Tidal modulation and triggering of low-frequency earthquakes in northern Cascadia

We analyze the influence of Earth and ocean tides on the triggering of low-frequency earthquakes (LFEs) in northern Cascadia using three LFE catalogs for southern Vancouver Island and Washington state from episodic tremor and slip events between 2003 and 2013. Sensitivities of LFE families to tidally induced fault normal stress, updip shear stress (UDSS), and corresponding time derivatives are computed and their geographic variability is mapped. We find localized areas showing higher sensitivity to UDSS than their surroundings, suggesting that tidal sensitivity depends on laterally heterogeneous physical properties such as variable pore fluid pressures and frictional properties along the plate interface. We observe that sensitivity of LFEs to UDSS rises dramatically from near zero on the first day of strong activity to a maximum ∼4 days later. In addition, the peak LFE rate transitions from a correlation with peak tidal shear stress rate to a correlation with peak tidal shear stress through large slow slip events. We identify 64 Rapid-Tremor-Reversals (RTRs) that start a few days after the main slip front. The RTRs have an average stress drop of ∼0.8 kPa and a majority (72%) occurs during periods of large positive UDSS. The combined observations imply that RTRs play an important role in slow slip processes and that modulation of creep rate due to tidal stress and tidal triggering of secondary events are jointly responsible for the observed tidal sensitivity.