The slow but continuous lowering of Mount Epomeo could be the cause of the major earthquakes that hit the island in the past, including that of August 21, 2017. This interpretation is given by a study conducted by Ingv and Cnr, in collaboration with Dpc, published on Geophysical Research Letters
What produced the magnitude 4 earthquake that struck Ischia on 21 August? The main cause could be the load exerted by the rocks that form the block of Mount Epomeo on others, less rigid and with a ductile behavior, which are located about 2 km deep. The subsidence of this block generates seismic activity along a surface of sub-vertical discontinuity, extending in an east-west direction for about 2 km and the same depth. To formulate this hypothesis, a study conducted by a team of researchers from the National Institute of Geophysics and Volcanology-Vesuvian Observatory (Ingv-Ov, Naples), the Institute for Electromagnetic Sensing of the Environment (Irea, Naples) and the Institute of methodologies for environmental analysis (Imaa, Potenza) of the National Research Council (Cnr), in collaboration with the Department of Civil Protection (Dpc, Rome). The results of the work, entitled "The 21st August 2017 Ischia (Italy) earthquake source model inferred from seismological, GPS and DInSAR measurements", have been published in Geophysical Research Letters.
"The availability of satellite radar data from the Sentinel-1 constellation, from the European Copernicus programme, and those from the COSMO-SkyMed constellation, from the Italian Space Agency (ASI) and the Ministry of Defence", highlights Riccardo Lanari, director of Cnr-Irea , "has made it possible to detect a subsidence of the ground up to a maximum of 4 cm, following the seismic event, in an area close to Casamicciola Terme". The fault, associated with the event, is located at a small depth in the northern sector of the island, at the base of Mount Epomeo.
"Its identification was possible thanks to a multidisciplinary approach that made it possible to integrate seismological and GPS (global positioning system) data from the Ingv networks with satellite radar data processed by the Cnr", underlines Francesca Bianco, director of Ingv-Ov. “The earthquake of 21 August 2017 was the first seismic event with destructive effects on Ischia, recorded by modern instrumental networks”. The island of Ischia is a volcanic field. Mount Epomeo, the main relief of the island, was formed due to the lifting of rocks deposited on the bottom of a caldera in the central part of the island, thanks to the thrust exerted by a magmatic intrusion. The Epomeo rocks are the product of the great eruption of the Green Tufo of Mount Epomeo, which occurred 55.000 years ago. Over the past ten thousand years, the island has been the site of numerous eruptions; the last one dates back to 1302. Today it is characterized by fumarolic and hydrothermal activity and by sporadic seismicity. Before 2017, the last earthquake with catastrophic effects on the island was the earthquake of 1883, which caused over 2300 deaths and the destruction of 80% of the building stock in the town of Casamicciola. That event was the first earthquake that the newborn Italian state had to manage. Among the victims, also the parents and sister of Benedetto Croce who, then seventeen years old, was miraculously extracted alive from the rubble.
This research, the authors conclude, shows the value of collaboration between research institutions, which has always been promoted by the Dpc on civil protection issues. Cnr and Ingv, together, have studied the phenomenon, contributing to the geological knowledge of Ischia. They elaborated an explanatory model of the dynamics of the area, providing the first instrumental data of moderate degree seismicity in Ischia, never recorded before, and calculating the characteristics of the source of the earthquake of August 21, 2017 with quantitative parameters.
Attached image: 3D view of the vertical component of ground displacement estimated from Sentinel-1 and COSMO-SkyMed data and of the discontinuity surface that generated the seismic event of August 21, 2017; the white dots represent the main seismicity recorded.