CASCADIA MEGATHRUST BEHAVIOR AND ITS CORRELATION WITH UPPER-PLATE STRUCTURE

Structural segmentation of the Cascadia forearc is revealed by isostatic gravity, active crustal faulting, GPS velocity gradients, and block rotations. Forearc segments appear to correlate with along-strike variations in megathrust tremor density, locking width, slip in the 1700 AD earthquake, and rupture history over the last 10 ka. The segmentation is primarily caused by northward migration and margin-parallel compression of forearc blocks as Cascadia is deformed in the Pacific-North America dextral shear couple. Segmentation boundaries correlate with Yakima fold belt-like structures that accommodate the N-S compression, extend westward across the forearc to the shoreline, and possibly merge with margin-parallel structures along the plate boundary. The correlation suggests a kinematic link between transverse forearc structures and the megathrust, possibly to be reflected in interactions between the megathrust seismic cycle and the cycles accommodating north-south compression on transverse, upper plate faults.