Strength dependency of frequency–magnitude distribution in earthquakes and implications for stress state criticality

Abstract Earthquake size distribution is characterized by the “ b -value” of the power law decay, which exhibits spatiotemporal variations. These variations are sometimes evident before a large earthquake. Understanding spatiotemporal 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 the 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 are 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.