Upper mantle structure in the Basin and Range province, western North America, from the apparent velocities of S waves

The variation of shear velocity with depth in the upper mantle for the Basin and Range province of western North America has been studied with direct measurements of dT/dΔ for S waves in the distance range 14° < Δ < 40°. Three orthogonal components of digital data were used and onset times were determined using the product of the horizontal radial and vertical components of motion and particle motion diagrams. A linear LRSM array in Arizona was used for the measurement of dT/dΔ. An S-wave velocity distribution is derived, compatible with P-wave velocity models for the same region. The derived model consists of a thin lid zone of shear velocity 4.5 km/sec overlying a low-velocity zone and a change in velocity gradient at a depth of 160 km. Two regions of high-velocity gradient are located at depths beginning at 360 km and 620 km.