"I can't remember an event that large on the East Coast," said Paul Segall, a Stanford geophysicist who studies earthquake faults. He called Tuesday's event "a significant earthquake for that part of the world. It could do significant damage."
But even smaller quakes in the eastern U.S. can cause damage, because eastern cities aren't as earthquake-ready as their West Coast counterparts.
"Basically, the building stock in the eastern part of the United States is not built for seismic shaking like we are in California," he said. "For that reason alone, we would expect more damage."
Asked where the vulnerabilities lie, he said, "The kinds of buildings we would worry about are unreinforced masonry: unreinforced brick or unreinforced stone or concrete that doesn't have enough rebar in it."
States along the Pacific Coast have strong rules for what buildings must be able to withstand. Newer buildings in California are buttressed but also designed to sway instead of snap if the ground shakes beneath them.
Older Eastern cities -- such as New York, Washington, Philadelphia and Boston -- have many more buildings made of brick, which can crumble in a violent earthquake. There's been no impetus to upgrade them.
Around Washington, D.C., where there was significant shaking and many evacuations, there's "a lot of old brick buildings built before anybody worried about those kinds of things," Segall said.
"People haven't invested in retrofitting as we have in California," he said.
For cities along the East Coast and in the Midwest, which had some of the historically strongest quakes that occurred in 1811-12 along the New Madrid fault, forcing the Mississippi River to begin flowing north for a while, today's quake may be "a good warning."
Disaster plans for most East Coast cities are less focused on quakes than other potential disasters.
"It's not good," said Dr. Irwin Redlener, director of the National Center for Disaster Preparedness. "Because resources are limited, because emergency agencies have had to make budget cuts, they have to make tough choices and plan for what's most likely.
"In California, that would mean focusing on earthquakes," he said. "In New York, that would mean worrying about coastal storms and terrorist attacks.
"But that doesn't mean there's no chance of something else happening. You're picking and choosing what to plan for. And essentially, you're taking a shot."
Some 80 million to 90 million Americans are estimated by the U.S. Geological Survey to live in seismically active areas. But the Eastern United States is very different from the West. Its faults are older and the shaking can be felt further away from the epicenter.
Among the most significant U.S. quakes outside the West Coast was the Cape Ann, Mass., earthquake of Nov. 18, 1755, with an estimated magnitude of 6.0 to 6.3. It knocked down chimneys and stone fences and produced the most damage around Cape Ann and in Boston, especially in landfill around the city wharfs. It could be felt from Halifax, Nova Scotia, south to the Chesapeake Bay. People aboard a ship 200 miles off the coast of Cape Ann felt the quake and feared they had run aground.
A magnitude 5.4 earthquake struck western Ohio on March 9, 1937, cracking a schoolhouse, breaking chimneys and walls, reducing the output of oil and gas wells and creating new springs where old springs had run dry. That shaker could be felt in tall buildings of Chicago, Milwaukee and Toronto, as well as in Kentucky, Michigan, Missouri, Pennsylvania and West Virginia.
On Tuesday, hospitals around the earthquake region contacted by ABC News did not report injuries. But they often seemed puzzled when asked, "Did you feel it?"
"YES! In Morgantown, W.V.," went one emailed answer. "Never felt anything like that before ... all shook up!"
"We felt the tremor in Columbia, S.C.," said another. "I was sitting at my desk and felt the quake. It was weird."