IMAGING THE JUAN DEFUCA PLATE BENEATH SOUTHERN OREGON USING TELESEISMIC P-WAVE RESIDUALS

The Juan de Fuca plate subducts under the western margin of the North American plate in southern British Columbia, Washington, Oregon, and northern California. Benioff zone seismicity delineates shallow parts of the plate in Washington and northern California, whereas in Oregon there is a lack of seismicity. This study images the Juan de Fuca plate in southern Oregon using seismic tomography. We inverted P wave travel time residuals from a 366-km-long seismic array operated in southern Oregon in 1982. The southeast striking array extended from the Coast ranges to the Modoc Plateau and crossed the High Cascades at Crater Lake, Oregon. We imaged three features under the array: one high-velocity zone and two low-velocity zones. The high-velocity zone is 3-4% faster than the surrounding upper mantle. It dips steeply at 65-degrees to the east beneath the Cascade Range and extends down to at least 200 km. We propose that this high-velocity feature is subducted Juan de Fuca plate. We also imaged two low-velocity zones, both of which are 3-4% slower than the surrounding earth structure. The southeastern low-velocity zone may be caused by partially molten crust underlying the Crater Lake volcano region.