Swarms and mainshock–aftershocks sequences are both triggered by fluids in the Ubaye Region (Western Alps)

Abstract

SUMMARY The Ubaye Region (French Western Alps) is one of the most seismically active regions in France. It is regularly struck by mainshock–aftershocks sequences like in 1959 (ML 5.3), 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, in this study, we compile a regional catalogue of existing focal mechanisms, completed by 100 new calculated focal mechanisms of aftershocks following the 07/04/2014 mainshock (ML 5.1). The oriented stress-state we reconstruct for different periods and subareas are similar to each other and to previous published values focusing on swarm periods. We then calculate fluid pressure required to trigger the earthquakes. Most of the events (65 per cent) need fluid overpressure between 15 and 40 MPa (17 to 40 per cent of the hydrostatic pressure) with a median value of 24 per cent. Moreover, even the largest events, like the mainshocks in the 2012–2015 sequence, appear to be triggered by fluid pressure, similarly as events within swarm sequences. However, 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.