Since the beginning of 2009, Central Oklahoma has experienced a substantial increase in seismic activity, too much to be random, researchers think. Drilling wastewater disposal in injection wells is a likely contributing factor, they said.
"We've statistically analyzed the recent earthquake rate changes and found that they do not seem to be due to typical, random fluctuations in natural seismicity rates," said Bill Leith, a U.S. Geological Survey (USGS) seismologist. "These results suggest that significant changes in both the background rate of events and earthquake-triggering properties needed to have occurred in order to explain the increases in seismicity. This is in contrast to what is typically observed when modeling natural earthquake swarms."
More than 200 earthquakes with magnitudes of 3.0 or greater have occurred in Central Oklahoma since January 2009. USGS and Oklahoma Geological Survey (OGS) have been looking into what could be the cause. Studies show that one to three magnitude 3.0 earthquakes or larger occurred yearly from 1975 to 2008, while the average grew to around 40 earthquakes per year from 2009 to mid-2013, they said.
Earthquakes of magnitude 2.5 to 5.4 are "often felt but only cause minor damage," according to seismologists. Magnitudes of 3 to 3.9 are classified as "minor." Quakes with magnitudes of 4 to 4.9 are considered "light," and moderate quakes have magnitudes of 5 to 5.9. An earthquake has to have a magnitude of at least six to be considered "strong," 7 to be considered "major," and 8 to be considered "great."
Injection-induced seismicity is thought to be the culprit. OGS has examined the behavior of the seismicity through the state assessing the optimal fault orientations and stresses within the region of increased seismicity, particularly the unique behavior of the Jones earthquake swarm just east of Oklahoma City. The USGS and OGS are now focusing on determining whether evidence exists for such triggering, which is widely viewed as being demonstrated in recent years in Arkansas (see Shale Daily, Oct. 9, 2012), Ohio (see Shale Daily, Sept. 6) and Colorado.
Additionally, fluid extraction is being eyed as a cause of seismic activity in the Barnett Shale in Texas (see Shale Daily, Aug. 28, 2013).
The Jones swarm includes the largest earthquake ever recorded in Oklahoma, a magnitude 5.6 that occurred near Prague, OK, on Nov. 5, 2011. It damaged a number of homes as well as the historic Benedictine Hall at St. Gregory's University, in Shawnee, OK. Almost 60 years earlier, in 1952, a comparable magnitude 5.5 quake struck El Reno and Oklahoma City. More recently, earthquakes of magnitude 4.4 and 4.2 hit east of Oklahoma City on April 16, causing objects to fall off shelves.
In Oklahoma, OGS operates a 15-station network of seismic monitors. Data from this system and from portable seismic stations installed in the Oklahoma City region are sent in real-time to the USGS National Earthquake Information Center, which provides continuous reporting on earthquakes worldwide.