Contrasts in seismic wave speeds and density across the 660-km discontinuity beneath the Philippine and the Japan Seas

[1] Using triplicate waveforms, we construct 34 seismic profiles east of Taiwan to investigate lateral variations of P and S wave speeds (V-P and V-S) near the mantle transition zone. Data from broadband and densely spaced, short-period arrays are combined to achieve high resolution. We find correspondingly high V-S in the back arc of Izu-Bonin where the Northern Philippine Sea Anomaly (NPSA) was previously delineated in the lower portion of the transition zone by an average V-P that is 1.5 +/- 0.5% higher than the surrounding regions. High V-S of the NPSA reduces the contrast in V-S across the 660-km discontinuity (DeltaV(S)) to about 3.9%, consistent with the notion that NPSA reflects a subhorizontal remnant of cold Pacific slab, resting immediately above the lower mantle. The aseismic NPSA contrasts the Fiji-Tonga region where many outboard earthquakes, low V-P and V-S and seismic anisotropy mark a large-scale, petrologic anomaly. The contrasting pattern implies that the NPSA is no longer cold enough to preserve materials such as metastable olivine or volatiles that can trigger earthquakes, reduce V-P and V-S, and cause anisotropy. In regions surrounding the NPSA, DVS is well constrained to be 5.9 +/- 0.5%. This precise result and known impedance contrast (similar to7%) under the Japan Sea lead to a density contrast (Deltarho) of about 8%, circumventing the trade-off between Deltarho and DeltaV(S). The magnitude of Deltarho is higher than recent estimates (similar to5%) without independent control on DeltaV(S), but close to longstanding global models, implying considerable resistance to slab penetration into the lower mantle north of the NPSA.