Internal structure of the Nojima Fault zone from the Hirabayashi GSJ drillcore

T. Ohtani et al., Internal structure of the Nojima Fault zone from the Hirabayashi GSJ drillcore, ISL ARC, 10(3-4), 2001, pp. 392-400
Citations number
Categorie Soggetti
Earth Sciences
Journal title
ISSN journal
1038-4871 → ACNP
Year of publication
392 - 400
SICI code
The internal structures of the Nojima Fault, south-west Japan, are examined from mesoscopic observations of continuous core samples from the Hirabayas hi Geological Survey of Japan (GSJ) drilling. The drilling penetrated the c entral part of the Nojima Fault, which ruptured during the 1995 Kobe earthq uake (Hyogo-ken Nanbu earthquake) (M7.2). It intersected a 0.3 m-thick laye r of fault gouge, which is presumed to constitute the fault core (defined a s a narrow zone of extremely concentrated deformation) of the Nojima Fault Zone. The rocks obtained from the Hirabayashi GSJ drilling were divided int o five types based on the intensities of deformation and alteration: host r ock, weakly deformed and altered granodiorite, fault breccia, cataclasite, and fault gouge. Weakly deformed and altered granodiorite is distributed wi dely in the fault zone. Fault breccia appears mostly just above the fault c ore. Cataclasite is distributed mainly in a narrow (approximate to1 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 relation ships and vein development, the lowest fault gouge in the fault core is jud ged to be newer than the other two. The fault zone characterized by the def ormation and alteration is assumed to be deeper than 426.2 m and its net th ickness is > 46.5 m. The fault rocks in the hanging wall (above the fault c ore) are deformed and altered more intensely than those in the footwall (be low the fault core). Furthermore, the intensities of deformation and altera tion 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 No jima Fault and/or the asymmetrical ground motion during earthquakes.