MINERAL, Va. -- Tens of millions of people from Georgia to Canada were jolted Tuesday by the strongest earthquake to strike the East Coast since World War II. Three weeks before the 10th anniversary of Sept. 11, office workers poured out of New York skyscrapers and the Pentagon, relieved it was nothing more sinister than an act of nature.

There were no known deaths or serious injuries, but cracks appeared in the National Cathedral and three capstones broke off its tower. Windows shattered and grocery stores were wrecked in Virginia, where the quake was centered. The White House and Capitol were partly evacuated.

The U.S. Geological Survey said the quake registered magnitude 5.8. By West Coast standards, that is mild. But the East Coast is not accustomed to earthquakes at all, and this one unsettled some of the nation's biggest population centers.

In New York and Washington, people said their thoughts were of an explosion or terrorist attack. In some cases, workers in Washington, D.C., mentioned the tremors in phone calls to colleagues in New York, and seconds later, the shaking reached there, too.

"We thought it was a bomb at first because everyone has 9/11 on the brain and that it's so close to September and the 10th anniversary," said Cathy McDonald, who works in an IRS office in downtown Washington.

Tony Williams surveys damage at his Mineral, Va. home after an earthquake struck Tuesday, Aug. 23, 2011. Items in his home were knocked over and displaced, and the home suffered some structural damage after one of the strongest earthquakes ever recorded on the East Coast shook buildings and rattled nerves from South Carolina to New England. (AP Photo/Scott K. Brown)

Hundreds of people spilled out of the federal courthouse blocks from ground zero after the quake struck just before 1 p.m. Workers in the Empire State Building rushed into the streets, some having descended dozens of flights of stairs.

"I thought we'd been hit by an airplane," said one worker, Marty Wiesner.

Bad vibrations

The quake was felt as far north as Toronto, as far west as Indiana and Kentucky and as far south as Atlanta and Savannah, Ga. It was also felt on Martha's Vineyard off Massachusetts, where President Barack Obama, who is vacationing there, was getting ready to tee off in a round of golf.

The a White House official said there were no reports of major damage to the nation's infrastructure, including airports and nuclear facilities. Two nuclear reactors at the North Anna Power Station in Virginia were automatically taken offline by safety systems, said Roger Hannah, a spokesman for the U.S. Nuclear Regulatory Commission.

Store owner B.J. Singh replaces bottles on shelves at Miller’s Mart food store in Mineral, Va., a small town northwest of Richmond near the epicenter of an earthquake Tuesday afternoon. (Steve Helber ~ Associated Press)

The plant is in the same county as the quake's epicenter, about 80 miles southwest of Washington and 40 miles northwest of Richmond, Va.

The Park Service closed all monuments and memorials on the National Mall, and ceiling tiles fell at Reagan National Airport outside Washington. Many nonessential workers in Washington were sent home for the day. The Capitol was reopened by late afternoon for people to retrieve their things.

At the Pentagon, a low rumbling built until the building itself was shaking, and people ran into the corridors of the complex. The shaking continued, to shouts of "Evacuate! Evacuate!" The main damage to the building, the largest single workspace for the federal government, came from a broken water pipe.

The National Cathedral said it had sustained "significant damage," with three capstones, each shaped like a fleur-de-lis, breaking off the main tower. Cracks appeared in the flying buttresses around the apse at the cathedral's east end, the oldest part of the building.

"Everyone here is safe," the cathedral said on its official Twitter feed. "Please pray for the Cathedral as there has been some damage."

Around Mineral, Va., a small town close to the epicenter, people milled around in their lawns, on sidewalks and parking lots, still rattled and leery of re-entering buildings. All over town, masonry was crumpled, and there were stores with shelf contents strewn on the floor. Several display windows at businesses in the tiny heart of downtown were broken and lay in jagged shards.

Carmen Bonano, who has a 1-year-old granddaughter, sat on the porch of her family's white-frame house, its twin brick chimneys destroyed. Her voice still quavered with fear.

"The fridge came down off the wall and things started falling. I just pushed the refrigerator out of the way, grabbed the baby and ran," she said.

By the standards of the West Coast, where earthquakes are much more common, the Virginia quake was not strong. Since 1900, there have been 50 quakes of magnitude 5.8 or greater in California alone. Quakes in the East tend to be felt across a much broader area.

"The waves are able to reverberate and travel pretty happily out for miles," said USGS seismologist Susan Hough.

The Geological Survey put the quake in its yellow alert category, meaning there was potential for local damage but relatively little economic damage.

The agency said the quake was 3.7 miles beneath the surface, but scientists said they may never be able to map the exact fault. Aftershocks may help to outline it, said Rowena Lohman, a seismologist at Cornell University. There were at least two aftershocks, magnitudes 2.2 and 2.8.

The last quake of equal power to strike the East Coast was in New York in 1944. The largest East Coast quake on record was a 7.3 that hit South Carolina in 1886. In 1897, a magnitude-5.9 quake was recorded at Giles County, Va., the largest on record in that state.

A 5.8-magnitude quake releases as much energy as almost eight kilotons of TNT, about half the power of the atomic bomb dropped on Hiroshima, Japan. The earthquake that devastated Japan earlier this year released more than 60,000 times as much energy as Tuesday's.

The Virginia quake came a day after an earthquake in Colorado toppled groceries off shelves and caused minor damage to homes in the southern part of the state and in northern New Mexico. No injuries were reported as aftershocks continued Tuesday.

For the most part, the East Coast quake was a curiosity, at least after the initial fear faded away. It disrupted what was, for millions of people, an ordinary workday.

On the West Coast, where the last major quake to strike a metro area was a magnitude-6.7 event that ravaged greater Los Angeles in 1994, what happened back East was cause for outright mockery.

"Californians yawn, shrug and go back to their iced lattes," Marcus Beer, who reviews video games for a local news broadcaster, said in a Twitter post.