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

Using waveforms and travel times from deep earthquakes, we constructed 16 seismic profiles, each of which constrains the radial variation in V-p over a small area beneath the northern Philippine Sea. Taken together, the azimuthral coverage of these profiles also places tight bounds on the lateral extent of a region of anomalously high V-p (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 is localized beneath the northwestern corner 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 to occur in the lower portion of the transition zone, extending all the way down to the 660-km discontinuity. Third, the presence of such a distinct anomaly reduces the contrast in V-p across the 660-km discontinuity from approximately 6% to 3%. Such a configuration is consistent with the interpretation that the anomaly is caused 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.