Seismicity in Cascadia

Abstract We examine spatio-geometric patterns in the density of seismicity within the Cascadia forearc to gain insight into controls on seismogenesis. Tremor epicenters exhibit the most regular distribution defining a 40–80 km wide band extending along almost the entire convergent margin from the southern terminus of Gorda plate subduction in northern California to the central Explorer plate below northern Vancouver Island. Based on prior characterization of constituent low-frequency earthquakes, the up- and down-dip limits of tremor are assumed to represent slab isodepth contours at nominal values of 28 and 45 km, between which fluids at near-lithostatic overpressures are trapped within the subducting slab. Epicenters of earthquakes within the North American crust are anticorrelated with those of tremor, and concentrated in Washington and northern California where they are sandwiched between tremor and the Cascade volcanic arc. Seismicity within the subducting plate possesses the most limited epicentral distribution. Seismicity is confined to shallow depths off Vancouver Island and in northern California, and projects to greater depths beneath Washington and southern British Columbia. Comparison of seismicity patterns with long-wavelength slab geometry and thermo-petrologic constraints suggests that seismicity occurrence in Cascadia is governed by an interplay between slab strain, metamorphic dehydration within the subducting oceanic plate, and a plate boundary seal that controls where fluids enter the overriding plate. The inferred fluid evolution model harbors interesting implications for mantle wedge hydrology, forearc crustal composition and volcanism in Cascadia and other warm subduction zones.