Structure of the upper mantle in the convex side of the New Hebrides island arc

S-wave arrival times from 30 earthquakes of the New Hebrides island arc are read from long-period components of the WWSS network, within the distance range 30–80°. S residuals from deep earthquakes of the New Hebrides and Tonga arcs are used to correct the residuals from the lower mantle effect and station corrections. An average delay of 3 to 4 s is found. This shows the existence of a low-velocity zone for S waves in the convex side of the arc. A three-dimensional inversion scheme is used to obtain the velocity structure of this zone. Other seismological data, such as pP attenuation, confirm the results of the inversion: two zones with low shear velocity (5 to 10 per cent lower than the J–B model) are defined. The first one is located beneath the d'Entrecasteaux fracture zone at a depth range of 150 to 350 km and the second one is in the south-west of the arc at the same depth range. Furthermore the deep upper mantle north of 16° S shows a normal S velocity, while south of 16°s it shows a low S velocity. A geodynamical model based on these results is proposed.