Magma Degassing as a Source of Long‐Term Seismicity at Volcanoes: The Ischia Island (Italy) Case

Transient seismicity at active volcanoes poses a significant risk in addition to eruptive activity. This risk is powered by the common belief that volcanic seismicity cannot be forecast, even on a long term. Here we investigate the nature of volcanic seismicity to try to improve our forecasting capacity. To this aim, we consider Ischia volcano (Italy), which suffered similar earthquakes along its uplifted resurgent block. We show that this seismicity marks an acceleration of decades‐long subsidence of the resurgent block, driven by degassing of magma that previously produced the uplift, a process not observed at other volcanoes. Degassing will continue for hundreds to thousands of years, causing protracted seismicity and will likely be accompanied by moderate and damaging earthquakes. The possibility to constrain the future duration of seismicity at Ischia indicates that our capacity to forecast earthquakes might be enhanced when seismic activity results from long‐term magmatic processes, such as degassing Plain Language Summary Seismic events that take place in volcanic areas are influenced by multiple volcanic processes, so seismicity is difficult to forecast. Sometimes seismicity may follow a recurrent behavior in time and mechanism. Therefore, understanding the main processes active at volcanoes may help to understand the causes of seismicity and contribute to forecast. The volcanic island of Ischia (Italy) shows decades‐long subsidence of its surface up to 1.2 cm/yr and experienced several destructive earthquakes in the last centuries. We use a multidisciplinary approach of mechanical and thermal simulations to understand the active processes and, in turn, to give insights on the causes of such seismicity. We find that the intense degassing of a magmatic body at 2 km depth drives the island‐scale subsidence and causes the observed recurrent seismicity. Our simulations show that degassing will continue for hundreds/thousands years. These results highlight that if seismicity is caused by recognizable magmatic processes, such as degassing, the capacity to forecast earthquakes at volcanoes may be significantly enhanced. Key Points About 30 years of leveling data show constant rate subsidence of the resurgent block at Ischia (Italy) Mechanical and thermo‐petrological simulations indicate magmatic degassing as main cause of the subsidence of the resurgent block Seismicity marks an acceleration of decades‐long subsidence, driven by degassing that will contin