Seismological study of meta-instability of the Yangbi M_S6.4 earthquake in 2021

Meta-instability is an irreversible precursor of earthquakes. To identify the meta-instability precursor of the Yangbi M_S6.4 earthquake (99.87°E, 25.67°N) that occurred on May 21, 2021, we selected seismic data from the pre-earthquake period between May 1 and May 21. We calculated the apparent velocity ratio and the apparent Poisson's ratio within the region (98.5°E−101°E, 24.6°N-27.1°N) and interpolated these values. Our findings revealed that the trends of the fitted straight lines at the maximum and minimum points of the gradient divergences of the apparent velocity ratio field and apparent Poisson's ratio field are consistent with the trend of nodal planes 1 and 2 of the focal mechanism, respectively. Interestingly, the trend of the fitted straight lines at the minimum and maximum points of their values is consistent with the trend of nodal planes 1 and 2 of the focal mechanism, too. Positive gradient divergence values indicate energy released, while negative values suggest energy absorption. This phenomenon is characterized by synergy and can be mutually explanatory. The observed stress state matches the experimentally demonstrated meta-instable state, indicating that it is a precursor signal. We propose that this method can be a reference for identifying the meta-instability of strike-slip strong earthquakes with a significant number of foreshocks. For seismically active regions, increasing the number of stations with rich data acquisition will facilitate more convenient stress field analysis.