Fluid-driven swarms and mainshock-aftershocks sequences in the Ubaye Region (Western Alps).

The Ubaye Region, located in the French Western Alps, is one of the most seismically active regions in France. It is regularly hit by mainshock-aftershocks sequences (1959, ML5.3), seismic swarms (2003-2004), and complex sequences (2012-2015) characterized by successive mainshocks clustered in time and space. This diversity of seismic behaviour highlights the complex processes at play in this area. To improve our understanding of these processes, we compile a regional catalogue of existing focal mechanisms, completed by 100 new calculated focal mechanisms of aftershocks following the 07/04/2014 mainshock (ML5.1). We reconstruct the stress-state orientation for different periods and sub-areas. We found that it is constant in time and space, and consistent to previous published values focusing on swarm periods in this area. We then calculate the fluid-pressure needed to trigger the events. Most of them (65%) need fluid-overpressure between 15 and 40 MPa (17-to-40% of the hydrostatic pressure) with a median value of 24%. Moreover, even the largest earthquakes, like the mainshocks in the 2012-2015 sequence, appear to be triggered by fluid-pressure, similarly as events within swarm sequences. While fluid-overpressure decreases with time in an aftershock sequence, it  varies randomly at high levels during a swarm sequence. Therefore, based on a fault-valve model, we propose that: 1) the fluids trapped in the fault plane tend toward lithostatic pressure and trigger the mainshock rupture and 2) part of the aftershocks are induced by the diffusing fluid-pressure. On the contrary, swarms need external, likely deep, fluid-pressure feedings. Fluid-pressure is likely to be a common triggering mechanism of the seismicity in the Ubaye Region, even if the involved processes should differ to explain the different types of seismic sequences.