Trench excavation study of the Kokura-higashi fault in northern Kyushu Island, southwest Japan

Abstract

The Kokura-higashi fault located in the northern Kyushu Island is an active fault extending in NNESSW direction with west-side-up and right-lateral displacement. The Earthquake Research Committee of the Headquarters for Earthquake Research promotion evaluated that the probability of the earthquake occurrence in the future on the Kokura-higashi fault is unknown because of the luck of paleoseismological data. After such situation, we carried out a trench excavation survey on this fault. A trench was excavated on the fault trace of the Kokura-higashi fault estimated from the continuation of the fault scarplet to reveal the past activity of this fault.

On the trench wall, a steeply dipping fault cutting bedrocks and overlying sediments was cropped out. The lower part of the sediments includes some humic soil layers with many wood fragments. The fault cuts until the top of the sediments just below artificial soil. Radiocarbon ages ranging from 15 ka to 40 ka in calendar year were measured for samples from humic soil layers on the trench wall and the borehole cores dug up before and after trenching. One faulting event recognized on the trench wall estimated to occur in 19 to 20 ka and one liquefied event estimated to occur in 29 to 35 ka. The recurrence interval including the last two events estimated by a former trenching study is calculated to about 9,000 years.