Oklahoma is a seismically active state crisscrossed with thousands of natural fault lines. But seismic activity has spiked over the past five years, leading scientists, regulators, oil and natural gas industry representatives and the general public to question what is different today — if anything — and what could be causing the rumblings.
The number of earthquakes measuring magnitude 3.0 or greater has jumped from an average of less than five a year to about 40 a year for the past five years and more than 200 so far in 2014, according to the Oklahoma Geological Survey.
Some researchers have attributed the swarm to natural causes. Others have pointed largely to the oil and natural gas industry, specifically at its practice of disposing of produced water by pumping it deep below ground. Others say the unusual swarm likely is a combination of factors.
One theory is that the state’s recent rumblings have natural origins.
Oklahoma — along with much of the central United States — is home to thousands of faults and has a history of quakes.
Earthquake swarms also are known to have occurred in other parts of the country. Before joining the Oklahoma Geological Survey, Oklahoma Geological Survey seismologist Austin Holland worked for the Idaho National Laboratory, where an earthquake swarm was recorded in an area without oil and natural gas activity.
“We had several thousand earthquakes in Idaho over a couple of years. Then it went quiet,” Holland said. “It was quiet before and after. Clearly the seismicity rates don’t remain constant over time. There were no human activities that could have affected that in any way.”
Continental Resources Inc. geologist Glen Brown has drawn a connection between the world’s largest earthquakes and seismic activity in Oklahoma.
Oklahoma has a history of seismic activity. Five of the state’s 15 largest recorded earthquakes occurred before the ongoing swarm, including the previous record-holding magnitude 5.5 in El Reno in 1952.
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Half of the world’s recorded magnitude 8.0 or greater quakes have happened over the past 10 years. The other half occurred during the 1950s. Both times of great seismic activity in other parts of the world have corresponded with quakes in Oklahoma, Brown said.
“This is compelling data. What’s happening in Oklahoma appears to be tied to global activity,” said Jack Stark, senior vice president of exploration at Continental Resources.
University of Texas seismologist Cliff Frohlich, however, dismissed the connection.
“I’m unaware of any evidence worldwide that there has been a change in earthquakes in the ’50s or presently,” said Frohlich, a senior research scientist at the University of Texas’s Institute for Geophysics.
Brown also has pointed to a swarm of earthquakes in northern Mexico over the past few years, including swarms in Chihuahua, Monterrey and Nicah, which are all hundreds of miles from oil and natural gas activity. While Oklahoma has experienced 20 magnitude 4.0 or greater over the past few years, Nicah has experienced 49, Brown said.
“What’s going on in Nicah is phenomenal,” Brown said.
Other possible explanations have to do with drought and lake and aquifer levels. Scientists have noted that earthquakes were created when Lake Mead was filled following construction of the Hoover Dam. Some researchers in Oklahoma and in other parts of the world are studying whether filling or draining underground aquifers could have a similar effect.
“Many times, the filling of dams has caused earthquakes because of the weight you’re putting on the earth’s crust,” said William Fleckenstein, professor and interim department head at the Colorado School of Mines.
Oklahoma and other Western states are three years into a drought that has strained aquifers, lakes and other water systems.
The U.S. Geological Survey has said man-made activity likely has at least contributed to the Oklahoma quakes.
“We’ve statistically analyzed the recent earthquake rate change and found that they do not seem to be due to typical, random fluctuations in natural seismicity rates,” USGS seismologist Bill Leith said in a May report.
Researchers from the U.S. Geological Survey, Cornell University, the University of Texas, Oklahoma State University and other sources have listed water disposal wells as a potential cause or contributor to the quakes.
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