In situ stress measurements in NIED boreholes in and around the fault zonenear the 1995 Hyogo-ken Nanbu earthquake, Japan

The 1995 Hyogo-ken Nanbu (Kobe) earthquake, M7.2, occurred along the north- east-south-west trending Rokko-Awaji Fault system. Three boreholes of 1001 m, 1313 m and 1838 m deep were drilled in the vicinity of the epicenter of the earthquake. Each borehole is located at characteristic sites in relatio n to active faults and the aftershock distribution. In particular, the Noji ma-Hirabayashi borehole [Hirabayashi National Research Institute for Earth Science and Disaster Prevention (NIED) drilling] in Awaji Island was drille d to a depth of 1838 m, approximately 320 m southeast from the surface rupt ure of the Nojima Fault, and it crosses fracture zones below a depth of 114 0 m. In situ stress measurements by the hydraulic fracturing method were co nducted in these boreholes within 1.5 years after the earthquake. Measureme nt results suggest the following: (i) Differential stress values are very s mall, approximately 10 MPa at a depth of 1000 m at each site; (ii) the orie ntation of maximum horizontal compression is almost the same in the borehol es, perpendicular to the surface trace of the faults, north-west-south-east ; (iii) fault types estimated from the state of stress differ among these s ites; and (iv) the differential stress value just beneath the fault fractur e zone decreases abruptly to one-half of that above the fault zone in the H irabayashi NIED drilling. These features support the idea that the shear st ress along the Rokko-Awaji Fault system decreased to a low level just after the earthquake.