A steeply dipping discontinuity in the lower mantle beneath Izu-Bonin

We analyze the coda of teleseismic P waves to deterministically map seismic scatterers in a 1000 km on-a-side cube beneath the Izu-Bonin trench by migrating and stacking waveforms. This method was applied to several hundred short-period vertical-component western United States seismometer recordings of 17 deep-focus Izu-Bonin earthquakes. Except for isolated arrivals, the midmantle appears devoid of sharp discontinuities. S-to-P conversions at the 660-km discontinuity generated the largest signals; the 410-km discontinuity occasionally generated signals. Horizontal discontinuities poorly explain other signals; however, S-to-P conversions generated at a nearly north-south trending, steeply dipping discontinuity at 30 degrees N, 145 degrees E, and 1000 km beneath and parallel to the Izu-Bonin trench explain most additional signals. Observed coherent S-to-P conversions of 2-s period waves limit the width of a gradient zone to <7 km. Compressional and shear tomographic models [Grand et al., 1997; Widiyantoro, 1997] image a fast velocity structure at a similar depth and orientation extending southward from the middle of the Izu-Bonin trench to the Mariana trench. We interpret the sharp scattering surface as a subducted crust and the tomographically imaged structure as the cold thermal anomaly associated with an ancient slab. We observe neither the deep scatterer nor high-seismic wavespeeds north of 32 degrees N. This feature is as much as 800 km east of recent deep earthquakes near 30 degrees N and is at a latitude where the slab appears to extend horizontally on the 660-km discontinuity to the west of the seismicity. We suggest that this deep slab fragment;was carried north 500-1000 km owing to oblique subduction. In this scenario, it would be associated with the near vertical subduction at the Mariana subduction zone.