Stein says this week's fore shock and main shock were too strong to attribute them to oil and gas extraction. Others note that this week's quakes have appeared at depths significantly deeper than deposits where fracking takes place.
The area hit by this week's quakes doesn't show up as vulnerable on the US Geological Survey's hazard map for the state. The vulnerable area sits in southwestern Oklahoma, along the Meers fault, the only fault in the state for which geologists have found evidence at the surface. Its last significant break occurred about 1,300 years ago in a quake estimated at magnitude 6.5 or 7.
Another case of migration? If so, it could fit a pattern Stein and other researchers posit after studying the New Madrid fault zone, centered under southeastern Missouri. That fault was responsible for a set of four intense quakes between 1811 and 1812. Each quake has been estimated to have reached at least magnitude 7, and perhaps as high as magnitude 8.
It has been shuddering at weaker levels ever since.
Stein and University of Missouri geophysicist Mian Liu have studied aftershock patterns from earthquakes along plate boundaries, in the wider regions around such boundaries, and in mid-continent.
Quakes along plate boundaries appear to have the shortest periods of aftershocks, on the order of 15 years. Mid-continent quakes appear to have the longest period of aftershocks – 200 years and counting.
Meanwhile, nearly a decade of GPS measurements around the region show that any "reloading" of the New Madrid fault is happening at a glacial pace. The crust is deforming at a pace of roughly half the width of a human hair per year. That's all that plate tectonics seems to be contributing to the stresses on the fault, compared with tens of millimeters a year along a plate boundary such as the San Andreas fault.
On the basis of those measurements, it's hard to see how the New Madrid fault stands a 25 to 40 percent chance of a magnitude 6 quake or stronger over the next 50 years, the researchers say.
These observations have led Stein, Purdue University geophysicist Eric Calais, and colleagues to propose that many quakes taking place in mid-continent may be aftershocks of large quakes that happened hundreds of years ago.
And if the contributions of plate tectonics to a rise in stress are as small as has been measured in the New Madrid area, geophysicists may have to look for other sources of stress.
In the northeastern quarter of the US, for instance, the crust is still heaving a sigh of geophysical relief at losing a two-mile-think layer of ice that covered Canada and the US northern tier during the last Ice Age. That seems to be more influential in setting up earthquake activity in that region than tectonics alone.
South and west of an arc stretching from Virginia into Illinois and the Dakotas, the stress buildup slows dramatically. This leads the team to suggest that the general motions of the continental plate alone don't account for the quake activity in the rest of the mid-continent, and that other factors need to be sought.