Injection test results on fault stability using a 3-components borehole deformation sensor in the non-active segment extending south of Shionohira Fault, NE Japan

Aoki, Kazuhiro ; Tanaka, Yukumo; Yoshida, Takumi; Seshimo, Kazuyoshi ; Guglielmi, Y.*; Cook, P.*; Soom, F.*

A slip in the Shionohira Fault caused the April 11, 2011 Fukushima-ken Hamadori Earthquake of Mw 6.7, the biggest aftershock occurring a month after the Mw 9.0 Tohoku-Oki earthquake. Co-seismic surface ruptures trending NNW-SSE indicate a normal slip faulting of maximum 2 m displacement. However, an N-S trending Kuruma Fault with lineaments along the southern extension of Shionohira Fault showed no surface ruptures. The key question of importance in better assessing the risk of seismic fault activation in Japan is to understand why some fault segments were activated and ruptured the land surface while others remained inactive although closely aligned on the same fault zone and most likely affected by the same stress conditions. Following up on the injection experiment using a 3 components borehole deformation sensor (SIMFIP probe: Step-Rate Injection Method for Fracture In-situ Properties) in the active segment of Shionohira Fault (Aoki, et al., 2018), this study reports on the second injection test in Minakami-kita, the non-active segment of Kuruma Fault extending south of Shionohira Fault.Kuruma Fault runs North-South with a dip of 70 degrees to the west. A borehole of 30 meters from the surface was excavated to the east with 50 degrees inclination. Test interval of injection was decided based on core observation, borehole TV, and caliper logging data at 19.75-21.70 m (Test 1) and at 21.65-23.6 m (Test 2). Clear fault reactivations have been observed in test 1 and 2 located across the main shear zones. The results of tests 1 and 2 show considerable difference from earlier test results of the active segment (given in parentheses): fracture opening pressures were 11.0 and 10.2 bars (2.0 to 2.3 bar), hydraulic permeability at 3.010 and 2.310 m, and fault displacements at 0.20 and 0.12 mm (1.39 mm), respectively. Detailed activated structures and modes will be explained in the poster.