Shrinking of Ischia Island (Italy) from Long-Term Geodetic Data: Implications for the Deflation Mechanisms of Resurgent Calderas and Their Relationships with Seismicity

The identification of the mechanisms responsible for the deformation of calderas is of primary importance for our understanding of the dynamics of magmatic systems and the evaluation of volcanic hazards. We analyze twenty years (1997-2018) of geodetic measurements on Ischia Island (Italy), which include the Mt. Epomeo resurgent block, and is affected by hydrothermal manifestations and shallow seismicity. The data from the GPS Network and the leveling route show a constant subsidence with values up to -15 & PLUSMN; 2.0 mm/yr and a centripetal displacement rate with the largest deformations on the southern flank of Mt. Epomeo. The joint inversion of GPS and levelling data is consistent with a 4 km deep source deflating by degassing and magma cooling below the southern flank of Mt. Epomeo. The depth of the source is supported by independent geophysical data. The Ischia deformation field is not related to the instability of the resurgent block or extensive gravity or tectonic processes. The seismicity reflects the dynamics of the shallow hydrothermal system being neither temporally nor spatially related to the deflation.