The swarm of earthquakes that have rumbled under the Oklahoma prairie for the past four years continues to intrigue scientists who are trying to determine how much influence oil and natural gas activity might have.
The U.S. Geological Survey last month released a statement, noting that the pace of earthquakes in the state has jumped to about 40 a year since 2009, up from one to three per year for the three previous decades.
“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 the statement.
But it may not be that simple, said Austin Holland, seismologist at the Oklahoma Geological Survey.
“There is a contribution of triggered seismicity by oil and gas activity in the state, but I think there's a combination of natural seismicity change and the triggered seismicity,” Holland said. “I think there clearly has to be a natural component to this.”
Studies are still ongoing, but Holland said research seems to indicate that hydraulic fracturing could be responsible for up to 10 percent of the increase.
Water injection wells also could be a contributing factor, especially in southern and northern Oklahoma, Holland said. Such wells have been attributed to quakes in Ohio and other areas.
But the increase cannot be fully explained by man-made causes, Holland said.
“Seismicity rates do not remain constant through time anywhere in the world,” he said.
Before joining the Oklahoma Geological Survey, Holland worked for the Idaho National Laboratory, where a similar 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.”
History of oil and gas activity
While oil and natural gas activity has picked up in Oklahoma in recent years, oil production and drilling activity still pale compared to levels seen in the 1980s and 1920s.
Katie Keranen, a former University of Oklahoma assistant professor now at Cornell University, said there is one clear difference in oil and natural gas activity today.
“The volumes of water being taken out and put back in are higher than ever before,” Keranen said. “The active wells today are very big. New injection wells are operating at very high volumes.”
The journal Geology in March published a report Keranen co-wrote stating that the magnitude 5.7 earthquake that struck in central Oklahoma in 2011 likely was caused by oil and natural gas drilling activity. It was the strongest earthquake ever recorded in the state.
“I've been really focused on the Jones, Luther, Prague area. I think it's all tied together in terms of these high-volume wells,” Keranen said. “Central Oklahoma has been the focus.”
Holland, however, said he has seen the most evidence of oil and natural gas related influence in northern and southern Oklahoma.
“In central Oklahoma, the volumes of water were greater a decade ago than they are today,” Holland said. “The largest growth in that area is in the Mississippian play in northern Oklahoma. That's where we likely may be seeing this contribution of induced seismicity.”
More study ahead
One thing all three groups agree on is that more research is needed.
“There's a lot more work to be done with identifying the contributing factors,” Holland said. “We need to do that in a very specific, data-driven determination such that we can demonstrate this sufficiently.”
The Oklahoma Geological Survey is looking to hire another seismologist, Holland said. The researchers have installed additional seismographs throughout the state to better detect exactly when, where, how strong and at what depth earthquakes strike.
When the monitors have recorded a sufficient amount of information, the suspected injection wells will return to operation while being closely monitored.