Independent component analysis of geoelectric field data in the northern Nagano, Japan.

Simultaneous anomalous change of geoelectric field was observed on January 17, 1999 at three far-distant stations in Nagano Prefecture, Japan. Eleven days after the anomalous change, an earthquake swarm with a M4.8 main shock started at a location within the triangle formed by the three stations. This change was rectangular in shape like the VAN-type anomalous change in Greece. The anomaly could be detected because it occurred at midnight when the noise level was low. In fact, there were a few M4-class earthquakes in the area for which no preceding anomalous electric changes were observed, perhaps because the changes occurred during the noisy daytime. In this paper, we examine if it is possible to extract the signal with the size of the anomalous change on January 17, 1999 from the noisy daytime data by Independent Component Analysis. For this purpose, a simulated noise was embedded in the real daytime data which were highly influenced by not only artificial noise but also by geomagnetic disturbances and rainfall. Even when the influence of geomagnetic disturbance or rainfall was large, the anomalous change was clearly recognized at least at two stations in all cases.