Seismic activity in the Ubaye Region (French Alps): a specific behaviour highlighted by mainshocks and swarm sequences

International audienceThe Ubaye Region is the most seismically active region in the Western Alps, with earthquakesthat were commonly felt by the population and that even damaged local villages and cities. Since thefirst testimonies in 1844, this area has been regularly struck by seismic swarms with a high number ofevents, such as in 2003–2004 or 2012–2015, or by mainshock–aftershock sequences with a magnitudeup to ML 5.3 in 1959. In this paper, we analysed both historical records and instrumental seismicityin the light of geological observations. Some earthquakes could be associated with known faults,even if most of them occurred on blind, unknown faults that reveal a highly fractured basement. Theabnormal level of seismicity, together with its peculiar behaviour, suggests complex driving processesinvolving not only tectonic loading but also fluid pressure