Abstract Anthropogenic activities like fluid injection can increase pore‐pressure and induce seismicity. Variations in the b‐value (slope of the frequency‐magnitude distribution) of induced and natural seismic events are thought to reflect the stress state, although recent laboratory results suggest that fault roughness also contributes. In nature, stress, fault roughness, and pore‐pressure effects can rarely be disentangled. Here, we investigate these effects and their relative contributions to b‐value variation in the laboratory and compare them with hydro‐shearing in Enhanced Geothermal System reservoirs. Spatial‐temporal variations in b‐values stem from three distinct factors: (a) Increasing differential stress shows an inverse linear relationship with b‐value, with a steeper slope at high pore pressure. (b) Higher pore‐pressures, on average, lead to lower b‐values. (c) Spatial variations during injection are potentially structurally controlled so that high‐damage zones promote higher b‐values. We conclude that b‐value variations outside the lab require cautious interpretation because of these multiple underlying causes.

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