Clay mineral analysis of the Hirabayashi NIED drill core on the Nojima fault that ruptured in the 1995 Kobe Earthquake, southwest Japan

A 1800-m-deep borehole was drilled at Nojima Hirabayashi and penetrated the Nojima fault that was activated at the time of the 1995 Hyogo-ken Nanbu Earthquake (Kobe Earthquake) in Japan. Three possible fracture zones were detected at depths of about 1140 m, 1300 m, and 1800 m. At first, we analyzed the mode of distribution of rocks, minerals and chemical elements in them. There is a foliated blue-gray gouge at a depth of 1140 m. So we infer that this is the central fault plane, and began our fracture zone analysis there, as follows. The degree of fracturing is evidently greater in the hanging wall than in the footwall. We estimated the relative amounts of minerals qualitatively, and we detected not only quartz, orthoclase, plagioclase, biotite and hornblende in the parent rock (granodiorite), but also kaolinite, smectite, laumontite, stilbite, calcite, ankerite and siderite, which are related to hydrothermal alteration. Biotite notably disappears in both the hanging wall and footwall across the central fault plane, although it disappears over a significantly greater distance in the hanging wall than in the footwall. Equally, we estimated the amounts of major chemical elements quantitatively. Al2O3, Fe2O3, MnO, TiO2, and P2O5 all decrease throughout this interval, except at a few points. H2O_{ and CO2 increase throughout the interval. Na2O increases in the region adjacent to the central fault plane, while MgO and CaO increase in the hanging wall and decrease in the footwall. SiO2 and K2O decrease in the hanging wall and increase in the footwall. Next, we particularly investigated about the clay minerals such as smectite. From the drill core, we separated the clay-size fraction and analyzed it by X-Ray Diffractometer (XRD). Incidentally, particle-size separations are based on Stokes_fs law. We prepared oriented samples for XRD and to make it, we used the glass slide method. We measured it both in the air-dried and ethylene glycol-solvated conditions. We analyzed the other fracture zones along this fault in the same way. As a result, about the mode of distribution of rocks minerals and chemical elements, the fracture zone at 1140 m depth is very similar to the fracture zone at 1800 m depth and differs significantly from the fracture zone at 1300 m depth. But, the results of the clay minerals are different among each fracture zone. In air-dried condition, d (001) of smectite are 15 angstrom (at 1140 m depth), 14 angstrom (at 1300 m depth), 14~12 angstrom (at 1800 m depth), respectively (In ethylene glycol-solvated conditions, they are all 17 angstrom). It may suggest the existence of different fluid circulation between shallower and deeper fracture zones.