An underground explosion at the MTA's Second Avenue subway construction site sent boulder-sized debris flying into Upper East Side streets Tuesday, shattering windows and frightening nearby residents and merchants.

The blast at East 72nd Street and Second Avenue occurred at 12:46 p.m. as workers with Schiavone Shea Kiewit Constructors were using dynamite-like explosives to excavate a planned escalator and elevator well, Metropolitan Transportation Authority officials said.

Although the explosion site was covered with protective metal plates, debris shot into the air, covering the area with rock fragments -- some more than a foot in diameter -- and creating a cloud of dust. The airborne debris shattered windows at an art gallery across from the blast site, and at least one apartment window above the gallery.

No one was injured in the blast, officials said. Construction workers routinely clear traffic and pedestrians before any underground blasting takes place.

But residents were shaken.

"This could have been a huge disaster -- the blast could have brought down buildings," said Joan Berlly, who lives across from the construction site. "My apartment shook and it was very scary."

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The $4.45 billion subway construction project to extend the Q line to lower Manhattan started in April 2007.

"We are investigating what caused this," said MTA Capital Construction president Michael Horodiniceanu, who oversees the so-called "megaproject" that is to be completed by 2016. "Things like this could happen, but they shouldn't happen. It's totally unacceptable."

The debris-spewing blast is the second in about two weeks in that construction area, although MTA officials said the earlier incident was less serious.

Carole Cusa, who lives in an apartment across from the construction site, said the blast Tuesday knocked her to the floor.

"I was in my kitchen working when I heard the blast and it threw me off my chair," she said, adding that the whole building shook. "I knew it wasn't normal."

Patrick Clancy, 41, who also lives in the building on the corner of 72nd Street and Second Avenue, said that after years of constant jarring blasts from the construction, he was not shaken by Tuesday's explosion. But his wife was. "She thinks the whole building is going to come down," Clancy said.

The cause of the explosion had not been determined Tuesday, MTA spokesman Adam Lisberg said. The agency ruled out an overloaded air compressor, he said, and did not believe too many explosives were used or that there was a problem with the protective plates.

"We're trying to figure out why," Lisberg said. "There's a million different possibilities."

Blasting was suspended until the city's Department of Buildings inspects surrounding buildings for structural damage.

"What happened at the Second Avenue subway construction site today is completely unacceptable," MTA chairman and chief executive Joseph J. Lhota said in a statement last night. "The MTA . . . will not resume work at the 72nd Street site until we receive a full explanation for what happened."

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City Councilman Daniel Garodnick, who went to the explosion site, said residents have suffered from noise and air quality issues throughout the subway project and deserve to have the safety issues "sorted out in 24 hours." Still, he said, he believes a Second Avenue subway will be "good for the community."

"But we have to survive the construction first," he said.

With Maria Alvarez

and Gary Dymski