Abstract
Analyses of electric field observations are presented providing evidence of anomalous change occurrence during seismic swarms. We used mainly the special antenna using casing pipe of the deep borehole to sense the vertical electric field change. Anomalous signals are found in good correlation with the swarm activity in terms of duration and field strength on the bases of almost continuous observational record of more than six years. It was very easy to discriminate anomalous signals from normal state owing to sufficiently large SN of some 20. Anomalies were found in the whole frequency bands of dc, ULF and ELF/VLF bands with persistent occurrences in the dc and ULF bands. But anomalies in the ELF/VLF bands appeared occasionally at around the period of high seismic activities. The anomalies were detected only at the nearest site Hodaka from the swarm activity zone. A sharp spatial delectability division sensing particular seismic swarm activity at the site is attributed to the geological setting. Sources of the anomalous electric signals are suggested to be deep underground by comparing anomalous signals at different depth. The generation mechanism is indicated to be of hydrological origin through the elector-kinetic effects on the basis of whole available data around the site.
Published Version
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