By Michael Wines, The New York Times, March 28, 2016
Californians have lived with the risk of a damaging earthquake for centuries. Now Oklahomans, and some Kansans, face the same threat, federal seismologists said on Monday.
In an assessment released by the United States Geological Survey, experts said the chance of a destructive temblor in the next year is as great in parts of north-central Oklahoma and southern Kansas — where oil-and-gas operations have set off man-made quakes for about five years — as it is in the shakiest parts of quake-prone California.
The warning came in the agency’s map of earthquake risks, a document that for the first time included the prospects for human-caused quakes.
“By including human-induced events, our assessment of earthquake hazards has significantly increased in parts of the U.S.,” Mark Petersen, the chief of the agency’s Natural Seismic Hazard Mapping Project, said in a news release.
Four other states where waste disposal has led to human-induced quakes — Texas, Colorado, New Mexico and Arkansas — face considerably smaller risks of damaging tremors, the agency said. About seven million people live in the areas at risk of a human-induced earthquake, most of them in Oklahoma and Texas.
Over the last 15 years, those states have experienced an explosion in oil and gas production, which releases huge amounts of toxic wastewater. That wastewater is disposed of by re-injecting it into the ground, into rock formations thousands of feet below the surface, increasing the pressure on existing subterranean faults, and causing them to slip and produce tremors.
Along with the economic boom from oil and gas exploration, Oklahoma has experienced a rising number of earthquakes. In an average year, Oklahoma has historically had fewer than two quakes of magnitude 3 or greater — roughly the level at which a tremor can be felt. Kansas has had even fewer such shocks. But last year, Oklahoma recorded 907 quakes at magnitude 3 and above, and Kansas registered 54.
Oklahoma now ranks behind only Alaska in earthquake frequency, followed by California.
Three of the quakes this year, measured at magnitudes of 4.7, 4.8 and 5.1, were among the largest in Oklahoma’s history.
Including Oklahoma and southern Kansas on the map reinforces what the Geological Survey’s scientists have said for some time: The huge number of small, human-caused quakes in the two states may have set the stage for a larger, more destructive one.
The area of greatest risk, the agency stated, is a swath of rural land along the Oklahoma-Kansas border that has been repeatedly rocked by tremors. Because the area is thinly populated, damage has so far has been limited.
The assessment said that in that area, there is a 5 to 12 percent chance of a level six earthquake on the Mercalli index of intensity, which rates the potential for damage. Level six indicates a strong earthquake that is widely felt, but that does not cause much damage. Level seven can cause moderate damage in ordinary buildings and considerable damage in lesser ones.
But the area of risk also extends through the central part of the state toward Oklahoma City, where the rate of quakes has recently dropped. Experts estimated the one-year risk there at between 5 and 10 percent.
Twenty other sites in the eastern and central United States have also experienced human-induced quakes, the agency said, but the threat of larger quakes in these areas is small, the agency said.
The Geological Survey emphasized that earthquake prediction was an uncertain science, and that some studies suggested that the maximum magnitude of human-induced quakes was less than that of natural ones.
Much of the region’s boom in oil and gas production comes from wells that employ hydraulic fracturing of shale deep in the earth, a process commonly called fracking. But the scientists said those wells likely are responsible for only a tiny share of quakes; fracking-related earthquakes typically are too small to be felt.
In both Oklahoma and Kansas, some of the tremors over the last five years have rivaled the largest in their histories.
This year, Oklahoma has recorded more than 160 quakes with a magnitude of 3 or more. But the pace appears to have slowed after the state’s oil and gas regulator, the Oklahoma Corporation Commission, effectively imposed steep reductions in underground waste disposal in February and March.
Earthquake scientists have nevertheless warned that the risk of a larger quake does not necessarily drop in lock step with a decline in smaller ones. The forces that have been loosed underground, they say, can take years to sort themselves out.
The Geological Survey report also said the chances of a damaging natural earthquake had risen in some parts of the United States. Scientists noted an increase in the number of small tremors along the New Madrid fault, near Memphis, which in the early 1800s was the site of one of the nation’s largest earthquakes. The increase in tremors there has slightly raised the prospects of a larger quake, the scientists said.