Journal of geomagnetism and geoelectricity
Online ISSN : 2185-5765
Print ISSN : 0022-1392
ISSN-L : 0022-1392
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Volume 39, Issue 1
Displaying 1-4 of 4 articles from this issue
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  • Electromagnetic Induction in an Irregular Layer Overlying the Earth. The 3rd Paper
    A Conducting Layer Having a Plane Top and an Undulatory Bottom
    Naoto OHSHIMAN
    1987 Volume 39 Issue 1 Pages 1-18
    Published: 1987
    Released on J-STAGE: September 14, 2010
    DOIhttps://doi.org/10.5636/jgg.39.1
    JOURNAL FREE ACCESS
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    Electromagnetic induction by a uniform magnetic field in a conducting layer having a plane top and an undulatory bottom is studied. A source field parallel to the surface is assumed. The apparent resistivity (ρa) vs period of magnetic variation (T) relation is obtained at various points over the top surface by means of conventional magneto-telluric technique. The phase vs T relation is also obtained from the electric and magnetic fields studied.
    It turns out that the ρa-T relation and the phase-T relation are the same as those of the two-layer model developed by Cagniard provided a small amplitude of the undulatory bottom is assumed. Also these relations are independent of the observation point. When the amplitude of undulation is large, however, the ρa-T and the phase-T relations are modified in such a way that an apparently low-resistive zone lies at the prescribed depth of undulatory boundary between the two layers of the original model, the conclusion being on the three-layer MT analysis due to Cagniard.
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  • Solar Cycle Variations in the Annual Mean Values of the Geomagnetic Components of Observatory Data
    Takesi YUKUTAKE, Joseph C. CAIN
    1987 Volume 39 Issue 1 Pages 19-46
    Published: 1987
    Released on J-STAGE: September 14, 2010
    DOIhttps://doi.org/10.5636/jgg.39.19
    JOURNAL FREE ACCESS
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    Solar cycle variations derived from annual mean values of all day data at observatories contain the effects of magnetic variations such as magnetic storms and solar diurnal changes with periods shorter than one year. The effects of the shorter period variations are, however, usually assumed to be averaged out, and the annual means are widely used for the analysis of the long term variations in the geomagnetic field. The effects of such shorter period variations are examined by comparing three kinds of annual means: the all day mean, the calm day mean and the night-time mean.
    The difference between the all day annual mean and the calm day annual mean, which is supposed to be caused by an accumulated effect of magnetic disturbances, has been found to vary almost in parallel with the solar cycle variations derived from the all day annual mean itself, exhibiting two peaks during a solar cycle. About 40% of the solar cycle variations in the all day annual mean values is due to this effect. Similarly the difference between the calm day annual mean and the night-time annual mean also shows solar cycle variations, but with a single peak for a solar cycle. The variations in this difference are interpreted to be caused by amplitude variations in the solar diurnal change.
    The effect of magnetic disturbances and that of diurnal changes contaminate the all day annual mean in different ways. This makes it difficult to extract the solar cycle variations in the geomagnetic field undisturbed by shorter period variations.
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  • Magnetic and Electric Signals Precursory to Earthquakes
    An Analysis of Japanese Data
    Tsuneji RIKITAKE
    1987 Volume 39 Issue 1 Pages 47-61
    Published: 1987
    Released on J-STAGE: September 14, 2010
    DOIhttps://doi.org/10.5636/jgg.39.47
    JOURNAL FREE ACCESS
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    Magnetic and electric data of earthquake precursor amounting to 61 in number so far obtained in Japan are analyzed. The relations between main shock magnitude (M), precursor time (T) and epicentral distance (D) are studied for precursors of five subdisciplines g (geomagnetic field change), earth currents (e), resistivity measured by electrodes having a long distance interval (R), resistivity measured by a resistivity variometer (r) and electromagnetic radiation (w).
    Linear relations between log T and M are obtained for subdisciplines g, e and R. T for the latter two subdisciplines is smaller than that for subdiscipline g for a fixed M. Such a difference can be understood by taking stress build-up and dilatancy-diffusion processes in the earth's crust into consideration.
    M-log D relations suggest that a subdiscipline g precursor represents a premonitory crustal strain of the order of 10-7-10-6, while that for subdiscipline r a strain of 10-9-10-8. Precursors for other subdisciplines represent a strain of 10-8-10-7. It is remarkable that precursors are sometimes observed at a point of which the epicentral distance is several times as large as that of epicentral area.
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  • ERRATA
    1987 Volume 39 Issue 1 Pages 63
    Published: 1987
    Released on J-STAGE: September 14, 2010
    DOIhttps://doi.org/10.5636/jgg.39.1_63
    JOURNAL FREE ACCESS
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