Extremal bounds on the seismic velocities in the Earth's mantle

Abstract

Summary Based on global travel-time constraints of the function τ(p) (Lee & Johnson), the confidence region for the lower mantle P and S velocity profiles have been determined. An exact method (Bessonova et al.) is used to compute the extremal bounds on velocity. Bounds on the lower mantle P velocity have an average resolution of 70 km in the depth range of ∼ 800–2900 km. The average bound width is ∼ 0.14 km s-1 at the 97 per cent confidence level. Bounds on the lower mantle S velocity have an average resolution of 85 km in the depth range ∼ 850–2900 km and an average width of 0.13 km s-1. These results indicate that significant differences exist among the published velocity models and that the bounds can be used to discriminate among them. The confidence regions are computed with an assumed upper mantle velocity model and are therefore subject to small baseline adjustments in velocity. The introduction of approximate bounds on the upper mantle τ(p) increases the lower mantle bound width by 30 per cent (for the P velocity). In this case, one obtains constraints on the absolute velocity function.