Researchers say injecting wastewater deep underground, a common method of disposing of the water produced from natural gas wells, caused the dramatic rise in earthquakes in Colorado and New Mexico since 2001.

A new study published in the Bulletin of the Seismological Society of America says there have been 16 earthquakes of 3.8 magnitude or greater in those two states since 2001. There was only one earthquake of that size in the previous 30 years.

“The increase in earthquakes is limited to the area of industrial activity and within 5 kilometers (3.1 miles) of wastewater injection wells,” the study says.

There are 21 high-volume wastewater injection wells in Colorado and seven in New Mexico. Since mid-2000, the amount of wastewater pushed into the wells each month has ranged from 1.5 million barrels to 3.6 million barrels.

Wastewater is produced along with oil and natural gas. In Colorado and New Mexico that water is separated out and then injected underground.