Subduction zones host the largest, most destructive earthquakes, along with a large variety of seismic and aseismic slip processes. Understanding these processes, and the relationships between them, is necessary to better constrain the mechanical properties of the fault and to better assess seismic hazard. For this purpose, repeating earthquakes are an important seismological tool, able to link aseismic slip and seismic behavior.

Subduction zones are home to some of the largest and most potentially damaging earthquakes on Earth. These regions often present a complex slip behavior, as areas of slow aseismic slip can neighbor earthquake rupture. The slip behavior is strongly controlled by the geometry, the structural heterogeneity and the stress state of the megathrust. To know whether a large and damaging earthquake can occur on a given portion of a fault, we therefore need to characterize the fault properties and processes acting upon it in detail. Characterizing the source physics of small to moderate earthquakes can allow us to gain key insights into the large-scale mechanical properties of a fault. In particular, stress drop (Δσ) indicates the difference in stress levels between the start and end of an earthquake, and can therefore be an indicator of the initial stress heterogeneities on the fault, as well as its shear strength.On average, stress drop is thought to be constant across scales (Abercrombie, 1995;Aki, 1967;Allmann & Shearer, 2009). However, in detail, variations in stress drops have been observed and linked to a variety of factors. Some studies have found an increase in stress drops with magnitudes on regional scales (Bindi et al., 2020),

Tel.: +33-48361-8779 † Presented as an expanded abstract: Dessa, J.-X.; Beslier, M.-O.; Schenini, L.; Chamot-Rooke, N.; Corradi, N.; Delescluse, M.; Déverchère, J.; Larroque, C.; Miguil, B.M.; Operto, S.; et al. (The SEFASILS Shipboard) Team. Seismic exploration of the deep structure and seismogenic faults in the Ligurian Sea by joint MCS and OBS acquisition: Preliminary results of the SEFASILS cruise.Abstract: The north Ligurian margin is a complex geological area in many ways. It has witnessed several phases of highly contrasting deformation styles, at both crustal scale and that of shallower cover tectonics, simultaneously or in quick succession, and with significant spatial variability. This complex interplay is mirrored in the resulting intricate structures that make it hard to identify active faults responsible for both, the significant seismicity observed, and the tectonic inversion undergone by the margin, identified at longer time scales on morphostructural grounds. We present here the first preliminary results of the leg 1 of SEFASILS cruise, conducted in 2018 offshore Monaco, in an effort to answer these questions by means of modern deep seismic acquisitions, using multichannel reflection and wide-angle sea-bottom records. Some first interpretations are provided and point towards an active basement deformation that focuses at the limits between main crustal domains.