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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy (S) - IASPEI - International Association of Seismology and Physics of the Earth's Interior JSS012 Poster presentation 2212 Implication of low velocity anomaly found beneath the oceanward side of Honshu subduction zone: Interaction between the sinking hot anomaly and exothermic phase change Dr. Satoru Honda Earthquake Research Institute University of Tokyo IASPEI Manabu Morishige, Yuji Orihashi Tomographic results are important to understand the dynamics of the mantle. Recently, Obayashi et al. (2006) confirmed the existence of slow velocity anomaly under the ocean side of the Honshu subduction zone and they interpreted as hot anomaly around 200 degrees. This hot anomaly is closely related to the 410 km seismic discontinuity which is supposed to be related to the exothermic phase change from olivine to wadsleyiite. Such a correlation appears to contradict our popular view of interaction between the exothermic phase change and convection that demands the promotion of the mantle flow. Here we consider a scenario in which a hot anomaly, whose origin might be a part of remains of hot plume(s) and/or stem(s), is dragged down by the slab and it interacts with the exothermic phase change near 410 km. The presence of exothermic phase change retards the downward movement of hot anomaly. Thus, the hot anomaly may stay near the 410 km for a while. To analyze this hypothesis, we have constructed a simple numerical model. We found that the hottest part is located above the 410 km discontinuity and stays there for a while. This may explain the observation and suggests that the determination of detailed distribution constrains the nature of the 410 km discontinuity. The time for the hot plume to stay near the 410 km discontinuity depends on many factors, such as the intensity of thermal anomalies and the viscosity. The hot anomaly whose size is equivalent to a few thousands km (horizontal) $times$ 100 km (vertical) may stay there ~100 million years. We shall discuss the geologic implication of this result. Keywords: 410kmdiscontinuity, tomography, convection

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy (S) - IASPEI - International Association of Seismology and Physics of the Earth's Interior JSS012 Poster presentation 2213 Mantle anisotropy beneath the eastern Asia Dr. Yoko Tono IFREE JAMSTEC IASPEI Yoshio Fukao, Takashi Kunugi, Seiji Tsuboi We made a detailed mapping of shear-wave splitting parameters of multiple ScS phases for the whole Japanese islands and their back-arc region. A set of multiple ScS phases (ScS, sScS, ScS2 and sScS2) has a mutually common source-receiver pair among the three nearby deep shocks and more than 550 stations of the Hi-net tiltmeter network and several IRIS stations situated in eastern Asia. The multiplicity of the ScS phases, three deep shocks with different focal mechanisms and an unprecedented number of stations made it possible to resolve mantle anisotropy into the two parts, anisotropy in the wedge mantle and the subducted Pacific-slab. The anisotropy of the wedge mantle shows a clear distinction of splitting pattern across the volcanic front. Such a distinction persists all along the southern Kuril, northern Honshu and Ryukyu arcs. On the Pacific side of the volcanic front vertically propagating shear wave is polarized with the fast direction approximately parallel to the trench, whereas it is polarized with the fast direction approximately parallel to the plate convergence direction on the marginal sea side of the volcanic front. This anisotropic system with the fast direction parallel to the plate convergence direction appears to extend to the Asian continent across the Japan Sea further away from the volcanic front. The Pacific-slab is anisotropic with the fast direction uniformly in the NNW, closely parallel to the Mesozoic fracture zones and perpendicular to the magnetic lineations on the northwestern Pacific seafloor. The NNW alignment extends into the β-spinel region well beyond the 400- km depth contour of the Wadati-Benioff zone but does not extend to the source region of the Vladivostok event at a depth of 566 km in the presumably oldest part of the subducted slab. The splitting parameters estimated from MDJ station, which is very close to the epicenter of the Vladivostok event, clearly shows a different anisotropy system with the fast direction roughly in EW. Keywords: anisotropy, pacific slab, wedge mantle

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