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

Abstract

Using triplicate waveforms, we construct 34 seismic profiles east of Taiwan to investigate lateral variations of P and S wave speeds (VP and VS) near the mantle transition zone. Data from broadband and densely spaced, short‐period arrays are combined to achieve high resolution. We find correspondingly high VS 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 VP that is 1.5 ± 0.5% higher than the surrounding regions. High VS of the NPSA reduces the contrast in VS across the 660‐km discontinuity (ΔVS) 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 VP and VS 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 VP and VS, and cause anisotropy. In regions surrounding the NPSA, ΔVS is well constrained to be 5.9 ± 0.5%. This precise result and known impedance contrast (∼7%) under the Japan Sea lead to a density contrast (Δρ) of about 8%, circumventing the trade‐off between Δρ and ΔVS. The magnitude of Δρ is higher than recent estimates (∼5%) without independent control on ΔVS, but close to longstanding global models, implying considerable resistance to slab penetration into the lower mantle north of the NPSA.