(CNN) -- Confusion and then an uneasy calm.
That's how residents up and down the East Coast described their reaction to the 5.8-magnitude earthquake that rattled millions of people Tuesday.
The quake, which struck at 1:51 p.m., was shallow -- just 3.7 miles deep. The epicenter was about 40 miles northwest of Richmond, Virginia, the U.S. Geological Survey said. It sent tremors throughout the region, surprising people at least as far away as New York, Ohio and Georgia.
Sarah Merz said her co-worker in Camden, New Jersey, was the first one to notice the rumbling.
"She said, 'What is that? Is it a train?' Everyone kind of started looking at each other. Everyone's eyes just kind of widened, and we all just ran to the front door."
People stood stunned outside for a few minutes, she said.
"All of the sudden your heart starts to pound because you know on the East Coast, this just doesn't happen. It made me nervous at first, but then at the same time it's kind of this crazy curiosity."
Even an hour after the quake, Merz, 28, said she still felt dizzy.
"You're like, wait, am I moving? You keep second-guessing yourself. ... You start thinking too much. You're like, is it happening again?"
When she sat down at her desk, Merz noticed her framed college diploma was still vibrating on the wall.
Stories like hers poured in from across the East Coast as residents, generally not used to earthquakes, struggled in the early moments of the temblor to understand what was happening.
Was is a helicopter? A truck? A train?
Kate Duddy was in an office building elevator in Manhattan, alone, when the shaking started.
"I have never felt a quake before. It was scary having no idea what the cause was," she said. "I felt the vibrations and the elevator stopped for a period of about five minutes."
Gabby Randle, 23, was in a meeting in an office building at New York Avenue and 15th Street in Washington.
"We could see a couple of things falling off of the building next to us," she said. "We sort of ran because the ceiling tiles were swaying."
An official who works in a federal building in New York said he thought someone was "playing a joke" on him when his desk started shaking. "And then," he said "I felt my whole body swaying and realized it was something much more."
Many buildings were evacuated in the U.S. capital, where huge crowds lined the streets as police cars sped by with sirens sounding.
CNN iReporter Susan Prahinski sent CNN images of her home office in Washington. The quake knocked shelves off her walls and sent books and her computer monitor crashing. A statue of the Greek philosopher Aristotle lost its head.
In Spotsylvania, Virginia, Tish Walker said she grabbed her dog and raced outside when the earthquake struck.
"I used to live in California, so I know shaking and this felt big," she said. "My first thought is always that the furnace might explode or a cabinet crashes down on top of us."
Courtney Thompson, a CNN iReporter from Lanoka Harbor, New Jersey, had no doubt it was a quake, either. The question for her was whether it had occurred out in the ocean.
She said everyone taking in the sun at Ortley Beach, New Jersey, sprang up at the same time and looked anxiously out to the Atlantic.
"The water did not recede (a telltale sign of an impending tsunami), but I'm sure that's what everyone was worried about," she said. "People started panicking. They didn't tell us we had to get off the beach, but I'm telling you, everyone got right off the beach."
Brendan Wein, a sales representative at Hoffman Nursery near Rougemont, North Carolina, said he first sensed rattling inside the building.
"We were thinking it was a helicopter," Wein said. "I was literally shaking in my chair."
Simon Griffiths, in Raleigh, North Carolina, compared the quake to a tractor-trailer truck roaring outside his house.
Billy Wray said his guitars are OK, but things felt pretty weird when the bank building that houses his music shop in Mechanicsville, Pennsylvania, started to sway.
"Over here, we were rocking and rolling," he said.
CNN iReporter Jeff Yapalater said he was in his backyard in New York's Long Island when the shaking started.
"Suddenly I felt this light swaying of the earth, I'd never felt that before, so I thought maybe I was experiencing vertigo for a moment, and it lasted maybe 30 seconds. ... We're feeling this really far away!" he wrote.
CNN's Susan Candiotti came across a couple waiting to get back into an evacuated courthouse, where they had been waiting to be married when the quake struck.
"I didn't feel the shaking at all," the groom said. He glanced over to his fiancee and joked, "Maybe it's a sign."
CNN's Philip Gast, Katy Byron, Brian Walker and Susan Candiotti contributed to this report.