抄録
Reverse and conjugate strike-slip faults are densely and complicatedly distributed around the Tsuruga Plain, a high strain rate region in central Japan. In this study, we revealed the existence of the Ikemi fault distributed in the northeastern part of the Tsuruga Plain and estimated its property and activity using drilling and percussion cores excavated across the estimated surface trace of the Uchi-ikemi lineament (total extension is over 3 km) in a waste-filled valley called the Uchi-ikemi. We analyzed the sedimentary facies, tephras and radiocarbon dating for ages and established stratigraphic correlation among the cores. As a result, we clarified the cumulative folded structure in the sediments filling the Uchi-ikemi. Although no clear fracture zone was recognized, we consider the folded structure as the flexure due to movement of the Ikemi fault because the structure was recognized in the Uchi-ikemi and the Naka-ikemi along the Ikemi lineament. Based on eruption ages of tephras and radiocarbon ages, at least four times of activity (>95, 38-30, 19-7, <7 ka) are estimated for the Ikemi fault. Using the eruptive age and amounts of offset of correlation layers, the average vertical slip rate of the Ikemi fault is estimated to be 0.21 mm/yr.
