9:45 AM - 10:00 AM
[HQR04-04] Ground-type classification based on Quaternary stratigraphy and sedimentary facies and the corresponding ground motion characteristics, with special reference to the 3D Urban Geological Map of Central Tokyo
★Invited Papers
Keywords:Quaternary stratigraphy, 3D geomodelling, borehole logs, microtremor observation, ground motion characteristics
Ground motion characteristics in an urbanized coastal plain are influenced by shallow subsurface geologic conditions. We classified ground types in central Tokyo based on stratigraphy and sedimentary facies in reference to the 3D Urban Geological Map of Central Tokyo published in April 2021 by our research group of GSJ, AIST, and conducted microtremor observations in each ground type to clarify the corresponding ground motion characteristics.
The Musashino Upland is composed of the Middle to Upper Pleistocene strata including the Yabu (MIS 9), Kamiizumi (MIS 7e), and Tokyo (MIS 5e) formations. Of them, the Tokyo Formation contains thick inner-bay soft muddy sediments as incised-valley fills. The detailed distribution of the incised-valley fills was demonstrated by 3D geomodelling. Microtremor observations reveal that the H/V spectra show a peak at 1–2 Hz at the areas of the incised-valley fills. Particularly, a remarkable peak at 1 Hz is observed in the H/V spectra of the Setagaya Valley fills with the high mud-content. On the other hand, the H/V spectra exhibit higher peak frequencies around 4 Hz at the areas outside the incised valleys because of the hard substrate composed mainly of marine sand beds of the Yabu and Kamiizumi formations.
The Tokyo Lowland is underlain by the post-LGM deposits composed generally of fluvial to inner-bay sand and mud. The 3D geomodelling based on the detailed stratigraphic correlation reveals that the buried geomorphology almost corresponding to the base of the post-LGM deposits comprises the deepest valley floor and four buried flat surfaces. We carried out microtremor observations along a line across the deepest valley floor and the flat surfaces 1 and 2 beneath the Tokyo Lowland. The H/V spectra at the areas both of the deepest valley floor and the flat surface 2 exhibit a peak at 1 Hz. The peaks of the flat surface 2 area are clearer than those of the valley floor area because of the high contrast of physical properties between the firm buried terrace gravelly sediments and the overlying soft post-LGM deposits. On the other hand, the H/V spectra at the flat surface 1 area show flat shapes in high frequency ranges due to relatively thin soft sediments above the Pleistocene hard substrate.
Thus, the ground-type classification based on the detailed stratigraphy and sedimentary facies using the 3D geomodelling enables the efficient and accurate assessment of the earthquake hazard risks.
The Musashino Upland is composed of the Middle to Upper Pleistocene strata including the Yabu (MIS 9), Kamiizumi (MIS 7e), and Tokyo (MIS 5e) formations. Of them, the Tokyo Formation contains thick inner-bay soft muddy sediments as incised-valley fills. The detailed distribution of the incised-valley fills was demonstrated by 3D geomodelling. Microtremor observations reveal that the H/V spectra show a peak at 1–2 Hz at the areas of the incised-valley fills. Particularly, a remarkable peak at 1 Hz is observed in the H/V spectra of the Setagaya Valley fills with the high mud-content. On the other hand, the H/V spectra exhibit higher peak frequencies around 4 Hz at the areas outside the incised valleys because of the hard substrate composed mainly of marine sand beds of the Yabu and Kamiizumi formations.
The Tokyo Lowland is underlain by the post-LGM deposits composed generally of fluvial to inner-bay sand and mud. The 3D geomodelling based on the detailed stratigraphic correlation reveals that the buried geomorphology almost corresponding to the base of the post-LGM deposits comprises the deepest valley floor and four buried flat surfaces. We carried out microtremor observations along a line across the deepest valley floor and the flat surfaces 1 and 2 beneath the Tokyo Lowland. The H/V spectra at the areas both of the deepest valley floor and the flat surface 2 exhibit a peak at 1 Hz. The peaks of the flat surface 2 area are clearer than those of the valley floor area because of the high contrast of physical properties between the firm buried terrace gravelly sediments and the overlying soft post-LGM deposits. On the other hand, the H/V spectra at the flat surface 1 area show flat shapes in high frequency ranges due to relatively thin soft sediments above the Pleistocene hard substrate.
Thus, the ground-type classification based on the detailed stratigraphy and sedimentary facies using the 3D geomodelling enables the efficient and accurate assessment of the earthquake hazard risks.