Active Faults and Surface Ruptures in the Low-Strain Ubaye–Mercantour Region (Western Alps)

The Western Alps have been the focus of detailed seismological investigations based on instrumental records, revealing diffuse seismicity predominantly expressed as earthquake swarms (M < 3.5), mainly concentrated along major inherited shear zones. Geological evidence indicates that these structures are compatible with a main cumulated strike-slip motion, whereas GPS data and instrumental seismicity suggest predominantly vertical deformation. Historical archives further document several moderate earthquakes (M > 5), particularly in the Ubaye–Mercantour region. The Durance–Sérenne–Bersezio fault system is identified as the main active structure in this area and is therefore the focus of a multidisciplinary study aimed at detecting and characterizing co-seismic surface ruptures.

At the Lombarde Pass (Mercantour), a 2 km-long fault scarp displays geomorphological markers indicative of right-lateral strike-slip motion along the Bersezio fault. Several ERT profiles across the fault highlight a very localized low-resistivity zone in the bedrock beneath the morphological scarp. Paleoseismological trenches excavated across the fault scarp reveal a clear, single co-seismic rupture, with a maximum vertical apparent offset of ~1 m at the bedrock–Quaternary deposits interface. Radiocarbon dating (¹⁴C) of bulk sediment samples from three trenches constrains this event to 7–6 ka cal BP, consistent with post–Younger Dryas deglaciation.

These results suggest the occurrence of large-magnitude earthquakes (M > 6) in a region currently dominated by swarm seismicity and provide new constraints on fault kinematics and deformation localization at the boundary between the internal and external Alpine domains.

This study sheds new light on discussions held during the PATA Days 2022 field trip, where this unusual tectonic structure in the Western Alps raised passionate questions about its Holocene activity and seismic potential.