Correlation of tremor activity with tidal stress in the northern Cascadia subduction zone

We analyze hourly data from five tremor episodes in the northern Cascadia subduction zone over the period 2003-2005 provided by the Tremor Activity Monitoring System (TAMS). All five tremor episodes correspond to slow slip events observed by GPS. Fourier decomposition is used to separate the hourly tremor counts for each episode into "long-period" (0<f<0.8 cpd), "tidal" (0.8<f<2.2 cpd), and "short-period" (f>2.2 cpd) components. The tidal component of the observations is compared with theoretical stress variations at depths of 20, 30, and 40 km, with 40 km being the depth of the interpreted subduction thrust interface. The stress variations are predicted by a 2-D ocean tide loading model combined with estimates of stress variations from Earth tides. We find that the shear stress in the thrust direction and the compressive normal stress on shallow dipping surfaces correlates with the data significantly better than the confining stress over the range of depths investigated. The relative amplitudes of tidal shear stress and compressive normal stress result in positive Coulomb stress favoring slip. Peak tremor activity occurs at times of maximum tidal shear stress in the thrust direction, which would assist slow slip and would suggest that tidal tremor and slip are colocated. The response of the tremor to tidal shear stress is roughly proportional to the mean activity level, controlled by tectonic conditions of stress and pore pressure. A significant, nontidal, daily variation in tremor activity of unknown origin is identified.