Anomalous radiation of environmental electromagnetic wave is reported as a portent of earthquakes. Then, we are observing environmental electromagnetic waves of the ELF band all over Japan. We have been regarding the large amplitude in the observed signal as anomalous radiation. However, the observed signal contains a lot of slight amplitude anomalies hidden by background noise. To detect the earthquake precursor, it is necessary to detect not only large amplitude anomaly but also slight amplitude anomaly. In this paper, we propose the method of detecting anomalous radiation using the magnetic field azimuth which does not depend on the amplitude level. It is assumed that magnetic field azimuth has notable fluctuation when anomaly EM radiation occurs. We detect this fluctuation by the azimuthal standard deviation calculated every 20 minutes. Moreover, we show that it is dominant in anomaly detection to use the fluctuation in magnetic field azimuth instead of the amplitude level.
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