Peak Tremor Rates Lead Peak Slip Rates During Propagation of Two Large Slow Earthquakes in Cascadia

We explore the evolution of slow slip on the Cascadia megathrust during two large episodic tremor and slip events and compare stress changes to the spatial evolution of tremor from Pacific Northwest Seismic Network tremor locations. We used displacement time series from ~72 GPS stations, along with the Extended Network Inversion Filter to solve for the time‐dependent fault slip. The 2010 ( Mw 6.8) and 2012 ( Mw 6.8) events propagated northward and southward, respectively, allowing us to assess directional effects on slip behavior. We observed that tremor occurs on the leading edge of propagating slipping regions, well ahead of the highest slip rates, independent of the along‐strike propagation direction. Resolution tests using the actual tremor distributions to generate synthetic data show that our result of peak tremor rates leading peak slip rates is not due to biases introduced by temporal smoothing. Calculated stress changes due to the time‐dependent slip distributions imply that tremor is sensitive to kilopascals of stress, consistent with studies of tidally triggered tremor. Within the resolution of our model, our results are consistent with the hypothesis that significant tremor is triggered by stresses ahead of the highest slip rates. We also observe ongoing slip continuing several days after tremor has passed. Our observations are consistent with some numerical models of tremor patches that suggest that this behavior can be explained by densely packed asperities resulting in somewhat crack‐like propagation rather than a slip pulse that is as concentrated as the tremor activity.