Abstract
In order to investigate fault activity, a self-potential survey was made near the surface faults of the 1896 Rikuu earthquake (M=7.2) in the Tohoku district of Japan. These surface faults are reverse and dip east. The following characteristic changes of the self-potential were observed; (1) the self-potentials at the east side of the fault were about 20 to 50mV larger than those at the west side, (2) the positions of the step of the self-potential profiles across the fault deviate more east from the surface fault trace as the survey line goes down to the south. These anomalies are successfully explained by the electrokinetic processes due to the fluid flowing out from the fault using the observed geoelectric structure of the fault. It requires that the fluid pressure (above hydrostatic) at the fault is a few bars higher than that around it. The electrokinetic current, however, produces only 0.2 nT on the surface, whereas the observed geomagnetic anomaly amounts to over 100 nT above the fault. The large magnetic anomaly should have a different origin from that of the self-potential anomaly.
