An underwater earthquake rocked western Indonesia on Wednesday, but there were no immediate reports of injuries or damage.

The U.S. Geological Survey put the preliminary magnitude of the quake at 5.5. It struck 160 miles (260 kilometers) south of Bengkulu, a city on Sumatra island, around 20 miles (35 kilometers) below the ocean floor.

Residents along the coast said they did not feel the afternoon tremor.

The country's geophysics agency put the magnitude at a much higher 6.1. Such discrepancies - though large - are not considered unusual here.

Indonesia straddles a chain of fault lines and volcanoes known as the Pacific "Ring of Fire" and is prone to seismic activity. A huge quake off western Indonesia caused the 2005 Asian tsunami that killed around 230,000 people.