The Edmond Sun
At 10:53 p.m. Nov. 5, 2011, a 5.6 magnitude earthquake located 44 miles east-northeast of the metro area near Prague was powerful enough to shake structures in Edmond.
The quake was felt from eastern Colorado to the Memphis, Tenn., area in much of Oklahoma, southern Kansas, southwestern Missouri, northwestern Arkansas and northern Texas. In Oklahoma, at least two people were injured, 14 homes were destroyed and many were damaged in the Shawnee-Sparks area, according to the U.S. Geological Survey. Parts of U.S. Highway 62 between Meeker and Prague buckled.
Dozens of aftershocks and debate about the cause of the temblor followed.
A study published online on March 26 by the scientific journal Geology reported that significant earthquakes are increasingly occurring within the U.S. continental interior, including five of magnitude 5.0 or greater in 2011 alone. The study identifies the Prague area temblor as the largest earthquake potentially related to injection.
On March 22, the Oklahoma Geological Survey released a paper by the organization’s director and state geologist G. Randy Keller and seismologist Austin Holland. They concluded the interpretation that best fits current data is that the Prague earthquake sequence was the result of natural causes.
Relatively large, natural earthquakes occur in Oklahoma, and a USGS map for the state shows the seismic hazard for the Prague-Wilzetta fault area was not zero before the sequences, according to Keller and Holland.
Oklahoma has experienced more than 10 magnitude 4.0 or greater earthquakes since the magnitude 5.0-plus El Reno temblor of 1952. This is statistically consistent with the Gutenberg-Richter relationship, which describes the distribution of earthquakes of differing magnitude over time, according to Keller and Holland.
In fracturing, a.k.a. “fracking,” after an oil or gas well is drilled a mixture containing mostly sand and water and a small amount of chemical additives is injected at a high pressure to fracture shale, according to various industry sources. The mixture keeps the fractured rock open, providing paths through which oil and natural gas are extracted.
The study published by Geology used aftershocks to show the faults that ruptured in the 2011 Prague earthquake sequence, and to show that the tip of the initial rupture plane is within 200 meters (656 feet) of active injection wells and within 1 kilometer (3,280 feet) of the surface.
Thirty percent of early aftershocks occurred within the sedimentary section, according to the study published by Geology. Subsurface data indicated fluid was injected into effectively sealed compartments.
Additionally, decades-long lags between the start of fluid injection and the onset of induced earthquakes are possible, modifying common criteria for fluid-induced events, according to the study published by Geology.
“The progressive rupture of three fault planes in the sequence suggests that stress changes from the initial rupture triggered the successive earthquakes, including one larger than the first,” the researchers stated.
Keller and Holland stated the Prague earthquake sequence, along with other current and historically active seismic areas in Oklahoma, would benefit from further study, including improved earthquake monitoring and acquisition of formation pressure data. Further studies, they concluded, will result in a better understanding of seismic activity statewide.
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