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OU study suggests contributing factors to state's largest quake

The Norman Transcript, Okla. — Adam Troxtell The Norman Transcript, Okla.

July 17--NORMAN -- It's been almost a year since a 5.8 magnitude earthquake shook just about the whole state and beyond.

Several people in Norman woke up when the state's largest earthquake, which originated near Pawnee, occurred on Sept. 3. Since then, researchers and scientists have been trying to find out how it grew to such a size, and a group led by a University of Oklahoma team is nearer to an answer.

Foreshocks, the Sooner Lake Fault and injection wells are all potential factors in creating the temblor that caused visible damage to Pawnee buildings, according to a study led by OU geophysics professor Xiaowei Chen. The work done by the team of OU, CalTech and Chinese researchers was published in Scientific Reports this week, according to a university release.

Chen said seismic data shows a series of foreshocks -- smaller earthquakes that occur in the buildup to a larger tremor -- happening for about two years in a concentrated area around the Pawnee quake epicenter. They occurred in two episodes.

"All of the foreshocks occurred in a very small area, about 1 kilometer in diameter," Chen said. "Some external force has been causing the foreshocks to migrate along the fault. In addition, the foreshocks, when they happen, they cause some stress changes that can promote the main shock."

Slow movement along the Sooner Lake Fault that is not picked up on a seismometer can lead to changes beneath the surface. Foreshocks change the stress levels along a fault, further altering the seismic makeup of an area, Chen said.

And the foreshocks line up with injection well activity in the area, she said.

"These occurrences correlate very well with injection wells in that area," Chen said. "There is a very strong correlation between injection wells and the seismicity before the earthquake happened."

The study doesn't suggest injection well activity caused the Pawnee earthquake. However, Chen said if the injection well activity caused the foreshocks, those can contribute to the final magnitude of a main shock.

"The key finding here is that we have found small earthquakes may contribute to the occurrence of the larger earthquake," Chen said. "This is what we need to pay attention to."

Seismicity in Oklahoma has dropped since the Pawnee quake, and earthquakes are felt at a much lower frequency. The state made changes to injection well regulations in an attempt to lessen the wells' potential impact on earthquakes.

But Chen said Oklahoma isn't necessarily in the clear. Seismicity could continue on for another decade after an initial spike.

A 4.2 magnitude quake near Stroud illustrated this on Friday. There were about five other tremors with magnitudes ranging from 2.7 to 3.8 following the main quake, according to the U.S. Geological Survey.

"We just need to keep watching the seismicity and be prepared," Chen said. "I don't think it will stop immediately."

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