Stress, fault roughness, and pore-pressure effects on frequency magnitude distribution of lab and induced seismicity

The b-value — the slope of the earthquake frequency-magnitude distribution — is widely used to infer stress conditions in seismogenic zones. Yet its interpretation is complicated by at least three factors that rarely act in isolation: differential stress, fault roughness, and pore pressure.

In this study, we disentangle these effects using controlled laboratory experiments and compare them with b-value variations observed during fluid injection in Enhanced Geothermal System (EGS) reservoirs. We find that increasing differential stress drives an inverse linear reduction in b-value, with the slope steepening at elevated pore pressures. Higher pore pressures, on average, are associated with lower b-values — contrary to a commonly held assumption. Spatial variations during injection appear structurally controlled, with high-damage fault zones promoting locally elevated b-values.

These results caution against single-factor interpretations of b-value in natural and induced seismicity settings, and highlight the need to account for the combined influence of stress, structure, and fluid pressure.