
Contrasts in seismic wave speeds and density across the 660‐km discontinuity beneath the Philippine and the Japan Seas
Author(s) -
Tseng TaiLin,
Chen WangPing
Publication year - 2004
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2003jb002613
Subject(s) - geology , seismology , mantle (geology) , discontinuity (linguistics) , transition zone , geodesy , olivine , geophysics , mineralogy , mathematics , mathematical analysis
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 (Δ V 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, Δ V S 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 Δ V S . The magnitude of Δρ is higher than recent estimates (∼5%) without independent control on Δ V S , but close to longstanding global models, implying considerable resistance to slab penetration into the lower mantle north of the NPSA.