Abstract: In order to further test and improve the judgment basis for identifying foreshocks through the characteristics of “narrow frequency band and low-frequency migration”, this paper compares and analyzes the FFT spectra of the Maduo M_S4.2 earthquake in Qinghai on December 24, 2020, and the Yushu M_S4.7 earthquake in Yushu on April 14, 2010. based on the waveform data recorded by digital seismic networks in Qinghai, Gansu, and other regions. The results show that the frequency band of Maduo M_S4.2 earthquake spectrum is wide, the dominant frequency band is 0~5.0 Hz. The frequency band of the Yushu M_S4.7 earthquake narrowed and shifted to the low-frequency end, the dominant frequency band of 0~1.5 Hz. The comparative analysis shows that the “frequency band narrows and shifts to low frequency” phenomenon is effective for direct foreshock identification. In the future, it is necessary to focus on the possibility of strong earthquakes occurring near the source area in the short term (1~3 months) after the frequency spectrum shift of small and medium earthquakes occurring.

Key words: Direct foreshock, FFT mean square spectrum, 2010 Yushu M_S4.7 earthquake, 2020 Maduo M_S4.2 earthquake