[S-SS14] Active faults and paleoseismology

[SSS14-03] A fault outcrop found in the Toikanbetsu fault zone, northern Hokkaido

*Tomonori TAMURA¹, Natsuko FUJITA¹, Ken-ichi YASUE² (1.Japan Atomic Energy Agency, 2.Toyama University)

Keywords:Toikanbestu Fault Zone, Fault outcrop

The earthquakes in northern Kamikawa area (Mj5.4 and Mj5.2), representative earthquakes in northern Hokkaido in recent years, were occured around the boundary between the Nakagawa town and Horonobe town in August 2022. In addition, 12 minor earthquakes have been also observed accompanied with the earthquakes in northern Kamikawa area ranging from Mj1.5 to Mj4.6 before and after only 1 weeks (Nakayama, 2023). In addition, it is familiar that the Toikanbetsu fault zone are distributed around near these epicenters of the earthquakes. The Toikanbetsu fault zone, mainly composed of the reverse faults forming fault scarps and flexure scarps, is crossing the Toikanbetsu area that are located in the eastern part of the Horonobe town Hokkaido (Imaizumi et al., 2018). In this area, Oka (1985) widely conducted topographical analysis and geological investigation and Ohtsu et al. (2007) estimated the latest activity of a part of the active fault. Oka (1985) identified 9 active faults tilting fluvial terraces and recognized active fault outcrop displacing fluvial terace deposits of the Eastern Toyoshin fault located in the northern part of the Toikanbetu area. Ohtsu et al. (2007) carried out the trench survey near the Pankerupeshupe river and recogized the fault clearly displacing fluvial teracce deposites. According to the radiocarbon dating, its latest activity is estimated to range from after 2400 ~ 2400 years ago to before 1800 years ago. On the other hand, most of the except the active faults mentioned above have not been recognized directly, and thus latest activity of most of scuh active faults are unknown. Therefore, clarifying the characteristics and activities of active faults whose activities are unknown is important for evaluation of earthquakes around the area. We newly found a fault outcrop near the Pankerupeshupe river fault shown in Oka (1985), whose peat and sand-gravel layers are deformed and displaced. This study will report the preliminaly results of geological descripition and radiocarbon dating and will discuss the activity of the fault.
In this fault outcrop, the Cretaceous sandstone sandwiching peat layer which are covered by sand-gravel layers. The fault gouge and breccia were confirmed between the sandstone and peat layer, accompanied with clasts and woods included in the peat layer that are dragged by shear movement. Because the R1 shear and P-foliation showing composite planar fabrics were recognized in the boundary fault, its sense of shear is estimated to be reverse fault with slight sinistral component. Shear planes with slickenlines were recognized in the peat layer and some of those displaces and deforms sand-gravel layers. The radiocarbon dating of 9 samples of humic soils and woods that were obtained from the peat layer were conducted. As a result, the ages of some samples indicated about 45,000 years ago as a latest one, while those of other samples showed the ages over the limit of radiocarbon dating. Thus, it is possible that the displacements and deformation in the fault outcrop are caused after around 50,000 years ago. However, since the the range of application of AMS is generally constrained from 60,000 ~ 50,000 years ago to present, the results are close to the limit of the radiocarbon dating. Although the treatment of the age is controversial, identification of woods have been being progressed to guarantee the results. We are planning to discuss the activity of the fault, including such analysis.