Strain accumulation in western Washington

Abstract

The Juan de Fuca plate is subducted beneath the North American plate off the coast of Washington at a rate of about 40 mm/yr N68°E. The average principal strain rates (extension reckoned positive) measured in northwestern Washington are as follows: Olympic peninsula 25 km south of Port Angeles from 1982 through 1990, and and near Seattle from 1972 through 1985, and . Both strain measurements are consistent with uniaxial contraction in the direction of plate convergence. Uplift rates inferred from tide gage recordings are about 4 mm/yr on the Pacific coast and near 0 mm/yr farther inland near Seattle. These deformation rates are consistent with a model of the Cascadia subduction zone in which the plate interface beneath the continental slope and outer continental shelf is locked but free to slip farther landward. The limited downdip extent of the locked segment of the plate interface is consistent with a shallow depth (∼20 km) of the isotherm (∼450°C) that defines the brittle‐ductile transition. Small thrust events diagnostic of seismic subduction should then occur only offshore and at shallow depths. The principal strain rates measured from 1972 through 1983 in the back arc region near Richland, Washington, are and .