A study on the characteristics of ULF electromagnetic spectrum before and after the Wenchuan MS8.0 earthquake

Based on the theory of electromagnetic spectrum, we analyzed the observation data of electromagnetic radiation with FFT and wavelet transform method. The data were recorded at Jinhe and Jiange stations in Sichuan province, and at Ersha station in Zhengzhou of Henan province before and after the Wenchuan earthquake. After studying the variation of the electromagnetic spectrum from FFT transform and the wavelet decomposition at different scales, it is found that there is anomalous information before the earthquake. The results show that, first, the dynamic FFT spectrum image shows obvious variations of the electromagnetic spectrum before the earthquake, with characteristic progressive stages in both time and frequency band, and with the increase of epicentral distance, the radiation energy is smaller and the anomaly appears later. Secondly, the detailed wavelet decomposition shows that abnormal low-frequency signals appear earlier than the high-frequency anomaly; at the station near to the epicenter, the high-frequency anomaly is more apparent, while at the far station, the low-frequency anomaly is relatively more apparent.