[S-SS15] Active faults and paleoseismology

[SSS15-05] Paleoseismological survey on the Shibetsu fault zone in the eastern part of Hokkaido, Japan

*Takashi Azuma¹, Daisuke Hirouchi², Koji Okumura³, Kaoru Taniguchi⁴, Masashi Omata⁴, Yorihide Kohriya⁴ (1.National Institute of Advanced Industrial Science and Technology, 2.Shinshu University, 3.Hiroshima University, 4.Pasco CORPORATION)

Keywords:active fault, paleoseismology, trenching survey, tephrochronology, the Shibetsu fault zone, Hokkaido

We conducted a paleoseimological survey on the Shibetsu fault zone in the eastern part of Hokkaido in 2018.
This fault zone has a length more than 52 km but no data was obtained about slip-rate and history of fault activeity in previous study (Hokkaido, 2004 and 2005). On a detailed topographic map with an air-borne LiDAR data, we found a new fault scarp with height of 2-5 m on fan produced the lastglacial age at Kotanuka site, and excurvated a trench (25 m in length, 8 m in width, 4 m in depth) and 4 drilling surveys (5 m in depth). We observed a high-angle fault cutting a tephra layer (Ma-l: ca.12,000 yrBP) and black soil with 40 cm on the trench wall. Based on drilling survey, the fan gravel layers, which covered with sandy sediment containing humic silt with 14C ages of 22,140-21,690 cal.BC and 20,587-20,343 cal.BC, is offset ca.3 m in vertical. We discuss the fault event and slip-rate of this fault with careful, because there are deformation of strata not only faulting but also periglacial process in the trench walls.