Condition of Occurrence of Large Man-Made Earthquakes in the Zone of Oil Production, Oklahoma
Abstract
Man-made seismicity is a response of the brittle crust to fluid injection at depth and to the subsequent increase in pore-pressure and stress field perturbations. In Oklahoma, where the sharp increase in earthquake rate correlates with injection operations, we show that the earthquake-size distribution can differ significantly on the volume of injected fluid. The size distribution of M < 3.5 earthquakes exhibits a near-constant slope b, while significant variation of b-values (from b ≈ 1 to b > 2) may be documented for larger magnitude ranges. This change shows statistically significant positive dependence on injection activity. In addition, largest events occur at the border of the injection area at some distance from massive injection, and in the periods of steady injection rate. These observations suggest that a deficit of large induced earthquakes under conditions of high injection rate can be accompanied by an overall increase of natural seismicity along pre-existing faults in the surrounding volume, where large events are more likely to be triggered over longer space-time scales.