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(CNN) -- An earthquake measuring a preliminary magnitude of 8.2 struck off the coast of Indonesia Monday -- on the same fault line that originated a December 26 earthquake that launched a deadly tsunami.

The director of the Pacific Tsunami Warning Center said scientists there feared another tsunami might hit the area.

Charles McCreary said he could not be certain that the quake, which was 30 km deep and 203 kilometer (126 miles) from Sibolga on Sumatra Island, would cause a tsunami.

"There is a potential for some wave activity," said Julie Martinez, a geophysicist at the U.S. Geological Survey's National Earthquake Information Center, in Golden, Colorado.

Thailand issued a warning that the quake could bring a tsunami to its southern provinces.

The warning, which was carried on national television, cautioned people in the six provinces to be careful and vigilant, but did not order evacuations.

The quake occurred at 11:09 a.m. ET (1609 GMT), and is considered a "great" earthquake, the largest of seven grades.

The grades are very minor, minor, light, moderate, strong, major and great.

Tsunamis are distinguished from normal coastal surf by their great length and speed. A single wave in a tsunami series might be 160 kilometers (100 miles) long and race across the ocean at 960 kph (600 mph).

When it approaches a coastline, the wave slows dramatically, but it also rises to great heights because the enormous volume of water piles up in shallow coastal bays.

On December 26, a 9.0-magnitude quake triggered a massive tsunami that devastated Asian and African coastlines in nearly a dozen nations, killing at least 175,000 people.