[S-SS14] Active faults and paleoseismology

[SSS14-P12] Drilling surveys across the northernmost part of onshore section of the Nishiyama fault zone, Katsuura, Fukutsu city, Fukuoka Prefecture, Japan

*Kosuke Ota¹, Takashi Ogami¹, Karin Nemoto^1,2, Kaoru Tabata^1,2, Masayuki Yoshimi¹, Tadashi Maruyama¹, Yukari Miyashita¹ (1.Research Institute of Earthquake and Volcano Geology, National Institute of Advanced Industrial Science and Technology, 2.Atmosphere and Ocean Research Institute, The University of Tokyo)

Keywords:Nishiyama fault zone, boring core, active fault

The Nishiyama fault zone, a NW-SE-trending left-lateral strike-slip fault extending for about 110 km, is the one of the longest active faults in the northern Kyushu, western Japan. Although its proximity to the densely populated Fukuoka City, recent faulting history is poorly understood, which hampers seismic hazard evaluation of the Fukuoka region and surrounding properly. To elucidate precise location, subsurface geometry and recent activity of the Nishiyama fault zone, we are carrying out an integrative study including high-resolution seismic survey, dense gravity survey and arrayed coring survey. Our study focuses on the Katsuura area in Fukutsu City at the northernmost of the onshore part of fault zone near the center of Fukuoka region. We drilled boreholes at three sites with closely spacing across the faults as inferred from interpretation of seismic reflection profiling and results of microtremor array survey. Three cores in depth ranging from 12.0 m to 43.0 m are successfully retrieved. Borehole core samples were analyzed using an X-ray CT scanner, split in half and photographed, lithology was described, and magnetic susceptibility were measured. The core is composed of unconsolidated sediments estimated to be Holocene and upper Pleistocene. Steeply dipping faults with an inclination of 70~85 degrees that clearly cut the core were also identified at the lower part of the core. The detailed descriptions and analytical results will be presented.

We appreciate landowners and cultivators of the drilling survey sites who kindly allowed us to conduct the survey. We thank the staff members of the Chuo Kaihatsu Corporation who took charge of the drilling. This study has been conducted as part of a project 'Development of High-Resolution Digital Geological Information for Disaster Prevention and Mitigation (FY2024)'.