We use a new method to construct an upper mantle model based on self-consistent computation of phase equilibria and physical properties. Computation of the isotropic elastic wave velocities of a pyrolytic bulk composition in thermodynamic equilibrium shows a distinct low-velocity zone with a minimum velocity VS = 4.47 km s-1 along the 100 Ma geotherm. In the vicinity of the low-velocity zone the velocity of this null hypothesis is approximated along oceanic geotherms by VS = 4.77 + 0.0380(P, z/29.80) - 0.000378(T - 300), with pressure P in GPa, depth z in km, temperature T in K, and velocity VS in km s-1. The null hypothesis predicts a minimum VS 0.1--0.2 km s-1 higher than that in seismological models of 100 Ma Pacific. We find that dispersion, estimated solely on the basis of seismological attenuation models, can account for this residual velocity deficit. Except in the immediate vicinity of the ridge (t < 5 Ma), a solid-state low-velocity zone provides a satisfactory quantitative explanation of seismic observations. We do not find a satisfactory explanation for the magnitude of the Gutenberg discontinuity or for the high shear wave velocity gradient zone.