Cascadia Tremor Muted Beneath Forearc Faults

Abstract:

Episodic tremor and slip on the Cascadia megathrust accommodates a significant portion of Juan de Fuca plate convergence with North America at depths between 25 and 40 km. Very high Vp/Vs values along the megathrust and tremor modulation by tidal forces indicate high fluid pressures in the tremor source region. Tremor density (number of events; i.e., five-minute windows during which coherent tremor is locatable)/km²) varies along strike and with plate depth. We show that variations in tremor density correlate with large faults in the forearc of the overlying plate. Strong gradients in tremor density outline a ~20 km-wide band of maximum tremor along the forearc that is segmented by transverse zones of lower tremor occurrence. These transverse zones coincide with isostatic gravity gradients caused by large forearc faults. The faults accommodate northward motion and rotation of forearc blocks in part driven by Pacific-North America dextral shear and oblique convergence. Short tremor bursts lasting hours to days commonly terminate at or near a block boundary and appear compartmentalized beneath fault-bounded forearc blocks. Some faults appear to be barriers to large-scale tremor migration, but others appear to have little effect on migration. Seismicity, seismic velocity tomography, potential fields, and structural geology indicate the faults extend deep into the forearc. If the faults extend to the megathrust, they could provide fracture pathways for fluids to escape from the over-pressured megathrust. We hypothesize that fluid pressure on the megathrust is reduced by the escape of fluids along the crustal faults and into the overlying plate, thus locally reducing tremor occurrence. The damping of tremor and related slow slip caused by fluid escape into upper plate faults could affect deep strain accumulation and fault properties along the megathrust, possibly influencing the behavior of great earthquakes.