2016 Volume 122 Issue 1 Pages 23-35
A small mound excavated at the Apporo 1 archaeological site, at the southeastern margin of the Ishikari Lowland, is inferred to be a landslide body originating from a modern pyroclastic fall deposit. Its generative process, inferred from observations of its interior structure, are discussed. The mound mainly comprises pumice and rock fragments originating from the Tarumai-d pyroclastic deposits (Ta-d: 8-9 ka). It rests upon humus overlying the Ta-d deposits on a river terrace, and is covered by the Tarumai-c pyroclastic deposits (Ta-c: 2.5-3 ka). On a longitudinal trench wall, we found an asymmetric extensional structure resembling a normal listric fault in the mound deposit, and an asymmetric duplex-like structure composed of a mixture of humic soil and loam at its base. On the trench wall, in the transverse direction, the observation of horst and graben structures in the mound deposit indicates a lateral tensile stress field. These observations suggest that the mound was a landslide deposit derived from a hill slope and emplaced on a terrace surface. The landslide, which developed within an unconsolidated, stratified pyroclastic deposit, was probably triggered by an earthquake. Conventional radiocarbon dating of humus, and dates based on the relationships of the strata, indicate that the slide occurred between ca. 4,600 and 2,500 years BP. This age may correspond the most recent seismic event on the Umaoi Fault in the Ishikari-teichi-toen fault zone.
