Abstract
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
Highlights
Long-term seismic forecast usually assumes constant stress accumulation resulting from regional tectonic processes
We find that the continuous release of excess fluids of a sill with initial volume of ~5 km3 injected at 6 ka best matches the currently measured deformation rate and it is sufficiently large to maintain eruptible magma during the period of volcanic activity of Ischia (Figures 1c, 4a)
The flux of deeply derived CO2 measured at Donna Rachele (~3 kt/yr; Chiodini et al, 2004) is orders of magnitude lower than that affecting the nearby volcanoes Campi Flegrei (400-1000 kt/yr; Cardellini et al, 2017) and Vesuvius (~55 kt/yr; Frondini et al, 2004), in agreement with our model of intrusion that has already degassed most of the fluids
Summary
Long-term seismic forecast usually assumes constant stress accumulation resulting from regional tectonic processes. Ischia experienced one of the highest uplift measured at any volcano, reaching 1000 m in 35-55 ka (de Vita et al, 2010; Sbrana et al, 2018 and references therein) This process, called resurgence, produced a fault-bounded block (Figure 1a), with topmost uplift to the NW and an overall tilt downward SE (Acocella & Funiciello, 1999). The faults along the northern boundary of the block (Casamicciola area, Figure 1a) triggered shallow (120 casualties), 1883 (estimated magnitude 4.3-5.2; >2300 casualties) and 2017 (M = 4.0; 2 casualties) (Cubellis & Luongo, 1998; D'Auria et al, 2018; De Novellis et al, 2018; Nappi et al, 2018) The similarities among these events (fault location, shallow depth and subvertical geometry, earthquake magnitude between 4 and 5, and dip-slip motion) provide favorable conditions to investigate their origin and to attempt to forecast future events. The two distinct minima (benchmarks 100A and 35A) show a stable ratio through time, indicating interdependence in the process/es controlling the deformation (Figure1d, inset)
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