2021 Volume 62 Issue 1 Pages 13-22
When considering the mechanism of earthquake occurrence, the stress state in the vicinity of the source fault is essential. In this study, we applied anelastic strain recovery method（ASR method）to a total of 20 rock core samples retrieved from a borehole penetrating the Futagawa fault, one of the source faults of the 2016 Kumamoto earthquakes and succeeded in determining the in-situ stress state using 6 core samples. The results reveal that the normal faulting stress regime dominates in the source area, which says that the vertical stress is the maximum principal stress（σ1）. This implicates the magnitude of the maximum horizontal stress（SHmax）, which is estimated to have been the maximum principal stress（σ1）before the earthquakes, has dropped significantly due to the strike-slip faulting during the earthquakes. The result that the stress state after the earthquakes is the normal faulting regime is consistent with the results obtained by hydraulic fracturing and focal mechanism conducted in the same area. We also reveal that the direction of the minimum horizontal stress（Shmin）is almost orthogonal to the strike of the Futagawa fault, which indicates that the horizontal shear stress that causes strike-slip faulting on the fault plane is very small.
