Following the earthquake that struck the island of Ischia on 21 August 2017, at 20:57 Italian time, teams of the QUEST (QUick Earthquake Survey Team) operational group of the National Institute of Geophysics and Volcanology (INGV) in collaboration with personnel of the National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA) began carrying out the macroseismic survey on the ground starting from the afternoon of 23 August. The survey was conducted according to the EMS98 European scale (European Macroseismic Scale [Grünthal, 1998] hereinafter EMS), which contemplates a detailed case history of construction typologies and levels of damage aimed at making the assessment of the intensity as objective as possible ) and concerned only external damage to buildings.
The area most damaged (and almost exclusively) was the hilly part of Casamicciola Terme, whose inhabited area is distributed on the northern side of the island. In order to clearly distinguish the areas of different types of damage within the municipal territory of Casamicciola Terme, two areas were kept separate in this phase of the survey: the hilly part of Casamicciola (Red Zone) and Marina di Casamicciola.
Marina di Casamicciola does not appear particularly damaged, having suffered minor damage, even if quite widespread (EMS VI intensity).
On the hilly part (Zona Rossa) the town of Casamicciola is built on the crests of small valleys or terraces and alluvial fans, like small separate settlements. Here the damage is locally very serious. In fact, although the majority of the houses were of type B, according to the classification of the EMS scale (buildings of good workmanship in bricks or blocks of tuff or squared stone), there were no tie rods and chains or other binding elements. In percentage terms, on the other hand, there are few type A houses (buildings in unworked stone, masonry with poor quality mortar sometimes absent, in general the most vulnerable) and type C (generally in reinforced concrete or curb masonry).
Casamicciola. Lesion on the wall with expulsion of masonry.
The observed damages are few total collapses, many damages such as cross cracks, loss of verticality and overturning of walls, ejection of edges, some partial collapses (see photos above and below). The reinforced concrete buildings have in rare cases suffered damage such as minor damage to the infill walls.
The complex of damages observed justifies the assignment of grade VIII to the red zone of Casamicciola Terme.
Widespread damage also in the Fango district of Lacco Ameno which was assigned an intensity equal to VII EMS. In Lacco Ameno capital sporadic very slight damages are observed (EMS intensity V).
The assignment of the lowest grades, where there is no manifestation of the damage, was carried out through interviews with the population, with the evaluation of the transient effects, such as falling objects and the level of perception of the population.
The concentration of the damages and their severity configure a clear site effect in the hilly area of Casamicciola Terme. This also agrees with the very strong attenuation observed: in fact, even at very small distances from the most damaged area, no damage effect appears (see table above).
The survey is still ongoing by the QUEST teams in order to complete the picture of resentment on the whole island.
Link to the INGVterremoti blog:
https://ingvterremoti.wordpress.com/2017/08/28/terremoto-dellisola-dischia-del-21-agosto-2017-il-rilievo-macrosismico/