SEISMIC IMAGING OF THE PACIFIC NORTHWESTERN UNITED STATES

The goal of this study is to characterize the seismic structure beneath the Pacific Northwestern United States to provide an improved understanding of the thermal, compositional, and dynamic state of the crust and mantle in the region. We utilize broadband seismic data from several arrays, including regional stations of the USArray Transportable Array and the 118 stations of the High Lava Plains broadband seismic array, which operated from January 2006 to September 2009. We present results of a host of studies undertaken as part of the High Lava Plains Project, including body and surface wave tomography, receiver function imaging of upper mantle discontinuities, and seismic anisotropy from body and surface waves.

From a structural standpoint, tomographic images show clear evidence for a laterally and radially complex Juan de Fuca / Gorda slab, which extends to depths of 500 km and perhaps deeper. Beneath southern Oregon and northern Nevada, the subducting slab appears to be bifurcated at depths of ~500 km. Upper mantle discontinuity imaging suggests cooler temperatures (perhaps the subducting slab) at greater depths, with an overall thicker mantle transition zone and a deeper 660 km discontinuity to the east. Beneath Yellowstone and the eastern Snake River Plain, reduced seismic velocities are not limited to a simple plumelike configuration in the upper mantle; rather, they are focused in a zone of extremely low velocities in the uppermost 200 km. From 200 to 700 km, the reduced velocity zone appears as a coherent, dipping sheetlike structure across the region. At shallow depths, very low velocities exist beneath the central Cascades and the eastern Snake River Plain.

From a dynamic standpoint, seismic anisotropy imaging for the region demonstrates a complex mantle fabric, likely due in large part to mantle flow. While the predominant fast direction is ~E-W, significant variations exist across the region, suggesting a complex flow field for many regions with the exception of the High Lava Plains.