Thermal monitoring to infer possible interactions between shallow hydrothermal system and slope-scale gravitational deformation of Mt Epomeo (Ischia Island, Italy)

Abstract

Geothermal and volcanic systems are prone to gravity-induced slope instability at different scales. Endogenous magmatic, hydrothermal and seismic forcings can significantly modify rock mass rheology and perturb the local stress field and gravitational equilibrium, inducing shallow or slope-scale processes. The island of Ischia, which is part of the Phlegrean Volcanic District (Italy), is a remarkable example of this kind of complex interacting system. This study focuses on monitoring the hydrothermal system located beneath the ongoing slope-scale deformation, which involves Mt Nuovo (the western part of Mt Epomeo) and is a complementary effect of the resurgence of an ancient caldera. Debris and rock avalanches have affected the slopes of this volcanic island, in response to the renewal of volcanic activity and caldera resurgence. Large parts of the corresponding mass-wasting deposits overlay the most active areas of the Ischia hydrothermal system, where ongoing slope-scale gravity-driven deformation owing to a mass rock creep (MRC) process is still evolving. To investigate possible relations between the perturbing shallow hydrothermal system and the MRC process, thermal monitoring of selected groups of fumarolic emissions located in several portions of the deforming sector has been carried out since 2008 on a monthly basis by means of direct (thermal probes) and remote sensing (IRthermography) techniques. Thermal monitoring of specific fumaroles reveals a peculiar seasonal trend characterized by a delayed inverse correlation with rainy periods and a short-term pulsating response to dry stages. The fumaroles also appear spatially correlated to the presence of MRC-related structures involving volcanic slopes. According to the measured thermal data, a conceptual model of the thermal interactions within the Mt Nuovo slope is provided, framing the potential role of thermal actions in accelerating the deformation process. In this view, possible chain effects, owing to magmatic or hydrothermal renovation, are depicted, delineating the most severe multihazard scenario consisting of an accelerating evolution of the MRC process towards paroxysmal collapse.