Unsteady along-strike propagation of slow slip events on a velocity-weakening fault

It has been shown previously that episodic slow slip events can occur on a purely velocity-weakening fault governed by rate and state friction if the size of the velocity-weakening region is properly tuned, though the range of sizes that allow for these events may be too small to account for the occurrence of slow slip events at many subduction zones (Liu and Rice, 2005, 2007; Rubin, 2008). We further investigate the propagation and episodicity of slow slip events that occur on a velocity-weakening fault with a simplified strip-like geometry. Events occur on a one-dimensional (line) fault, but we adjust the elasticity equations to mimic the elongate geometry of the slow slip region in Cascadia. We determine the parameterized along-dip lengths that allow for slow slip events, either in cycle simulations or in single-event simulations where we specify the initial conditions. As expected (e.g. Rubin and Ampuero, 2005), in single-event simulations the maximum along-dip length that allows for slow slip events increases as the initial conditions get further below steady state. In cycle simulations, the maximum along-dip length that allows for slow slip events is close to the length expected for a fault that begins near steady state. The simulated slow slip events differ from the large episodic events observed in Cascadia in that their maximum along-strike extent is usually not much more than the along-dip length, and they rarely exhibit a simple periodicity. This lack of periodic large events can be explained if we note that the criterion for the slow slip front to propagate long distances along strike is nearly identical to the criterion for the fault to accelerate to seismic speeds. There is a small part of parameter space, where the fault begins below steady state and the along-dip length is properly tuned, that allows for episodic slow slip events that propagate long distances along strike. However, we see no reason why the plate interface in Cascadia should consistently return to this small part of parameter space before each slow slip event.