How the Oklahoma earthquake showed that danger remains after years of earthquakes becoming less frequent | The nation and the world

OKLAHOMA CITY (AP) — After a spike in earthquakes in early 2010, Oklahoma regulators 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 earthquakes, has steadily decreased over the years.

But two larger quakes in recent weeks, including a 5.1-magnitude quake over the weekend that was one of the strongest in years, serve as a reminder of the danger after the latest quake rocked an area filled with such injection wells.

The quake will be tied as the fourth strongest in Oklahoma history if seismologists maintain the rating, according to Oklahoma Geological Survey data.

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

What causes Oklahoma earthquakes?

It is difficult to determine the exact cause of any earthquake. The general scientific consensus is that high-pressure injection of wastewater is activating ancient faults deep in the Earth's crust, said Nick Hyman, director of the Oklahoma Geological Survey.

Wastewater consists of the remains of oil and natural gas production, which includes saltwater, drilling fluids and other mineral water, thousands of feet (meters) into the ground. The late Friday night quake, which shook an area near Oklahoma City, was in an area with nine waste disposal wells within a 10-mile (16-kilometer) radius, although not all of those wells were active.

“The problem is that you're talking about a lot of fluid-injecting zones, a lot of faults. It's very difficult when an earthquake happens to know exactly why it happened,” Hyman said.

What did Oklahoma do to stop them?

A significant increase in earthquakes more than a decade ago has led state regulators to place restrictions on wastewater disposal, especially in areas surrounding the epicenter. Since then, the number of earthquakes has begun to decrease significantly.

In general, earthquakes with a magnitude of 2.5 to 5.4 can be felt but cause only minor damage, while earthquakes with a magnitude of 5.5 to 6.0 can cause minor damage to buildings. In Oklahoma, the number of earthquakes in the past decade with a magnitude greater than 2.7 fell from about 2,000 in 2015 to fewer than 40 last year, according to Oklahoma Geological Survey data.

Where were the last earthquakes?

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

In the wake of that earthquake, the Oklahoma Corporation Commission ordered all waste disposal wells to pump into the Arbuckle Formation, a deep layer of rock beneath the Earth's surface, 10 miles (16 kilometers) from the epicenter to begin slowly reducing disposal operations. Spokesman Matt Skinner, whose agency regulates Oklahoma's oil and gas industry, said a gradual reduction is necessary to avoid sudden pressure changes that could trigger more seismic activity.

In the 30 days before the quake, Skinner said nearby operators were disposing of a total of 10,000 barrels of wastewater per day into six active disposal wells, an amount Skinner said was not abnormally high. By comparison, before the wave of earthquakes in the 2000s, some wells were routinely pumping 50,000 barrels of wastewater or more per day into the formation, he said.

What type of damage do earthquakes cause?

In Prague, a small town of about 2,300 people known for its annual Kolači festival, City Manager Jim Greif said the initial damage was mostly bricks falling from the exterior of homes, broken glassware and dishes, cracks in sheetrock and open cabinet doors. There were no reports of injuries.

“Some people had damage to the chimney near the epicenter,” Greif said. “It was basically just things falling off the shelves.”

Tim Holick, who runs an auto repair shop in Brack, said he noticed the heater had separated from the wall inside the house.

“We were about to run out of our earthquake insurance because we hadn't had insurance in a while,” Holick said.

How big are the strongest earthquakes in Oklahoma?

The strongest earthquake ever recorded in Oklahoma was a 5.8 magnitude quake near the town of Pawnee in 2016. Some reported feeling the quake as far away as Illinois and southern Texas. After that earthquake, a state of emergency was declared and regulators ordered the closure of wastewater disposal wells within a 500-mile (800 km) radius of the epicenter. No serious injuries were reported, but a man was treated and released from the hospital after part of the heater fell on him.

Hyman, the OGS scientist, said that although it is impossible to accurately predict earthquakes, he does not believe Oklahoma is at high risk for a large damaging earthquake greater than 6.0.

“The faults we're seeing don't seem to be that vulnerable at magnitude 6 and above where you actually start to see significant damage,” he said. “We know scientifically that we are not sitting in the (California) Bay Area waiting for the big event.”

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