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