SUMATRA, Indonesia - An earthquake with a magnitude of 7.7 rattled the Indonesian island of Sumatra on Wednesday, shaking buildings, causing blackouts and prompting a tsunami warning that sent people rushing to higher ground. There were no immediate reports of injuries or widespread damage. The quake struck at 5:15 a.m. local time and was centered 125 miles northwest of Sibolga, in Sumatra, a major island in western Indonesia, the U.S. Geological Survey reported. It was about 28 miles deep, the agency said. The Indonesia Meteorology and Geophysics Agency issued a tsunami warning following the quake, which struck as people in the predominantly Muslim region were preparing for morning prayers. The warning was later lifted.