Arrayed-boring survey near the eastern end of the Kawakami fault of the Median Tectonic Line Active Fault Zone in central Shikoku, southwest Japan

Four cores (Br. A-D, from north to south) were taken across the estimated trace of the Kawakami Fault at Yokoguro, Saijo City, in order to obtain geological evidence regarding the nature of the geomorphologically defined fault trace. The Kawakami Fault is part of the Median Tectonic Line Active Fault Zone, which is a 400-km-long right-lateral strike-slip fault. The Kawakami Fault is ~30 km long and strikes N70°E. Sedimentary textures in the cores were analyzed in terms of sedimentary structure, color, volcanic ash content, magnetic susceptibility, and micro fossil content. Four units (Units 1-4) are recognized in Br. A, B, and C, whereas Unit 4 is not encountered in Br. D, which was obtained from the downthrown side of the fault. The K-Ah horizon (7300 yrs BP) in Br. C is ~2.24 m shallower than in Br. D. The boundary between Unit 3 and Unit 4, which is located below the K-Ah horizon, occurs > 6.0 m higher in Br. C than in Br. D. The differences in depth between corresponding layers in Br. C and Br. D increase progressively with depth. In addition, each unit in Br. C is thinner than in Br. D. These observations, combined with groundwater level information, indicate that the Kawakami Fault is located between Br. C and Br. D, and that the fault has moved repeatedly with a down-to-the-south sense of displacement. Depth differences between corresponding layers in Br. B and Br. C show a similar trend to the differences between Br. C and Br. D, suggesting that a flexure structure exists between Br. B and Br. D. Using the calculated fault displacements (~2.24 m between Br. C and Br. D, and ~4.03 m between Br. B and Br. D) that have occurred since ~7300 yrs BP (as calculated using the K-Ah horizon), the vertical slip-rate of the Kawakami Fault at Yokoguru is ~0.3 mm/yr, or ~0.55 mm/yr in the case that a flexure structure exists between Br. B and Br. D.