Geologists explain Oklahoma earthquakes, ponder ‘unknown’

WICHITA, Kan. (AP) — Thousands of earthquakes have rattled Oklahoma and neighboring states in recent years, with the most significant portion traced to the underground injection of wastewater left over from oil and gas production. The latest strong temblor — a magnitude 5.0 — damaged dozens of buildings in Cushing, Oklahoma, but spared a major oil terminal and caused no major injuries.

Here’s a look at earthquakes in Oklahoma and what the future may hold:

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WHY DOES OKLAHOMA HAVE SO MANY EARTHQUAKES?

Scientists have linked Oklahoma’s earthquakes to the underground disposal of wastewater during oil and gas production. Virtually every well produces some water along with the oil and gas that is pumped out of the ground. But wells in Oklahoma bring up a lot more — as much as 10 to 50 barrels of water for every barrel of oil produced.  That is far more water than is produced from wells in other U.S. energy producing areas like those in North Dakota where wastewater injection has not triggered earthquakes.

All that wastewater, along with a small amount of water used in the drilling process itself, is contaminated. To get rid of it without contaminating surface waters, producers have been injecting it using disposal wells drilled deep into the Arbuckle formation, a sandstone layer that underlies Oklahoma and southern Kansas.

But so much wastewater has now been injected into it that it has overloaded the formation. This “pulse of water” is now spreading out across the formation, changing the underground pressure and triggering earthquakes along what had been inactive, stable faults millions of years old.

“We have earthquakes telling us where the faults are,” said Jeremy Boak, director of the Oklahoma Geological Survey.

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ARE THE OKLAHOMA EARTHQUAKES GETTING WORSE?

For about 100 years or so, there had been only two earthquakes that came close to a magnitude 5.0 in Oklahoma, said George Choy, a geophysicist with the U.S. Geological Survey. During the past five years, there have been at least four 5.0 or greater earthquakes — including the one Sunday that was the third such earthquake this year.

The number of Oklahoma earthquakes peaked in June 2015 and the amount of wastewater injection peaked in late 2014. The rate of earthquakes has been cut in half as producers have been decreasing the amount of water they had been injecting into the formation.

Magnitude 2.8 or greater earthquakes have decreased from 4.5 to 2.3 earthquakes per day in Oklahoma, Boak said.

“I would argue it is declining, even as we have these large earthquakes,” Boak said.

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WHY HASN’T THERE BEEN MORE DAMAGE?

As a rule-of-thumb, it usually takes a 6.0 magnitude or greater earthquake in that part of the country before you can see major damage, said Rex Buchanan, director emeritus of the Kansas Geological Society.

But even then a lot depends on the earthquake’s location and depth, the time of day, and the type of geology.

Sometimes a little bit of luck helps, too. Until now, Oklahoma’s strongest earthquakes have registered in less-populated areas.

Cities built over hard bedrock shake more than those built on alluvial areas with more sand and gravel.

“What matters here is not how much energy was released by the earthquake but how intensely does the ground shake,” Buchanan said.

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WHAT DOES THE FUTURE HOLD?

Oklahoma and its neighboring states can expect more earthquakes for at least another year — and probably for several years — even after wastewater injection into the Arbuckle formation ends, Boak said.

It takes a while for the pressure in that wastewater to reach equilibrium and spread out enough so it is not sending those pulses of pressure in such concentrated areas, and triggering earthquakes.

“We are still learning about this stuff every day, Buchanan said. “So it is an unknown.”

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