[H-DS11] Geohazards in humid, tectonically active countries and their precursors

[HDS11-02] Spread-type landslide in the Shimogo-area, Fukushima Prefecture, Northeast Japan: Characteristics of sediments accumulated in the rift between the sliding blocks and mudstone along the sliding surface

*Satoru Kojima¹, Ibuki Inoue¹, Shoutaro Ukai¹, Katoh Yasuo^1,2 (1.Department of Civil Engineering, Gifu University, 2.Kawasaki Geological Engineering Co Ltd., Nagoya 465-0025, JAPAN)

Keywords:spread-type landslide, Fukushima Prefecture, Shimogo area

A spread-type landslide is recognized in the Shimogo area, Fukushima Prefecture, Northeast Japan. We report the lithological characteristics of sediments accumulated in the graben between the landslide blocks, describe geological structures in silt-mud formation along the slip surface, and present a 14C age of plant remain included in the silt-mud, following the presentation by Kato and Kojima (2022) on the results of geological and geomorphological investigations on this landslide. We drilled sediments in the graben by using a hand-auger drilling system and got a 3 m-core composed of volcanic mud, silt, and sand including mm- to cm-scale pumice grains. Very fine-grained fractions of the sediments consist of volcanic glass shards of pumice, microlite-bearing, blocky, vesicular types, and minerals like quartz, feldspar, hornblende, and pyroxene. The grain compositions suggest that the volcanic sediments are correlated with the Numazawa Lake volcaniclastic rocks erupted about 3,400 BC from the Numazawa volcano about 40 km northwest from this landslide (Yamamoto, 2003). Plastically deformed landslide mud-silt extruded from the slip surface horizon at the toe of this landslide includes the andesite blocks of the slide mass and clasts on the riverbed. Also included are the plant remains with the 14C age of 5628-5524 cal BC (93.25 %). These facts indicate that the landslide was active, at least, about this age. The XRD analyses show that the landslide silt-mud include smectite common to landslides. Observations of polished surface by naked eyes and thin sections by polarized microscope of the landslide silt-mud revealed that the rock is tuff rich in volcanic glass shards, include mud clasts with zigsaw cracks formed by brittle deformation, black-linear structures developed parallelly in two-directions. These deformation structures were formed by the spread-type landslide movements along the alternating silt and mud formations.