Volcanic Hazards Assessment at the Island of Ischia, Within the Neapolitan Area (Italy)

The island of Ischia is one of the three active volcanoes of the Neapolitan area. It hosts a permanent population of about 50,000 people, which increases up to 200,000 in summer time. The volcanic hazard of the island is extremely high also because of its explosive character. The intense population of both island and surrounding Neapolitan area, thriving farms and a complex trade-network with the near city of Naples, contribute to determine a high volcanic risk in the area. Volcanic hazards assessment, including possible eruption scenarios, is critically based on knowledge of the volcano past behavior and the definition of its present structural setting. Volcanism at Ischia began prior to 150 ka bp and continued until the 1302 A.D. last eruption. It is dominated by the Mt. Epomeo Green Tuff caldera-forming eruption (55 ka), followed by resurgence, which has caused a net uplift of the central part of the island of about 900 m over the past 33 ka. The most recent period of activity began at about 10 ka, with volcanism mainly concentrated around 5.5 ka and in the past 2.9 ka. During the past 5.5 ka, about 45 effusive and explosive eruptions took place, with almost all the vents located in the eastern portion of the island. The time-space vents distribution has been directly related to a simple-shearing block resurgence mechanism. Effusive eruptions emplaced lava domes and lava flows moving along the valleys of the north-eastern sector of the island. Explosive eruptions, both magmatic and phreatomagmatic, generated tuff cones, tuff rings and variably dispersed pyroclastic-fall and -current deposits in the eastern sector of the island. Areal distribution maps of these deposits do not permit to estimate the magnitude of the explosive eruptions, as a large amount of tephra was deposited into the sea. Maps of the frequency of deposition show the areas that more frequently have been covered by fallout deposits and invaded by pyroclastic currents. Three classes of frequency have been defined: 1) areas affected by the fallout deposits of 5 to 6 eruptions, and by the pyroclastic current-deposits of 5 to 8 eruptions; 2) areas affected by 3 to 4 eruptions; 3) areas affected by 1 to 2 eruptions. The fallout frequency map shows that only one area is included in class 1, and almost all the eastern sector of the island has been covered by fallout deposits at least once in the past 10 ka. The pyroclastic-current frequency map shows that three areas are included in class 1. All the eastern sector of Ischia and a small area along the northern coast were invaded by pyroclastic currents originated by vents located east of the Mt. Epomeo resurgent block. As in the case of fallout deposits, only one eruption emplaced pyroclastic-current deposits in the south-western part of the island. The results of stratigraphical, volcanological and structural investigations suggest that the possible eruption scenarios in case of renewal of volcanism in short-mid terms, that is in years or tens of years, include either an effusive or an explosive eruption. The north-western part of the area active during the past 2.9 ka has the highest potential for opening of a new vent and invasion by lava flows and pyroclastic currents. All the eastern portion of the island could be affected by tephra fallout.