Seismic Events: U.S. Midwest – ‘There’s something happenin’ here, and what it is ain’t exactly clear..’
For Updates go to: CERI – Recent Central and Eastern U.S. Earthquake Map (interactive);
The earthquake that rattled at least seven mid-western states on Saturday morning was a stark reminder that a new type of seismic hazard exists, which is associated with human activity. The temblor with a magnitude of 5.6 was located near the town of Pawnee, about 75 miles north-northeast of Oklahoma City. Although the quake caused only minor damage, it shook an area of the United States which – until a few years ago – was considered basically free of any significant seismic activity. But ever since an earthquake of the same magnitude struck a region immediately east of Oklahoma City in November 2011, it has become clear that a sizable seismic hazard, widely ignored until then, lurks under the oil producing states in the Midwest.
When seismologists started to study the enormous increase in earthquake activity in Oklahoma, it very quickly became clear that it was not caused by natural phenomena. Instead, the cause was human activity. The first suspect was fracking. Since the beginning of this century the oil and gas fields in Oklahoma, Kansas, Texas and elsewhere have seen a boom in this procedure, which helps improve the extraction of hydrocarbons from underground. Geologic formations are broken and cracked by injecting water under high pressure from the wellhead. Because such cracking of rocks is a process very similar to the generation of earthquakes in nature, it seemed plausible to blame the sudden increase in the seismicity rate on fracking. However, the temblors resulting from fracking are way too small to be felt by humans. Bill Ellsworth, a former seismologist at the USGS now at Stanford, summarized in the journal “Science” in 2013: “More than 100,000 wells have been subjected to fracking in recent years, and the largest induced earthquake was magnitude 3.6, which is too small to pose a serious risk.”
The real culprit, it turned out, also had something to do with oil and gas production. Virtually every hydrocarbon field in the world contains a significant amount of water, which is pumped to the surface together with the oil. In the United States alone, about 21 billion barrels of this so called “produced water” are generated each year from about 900,000 wells. This is equivalent to a volume of 2.4 billion gallons per day. Because this produced water contains salts, remnants of the oil and many other organic and inorganic chemicals, it cannot be flushed into regular wastewater treatment plants or even released untreated into rivers or creeks. It is simply too hazardous. To dispose of this water, it is pumped back into the ground, usually in injection wells deep below the groundwater aquifers and the oil producing formations. There are more than half a million such injection wells in the US, most of them in Texas, California, Oklahoma and Kansas.
Berkely Seismo Blog
Oklahoma Geological Survey – Mapping
Seismicity Map of Oklahoma
Seismic Hazard Map of Oklahoma
Animation of Oklahoma Seismicity
Animation of M5.6 Foreshocks and Aftershocks-Nov 5, 2011 to July 31, 2012