How noteworthy was Tuesday's quake across Virginia, Maryland, Delaware and much of the American East Coast?
Enough to raise the eyebrows of an earth scientist in central Pennsylvania whose work has taken him onto some of the world's shakiest real estate.
"This was probably the strongest earthquake I've ever been in, and my research has taken me to active volcanoes," said Peter LaFemina, an assistant professor of geosciences in State College, Pa. "We felt it very strongly here, most likely because we're on very stable rock in this region."
It was that same usually steady, deeper rock that helped to send shaking from the quake near Mineral, Va., across Delaware and the rest of the Northeast on Tuesday, researchers said.
Although most of sandy-soiled Delaware is considered to be in a medium hazard area for earthquakes, the entire mid-Atlantic sits atop ancient bedrock crisscrossed by cracks and faults that periodically let go with a bang.
Just that happened Tuesday about 3.75 miles below the surface near Mineral, in an area known as the "Central Virginia seismic zone," a place where few clues are available about the number or location of cracks and weaknesses that can spawn earthquakes.
"Where harder rocks are deep down, those energies can be transmitted up and outward," when something moves, said Peter P. McLaughlin Jr., Delaware's state geologist and interim director of the Delaware Geological Survey. "That's why the shaking was felt as far north as Canada and as far south maybe as Georgia.
The quake itself, McLaughlin said, might have been the release of strains created 150 million or more years ago, during the most recent splitting of the continents and the re-opening of what is now the Atlantic Ocean, at a time when the Appalachian Mountains rose up.
Stefanie Baxter, a geologist at the Delaware Geological Survey, said the effects of Tuesday's tremor were not the most intense in state history. But it might have been the first on record to be felt throughout the state.
Although the magnitude can be measured only at the epicenter, Baxter said the effects reported in Delaware would be comparable to the intensity of a quake of about 2.0 near its epicenter. That might not sound like much, but for the First State, it's quite high, she said. The Geological Survey's tectonic instruments on the University of Delaware campus were designed to measure much smaller geological events. Tuesday's readings sent the seismographs measuring activity at Bellevue and Brandywine Creek state parks spiking off the charts.
Unlike the active earthquake zones of the Pacific, where global plates are grinding alongside, over or under one another, the bedrock sheets of the East are far from their nearest continental neighbor, at the mid-Atlantic Ocean ridge. The land under the eastern United States is nevertheless crisscrossed by deep faults, including the Piedmont area below Mineral, Va., and northern New Jersey.
And from time to time, those faulted areas move and shake.
"There's a fair amount of historic activity in that area, but not this big," McLaughlin said. "This is the biggest that relates to those faults running through there."
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