Mapping of P-wave slab anomalies beneath the Tonga, Kermadec and New Hebrides arcs

Abstract

Abstract P-wave velocity anomalies of several per cent fast relative to the Jeffreys-Bullen (JB) model are seen around subduction zones under the Tonga, Kermadec and New Hebrides trenches from inversion of International Seismological Centre (ISC) travel-time residuals from intermediate and deep-focus earthquakes. Mantle heterogeneities outside the model are corrected using previous velocity models. Ray paths inside the model region are traced allowing two-dimensional velocity variations. The rays of similar source and receiver locations are combined into summary rays, and the hypocenters are relocated as part of the inversion iterations. The resulting images are interpreted in conjunction with resolution and error estimations. The slab-like anomalies are generally continuous in places of high seismicity, and decrease in amplitude with depth. Most coherent slab anomalies are surrounded by large slow patches between the surface and 350-km depth. The fast slab beneath the Tonga arc appears to end at ∼ 550-km depth, before the seismicity ends, but there is a fast band at 750–1000-km depths below the deepest Tongan earthquakes. The fast slab-like anomaly beneath the Kermadec arc tends to flatten to subhorizontal near the bottom of upper mantle, and it occurs aseismically near New Zealand down to 500–550-km depth. Interpretations in the lower mantle in many places, such as Kermadec and New Hebrides arcs, however, are hampered by poor resolution.