Seismic Activity in the Ubaye Region (french Alps): a Specific Behaviour Highlighted by Mainshocks and Swarm Sequences

The Ubaye Region is the most seismically active region in the Western Alps, with earthquakes that were commonly felt by the population and that even damaged local villages and cities. Since the first testimonies in 1844, this area has been regularly struck by seismic swarms with a high number of events, such as in 2003–2004 or 2012–2015, or by mainshock–aftershock sequences with a magnitude up to ML 5.3 in 1959. In this paper, we analysed both historical records and instrumental seismicity in 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. The abnormal level of seismicity, together with its peculiar behaviour, suggests complex driving processes involving not only tectonic loading but also fluid pressure.