Physics
Scientific paper
Apr 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993pepi...77...33t&link_type=abstract
Physics of the Earth and Planetary Interiors, Volume 77, Issue 1-2, p. 33-38.
Physics
Scientific paper
Electrodes consisting of a steel pipe in a deep borehole and a grounded wire surrounding the borehole were constructed to measure vertical electric fields, whereas conventional electrodes measure horizontal fields. Three years of monitoring showed that the anomalous variations in vertical underground electric fields preceding earthquakes and volcanic eruptions are much clearer than the variations in horizontal fields. The data also showed that there is much less man-made noise in the vertical fields. To determine whether these observed anomalies are forerunners of seismic disturbance, a system developed to locate precisely the source regions of underground vertical electric fields or volcanic tremors has been continuously operated. The system uses three or more time lags calculated by cross-correlating the electric fields or volcanic tremors recorded at four, or more, monitoring stations. If this system reveals the intensity distributions of the sources, prediction of imminent earthquakes and volcanic eruptions will become possible. To explain the mechanisms by which precursory electric field variations are generated, a model is proposed in which the electric field variations are generated by Earth current variations caused by increased electric conductivity in source regions before an earthquake. The conductivity is increased by free electrons and positive holes created on the fresh fracture surfaces caused by microcracks that occur before rock failure in the Earth's crust. The model can explain precursory electrical phenomena which are observed not only in the field before an earthquake but also in the laboratory before rock failure.
Fujinawa Yukio
Takahashi Kozo
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