A decade ago, there were typically 20 earthquakes a year that were large enough to feel in the central and eastern U.S. But in 2015, there were over 1,000 of them.

Why? It’s mostly because we’re pumping more water into the ground.

The boom in U.S. oil and gas production over the last decade has brought many more oil wells —which also produce water.

Most is naturally occurring in the formation and some was injected by operators to allow or improve the recovery of oil and gas.

In both cases, the water will likely have picked up salt and other minerals from the rock, making it many times saltier than seawater.

Operators may reinject this water to continue to liberate oil and gas.

But more often, there’s too much to handle. So it’s trucked or piped to disposal wells, where it’s pumped down into deep saltwater reservoirs.

Adding large volumes of wastewater increases the pressure in these rock formations—which can allow natural faults to slip more easily than they normally would, causing earthquakes.

To address these quakes, regulators and the petroleum industry are monitoring disposal wells and shutting down those that could cause damaging seismic activity. And they now think that managing wastewater injection more carefully should help.

There’s still more work to be done, and university research centers like the Bureau of Economic Geology are conducting major studies, with the aim of minimizing the risk of earthquakes while maintaining the benefits of domestic energy production.