VARIATIONS OF P-WAVE SPEEDS IN THE MANTLE TRANSITION ZONE BENEATH THENORTHERN 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 toge ther, the azimuthral coverage of these profiles also places tight boun ds 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 ar rival times of the direct P phase, we utilize the waveforms and move-o ut 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 result s. First, being approximately a subhorizontal, laterally-uniform featu re, the anomaly is localized beneath the northwestern corner of the Ph ilippine Sea, within a region of approximately 500 x 500 km(2) immedia tely east of the Ryukyu arc. Second, the anomaly is well constrained t o 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 d istinct anomaly reduces the contrast in V-p across the 660-km disconti nuity from approximately 6% to 3%. Such a configuration is consistent with the interpretation that the anomaly is caused by a remnant of sub ducted 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 transitio n.