The Catania region and nearby offshore, showing the faults on the flank of Mount Etna and the offshore North Alfeo fault (red lines), the MEOC (Main Electro Optical Cable) operated by INFN-LNSa (thick black line). A small earthquake (magnitude 2.6) occurred on 21 Nov. 2020 at 13:55 (UTC) in Santa Venerina village (green star). The small white star indicates the position 6 km from the port of Catania where the cable was disturbed (possibly by a submarine landslide). Credit: Gutscher et al., Earth and Planetary Science Letters (2023).
A cable placed at the bottom of the sea off the shore of Catania, in Sicily, may have detected movements of the underlying tectonic fault in November 2020, according to findings published in Earth and Planetary Science Letters1. If the observation is confirmed, signals from the cable may provide early warnings of future shocks in a zone that has seen major disasters, such as the 1693 and 1908 earthquakes.
In October 2020, researchers at the University of Catania and the South National Laboratories (INFN-LNS) also in Catania, among other institutions, placed a 6-kilometre fibre optic cable crisscrossing the surface of the Alfeo fault, off the coast of eastern Sicily. The cable was plugged to an existing 29-kilometre line running all the way back to Catania’s harbour, that was originally used for the Neutrino Ettore Majorana Observatory (Nemo) and later repurposed for marine research.
The scientists sent laser pulses through the cables and analysed how they were reflected over 21 months. They used an engineering technique taken from engineering and applied for the first time for this purpose, called Brillouin reflectometry, which allows to deduce strains put on the cable from the laser signals.
Between 19 and 21 November 2020, signals of strain emerged from two out of the three points in which the cable crosses the fault, leading to its elongation by 1.5 cm.
Seismic sensors around the area did not detect anything relevant in those days. That strain may be compatible with non-seismic phenomena like seabottom currents or submarine landslides. However, the location and timing of the signal suggests a slow fault slip may have also happened.
Researchers are continuing to collect data and hope that at some point they may detect strain in the cable followed by an actual seismic signal. “Offshore cables cross faults throughout the world,” says Giovanni Barreca, a geologist at the University of Catania. “If they have some prediction capacity, the system may be scalable to many other cases”.
Giorgio Riccobene, a physicist at INFN-LNS, notes that “beyond these cables, the sea of eastern Sicily has come to host many underwater sensors and is becoming a unique place to carry out a variety of correlated analysis”.
