Variations in the Power Spectral Density of Thunderstorm Discharge Pulses in the Frequency Band of 12–40 kHz Associated with Earthquakes in Japan and Taiwan

Аннотация

Records of the electric component of the electromagnetic field in the Magadan region indicate abnormal variations in the power spectral density of pulsed signals of thunderstorm discharges (atmospherics) in the 12–40 kHz frequency band during the preparation of several earthquakes on Kyushu Island (Japan) in April 2016. The anomalies were manifested by the unusual behavior of the ratio of spectral densities of atmospherics for higher and lower frequencies. The time dependence of the ratio of spectral densities was slightly distorted, and its values were decreased in the daytime for 3 weeks with a subsequent increase in the daytime minima and emerging outliers of daytime values 3 days before the start of earthquakes. The combination of these anomalies was a precursor of the earthquake series. The detected anomalies were partially observed before the strong earthquakes in Taiwan from October 2013 to February 2016. The anomalies are not associated with geomagnetic activity.