Physical property distribution and fracture density of a ~700 m-deep borehole penetrated through the Futagawa fault ruptured during the 2016 Kumamoto earthquake mainshock

To reveal depth distribution of physical properties and fracture density in a seismic fault drilling borehole called FDB-1, wireline log data and core descriptions were analyzed. The borehole was penetrated through the Futagawa fault ruptured during the 2016 Kumamoto earthquake mainshock. Its location is in the active Aso volcanic region, SW Japan. The borehole drilling and data acquisition were conducted in 2017–2018, ~1.5 years after the earthquake, by a consignment project conducted by Kyoto University, Japan funded by the Nuclear Regulation Authority, Japan. The log data and the core descriptions used for fracture density analyses were originally summarized in a free-access report in Japanese by Kyoto University in 2018 (https://www.nsr.go.jp/data/000256426.pdf). The data sets consist of (i) log data including borehole diameter, temperature, gamma ray, spontaneous potential, resistivity, P-wave velocity obtained by downhole borehole logging; (ii) depth and dip angle categories of all fractures observed from drill core samples, (iii) fracture density for each dip angle category in each one-meter intervals from 302 m to 666 m; (iv) dip angle and downdip azimuth of fractures from borehole televiewer images. \n\nAll the data was obtained from the same depth interval from ~302 m to ~666 m in the vertical borehole FDB-1, where core samples were retrieved, and wireline downhole geophysical logs were conducted. The conventional log operations providing (i) log data were carried out by Geophysical Surveying Co., Ltd. (https://www.gsct.co.jp/). Digital log data were sampled at an interval of 10 cm. All fractures were macroscopically identified by core description, and (ii) number of fractures, depth and their dip angle categories were recorded. The categories were defined as L, M, and H indicating fracture dip angles; “L” means <30° from the horizontal plane; “M”: ≥30° and