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≈1to 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.
Figure 1. Seismicity and fluid injection in Oklahoma. (a) Map of earthquakes and injection wells: M≥2.7 earthquakes in the ANSS catalog (red dots); M≥5.0 earthquakes (stars); injection depth and injected volume from 2006 to 2018 (filled circles). (b) Earthquake sequence (black bars); monthly number of M≥2.7 earthquakes (red line) and monthly volume of fluid injected (blue line) with respect to time. The 4 largest earthquakes shown by stars in (a) give the magnitude scale.
Figure 2. Changes in earthquake-size distribution with respect to injection activity. (a) Map of the related injected fluid volume VE attributed to M≥2.7 earthquakes. Stars show epicenters of large earthquakes. (b) Earthquake-size distributions for 1000 events attributed the lowest (dots) and the highest (+) related fluid volume, respectively. (c) Slopes bM3.5, and bM2.7 of the earthquake-size distribution with respect to the related fluid volume in two magnitude ranges [2.7; 3.4] (blue dots) and [3.5;Mmax] (red dots), respectively. Error bars are standard deviations obtained by bootstrap resampling, dashed red line shows a trend, the best linear fit between bM3.5 and log10VE. (d) Distribution of the slope of the best fit for 1000 semi-synthetic catalogs and its approximation by Gaussian distribution; the observed slope is shown by red bar.
Source: Vorobieva I., Shebalin P., Narteau C. Condition of Occurrence of Large Man-Made Earthquakes in the Zone of Oil Production, Oklahoma // Izvestiya, Physics of the Solid Earth. 2020. V. 56. № 6. P. 911-919. DOI:10.1134/S1069351320060130