We examine two prominent upper mantle velocity anomalies in the southwest Pacific, the Tonga slab anomaly and the corresponding overlying mantle wedge anomaly, using data collected during a combined land-sea deployment of temporary seismometers. The linear geometry and small interstation spacing of the instruments yield high-resolution data along a cross section of the Tonga subduction zone, including the actively spreading Lau back arc basin. We estimate the relative variation of P and S velocity, often described as &ngr;=Δ ln Vs/Δ ln Vp, for the slab and mantle wedge anomalies using two distinct methods: a linear regression of the P and S travel time residuals, and detailed modeling of the velocity structure using a three-dimensional finite difference travel time algorithm. The two methods yield similar results, with &ngr; of the slab being 1.1--1.5 and &ngr; of the mantle wedge being 1.2--1.3. These values are consistent with experimental data concerning the effect of temperature on P and S wave velocities in the upper mantle and are lower than what is expected for velocity anomalies generated by the presence of partial melt. These observations imply that either the theoretical estimates of &ngr; for partial melt are too large or very little partial melt is present beneath the Lau basin. In the latter case, melt must be quickly removed from the rock matrix, such that the velocity anomalies are due to increased temperature, and not melt. The bulk of the velocity anomaly in the mantle wedge can be explained by temperature anomalies of 400--600 ¿C because of the amplification of temperature derivatives of seismic velocity by anelastic effects. Such large thermal anomalies, generated by decreased lithospheric thickness and mantle upwelling beneath the fast spreading Lau back arc basin, can still leave the mantle near the solidus, even after accounting for the effect of increased volatile content in the mantle wedge. The lower-amplitude velocity reductions in the deeper wedge are likely related to an increased concentration of volatiles from the subducting slab. ¿ 1999 American Geophysical Union