The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
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Volume 126, Issue 6
Displaying 1-4 of 4 articles from this issue
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Articles
  • Stratigraphy, age and depositional processes of Yosasagawa Debris Avalanche deposit, North Tochigi Prefecture, Japan
    Akihiko Kikuchi, Takeshi Hasegawa
    2020 Volume 126 Issue 6 Pages 293-310
    Published: June 15, 2020
    Released on J-STAGE: October 14, 2020
    DOIhttps://doi.org/10.5575/geosoc.2020.0014
    JOURNAL FREE ACCESS
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    The Yosasagawa Debris Avalanche (YDA) is the oldest and largest debris avalanche associated with sector collapse of the Nasu Volcanic Group, North Tochigi Prefecture, Japan, during the middle Pleistocene. The deposit is widely distributed along the rivers that cut the Nasu Volcanic Group, such as the Yosasagwa and Naka rivers. Here, we report an investigation of the relative age, distal correlation, and depositional processes associated with the YDA. The YDA is characterized by the presence of matrix and block facies containing quartz-bearing pyroxene andesite to dacite (SiO2 = 58-63 wt.%) lava blocks and clasts that occur stratigraphically between widespread biotite-bearing tephras, such as KMT (Kaishio Kamitakara Tephra, 0.62 Ma) and APm (Ohamachi APm tephra, 0.36-0.33 Ma), in the Chubu region (Central Japan). Although previous research has reported the age of the YDA as 0.3 Ma, the tephra-based stratigraphy suggests that the YDA occurred before 0.36-0.33 Ma. Using lithofacies (debris avalanche-like), petrography, and the whole-rock geochemistry of lava blocks in the deposit, the YDA can be correlated with the Awakawa Pumice Bed in northern Ibaraki Prefecture. Based on this correlation, the runout distance can be estimated to be more than 100 km from the source, Sanbonyaridake Volcano, Nasu Volcanic Group. Although the lithofacies of the upper part of the YDA are uniquely massive and contain mega-blocks at all outcrops, the lithofacies in the lower part of the YDA show remarkable lateral variation; i.e., an increasingly proportion of lava blocks with clast-supported structure is observed downstream. This indicates that the YDA was water-saturated and fluidized by river transport and thus able to travel large distances as lahar.

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  • S-wave velocity structures and ground motion characteristics of loam terraces: The case of the Utsunomiya area, Tochigi Prefecture, central Japan
    Tsutomu Nakazawa, Ikuo Cho, Kentaro Sakata
    2020 Volume 126 Issue 6 Pages 311-326
    Published: June 15, 2020
    Released on J-STAGE: October 14, 2020
    DOIhttps://doi.org/10.5575/geosoc.2020.0012
    JOURNAL FREE ACCESS
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    We examined the influence of volcanic ash soil (loam beds) overlying fluvial terraces on ground motion characteristics in the Utsunomiya area, Tochigi Prefecture, central Japan. Microtremor surveys reveal that the loam beds exhibit relatively low S-wave velocities (130-150 m/s) and that the average S-wave velocities to the depth of 30 m (AVS30) and the peak frequencies of horizontal-to-vertical (H/V) spectral ratios become lower in higher terraces, due to the influence of thick loam beds overlying the gravelly terrace deposits. In addition, high AVS30 values and high peak frequencies of H/V spectra are observed in the lowlands along rivers, because these areas are underlain mainly by fluvial gravelly deposits without loam beds. Ground motions in a relatively low frequency range (e.g., 1.5-2.0 Hz) are more amplified on higher terraces and hills than on lower terraces or lowlands.

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