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

为了观测地震波诱发的地球电位差信号，在仙台青山进行了改进的地下垂直方向测量方法的连续试验。该方法成功地减少了人工地电流源和天然地电流源产生的不必要信号。在所有震级大于日本震级的地震中，我们都能清晰地观测到EPD信号。纵波到达时间与气象机构地震仪测得的时间吻合得很好。虽然仙台地震小于1级，但该方法已清晰地记录了南兵库县地震（神户地震）的EPD信号。在青山场址的地下三个垂直不同位置观测到环境保护署的信号。地震波在地下诱发的地电场具有以下特点：(1)垂直方向的电场比水平方向的电场高一个数量级；(2)上升到地表后地电场迅速增大。我们从东北大学地震观测站获得了地震波形记录，该观测站位于距青山遗址700米的地方。对两种波记录进行了谱分析。在地球的加速度、速度和位移中，EPD信号似乎与地球的加速度信号成正比。我们估计了地球上电场与压降的比值。在假定由地球加速度引起的流势的前提下，这个比值是合理的。

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.