Stratigraphy, distribution patterns, and ground motion characteristics of the Pleistocene Setagaya and Tokyo formations beneath the Musashino Upland, Setagaya, Tokyo, central Japan

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  • 東京都世田谷区，武蔵野台地の地下に分布する世田谷層及び東京層の層序，分布形態と地盤震動特性
  • トウキョウト セタガヤク,ムサシノ ダイチ ノ チカ ニ ブンプ スル セタガヤソウ オヨビ トウキョウソウ ノ ソウ ジョ,ブンプ ケイタイ ト ジバン シンドウ トクセイ

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<p>The stratigraphy and characteristics of ground motion in the Pleistocene Setagaya and Tokyo formations were investigated. These strata, which occur beneath the Musashino Upland, southwestern Tokyo, central Japan, were studied using drilling surveys and observations of microtremors. Facies analysis and tephro- and palyno-stratigraphic examination of sediment cores reveal that the Setagaya Formation is composed mainly of muddy sediments deposited as incised-valley fills in a bay environment during the early to middle Marine Isotope Stage (MIS) 5e. The valley is thought to have formed during MIS 6. The valley-filling muddy sediments are soft with standard penetration resistance (SPT N-values) between 1 and 5, and S-wave velocities of ~150 m/s. The overlying Tokyo Formation comprises gravelly and muddy sand that accumulated in the inner parts of a larger bay, most likely during the middle to late MIS 5e. Microtremor observations show that ground motion in the Setagaya area reflects the distribution of these Pleistocene strata. There appears a peak at 1 Hz in the microtremor horizontal to vertical (H/V) spectral ratio in locations where the Setagaya Formation is relatively thick (~20 m) and lacks overlying gravelly terrace deposits. When considering either geotechnical problems or geological disasters that include amplification of earthquake motions in the metropolitan area, we need to focus on the soft muddy valley-fills beneath uplands, such as those of the Setagaya Formation.</p>

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