The environmental effects of the 2017 Casamicciola earthquake for the seismic hazard assessment of the Ischia island (Tyrrhenian Sea)

The Ischia island is located on the northwest side of the Gulf of Naples. In historical times it was characterized by strong seismic events and tsunami correlated to mass instability related to the volcano-tectonic dynamics of the island itself. In particular the Casamicciola Terme village, in the northern area of the island, was heavily hit by strong earthquakes, the most important occurred in 1762, 1796, 1828, 1881, 1883 (Alessio et al., 1996), which allowed to recognize an active seismogenic area along the northern slope of the Mt. Epomeo resurgent block. Most of these earthquakes show parameters comparable with the August 21, 2017 seismic event, both for the epicentral zone, and for the macroseismic and environmental effects. On August 21, 2017 a shallow earthquake of Md 4.0 struck the Casamicciola village and the neighboring localities, causing 2 fatalities and about 40 people injured. Although the 2017 Casamicciola earthquake was a moderate size volcano-tectonic event, we observed several ground effects both primary (surface ruptures), and secondary (landslides, hydrological variations etc.). The main earthquake-induced effects detected during dedicated field surveys were: ruptures, fractures, landslides, variations in fumarolic activity and collapse of drywalls. The pattern of primary coseismic ground effects is represented overall by 62% of ruptures and 17% of fractures; the secondary effects consisted in 14% of drywall collapses; 6% of landslides phenomena; 5% of steam variations in the fumaroles. Considering the distribution of the primary and secondary coseismic effects we have assessed an epicentral intensity of VII degree ESI-07, taking into account the total length of the fault segment, about 2 km, as well as the area affected by other secondary coseismic effects, which is only of a few km2 (Nappi et al., 2018, Porfido et al., 2018). The study of the environmental seismic effects is fundamental both for the seismic hazard evaluation of Ischia, one of the most crowded touristic destinations worldwide, and also for the reconstruction of coastal and inland villages after the earthquake.