LITHOSPHERIC STRUCTURE OF NORTHERN CALIFORNIA FROM TELESEISMIC IMAGES OF THE UPPER MANTLE

Teleseismic P wave travel time residuals from 120 earthquakes recorded across the U.S. Geological Survey California seismic network were used to determine the lithosphere P wave velocity structure beneath northern California, a region characterized by complex interactions between the Pacific, North American, and Gorda plates. Lateral P wave velocity variations beneath the array were determined by inversion of 9383 travel time residuals. Inversion results for the crust show strong correlations to volcanic features. The active volcanic fields, Shasta-Medicine Lake, Lassen, and Clear Lake, are characterized by crustal low-velocity anomalies that average approximately -6%, possibly identifying partially molten magma bodies. Cooled, solidified magma bodies beneath the extinct volcanic fields, Sonoma, southern Clear Lake, and Sutter Buttes, are denoted by relative velocity highs averaging +3%. The largest upper mantle velocity variations occur in the depth range 30-110 km, where velocities vary from -5.5% to +9.5%. These velocity variations reflect changes in the thickness and geometry of the Pacific, North American, and Gorda plates where they interact at the Mendocino Triple Junction. North of the Mendocino Triple Junction, the steep 70-degrees east dipping portion of the Gorda plate is imaged as a +5% velocity high to depths near 270 km. A presumed segment of the Gorda plate, observed beneath the northern Great Valley and south of the inferred edge of the plate, is characterized by a +9% velocity high in the depth range 30-70 km. Beneath the northern Coast Ranges, shallow asthenosphere is imaged in the depth range 30-100 km as a pronounced southward tapering -4% low-velocity zone, which we interpret as the slab window. Results from this study provide improved constraints on Gorda plate subduction, evolution of the San Andreas fault system, and development of the lithosphere beneath western North America.