The risk of a large earthquake striking Oklahoma next year — and rattling buildings in Wichita — has decreased, according to a study published last week in Science Advances.
But it will still be more than 50 times higher than it would be without wastewater injections from the oil industry, according to the study by one of the world’s leading researchers on man-made earthquakes, Mark Zoback of Stanford University.
Before 2009, a large earthquake of 5.0 magnitude would strike Oklahoma about once every 150 years, with less than a 1 percent chance in any single year. But in 2017 there will be about a 37 percent chance of an earthquake of that size.
In November, a 5.0-magnitude earthquake struck Cushing, Okla., damaging dozens of buildings and knocking over brick walls and chimneys. It was felt about 145 miles away in Wichita.
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The earthquake risk was at its peak in 2015 when it was 110 times above normal, according to the study, because of how much wastewater from fracking was being injected into the ground then.
Since then the price of oil has fallen and Oklahoma has started to regulate injections more aggressively, so the number of earthquakes has fallen from about three per day to about two, and the study predicts the decrease will continue.
The risk of a large earthquake will linger for several years after the current reductions in wastewater, according to the study.
The model assumes that oil companies can pump a certain amount of water into the ground before the pressure rises enough in the deep basement rock to cause earthquakes.
Oklahoma has been pumping wastewater above this earthquake “triggering threshold” for several years, starting in 2012 and continuing through the end of 2015.
The “triggering threshold,” according to the study, is the minimum level of wastewater that will increase underground water pressure sufficiently to cause already existing cracks in basement rock to slip and cause earthquakes.
These predictions are part of a new model that the authors believe can be used to predict increases and decreases of earthquake activity based on how much wastewater is being injected into the ground.
Two recent earthquakes, however, don’t fit the new model, and those are the two largest quakes ever recorded in the area.
The 5.6-magnitude earthquake that struck near Prague, Okla., in 2011 would not have been predicted by the new model, because it occurred before some of the largest increases in wastewater started in 2012. This could be because the Prague earthquake was natural. Or it could be because of a limitation in how much scientists know.
The locations of most earthquake faults in Oklahoma are nearly impossible to determine, according to Cornelius Langenbruch, also of Stanford, a coauthor of the study. So even if the water pressure underground has been lowered overall, an increase in one small area could still trigger a large earthquake.
The 5.8-magnitude earthquake that struck near Pawnee, Okla., in September was seven times more powerful than the new model predicted for 2016. But over the course of the past five years, the model predicted a 25 percent chance of such a large earthquake striking, so it wasn’t totally unexpected. Without any oil activity, there would be a 5.8 earthquake in Oklahoma every 1,900 years, according to the study.
Zoback’s research is frequently cited by the Oklahoma Geological Survey and the Oklahoma Corporation Commission.