Short- and long-term velocity variations and strain evolution at Ischia (ITALY) and their implications for dynamics of the hydrothermal system

Abstract

In active volcanic systems, the elevated pressurization of fluids and the movement of melt materials have an enormous influence on the stress-state of rocks and their mechanical behavior. We use seismic ambient noise to evaluate the static seismic velocity variations related to long-term volcanic deformation, and the dynamic changes associated with the 2017 Casamicciola earthquake (Mw 3.9), in the active volcanic complex of the Ischia Island (Italy). Our study reveals a significant dynamic velocity reduction mostly related to the near-surface damage, with a permanent drop near the red zone, that we posit to be related to the documented landslides and the subsidence observed immediately after the earthquake. We also report a positive long-term linear trend of velocity variations, sensitive to a generalized contraction of the Ischia Caldera that we revealed with geodetic modeling. Our results suggest a depressurization of the shallow hydrothermal system, through degassing along faults or sills.