Strain accumulation and rotation in western Oregon and southwestern Washington

Velocities of 75 geodetic monuments in western Oregon and southwestern Washington extending from the coast to more than 300 km inland have been determined from GPS surveys over the interval 1992–2000. The average standard deviation in each of the horizontal velocity components is ∼1 mm yr⁻¹. The observed velocity field is approximated by a combination of rigid rotation (Euler vector relative to interior North America: 43.40°N ± 0.14°, 119.33°W ± 0.28°, and 0.822 ± 0.057° Myr⁻¹ clockwise; quoted uncertainties are standard deviations), uniform regional strain rate (ε_EE = −7.4 ± 1.8, ε_EN = −3.4 ± 1.0, and ε_NN = −5.0 ± 0.8 nstrain yr⁻¹, extension reckoned positive), and a dislocation model representing subduction of the Juan de Fuca plate beneath North America. Subduction south of 44.5°N was represented by a 40‐km‐wide locked thrust and subduction north of 44.5°N by a 75‐km‐wide locked thrust.