Off-fault plasticity limits the speed of elongated earthquake ruptures

Conventional earthquake rupture models in elastic media with scale-independent dissipation predict rupture acceleration up to a limiting speed dictated by seismic wave speeds. However, most earthquakes are inferred to be significantly slower than these theoretical limiting speeds. Previous two-dimensional modeling shows that inelastic deformation of the material surrounding the fault can stabilize rupture propagation at velocities below the limiting speed, owing to scale-dependent dissipation due to the expansion of the off-fault inelastic zone with rupture length. An essential ingredient missing from such models is that large earthquake ruptures become elongated after they saturate the seismogenic width, which prevents a length-dependent growth of the off-fault dissipation zone. Here, we carry out a computational study of elongated ruptures with off-fault plasticity, with a particular focus on steady rupture speeds. We combine a 2.5D (width-averaged) modeling approach and Mohr-Coulomb plasticity to simulate width-limited strike-slip earthquake ruptures propagating in an inelastic medium. We observe steady pulse-like ruptures propagating at speeds below the Rayleigh wave speed. Through a systematic parametric study, we characterize the dependence of the steady rupture speed on the initial fault stress, closeness to plastic failure, and seismogenic zone width. Our simulation results show that off-fault plastic deformation allows the rupture velocity on long faults to saturate at speeds between 6.5 and 99% of the Rayleigh wave speed, encompassing the common range of speeds of real earthquakes as well as those of tsunami earthquakes. Our results indicate that off-fault plasticity is a viable mechanism for achieving realistic earthquake rupture speeds, even on planar faults with homogeneous stress and strength. The model also provides insights on the factors controlling other earthquake source properties, including pulse width, peak slip velocity, inelastic off-fault deformation and inelastic zone width.