Cluster of seismic events strike shale-rich Oklahoma

A cluster of seismic events in shale-rich Oklahoma culminated with a magnitude-4.5 quake early Wednesday morning, the USGS reported.

Five seismic events were recorded over the last 24 hours by the U.S. Geological Survey. The strongest of the events occurred around 4:30 a.m. local time about 8 miles southeast of Pawnee, Okla. A smaller tremor was recorded by the USGS a few hours later.

On Sept. 3, Pawnee was the center of a magnitude-5.6 quake, which was felt across much of the region.

One of the U.S. states with a significant amount of shale oil and natural gas, a study from the USGS found the disposal of oil and gas-related wastewater is the "primary reason" for an increase in seismic activity in central states like Oklahoma. That process is different from hydraulic fracturing.

In response to the Sept. 3 quake, the Oklahoma Corporation Commission's Oil and Gas Division said it was nearly doubling the size of a so-called area of concern. Combined with areas under the authority of the federal Environmental Protection Agency, the state government said it also ordered the closure of 32 wells.

According to the commission, wastewater injected into the so-called Arbuckle formation near Pawnee "poses the largest potential risk for earthquakes in Oklahoma." A special order from the commission related to the area of interest translates to a volume reduction of wastewater injections for wells under state jurisdiction of 40,000 barrels per day. The order is mandatory and relates in large part to new fault data coming out of the Oklahoma Geological Survey and the USGS.

Oklahoma Gov. Mary Fallin issue a declaration of a state of emergency for the county impacted by the September quake, but has so far not responded with an official statement on the latest outbreak.

Oklahoma is one of the most significant producers of crude oil in the United States, accounting for about 4 percent of the nation's total. It hosts some of the largest deposits of shale oil in the country and the trading hub in Cushing is considered the most significant entity of its kind in North America.