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We obtained phase velocities of fundamental Rayleigh waves in the northwestern Pacific ocean and the northern Philippine sea from seismograms retrieved from a long-term broad-band ocean bottom seismometer (LT-OBS) array deployed in 1999–2000 and from those recorded in Japan, Guam and Ponape islands. This array observation was one of the first attempts to record continuous seismograms on the seafloor using broad-band seismographs on a long-term basis. The geographical distribution of the LT-OBS stations and the surrounding land stations enabled us to determine pure-path phase velocities in a frequency range from 0.01 to 0.05 Hz across the Mariana trough, the Parece Vela basin, the Shikoku basin and the Minami Daito basin, which were found to be systematically lower than those of the Pacific seafloor at comparable ages. This result, along with other geophysical and geological observations in the Philippine sea, may be explained by an uppermost mantle that is relatively Fe rich. [ABSTRACT FROM AUTHOR]

Abstract: We determined the depths of the 410-km and 660-km seismic discontinuities (called “410” and “660,” respectively, hereafter) beneath the Philippine Sea and the northwestern Pacific by the receiver function method; we used the broadband data obtained from broadband ocean-bottom seismographs. We found a very deep “660” at 691km in the stagnant slab beneath the Philippine Sea. In the surrounding Philippine Sea regions and northwestern Pacific, the “660” appears at depths of 659–674km. Comparison of the “660” depth and P-velocity anomalies indicated that temperatures in the stagnant slab are lower than that in the mantle transition zone (MTZ) beneath the northwestern Pacific by about 500K. The temperature in the MTZ beneath the Parece Vela Basin and West Philippine Sea Basin, which surrounds the stagnant slab, is lower than that in the MTZ beneath the Pacific by about 200K. The water content in the stagnant slab (∼0.2wt% H2O) is higher than that in the other regions; however, the large uncertainty in the estimated water content (±0.2–0.3wt% H2O) prevents us from arriving at a definitive conclusion regarding the presence of water in the stagnant slab. Therefore, it is desirable to use other geophysical parameters (e.g., electrical conductivity) along with the seismic parameters in order to obtain an accurate estimate of the water content in the MTZ. [ABSTRACT FROM AUTHOR]

Abstract: The three-dimensional (3-D) electrical conductivity distribution in and around the mantle transition zone (MTZ) beneath the north Pacific is imaged by inverting revised geomagnetic responses at 13 geomagnetic observatories and cable responses of 8 long baseline submarine cables in the area. Two one-dimensional (1-D) models representing the average structure beneath the north Pacific were employed as the reference and initial models for the 3-D conductivity inversions. The features in the revised semi-global conductivity model in the MTZ are (1) a high conductivity anomaly beneath the Philippine Sea, (2) a high conductivity anomaly beneath the Hawaiian Islands, and (3) a low conductivity anomaly beneath and in the vicinity of northern Japan. Arrhenius plots of the conductivity and temperature estimated from global seismic tomography models indicate that the anomalies in the MTZ beneath northern Japan and most areas under Hawaii can be explained mainly by the temperature effect, but that the conductivity beneath the Philippine Sea requires an additional cause to explain the higher conductivity. If the cause is the effect of water, then the present result suggests high water content of 1wt% or more throughout the MTZ beneath the Philippine Sea. [ABSTRACT FROM AUTHOR]

Abstract: Previous tomographic studies using short-period P times have typically shown that the fast Pacific slab is stagnant in the mantle transition zone beneath the northern Philippine Sea and eastern Asia. However, there has been no consensus about the S-wave velocity anomalies of the stagnant slab, so that it has remained difficult to interpret the stagnant slab in terms of thermal and/or compositional state. Here we analyze both P- and S-wave travel time anomalies associated with the stagnant slab in the Izu-Bonin subduction zone, based on broadband records from Japan, Korea, and China. We found that P and S waves traveling through the Izu-Bonin stagnant slab exhibit systematically travel time residuals of −3 to −4s for P waves and −6 to −8s for S waves with respect to the Iasp91 model. The observed P residuals are consistent well with an existing three-dimensional P-velocity model, indicating that the stagnant slab beneath the studied region has fast P-wave velocity anomalies of 1.4%. The observed S residuals are consistent with an S-wave velocity model constructed from the P-wave velocity model with a constant scaling factor ∂ln V s /∂ln V p (=R) of 1.4. Agreement of the experimentally estimated R (1.38) and the seismologically obtained R (1.4) values suggests that both P- and S-velocity anomalies can be explained by a stagnant slab colder than the surrounding mantle by about 250–300K. [Copyright &y& Elsevier]

Abstract: We have determined the three-dimensional shear wave speed structure of the upper mantle in and around the Philippine Sea region using seismograms recorded by dense land-based and long-term broadband ocean bottom seismographic stations. We used a surface wave tomography technique in which multimode phase speeds are measured and inverted for a 3-D shear wave speed structure by incorporating the effects of a finite frequency and ray bending. The new ocean bottom data provided us with improved spatial resolution (~300 km) in the Philippine Sea region. In the upper 120 km, the shear wave speed structure is well correlated with seafloor age. At depths greater than 160 km, fast anomalies of the subducting Pacific Plate are clearly defined. We also found slow speed anomalies beneath the Kyushu–Palau ridge at depths greater than 120 km. Along the Izu–Bonin(Ogasawara)–Mariana arc, we have detected three separate slow anomalies in the mantle wedge at depths shallower than 100 km beneath the back arc. Each anomaly has a width of ~500 km. Moreover, these three anomalies have a close relationship with the three groups of frontal and rear arc volcanoes having distinct Sr, Nd, and Pb isotope ratios. We suggest that each of the anomalies is a site of large-scale flow of upper mantle into the mantle wedge, and that each already contains a component from the adjacent subducting slab. [Copyright &y& Elsevier]