In another sign that the US shale gas boom may be contributing to higher seismic activity, regulators in the state of Colorado have said they believe the disposal of oil and gas wastewater at a well in Weld County may have caused a series of small earthquakes this year.

The Colorado Oil and Gas Conservation Commission (COGCC) halted activity at a wastewater disposal site owned by NGL Water Solutions on June 23 after a 3.2 magnitude quake on May 31 was followed by other smaller quakes in the area. COGCC is investigating whether the company exceeded permitted injection volumes.

Limited operations have been allowed to continue at the disposal site after NGL sealed off what the COGCC described as a "preferential pathway" for the wastewater to flow from the bottom of the well into basement rock beneath.

COGCC will continue to monitor the Weld County well and will halt work if another seismic event above 2.5 occurs.