Irreversible precursors before the 2021 Yangbi earthquake

The sub-instability stage refers to an irreversible seismogenic precursor state prior to fault macroscopic rupture. Laboratory rock friction experiments have verified that abundant precursory anomalies emerge during this stage, which is critical for short-term and imminent earthquake prediction. At present, most validations of the sub-instability theory rely on surface observations such as water level, gravity, and crustal deformation. However, effective approaches to identify sub-instability characteristics directly from the perspective of stress fields remain limited. Given that deep crustal stress cannot be directly measured, indirect geophysical parameters such as seismic wave velocity are essential to invert stress evolutionary processes. Previous studies have demonstrated that seismic velocity variations are governed by regional stress regimes and fault structures. Under relatively stable tectonic conditions, the velocity field can reliably characterize the spatial distribution of subsurface stress.

The 2021 Ms6.4 Yangbi earthquake in Yunnan was a shallow strike-slip strong earthquake with abundant pre-seismic foreshock sequences, providing an ideal natural case for retrospective investigation of pre-earthquake sub-instability. Based on dense pre-shock seismic data, this study constructs regional geophysical fields using apparent velocity ratio and apparent Poisson’s ratio. We calculate the gradient and gradient divergence to characterize the accumulation and release of tectonic stress and strain energy. By matching the spatial features with focal nodal plane orientations, we identify precursory sub-instability responses and establish a sub-instability discrimination method applicable to strike-slip strong earthquakes with active foreshocks, providing a new insight for regional seismic precursor identification. This study adopts seismic observations from May 1 to May 21, 2021, with a research area ranging from 98.5°E–101.0°E and 24.6°N–27.1°N. A total of 100 qualified earthquakes are screened, with epicentral distances of 20–100 km, clear Pg and Sg phases in all four quadrants, and positioning residuals less than 0.5. The average positioning residual is 0.3029, ensuring high data quality. The study area is gridded with an initial resolution of 10′×10′, and the median values of apparent velocity ratio and apparent Poisson’s ratio are adopted for grids with multiple earthquakes. A continuous curvature spline interpolation with a tension factor of 0.2 is applied to achieve refined 1′ gridding, and gradient vector fields are extracted within a 4′ grid window. Mathematical formulas for apparent P/S wave velocity, apparent velocity ratio, apparent Poisson’s ratio, gradient, and gradient divergence are presented in this study. The gradient represents the direction of maximum stress variation, while positive and negative gradient divergence corresponds to energy release and accumulation, respectively. The extreme values of the geophysical fields match well with the divergence extremes: low field values correspond to high-divergence energy accumulation zones, whereas high field values correspond to low-divergence energy release zones, demonstrating consistent synergistic responses.

For the apparent velocity ratio field, the orientations of minimum and maximum value trends are 142.8° and 40.5°, with corresponding gradient divergence extreme orientations of 141.8° and 39.1°. For the apparent Poisson’s ratio field, the trending orientations of minimum and maximum values are 143.9° and 34.2°, and the corresponding divergence extremes are 144.0° and 49.9°. The dominant orientations of both geophysical fields are highly consistent with the two focal nodal plane orientations (138° and 45°) of the Yangbi earthquake. Positive gradient divergence indicates outward regional energy release, while negative values represent continuous energy accumulation. The two geophysical fields exhibit fully consistent pre-seismic energy evolution patterns. Mechanically, energy accumulation increases crustal medium temperature, whereas energy release reduces temperature, thereby altering wave velocity and Poisson’s ratio and forming stable pre-seismic precursory anomalies. This stress-energy evolution is highly consistent with laboratory sub-instability experimental results, confirming that the identified anomalies are reliable pre-earthquake sub-instability precursors. Accordingly, this study proposes a sub-instability discrimination framework suitable for strike-slip strong earthquakes with abundant foreshocks. Meanwhile, the limitations of the method are clarified: the approach relies on sufficient dense seismic data and high-density station observations. In addition, surface observation fields need to be approximately orthogonal to the deep stress field, and crustal medium heterogeneity may induce local identification errors.
 

Title:
Seismological study of meta-instability of the Yangbi MS6.4 earthquake in 2021
Journal Name: Geodesy and Geodynamics
DOI: https://doi.org/10.1016/j.geog.2025.07.002
URL: https://scholarcast.cast.org.cn/top-news/2077962079733583872