11:30 AM - 11:45 AM
[HQR05-10] Variation of ground motion characteristics influenced by lateral facies change of Pleistocene deposits in the Tokyo metropolitan area
Keywords:ground motion characteristics, Pleistocene, Tokyo metropolitan area
The Kanto Plain, the largest plain in Japan with the capital city, Tokyo, is known for its large proportion of uplands. We investigated how the ground motion characteristics change influenced by lateral change in sedimentary facies of Pleistocene deposits in the Tokyo metropolitan area, by drilling surveys and microtremor observations.
At Kami-yoga, Setagaya-ku, where the muddy lower part of the Kioroshi (Tokyo) Formation (MIS 5e) is distributed and the overlying strata do not contain coarse-grained sediments down to the ground surface, a remarkable peak in the microtremor H/V spectrum is observed in the low frequency band around 1 Hz, similar to lowland areas despite the upland location. On the other hand, even in areas where the lower part of the Kioroshi Formation is distributed, when the upper part of the Kioroshi Formation is sandy, or when the Joso Formation/younger terrace deposits (MIS 5c and younger) are thickly distributed, two indistinct peaks are observed in the H/V spectra. The H/V spectral peaks are more distinct in the area whose basement is composed of the consolidated Kazusa Group than the unconsolidated lower Shimosa Group. In other words, the ground motion characteristics in the Tokyo metropolitan area are strongly influenced by three geological factors: 1) the presence and thickness of valley-fill deposits in the lower part of the Kioroshi Formation, 2) the lateral facies change of terrace deposits, and 3) the lithofacies of the basement.
At Kami-yoga, Setagaya-ku, where the muddy lower part of the Kioroshi (Tokyo) Formation (MIS 5e) is distributed and the overlying strata do not contain coarse-grained sediments down to the ground surface, a remarkable peak in the microtremor H/V spectrum is observed in the low frequency band around 1 Hz, similar to lowland areas despite the upland location. On the other hand, even in areas where the lower part of the Kioroshi Formation is distributed, when the upper part of the Kioroshi Formation is sandy, or when the Joso Formation/younger terrace deposits (MIS 5c and younger) are thickly distributed, two indistinct peaks are observed in the H/V spectra. The H/V spectral peaks are more distinct in the area whose basement is composed of the consolidated Kazusa Group than the unconsolidated lower Shimosa Group. In other words, the ground motion characteristics in the Tokyo metropolitan area are strongly influenced by three geological factors: 1) the presence and thickness of valley-fill deposits in the lower part of the Kioroshi Formation, 2) the lateral facies change of terrace deposits, and 3) the lithofacies of the basement.