Development of slow slip front during the nucleation of laboratory fluid-induced earthquakes

Fluid injections are known to induce earthquakes in the upper crust. Recent studies have highlighted that fluid injections can contribute to the nucleation of instabilities close to or far from the injection site due to stress transfer induced by poroelastic processes. In addition, recent studies have suggested the maximum magnitude earthquake is expected to be a function of the volume injected. However, the development of the slip front related to the fluid pressure front, as well as its implications on the induced seismic sequence in time and space, remain poorly constrained in the laboratory and in natural fault systems. Here, we investigated the influence of the initial normal stress (i.e., the permeability of the fault plane) and of the injection rate on the development of both the fluid pressure front and associated slip front during the nucleation stage of laboratory fluid-induced earthquakes.

Experiments were conducted on saw cut samples of andesite, presenting a negligible bulk permeability compared to the fault plane one. Strain gauges were glued all around the fault surface to track the strain transfer associated with slip front propagation during injection and the rupture velocity during dynamic rupture propagation. The dynamics of the fluid pressure front was inverted from pore pressure measurements located at both edges of the fault. The evolution of the slip distribution due to the change in fluid pressure around the injection site was inverted from strain gauge measurements, assuming a 3D modelling of the sample specimen using the Finite Element Method and a crack-like propgation. Remarkably, this method allowed to map and quantify the development of slip instabilities during the nucleation stage of stick slip events. Our experiments highlitghted that increasing the injection rate leads to a transition from slip front propagated behind the fluid pressure front, to slip front propagation far away from the diffusion front, even at the scale of our experimental fault. Finally, we estimated the premonitory seismic moment released during the nucleation stage of instability, and we compared it to the injected volume. This scaling differs from the one relating the volume injected to the seismic moment of the main events.