A second earthquake struck Greeley in northeastern Colorado on Monday, June 23 prompting the state’s Oil and Gas Conservation Commission to order a halt to the injection of contaminated drilling wastewater into a deep disposal well in the area.

The ban on injecting wastewater will last for 20 days as officials explore a potential link between the injection activity and the sudden jump in seismicity in the area. The most recent quake was a 2.6 magnitude temblor that hit about five miles north of Greeley at 12:27 p.m. It follows a 3.4 magnitude quake which struck the same area May 30.

Two quakes in less than a month, in an area the U.S. Geological Survey formerly called “aseismic,” has led to speculation that the temblors are “frackquakes,” seismic activity induced by the injection of drilling wastewater into deep rock formations. 

High Sierra Water Services, the company doing the injecting, was ordered to stop its activities in light of the quakes. 

It is accepted scientific fact that earthquakes are induced by fluid injection into deep geological formations. The U.S. Geological Survey has refused to rule out the possibility that such injection wells can interact with nearby faults to cause major earthquakes.

Earthquakes induced by fluid injection can have magnitudes of up to 5, and injection activity has been known to cause significant earthquakes in eastern Colorado in the past.