Comparison of fault gouges in the aftershock area and the non aftershock area of the 2000 Tottori-ken Seibu earthquake

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We examined and compared mineralogical and geochemical properties of fault gouges in the aftershock and non-aftershock areas of the 2000 Tottori-ken Seibu earthquake, to establish a new method for evaluating the activity of low-activity faults. The gouges were examined using X-ray powder diffraction analysis, sequential selective extraction tests, and color measurements. Results show that the aftershock-area gouge is mainly composed of illite and chlorite, while the non-aftershock-area gouge is mainly composed of halloysite. Iron in the aftershock-area gouge is mainly contained in illite, while in the non-aftershock-area gouge, it is mainly in the form of amorphous and crystalline iron oxide. Results of color measurements (L∗a∗b∗ color space) show that differences in L∗ values represent differences in the presence/absence of illite and halloysite, negative a∗ values from the aftershock-area gouge represent the presence of chlorite, and positive a∗ values from the non-aftershock-area gouge represent the presence of crystalline iron oxide. These results indicate that mineralogical and geochemical characteristics can clearly distinguish fault gouges in aftershock and non-aftershock areas of the 2000 Tottori-ken Seibu earthquake, and that color measurements can be an effective and simple proxy method for the screening of gouges that require further analysis in the field.