Abstract
The island of Ischia, an active volcanic field emerging in the western sector of the Gulf of Naples (Southern Italy), represents an archetypal case of caldera that underwent a very large resurgence related to the intrusion of a shallow magma body. The resurgence culminated with the formation of a structural high in the central sector of the island, i.e., the Mt. Epomeo block. This is bordered by a system of faults along which volcanic activity occurred up to 1302 A.D., and damaging earthquakes were generated in historical and recent time. The seismicity is located prevalently in the northern sector of the island and appears to be correlated with the most recent phase (<5 ka) of ground movement (subsidence), although the mechanism of earthquakes’ generation is still debated. By jointly analyzing offshore and onshore data (seismic profile and stratigraphy wells, respectively) and new petrological and geochemical data related to the most recent phase of volcano-tectonic activity, we develop a geological and structural layout of the northern sector of the island. In particular, we identify the seismogenic fault associated with the historical and recent destructive earthquakes of Ischia. This fault formed in the northern sector of the island during the final stage of the resurgence. We also propose a conceptual volcano-tectonic model of the northern sector of the Ischia Island, depicting the displacement of the fault zones in the off-shore area and the possible mechanism of stress loading and release in the on-shore zone, which is mainly driven by the subsidence of the Mt. Epomeo block. Our results are crucial for evaluating the dynamics of the seismogenic structures in the framework of the general subsidence of the island, as well as the related seismic hazard.
Highlights
Caldera long-term resurgence and subsidence represent primary processes that are generally associated alternatively to renewal of magma activity and following drainage/degassing or to fluid pressurization and depressurization (Marsh, 1984; Hurwitz et al, 2007; Kennedy et al, 2008)
North of Ischia, we observe an interdigitation of 1) thick volcanic units linked to the calderaforming phase of the volcanic field, 2) clay-rich marine sedimentary bodies from Campanian plain, and 3) thick epiclastic units formed during the caldera filling and the successive resurgence
After long-term resurgence of about 1,000 m lasting about 50 kyr, the island of Ischia experienced a generalized subsidence—at least in historical time–and low-magnitude but high-intensity earthquakes, which occurred in the northern sector, along volcano-tectonic structures formed during the most recent uplift phase (Sbrana et al, 2018)
Summary
Caldera long-term resurgence and subsidence represent primary processes that are generally associated alternatively to renewal of magma activity and following drainage/degassing or to fluid pressurization and depressurization (Marsh, 1984; Hurwitz et al, 2007; Kennedy et al, 2008). The general model of caldera resurgence is associated with magma intrusion in the shallow crust (after a caldera collapse), which perturbs the shallow aquifers generating large hot fluids advection (i.e., the geothermal system) and stressing the above crust, producing bending, faulting, volcanic activity, and earthquakes (Cole et al, 2005; Kennedy et al, 2012; Branney and Acocella, 2015; Galetto et al, 2017). In spite of their inferred relatively low magnitude (Table 1), the high damaging level of these events has been mainly ascribed to the
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