An earthquake with a preliminary magnitude of 6.3 rattled the western coast of Sumatra on Friday, the local Meteorological and Geophysics Agency said, causing frightened residents to flee their homes. There were no immediate reports of damage or casualties in Bengkulu province, about 600 kilometers west of the capital, Jakarta. The quake was not believed to have been powerful enough to generate a tsunami, the agency said. The US Geological Survey put the tremor at a magnitude 5.9. The shaking sent residents running out of their houses in panic, Silahuddin, who like many Indonesian uses one name, told El-Shinta radio. Indonesia, which straddles a series of active fault lines, is prone to seismic and volcanic activity. A giant earthquake along the same coast spawned the killer Asian tsunami that killed more than 230,000 people in December 2004.