Variations of the electric potential in the vicinity of the Nojima fault during the activity of aftershocks of the 1995 Hyogo-ken Nanbu earthquake

We made observations of the electric potential at three sites located near the Nojima Fault on Awaji Island, aiming at examining whether any anomalous changes appear in association with aftershocks of the 1995 Hyogo-ken Nanbu earthquake of M7.2. Unfortunately however, the observation area turned out to be highly contaminated by noise currents arising from the remote DC-operated railway systems. In order to avoid noise contamination, we attempted to apply the BAYTAP-G analysis method to the data obtained during the aftershock activity and found a gradual change which appeared several days before an aftershock of M4.8. We also show time changes in the preferential direction of noise currents, due to the inhomogeneous resistivity structure in the vicinity of the fault, and suggest that they are likely to reflect changes in the electrical property of the fault.