Catastrophic landslides off Ischia volcanic island (Italy) during prehistory

Generally the evaluation of the geological hazards from active volcanoes chiefly concerns the prediction of eruptions whereas less attention is generally paid to other volcanicrelated phenomena, such as avalanching–landsliding. This is the case for Ischia (Italy), an active volcanic complex, whose collapse behaviour is only now being evaluated and recognized following extensive marine geophysical and geological investigations. The island of Ischia represents the emerged section of a larger east–west-trending volcanic ridge. The central sector of the island, Mt. Epomeo, has risen to at least 800 m above sea level (a.s.l.) in the past c. 30 ka, at an average rate of 20 mm a. The major consequence of such volcano-tectonic uplift includes either sudden collapses, with attendant debris avalanches, or other mass movements in the form of mud-debris flows, debris slides and rock-falls, all radiating out from Mt. Epomeo and most of them entering the sea. During prehistoric times the island of Ischia underwent major catastrophic collapses resulting in debris avalanche deposits of .1 km to ,0.5 km that have been recognized offshore both NW and south of the island. This study provides possible scenarios for the emplacement of these deposits, with particular reference to the resulting landslide-related tsunami hazard. Ischia is a densely populated volcanic island bordering the Bay of Naples, Italy, to the west (Figs 1 and 2). The island is inhabited by some 45 000 people, and the population increases several-fold during the summer as a result of tourism. Ischia’s natural history includes a broad range of volcano-related phenomena such as eruptions, shallow earthquakes and landslides, whose influence on human activity is proven since Neolithic (3000–4000 years BP), when the island was already inhabited by Italic populations. Then, after a long unwitnessed interval, the island was settled by Greeks around the seventh century BC and became the first Greek colony in southern Italy under the name of Pithecussai (Buchner 1986). The first traces of the Roman civilization date back to the fourth century BC and partly overlap with the Greek former colonization. According to archaeologists the Roman village of Aenaria suddenly disappeared around AD 130–150, probably after a natural disaster (i.e. an eruption or a volcanic earthquake; see Buchner 1986). Eruptions lasted until the Middle Ages, and the last occurred in AD 1302 when the Arso lava flow covered the eastern side of the island (Vezzoli 1988). The last destructive earthquake occurred in 1883 at Casamicciola village on the north side of the island (Cubellis & Luongo 1998). Volcano-tectonic ground uplift, coupled with intense hydrothermal weathering and seismic shaking, has strongly favoured recurrent landsliding over the subaerial flanks of Mt. Epomeo, the major relief of the island. These landslides, in the form of debris slides, debris flows or mud flows, and their effects on humans have been frequently reported in historical chronicles from the Middle Ages to the present (see Guadagno & Mele 1995; Mele & Del Prete 1998). Ischia is one of the seven Italian volcanoes considered ‘active’ by the national scientific community, the others being Campi Flegrei and Vesuvius in the Neapolitan region; the islands of Stromboli, Volcano and Pantelleria; and Etna in Sicily. Provided an average recurrence interval can be determined (based on its historical–stratigraphical record) a volcano may be considered active if the recurrence period is greater than the time elapsed since the last eruption. This definition suggests the presence of a magma chamber at relatively shallow depths, with the potential hazard of future eruptions. After the discovery of large underwater hummocky fields in deep water to the south of Ischia (Chiocci et al. 1998) the island offshore has been the subject of a wide range of marine geophysical and geological surveys. The major outcome was the recognition of the ‘Ischia debris avalanche’, the first unequivocal example of a mass-transport deposit related to a prehistoric collapse so far recognized in the Mediterranean (Chiocci & de Alteriis 2006). From: VIOLANTE, C. (ed.) Geohazard in Rocky Coastal Areas. The Geological Society, London, Special Publications, 322, 73–104. DOI: 10.1144/SP322.3 0305-8719/09/$15.00 # The Geological Society of London 2009. At the same time, previously unreported hummocky terrains were detected at shallow depths offshore to the north and west of the island (Violante et al. 2004). These studies have revealed the importance of gravity-driven processes leading to the development of extremely rapid (catastrophic) landslides, such as debris avalanches and debris flows, entirely or partially underwater. The studies also established that such deposits are all related to the volcano-tectonic uplift of Mt. Epomeo, and that they are one or more orders of magnitude greater than the on-land landslides mentioned above. Such behaviour makes this volcanic complex, together with the island of Stromboli, among the most likely potential sources for landslide-related tsunami in the Tyrrhenian Sea. The aim of this paper is to provide new geophysical and geological data on Ischia submarine mass movements and to discuss their origin and behaviour. In addition, an attempt is made to assess the geological hazard affecting the island and the neighbouring coastal zone.