SEISMIC VELOCITY-GRADIENTS IN THE UPPER MANTLE

The radial velocity gradients for P- and S- waves in the upper mantle are of considerable interest for petrological studies of the upper mantle but are difficult to constrain directly. Comparison of three different styles of velocity model, constructed within a common parametrisation to give a good fit to the travel times of P- and S- waves to 30-degrees, provide a useful framework for constructing bounds on the radial gradients. For depths from 250-1000 km a somewhat subjective set of bounds are suggested for the velocity gradients and velocity contrasts at the major discontinuities in an 'average' mantle. One of the most significant factors affecting the estimation of radial gradients is the presence of small-scale heterogeneity in the mantle. Modest heterogeneity, around 1 percent perturbations on scale lengths of about 300 km, can give rise to horizontal velocity gradients which can overwhelm the radial gradients in the shallower parts of the mantle and still be a quarter of the size of the radial gradients at greater depths. However such scales of heterogeneity are unlikely to simulate the presence of additional upper mantle discontinuities for ray paths travelling near vertical.