Modeling Spontaneous Generation of Off-Fault Plastic Strain Localization During Dynamic Earthquake Rupture

We extend an elastodynamic finite element method to incorporate off-fault plastic yielding of Mohr-Coulomb form into a 2D spontaneous dynamic earthquake rupture model. For straight faults under uniform stress conditions, we find that rupture-induced plastic strain tends to spontaneously localize into discrete bands if the off-fault material is close to the yielding strength in the initial stress state. While fully regularized, convergent solutions of the strain localization are difficult to be achieved, the numerical simulations can capture the onset of localization. Attempts at regularization via viscoplastic relaxation sometimes result in a smooth, numerically convergent solution , while in other cases the result is simply to delay the onset of localization, without suppressing the smallest, numerically irresolvable scale lengths of localized plastic strain. For non-straight faults, we find that a discrete kink in fault strike also localizes plastic strain into several discrete bands whose orientations are consistent with the accommodation of complex fault geometry at the kink. Apparently convergent numerical solutions (as assessed by numerical element size reduction) can be obtained for the strain localization at the kink. Off-fault plastic yielding and strain localization can have significant effects on spontaneous dynamic rupture propagation and seismic radiation. It lengthens the cohesive zone at the rupture front, such that the width of this zone tends to stabilize with rupture propagation distance, with the fault behavior under slip-weakening friction essentially equivalent to a time-weakening friction law. It also reduces seismic radiation from fault kinks by filtering high-frequency contents above several Hertz.