[S-SS11] Active faults and paleoseismology

[SSS11-09] Offshore active fault survey of the Ogori fault, off the coast of Ube city, Yamaguchi prefecture, western Japan

*Takashi OGAMI¹, Tomoyuki SATO¹, Kosuke OTA¹, Karin NEMOTO^1,2, Sabrina LLOYD^1,2, Yuka YOKOYAMA³, Seishiro FURUYAMA⁴, Nobukatsu TAKEDA⁵, Kyohei TAKAHASHI⁵, Takashi SATO⁵, Takashi HOSOYA⁶, Chiharu NAKASE⁶, Tadashi MARUYAMA¹, Yukari MIYASHITA¹, Osamu FUJIWARA¹ (1.Geological Survey of Japan, AIST, 2.Atmosphere and Ocean Research Institute, the University of Tokyo, 3.Tokai Univeristy, 4.Tokyo Univerisity of Marine Science and Technology, 5.Sogo Geophysical Exploration Co., Ltd, 6.Chuo Kaihatsu Corporation)

Keywords:Ogori fault, Offshore active fault survey, Suo nada sea, Seismic reflection survey, Offshore drilling survey

We conducted an offshore active fault survey of the Ogori fault, which extend 31 km long from Yamaguchi city to off the coast of Ube city, Yamaguchi prefecture (HERP, 2016). At the off the coast of Ube city, three active faults (Kikugawa fault zone, South-off Ube fault and Ogori fault) are located closely to each other. To evaluate and mitigate seismic hazard from these faults, information of these faults including precise distribution, past activities, and linkage of these fault is important. However, there are few studies of Quaternary geology of this area, and seismic survey with enough specifications (resolution and density) to elucidate such complex geological structure is not conducted yet. Past activity of the Ogori fault is investigated at onshore by trenching and drilling surveys (Komatsubara et al., 2005; Aiyama and Kanaori, 2011). According to the results, the latest activity of the fault occurred during the range of 13,300 cal BP to 17th century. To advance evaluation of earthquake related hazard in the future, the time window of the latest event should be narrowed down to upgrade the accuracy of earthquake occurrence probability and understand linkage with other active faults.
We conducted multi-channel seismic survey with boomer and mini GI-gun source at Suo nada sea in 2022, and found a flexure scarp at the offshore extension of the Ogori fault (Ogami et al., 2023). Cumulative deformation is observed, and the Holocene is also deformed by the fault. To elucidate development process of the flexure scarp, we also conducted SBP survey using SES2000 as a sound source. In 2023, we obtained 45 m long sedimentary core by offshore drilling survey to investigate sedimentary age and environment around the flexure scarp. We report preliminary result of past activity of the Ogori fault based on the offshore active fault survey including seismic reflection surveys and offshore drilling.

References
Aiyama and Kanaori (2011) Journal of the Japan Society of Engineering Geology.
HERP (2016) Longterm evaluation of Oharako fault and Ogori fault.
Goto et al. (2021) 1:25,000 Active Fault Map “Ube east”.
Kagohara et al. (2021) 1:25,000 Active Fault Map “Yamaguchi”.
Kanaori et al. (2006) Journal of the Japan Society of Engineering Geology.
Komatsubara et al. (2005) Annual Report on Active Fault and Paleoearthquake Researchs.
Mizuno et al. (2003) Annual Report on Active Fault and Paleoearthquake Researchs.
Ogami et al. (2023) JpGU2023.