Electromagnetic field anomaly associated with the 1998 seismic swarm in Central Japan

Analyses of electric field observations are presented providing evidence of anomalous change occurrence during seismic swarms. We used mainly the spec ial antenna using casing pipe of the deep borehole to sense the vertical el ectric field change. Anomalous signals are found in good correlation with t he swarm activity in terms of duration and field strength on the bases of a lmost continuous observational record of more than six years. It was very e asy to discriminate anomalous signals from normal state owing to sufficient ly large S/N of some 20. Anomalies were found in the whole frequency bands of de, 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 t he period of high seismic activities. The anomalies were detected only at t he nearest site Hodaka from the swarm activity zone. A sharp spatial delect ability division sensing particular seismic swarm activity at the site is a ttributed to the geological setting. Sources of the anomalous electric sign als are suggested to be deep underground by comparing anomalous signals at different depth. The generation mechanism is indicated to be of hydrologica l origin through the elector-kinetic effects on the basis of whole availabl e data around the site. (C) 2000 Elsevier Science Ltd. All rights reserved.