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Abnormal Variations of Earth−Current Potentials accompanied with the Earthquake, off the Cape of Sioya, May 23, 1938
Abnormal variations of earth-current potentials accompanied with the conspicuous earthquake May 23, 1938, are studied by the same method of ″Difference-Curves" as in previous papers ;that is, the difference of potential differences of two independent base lines of different length in the same direction, E-W., reduced to the equal base length, in which any variation of universal type such as recorded simultaneously on the magnetograph is eliminated. The same result showing the relation between the earth-current and earthquake is obtained by both the calculation from the two independent potential differences and a direct measurement with a special differential galvanometer (Fig. 1). We can find also some variations of earth-currents varying with the activity of aftershocks (Fig.2).
It is very interesting that a transient current of about 66 mV/km are observed five hours before the earthquake (Fig. 3-4). The direction of the current is directed towards the epicenter; strictly speaking, it makes an angle θ=37°.5 with the east-west direction, while the straight line joining the epicentre and the observatory makes 24°. Such a type and magnitude of variation as this transient current is observed only on days of near thunderstorm, while no thunder was heard near the observatory on that day. Then it seems likely that this variation may be related to the earthquake, considering our experiences on earth-currents hitherto published and some electric and luminous phenomenon reported on the occasions of some great earthquakes.
An electric model of this transient current is constructed; that is, a dipole contained in the east-west vertical plane passing through the epicentre lies approximately at the hypocentre, and its axis of l makes an angle θwith the earth's surface, having the currents of +I and -I on both ends (Fig. 6).
We find a very satisfactory coincidence of both calculated and observed values of θ, when θ=0, that is, the axis of the dipole is parallel to the earth's surface. The determined axial length l = 10 km., when θ=0. This is the same magnitude as the radius of the sphere obtained by some seismologists, on which the destructive force acts at the hypocentre. The value of I is also determined as 5.4x105 Amps , when the specific resistance of the homogeneous and isotropic earth's crust taken as 10４ ohms/cm3. The order of magnitude of the potential difference between both ends of the dipole is estimated larger than 10４ volts which may explain some curious luminous phenomenon reported sometimes before the destructive earthquake. From the facts obtained above, we may say that the earthquake and accompanied transient earth-current are of two different aspects of the same crustal energy transformation and the latter may be called an "Unseen-Earthquake".
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