Some Considerations of the Recent Faulting in the Western Fringe of the Fukushima Basin

Abstract

The afore-mentioned observations and their explanations will be summarized as follows:1. Three faults can be recognized along the western fringe of the Fukushima Basin, and under all of them there are fault scarps displaced in and after the Diluvium.2. The Kori fault is a thrust one. At the period of the most recent displacement, narrow troughs caused by depression, or shallow hollows as a result of reverse dipping were formed under the fault bench.3. The Kari and Daiyama faults cross each other in the Iizaka. area. For this reason, each activity of these faults, that is, the up-thrust of the Kori fault block and the subsidence of the fallen side of the Daiyama fault made the area dip toward the basin with the result that the river terraces were formed there.4. The arrangement and forms of the new alluvial fans under the western mountain foot were decided by the modes of faulting. That is to say, under the northern half of the Daiyama fault scarp the fans developed to the south-east, and at the eastern foot of Azuma Volcanic Group, that is, under its southern half, -they shifted to the north. These phenomena were due to the mode of the subsidence of this basin. Although the decisive conclusion must be avoided because the concrete data have not yet been obtained, the geological structure is likely to give a powerful suggestion to the mode of the subsidence.5. At the eastern foot of Azuma Volcanic Group, the form and the materials of the fans did not necessarily depend upon only the displacement of the fault there. In this area they were more sensitive to the volcanic activity in the drainage area of each rivers constructing the fans.6. In the central part of the basin there was a large lake during the period from the higher surface to the middle (the old fan), and was deposited the thick hed of not only lake deposits but also the voclanic ash from the western volcanoes. For this reason ground-water is prevented from percolating down in this part of the basin, where drinking water is provided with by the artensian water contained in the basal gravel bed of the higher surface. The lake gradually decreased in size, and at the period when the new fans were formed, it was retained only in the Nakayachi area.