[S-SS14] Active faults and paleoseismology

[SSS14-P09] Shallow seismic reflection survey and gravity survey across the Oritsume Fault, Northeast Japan

*Yohei Tasaki¹, Shinsuke Okada², Ryo Honda³, Yoshihiro Hiramatsu⁴, Shin Koshiya⁵ (1.Graduate School of Arts and Sciences, Iwate University, 2.Faculty of Science and Engineering, Iwate University, 3.Mount Fuji Research Institute, Yamanashi Prefectural Govermment, 4.Faculty of Geosciences and Civil Engineering, Institute of Science and Engineering, Kanazawa University, 5.Research Center for Regional Disaster Management, Iwate University)

Keywords:Oritsume Fault, gravity survey, seismic reflection survey, reverse fault, tectonic inversion

The Oritsume Fault is distributed about 47 km long, NNW-SSE trending western dipping reverse fault, which is located from Kuraishi-mura, Sannnohe-gun Aomori Prefecture to Kuzumaki-cho, Iwate-gun, Iwate Prefecture (Figure 1). In the northern part of the Oritsume Fault, a flexure of Pliocene strata distributed (Tatsunokuchi Flexure), however the activity of the fault during late Quaternary is not so cleared (Headquarters for Earthquake Research Promotion, 2004). In addition, the Oritsume Fault was activated as a normal fault in Miocene, since Pliocene the fault reactivated as a reverse fault in the compressional stress regime (Regalla et al., 2017). In the central part of the Oritsume Fault (southern side of the Sarugoe pass), Okada et al. (2004) conducted a shallow seismic reflection survey in 2023. The result of seismic survey shows three western dipping faults (Figure 2) and their associated subsurface geological structures. In particular, the westernmost fault is considered as a reverse fault that was generated as a normal fault associated with Japan Sea opening in Miocene, and it was displaced significantly by the reverse fault since tectonic inversion occurred. The location of the easternmost fault is roughly corresponding to the estimated active fault interpreted by geomorphology. Then, the easternmost fault also has same tectonic evolution and it is still active in Late Quaternary.
In this study, the subsurface geological structure is not clear from the seismic reflection surveys due to noise and other factors, so we conducted gravity surveys along the seismic reflection survey line and its extensions to calculate the Bouguer gravity anomaly and to reveal the subsurface density structure.
The gravity survey was conducted along a shallow seismic reflection survey line (Okada et al., 2024) in Karumai-cho, Kunohe-gun, Iwate Prefecture. In addition to the seismic reflection line, we extended the survey line about 3 km to the west and about 1.5 km to the east. A total gravity station in this study were 47, standard interval of gravity stations was 50 m on the seismic reflection survey line and 500 m on its east and west extensions. A Scintrex CG-3M gravimeter was used in this survey. A loop-closing observation method was adopted in daily measurement for the correction of gravimeter drift. The positions of each gravity stations were obtained by using VRS-GNSS and total station survey.
For the gravity analysis, we applied tide and drift corrections for the obtained gravity reading values, then absolute gravity values of each stations were calculated. The absolute gravity value of the temporary reference station was determined by difference of gravity reading values between the Mt .Fuji Research Institute and the temporary reference station. Afterward, we calculated Bouguer gravity anomaly by using standard gravity reduction methods. Terrain correction was calculated according to the method of the GSJ gravity survey group (1989). The reduction density of 2.67 g/cm³ was applied for the gravity reductions.
The overall Bouguer gravity anomalies along the seismic line are gently decreasing toward the west, but negative gravity anomalies are observed in the central and western part of the seismic line. According to the result of the seismic survey and geological map (Tsujino et al., 2018), these bouguer anomalies correspond roughly to the section among three faults, where the Miocene Kadonosawa and Jumonji Formation were thickly deposited by normal fault movement associated with the Japan Sea opening and subsequent tectonic inversion. In this section, the upper surface of the Jurassic Kuzumaki Formation is deeper than both side of the section, which is consistent with changes in the Bouguer gravity anomaly.