A sharp earthquake in central Oklahoma last weekend has raised fresh concern about the security of a vast crude oil storage complex, close to the quake's center, that sits at the crossroads of the nation's oil pipeline network.
The magnitude 4.5 quake struck Saturday afternoon about 3 miles northwest of Cushing, roughly midway between Oklahoma City and Tulsa. The town of about 8,000 people is home to the so-called Cushing Hub, a sprawling tank farm that is among the largest oil storage facilities in the world.
Scientists reported in a paper published online last month that a large earthquake near the storage hub "could seriously damage storage tanks and pipelines." Saturday's quake continues a worrisome pattern of moderate quakes, suggesting that a large earthquake is more than a passing concern, the lead author of that study, Daniel McNamara, said in an interview.
"When we see these fault systems producing multiple magnitude 4s, we start to get concerned that it could knock into higher magnitudes," he said. "Given the number of magnitude 4s here, it's a high concern."
The federal government has designated the hub, run by energy industry companies, a critical national infrastructure. Major tank ruptures could cause serious environmental damage, raise the risk of fire and other disasters and disrupt the flow of oil to refineries nationwide, said McNamara, a research geophysicist at the National Earthquake Information Center in Colorado.
The Cushing quake is among the largest of thousands of temblors that have rocked central and northern Oklahoma in the past five years, largely set off by the injection of oil and gas industry wastes deep into the earth. The watery wastes effectively lubricate cracks, allowing rocks under intense pressure to slip past one another, causing quakes.
The tens of millions of barrels of injected wastewater have helped make Oklahoma the second most seismically active state, behind Alaska. Although quakes have damaged or destroyed buildings and roads and, in a few instances, injured people, regulators do not have the authority to seriously curb waste disposal, and politicians in a state dominated by the energy industry have made no move to give it to them.
The state had three earthquakes of magnitude 3 or greater in 2009. Last year, it had 585, and this year's total exceeds that.
Many scientists say the largest earthquake recorded in Oklahoma, a magnitude 5.7 temblor in 2011, was apparently unleashed by injected waste. Research suggests that the Cushing faults hold the potential for a quake as large as magnitude 6, McNamara said.
The Cushing oil hub stores oil piped from across North America until it is dispatched to refineries. As of last week, it held 53 million barrels of crude, said Afolabi Ogunnaike, an industry analyst at Wood Mackenzie, a natural resources analytics firm.
The earth beneath the tanks was comparatively stable until last October, when magnitude 4 and 4.3 earthquakes struck nearby in quick succession, revealing long-dormant faults beneath the complex. Three more quakes with magnitudes 4 and over have occurred within a few miles of the tanks in the past month.
The Department of Homeland Security has gauged potential earthquake dangers to the hub and concluded that a quake equivalent to the record magnitude 5.7 could significantly damage the tanks. McNamara's study concludes that recent earthquakes have increased stresses along two stretches of fault that could lead to quakes of that size.
The vice chairwoman of the state's oil and gas regulatory body, the Oklahoma Corporation Commission, said in an interview that the potential for a large earthquake in Cushing was among her biggest worries.
"It's the eye of the storm," said the vice chairwoman, Dana Murphy.
Nevertheless, Oklahoma's attempt to deal with the earthquakes this autumn faces continuing obstacles.
The government's chief seismologist, who came under oil industry pressure to minimize the quakes' origins in waste disposal, left this fall, and his successor is scheduled to depart soon. The state budget for the fiscal year that began in July slashed appropriations to the Corporation Commission by nearly 45 percent.
The commission has used its limited power over oil and gas exploration and production to persuade some companies in quake-prone areas to reduce the amount of waste they inject underground. This week, however, a Tulsa energy company filed the first challenge to those efforts, calling them arbitrary and a violation of due process. The two sides are negotiating an accord.