Seismic event update near the island of Ischia (NA), the historical seismicity of the island

To beyond 12 hours fromseismic event located by the National Institute of Geophysics and Volcanology 3 kilometers north of the island of Ischia, no other relevant shocks were recorded but only about thirty small earthquakes of very low magnitude (less than 1).

The island of Ischia, the largest of the Partenopee Islands which closes the Gulf of Naples to the west, has one historic seismicity well known.

Il Parametric Catalog of Italian Earthquakes CPTI15 (Rovid et al., 2016) contains the epicentral parameters of 12 earthquakes located on the island or in the sea in the immediate surroundings(represented and listed in the figure and table below), with magnitude Mw between just under 3.0 and just over 4.0. The oldest of these earthquakes occurred on November 2, 1275, the most recent dates back to April 23, 1980.

Ischia earthquakes in the CPTI15 catalog (Rovida et al., 2016)

Most of these events were already known in the large descriptive repertoire of Italian earthquakes compiled by Mario Baratta at the end of the 1901th century and published in XNUMX. The studies carried out over the last twenty years have documented the effects of all the known earthquakes affecting the island.

The main earthquakes are dated respectively 1275, 1796, 1828, 1881 and 1883.

Characteristic of these earthquakes is that they have rather modest magnitude estimates (values ​​calculated with specific procedures for earthquakes in volcanic areas and with very strong uncertainty values) corresponding effects of very high and destructive macroseismic intensity (X MCS in 1883 in Casamicciola), but generally affect a very limited area, while the area of ​​resentment is generally not very extensive.

These particularly dramatic scenarios – found in the seismic history of Casamicciola – are undoubtedly the result of a combination of complex factors and to be analyzed on a case-by-case basis.

Observed seismic history of Casamicciola (Locati et al., 2016)

Among the contributing causes that in the past determined the high consistency of the effects are the very superficial hypocentres, the geology of the island, the vulnerability of the building heritage and the high population density.

The island is included in the Phlegrean volcanic region and has had numerous eruptions in protohistoric and historical times, but the last of these phenomena (Arso eruption) dates back to 1301; the earthquakes that make up the seismic history of Ischia are not, however, accompanied by eruptive activity.

The fragility of the soil is instead evident, as evidenced by the frequency of landslides: the oldest historically well-documented dates back to the second half of the thirteenth century, the most recent occurred on November 10, 2009 when a ridge of the Mount Epomeo it detached due to heavy rains causing a landslide that reached the port of Casamicciola, causing one victim and numerous injuries.

Anthropization is very ancient, with prehistoric traces from 1400 BC and a Greek colonization from the XNUMXth century BC. The presence of minerals, the fertility of the volcanic soil and the presence of thermal springs have favored human presence throughout the historical period. The earthquake of July 28, 1883 it caused more than 2000 victims because it happened in the height of the tourist season, when the hotels were very crowded (the press of the time renamed it, not surprisingly, "the earthquake of the rich"). But we must not forget that just over two years before another strong earthquake, the March 4, 1881, it had caused equally serious damage, and the building stock of the island was evidently in very bad condition.

A particular mention deserves the case of the most recent earthquake, that of April 23, 1980, which in the CPTI Catalog15 has a relatively high magnitude value for this area (Mw 4.37); this value derives from an instrumental datum (Mw 4.6, ISC) affected by many uncertainties (uncertainties that also concern the location: the Vesuvius Observatory believed it to be located 20 km south of Ischia) combined with the macroseismic magnitude (3.1) due to an earthquake that caused fear but no damage. This earthquake clearly exemplifies the variability of location, magnitude and even macroseismic intensity estimates with their uncertainties.