CASCADIA SUBDUCTION TREMOR MODULATED BY UPPER PLATE STRUCTURE AND COMPOSITION (Invited Presentation)

Episodic tremor and slip occurring at depths of 30 to 40 km on the Cascadia megathrust accommodate a significant portion of Juan de Fuca plate convergence with N. America. Tremor modulation by tidal forces and very high Vp/Vs indicate near lithostatic fluid pressures in the tremor source region. Tremor density (events/km²) varies substantially along strike and is manifest at two different scales. Shorter wavelength variations over tens of km appear related to forearc faults, with tremor lows occurring beneath major transverse faults. These faults accommodate northward motion and rotation of forearc blocks, in part driven by Pacific-North America dextral shear and oblique convergence. A K-S test shows that coincidence of tremor lows with faults is not a random occurrence (p < 10^-7). Seismicity, tomography, and potential fields indicate the faults extend deep into the forearc. Faults that reach the megathrust may provide fracture pathways for fluids to escape from the over-pressured megathrust, thus reducing fluid pressure and tremor occurrence. Longer wavelength variations in tremor density over hundreds of km have been correlated with forearc terranes, with a pronounced low located beneath accreted basaltic crust of Siletzia in the Oregon forearc. Adjacent, broad tremor density highs beneath the Olympic and Klamath Mountains correlate with forearc elevation, reflecting underplating of sedimentary accretionary complexes, which also provide a quartz-rich seal for the over-pressured megathrust. Similar relations occur along the Nankai margin of Japan, where low frequency earthquakes (a proxy for tremor) occur less frequently where the megathrust is well-drained, as inferred from seismic velocity perturbations in the overlying forearc. In both Cascadia and Nankai, fluid trapping vs. migration into the upper plate may partly explain the anti-correlation between tremor on the megathrust and seismicity in the overlying forearc crust.