The total number of earthquakes occurring in central and eastern parts of the United States fell last year, and federal scientists are predicting fewer foundation-shaking quakes for 2017 as well, though people living in at-risk areas may experience some stronger shocks.
In a new report published Wednesday, the U.S. Geological Survey said that nearly 3.5 million people live and work in areas where there is a significant risk of earthquakes triggered by wastewater disposal from hydraulic fracturing and oil drilling practices. The forecast is an improvement over last year when the agency reported that 7 million Americans were at risk—and it's a sign that regulations on how much wastewater can be injected into the ground are starting to work.
"This is great news," Mark Petersen chief of the USGS National Seismic Hazard Mapping Project said in a press conference on Wednesday. "These reductions indicate that this type of seismicity is manageable or controllable."
Wastewater disposal is the primary cause of human-induced seismic activity in oil country. Fracking operations produce a lot of briny wastewater that is disposed of by injecting it into deep underground wells. The pressure from injected fluids can weaken faults, causing them to slip and the ground to shake.
Last year, the USGS noted that Oklahoma had become a hotspot for such manmade earthquakes. The overall rate of earthquakes in the state was a hundred times higher than what it was before oil and gas production boomed. Between 1980 and 2000, Oklahoma averaged about two earthquakes a year with a magnitude of 2.7 or higher on the Richter scale.
In 2008, Petersen said, Oklahoma started seeing an uptick in quake frequency, and by 2014, the number skyrocketed to about 2,500 earthquakes each year. In 2015, it increased again, to 4,000, but dropped back down to 2,500 in 2016. Scientists believe that the decrease may be partly due to regulations, implemented over the past two years, that have limited the amount of water than can be pumped into the underlying sedimentary rock. Less artificially inject water in the ground probably placed less pressure on vulnerable faults. Last week, the Oklahoma Corporation Commission added new restrictions that might help alleviate quake risk even further by allowing companies to distribute water over multiple wells, further reducing the pressure on quake-prone hotspots.
But despite the decrease in the overall number of earthquakes, Oklahoma has experienced quakes that were stronger in magnitude, Petersen said. Of the earthquakes last year, 21 were greater than magnitude 4.0 and three were greater than magnitude 5.0. In fact, the 5.8-magnitude quake that rocked Pawnee, Oklahoma, in September was the state's largest ever recorded.
This year, large portions of Oklahoma still have the highest risk for damaging quakes—on par with parts of central California. And Petersen said scientists don't know if Oklahoma will ever get back to pre-2008 activity levels of less than one quake a day.
Predicting earthquakes is a complex task. While the USGS forecast may not be able to pinpoint specific sizes or locations of quakes, its maps are useful for anticipating where future earthquakes will occur, how often they will occur, and how extensive the resulting damage may be. Petersen said the annual earthquake hazard report has been used by engineers to evaluate earthquake safety of buildings, bridges, and pipelines. Risk modelers have used the data in developing new risk assessments and calculating insurance premiums, and the U.S. Army Corps of Engineers has used the information for safety assessments.
But the work's not done yet, according to Petersen. Studying the differences between natural and manmade quakes, and incorporating additional data from industry about wastewater injection sites and volumes will help scientists make the prediction model more accurate, he said. Of course, if increasing oil prices lead to more drilling and more wastewater injection, the ground could still get shakier in the future. "There's still a lot of concern in Oklahoma," he said.