Earthquakes and Tide Response of Geoelectric Potential Field at the Niijima Station

We adopted the spectrum analysis method and the BAYTAP-G (Bayesian Tidal Analysis Program-Grouping Model) method to analyze the geoelectric potential data at the Niijima station during 1998-2000. After the quantitative analysis of the tide response of geoelectric potential changes at the above station, we obtained some characteristics of the tide response. Besides the tidal components with various periods such as diurnal tide, semi-diurnal tide, and so on, semi-monthly and semi-yearly variations of tide response were also detected from the geoelectric potential data at the Niijima station. Further quantitative analysis showed that some anomalous tide response for the surveying dipoles associated with the Wakago site occurred prior to the earthquake swarm in the summer of 2000, while there was no clear anomalous tide response for another surveying dipole at the same station. The above result may become a kind of new evidence of the so-called “selectivity” phenomena of seismic electromagnetic signals.