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Short‐ and Long‐Term Seismic Velocity Variations and Strain Evolution at Ischia (Italy): Implications for Dynamics of the Hydrothermal System
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Short‐ and Long‐Term Seismic Velocity Variations and Strain Evolution at Ischia (Italy): Implications for Dynamics of the Hydrothermal System
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Journal Article
Short‐ and Long‐Term Seismic Velocity Variations and Strain Evolution at Ischia (Italy): Implications for Dynamics of the Hydrothermal System
2024
Overview
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 (Mw 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. Plain Language Summary Volcanic systems are influenced by a variety of complex and rapid processes, including significant temperature variations, intricate stress patterns, and changes in fluid pressure. In our study, we used seismic wave velocity measurements from ambient noise recordings and GPS data to show that seismic velocity changes are highly sensitive to depressurization, ground shaking, and damage levels. These changes reflect the pressure levels of hydrothermal and magmatic fluids in volcanic regions. Our results indicate varied shallow degassing, mainly in the northern part of the island, and lower effective stress in the southern part, where there is higher geothermal activity and highly pressurized fluids. While GPS data provide surface measurements, seismic velocity variations offer insights into the Earth's crust. Together, these measurements help us understand deformation processes at different depths, which is crucial for monitoring volcanic activity. Key Points We characterize short and long‐term seismic wave velocity variations during 8 years at the volcanic Island of Ischia (Italy) We revealed a significant coseismic drop in occurrence of the 2017 Mw 3.9 Casamicciola earthquake tracking the near‐surface damage We observe a remarkable sensitivity of the seismic wave velocity to depressurization processes of the hydrothermal system
