Research on Source Parameters of Earthquakes before and after 2021 Yangbi M_S6.4 Earthquake

Based on the digital waveform data recorded by 22 fixed and temporary stations located around the Yangbi area, the inelastic attenuation coefficient and site response of 22 stations are calculated by the Atkinson method and Moya method. Source parameters of 819 earthquakes with M_L≥1.0 before and after 2021 Yangbi M_S6.4 earthquake sequence are accurately calculated after removing the effects due to earthquake inelastic attenuation and site effect. The results show that: (1) There is a good linear relation between seismic moments M₀ and local magnitude M_L; the source rupture radius range have a certain positive correlation with M₀; the corner frequencies negatively correlated with M₀; when M_L≤2.8, the stress drops increase with M_L, while when M_L＞2.8, there is no obvious relationship; (2) The temporal and spatial evolution process of stress drops derived from seismic data with M_L2.9 to 3.8 shows that the spatial distribution of the stress drops before and after Yangbi M_S6.4 earthquake sequence has obvious difference, mainly controlled by the respective seismogenic fault in Yangbi area. In the foreshock sequence, the stress drop in the middle part of the focal area shows a trend of increasing gradually, which is in the process of accelerating stress accumulation. The stress disturbance caused by a series of highly clustered moderate seismic activities may trigger the Yangbi M_S6.4 mainshock in the northern part of the source area with long-term stress accumulation. The results indicate that the temporal and spatial evolution process of stress drops of foreshock sequence has a certain indication effect on the occurrence of the M_S6.4 mainshock. After the M_S6.4 mainshock, its aftershock activity broke the whole aftershock region from the northwest to southeast. Due to the occurrence of the mainshock in the northwest section of aftershock region, the background stress in this region was enhanced, and the spatial and temporal distribution of stress drop showed a high-low value fluctuation state. The area from the middle of aftershock region to the area near the largest foreshock epicenter may be characterized by relatively low stress drop due to frequent foreshock activities and more stress release. The southern section of the aftershock area has abundant aftershock activities, which is the area with high stress drop. The activity of moderate-strong earthquakes area is consistent with the area of high stress drop in Yangbi earthquake sequence.