Research on the foreshock characteristics of earthquakes in Chinese Mainland using waveform envelope method

Regarding foreshocks, it is believed that the percentage of the mainshock events with observable foreshocks is very low. But a recent study suggests that 15%–43% of large mainshocks have at least one foreshock. That is to say, if we can effectively identify foreshocks, it can help us carry out short-term and imminent earthquake forecasting, which is of great significance to the local people.

To identify the foreshock, the first step is to understand the characteristics. For example, the low b-value of the foreshock sequence, the cumulative number of foreshock sequences satisfying the anti-Omori law,  the foreshock sequences having similar focal mechanism solutions, and having the characteristics of migration towards the mainshock. Among them, the low b-value of foreshock sequence is widely recognized as a typical feature of foreshocks. However, this method is mainly based on the earthquake catalogs, and requires the number of earthquakes to meet the calculation conditions. Actually, it is difficult to obtain the b-value results in a short time after the earthquake.

In this study, we selected the foreshock event waveform recorded by the station closest to the earthquake epicenter and obtained the envelope function of the waveform. Each peak of the envelope can be regarded as an earthquake event. The amplitude and time of the function peak correspond to the magnitude and time of the earthquakes. Similar to the G-R relationship, the β-value, corresponding relationship between magnitude and peak number, is preliminarily obtained.

For all the M≥6.0 earthquakes with foreshocks in Chinese Mainland since 2010, we calculate the β-values of foreshocks and mainshocks using waveform envelope method. Most β-values of the foreshocks and mainshocks are calculated except the Yushu mainshock for its unsatisfactory data quality. In addition, we also apply this method to the M≥5.0 earthquakes occurred in Chinese mainland since 2022. The results indicate that the β-values of foreshocks are all lower than its mainshocks. If the β-value is less than 0.7, the earthquake can be considered as a foreshock. The β-value could also provide us with some hints about the magnitude and time interval between the foreshock and mainshock.