The number of earthquakes rattling the U.S. shale patch is growing exponentially as producers pump massive amounts of dirty water from their oil and gas wells back underground. Tremors registering at least a 2 on the Richter scale quadrupled from 2017 levels to a record 938 last year and are on pace to top that this year, according to a Rystad Energy analysis of data in Oklahoma, Texas, Louisiana and New Mexico. Though earthquakes have long been linked to activity related to shale production, the report provides more evidence of the connection in the U.S. Southwest, where oil drilling has intensified in the past decade.

Last year’s jump in earthquakes came even as production tumbled amid the pandemic and the amount of wastewater injected into disposal wells slipped. But water disposal — the biggest culprit behind earthquakes in oil and gas fields — climbed sharply from 2011 through 2019 to more than 12 billion barrels, creating conditions that can make the ground more unstable overall.

Earth Shaking

Seismic activity greater than 2.0 magnitude on the rise in U.S. southwest

Source: Rystad

Note: 2021 data is through first five months

The increased tremors and huge volumes of wastewater have added to environmental concerns surrounding oil and gas production from shale fields. Drillers have come under increasing scrutiny in recent years, with companies under pressure from investors to disclose climate risks and reduce greenhouse gas emissions.

“Earthquakes are not the only environmental issue caused by water disposal,” Ryan Hassler, an analyst at Rystad, said Thursday in a report. “Fresh water sourcing in arid regions of West Texas and New Mexico threaten the water supply of local communities and essential agriculture activities, while environmental concerns surrounding the chemical composition of produced water serve only to fan the flames of public antipathy.”

Tremors in shale fields can also be caused by fracking or blasting apart rock to release oil and gas. Most of those earthquakes, though, are imperceptible. The real danger is that if the oil field is situated along a geological fault, the fractures can cause a chain reaction and trigger a bigger seismic event.

To maintain water disposal at 2020 levels, the industry has to treat and recycle it, which could cost producers more than $1 billion a year, Hassler said.