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This work is devoted to the study of the background seismic noise at Ischia Island. We highlight two persistent independent signals, with dominant frequency peaks in the range 1−2 Hz and 3−4 Hz, respectively. The first signal is the most energetic and persistent and its polarization properties are well defined, revealing a shallow propagation likely controlled by both tectonic/volcanic structures and morphological features. This signal is well correlated at all the stations in the Casamicciola area and its group velocity is estimated as 500−600 m/s. The second signal shows similar polarization properties but the temporal pattern of its amplitude is strongly affected by anthropogenic noise, revealing a modulation on a diurnal scale. On the investigated spatial scales, any clearly correlated signal is not evidenced. We interpret the 1−2 Hz correlated wave-packets as the persistent whisper of the shallow circulation of fluids, which are a mixing of sea and meteoric water and thermal fluids of the hydrothermal reservoir. In the conceptual scheme of the self-sustained musical instruments, we hypothesize that the solid structure of the shallow hydrothermal system of Ischia is formed by a network of channels, continuously excited by the circulating hydrothermal fluids, which produce a whispered sound (i.e. the seismic noise). This mechanism is able to produce persistent self-sustained oscillations that we observe as the 1−2 Hz seismic signal and, eventually, higher modes.

In this paper we give an overview of the recent geophysical, geochemical and volcanological studies concerning the island of Ischia within the geological and tectonic framework of Southern Italy. Ischia is an active volcanic field that had a complex volcanic history resulting from dominant explosive and minor effusive activity, several caldera collapses, and renewed volcanism from vents located inside the collapsed area. The island is morphologically dominated by Mt. Epomeo, the result of a prominent resurgence phenomenon taking place since ca. 33 ka BP, and responsible for ca. 900 m of total uplift, one of the largest known compared to the relatively small size of the caldera. The uplift was accompanied by activation of faults, seismic activity and renewal of volcanism, and may be considered a main factor for inducing slope instability. For Ischia, volcanological, petrological and geophysical studies are, at present, limited compared to the other active volcanoes of the Neapolitan Area. Furthermore, the island is characterized by high volcanic, seismic and hydrogeological risks. Thus, this review is aimed at highlighting aspects of the knowledge on Ischia that need more investigations, in order to better assess some characteristics of its structural setting. Features such as the precise location of the caldera boundaries and the depth of the magma chamber representing the drive for the resurgence still need to be well defined. A critical analysis of all lines of evidence relevant to the current theories about the island resurgence ( resurgent block vs. resurgent dome ) has been carried out. Our analysis reveals that the resurgent block model, differently from the resurgent dome model, is consistent with the most significant features, such as tilting of the resurgent block, faults type, dip and distribution at the edges of the block, and occurrence of most of the past 10 ka eruption vents on the eastern sector of the island. However, as both model require an input of fresh magma into the shallow plumbing system, it is not clear at present how much magma was necessary to achieve the measured uplift, and whether the drive was provided by magma or magmatic volatiles.

In this paper we present an approach for evaluating landslide susceptibility in seismic areas. It is known that earthquake-induced landslide susceptibility is related to several, often interplaying, factors. Nevertheless, an effective grade-2 zonation should be characterized by a good balance between simplicity, quickness and reliability. The GIS-based procedure we present employs only three factors that we believe are the most significant in this susceptibility assessment: the type of outcropping rocks/soils, the slope angle and the MCS intensity. The local annual precipitation, certainly an essential factor, is considered here as a parameter whose seasonal pattern is constant in time and space. Each of the three parameters is expressed as a Significance percentage and the resulting Seismic Landslide Susceptibility level of an area is given by the average of the significances of the first two factors multiplied by the significance of the third factor. The procedure was set and tested on the volcanic island of Ischia (southern Italy), which was affected by several historical earthquake-induced landslides. The results of this susceptibility zonation test at Ischia show a very good match between the distribution of the sources of historical landslides and the areas we identified as the most susceptible ones.

The areas exposed to pyroclastic flow invasion in case of explosive eruptions at Ischia island (Italy) have been evaluated, together with the related volcanic hazard and risk. Ischia island lacks a central vent so, for the evaluation of volcanic hazard, it is necessary to forecast where a future explosive event might occur. At this purpose, we have evaluated the Vent Opening Probability by taking into account all the relevant geological information and processing them through spatial analysis. The whole Ischia island has been subdivided into 358 cells, in which we take into account the contribution of volcanological, geophysical and geochemical data to define the probability of opening of the vents. As a result, the highest spatial probability is found on the eastern sector of the island. Two series of simulations, by using the “energy cone” model [Malin, M.C., Sheridan, M.F., 1982. Computer-assisted mapping of pyroclastic surges. Science 217, 637–640] and assuming column collapse heights of 100 m, for eruptions of Volcanic Explosivity Index (VEI) = 3, and of 300 m for eruptions of VEI = 4, with Heim coefficient equal to 0.1, were performed. Through the overlay of the energy cone to the Digital Elevation Model of Ischia, it has been possible to automatically detect the invaded areas for each hypothesized vent. The intersection of this areas made it possible to define the different spatial probabilities of invasion by pyroclastic flow (PyF) for each point of the Ischia territory. Two different types of hazard maps have been derived: an equi-probability hazard map, sensitive to the distance from the vents and the presence of topographic obstacles, and a weighted hazard map, in which the probability of vent opening has also been taken into account. For both types of eruptions, the highest spatial probability of invasion are the so-called “graben of Ischia”, and the coastal plain between Monte Vico and Ischia Porto. Finally, by intersecting the exposed value map with the weighted hazard maps, the classification of the Ischia territory into three risk classes has been performed. The villages located in the Ischia graben and in the northern sector of the island have resulted to be exposed to the highest risk.