HR: 1330h
AN: T22B-0521    [PDF]
TI: Characteristics of logging data for fracture zones in Hirabayashi NIED borehole drilling through Nojima fault
AU: * Omura, K
EM: omura@bosai.go.jp
AF: National Research Institute for Earth Science and Disater Prevention(NIED), 3-1, Tennodai, Tsukuba, 305-0006 Japan
AU: Ikeda, R
EM: ikeryu@ep.sci.hokudai.ac.jp
AF: Hokkaido Univ., Nishi8, Kita10, Kita-ku, Sapporo, 060-0810 Japan
AU: Iio, Y
EM: iio@rcep.dpri.kyoto-u.ac.jp
AF: DPRI, Kyoto Univ., Gokasho, Uji, 611-0011 Japan
AU: Arai, T
EM: t.arai@aist.go.jp
AF: AIST, Central7, 1-1-1, Higashi, Tsukuba, 305-8567 Japan
AU: Kobayashi, K
EM: kenkoba@gs.niigata-u.ac.jp
AF: Niigata Univ., 8050, Ninomachi, Igarashi, Niigata, 950-2181
AU: Shimada, K
EM: shimada@eps.s.u-tokyo.ac.jp
AF: Tokyo Univ., 7-3-1, Hongo, Bunkyo, 113-0032 Japan
AU: Tanaka, H
EM: tanaka@eps.s.u-tokyo.ac.jp
AF: Tokyo Univ., 7-3-1, Hongo, Bunkyo, 113-0032 Japan
AU: Hirano, S
EM: hiranos@jamstec.go.jp
AF: JAMSTEC, 2-15, Natsushima-cho, Yokosuka, 237-0061 Japan
AU: Matsuda, T
EM: mtatsuo@bosai.go.jp
AF: National Research Institute for Earth Science and Disater Prevention(NIED), 3-1, Tennodai, Tsukuba, 305-0006 Japan
AB: The Hyogo-ken Nanbu earthquake (1995.1, MJMA=7.2) activated the Nojima fault in the northern part of Awaji Island, southwest Japan and a surface rupture appeared more than 10km long. After the earthquake, the Hirabayashi NIED borehole was drilled penetrating through the fault zone to a depth of 1838m from a point about 302m SE from the surface trace of the Nojima fault. In the borehole, physical well logging was done from 251m depth down to the bottom. At the same time, cores were collected from 1000m depth with an almost 100% recovery rate and remarkable fractured zones containing cataclastic rocks were confirmed at three depths, around 1140m, 1300m and 1800m. Gamma ray logging survey indicates that the intensity of natural gamma ray changes abruptly within the interval of intrusive rocks. Matching values of depths at the boundaries between intrusive rocks and host rocks, the calibration equation was obtained between "core depth" and "logging depth"; "core depth" is measured by adding up lengths of drilling pipes and "logging depth" is measured by the total length of the cable taken down a logging tool. Results of well logging show that, in the depth interval of host rocks, normal resistivity is from several hundreds to several thousands ohm m, micro resistivity is several tens ohm m, P wave velocity is 5 - 6km/sec, density is about 2.6g/cm3 and neutron porosity is several %. On the other hand, in the depth interval of the fracture zone, those properties decrease down to several tens ohm m, several ohm m, 2 - 4km/sec, 1.5 - 2.0g/cm3 and increase up to several tens %, respectively. Investigating correlations between physical properties measured by well logging in the Hirabayashi NIED borehole, three fracture zones are characterized. In fracture zones, neutron porosity is beyond 10%, P wave velocity is less than 5 km/sec. The decreasing rate of density is higher and the logarithm of normal resistivity increases more gently with the increase of neutron porosity than in the depth interval of host rocks. The correlation between neutron porosity and P wave velocity is not clear and normal resistivity does not obey the Archie's relation in the fracture zones. The fracture zone at a depth of 1800m indicates some different characters from other two fracture zones; P wave velocity, density and neutron porosity appear to change gently contrasting to a remarkable decrease of normal resistivity.
DE: 8010 Fractures and faults
DE: 8025 Mesoscopic fabrics
DE: 8164 Stresses--crust and lithosphere
SC: Tectonophysics [T]
MN: 2003 Fall Meeting