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

A.P. Schultz, R.S. Crosson
University of Washington, Geophysics Program, Box 351650, Seattle, Washington, 98195-1650, USA

Abstract:

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.

AGU Index Terms: 7205 Continental crust; 7200 SEISMOLOGY; 7218 Lithosphere and upper mantle; 8145 Physics of magma and magma bodies
Keywords/Free Terms: Cascades, Crust, Root.

JGR-Solid Earth 96JB02289
Vol. 101 , No. B12 , p. 27,899