[1] Several studies have shown that the seismic tremor in episodic tremor and slip is tidally modulated, suggesting a sensitivity to the rather small tidal stresses. We address whether the slip rate in slow slip events is also tidally modulated by examining data from six borehole strainmeters in northwest Washington and southern Vancouver Island. We simultaneously fit data from multiple stations and from slow slip events occurring over a 3 year interval from January 2007 to June 2009, as we are unable to extract a meaningful signal from a single record. We find modulation of the strain rate with a 12.4 h period, that of the tide with the largest amplitude, that is significant at the 99% level. The amplitude of this modulation suggests that the slip rate during slow slip events oscillates, on average, 25% above and below its mean value during a tidal cycle. Tidal modulation estimates at three other periods are significant with more than 70% probability. The phase of maximum strain rate in the 12.4 h M2 period coincides with the phase of the maximum tremor rate taken from a catalog in an overlapping region. Comparison with a simple tidal loading model shows that the phase of maximum strain rate in the M2 period may occur at the maximum shear stress or up to 90°before it, depending on the location of slip in the subduction zone.

[1] We investigate the behavior of simulated slow slip events using a rate and state friction model that is steady state velocity weakening at low slip speeds but velocity strengthening at high slip speeds. Our simulations are on a one-dimensional (line) fault, but we modify the elastic interactions to mimic the elongate geometry frequently observed in slow slip events. Simulations exhibit a number of small events as well as periodic large events. The large events propagate approximately steadily "along strike," and stress and slip rate decay gradually behind the propagating front. Their recurrence intervals can be determined by considering what is essentially an energy balance requirement for long-distance propagation. It is possible to choose the model parameters such that the simulated events have the stress drops, slip velocities, and propagation rates observed in Cascadia.Citation: Hawthorne, J. C., and A. M. Rubin (2013), Laterally propagating slow slip events in a rate and state friction model with a velocity-weakening to velocity-strengthening transition,

No abstract

[1] 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.Citation: Hawthorne, J. C., and A. M. Rubin (2013), Tidal modulation and back-propagating fronts in slow slip events simulated with a velocity-weakening to velocity-strengthening friction law,