SEISMICITY AND ATTENUATION IN THE CENTRAL VANUATU (NEW HEBRIDES) ISLANDS: A NEW INTERPRETATION OF THE EFFECT OF SUBDUCTION OF THE D'ENTRECASTEAUX FRACTURE ZONE

The spatial distribution of the intermediate-depth earthquakes in the central Vanuatu (New Hebrides) islands includes a remark- able gap in seismic activity located between depths of 100 and 200 km and having a lateral extent of about 150 km. No well-located earth- quakes determined from teleseismic d ata during a 16-year period nor any microearthquakes located by a local seismograph network during a 5-year period have occurred in the gap. A zone of attenuation of high-frequency shear waves over- laps the seismicity gap. No contortion of the Benioff zone in the region of the gap indicative of major disruption of the subducted plate is apparent. However, the gap is close to the extrapolated location of the subducted part of the D'Entrecasteaux Fracture Zone (DFZ). The DFZ is a ridgelike bathymetric feature on the oceanic plate being subducted beneath the island a rc. A pogitive thermal anomaly associated with the DFZ may cause the gap of intermediate-depth seismic- ity and produce anomalous absorption of high- frequency shear waves. Alternatively, a zone of strong scattering associated with the subducted part of the DFZ may also account for the anoma- lous attenuation.