The eastern margin of the Yokote basin fault zone is primarily composed of two kinds of fault : frontal fault and boundary fault. Senya fault and Shiraiwa fault along the eastern margin of the Yokote basin are classified as frontal faults. On the other hand, Kawaguchi fault extending along the boundaries between Mahiru mountains and Konuma or Senya hills is classified as a boundary fault. Previous researches revealed that the boundary fault had been active during the early Pleistocene. However, the development history of the boundary fault is still unknown in detail. We conducted field investigations in and around the Konuma hills and found an expansive fault outcrop excavated with felling works around the boundary fault at the south side of the Irisumi-sawa. Pleistocene deposits, which are referred to as the Kurisawa formation, and Neogene volcaniclastic rocks are exposed at the outcrop. These sediments and rocks are highly deformed and cut by thrusts. This outcrop is important because there is little research reporting fault outcrop of the boundary fault in the Pleistocene around the eastern margin of the Yokote basin fault zone. Revealing the development history of the boundary fault during the early Pleistocene is expected. Detailed observations of the fault outcrop revealed some N-S thrusts, which aligned at 10-40m intervals, and deformed layers of Kurisawa formation. Lower layers were more highly deformed by thrusts than upper layers. This suggests that the activity of the boundary fault had been gradually reduced during the deposition of Kurisawa formation. The closest thrust to the mountain in the fault zone might have been formed first.
The valley in the upper reaches of the Abe River, Shizuoka Prefecture, was filled with an “Oya-Kuzure” slope failure deposit from the early 18th century Hoei Earthquake. The Akamizu Fall, which has maximum river-bed gradient in the distribution area of the slope failure deposit, is located on 0.4km downstream from the confluence of the Oya River originating from the “Oya-Kuzure” on the Abe River mainstream. Distribution of slope failure deposit (mainly debris flow deposit) and paleocurrent estimated from imbrication of gravels in the debris flow deposit around the Akamizu Fall showed that the waterfall was formed by a shortcut of the incised meander valley across a shale ridge due to deposition of slope failure deposit filling the valley. Similarly, constrained gorges located in the distribution area of slope failure deposit were formed by filling an old valley with slope failure deposit and subsequent downward erosion of basement rock (Neogene shale) slope. Sedimentological features of the slope failure deposit and the relationship of depositional surfaces of the deposit imply that transportation mode of the slope failure deposit might have transformed from debris flow to hyperconcentrated flow 0.5km downstream of the Akamizu Fall.