Distribution of fault rocks in the fracture zone of the Nojima Fault at a depth of 1140 m: Observations from the Hirabayashi NIED drill core

Characteristics of deformation and alteration of the 1140 m deep fracture z one of the Nojima Fault are described based on mesoscopic (to the naked eye ) and microscopic (by both optical and scanning electron microscopes) obser vations of the Hirabayashi National Research Institute for Earth Science an d Disaster Prevention (NIED) drill core. Three types of fault rocks; that i s, fault breccia, fault gouge and cataclasite, appear in the central part o f the fault zone and two types of weakly deformed and/or altered rocks; tha t is, weakly deformed and altered granodiorite and altered granodiorite, ar e located in the outside of the central part of the fault zone (damaged zon e). Cataclasite appears occasionally in the damaged zone. Six distinct, thi n foliated fault gouge zones, which dip to the south-east, appear clearly i n the very central part of the fracture zone. Slickenlines plunging to the north-east are observed on the surface of the newest gouge. Based on the ob servations of XZ thin sections, these slickenlines and the newest gouge hav e the same kinematics as the 1995 Hyogo-ken Nanbu earthquake (Kobe earthqua ke), which was dextral-reverse slip. Scanning electron microscopy observati ons of the freeze-dried fault gouge show that a large amount of void space is maintained locally, which might play an important role as a path for flu id migration and the existence of either heterogeneity of pore fluid pressu re or strain localization.