Fracture analysis in the Nojima fault from FMI images
Tsutomu KIGUCHI, Hisao ITO, Yasuto KUWAHARA and Tomoyuki OHTANI
Geological Survey of Japan,
Higashi 1-1-3, Tsukuba, Ibaraki, 305-8567, Japan,
E-mail: kig@gsj.go.jp
Abstract
The borehole was drilled by the Geological Survey of Japan to penetrate the
Nojima fault. The coring , borehole logging and hydrophone VSP experiment
were carried out. The fault zone is characterized by altered and deformed
rocks from 426.1 m to 746 m, with coaxial zone of fault gouge at 623.3 m to
625.1 m. We detected a few thousands of fractures from FMI images. The
trend of the fracture strike at the depth range where the conventional
logging data showed extremely anomaly in the fault zone roughly parallel to
the surface strike of the Nojima fault, although the fracture strike of all
fractures inside of the fault zone is widely dispersed. Seismic anomaly
outside of the fault zone estimated from the DSI data is considered to be
caused by the aligned fracture with NE - SW trend. It is noteworthy that
almost all the resistive fractures are distributed in the shallower depth
than the coaxial zone. The strike and dip of some fractures at coaxial zone
are consistent with the surface strike of the Nojima fault and the dip
estimated from the depth of coaxial zone. The fractures at coaxial zone are
considered to be permeable from the results of hydrophone VSP analysis.
raeg99@tansa.kumst.kyoto-u.ac.jp
Last modified: Wed Nov 24 09:38:29 1999