JAKARTA, Indonesia --A powerful, shallow earthquake hit off the southwestern coast of Sumatra in Indonesia on Wednesday, officials said. There were no immediate reports of injuries or damage.
The U.S. Geological Service said the earthquake had a preliminary magnitude of 7.8. It was centered under the ocean at a depth of 24 kilometers (15 miles), it said.
Shallow earthquakes are more likely to cause damage, but the USGS said the quake was located far from land, about 659 kilometers (409 miles) from the town of Muara Siberut.
Indonesian officials warned that the quake might trigger a tsunami.
Marjina, a resident of Sikakap in the Mentawai islands, about 690 kilometers (430 miles) from the epicenter, said the quake was felt only weakly there, but the tsunami warning caused panic among villagers, who ran to higher ground.
Australia's Bureau of Meteorology issued a marine warning for the distant Cocos and Christmas islands. It did not advise evacuations, but said strong and dangerous currents were possible and people should secure boats and avoid waterfront areas. The Joint Australian Tsunami Warning Center later issued but then canceled a tsunami watch for Western Australia.
Indonesia, the world's largest archipelago, is prone to seismic upheaval because of its location on the Pacific "Ring of Fire," an arc of volcanoes and fault lines circling the Pacific Basin.
A massive magnitude-9.1 quake off Indonesia in 2004 triggered a tsunami that killed 230,000 people in a dozen countries. Most of the deaths were in Aceh province.