From The Greeley Tribune (Trenton Sperry):

As the annual number of earthquakes in the United States has increased, some have pointed to the oil and gas industry as a cause. But while scientists say there is evidence to suggest wastewater injection wells used by the industry could be linked to the increase, there is little or no evidence to suggest a similar link for fracking operations.

“Hydraulic fracturing almost never causes true earthquakes,” University of Texas seismologist Cliff Frohlich told the Associated Press in September during a gathering at West Virginia University for a National Research Council workshop. “It is the disposal of fluids that is a concern.”

Frohlich was referring to the disposal of wastewater, a byproduct of oil and natural gas production from tight shale formations and coal beds, according to the U.S. Department of the Interior’s website. Wastewater produced from many oil and gas production wells within a field may be injected through a single or just a few disposal wells, according to the website.

The question of whether oil and gas operations cause earthquakes was on the minds of Weld County residents Sunday after a 3.4-magnitude earthquake struck 4.8 miles northeast of Greeley about 9:35 p.m. Saturday night, according to the U.S. Geological Survey. The epicenter was near Weld County roads 66 and 43, which is about 3 miles northeast of Greeley.

The epicenter of the quake was about 1.5 miles from two oil and gas wastewater injection wells, both operated by High Sierra Water Services LLC of Denver, according to data from the Colorado Oil and Gas Conservation Commission. They are the only injection wells in at least a 5-mile radius of the quake’s epicenter.

Injection wells provide one of the most economical ways to dispose of wastewater, according to the USGS website, forcing the wastewater deep below aquifers that provide drinking water.

The USGS website also notes, however, wastewater injection increases the underground pore pressure, which may, in effect, lubricate nearby faults, thereby weakening them. If the pore pressure increases enough, the weakened fault will slip, releasing stored tectonic stress in the form of an earthquake. Even faults that have not moved in millions of years can be made to slip and cause an earthquake if conditions underground are appropriate, according to the USGS website.

USGS scientists have found the increase in seismicity in some locations coincides with a significant increase in the injection of wastewater into disposal wells, mostly in Colorado, Texas, Arkansas, Oklahoma and Ohio, according to the Department of the Interior’s website.

Saturday night’s quake near Greeley provided minor shaking that was felt as far south as Longmont and as far north as Fort Collins, according to the USGS website.

The 10,800-foot injection wells near the epicenter were last inspected by the Weld County Department of Public Health and Environment in October 2013, according to COGCC records. State inspectors last checked the wells in August 2012, about four months before one of the wells was completed as a wastewater injection well, according to COGCC records.

Representatives of High Sierra Water Services and the COGCC did not immediately respond to requests for comment Sunday.

The vast majority of wastewater injection wells do not cause earthquakes. According to the Department of the Interior’s website, of approximately 150,000 Class II injection wells in the United States — including roughly 40,000 wastewater disposal wells for oil and gas operations — only a tiny fraction have induced earthquakes large enough to be of concern to the public.

However, injection wells in Colorado causing earthquakes would not be without precedent. In 1961, a 12,000-foot well was drilled at the Rocky Mountain Arsenal, northeast of Denver, for disposing of waste fluids from Arsenal operations, according to the USGS. Injection began in March 1962, and an unusual series of earthquakes erupted in the area shortly after. The U.S. Army ceased use of the injection well in 1966, and in 1990 a solid link was established between the injection of fluids and the subsequent rash of earthquakes.

But Paul Earle, a seismologist with the USGS, said there’s still plenty to consider to determine if the Greeley earthquake was natural or man-made.

“Just because there are injection wells near there doesn’t necessarily mean they caused the earthquake,” Earle said. “There are a number of things you have to address to make that determination. But it’s certainly something we need to look at and will look at.”