Oklahoma 5.6 Earthquake on November 6: UPDATED

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Location of the November 2011 Oklahoma earthquake (Image from USGS)

Oklahoma isn’t usually associated with earthquakes, but was struck by a series of tremors on 5-6 November, culminating in the largest recorded event to be felt in the state, a magnitude 5.6 event. Though small on a global scale, initial data from the United States Geological Survey show that shaking was widely felt across the state and beyond.

The November 5-6 Earthquakes in  Oklahoma

Preliminary data published by the USGS show that the largest of the series of the tremors, which occurred 21 miles from the town of Shawnee, had a magnitude of 5.6. It had been preceded by a series of minor tremors, with dozens of smaller events recorded in the area: all occurred at a shallow depth, around 5km. At the time of writing, no damage or injuries had been reported.

The Causes of the Oklahoma Earthquake

Faulted rocks in Oklahoma (Photo from USGS)

Most earthquakes are associated with major seismic zones, where the giant plates which make up the earth’s crust move relative to one another. Although the Pacific coast of North America is associated with such plate boundaries, and is consequently earthquake-prone, the central and eastern parts of the continent are isolated from such zones and generally regarded as being tectonically stable.

Earthquakes which occur in such areas are not, therefore, related to such large-scale plate dynamics, but are caused by movement along faults in the underlying bedrock. Tremors occurring on such fault systems are typically both smaller and shallower than those occurring at major plate boundaries – as occurred in Oklahoma.

Although the USGS notes that such faults are often deeply-buried, and can be undetected until earthquake activity occurs, an initial location map produced by the Oklahoma Geological Survey clearly shows that the main recorded tremors for the November 2011 earthquake occurred on a known fault line.

Earthquake History of Oklahoma

Although Oklahoma has never experienced a major earthquake, the events of November 2011 built up to the largest event so far recorded in the state’s earthquake history. The largest to be previously recorded occurred in 1952 and, with a magnitude of 5.5, released a little less energy than the most recent tremors but the state’s other earthquakes occur rarely and are usually of a magnitude 3-4.

The USGS’s review of significant earthquakes in the state includes estimates of damage. No deaths have been recorded, and most events cause only minor damage – though it’s worth noting that although the tremors were relatively small in scale, the fact that they were felt so widely is typical of tremors east of the Rockies, where shaking is generally more widely felt than those to the west.

Oklahoma Earthquake Map: Image courtesy USGS

The reasons for this are complex, but the British Geological Survey’s Dr. Roger Musson told Decoded Science in a recent interview that a significant factor is the distance from major plate boundaries: in areas made up of older, colder continental rock, the seismic waves generated by an earthquake spread further before their energy is dissipated.

Sources 
Oklahoma Geological Survey. “Recent Earthquakes.” November 6, 2011. Accessed 6 November, 2011.

United States Geological Survey. “Magnitude 5.6 – Oklahoma.” Accessed 6 November, 2011.

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