Can earthquakes nucleate on nominally stable velocity-strengthening faults?

Tectonic faults are often assumed to slip either slow due to stable, velocity-strengthening frictional behavior, or fast as a result of velocity-weakening friction leading to dynamic (seismic) rupture. As a consequence, velocity-strengthening faults may be regarded as intrinsically stable as they do not spontaneously nucleate seismic events. However, recent laboratory and in-situ experiments of fluid injection challenged such assumptions. Here we present a fully coupled hydro-mechanical fault model in which stable, rate-strengthening frictional behavior is combined with dynamic weakening due to rapid poroelastic effects, allowing unstable (seismic) slip to occur on nominally stable faults. In our numerical experiments, fluid injection reduces the effective normal stress and frictional resistance, thus bringing the fault to failure. The onset of fault failure is controlled by competing mechanisms of shear-induced dilation and shear-enhanced compaction, which cause fault weakening and the propagation of a slow-slip transient from the fluid injection point. When a critical size of the slow slip patch is reached, dynamic rupture eventually nucleates at the slow-slip event front and propagates beyond the fluid pressure perturbed region. Further numerical experiments indicate that, when the fault is critically stressed, the growth of the aseismic patch – prior to dynamic rupture – occurs in a few seconds, whereas at lower stress levels, the aseismic slip phase propagates slowly over hundreds of seconds. These results predict that poroelastic compaction and fluid pressurization can cause the transition from aseismic slow-slip to fast seismic slip and the propagation of dynamic rupture on velocity-strengthening faults. In particular, they demonstrate that compaction-induced fluid pressurization can overcome the initial phase of shear-induced dilatancy, thus allowing the propagation of dynamic rupture in the form of pulse-like pore-pressure waves. The implication that earthquake rupture may nucleate on rate-strengthening faults, presently considered to be nominally stable, requires a re-evaluation of seismic hazard in many areas, particularly in the case of fluid injection in enhanced geothermal systems and CO2 storage.