Dr. Bill Miller, photo courtesy of UNCA.
Miller was raised in Virginia, and his Master's thesis focused on the geologic area near Mineral, Va., where yesterday's quake was centered. "The Appalachians, including the Piedmont, are loaded with faults like this," he told Xpress. "The rocks in the East don't have a lot of tectonic activity moving them around, which means that they've had time to get cemented by percolating fluids and settling and all kinds of geologic processes that glue everything together. Therefore, when you have a rupture somewhere, [the earth] transmits that vibration efficiently." And, at around 3 miles deep, it was a fairly shallow quake, "so more of the energy is exerted on the surface than if it were deep. So that's why it covered such a huge area."
The quake's epicenter was in the middle of a huge and relatively stable geologic formation known as the North American Plate, which extends many miles out into the Atlantic; but it's still subject to various kinds of earthly movement. The proof, says Miller, lies in the fact that the Appalachian mountains should have been eroded a long time ago. The big geologic upthrust that gave rise to these mountains occurred hundreds of millions of years ago, he says, and "it usually takes about 50 million years to erode such mountains completely flat. But they're still here. So that leads some people to conclude there must be some minor, continuous uplift going on. Otherwise, we wouldn't be here, at this elevation." What's that continued upthrust ultimately caused by? "There's quite a little speculation on that," Miller says, "but nobody knows for sure."
As for why folks standing on the ground might not have felt yesterday's quake, while those in tall buildings did, Miller tells Xpress, "There are certain places where earthquake waves will be amplified. It's like a spring effect. You need a lot more force to feel it when you're standing on the ground, versus when you're in a building," he says, where the floors and walls tend to amplify the movement of the earth in which the building's foundation is buried.
And if you're wondering whether we can expect more of this sort of thing in the future: "We're going to have small earthquakes here and it's completely normal," Miller says, recalling the record-breaking NC earthquake of magnitude 5.2 centered in Waynesville back on February 21, 1916.
"That's not all that different from the one that struck in Virginia yesterday," which was measured at magnitude 5.8, he says. "Out in California, they have earthquakes of that magnitude about once a year; we have them here in WNC about once per century."
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