Internal structure of the Nojima Fault zone from the Hirabayashi GSJ drill core

Abstract The internal structures of the Nojima Fault, south-west Japan, are examined from mesoscopic observations of continuous core samples from the Hirabayashi Geological Survey of Japan (GSJ) drilling. The drilling penetrated the central part of the Nojima Fault, which ruptured during the 1995 Kobe earthquake (Hyogo-ken Nanbu earthquake) ( M 7.2). It intersected a 0.3 m-thick layer of fault gouge, which is presumed to constitute the fault core (defined as a narrow zone of extremely concentrated deformation) of the Nojima Fault Zone. The rocks obtained from the Hirabayashi GSJ drilling were divided into five types based on the intensities of deformation and alteration: host rock, weakly deformed and altered granodiorite, fault breccia, cataclasite, and fault gouge. Weakly deformed and altered granodiorite is distributed widely in the fault zone. Fault breccia appears mostly just above the fault core. Cataclasite is distributed mainly in a narrow (≈1 m wide) zone in between the fault core and a smaller gouge zone encountered lower down from the drilling. Fault gouge in the fault core is divided into three types based on their color and textures. From their cross-cutting relationships and vein development, the lowest fault gouge in the fault core is judged to be newer than the other two. The fault zone characterized by the deformation and alteration is assumed to be deeper than 426.2 m and its net thickness is > 46.5 m. The fault rocks in the hanging wall (above the fault core) are deformed and altered more intensely than those in the footwall (below the fault core). Furthermore, the intensities of deformation and alteration increase progressively towards the fault core in the hanging wall, but not in the footwall. The difference in the fault rock distribution between the hanging wall and the footwall might be related to the offset of the Nojima Fault and/or the asymmetrical ground motion during earthquakes.