Earthquakes in the Tyrrhenian Sea: how and why they occur

Yet, as the INGV reports in one of its articles, the Marsili is characterised by ‘hydrothermal activity and very low-energy seismic activity’.

No direct correlation can be found for two other factors either: the 20 km distance between the volcano and the area affected by the seismic sequence of recent days, and the discrepancy between the extensional volcanism of Marsili and the compressional source that characterised the earthquake on 23 October.

Salvatore Stramondo, the head of the Earthquake Department at the INGV, explains that the Tyrrhenian Sea, particularly its southern part, is affected by three main seismic sources: 1) crustal sources; 2) deep sources; 3) volcanic sources.

Whilst underwater volcanic eruptions can be ruled out in this case, the characteristics of the deep-sea vents do not match those of the October tremors either.

Earthquakes of this type can have a hypocentral depth of between 400 and 600 km and are caused by the subduction of the Ionian plate beneath the Calabrian arc: the plate gradually descends to a depth of hundreds of kilometres, until it reaches the lithosphere and collides with it.

The October event, on the other hand, was a crustal seismic sequence, caused by an accumulation of energy generated by the continuous northward movement of the African plate along its boundary with the Eurasian plate.

Stramondo points out that, whilst seismic zones are well known and monitored, the timing – that is, when the next earthquake will occur – cannot yet be predicted.

Meanwhile, the Phlegraean Fields recorded further tremors on Tuesday 27 October – albeit minor ones – following the swarm of tremors with a magnitude of 4.4 last May. ‘It is important to remember that the Phlegraean Fields are a volcanic area, one of the highest-risk areas in the world,’ emphasises Stramondo. “It is an active area: the ground is rising due to gases heated by the magma beneath. Furthermore, the risk – which is the product of hazard, vulnerability and exposed value – is due to the presence of numerous homes and infrastructure on the surface of the area in question.”

As for the ever-present fear of a possible eruption of Vesuvius, Stramondo reassures us that ‘Italy’s volcanoes are the best monitored in the world’, thanks to a system that provides benchmark parameters. Furthermore, consulting past reports makes it possible to identify potential warning signs and activate any necessary alert measures.