Heather Savage was struck by the date of the Oklahoma earthquake -- Feb. 27, 2010.
At magnitude 4.1, it wasn't all that big or damaging. But it was the same day as one of the largest earthquakes ever recorded, a magnitude-8.8 rupture that killed more than 500 people and wreaked havoc thousands of miles south in Chile.
"They were only about 16 hours apart," said Savage, a geophysicist at Columbia University's Lamont-Doherty Earth Observatory.
Savage and her fellow researchers at Lamont-Doherty now believe a "seismic wave" from that megaquake set off the smaller quake near Prague, Okla.
And they believe the rumbling that day in Oklahoma foretold a larger, magnitude-5.7 earthquake that occurred a year and a half later in nearly the same place. The biggest earthquake recorded in Oklahoma, it injured two and damaged more than 200 homes and businesses.
Researchers believe the November 2011 quake and its precursor, along with dozens of foreshocks and aftershocks, occurred because of deep injection of wastewater from oil and gas drilling. More than 15 years of deep injection had built up pressure in a little-known fault, causing it to rupture.
And that's not the only place it has happened. In a study released yesterday, the Lamont-Doherty team reported that "seismic waves" rippling from megaquakes halfway across the globe can set off spates of minor quakes near industrial injection sites. Sometimes, those minor quakes have been followed by larger, damaging quakes in the same area linked to injection.
They haven't found a way to predict such quakes at this point, but they say they're seeing intriguing clues.
"It acts as something of a stress test," said Lamont-Doherty researcher Nicholas van der Elst, lead author of the study.
The study focuses on quakes concentrated around Trinidad, Colo., and Snyder, Texas, that have been linked to deep injection of underground drilling wastewater, in addition to the one in Oklahoma.
In southern Colorado, the 2010 earthquake in Chile triggered a "small, but statistically significant" spurt of four earthquakes in one day near underground injection of water produced in coalbed methane drilling. A year and a half later, in August 2011, the area experienced a "seismic swarm" that included a magnitude-5.3 quake felt as far away as Nebraska.
The massive, magnitude-9 earthquake off the coast of Japan in March 2011 led to the Fukushima Daiichi nuclear meltdown. But the study says it also appears to have set off a series of small earthquakes in the Cogdell oil field near Snyder, Texas. The same area experienced a "seismic swarm" six months later, topped off by a magnitude-4.5 quake.
And waves from a magnitude-8.6 earthquake in Sumatra in April 2012 appear to have triggered small upticks in seismic activity near the previous earthquakes in Oklahoma and Colorado.
But there are other areas with drilling linked earthquakes, such as Youngstown, Ohio, and Guy, Ark., that experienced no such warning quakes before full-scale seismic ruptures. The researchers think that the pressure in those faults built up more quickly and ruptured before any distant mega-quakes could affect them.
Geologists have known for decades that deep injection of industrial waste can lubricate faults and unleash earthquakes. One of the most famous instances of man-made earthquakes, or "induced seismicity," occurred in the late 1960s at the Rocky Mountain Arsenal near Denver, where the Army manufactured chemical weapons.
Some earthquake researchers now say the nation's drilling boom, fueled by advances in high-volume hydraulic fracturing, could be spurring a rash of such man-made quakes. The quakes in Oklahoma and Colorado have been cited as prime examples of this trend. But state officials have dismissed the researchers' findings.
Most seismologists agree that the specific process of hydraulic fracturing presents little or no risk of triggering earthquakes that damage property or injure people.
But fracking and other oil and gas production processes produce millions of gallons of wastewater. The fluid is far saltier than seawater and is laced with toxic and even radioactive chemicals. Some can be reused, but eventually, what comes to the surface must be disposed of. Usually, that's into injection wells.
Not long after van der Elst started at Lamont-Doherty nine months ago, Savage stepped into his office and brought up the connection between the quakes in Chile and Oklahoma.
"That was the flag that caught our attention," van der Elst said.
Both had a keen interest in "remote triggering" -- waves from big earthquakes setting off smaller ones far away. Van der Elst had done his doctoral dissertation on the topic.
Savage had been studying the phenomenon for years. And she was working on another study, released in March, asserting the connection between drilling activities and the Oklahoma earthquake (EnergyWire, March 27). They joined with the leader of that study, University of Oklahoma professor Katie Keranen (now at Cornell University) and another Lamont-Doherty scientist, Geoffrey Abers, to examine the connections.
Remote triggering was more commonly understood to occur in natural settings such as Old Faithful in Yellowstone and geysers in California. Tying it to man-made quakes was something new.
Now that the trend has been identified, the question is whether the next megaquake will set off small quakes in faults filling up with drilling wastewater, and whether that might predict another, larger quake.
"We're going to be looking very closely at that," van der Elst said. "That will be very powerful confirmation or contradiction of this idea."