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By Mica Rosenberg

GUATEMALA CITY, June 13 (Reuters) - A powerful earthquake rattled Guatemala and El Salvador on Wednesday, forcing terrified residents to flee shaking buildings, but there were no reports of casualties.

The U.S. Geological Survey said the earthquake had a magnitude of 6.8 and was centered in the Pacific Ocean some 46 miles (75 km) south of the Guatemalan town of Escuintla. The quake occurred at a depth of about 40 miles (65 km).

Buildings in Guatemala City swayed for about 30 seconds and people ran into the streets. Parents formed long lines outside schools to look for their children, and authorities asked residents to stay out of high buildings.

"I was kind of scared that something would collapse. It was strong," said Marcelo Rocha, a 19-year-old security guard at a sugar export facility in Puerto Quetzal, which is Guatemala's busiest port and close to the earthquake's epicenter.

Emergency services said the country escaped virtually unscathed.

"We are all on alert but we've got to keep calm," Guatemalan President Oscar Berger said in a public address. "There have been no reports of any harm to people or structures."

El Salvador's Interior Ministry said there were no reports of fatalities or injuries there.

Twin earthquakes just one month apart killed around 1,150 people in El Salvador in 2001, most of the victims buried in a huge mudslide near the capital San Salvador.

The impoverished countries of Central America are susceptible to earthquakes, hurricanes and volcanic eruptions, often followed by dangerous mudslides.

SCARY MEMORIES

Earthquakes in Guatemala evoke frightening memories of a 7.5 magnitude quake that struck the country's poor indigenous highlands in 1976, killing more than 23,000 people, many of them crushed under collapsed adobe houses.

"It felt just like that other quake," said Ponciano Martinez, a 49-year-old fireman in Escuintla. "We have some very big water tanker trucks here that were moving, that's how we know it was strong."

The Pacific Tsunami Warning Center in Hawaii issued a notice of Wednesday's earthquake but said there was no danger of a tsunami.

"The earthquake was too deep to generate a tsunami hazard," said Gerard Fryer, a geophysicist at the center.

"We don't think there is going to be a dangerous Pacific-wide tsunami but we will monitor the situation," said David Walsh, an oceanographer with the warning center.

A spokeswoman for Guatemalan coffee growers group Anacafe said there were no reports of any damage to crops while sugar mills said they felt the quake but were not damaged.

(Additional reporting by Alberto Barrera in San Salvador, Frank Jack Daniel and Jason Lange in Mexico City and Brian Harris in Costa Rica)

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