Short‐ and Long‐Term Seismic Velocity Variations and Strain Evolution at Ischia (Italy): Implications for Dynamics of the Hydrothermal System

Abstract In active volcanic systems, the elevated pressurization of fluids and the movement of molten materials influence the stress state and mechanical behavior of rocks, but the direct measurement of these processes and the related evolution of rocks properties is difficult. By studying seismic velocity variations, we quantify the physical changes in rocks induced by long‐term volcanic deformation and the dynamic changes associated with the 2017 Casamicciola earthquake (M w 3.9) in the active volcanic complex of Ischia Island, Italy. Our study reveals a significant dynamic velocity reduction (∼0.2%), primarily due to near‐surface damage, with a permanent drop linked to documented landslides and subsidence observed immediately after the earthquake. We also identified a positive long‐term linear trend in velocity variations, indicative of a generalized contraction of the Ischia Caldera, as revealed by geodetic modeling. Our results suggest a depressurization of the shallow hydrothermal system through degassing along faults or sills.