Tidal modulation and back-propagating fronts in slow slip events simulated with a velocity-weakening to velocity-strengthening friction law
We examine tidal modulation and back-propagating fronts in simulated slow slip events using a rate and state friction law that is steady state velocity weakening at low slip rates and velocity strengthening at high slip rates. Tidal forcing causes a quasi-sinusoidal modulation of the slip rate during the events, with the maximum moment rate occurring close to or slightly after the maximum applied stress. The amplitude of modulation scales linearly with the tidal load and increases as the tidal period increases relative to the timescale for state evolution. If we choose parameters so that the model matches the observed tidal modulation of slip in Cascadia, it can reproduce only a subset of the stress drops inferred from observations and only in a limited portion of parameter space. The tidal forcing also causes back-propagating fronts to form and move back through the region that has already ruptured. The stress drop that drives these back-propagating fronts sometimes comes from the tidal load and sometimes from a stress recovery that occurs behind the front in tidal and non-tidal simulations. We investigate the slip and propagation rates in the back-propagating fronts and compare them with observations. The modeled fronts propagate too slowly to be good representations of the fronts inferred from tremor observations. For the simulated fronts to propagate at the observed speeds, the stress drops driving them would have to be more than 70% of the stress drop driving the forward-propagating front.