Earthquakes caused by human activity will now be included in the US Geological Survey's seismic risk maps, the agency says after a sharp rise in tremblers linked to wastewater disposal wells used by the oil and gas industry in Oklahoma.
The seismic risk maps are used by emergency management officials as well as the country's major engineering and design associations to guide how strong to construct buildings.
"By including human-induced events, our assessment of earthquake hazards has significantly increased in parts of the US," Mark Petersen, chief of the USGS National Seismic Hazard Mapping Project, said in a statement.
About seven million people in the central and eastern United States live or work in areas threatened by so-called induced seismicity, and in parts of these regions, the damage caused by earthquakes could be at parity with that seen in high-hazard regions of California, the USGS said.
Oklahoma is at the greatest risk for hazards associated with induced seismicity, the USGS said, followed by Kansas, Texas, Colorado, New Mexico and Arkansas.
Oklahoma in 2015 experienced 907 magnitude-3.0 or greater earthquakes, compared with just two of similar size in 2009. In February, a magnitude-5.1 tremblor shook the area around Fairview, Oklahoma - the third strongest recorded in the state.
The uptick in quakes has prompted serious concern among those near the oil storage hub at Cushing, Oklahoma, home to about 66 million barrels of oil and the delivery point for the West Texas Intermediate futures contract.
"We have had some earthquakes that were way to close to those tanks," said Michael Teague, Oklahoma's Secretary of Energy and Environment.
The disposal of saltwater - a natural byproduct of oil and gas drilling - into wells has been tied to earthquakes. Oklahoma regulators have already ordered many disposal wells to curb operations.
"The good news is that we are already seeing a very positive response to those actions in the form of reduced seismic activity in the central and north central areas of Oklahoma," the Oklahoma Oil & Gas Association said in a statement.
Saltwater injections into disposal wells is down by roughly half from a peak of 1.8 million barrels per day in 2014, the Oklahoma Geological Survey said, with half the wells shutting due to low oil prices.
The USGS said building code committees are still deciding whether to include induced earthquakes in their revisions, in part because they could be temporary.
The American Society of Civil Engineers' 2016 guidelines do not take into account man-made earthquakes and it does not plan to update them until 2022.
"There is always a delay in design codes adapting the USGS Seismic Hazard Maps," said Muralee Muraleetharan, a civil engineering professor at the University of Oklahoma.