Many fluvial terraces formed by Shinano River can be seen in the Tokamachi Basin, which is located between Uonuma and Higashikubiki Hills. Previous researchers reported that some thrusts along the eastern and western margin of the basin cut and deformed those fluvial terraces, and have surveyed their distribution, activities, and structures. Earthquake Research Committee named these faults as western and eastern Tokamachi Fault Zone (TFZ) and revealed each last event based on the results of previous papers and contract works. However, the relationships with other faults around TFZ and their recurrence interval are still debatable. We found an anticline (hereinafter referred as “Hosoo-Nyoraiji anticline”) on the terrace surface by detailed geomorphological analyses around the eastern TFZ, as a part of the evaluation survey of active faults in 2015 conducted by Geological Survey of Japan. Hosoo-Nyoraiji anticline (HNA), which is likely to be symmetric fold, has about 1.5 km wavelength and uplifted the top terrace classified as Maibara group (formed in 140-300 ka) about 20-25 m high. Near Nakazaike village, some steps below of the top terrace classified as Hoonokizaka or Kaisaka (55 ka) groups have been uplifted progressively. In addition, geological survey of the Uonuma Formation covered with the terrace deposits revealed that the upper layer has deformed in line with the deformation of HNA. These results suggest that the anticline had deformed in a time period when the upper Uonuma Formation deposited. Wavelength of the anticline yields that the thickness of deformed layer is about 0.3 km, suggesting that HNA was formed by the slip on the detachment fault inside the Uonuma Formation. Yokokura et al. (Chishitsu News, 2008) also suggested that there is a detachment fault under HNA, and it is connected to the eastern TFZ. This suggests that the detachment fault was formed by horizontal compression along the NW-SE direction, possibly as a secondary fault.
To reveal the eastern extension of the Futagawa fault zone and its paleoseismicity, we excavated a trench across the surface rupture of the 2016 Kumamoto earthquake extended to the Aso caldera. The six-meter-deep trench at Kurokawa, the town of Minami-Aso exposed young sediments mainly composed of multiple massive loam units, volcanic sand layers, fall-out tephra units, and organic rich fine sediments that are evidently warped and cut by a group of numerous faults. The Kikai-Akahoya tephra layer of 7.3 ka exposed near the bottom of the trench, shows up to two-meter vertical offset, which suggests cumulative slip due to the multiple paleoseismic events. As a result of our detailed observation and radiocarbon dating, we interpret four paleoseismic events, the 2016 Kumamoto earthquake, the penultimate event occurred any time between 2,132 and 1,900 cal BP, antepenultimate event of 4,237-4.1 cal ka BP, and the oldest one of 7,300-4,090 cal BP. The average inter-event time is 1,400-2,400 years. Even though we eliminated the antepenultimate event as an uncertain event, the inter-event time would be 2,000-3,700 years. These estimates would be consistent with the ones estimated from other post-Kumamoto earthquake studies along the Futagawa fault zone in and out of the Aso caldera. It suggests that the similar type of the earthquakes of the 2016 Kumamoto earthquake might have repeatedly occurred during the Holocene period.
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.
Re-examination of paleo-tsunami deposit ages was conducted to contribute establishment of paleo-tsunami scenarios and risk assessment along the Pacific coast of Tohoku region. Fifteen new radiocarbon dates were obtained from handy-Geoslicer samples drilled at Onuma, Minami-Sanriku Town Miyagi Prefecture. These new dates consistent to stratigraphy and tephras enabled us to establish an age model and to estimate ages of paleo-tsunami deposits. Ages of six paleo-tsunami deposits were estimated: S1 (680-1010 cal BP), G1 (1540-2350 cal BP), G2 (2730-3630 cal BP), G3 (3850-4080 cal BP), S2 (4450-4780 cal BP), and S3 (5540-5730 cal BP). Moreover, there was no paleo-tsunami deposit correlated with the AD 869 Jogan tsunami, indicating the tsunami behavior differently to the 2011 Tohoku-oki tsunami. In the future, a tsunami deposit survey should be conducted on the southern part of the Sanriku Coast.