Search for deep slabs in the Northwest Pacific mantle

A residual sphere is formed by projecting seismic ray travel-time anomalies, relative to a reference Earth model, onto an imaginary sphere around an earthquake. Any dominant slab-like fast band can be determined with spherical harmonic expansion. The technique is useful in detecting trends associated with high-velocity slabs beneath deep earthquakes after deep-mantle and near-receiver effects are removed. Two types of corrections are used. The first uses a tomographic global mantle model; the second uses teleseismic station averages of residuals from many events over a large area centered on the events of interest. Under the Mariana, Izu-Bonin, and Japan trenches, the dominant fast bands are generally consistent with seismicity trends. The results are unstable and differ from the seismicity trend for Kurile events. The predominant fast band for most deep earthquakes under Japan is subhorizontal rather than near vertical. We find little support for the deep slab penetration hypothesis.