Tidal modulation of slow slip in Cascadia

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 degrees before it, depending on the location of slip in the subduction zone.