山形県鶴岡市西目地区‘丸山’の地質：令和4年12月に発生した地すべり災害に関連して

抄録

A landslide occurred on the south slope of the hill, so-called ‘Mt. Maruyama’, Nishime, Tsuruoka, Yamagata Prefecture, on December 31, 2022, destroying 17 houses and causing two deaths. We surveyed the geology of ‘Mt. Maruyama’ and describe its rock facies and minerals that have formed from weathering. The landslide head scarp is composed of weakened basaltic andesite and minor sedimentary rocks. More broadly, the facies at ‘Mt. Maruyama’ consist mainly of sedimentary rocks, including tuffaceous conglomerate, tuffaceous sandstone, lapilli tuff, and pumiceous tuff associated with a lens-shaped body of basaltic andesite. These rocks might belong to the lower to middle Miocene Zenpoji or Oyama Formations. Powder X-ray diffraction (XRD) analyses identified halloysite, kaolinite, and goethite in all facies, suggesting that the igneous and pyroclastic rocks have been strongly weathered by groundwater. The weakened nature of the rocks indicates that ‘Mt. Maruyama’ has been affected by deep weathering.