Abstract

Seismic swarms, in natural contexts or induced by anthropogenic fluid injections, commonly show migrations of earthquake hypocenters. Classically, such migrations are interpreted as the diffusion of the fluid pressure. However, recent studies show that the seismic front maps the stress concentration at the tips of propagating aseismic slip that is primarily induced by the fluid pressure increase. In this case, seismic migration might depend on the hydromechanical properties that control the dynamics of aseismic slip propagation rather than the hydraulic diffusivity. Here, we use synthetic seismic catalogs obtained from the hydromechanical modeling of a slip‐weakening, permeable fault response to a fluid injection. By varying the fault permeability and the stress state, we show that the shape of the seismic front in a distance‐time plot depends on the initial fault criticality to failure, and not on the hydraulic properties. We then extrapolate this numerical result to seven observed injection‐induced earthquake swarms by showing that the shape of the migration front directly depends on the seismogenic index. The seismic front has a diffusive behavior when the aseismic slip is directly driven by the pressure increase, either at the swarm beginning or later if the faults are not initially critically stressed. On the contrary, an accelerating seismic front indicates a high‐criticality stressed fault, on which the aseismic slip runs away from the fluid pressurized area. Therefore, this study highlights that the seismic front shape may prove a useful indicator to characterize the initial stress state of faults over which the swarms occur.