In a Decade, Oklahoma's Earthquakes Will Be Normal Again

Next year, though, the state faces a 40-percent risk of a major tremor.

Oil storage tanks outside Cushing, Oklahoma (Nick Oxford / Reuters)

Earlier this month, a disaster threatened the American economy—but no one much noticed it because of the presidential election.

On the evening of Sunday, November 6, a 5.0-magnitude earthquake shook Cushing, Oklahoma. The trembling dislodged bricks, broke windows, and forced 40 people out of their homes. Some of the city’s downtown remains closed three weeks later.

Yet the quake could have had farther-reaching consequences. Cushing, an 8,000-person city in the state’s center, is also one of the largest oil trading hubs in North America. Crude oil from Texas and the lower Midwest is stored at Cushing on its journey to refineries and the coasts, filling the hundreds of circular towers that surround the plains around the city. On any given day, more than 60 million barrels of crude sits in or around Cushing.

Oklahoma’s earthquakes had suddenly become more than a threat to local property and human life. Given how much American economic power rests on the fossil fuel business, they had become, as Bloomberg puts it, a national-security issue.

The Cushing quake also seemed like a discouraging sign. It’s increasingly understood that Oklahoma's earthquakes are caused by the underground injection of wastewater from fracking and other mining operations; the water trickles through the sediment and irrigates ancient fault lines, allowing them to slip. After the Oklahoma state government started to regulate wastewater injection in May of this year, earthquakes have generally been on the decline. Though a 5.8-magnitude earthquake struck Pawnee in September—the largest earthquake ever measured in the state—the number of smaller quakes has fallen.

Was the Cushing quake an end to that trend?

A new study, published Wednesday in Science Advances, says no, and it asks Oklahoma to stay the regulatory course. In five to 10 years, the state should return to experiencing a normal number of earthquakes.

“We can already see the effect of the reductions, but the risk is still high and it will be high for some more years. But our model actually predicts the rate will keep decreasing and slowly approach the tectonic background level in five to ten years,” said Cornelius Langenbruch, a geophysicist at Stanford University and one of the authors of the new study.

Between now and then, though, things could get hairy, even if the number of earthquakes overall keeps trending down. Before 2009, the state saw one to three minor earthquakes every year. It now sees between one and three quakes per day.

The state will also see a highly elevated risk of major earthquakes for the next few years. In 2017, for instance, Oklahoma faces a 40 percent risk of experiencing a major earthquake with a magnitude larger than 5.0, an earthquake strong enough to damage structures.

Langenbruch’s paper, written with fellow Stanford researcher Mark Zoback, is a completed seismic risk model for the state of Oklahoma. It’s the first risk model to factor in not only local fault lines and historical seismicity, but wastewater injection rates. Langenbruch said the study was only possible because the state now requires that oil companies make their underground water injection data available every week.

It wasn’t always this way. For years, even as neighboring states began to regulate wastewater injection, Oklahoma officials doubted that pumping water back underground made earthquakes more likely. Eventually, companies in Kansas and Arkansas began shipping additional water into Oklahoma to inject it underground.

In the past year, though, Oklahoma oil companies, regulators, and government officials have worked together to reduce the rates of water injection. (They’ve also been helped by lower oil prices, which reduce the amount of drilling in the state.) In fact, scientists are meeting this week for a special conference on “induced seismicity,” the geological term for man-made earthquakes.

Maps like this one, in Langenbruch and Zoback’s article, have helped. It shows that while historical quakes have happened throughout Oklahoma, the post-2009 earthquake boom has been concentrated around fracking sites. (The “heat map” shows where underground water pressure has been increased by fracking injection.)

A map of historical and recent earthquakes in Oklahoma. The colored heat map shows where wastewater has been injected below the surface in the years since 2009. The black dots, spread throughout the state, show earthquakes that occurred between 1979 and 2008. The gray dots, concentrated around the fracking formations, show post-2009 earthquakes. (Cornelius Langenbruch / Mark Zoback / Science Advances)

“For a long time, people involved in the business denied this was taking place,” said Langenbruch. “Finally, everybody’s working together. Everybody understood the seismic hazard in Oklahoma had to be reduced.”

If Oklahoma sticks to its policies, that may come to pass.