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 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.