著者

小野 有五 斉藤 海三郎

出版者

一般社団法人 日本活断層学会

雑誌

活断層研究 (ISSN:09181024)

巻号頁・発行日

vol.2019, no.51, pp.27-52, 2019-12-25 (Released:2020-01-27)

参考文献数

48

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Geomorphological and geological studies made clear the distribution and development of three major marine terrace surfaces, Iwanai Upland and Kyowa III surfaces (about 30m in eleveation), Kyowa II (about 40~50m ditto) and Kyowa I (about 60m ditto) in the Iwanai Plain, western Hokkaido. The Iwanai Upland surface only, covered by dune sand with Toya ash (115ka) at the top, gives the absolute age of the marine terrace formation, corresponding to MIS5e. A 100m-deep boring at the Iwanai Upland by Hokkaido Electric Power Company (HEPCO) shows two clear transgression-regression sequences: Sequence I, the uppermost 30m, corresponding to MIS5e, and II, between -33 and -65m, most possibly to MIS7. Base gravel beds, located at about -30m and -65m correspond to the river bed gravel deposited on the lower sea-levels, MIS6 and MIS8. This suggests a continuous tilting at the central part of the Iwanai Plain during the Middle Pleistocene. After MIS5e, this tilting changed to the uplift movement which elevated the Iwanai Upland surface. In the marginal area of the plain, the uplift movement continued to elevate the Kyowa II and I surfaces, also: the Kyowa II, covered by a thick dune sand with several paleosoils, is correlated to most probably MIS7, and the Kyowa I is characterized by a thick delta sequence in the Iwanai Plain, and a sand barrier or spit deposit on the site of Tomari Nuclear Power Station which sits at the base of Shakotan Peninsula. Both can be correlated by the top elevation of these sediments, higher than MIS7 terrace level, and also by their spatial distribution: barrier or spit at the bay mouth and the delta at the bay head. The Kyowa I can be correlated to MIS9, and a mean uplift rate seems to be 0.1~0.2mm/y after MIS9 to the present. Since the uppermost trace of F-1 fault attains to the sediments of MIS9 in the site of Tomari Nuclear Power Station, the fault movement occurred at least later than 330ka. Trace of dislocation by F-1 fault disappears gradually in the upper sediments which are covered by the slope deposits including the pumice grains of Spfa1 (42ka), Toya and one from Niseko volcano (around 220ka). As these pumice grains are largely moved by cryoturbation of MIS2, the coldest phase of the last glacial period in Hokkaido, it is evident that those slope deposits were finally settled around MIS2. This fact suggests the movement of F-1 later than 125ka cannot be deniable, which means the F-1 is regarded as the “active fault” according to the New Regulatory Requirements for Light-Water Nuclear Power Plants.