[S-SS14] Active faults and paleoseismology

[SSS14-17] High-resolution seismic reflection survey at northern part of Iyo-nada Sea, western Japan

*Takashi OGAMI¹, Tomoyuki SATO¹, Tadashi MARUYAMA¹, Masayuki YOSHIMI¹, Yukari MIYASHITA¹, Kosuke OTA¹, Masayoshi SATO², Yuki NAKAMURA², Moeko TAKAHATA², Takashi SATO², Nobukatsu TAKEDA² (1.National Institute of Advanced Industrial Science and Technology, 2.Sogo Gephysical Exploration Co., Ltd)

Keywords:Offshore active fault, High-resolution seismic reflection survey, Seto Inland Sea

We conducted offshore active fault surveys at Iyo-nada Sea (western part of the Seto Inland Sea), western Japan. The Seto Island Sea is the largest occluded sea in Japan, and is important for maritime traffic, fisheries, industries (including energy industry) and tourism. The southern margin of the sea is bounded by the Median Tectonic Line Fault Zone (MTL), which is the largest active fault zone in Japan (HERP, 2017). According to Tsukuda (1990, 1992), landscape of the Seto Island Sea is formed as a result of the “Setouchi Shear Zone”, which have developed along north side of the MTL. However, detailed study on the “Setouchi Shear Zone” is not conducted and some areas remain as “blank area” of offshore active fault surveys. In such a “blank area” of active fault surveys, offshore active faults are recently revealed by high-resolution seismic reflection surveys in Suo-nada and Iyo-nada Seas (Tanaka et al., 2010, 2011). To mitigate seismic hazards of large earthquakes which may be generated by the newly revealed offshore active faults, we need to understand magnitude and occurrence possibilities of them as soon as possible.
In 2024FY, we conducted high-resolution seismic reflection surveys on the northern part of the Iyo-nada Sea, to figure out exact position, extents and characteristics of the newly revealed offshore active faults reported by Tanaka et al. (2010, 2011). We choose mini-GI and Sparker as sources of our multi-channel seismic reflection surveys systems (MCSs). We used streamer cables of 24 ch with 6.25 m group interval and 16 ch with 2.5 m group interval, as receiver of mini-GI MCS and Sparker MCS, respectively. We monitored shallow sediments using a parametric SBP (formerly called as SES2000) during the survey of Sparker MCS.
We obtained time-migrated sections based on the records of MCS surveys. Well stratified sedimentary body and its basement is recognized, and the depth of the basement is 0.6-0.7 s at most. As shown by Tanaka et al. (2010, 2011), series of offshore active fault is recognized between the bay of the Ageno-sho which locates at the middle of southern coast of Yashiro Island (Suo-Oshima town, Yamaguchi prefecture) and off the coast of Kunisaki Peninsula (Kunisaki city, Ooita prefecture). We will report the result of our seismic surveys and discuss the extent and characteristics of the offshore active faults and further study planned in 2025FY.

Acknowledgements: The local governments, related organizations and fisheries in and around the Iyo-nada Sea helped us a lot during planning and conducting our field observation. We deeply appreciate all the people concerned with this study for their kind corporation.

Refference: HERP(2017) Long-term evaluation of Median Tectonic Line Active Fault Zone (2nd edition). Tsukuda (1990) Structural Geology. Tsukuda (1992) Memoirs of the Geological Society of Japan. Tanaka et al.(2010) Annual Meeting of JSEG. Tanaka et al. (2011) JpGU2011.