Earthquake will give Oklahoma experts an education

Seismologists say the earthquakes that shook Oklahoma on Saturday, including the largest tremor in state history, were along one of the few known fault lines in the state.
BY MICHAEL KIMBALL mkimball@opubco.com Published: November 8, 2011

The biggest earthquake in Oklahoma's recorded history was a natural occurrence along a known fault line and likely had nothing to do with oil and gas exploration, geological experts said Monday.

All of the tremors Saturday seem to be part of an isolated and inevitable release of seismic energy, G. Randy Keller, director of the Oklahoma Geological Survey, said before another quake shook central Oklahoma on Monday evening.

That quake registered at 4.7 magnitude and was centered about five miles northwest of Prague, in the same area as Saturday's record-setting tremor. The 4.7 quake, the second one registered Monday, came in the midst of heavy thunderstorms when a large portion of the state was under a tornado watch.

These will become the most studied earthquakes ever in the state. A wide array of sensors deployed after a recent surge in smaller earthquakes will provide experts plentiful data on the ground movement.

The magnitude-5.6 earthquake that struck at 10:53 p.m. Saturday northwest of Prague in Lincoln County most likely occurred along the Wilzetta fault line, which stretches across part of central Oklahoma east of Oklahoma City, the OGS and the U.S. Geological Survey reported. A magnitude-4.7 foreshock occurred nearby at 2:12 a.m., along with numerous smaller foreshocks and aftershocks.

No major injuries were reported. Several structures sustained cracked walls and damaged brickwork.

Similar to 2010 quake

The quakes were probably the result of two tectonic plates moving past each other laterally, called a “strike-slip” fault line, Keller said. The same movement on a different fault line was responsible for a magnitude-4.7 earthquake centered near Norman on Oct. 13, 2010.

“The fact that this stress has been released is a good thing,” Keller said. “It's unlikely to occur there again soon.”

The quake last year and Saturday's tremors were likely unrelated to a series of smaller earthquakes, called swarms, affecting central Oklahoma for the last few years, Keller said. The origins of those quakes are harder to pinpoint.

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