For 30 years geologists have been puzzled by a remarkably straight magnetic line that runs between New York and Alabama along the Appalachians.
A more recent aerial magnetic survey of the Alabama end of the line suggests that it's probably a 500-million-year-old San Andreas-style fault that appears to have slipped 137 miles to the right in the distant past.
If so, it's no surprise that the most dangerous part of the eastern Tennessee seismic zone is right next to part of this magnetic line and has the second-highest earthquake frequency in the eastern United States.
"It's most likely a strike-slip fault," said Mark Steltenpohl of the University of Alabama at Auburn. “But it's all buried.”
The fault is invisible from the surface and there is very little information about it because no one has actually drilled down through it to investigate, Steltenpohl told Discovery News.
That would, in fact, be pretty hard to do, since the fault zone is very narrow and it would be hard to find with a drill using just magnetic maps to set up a drill rig.
“It's almost a needle in a haystack,” said Steltenpohl.
Both steep and deep
The New York-Alabama Lineament, as geologists call it, was first revealed by aerial magnetic mapping in 1978. Since then people have looked at smaller sections of it to try and understand it, with little success. Seismic surveys across the feature indicated it is very steep and runs very deep.
“It's been sort of enigmatic,” said U.S. Geological Survey geologist Wright Horton, a co-author with Steltenpohl on a paper in the June issue of the journal Geology about the fault.
The key to seeing it as a strike-slip fault is detecting features that are cut off by the fault and offset. Those sorts of offsets were finally found in maps from a 2002 aerial magnetic survey of the Alabama part of the lineament, said Horton.
“Once we got the south end of it pinned down, the rest of it fell into place,” Horton said.
Likely not active
The fact that the fault has not cut through the layers of earth above it and shown itself on the Earth's present surface suggests it's not active and so people can probably rest easy.
However, the fault and fractures related to it — like the probably similarly-ancient faults of eastern Tennessee — are not incapable of quakes. In fact they are perfect places for stresses in the crust to be released, so long as they are weakened by water, explained geophysicist John Costain of Virginia Tech.
“If the lineament is there, then you're sure to get earthquakes more than otherwise,” said Costain.
That's because faults, however ancient can serve as conduits for water that weakens fault zones and can cause regional stresses in the crust to be relieved as an earthquake.
This is, in fact, the likely secret to how all big and small mid-continent quakes can happen, so far from the more active and obvious zones where tectonic plates are smashing together, he explained.
“The crust is full of fluids and looking for an excuse to break,” said Costain.
The New York-Alabama Lineament is one more place where that can happen.