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CATANIA – «It is clear that there is no direct physical connection between the earthquake last night and Etna and the reason is very simple. The two geological objects are separated by about a hundred kilometers. So, either one imagines that under Eastern Sicily there is a huge magma basin that on the one hand causes the eruptions of Etna and on the other moves the faults, or, as is obvious, this connection does not exist. “Thus, Marco Neri, first researcher and volcanologist at Ingv.
«It is not Etna that produces the earthquakes in Ragusa – Neri emphasizes – but both the volcano and the earthquakes in Ragusa originate from a geodynamic process that moves the tectonic plates: it is not that an earthquake triggers an eruption on Etna or vice versa, but both are manifestations that somehow originate in the movement of the plates “
“Until a million and a half years ago Which seems like a lot, but it is very little from the geological point of view -explains Neri-, the Iblei mountains were home to many volcanoes and Etna, which is much younger, did not yet exist. In the last million years, eruptive activity has shifted north into eastern Sicily and has been focused on the Etna area.
«Etna, therefore, was not the only active volcano in eastern Sicily – explains the expert – and it is clear that there must still be a certain degree of kinship between its eruptive manifestations and the earthquakes in the Iblea area, although there is no direct connection. Both the volcano eruptions and the earthquakes that occur in southeastern Sicily are in some sense related, albeit a little distant. They are cousins. “The single parent resides in the faults that delimit the tectonic plates.”
“In this case, then -continues-, in the same tectonic line that separates the European plate from the African there are areas that are marked by the presence of faults, for example in front of Syracuse or like the fault that moved last night off the coast of Ragusa and when these faults move generate earthquakes. But the same boundary that separates the European plate from the African, further north, allows the 30-km-deep magna to reach the surface and form the Etna volcano. These Iblei earthquakes and the manifestations of Etna, therefore, it is as if they had a single father but generated by different mothers without therefore a direct connection between them ”.
“An earthquake of magnitude 4.4 – Neri adds, about yesterday’s earthquake in the Ragusa area – it is quite strong, but the consequences of this earthquake also and above all depend on the depth of the hypocenter. This earthquake occurred at a depth of 30 kilometers with a space that served to slightly attenuate the propagation of seismic waves, reaching the surface in a less violent way – so much that people perceived it – but without generating any damage. A strong earthquake, yes, because people feel it, but it does not cause permanent damage in the area. If the same earthquake, instead of occurring at a depth of 30 kilometers, were 10 kilometers away, the situation would be a little different. If the same earthquake occurred in the Etna area where earthquakes tend to be with much shallower hypocenters, the earthquake could cause much more significant damage.
“The fact that people have warned In various parts of Sicily the shock -explains Neri- also depends on the type of geological substrate that -in this case- make up the Iblei mountains, formed mainly by very compact limestone rocks and very different from the clay rocks that, for example, emerge in the center. from Sicily, in the Catania-Messina motorway area, very rich in clay. Clays, in general, are quite plastic sediments, while carbonate rocks like those of the Iblei are very rigid. So when an earthquake travels between rigid rocks, the propagation wave travels very quickly over great distances. If, on the contrary, seismic waves pass through clay rocks, such as those in central Sicily, this transmission slows down and the seismic wave changes its shape a little. An earthquake that occurs in the Iblei is usually affected in a large area precisely because these mountains, being rigid, transmit and propagate seismic waves more efficiently.
And to the question, if it is possible to foresee other tremors, he answers: “Usually when an earthquake occurs it is generated by the movement of a fault that is nothing more than a fracture of the crust. In a fault, the energy accumulates until it exceeds the rock resistance threshold, the rock splits and generates the earthquake. It is very common that when this occurs the earthquake is not isolated, but others can occur, which are defined as aftershocks. When there is an earthquake of a fairly good magnitude, like the 4.4 one, a seismic swarm is possible, but I challenge anyone to predict what will happen in the immediate future. The truth is that all of eastern Sicily is in an area where earthquakes are frequent. So more than worrying about the next earthquake, we should worry about how our houses are made, how safe they are so that we can focus our attention on the resilience of the places where we are. In a word, earthquake-proof houses … ».
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