Anomalous variation of the electric self-potential difference between carbon rod electrodes fixed in an underground geophysical observation tunnel took place preceding a considerably strong earthquake (M=5.6) which occurred at an epicentral distance of 3.2km. The precursor-like changes ranging 40 mY at maximum began about 40 days before and also 55 hours before the earthquake occurrence and the recovery began co-seismically coming back to the initial level after 13 hours. The geophysical observation tunnel is located in the fractured and electrically conductive zone of the Yamasaki active fault, a target field for the earthquake prediction study.It was elucidated from the results of observation of the electric self-potential differences with the components of various directions and electrodes spacing and from the measurements of the self-potential across the fault and in the tunnel that the observed anomalous variation does not imply the variation of the earth-current observed due to the voltage drop through the ohmic resistance between the electrodes, but implies the decrease of the self-potential of the electrode, fixed in the fractured zone of the fault, itself, to that of another which served as a reference.As for the cause of the decrease of the self-potential, there may be two possibilities: (1) it was due to the time variation of the contact electric potential difference between the electrode and the surroundings at each electrode, (2) it was caused by the time variation of the self-potential of the fractured rock or of the water in the cracks of the fault where the electrode is fixed.As the electrodes used are carbon rod electrodes, non-equilibrium balanced type electrodes, the first possibility may be plausible. However, as all electrodes had already aged and become stable enough at that time and also considering the other observational results, the second possibility is likely to be more probable.

Geomagnetic and telluric current observations were conducted on and around the seismically active fault, Yoshioka-Shikano Fault, from 1976 to 1977 The confined plane of the geomagnetic variation expressed as 4Z=A4X+Bd fir, was calculated for each station by applying the transfer-function techniques or the least-square method to the records of Pi 1-2 type pulsations.It was found that the landward increasing tendency of the A values of the confined planes caused by the electromagnetic coastal effect of the Sea of Japan, was slightly interrupted on the northern edge of the fault and also, that the amplitude of the N-S component of geomagnetic variation (4X) was considerably enhanced upon the fault. These results are proposed to be due to the effect of the swelling of the electrically conducting medium in the crust beneath the seismically active fault.A tentative analysis was also made for the telluric current records observed on the fault, comparing it with the data obtained at the same site just after the Tottori Earthquake. As for the direction of polarization of the electric field there was no noticeable difference between them.

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