Earthquakes in Oklahoma? Rodgers and Hammerstein didn't see that one coming.
Indeed, this week's temblors in the Sooner State highlight the challenge scientists face as they try to improve earthquake-hazard assessments in the central and eastern United States, particularly across the lower half of the country.
In regions known for relatively frequent, large quakes, such as the west coasts of North and South America or deep in the heart of Turkey, sources of stress on faults are well known. And the faults themselves are increasingly well-studied, allowing scientists to estimate repeat rates for major temblors along these shifting cracks in Earth's crust.
In the middle of the continental US, however, research over the past decade suggests that trying to estimate future quake activity may be more like a high-stakes game of "Whac-A-Mole."
One fault system might generate a cluster of quakes over a period of a few years, then it delivers diminishing set of aftershocks for centuries while stress migrates to a new fault system, explains Seth Stein, a geophysicist at Northwestern University in Evanston, Ill.
The past of any one fault system may not be a prologue to its future. Trying to divine that future may represent "an exercise in closing the barn door after the horse has gone," he says.
Oklahoma sits squarely in the portion of the country that appears most susceptible to this wandering seismic activity, some studies suggest.
That's likely to be of little comfort to people rattled by this week's quakes. The first shock struck central Oklahoma early Saturday morning, with a magnitude of 4.7. This turned out to be a fore shock in advance of a magnitude 5.6 quake that struck the same fault Sunday night. The area has experienced a series of aftershocks, including another magnitude 4.7 quake Monday night. A magnitude 3.6 aftershock hit early Tuesday afternoon.
Throughout 2009 and 2010, the state has experienced unusually high earthquake activity, although the activity may be typical for the state when viewed over long periods of time, according to the state geological-survey office in Leonard.
No one has a good handle on why the activity has increased. This week's quakes may well be part of this "swarm," Dr. Stein says.
Some residents are asking whether oil and gas extraction in the state may have triggered the quakes, particularly via the practice of hydraulic fracturing, or "fracking." This involves pumping fluids into the ground to force more oil or gas out. Fluids can in effect lubricate faults, reducing the friction that may be holding back an earthquake.
Several studies in the US and overseas have found evidence that fracking and other techniques for injecting fluids into oil and gas formations have triggered small quakes, with varying degrees of certainty.
In August, for instance, researchers with Oklahoma's geological survey looked into complaints that fracking had triggered a series of small earthquakes ranging in magnitude from 1 to 2.8 south of Elmore City.
After sorting through the evidence, the survey concluded that while it was possible fracking triggered the quakes, the data available weren't good enough to support "a high degree of certainty" that fracking was the culprit.
Studies of fracking activities in England have come to stronger conclusions, but the magnitude of the quakes were similarly small, although strong enough for people to feel them.