[S-SS13] Active faults and paleoseismology

[SSS13-P07] Shallow seismic reflection survey across the northern part of the Itoigawa–Shizuoka tectonic line active fault system around Uminockchi, Taira, Omachi city, central Japan

*Shinsuke Okada¹, Tatsuya Ishiyama², Nobuhisa Matta³, Shin Koshiya¹, Katsuya Noda⁴, Nene Katayama¹, MA Bowen², Kenshiro Oikawa⁵, Tomo Tamura¹, Urara Ota⁶, Miri Shirokane⁶ (1.Faculty of Science and Engineering, Iwate University, 2.Earthquake Research Institute, University of Tokyo, 3.Okayama University Graduate School of Education, 4.GEOSYS,INC., 5.Graduate School of Arts and Sciences, Iwate University, 6.School of Education, Okayama University)

Keywords:Seismic reflection survey, Itoigawa–Shizuoka tectonic line active fault system, reverse fault, strike-slip fault

The northern extent of Itoigawa-Shizuoka tectonic line (ISTL) active fault system consists of the Kamishiro Fault and the Eastern Boundary Fault of Matsumoto Basin (section north of Akashina) with length about 50 km from north to south. The Northern ISTL is considered about east dipping reverse fault that distribute along the eastern boundary between basin and hills (Headquarters for Earthquake Research Promotion, 2015). However, in the section north of Lake Kizaki, active faults with left-lateral offsets of channels distributed behind (east side of) the frontal reverse fault have recently been pointed out based on topographic interpretation using aerial photo and DEMs, as well as trenching survey (e.g., Hirouchi et al., 2018). To reveal the subsurface structure and relationship between reverse fault and left-lateral fault, we carried out a high-resolution shallow seismic reflection survey around Uminokuchi, Taira, Omachi city (northern area of Lake Kizaki).
The seismic line starts from western side of the basin and extends approximately 1.3 km to the northeast across the JR Oito line, Route 148, the east-dipping reverse fault, and left-lateral fault. The source used in the seismic survey was IVI mini-viberator (T-15000). Sweep length was 16 sec and frequency range beginning at 10 Hz up to 120 Hz. The total number of recordings were 239. The receiver was GS11-D (natural frequency, 4.5 Hz, GEOSPACE Inc.). The source and receiver interval were both 5 m. We used fixed spread of 243-ch of receivers with GRS stand-alone recording system (GEOSPACE Inc.) with 1 msec sampling rate.
The seismic reflection data was processed using the standard common mid-point (CMP) method using seismic processing software SuperX-C (JGI Inc.). The CMP spacing was 2.5 m. The CMP line is across the east-dipping reverse fault and left-lateral strike-slip fault at CMP 152 and CMP 310, respectively.
In the seismic section, deeper extension of Pre-Miocene basement rocks distributed western part shows strong and continuous eastern dipping reflectors at depth. In the sallower domain from the middle to eastern part of this seismic image, eastern dipping continuous reflectors are observed. In the domain between upper side of Pre-Miocene basement rock and the deeper part of eastern dipping continuous reflectors, we can see the sub-horizontal reflectors which is no dislocation and deformation by active faults. In the eastern side of the left-lateral fault, highly continuous reflectors are distributed near the surface, but in the western side of this fault, discontinuity of reflectors can be seen along the deeper extension of the strike-slip fault.