P velocity models of the mantle transition zone beneath pacific margins

Abstract

We present P velocity models of the mantle transition zone beneath the Pacific margins, inferred from velocity determinations at depths of foci of deep earthquakes. The velocity models, essentially applicable to various subduction zones in the Pacific margins, reveal a sharp first-order discontinuity with an abrupt P velocity jump of 7–9% at depths of 365 km in the Japan, 390 km in the Okhotsk Sea and 400 km in the Solomon Islands regions. This P velocity discontinuity is also brought out in the Tonga-Kermadec region at 410 km depth, although with a smaller velocity jump. The velocity models in the Tonga-Kermadec and Peru-Chile regions consistently reveal another prominent discontinuity with an abrupt P velocity jump of 8–10% at depths of 600 km and 570 km, respectively. A comparison with the latest global average estimates of the depths of the ‘410 km’ and ‘660 km’ velocity discontinuities in the transition zone reveals that both the discontinuities are relatively elevated to shallower depths within subducting slabs. The P velocity-depth data in the Tonga-Kermadec and Peru-Chile regions further suggest that the subducting slabs probably penetrate into the lower mantle well below the relatively elevated ‘660 km’ discontinuity.