Schematic illustration of the cause of seismicity at Ischia. The magma injected approximately 6000 years ago (A) is now degassing (B), leading to volume decrease and a general deflation of the island. The movement is rather continuous in time, but as it is mostly accommodated by faults on the Norther portion of the island, can occur with rapid accelerations, producing disastrous earthquakes such as those of Casamicciola in 2017.
Trasatti E.1, Acocella V.2, Di Vito M.A.1, Del Gaudio C.1, Weber G.3, Aquino I.1, Caliro S.1, Chiodini G.1, de Vita S.1, Ricco C.1, Caricchi L.3
1 INGV
2Università Roma Tre
3Università di Ginevra
In volcanic areas seismicity provides a further hazard, in addition to those deriving from eruptions and related phenomena. Seismicity usually results from various transient processes, and it is thus difficult to be forecast. In some cases, however, seismic events may follow a recurrent behavior, thus allowing understanding its causes and enhancing forecast.
The volcano of Ischia island (Italy) showed a strong uplift of its caldera floor, a process dubbed resurgence, and decades-long subsidence. Ischia experienced several shallow and destructive earthquakes with recurrent features in the last centuries, the last occurring in 2017. Here, we use geologic and geodetic data along with mechanical and thermal models to understand the causes of such a recurrent seismicity. We show that: 1) seismicity results from an acceleration of the constant (~1 cm/yr) subsidence of a part of the resurgent block; 2) this subsidence is driven by the degassing of the magma emplaced below the resurgent block in the last ~6 ka and previously responsible for part of its uplift. Our results suggest that degassing, driving subsidence and seismicity, should continue for a few hundreds to thousands of years. The possibility to constrain the future duration of seismicity at Ischia indicates that our capacity to forecast earthquakes might be significantly enhanced when seismic activity results from long-term magmatic processes, such as degassing.