Shear wave velocities in the Earth's mantle

Abstract

Direct measurements of the travel time gradient d T/d Δ for S waves over the distance range 14° < Δ < 100°, together with travel time data, are used to derive a shear velocity model for the Earth's mantle. A network of seismograph stations in Arizona operated as an array was used for the measurement of dT dΔ . The complex velocity structure in the upper mantle makes the use of multiple arrivals necessary to define the dT dΔ −Δ curve for distances less than 55°. In order to satisfy the data it is necessary to discard the usual assumption of lateral homogeneity below shallow depths and a shear velocity differential of up to 0.1 km/s down to a depth of 1000 km is proposed between western North America and areas of the Pacific Ocean. Distinctive features of the velocity model for the upper mantle beneath western North America are a low-velocity zone centred at 100 km depth and zones of high velocity gradient beginning at 400, 650 and 900 km. Also, the moderate velocity gradient derived for the depth range 150–400 km implies a density reversal in the same area if Love wave dispersion data are to be satisfied. The velocity model derived for the lower mantle beneath oceanic areas has a zone of high velocity gradient at 1200 km depth and a pronounced low-velocity zone at the core-mantle boundary.