Electric and magnetic phenomena observed before the volcano-seismic activity in 2000 in the Izu Island Region, Japan

摘要

Significant anomalous changes in the ultra low frequency range (≈0.01 Hz) were observed in both geoelectric and geomagnetic fields before the major volcano-seismic activity in the Izu Island region, Japan. The spectral intensity of the geoelectric potential difference between some electrodes on Niijima Island and the third principal component of geomagnetic field variations at an array network in Izu Peninsula started to increase from a few months before the onset of the volcano-seismic activity, culminating im- mediately before nearby magnitude 6 class earthquakes. Appear- ance of similar changes in two different measurements conducted at two far apart sites seems to provide information supporting the reality of preseismic electromagnetic signals.