Crack-filling clays and weathered cracks in the DPRI 1800 m core near the Nojima Fault, Japan: Evidence for deep surface-water circulation near an active fault

Crack-filling clays and weathered cracks were observed in the Disaster Prev ention Research Institute, Kyoto University (DPRI) 1800 m cores drilled fro m the Nojima Fault Zone, which was activated during the 1995 Hyogo-ken Nanb u earthquake (Kobe earthquake). The crack-filling clays consist mainly of u nconsolidated fine-grained materials that fill opening cracks with no shear textures. Most of the cracks observed in the DPRI 1800 m cores are yellow- brown to brown in color due to weathering. Powder X-ray diffraction analyse s show that the crack-filling clays are composed mainly of clay minerals an d carbonates such as siderite and calcite. Given that the top of the boreho le is approximately 45 m above sea level, most of the core is far below the stable groundwater table. Hence, it is suggested that the crack-filling cl ays and weathered cracks in the cores taken at depths of 1800 m were formed by the flow of surface water down to the deep fractured zone of the Nojima Fault Zone during seismic faulting.