Distribution and properties of fractures in and around the Nojima Fault in the Hirabayashi GSJ borehole
H. Ito, T. Kiguchi, 2005. "Distribution and properties of fractures in and around the Nojima Fault in the Hirabayashi GSJ borehole", Petrophysical Properties of Crystalline Rocks, P. K. Harvey, T. S. Brewer, P. A. Pezard, V. A. Petrov
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A borehole penetrating the Nojima Fault was drilled in 1996, one year after the magnitude 7.2 1995 Kobe earthquake. The major fracture distribution detected by a borehole imaging tool (FMI) in the borehole was compared with the S-anisotropy from aftershocks. The fractures far from the Nojima Fault (more than 50 m) are aligned in an east—west direction, that is, parallel to the tectonic stress direction, whereas the fractures close to the Nojima Fault are normal to the fault. In the fault zone, a fault-parallel fault system was observed over short depth intervals, otherwise the fracture system in the fault zone is random. The fault-normal fracture suggests the complete reduction of the shear stress of the Nojima Fault.