Seismic surface waves recorded by the Berkeley Digital Seismic Network have been analyzed in order to constrain three-dimensional lateral heterogeneity of the upper mantle under northern California. A total of 2164 seismograms from 173 teleseismic events were windowed for the fundamental mode Rayleigh wave, followed by estimation of complex amplitude spectra over the period range 16 to 100 s using a multiple-taper method. Since Rayleigh waves at shorter periods, particularly below 35 s, suffer from serious multipathing or non-plane wave arrivals, these amplitude spectra have been interpreted as the product of wavefront distortion along the teleseismic propagation path and seismic structure beneath the network. The amplitude spectra are first modeled in terms of non-plane incoming wavefields and structural phase velocity perturbations period by period. After corrections for Moho and surface topography, the phase velocity maps are inverted for three-dimensional shear velocity perturbations Δvs down to a depth of 200 km. The Δvs maps are in good agreement with the results of body studies over a broad spatial scale. The dominant signals are associated with the thermal effects of the active Gorda and fossil Farallon subducted slab stretching from Mount Shasta through the western Sierran foothills to the southern Great Valley and asthenospheric upwelling beneath the northern Coast Ranges. The southern Sierra Nevada Range is characterized by fast Δvs down to ~50 km and slow velocities between ~60 and 120 km depth, in agreement with independent interferences of a cold crust and warm upper mantle, which may provide the buoyancy forces necessary to support the elevation of the range. ¿ 1999 American Geophysical Union