The 2021 La Palma (Canary Islands) Eruption Ending Forecast Through Magma Pressure Drop

Forecast of eruptive activity is a core challenge in volcanology. Here, we present an actual forecast for the end of the 2021 La Palma eruption. Using continuous GNSS data, we identified a co-eruptive quasi-exponential deflation trend. Assuming mass conservation, magma upflow from an overpressurized reservoir drives the eruptive process. The forecast was carried out during the eruption, however there was uncertainty in the key percentage of drop in driving pressure necessary to stop this eruption. In hindcast, we explore how forecast uncertainty reduces with increase in ingested near-real time data. We conclude that precise forecasts could have been possible, but only after twice a characteristic exponential decay time-scale, providing error estimates of 45% of the actual duration. We verify the mass conservation assumption using eruptive material volumes and propose that the eruption dynamics was controlled by a main reservoir at a depth close to Moho discontinuity beneath Cumbre Vieja volcano. The forecast, an actual prediction of the temporal or spatial characteristics of a future event, of when a volcanic eruption will end is challenging. This is reflected in the scarcity of literature about it. During the 2021 eruption of La Palma, we used GNSS geodetic data that tracks the change in shape of a volcano to make such a forecast. We exploited a temporal decaying pattern of this data and some basic assumptions that could be interpreted as when the eruption would stop. The forecast was made before the eruption ended, although it was too uncertain to have practical implications. With the eruption already over, we looked back at the data and found that the more information we could have analyzed, the more accurate the forecast could have been. We conclude that accurate forecasts could be possible after twice the characteristic time of the decay process has passed. Using deformation data, the end of La Palma eruption, although uncertain, was possible to forecast Extensive end of eruption hindcasts to understand bounds on method applicability for La Palma eruptions Real-time ground deformation interpretation could represent a simple and powerful tool for La Palma volcano monitoring