Distribution of fossil seawater and its role in Neogene sedimentary rock landslides in Niigata, eastern marginal region of the Japan Sea

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  • Masahiro Chigira
    Disaster Prevention Research Institute Kyoto University, Gokasho (currently Fukada Geological Institute), Uji‐shi Kyoto 611‐0011 Japan
  • Koichi Suzuki
    Civil Engineering Research Laboratory, Central Research Institute of Electric Power Industry (currently Hokkaido University), 1646 Abiko, Abiko‐shi Chiba 270‐1194 Japan
  • Naoki Watanabe
    Research Institute for Natural Hazards and Disaster Recovery Niigata University, 8050 Ikarashi‐nino‐cho, Nishi‐ku Niigata 950‐2181 Japan
  • Nariaki Nishiyama
    Graduate School of Science, Kyoto University (currently OSPEC Inc.), Gokasho, Uji Kyoto 611‐0011 Japan

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Published
2020-01
Resource Type
journal article
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  • http://onlinelibrary.wiley.com/termsAndConditions#vor
DOI
  • 10.1111/iar.12351
Publisher
Wiley

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<jats:title>Abstract</jats:title><jats:p>The Neogene marine sedimentary rock area in the eastern marginal region of the Japan Sea is an area with some of the highest landslide densities in Japan. Some of the landslides in this area have been known to involve saline groundwater, which can be the cause of these landslides. In order to demonstrate the relationships between landslides and saline water, topographic, geological, groundwater, and electromagnetic surveys were performed in the eastern marginal region of the Japan Sea. Many landslides and gravitational slope deformations with linear depressions and small scarps were recognized in the study area. The resistivity profile obtained by an electromagnetic survey suggests that there is a wide zonal distribution of saline water with salt concentrations equivalent to seawater at depths of 50–100 m or more and that the groundwater shallower than 50 m has an electrical conductivity of less than 100 mS/m. The shallow resistive groundwater is inferred to be meteoric water that replaced the saline groundwater, which likely weakened the bedrock, resulting in landslides. A ridge of competent tuff overlying mudstone has many linear depressions from gravitational slope deformation and low‐resistivity water to a depth of 600 m, which suggests that the mudstone was weakened by water replacement and deformed under the tuff caprock. The saline groundwater is inferred to be fossil seawater trapped in pores during sediment deposition, which is brought near the ground surface along with rocks by tectonic movement in the hills. Thus, the saline water and its fresh water replacement are among the important basic causes of the landslides. The oil well data obtained in the eastern marginal region of the Japan Sea suggest that such saline water replacement has occurred widely and that replacement is likely one of the predispositions for the frequent landslides there.</jats:p>

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