The magnitude-5.9 jolt matched the strongest of the aftershocks that have followed the huge quake of Jan. 12 that devastated Haiti's capital.
Wails of terror erupted in Port-au-Prince, where the aftershock briefly interrupted rescue efforts amid the broken concrete of collapsed buildings, and prompted doctors and patients to flee the University Hospital.
Hundreds of thousands of Haitians remain homeless, hungry and in mourning — most still waiting for the benefits of a nearly $1 billion global aid campaign that has brought hundreds of doctors and thousands of troops to the impoverished Caribbean nation.
The U.S. Navy's floating hospital, USNS Comfort, dropped anchor in view of the capital on Wednesday with about 550 medical staff, joining teams from about 30 other countries trying to treat the injured. About 250,000 people were hurt in the quake and aid groups say many people have died for lack of medical care or adequate equipment.
And the Pentagon announced that 2,000 more U.S. Marines would be sent to Haiti, adding 11,500 U.S. military personnel already on the ground or on ships offshore — a number expected to reach 16,000 by week's end.
At a golf course where U.S. troops have been trying to help 25,000 people living under sheets of plastic and old cloth, soldiers and quake victims alike raced for open ground as the quake began.
A slow vibration intensified into side-to-side shaking that lasted about eight seconds. Some in Port-au-Prince said the far stronger Jan. 12 quake seemed to last for 30 seconds.
"It kind of felt like standing on a board on top of a ball," said Staff Sgt. Steven Payne. The 27-year-old from Jolo, West Virginia, who was part of the U.S. Army's aid mission.
At least one woman to die of a heart attack, according to Eddy Thomas, a private undertaker who was wheeling her body along a street in Port-au-Prince: "She had a heart condition, and the new quake finished her."
The U.S. Geological Survey said the aftershock was centered about 35 miles (60 kilometers) west-southwest of Port-au-Prince and 6.2 miles (10 kilometers) below the surface.
It was a little further from the capital than last week's magnitude-7.0 quake, which the killed an estimated 200,000 people and made as many as 2 million homeless, according to the European Union.
Wednesday's temblor matched the strongest of 49 aftershocks of magnitude-4.5 or greater that have followed the Jan. 12 quake and USGS geophysicist Bruce Pressgrave said nobody knows if a still-stronger aftershock is possible.
"Aftershocks sometimes die out very quickly," he said. "In other cases they can go on for weeks, or if we're really unlucky it could go on for months" as the earth adjusts to the new stresses caused by the initial quake.
The shaking ripped 8-inch (20-centimeter) cracks in a road west of the capital near Leogane, where U.S. Marines were setting up a post to aid quake victims who are sleeping in streets, culverts and driveways, often under tree branches draped with sheets to guard against the sun.
The latest quake, combined with a light rain on Tuesday, has complicated rescue efforts, said Dr. Yi Ting Tsai, part of a Taiwanese crew digging for survivors near the ruined cathedral.
"The problem is the rain and the new quake this morning has made the debris more compact," he said.
International aid teams have saved 121 people from the rubble, an unprecedented number, according to aid organization. Dr. Jon Kim Andrus, deputy director for the Pan American Health Organization, said that "countless more have been rescued by Haitians working with no equipment at all," he said.