Backward earthquake ruptures far ahead of fluid invasion: Insights from dynamic earthquake-sequence simulations

The 2011 M9 Tohoku-Oki earthquake caused widespread seismicity including that associated with upward fluid migration within the overriding plate. Such fluid-driven earthquake sequences have a peculiar behavior whereby the ruptures of individual small earthquakes propagate in the opposite direction to the direction of pore pressure invasion. This backward rupture propagation is not predicted by the conventional fluid-driven seismicity model that only considers the evolution of pore pressure on the fault, thus ignoring the effect of stress redistribution. Here, we investigated the characteristics of earthquakes under the influence of upward fluid invasion using numerical simulations considering realistic frictional behavior and fault creep. Specifically, we examined the slip behavior of a fault governed by rate-and-state dependent friction when imposing upward pore pressure diffusion. We assumed multiple velocity-weakening patches on an otherwise velocity-strengthening fault. Our results show that the seismicity front can migrate much faster than predicted by the conventional model because of the redistribution of shear stress on the fault. The recurrence of earthquakes and the directions of ruptures are controlled by the brittleness, β, i.e., the patch size divided by the critical nucleation size. When β > 1, earthquakes repeatedly occur at the same patches, and the swarm activity expands with time. The first rupture at each patch propagates updip, while the directions of subsequent ruptures are variable. When β < 1, earthquakes occur only once in each patch, followed by repeating slow slip events, and the source region migrates rather than expands. In this case, individual earthquake ruptures propagate downdip in the opposite direction to the pore pressure invasion, explaining the observed peculiar rupture characteristic. The consistency obtained between the simulated β < 1 characteristics and the observed ones suggests that the fluid-driven seismicity following the Tohoku-Oki earthquake was only a fraction of the total deformation in which aseismic slips were dominant.