Measurement of Seismic Wave Using Vertical Earth Potential Difference Method and Its Wavelet Analysis

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To observe the earth potential difference (EPD) signals induced by seismic waves a modified method, measured in the vertical direction underground, has been operated continuously at Aobayama in Sendai. This method is succsesfully used to reduce the unnecessary signals induced by artificial and natural earth current sources. On all earthquakes of more than I Japanese intensity scale we can clearly observe EPD signals. The P-wave arraival times agree quiet well with ones obtained by meteorological agency seismograph. Though less than 1 scale in Sendai the EPD signals of the Southen Hyougo Prefecture earthquake (the Kobe eartquake) have been clearly recorded in this method.The EPD signals are observed at three vertical different positions underground in the Aobayama site. We have following characteristics of the earth electric field underground induced by seismic waves, (1)the electrical field in the vertical direction is higher than in the horizontal direction by one order, (2)this field increases repidly coming up to the ground surface.We have obtained the seismic waveform records from the Earthquake Obsrervatory of Tohoku University which is located apart 700m from the Aobayama site. Spectrum analysis has been performed for both wave records. Of accelaration, velocity and displacement of the earth, the EPD signal seems to be proportion to the earth accelaration signal. We estimate the ratio of electric field to pressure drop in the earth. This ratio is reasonable on the assumpution of the streaming potential which is induced by the earth accelaration.