Arrival times of compressional (P) and shear (S) waves generated by earthquakes located in the New Hebrides subduction zone and recorded by local and regional arrays of seismographs are used to determine large-scale one-and three-dimensional elastic wave velocity structures of the subduction zone between 15 ø and 20øS and from the surface to about 250 km depth. The results obtained from inverting the locally and regionally recorded arrival times individually corroborate each other, and they are inverted jointly in order to improve the resolution of shallow to intermediate depth structures. The results for one-dimensional structure indicate a gradual increase of velocity with depth until a 9% reversal appears between 60 and 100 km depth. The three-dimensional structure determined from the joint inversion shows that these low velocities lie within a sizable seismic gap in the descending Benloft zone. Taken with other observations such as the attenuation of high-frequency shear waves travelling across this gap and the locations of active volcanoes at the surface, we infer that the low-velocity region represents a thermal anomaly of about 750øC which alters the physical properties of the descending plate in this region. At shallower depths there is evidence of low-velocity structures included in the descending plate: one beneath north Malekula island and another between Malekula and Efate islands corresponding to the large embayment of the leading edge of the upper plate. The locations of these structures combined with previous investigations of the area lead us to infer that these low velocities are due to the subduction of small-scale features such as seamounts and accretionary wedges. INTRODUCTION The New Hebrides arc is part of a larger island arc system that forms the boundary between the subducting Indo-Australian plate and the overriding Pacific plate (Figure 1). Convergence between the two plates occurs here at a rate of about 11 cm/yr and in a direction approximately normal to the trench [Dubois et al., 1977]. The present New Hebrides arc has been a region of active subduction for probably less than 6-8 m.y., having inherited this role when an ancestral Solomon-New Hebrides-Fiji-Tonga subduction zone was disrupted in the late Miocene [e.g., Karig and Mammerickx, 1972; Hamburger and Isacks, 1987]. The arc appears simple on a regional scale, being nearly linear along its 1200 km length from the Santa Cruz islands to the Hunter fracture zone. The upper plate is morphologically complex, however, with the most striking structural anomalies occurring in the central part of the arc between 14øand 17øS. In this area, the normal trench, island arc and back arc rift systems that occur elsewhere along the upper plate are replaced by the western island blocks of Santo and Malekula, the central Aoba basin and the horst-like structure beneath the eastern islands of Maewo and Pentecost, respectively. Directly to the west of these structures, on the subducting plate, is a broad, east-west oriented topographic high known as ...
