[S-SS14] Active faults and paleoseismology

[SSS14-P13] Graben structure of Nishiyama fault revealed by microtremor measurement and S-wave reflection survey in Katsuura, Fukutsu, Fukuoka prefecture

*Masayuki Yoshimi¹, Tadashi Maruyama¹, Yukari Miyashita¹, Takashi OGAMI¹, Ota Kosuke¹, Koji Yamada², Yosuke Kato² (1.National Institute of Advanced Industrial Science and Technology, Geological Survey of Japan, AIST, 2.Hunshin Consultants)

Keywords:Nishiyama fault zone, subsurface structure, microtremor survey, S-wave reflection survey

The Nishiyama fault zone is a 110-km-long active fault zone extending in a northwest-southeast direction from the south of Okinoshima Island in the Genkai Sea to near the Fukuoka-Oita prefectural border. Morphological features implies this fault to be a left lateral fault with uplift mainly southwest side. At the north-west tip of the Nishiyama section in the central part of the fault, in the Nishi-Higashi to Katsura-Ku, Katsuura area, multiple active fault maps show fault traces in almost the same position based on the left bending of river valleys and the displacement topography of fan terraces, showing the fault has a southwest dipping component. Miyashita et al. (2013) conducted array borings and trenching on the fault trace in this area and recognized a fault in the Cretaceous Kanmon Formation, but no significant drop of the rock surface was identified.
In this study, we conducted single-point microtremor observations in the Katsuura area to elucidate the basement structure of the area. We found that the peak frequency of the microtremor H/V spectral ratio was 4 Hz or higher on the northeast side and 1-2 Hz on the southwest side, indicating that the basement is deeper in the southwest side of the fault . The fault location is estimated at the end of the fan terrace, from the location of the abrupt change in the peak frequency, which is southwest to the published fault trace. The peak frequency is again increase to above 4 Hz at the far southwest, suggesting a depression zone near the fault.
We conducted a 300-m-long S-wave reflection survey including the depression zone. A Graben structure having dipping faults at the both ends was imaged within the depression zone. Borehole drilling within this depression identified a fault in the sedimentary layer. In this presentation, the details of this investigation and the relationship between the depression zone and the fault will be discussed.