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Japan Marine Science and Technology Center Institute for Frontier Research on Earth Evolution (IFREE) estimate the thermal and chemical anomalies, we determined mantle discontinuity depths using semi-broadband waveform data from the Trans-Philippine Sea Array. We employed Velocity Spectrum Stacking of the receiver functions. First, we computed receiver functions from three-component seismograms for each event-station pair. After discarding the receiver functions with low S/N ratios, we divided the receiver functions into three groups, those at stations in the Mariana Trough, in the Parece Vera Basin, and in the Daito Ridge. We then stacked the receiver functions to enhance the Pds converted signals from the mantle discontinuities, where 'd' denotes a discontinuity depth. We measured Pds-P times to determine the discontinuity depths, taking into account the S-velocity structure above the discontinuities, which was taken from existing tomographic models. The discontinuity depths thus determined are: km for "" and km for "" beneath the Mariana Trough, km for "" and km for "" beneath the Parece Vera Basin, and km for "" beneath the Daito Ridge. Signals from the "" were not detected beneath the Daito Ridge (Fig.). The "" discontinuity depths beneath the Philippine Sea are substantially deeper than the global average (km). We will inter- Fig.2 Lateral variations in Rayleigh wave phase velocities. Blue lines denote higher velocities than average and red ones lower velocities. 2.3. Depths of the mantle discontinuities beneath the Philippine Sea using semi-broadband data from the Trans-Philippine Sea Array Studies of the depths of the "km" and "km" mantle discontinuities indicate thermal and chemical anomalies in the mantle transition zone independently of tomographic studies. Recent seismic tomography studies indicate that the Pacific slab is stagnant in the mantle transition zone, which could impart thermal and/or chemical anomalies in and around the stagnant slab. To Fig. 3 Depths of the "410" and "660" and the thickness of the mantle transition zone beneath the Philippine Sea. Green triangles are the OBS stations and the red cross denote the conversion points of P660s. 89
JAMSTEC 2002 Annual Report Institute for Frontier Research on Earth Evolution (IFREE) pret the present results, along with existing tomographic results and high-pressure high-temperature experimental results, in terms of thermal and chemical anomalies. temperatures, which may be associated with a hot plume, and the latter as -˚ lower temperatures, which may indicate the cold stagnant Pacific slab. 2.4. Thermal structure between Hawaii and the Philippine Sea as inferred from seismic and electromagnetic tomography. An integrated interpretation of seismic and electromagnetic tomography models would provide strong constraints on thermal and chemical heterogeneities in the mantle. We estimated the temperature field at depths between and km across the Pacific Ocean from Hawaii to the Philippine Sea, by comparing seismic and conductivity structures (Fig.). There are two sites of seismic and conductivity anomalies beneath Hawaii and the Philippine Sea: (a) Low seismic velocities and high conductivities at depths between and km beneath Hawaii; and (b) High seismic velocities and low conductivities in the uppermost lower mantle beneath the Philippine Sea. The former can be interpreted as -˚ higher 3. Marine experiments and data analysis for seismic and electromagnetic studies The marine maneuver observation group uses seismic and electromagnetic approaches to image mantle structure where it is not well resolved by only on-land data. Main targets are the northwest Pacific and adjoining back arc basins, where there is a region of mantle downwelling, and French Polynesia, where there is a region of mantle upwelling. Since the mantle dynamics group was established, we have been designing experiments and acquiring data. The achievements in were: ) acquisition of both seismic and electromagnetic data in the Mariana arcback arc system and their preliminary analysis, ) deployment of electromagnetic observation sites in the eastern Japan sea, and ) deployment of seismic observation sites in French Polynesia. Comparison of Seismic & EM Tomography and estimated temperature anomalies Depth (km) 300 1000 0 (a) Philippine Tr. Mariana Tr. Hawaii Is. 20 40 60 80 100˚ (b) Philippine Tr. Mariana Tr. Hawaii Is. 0 20 40 60 80 100˚ -1.5 0.0 1.5 -0.8 0.0 0.8 Slowness perturbation (%) Electrical conductivity anomaly log10 3D/1D) Depth (km) 300 1000 0 (c) Philippine Tr. Mariana Tr. Hawaii Is. 20 40 60 80 100˚ (d) Philippine Tr. Mariana Tr. Hawaii Is. 0 20 40 60 80 100˚ -400 0 400 -400 0 400 Temperature anomaly (K) Temperature anomaly (K) Fig. 4 Cross sections along the great circle from the Philippine Sea to Hawaii. (a) Seismic velocity anomalies; (b) Electric conductivity anomalies; (c) Temperature field deduced from the seismic velocities; (d) Temperature field from the conductivity. 90
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