The series of earthquakes in Oklahoma continued this weekend with a magnitude 3.8 (M3.8) tremor in northern Oklahoma.
The tremor occurred at a depth of 5.5km in the early hours of the morning, local time, with the epicenter (the point on the earth’s surface immediately above the fracture in the rock) – 9km from Medford and 154km from the state capital, Oklahoma City.
Tectonic Setting: Stable Continental Interiors
Most earthquakes are associated with the margins of the Earth’s tectonic plates, although some (usually smaller in magnitude) are caused by movement on shallow faults at significant distances from these more active zones.
Typically such stable continental interiors are free of major tremors and the United States Geological Survey, commenting on the seismicity of the central and eastern USA, remarks that “most of the enormous region from the Rockies to the Atlantic can go years without an earthquake large enough to be felt, and several U.S. states have never reported a damaging earthquake.”
To illustrate this, a look at the largest tremors on record in the central US states shows that states such as Florida, Georgia, Louisiana and Maryland (among others) have never experienced an earthquake of M5.0 and many others have never had a tremor of much larger than M5.0.
There are, of course, exceptions. Historically active zones can produce major tremors in stable interiors; the most notable example in the US is the New Madrid seismic zone, where a long-buried ancient rift (a former tectonic margin) remains capable of producing significant earthquakes such as those in 1881, which may have exceeded M8.0. (The exact magnitude is based on estimates because the tremors occurred prior to the development of modern seismology.)
Seismic History and Induced Seismicity
Oklahoma is different. The increasing number of small tremors which have occurred in recent years has attracted attention from seismologists. The USGS identified over 300 tremors of at least M3.0 between January 2009 and October 2013, a pattern which “do[es] not seem to be due to typical, random fluctuations in natural seismicity rates” according to a USGS seismologist quoted in a press release.
The current consensus is that a contributory cause of this swarm in human activity – specifically, wastewater disposal as part of the process of energy exploitation – fracking.
Although in some areas such activity does not appear to generate seismic activity (there are many factors influencing seismicity, including local geology and depth of water injection) the pattern of Oklahoma earthquakes is sufficient to raise concerns.
The largest earthquake in the series to date is the M5.6 of November 2011. Research from the USGS, published in June 2014, considers that this earthquake may have been triggered by a previous, human-induced, event increasing stresses on a section of the fault where it occurred. Previously, the largest tremor to strike the state was an M5.5 in 1952.
Oklahoma Seismic Hazard
Recent seismicity maps produced by the USGS indicate that seismic hazard in Oklahoma has increased – and that residents should expect, and prepare for, further earthquakes in the central and northern areas of the state.
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