Plate boundary segmentation and upper/lower plate structure in Cascadia

A number of studies have suggested correlations between structural features of the upper or lower plates in Cascadia and segmentation of current episodic tremor and slip (ETS) behavior and of paleoseismically-defined megathrust earthquakes. I synthesize constraints on crustal structure to explore these correlations and conclude that projecting the surface terrane boundaries in the North American plate into the crust can explain some apparent inconsistencies between ETS segmentation and upper plate structure. The primary control on ETS segmentation appears to be the composition of the upper plate immediately above the plate boundary. Segmentation of megathrust behavior, on the other hand, appears to be controlled in large part by topography and structure of the subducting plate as well as its interaction with structural variations in the outer edge of the upper plate. A primary motivation for this synthesis is to identify key locations where new high-resolution information on upper and lower plate structure should be acquired to test these ideas.