Examining the strength dependency of frequency-magnitude distribution in small Earthquakes: implications for stress state criticality

The distribution of earthquake sizes, characterized by the power law decay as “b-value”, exhibits spatiotemporal variations detected by seismic networks. These changes are sometimes evident before a large earthquake. Understanding these variations is key to developing a model for large-earthquake generation. Previous studies have shown that changes in the b-value are caused by the tectonic stress regime. Furthermore, lab experiments have demonstrated b-value dependency of acoustic emissions on the criticality of the failure condition. However, the factors controlling the b-values during natural seismic activity are unclear. In this study, changes in the b-value in small earthquake sequences were investigated, focusing on failure criticality. Based on our high-precision focal mechanism dataset, we conclude that the b-value decreases as it nears a critical failure condition, providing a physical explanation for the reduction in b-value before a major earthquake. Our findings elucidate fault failure models, facilitating improvement in earthquake alerts and disaster mitigation.