Geological and geophysical studies of the Nojima Fault from drilling: An outline of the Nojima Fault Zone Probe

The Nojima Fault Zone Probe was designed to study the properties and recovery processes of the Nojima Fault, which moved during the Hyogo-ken Nanbu earthquake (M(JMA)7.2) of 1995. Three holes, 500 m, 800 m and 1800 m deep, were drilled into or near the fault zone by the Disaster Prevention Research Institute, Kyoto University. The 500 m and 800 m holes were drilled in November 1995, and in December 1996 the last hole reached its final depth of 1760 m. The significant results are: (i) Geological and geophysical reconstruction of the structure and evolution of the Nojima Fault was obtained; (ii) the maximum compression axis was found to be perpendicular to the fault, approximately 45 degrees to the regional compression stress axis; (iii) micro-earthquakes (m = -2 to +1) were induced by water injections 1-3 km from the injection points in the 1800 m hole; (iv) the fault zone was measured to be 30 m wide from microscopic studies of core samples. Instruments such as three-component seismometers, crustal deformation instruments, and thermometers were installed in the holes.