He added that to make the scenario realistic, the model also includes the mechanical properties of the rocks in Oklahoma. The result was that the model successfully predicted changes in the crustal stress that come from brine injection. 

For the final step, Shirzaei said, "We used a well-established physical model of how earthquakes begin so we could relate stress perturbations to the number and size of earthquakes."

The team found that the physics-based framework does a good job of reproducing the distribution of actual earthquakes by frequency, magnitude and time. 

"An interesting finding, was that a tiny change in the rocks' elastic response to changes in fluid pressure can amplify the number of earthquakes by several times," Zhai said. "It's a very sensitive factor." 

Making production safer

While wastewater injection can cause earthquakes, all major oil and gas production creates a large amount of wastewater that needs to be disposed of, and injection is the method the industry uses.

"So to make this safer in the future, our approach offers a way to forecast injection-caused earthquakes," Shirzaei said. "This provides the industry with a tool for managing the injection of brine after fracking operations." 

Knowing the volume of brine to be injected and the location of the disposal well, authorities can estimate the probability that an earthquake of given size will result. Such probabilities can be used for short-term earthquake hazard assessment. 

Alternatively, the team says, given the probability that an earthquake of certain size will happen, oil and gas operators can manage the injected brine volume to keep the probability of large earthquakes below a chosen value. 

The end result, said Zhai, "is that this process will allow a safer practice, benefiting both the general public and the energy industry."

Robert Burnham

Science writer, School of Earth and Space Exploration