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 (MJMA7.2) 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° 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.