-south component appears to be the most reactive with repeated deformation of approximately 5 mm compared to ~2 mm for the east-west component. The vertical component does not show a similar signal prior to single events, however it does show significant uplift (up to 20 mm) during periods of high eruptive activity. This signal is most prominent in 2020 and 2022 when Rincon experienced hundreds of eruptive events per year resulting in a saw-tooth pattern of deformation. Similar signals are not visible on the flank station, potentially indicative of a shallow pressure source close to the vent itself and consistent with pressurization within the shallow hydrothermal system. To date, a phreatic eruption has not been successfully forecast owing to a lack of reliable precursory signals. Improved eruption forecasting likely requires a combination of measurements to document changes in gas flux, seismic signals, thermal signatures, and deformation.