US earthquakes linked to oil & gas drilling

Mar 12, 2014 12:00 AM

A series of earthquakes that shook the US state of Oklahoma in November 2011 are being attributed to the disposal of wastewater from nearby oil & gas drilling sites, according to a study from the US Geological Survey.

News agencies are reporting that a magnitude 5.0 earthquake on November 5th which has already been linked to oil and gas wastewater disposal nearby, likely caused another magnitude 5.7 quake the next day. That, in turn, caused "thousands of aftershocks", the USGS said in a statement.

The report adds to previous studies suggesting that pumping millions of gallons of drilling fluids underground puts stress on fault lines and can cause earthquakes.

While the USGS study concentrates on wastewater, it also draws attention to the oil and gas production technique known as fracking, which involves pumping a mix of chemicals, water and sand deep underground to release oil and gas. Much of that water flows back to the surface and is disposed of in caverns.

The USGS said the 5.7 magnitude quake was the largest earthquake associated with wastewater injection, but said that the earthquakes have not been directly linked to fracking.

"The observation that a human-induced earthquake can trigger a cascade of earthquakes, including a larger one, has important implications for reducing the seismic risk from wastewater injection," Elizabeth Cochran, USGS seismologist and coauthor of a study on the earthquakes, said.

Historically, earthquakes in the central United States have been uncommon. Yet in the year 2011 alone, numerous moderate-size earthquakes occurred in Colorado, Texas, Oklahoma, Ohio and Arkansas, the USGS said.

Oklahoma, the nation's fifth-largest oil-producing state, recorded 238 earthquakes through Nov. 18 last year. More than 100 of those were at least a magnitude 3.0, tremors large enough to be felt.

From 1991 to 2008 there were no more than three quakes a year of that size in the state.

Many of these earthquakes occurred near waste-water injection wells, and some have been shown to be caused by human activities, the USGS said.

In August, five Arkansas residents who sued oil companies Chesapeake Energy Corp and BHP Billiton Ltd , claiming wastewater disposal wells from fracking caused earthquakes that damaged their homes, settled with the companies for an undisclosed sum.

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