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Earthquake hypocenters within the subducting Juan de Fuca plate beneath Washington and N Oregon are interpreted as showing that the direction of plate dip changes from northeast beneath the Puget Sound region to E-SE beneath SW Washington. The shallowest hypocenters within the Juan de Fuca plate are between 30-40 km depth, and the distribution of these events strikes N-NE from near the mouth of the Columbia River to the northern Olympic Mountains. The distribution of hypocenters between 40 to 50 km generally strikes parallel with the shallowest events, but shows a significant broadening beneath the eastern Olympic Mountains and Puget Sound. This change in the distribution of earthquake hypocenters reflects an upward arching of the Juan de Fuca plate beneath Puget Sound compared with the depth of the plate beneath SW Washington. Concludes that the 1949 earthquake resulted at least in part from down-dip tensional forces within the subducting Juan de Fuca plate. One consequence of the change in the direction of plate dip is that volcanic front in Washington is everywhere perpendicular to the dip of the Juan de Fuca plate.-from Authors
Additional Publication Details
Geometry of the Juan de Fuca plate beneath Washington and northern Oregon from seismicity ( USA).