東京直下に沈み込む2枚のプレートと首都圏下の特異な地震活動  [in Japanese] Subduction of Two Oceanic Plates and Unique Seismic Activity beneath the Tokyo Metropolitan Area  [in Japanese]

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Author(s)

    • 長谷川 昭 HASEGAWA Akira
    • 東北大学大学院理学研究科地震・噴火予知研究観測センター Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University
    • 中島 淳一 NAKAJIMA Junichi
    • 東北大学大学院理学研究科地震・噴火予知研究観測センター Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University
    • 内田 直希 UCHIDA Naoki
    • 東北大学大学院理学研究科地震・噴火予知研究観測センター Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University
    • 海野 徳仁 UMINO Norihito
    • 東北大学大学院理学研究科地震・噴火予知研究観測センター Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University

Abstract

 Recent investigations based on seismic tomography, hypocenter determinations and focal mechanism analyses using dense seismic network data reveal the precise configurations of the Pacific (PAC) and Philippine Sea (PHS) plates subducting beneath the Tokyo metropolitan area. Estimated geometry shows a broad contact zone between the two plates located directly beneath the Kanto plain. The overlap with the PHS plate subducting above it provides the PAC plate with protection from being heated by the hot mantle wedge. Moreover, the fore-arc portion of the PHS plate, before its subduction beneath Kanto, had been cooled by the subduction of the PAC plate from the Izu-Bonin trench. These cause lower-temperature conditions within the two oceanic plates and the upper continental plate beneath the Tokyo metropolitan area. As a result, depth limits of seismic activities within the plates and along their boundaries are anomalously deep. Seismic tomography studies show that the easternmost portion of the PHS slab mantle is serpentinized. The PHS slab may have been torn in two along the western boundary of this serpentinized mantle, with the eastern portion being left behind relative to subduction of the western portion. This is accompanied by the generation of large intraslab earthquakes along the boundary. We need to take these observations into consideration to understand the mechanism generating <i>M</i>7-class earthquakes, which are anticipated to occur in the southern Kanto region with a high probability.

Journal

  • Journal of Geography (Chigaku Zasshi)

    Journal of Geography (Chigaku Zasshi) 122(3), 398-417, 2013

    Tokyo Geographical Society

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