Mr. Brudzinski et al., VARIATIONS OF P-WAVE SPEEDS IN THE MANTLE TRANSITION ZONE BENEATH THENORTHERN PHILIPPINE SEA, J GEO R-SOL, 102(B6), 1997, pp. 11815-11827
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.