Human-induced earthquakes could affect about 3.5 million people this year, according to a U.S. Geological Survey report published Wednesday that cites wastewater injections as the primary cause.
The report shows about half the number of people are expected to be affected by human-induced seismic risk this year, compared to projections for 2016, when the USGS estimated about 7 million people would be affected.
Almost all of those affected are projected to be in Oklahoma and southern Kansas, which have seen a sharp spike in earthquakes in recent years due to natural gas companies injecting wastewater deep underground into wells, according to USGS.
Hydraulic fracturing, which some environmentalists say has led to an increase in seismic activity, “is only rarely the cause of felt earthquakes,” the report says. USGS has previously said fracking can cause “microearthquakes.”
Texas and Arkansas last year experienced less seismic activity than expected, partly because of reduced natural gas production and therefore less wastewater injection, Mark Petersen, chief of the USGS National Seismic Hazard Mapping Project, said in a conference call with reporters. That decrease led the agency to project less seismic activity in those areas for this year than it did in its 2016 report, he said.
Oklahoma experienced about 2,500 earthquakes of at least 2.7 magnitude in 2014, followed by 4,000 in 2015 and 2,500 last year after experiencing an average of about two per year between 1980 and 2000, according to the report.
In September, state officials in Oklahoma ordered the shutdown 37 wastewater wells after a 5.8-magnitude earthquake prompted Gov. Mary Fallin (R) to issue a state of emergency.
Despite an expected drop in seismic activity for this year, the risk is “hundreds of times higher than before the man-made activity began,” Petersen said.
Still, the anticipated decrease in activity in human-caused seismic activity is a sign that the natural gas industry and regulators have taken positive steps to reduce risk, he added.
“I feel this is a success story in how we can reduce the seismic hazard,” Petersen said.
Other than Oklahoma and Kansas, the report projects significant naturally occurring seismic activity in California and near Seattle, but the risk level is not greatly affected by human activity there.