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 |
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| Authors: | Shin-Ichi Uda Aiming Lin Keiji Takemura |
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| Institution: | Earthquake Research Institute, University of Tokyo, Bunkyo-ku, Tokyo 113-0032, Japan,;Faculty of Science, Kobe University, Nada-ku, Kobe 657-8501, Japan and;Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan |
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| Abstract: | Abstract Crack-filling clays and weathered cracks were observed in the Disaster Prevention Research Institute, Kyoto University (DPRI) 1800 m cores drilled from the Nojima Fault Zone, which was activated during the 1995 Hyogo-ken Nanbu earthquake (Kobe earthquake). The crack-filling clays consist mainly of unconsolidated 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 analyses show that the crack-filling clays are composed mainly of clay minerals and carbonates such as siderite and calcite. Given that the top of the borehole 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 clays 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. |
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| Keywords: | 1995 Hyogo-ken Nanbu earthquake carbonate clay mineral crack-filling clay fault drilling groundwater circulation Nojima Fault weathered crack |
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