The epicenter of the 2.8 magnitude quake, which began at 9:44 a.m., was two miles east-southeast of Pennsville, according to a report by the U.S. Geological Survey. It originated 3.1 miles beneath the earth's crust.

The loud rumbling made officers at the Delaware Memorial Bridge think at first that two tractor trailers might have collided, or an oil tanker might have exploded on the river, said Master Cpl. Joseph DiStefano, spokesman for the bridge police.

But the bridge was not damaged, and no accidents resulted from the quake, he said.

Jessica Dougan, an interpreter at Fort Mott State Park, within a few miles of the epicenter, thought it was thunder and then panicked when she heard a rumor that it was an explosion at the DuPont chemical plant in Penns Grove.

"I called my brother, who works there, and he told me he was fine," she said.

The Salem nuclear plants "are running operationally fine," and no critical infrastructure was harmed, said Robin Weinstein, spokesman for the county department of emergency services.

The rattling lasted only a few seconds, according to Weinstein, who is based in Woodstown, about eight miles from Pennsville.

To put the event into perspective, 10 stronger quakes occurred before noon yesterday in North America, including in Puerto Rico, Alaska, California, Washington state, the British Virgin Islands, Mexico, and the Dominican Republic.

The South Jersey quake, though, was the only Northeast quake in the last seven days of 1.0 or stronger.

Although Salem County 911 received about 150 calls, no injuries were reported, Weinstein said.

The engineering department went out doing site inspections yesterday, checking roads, bridges, and buildings, he said.