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Abstract

The M6.4 Yangbi earthquake, which occurred in Yunnan Province on May 21, 2021, has attracted a lot of attention for its abundant foreshocks and superior monitoring capability around the mainshock. It seems to be a typical foreshock-mainshock-aftershock sequence in China Mainland in recent decades, and the key question is whether it is possible to distinguish foreshocks from normal seismic activities. The b-value derived from the Gutenberg-Richter (GR) law, as an important seismicity parameter, has the characteristics of decreasing during foreshock activity and increasing during normal aftershock sequence by previous studies. At present, the sequence’s b-value is usually calculated by using the earthquake catalog, which makes it difficult to obtain the b-value results in a short time after the earthquake. In this study, we select the foreshock event waveform recorded by the station closest to the earthquake epicenter and obtain the envelope function of the waveform. Each peak of the envelope can be regarded as an earthquake event. The peak amplitude and time correspond to the magnitude and time of earthquakes. Similar to the GR relationship, the corresponding relationship between magnitude and peak number is preliminarily established, and the corresponding b-value results are obtained. This study aims to explore the effectiveness of the waveform envelope method to identify foreshocks in China mainland, and tries to solve the problem that the b-value cannot be accurately calculated due to the lack of catalog after a large earthquake.