By Lincoln Anderson

Tuesday’s tremblor was reportedly the most powerful such event the city has felt in the past 60 years. Luckily, the 5.8-magnitude earthquake’s epicenter was 300 miles away from the Big Apple in Virginia, and almost 4 miles underground, so its force here was only a 2 on the Richter scale.

Swaying buildings, vibrations and one scarily powerful thump were among the early afternoon quake’s most obvious signs. But it wasn’t obvious, at first, to most people that it was, in fact, an earthquake.

As they did elsewhere around the city, people in Hudson Square and Tribeca — along the west end of Canal St. — instinctively emptied out of their buildings, not sure what was going on at first. At Greenwich and Canal Sts., office workers hurried outside, then crossed the street to stand on the sidewalk of the new Zinc Building luxury residence. Their first thought was to get away from their own buildings.

Used to pile driving and vibrations from all the new construction in the area, some initially thought it was construction related.

But the question, “What the hell WAS that?” was quickly answered thanks to ubiquitous smartphones. Yes, believe it or not — it was an earthquake. Whoah! This was something new for New York.

In addition to vibrations from new construction, buildings being rattled by traffic along Canal St. is common.

“The building usually shakes because of trucks going by and also the billboard on top, which catches the wind — so we were like, ‘Whatever,’” noted Andrew Mak, who works on the top floor of a building at Renwick and Greenwich Sts.

But it was soon clear that this was more than just the usual billboard-shaking-the-building thing.

“I said to Amanda, ‘Your cup is ‘Jurassic Park’’ing,” recalled Ellen LaVery.

People stood outside for about 40 minutes. It wasn’t exactly clear what to do next.

How soon do you go back indoors after an earthquake? Does someone come around and tell you, “Hey, it’s O.K.! Go back in!” Do you do some kind of test — with some sort of instrument?

However, since there didn’t seem to be any aftershocks, after a while it seemed like it might be all right to go back inside.

So people did.

A couple of hours later, Mayor Bloomberg held a City Hall press conference to explain what had just happened. He and everyone else in the building had evacuated it two hours earlier. In his own case, he said he’d felt vibrations through his right elbow, which had been resting against something.

“I think people realized what it was — and went back into buildings after a period of time,” he said.

Bloomberg said it was fortunate the event had occurred during the day and in good weather. He likened the quake, in a way, to “a fire drill,” where people go out of their building, briefly gather on the sidewalk, then go back in.

As for his own experience, the mayor said, at first, he thought what he was feeling was from construction, since renovations are being done in City Hall. Asked by a reporter if he also had initially suspected terrorism, he said no.

“Terror doesn’t start with a small vibration and peter out — I wish it did,” he said.

The mayor reported that there had been no injuries and only minimal damages: A chimney collapse was reported in the Red Hook public housing project. Every day bricks are falling off buildings in the city, he noted.

Governor Cuomo said Tuesday’s earthquake didn’t change his feelings about Indian Point nuclear plant, which he has previously said should be shut down as soon as possible. Indian Point is located only 32 miles from Midtown Manhattan. The plant is reportedly built to withstand a 6.0-magnitude quake.

“My position on Indian Point is one accident could be one accident too many,” Cuomo said. “My opinion on Indian Point has been long held and has nothing to do with today’s events.”

In the aftermath of the — knock on wood — harmless incident, the question became: “What were you doing during the earthquake?”

Lower East Sider Elsa Rensaa said she was at the World Trade Center Health Clinic at Bellevue Hospital when the tremblor hit. She was getting prescription medicine for “9/11 rash,” which she got on her forearms when she was exposed to the Twin Towers’ fallout while photographing the disaster.

“Everybody shut up,” she said of the reaction to the quake. “It kind of knocked out electrical equipment and some cell phones. Everybody just stood there and looked at each other. It was like a tremor, a slight wiggle.” She said the hospital wasn’t evacuated, at least not where she was.

True to form, activist John Penley was involved in planning a protest against the closing of the Lower East Side’s Bialystoker nursing home.

“I was actually on a bus heading up to check out the location of Ira Meister’s office for a protest I’m doing outside his office next Thursday at 3:30 p.m.,” he said, referring to the Bialystoker board chairperson. Penley noted he’s experienced earthquakes that were more powerful in Mexico, so he wasn’t that worried — but he still wasn’t taking chances.

“You never know if it’s going to get worse — that’s the thing,” he said. “I did get off the bus. I didn’t want to be on a bus in case it got worse.”

A couple of artists didn’t notice the event.

Jim Power, the “Mosaic Man,” called in about an hour after the quake, saying he was getting ready to install a new mosaic birdhouse in an E. Sixth St. community garden, and, no, he had never felt a thing. Anyway, he used to live in California and had seen much worse, he said.

East Village artist Theresa Byrnes said, “Truth is I didn’t even notice. Maybe because my studio and gallery is a ground-floor storefront. My assistant, Domo, and I were hanging a small painting on Ninth and then we rolled down Avenue C to Guerra Paint on 13th St. The gals at the store said pigments were falling off shelves. There was a crowd of people outside the Ninth St. precinct [Housing Police stationhouse]. People were gathered on our way to 13th, swapping quake stories.”

Soho’s Ian Dutton is sort of the Zelig of major incidents. It probably helps that he’s a pilot: He flew a relief mission to Haiti after that country’s devastating earthquake last year, and, by coincidence, was in Oslo last month when a bomb was set off outside the prime minister’s office.

This time, though, he was on vacation in South Dakota and about to enter the Minuteman Missile National Historic Site — a decommissioned I.C.B.M. silo — when the quake hit. He obviously didn’t feel it that far away, but — showing how news travels almost instantaneously today — he knew about it almost as soon as it happened via his iPhone.

He and his wife, Shea, decided to “check in” on Facebook before entering the silo, and saw that their friends had posted news of the quake just seconds before.

In general, the higher their floor, the more that people felt vibrations and swaying.

Filmmaker Michael Schiller said, “I was on the seventh floor of an office building on Hudson St. I felt it swaying a little and didn’t think anything of it. Then someone screamed out, ‘Why are the lights shaking?’ I stepped out into the hall and everyone had their heads poked out, looking bewildered while the building started to shake for real. I stood in the doorway, like any earthquake-preparedness-trained individual should.

“When the shaking stopped, I looked out the window and saw hundreds of people in the street. I went outside and felt really unsafe, waiting for the next shock to hit harder. Over a loud speaker a voice said, ‘It is safe to return to the building.’ I turned to the lady next to me and said, ‘That’s what they said on 9/11,’ and walked over to the Hudson River where no buildings could fall on my head.”