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Constraints on the subduction geometry beneath western Washington from broadband teleseismic waveform modeling

DEPARTMENT OF GEOLOGY UNIVERSITY OF MISSOURI, COLUMBIA, MISSOURI 65211
GEOPHYSICS PROGRAM (AK-50) UNIVERSITY OF WASHINGTON, SEATTLE, WASHINGTON 98125

Abstract

Source-equalized P-wave receiver functions from an array of broadband portable instruments located about 50 km from the coast (near 46.8°N, 123.4°W) in western Washington are interpreted to determine the subduction geometry of the Juan de Fuca plate beneath the Pacific Northwest. Waveforms from more than 50 teleseismic events were available from the temporary array. These data display large variations in both amplitude and timing of large secondary arrivals interpreted to be P- to S-converted phases from both the bottom of the subducting oceanic crust and the overlying continental crust-mantle boundary. This behavior is consistent with that expected for plane P waves interacting with a dipping interface at depth. Our model suggests that the Juan de Fuca plate is dipping 20° ± 3° in the direction 110° ± 20°. The continental Moho is at about 31 km depth. Only a thin wedge of upper mantle material exists between the overlying crust and the subducting slab. This wedge may be responsible for anomalous arrivals in the observed receiver functions. The model determined in this study is consistent with large-scale deformation in the Juan de Fuca plate that has been recently proposed based on analysis of seismicity data in the Puget Sound region. The southeasterly dip direction at our site is consistent with its location on the southern flank of an arch in the slab in the vicinity of Puget Sound caused by the change in strike of the subduction zone between latitudes 47° and 48° north.

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