Injection‐Induced Seismicity Size Distribution Dependent on Shear Stress

Like natural seismicity, induced seismicity caused by injection also shows a power law size distribution, and its gradient b‐value is used for seismic hazard analysis. Despite the well‐known result that b‐value is negatively correlated with differential stress for natural earthquakes, there is no similar correlation for b‐value variations because the differential stress is nearly constant for injection‐induced seismicity, we thus investigate the b‐value dependence on the relative shear stress acting on existing fractures, using the fault orientation and in situ stress information. The seismicity occurring along existing fractures having high shear stress has significantly lower b‐values than does that associated with lower shear stress fractures. We examine the b‐value dependency on slightly changing differential stress, but the relationship is not clear. The b‐value for injection‐induced seismicity is dependent on relative shear stress on faults, which is a novel physical explanation for the b‐value variations of induced seismicity. Plain Language Summary Frequency magnitude distribution of a series of natural earthquakes and laboratory earthquakes correlates with the applied stress. However, the variation of the frequency magnitude distribution for injection‐induced seismicity has not been understood well since the stress state is rather constant in reservoir scale. This study investigates the stress state of the faults that caused the injection‐induced seismicity. We found that the events that occurred from the fault that oriented to have relatively higher shear stress caused the b‐value reduction. This finding provides the new perspective of the scaling law of frequency magnitude distribution for induced seismicity. This insight leads to seismic hazard mitigation due to fluid injection. Key Points We estimate shear stresses on the faults from in situ stress and fault orientation, and study the correlation to magnitude and b‐value b‐value negatively correlates with the shear stress rather than differential stress Shear stress is the cause of low b‐value for injection‐induced seismicity case