Variations of P wave speeds in the mantle transition zone beneath the northern Philippine Sea

Using waveforms and travel times from deep earthquakes, weconstructed 16 seismic profiles, each of which constrains theradial variation i Vp over a small area beneath e northern Philippine Sea. Taken together, the azimuthal coverage of these profiles also places tight bounds on the lateral extent of a region of anomalously high Vp (up to 3% faster than average Earth models) originally suggested by travel time tomography. Unlike travel time tomography, which relies heavily on arrival times of the direct P phase, we utilize the waveforms and move-out of later arrivals that mainly sample the mantle transition zone of interest. Our results identify three important characteristics of the northern Philippine Sea anomaly that are distinct from previous results. First, being approximately a subhorizontal, laterally uniform feature, the anomaly islocalized beneath the northwestern comer of the Philippine Sea, within a region of approximately 500 x 500 km 2 immediately east of the Ryukyu arc. Second, the anomaly is well constrained tooccur in the lower portion of the transition zone, extending allthe way down to the 660-km discontinuity. Third, the presence ofsuch a distinct anomaly reduces the contrast in V, across the 660-km discontinuity from approximately 6%to 3%. Such a configuration •s consistent wath the interpretanon that the anomaly iscaused by a remnant of subducted slab, as negative buoyancy should rest the slab just above the 660-km discontinuity where resistance to subduction is expected from a negative Clapeyron slope during the spinel-Mg-Fe-perovskite transition.