Mid-Continent Section

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Oklahoma's Recent Earthquakes and Saltwater Disposal


The number of small-to-moderate sized earthquakes in much of the central and eastern United States began to increase markedly in 2009. About three quarters of the anomalous seismicity in the central and eastern U.S. has been in Oklahoma, which went from approximately one M≥4 earthquake every decade, to one nearly every 2 weeks. In five study areas that encompass 82% of the recent M≥3 seismicity, we show that the increases in seismicity follow increases in the rates of saltwater disposal with varying temporal relationships. Adjacent areas where there is relatively little saltwater disposal have had comparatively few earthquakes. In the areas of greatest seismic activity, the saltwater disposal comes principally from produced water, saline pore water that is co-produced with oil and then injected into deeper sedimentary formations, not flowback water. The injection formations appear to be in hydraulic communication with potentially active faults in crystalline basement, where nearly all the earthquakes are occurring. Although the majority of the recent earthquakes have posed little danger to the public, the possibility of triggering damaging earthquakes on potentially active basement faults cannot be discounted. Injection of the produced water into depleted portions of the reservoirs from which it was produced should reduce the rate of seismicity.