The discovery of episodic non-volcanic tremor and slip (ETS) events in multiple subduction zones is arguably the most intriguing new phenomenon in earthquake seismology in the past two decades. In this presentation, I will first discuss numerical modeling effort to understand the physical mechanism of episodic slow slip events (SSE) in the framework of rate-state friction. We have implemented realistic three-dimensional subduction fault geometry, and used tremor distribution and gravitational anomalies along the Cascadia margin to constrain frictional parameters in the SSE cycle model. The modeled SSEs capture the major characteristics revealed by GPS observations in northern Cascadia. Two minor slip patches with smaller areas and cumulative slips straddle a central slip patch beneath Port Angeles where SSEs appear every ∼1.5 years with a maximum slip of ∼2.5 cm. The along-strike segmentation of slow slip is inversely related to the local fault dip and strike angles of the slow slip zone, suggesting strong geometrical control on the slow slip process. Segmentation of Mw6.0 SSEs, represented by the average slip released over many SSE cycles, appears spontaneously mimicking the tremor distributions, while individual SSEs can still propagation across the segmentation boundaries, indicating adjacent SSE interactions as observed in time-dependent GPS inversions.
I will also present our recent work on stress inversion and implications for a weak subduction plate boundary at seismogenic depths in southern Cascadia near the Mendocino Triple Junction.