Italy is one of the nations most exposed to natural risks. As a matter of fact, the whole territory is characterised by an intrinsic fragility deriving from its peculiar morphologic, geologic, seismic, hydrogeologic features. Such a fragility is enhanced by the actions deriving from the unrelentless climate change that influences both intensity and frequency of meteoric hazards, and from the anthropogenic pressure related to the rapid and unregulated growing of urban environment.
The combination of these predisposing actions results in an extremely high susceptibility of the whole territory to triggering events, such as intense and prolonged rainfalls, land and coastal storms, unusual drought periods, strong-motion earthquakes, volcanic phenomena, and other natural hazards frequently occurring in whole Italian peninsula. The enhanced vulnerability of the urban environment due to its progressive aging as well as the lack of maintenance and the growth of its exposure related to the raising intensity and interconnection of human activities lead to an increased complexity of the resulting risks.
As a matter of fact, last disaster events, related or not to COVID 19, demonstrated that it is no more possible to apply mitigation strategies considering one risk at a time, but it is compulsory to analyse risk considering its complex nature deriving from the strong interaction among the different predisposing and triggering factors. This is true at any considered scale and with reference to any disaster cause.
A current key issue towards mitigation of natural risks is the transition from the conventional single hazard evaluation to multi-hazard approaches, in order to account for the possible combination of predisposing factors, preparatory phenomena and triggering events inducing cascading effects. In this study, a multi-hazard approach is tentatively adopted with reference to the evaluation of ground instability and permanent deformation related to liquefaction and landslides in the volcanic island of Ischia. This environment, well known for the richness of its hydro-thermal resources, was frequently interested by instability caused by hydro-meteoric and seismic events, the latest of which occurred with an unusual spatial and temporal concentration in 2009, 2017 and 2022 in the same area of Casamicciola. A multi-disciplinary multi-level approach has been carried out to accurately define the predisposing and preparatory factors, as well as the critical events triggering ground instability in the studied area. A careful collection of geological, geophysical and geotechnical data has been carried out aimed at defining geometric and mechanical properties of the volcanic granular deposits covering the island, which are characterised by a high degree of heterogeneity and, insofar, have been poorly investigated under the geotechnical viewpoint.
A specific study has been conducted in cooperation with researchers operating in the fields of geology and seismology aimed at better characterising the peculiar seismicity of the island related to its volcanic nature. Synthetic reference input motions were defined, based on specific simulations of the source slip and deep propagation mechanisms. Different quantitative scenarios of ground instability caused by the seismic triggering events have been produced by adopting a multi-level approach for both slope displacement and liquefaction. From the perspective of multi-hazard analysis (Zschau, 2017), potential cascading effects related to ground instabilities were investigated considering both the ‘multi-layer single hazard’ (i.e. no interaction between hydro-geological and seismic hazard) and the ‘multi-hazard’ approaches (i.e. hazard interactions considered). In the first approach, the hydro-meteoric action has been considered as a predisposing factor preceding a strong-motion earthquake, i.e. the variable susceptibility to seismic instability was related to the seasonal groundwater fluctuation and to a maximum potential earthquake. In the second case, the annual rate of occurrence of a strong-motion earthquake was probabilistically combined with the chance of having a concurrent extreme meteoric event.
The procedure adopted may represent a methodological proposal for the definition of realistic scenarios induced by cascading hazards, and to provide an estimate of their impact on the built environment through the determination of suitable engineering demand parameters related to co-seismic soil deformation. This in turn may contribute to the objective of vulnerability reduction and risk mitigation through adequate engineering and urban planning solutions.