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 dept