Dynamic and static strain sensitivity of seismic velocity variations at Ischia Island, southern Italy 

Ischia Island is the westernmost, active volcanic complex of the Campanian plain (Southern Italy, Civetta et al., 1991). A long-term depressurization (Sepe et al., 2007) in the local hydrothermal system is causing deflation and contraction (Trasatti et al., 2019) of the surrounding volcanic edifice. In 2017 a Mw 3.9 shallow earthquake occurred in Casamicciola, in the northern part of the island, causing landslides and several collapses (Nappi et al., 2018). Here we present seismic velocity variation measurements δv/v over 8 years (2016-2023) for variable coda waves time lapse using empirical Green's functions reconstructed by autocorrelation of seismic noise recorded at local velocimeters. We compared velocity variations time series with the temporal evolution of the strain, obtained from displacements measured at the GPS network deployed on the island.  We focused on short-term velocity variations caused by the earthquake and on the long-term trend of δv/v measurements. This latter shows to be related to the deformation mechanism affecting the volcanic edifice. We found high values in both dynamic and static strain sensitivity of velocity variations with appreciable differences on the island, reflecting the anisotropic pattern of depressurization. This also proves a significant non-linearity in the elastic properties of the local volcanic materials. The joint use of geodetic methods and ambient noise monitoring revealed a remarkable sensitivity of δv/v to depressurization processes and its potential to enhance our understanding of the dynamics of the magmatic system. References Civetta, L., Gallo, G., & Orsi, G. (1991). Sr- and Nd-isotope and trace-element constraints on the chemical evolution of the magmatic system of Ischia (Italy) in the last 55 ka. Journal of Volcanology and Geothermal Research, 46(3–4), 213–230. https://doi.org/10.1016/0377-0273(91)90084-D Nappi, R., Alessio, G., Gaudiosi, G., Nave, R., Marotta, E., Siniscalchi, V., Civico, R., Pizzimenti, L., Peluso, R., Belviso, P., & Porfido, S. (2018). The 21 August 2017 Md 4.0 Casamicciola Earthquake: First Evidence of Coseismic Normal Surface Faulting at the Ischia Volcanic Island. Seismological Research Letters, 89(4), 1323–1334. https://doi.org/10.1785/0220180063 Sepe, V., Atzori, S., & Ventura, G. (2007). Subsidence due to crack closure and depressurization of hydrothermal systems: a case study from Mt Epomeo (Ischia Island, Italy). Terra Nova, 19(2), 127–132. https://doi.org/10.1111/j.1365-3121.2006.00727.x Trasatti, E., Acocella, V., Di Vito, M. A., Del Gaudio, C., Weber, G., Aquino, I., Caliro, S., Chiodini, G., de Vita, S., Ricco, C., & Caricchi, L. (2019). Magma Degassing as a Source of Long‐Term Seismicity at Volcanoes: The Ischia Island (Italy) Case. Geophysical Research Letters, 46(24), 14421–14429. https://doi.org/10.1029/2019GL08537