How an Oklahoma earthquake showed danger remains after years of quakes becoming less frequent

OKLAHOMA CITY — After a dramatic spike in earthquakes in the early 2010s, state regulators in Oklahoma began taking steps to limit the injection of wastewater from oil and gas extraction deep into the ground. As a result, the number of earthquakes, especially large ones, has steadily decreased over the years.

But a pair of larger quakes in recent weeks, including a magnitude 5.1 quake over the weekend that was one of the strongest in years, are a reminder of the danger after the last shook an area dotted with such injection wells.

According to data from the Oklahoma Geological Survey, the earthquake would be the fourth strongest in Oklahoma history if seismologists maintain this rating.

Here’s the latest on what’s happening in Oklahoma.

Determining the exact cause of an earthquake is difficult. But the general scientific consensus is that the injection of high-pressure wastewater activates ancient fault lines deep in the Earth’s crust, says Nick Hayman, director of the Oklahoma Geological Survey.

The wastewater is a remnant of oil and natural gas production, including salt water, drilling fluids and other mineralized water, that sits thousands of feet underground. The earthquake late Friday evening, which shook an area near Oklahoma City, occurred in an area with nine disposal wells within a ten-mile radius, although not all of those wells were active.

“The problem is you’re talking about a lot of areas where fluids are being injected and a lot of errors,” Hayman said. “It’s very difficult when there is an earthquake to know exactly what caused it.”

The large increase in earthquakes more than a decade ago prompted state regulators to impose restrictions on wastewater disposal, especially in areas around the epicenter of the earthquakes. Since then, the number of earthquakes began to decrease dramatically.

In general, earthquakes between magnitude 2.5 and 5.4 are felt but cause only minor damage, while earthquakes between magnitude 5.5 and 6.0 can cause minor damage to buildings. In Oklahoma, the number of earthquakes with magnitudes greater than 2.7 has fallen over the past decade from nearly 2,000 in 2015 to fewer than 40 last year, according to data from the Oklahoma Geological Survey.

At least six earthquakes, including two with magnitudes greater than 4, were recorded near the Oklahoma City suburb of Edmond in mid-January. Then, shortly before midnight Friday, a magnitude 5.1 earthquake occurred about 5 miles (8 kilometers) northwest of Prague, Oklahoma, the Oklahoma Geological Survey reported.

After that earthquake, the Oklahoma Corporation Commission ordered all disposal wells to be pumped into the Arbuckle Formation, a deep layer of rock beneath the Earth’s surface within 10 miles of the earthquake’s epicenter, to slowly reduce disposal. The gradual reduction is necessary to avoid sudden pressure changes that could result in more seismic activity, said spokesman Matt Skinner, whose agency regulates Oklahoma’s oil and gas industry.

In the 30 days before the earthquake, Skinner said area operators discharged a total of 10,000 barrels of wastewater per day at six active storm drains, an amount Skinner said is not abnormally high. By comparison, before the wave of earthquakes in the 2010s, some wells routinely pumped 50,000 barrels of wastewater or more into the formation every day, he said.

In Prague, a small town of about 2,300 residents known for its annual Kolache festival, city manager Jim Greff said preliminary damage consisted mainly of rocks falling from the exterior of homes, broken glassware and dishes, cracks in plaster and thrown open cupboard doors. There were no reports of injuries.

“Some people had chimney damage near the epicenter,” Greff said. “It was mainly things that fell off the shelves.”

Tim Holik, who runs a car repair shop in Prague, said he saw his fireplace come loose from the wall in the house.

“We almost let our earthquake insurance lapse because we haven’t had one in a while,” Holik said.

The strongest recorded earthquake in Oklahoma was a magnitude 5.8 earthquake near the city of Pawnee in 2016. Some reported feeling the quake as far away as Illinois and southern Texas. Following that unrest, a state of emergency was declared and regulators ordered the closure of wastewater treatment wells within a 500-mile radius of the epicenter. No serious injuries were reported, but one man was treated and released from hospital after part of a fireplace fell on him.

Hayman, the OGS scientist, said that while it is impossible to predict earthquakes accurately, he does not believe Oklahoma is at high risk of a major destructive earthquake greater than 6.0.

“The ruptures we’re seeing don’t seem to be sensitive to magnitude 6 and above, where you really start to see major damage,” he said. “We know scientifically that we are not in the (California) Bay Area waiting for the big one.”