2016 Volume 27 Pages 13-26
The Yamasaki fault zone comprises six left-lateral active faults including the Ohara, Hijima, Yasutomi, and Kuresakatouge faults. These faults, exclusive of the Hijima fault, have proved to reactivate during AD 868 Harima Earthquake. We explored three trenches across the Hijima fault at the Aoki district in Shiso City to reveal its late Holocene activity. Although no fault planes appeared at any trench sites, a buried creek outcropped at one of them. The radiocarbon dates of humic layers on the creek bottom show that the creek was constructed after 1,600 years ago. We also detected the deformed soil layers and cracks in the terrace deposits at another site caused by the strong motion during a large earthquake after 1,600 years ago. An old oral tradition in the Aoki district suggests that a fault pond appeared with a surface rapture by an ancient earthquake and then artificial creeks were dug to drain the pond water. The excavated creek, cracks, and deformed soil layers support the reality of the oral tradition. Because AD 868 Harima Earthquake is the only large earthquake since 1,600 years ago in the region, the Hijima fault possibly reactivated and produced the surface rapture during the earthquake.