Abstract
We analyze the ULF magnetic field emissions observed at three different field sites. Large seismic events occurred near each station. Temporal dependence of the polarization ratio (ratio of horizontal to vertical magnetic field components) has been calculated in the frequency range between 1.17 mHz and 0.5 Hz with longer than 2-year data for each event. Two of three seismic events were associated with anomalous change in the polarization ratio. Remarkable similarities were found in the ratio between these two different seismic events: (1) The ratio has a significant increase about 1 month before the earthquake (about two times larger than usual level), (2) the increase is seen in the nighttime period (22–02 LT). The frequency range of increase in the ratio depends on the earthquake (0.01–0.022 Hz for the Izu Island earthquake swarms and 0.05–0.1 Hz for the Guam earthquake). Calculated ratio based on the simple theoretical model has a good agreement with observed values for these two earthquakes.
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