Seismic velocity structure across the central Washington Cascade Range from refraction interpretation with earthquake sources

A two-dimensional seismic P wave velocity model, extending to a depth of ∼50 km, is interpreted for a 425 km long profile extending from Hood Canal in western Washington across the central Cascade Range to Walla Walla in eastern Washington. Existing Pacific Northwest Seismograph Network stations and earthquake sources are employed in a refraction/wide-angle reflection interpretation. In the preferred model, crustal velocity decreases slightly from west to east in the depth range 10–25 km beneath the Cascade Range. The continental Moho is estimated to dip 2.7° to the west in eastern Washington and 4.4° to the east beneath Puget Sound forming a distinct crustal root for the Cascade Range. Crustal thickness ranges from ∼35.5 km on the west end beneath Puget Sound and 34 km on the east end near Walla Walla to 47 km under the high Cascades. Seismic velocity in the interpreted crustal root suggests rock of mafic composition which is consistent with a process of underplating resulting from dehydration of the subducting Juan de Fuca plate. Simplified analysis also suggests that the present topography of the Cascades in the vicinity of our profile is supported by the isostatic response of this root zone.