P-waves propagating from shallow earthquakes in the Tonga-Kermadec seismic zone to regional stations in Raratonga and Niue are associated with large negative travel-time residuals (observed minus Jeffreys-Bullen time) of the order of -10 s. For focal depths below 100 km the magnitude of residuals decreases sharply, to about -2.5 s for depths near 200 km. The pattern of residuals is explained by a model of oceanic lithosphere possessing an upper mantle P-wave velocity of 8.3 km/s and having its base at a depth near 100 km. ¿ American Geophysical Union 1987