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.