According to the latest report from the National Academies of Science, the risk of earthquakes from hydrofracking is low. There is a potential for induced seismicity - that's what they call man-made quakes - any time humans inject fluid into the earth.
In the past year fracking has been pointed to as the cause of two quakes - one in Oklahoma (measuring 2.8) and another in England (2.3). But injecting shale gas wastes into injection wells has caused hundreds of tremors. According to the report, California and Oklahoma feel most of these quakes, though Arkansas gets its fair share. And Colorado has measured three injection well-caused quakes at 5.0 to 5.5 - enough to rattle the china and shake you out of bed.
It's not just injection wells, either. Northern California has recorded anywhere from 300 to 400 small quakes a year (since 2005) due to geothermal energy extraction.
Injecting fluids deep, and under pressure, can trigger tremors because it changes the pressure of the soil and rock - pressure that keeps faults from moving. However, those technologies that are designed to keep a balance between the amount of fluid being injected and withdrawn appear to produce fewer tremors, says the report. Injection wells, on the other hand, don't maintain a fluid balance because nothing is removed.
Earlier this spring USGS geophysicist William Ellsworth and his research team presented data showing a spike in earthquakes near oil and gas drilling operations. The number of earthquakes last year was a sixfold increase over previous levels.
So while fracking shale to extract gas may cause only the occasional tremor, it's getting rid of the millions of gallons of waste produced by unconventional gas extraction that's the real problem.