Paleoseismological record of a major Holocene earthquake in the southwestern Alps

The Western Alps are characterized by moderate historical and instrumental seismicity, with no earthquakes larger than M 5.3. Whether regional faults are capable of generating larger surface-rupturing earthquakes therefore remains uncertain. Geological investigations in the Argentera Massif (Maritime Alps) provide new constraints on this issue. Along the Bersezio Fault, at the Italy–France border, we describe a ~ 2 km-long section of the fault that right-laterally offsets geomorphic markers. Three paleoseismological trenches along its associated fault scarp reveal evidence for a single coseismic rupture, with an apparent vertical offset of ~1 m. Radiocarbon dating of bulk sediment samples constrains this event to between 6.5 and 6.0 kyr. This represents the first direct observation of a Holocene paleoearthquake in the Western Alps. The estimated coseismic slip is consistent with an earthquake of M ≥ 6.5, while the fault-scarp geometry indicates cumulative strike-slip deformation. These observations raise questions about whether the present-day seismicity, dominated by extensional focal mechanisms, adequately represents the long-term deformation regime. These findings highlight the value of paleoseismic investigations for revealing the complexity of fault behavior and constraining earthquake potential in slowly deforming tectonic regions.