Injection induced seismicity size distribution dependent on shear stress

TL;DR

This study reveals that the size distribution of injection-induced earthquakes depends on the shear stress on existing faults, providing a physical explanation for variations in b-values observed in seismic hazard assessments.

Contribution

It demonstrates that b-value variations in induced seismicity are physically linked to the relative shear stress on faults, a novel insight into seismic hazard analysis.

Findings

High shear stress faults have lower b-values.

b-value depends on relative shear stress.

Provides a physical basis for b-value variations.

Abstract

Like natural seismicity, induced seismicity caused by fluid injection also shows a power law size distribution, and its gradient b-value (ratio of small to large earthquakes) is often used for seismic hazard analysis. Despite well-known relationship that b-value is negatively correlated with differential stress for natural earthquakes, there is no understanding of the physical causes for b-value variations in injection-induced seismicity in the scale where the differential is nearly constant. We investigate a b-value dependence on the relative shear stress acting on existing fractures and show that the seismicity occurring along existing fractures with high shear stress have significantly lower b-values than does that associated with lower shear stress fractures. The b-value for injection induced seismicity is dependent on relative shear stress on faults. Our results provide a novel physical explanation for the b-value variations of induced seismicity.