Fluid Processes Highlighted by Temporal Variations of <i>b</i> ‐Value During Swarms and Aftershocks Sequences in the Ubaye Region (Western Alps, France)

Abstract The b ‐value from the Gutenberg‐Richter law is a crucial parameter in the assessment of seismic hazard. Its temporal variations may also bring useful insights on the processes driving seismicity at depth, even if not yet fully understood. In this paper, we focus on the temporal evolution of the b ‐value in the Ubaye Region (French Western Alps) which was hit by seismic swarms (2003–2004) and complex sequences with several mainshocks (2012–2015). The swarm‐like sequences show a common temporal behavior of b ‐value characterized by an increase and then a return to the initial level. The temporal b ‐value pattern for the mainshock‐aftershock‐like sequences is quite different. After a drop in the b ‐value that may follow the mainshock, the b ‐value increases above the background level before going back to it. Moreover, no precursory pattern can be identified before the mainshock. Fluid processes are recognized to play a crucial role in the driving mechanisms of these seismic sequences. Drawing parallel between swarms and aftershock sequences suggests that the b ‐value depicts fluid‐processes in the Ubaye Region seismicity. We propose that b ‐value shows a complex behavior, with variations due to Coulomb stress‐transfer from the mainshock and fluid‐pressure processes. Therefore, even with a catalog made at the French national scale, the b ‐value variations may help to monitor the on‐going processes at depth.