HR: 13:55h
AN: S53C-02    [Abstracts]
TI: Comparison Of The Fault Detected By Active Fault Drilling And Fault Zone Trapped Wave
AU: * Ito, H
EM: hisao.itou@aist.go.jp
AB: After the 1995 Hyogoken-nanbu earthquake (M 7.5), we drilled a 747 m deep borehole at Nojima Hirabayashi penetrating the Nojima fault to elucidate the fine structure of the fault just after the large slip. The fault zone is characterized by altered and deformed granodiorite. The fault zone has low resistivity, low density, low velocities (both P and S wave; more than 50% decrease at the fault gouge), high porosity, and high Vp/Vs. The borehole observations, such as FMI logging, also revealed fine structure of the Nojima fault that corresponds to the changes in the degree of deformation within the fault zone. The width of the fault zone from the drilling result is about 50 m. From the fault zone trapped wave observed by borehole seismometers, the fault zone width is 20-110 m, and is consistent with direct observations by drilling. This shows a good correlation between in-situ physical properties measured in boreholes with remote geophysical observations.The shallow structure of the Mozumi-Sukenobu fault, central Japan, had been investigated by the array observation of trapped waves and geological surveys within the research gallery penetrating the fault. The modeling of fault zone head waves and trapped waves generated by a explosive source indicates that the fault zone can be modeled as a 200-m-wide low-velocity zone extending at least 2 km from the array with a 20 % decrease in the P-wave velocity relative to the host rock. The width of the fault zone estimated by the trapped waves is consistent with the estimate by geological surveys at the gallery, suggesting that the width of the fault zone maintains at least 2 km from the gallery.
DE: 7230 Seismicity and seismotectonics
DE: 8010 Fractures and faults
DE: 8123 Dynamics, seismotectonics
SC: Seismology [S]
MN: 2004 AGU Fall Meeting