JAKARTA ~ A strong 6.6-magnitude earthquake struck off Sumatra island on Thursday, seismologists said, but there were no reports of casualties or damage.

The quake hit at 03:01am and was centered 239 kilometers south of Padang in Sumatra, at a depth of 42 kilometers under the sea, the United States Geological Survey said.

The Indonesian archipelago sits on the Pacific Ring of Fire where continental plates meet, causing frequent seismic and volcanic activity.