Journal of geomagnetism and geoelectricity

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Dynamo action of the turbulence which is highly anisotropic under the influence of Coriolis and Lorentz forces, is investigated. The turbulence is taken to be driven by a random body force which is nearly homogeneous, isotropic and mirror-symmetric. Both the rotation and the magnetic field cause the intensity of the turbulent motion parallel to the rotational axis (or the line of force) to become larger than that of the perpendicular components by a factor √2.
The mean electromotive force of the anisotropic turbulence is evaluated separately to isolate the effects of the rotation and the magnetic field. The locally homogeneous turbulence creates two kinds (α-, and β-type) of electromotive forces under the influence of the rotation. The α-type includes a term proportional to the mean field (α-effect), and the β-type a term proportional to Ω×J (Ω is the rotation rate and J the mean current density). It is found that in the highly rotating systems both α-effect and the Ω×J term are effective for the dynamo action. The magnetic field results in only the effect of producing the magnetic helicity which can be retained in derivation of the α-effect for the isotropic turbulence.

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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.

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