Imaging the North Ligurian Fault System (NLFS) with a submarine DAS cable

The NLFS extends along the French–Italian Riviera, offshore one of Europe's most densely populated coastlines. Originally formed as a normal-fault system during back-arc opening of the Ligurian basin, it is now being reactivated in reverse motion under regional compression driven by Africa–Eurasia convergence. Its most recent large rupture, the 1887 Mw ~6.8 Imperia earthquake, triggered a damaging tsunami along the Riviera - mostly on the Italian side. Yet the geometry of the fault system at seismogenic depth remains essentially unresolved. Dedicated marine surveys have imaged the fault traces at the seafloor and characterised the shallow structure through multibeam bathymetry and seismic reflection, but the thick Messinian salt layer prevents the imaging of deeper structures. Offshore earthquakes are located by land networks with kilometer-scale depth uncertainty, and small-magnitude seismicity (Mw < 2) around the fault is neither reliably detected nor accurately located.
We use the Lido submarine dark-fibre cable (ANR MARMOR), which runs from Monaco to Savona directly over the eastern part of the fault system, as a dense seismic array. Distributed acoustic sensing turns this 160-km cable into tens of thousands of channels, filling the offshore coverage gap directly above the active structures.
We present results from continuous DAS recordings acquired over the fault system: a two-year catalogue of submarine events including a wealth of previously undetected signals. The detection of very small earthquakes allows us to place new constraints on the fault geometry at depth. These new submarine observations extend the community's decade-long effort to image the Ligurian margin and assess its seismic risk.