(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, Colo.

The quake occurred at 1109 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.