In This Issue

Over the past decade, the central and eastern United States have experienced an increased number of earthquakes, which are mostly attributed to waste fluid disposal through subsurface injection of saltwater produced from oil/gas production. Guang Zhai et al. (pp. 16228–16233) combined data on fluid injection over time with a physics-based model of crustal stress to calculate pore pressure and poroelastic stresses, and estimate relative seismicity over time for central and western Oklahoma. The authors found that pore-pressure diffusion was the dominant determinant of seismicity rate. However, poroelastic stresses amplified the effect of pore pressure on seismicity by a factor of 2–6. The model reproduced the observed frequency and magnitude patterns of earthquakes of magnitude 3 or greater over time for the years 2008–2017. A mandatory reduction in injection volumes, imposed in 2016, substantially reduced the probability of an earthquake exceeding magnitude 5 in western Oklahoma, but not in central Oklahoma. Imposing a hypothetical injection shut-in in April 2017 led to a gradual decrease in earthquake probability, approaching background levels by approximately 2025. The results highlight the importance of fluid diffusion in determining earthquake risk, according to the authors. — B.D.