States where hydraulic fracturing is taking place are experiencing unprecedented earthquake activity.
The drilling method involves sending high-pressure blasts of water, sand and chemicals into deep shale rock formations to free up oil and gas. The process produces millions of gallons of wastewater which operators send underground into injection wells.
Scientists are researching whether these injections are causing quakes.
A study published earlier this month in the journal Science suggests that just four injection wells in Oklahoma have caused about 20 percent of the quakes from the eastern border of Colorado to the Atlantic coast since 2008.
However, seismologists and the oil and gas industry stress that the number of earthquakes tied to injection wells is small relative to the thousands of wells in operation in recent decades.
Explore further: Expert: Wastewater well in Ohio triggered quakes