3. Geophysical logging
The fault zone is characterized by gradual decreases in electrical resistivity, density, P-, and S-wave velocities towards the coaxial zone (Fig. 3). The boundary of the fault zone is gradual, however, the coaxial zone is characterized by sharp decreases in resistivity, density, and P- wave velocity and almost no shear or Stoneley wave arrivals. Geophysical properties, including Gamma ray logging results, are asymmetric between the hanging wall and the footwall of the Nojima fault. This reflects the difference in the deformation degree and the deformation history during the earthquake cycle.
The permeability distribution estimated from Stoneley wave analysis shows that permeability is higher in the fault zone and the extremely high permeable zones are localized in several narrow zones (fault gouges and cataclasite zones) in the fault zone.
Figure 3. Logging results of resistivity, FMI and sonic waves. The vertical scale is the same as that of Fig. 4.