The 2017 Ischia earthquake, which caused extensive damage between Casamicciola and Lacco Ameno, must be seen as a wake-up call. The past history of the island, in fact, shows that the strongest earthquakes often occur in 'swarms', in the same area, separated in time from each other by a few years, and with a total duration of a few decades. Therefore, the safety of buildings in an area, about 20 km2 wide, where past earthquakes have been highly destructive appears to be a priority
Following the seismic event that struck the island of Ischia on 21 August 2017, a team of researchers from the Vesuvius Observatory of the National Institute of Geophysics and Volcanology (OV-INGV), together with colleagues from the University of Trieste and of the Institute of Geophysics, China Earthquake Administration (IGPCEA), has analyzed in detail the earthquakes of the past in Ischia in correlation with the distribution of the damages produced to the urban centers which, in the case of the disastrous earthquake of 1883, were very well described by Giuseppe Mercalli.
In the study "Seismic risk mitigation at Ischia island (Naples, Southern Italy): an innovative approach to mitigate catastrophic scenarios" just published in the journal Engineering Geology, the researchers, in particular, analyzed the seismic sequence of the 800s, which began in 1828 and continued with five other important events, two of which, in 1881 and 1883, with enormous damage to people and things.
“Following the seismic events of the nineteenth century, Mercalli exhorted the authorities of the time to avoid rebuilding with the same techniques and in the same places destroyed in 1881. Unfortunately, the 1883 seismic event was not listened to, was particularly intense and the damage to housing structures and, consequently, the victims were much more numerous" - recalls Giuseppe De Natale, a researcher at INGV, who adds "in consideration of the current urbanization of the area, our work has also calculated, in a necessarily approximate way, the effects which would probably occur if a similar earthquake occurred, respectively, in 1883, 1881 or 1828".
"The expected scenarios, although they would imply less damage than the 'twin' earthquakes of the past, would still be very important, especially in the upper Casamicciola area which should, if anything, be dedicated to particular construction techniques".
“The work demonstrates that the most effective method for securing urban areas at greatest risk is to consolidate buildings so that they can withstand, in each area, the same intensities experienced during the 1883 earthquake, which can be reasonably considered the strongest expected event", underlines De Natale again, who concludes as follows: "The need for new research on seismic hazard and rapid safety measures for the most exposed urban centers of Ischia is evidently urgent, but nonetheless represents a paradigm for the entire Italian territory where, unfortunately, earthquakes of even extremely modest magnitude cause unacceptable victims and damages”.
It is necessary to remember that other recent studies, carried out by other researchers of the Institute, propose different models and interpretations both of the evolution of the seismogenic structure of the island of Ischia and of its dangerousness.
In particular, in the current state of knowledge, it is not possible to obtain a certain and unambiguous interpretation of the processes currently underway in the subsoil of Ischia.
INGV is, however, daily committed to achieving this fundamental scientific and social objective.
The published research has an essentially scientific value, without immediate implications regarding the aspects of civil protection at the moment.
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