The Yamasaki Fault System is the longest active fault system in the Chugoku Mountains, western
Japan, and is composed of seven active faults named the Ohara, Hijima, Yasutomi, Kuresakatouge, Biwakou, Miki, and Kusadani Faults from northwest to southeast. Among these active faults, the latest activity and recurrence interval of the Kuresakatouge Fault have not been clarified in spite of the intense researches after the 1995 Hyogo-ken Nanbu (Kobe) Earthquake. Reexamination of the fault outcrop and a trenching survey were conducted to reveal the recent activity of this fault, at Goji and Okugoji sites, Himeji City, Hyogo Prefecture, respectively. At the Goji site, the sediments deposited after the fall of the Kikai-Akahoya tephra (K-Ah) erupted at 7,300 cal BP were displaced by active faulting, which is considered to be the latest event of the Kuresakatouge Fault. On the other hand, at the trench site of Okugoji, fault displacement was recognized only within the lower part of the gravel beds deposited after the fall of the Sanbe-Ukinuno tephra (SUk) erupted at 20,000 cal BP, and the upper part of the beds and the younger sediments deposited after the fall of K-Ah show non-fault-related deformation. Judging from the small amount of vertical displacement less than 30 cm at the Okugoji trench site, the deformation of the sediments indicates a result by the only one event between 20,000 and 7,300 cal BP. The Kuresakatouge Fault has been activated at least two times during the last 20,000 years, suggesting its longer recurrence interval than those of the other six faults comprising the Yamasaki Fault System.
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