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Abstract

There are many reports on variations in the electromagnetic (EM) field in association with occurrence of earthquakes. However, the reliability of their existence is still a topic of dispute. It should be tested by referring many events to secure the statistical significance. In the present study, we referred EM dataset acquired just after the 2016 Central Tottori earthquake. The data were originally recorded for the EM sounding around the focal area. The characteristic variations in the EM time series were tried to be identified and the timing of their occurrence was compared with the aftershocks to confirm the hypothesis that variations in the electromagnetic do occur in association with earthquakes. To perform an exhaustive and objective counting, we proposed an automatic procedure to detect EM signals in the time series. A template waveform was defined by stacking representative waveforms that were picked up by visual inspection. The EM signals were identified by comparing the EM time series and the template waveform. By using the proposed procedure, we confirmed that the number of signals occurred during a several time-range before and after earthquakes are larger than that expected when it occurred randomly. We also performed a statistical test for the increase in the signal occurrence rate and confirmed that the recognized increase in the signal occurrence rate was significance with the 95% reliability. These results suggest that EM variations recorded in the EM time series include earthquake-related signals, at least for the case of aftershocks of the 2016 Central Tottori earthquake.