A study by the U.S. Geological Survey has concluded that underground disposal of waste from hydraulic fracturing, or "fracking," has caused earthquakes in Colorado, the Denver Post has reported.

That finding, which is still under review by Colorado authorities, adds to previous assertions that such injections -- which are common in Kern County -- can trigger seismic activity.

The petroleum industry has disputed such claims, saying the decades-old process is safe and that there is no evidence of seismic activity induced by fracking.

Fracking injects sand, large quantities of water and small concentrations of sometimes toxic chemicals underground to free up deposits of oil and gas.

But fracking is not the primary issue in the USGS study. Rather, it focused on "flowback," the fluid that comes to the surface after fracking.

Although some companies recycle part or all of their flowback, the more common practice is to dispose of it underground by injecting it at high pressure into a zone other than where the fracking was done.

Here's a link to the Denver Post story:

http://www.denverpost.com/environment/ci_22118583/colo-officials-question-link-fracking-waste-disposal-quakes

-- John Cox