In-situ stress at a site close proximity to the Gofukuji Fault, central Japan, measured using drilling cores

The stress state near the Gofukuji Fault, central Japan, was estimated by simultaneously applying deformation rate analysis (DRA), the acoustic emission (AE) method, and AE rate analysis (AERA) to drilling core samples recovered from depths of 327 and 333 m at a site close to the fault. The obtained stress state was for a strike‐slip fault. It is predicted from the estimated stresses that a tensile stress of 6.4 ± 1.1 MPa acted on the borehole wall at 333 m depth during drilling. This is comparable to the typical tensile strength of granodiorite. The tensile stress estimated at 327 m depth is less than the tensile strength. This is consistent with observations of drilling‐induced tensile fracture (DITF) at depths between 329 and 334 m. Ratios of the shear stress to normal stress (normalized shear stress) acting on the fault are calculated to be 0.4–1.0, which are comparable to friction coefficients of rocks measured in laboratories. The large normalized shear stress may represent strength recovery of the Gofukuji Fault. The impendency of earthquake occurrence on this fault is evaluated to be high from the recurrence interval of earthquakes on the fault.