A powerful earthquake has struck off the west coast of the Indonesian island of Sumatra.

The quake which struck at 1508 local time on Wednesday had a preliminary magnitude of 7.6 according to the US Geological Survey.

It had an epicentre close to Simeulue island, about 300km west of the main city of Medan in northern Sumatra.

Indonesia is prone to seismic upheaval due to its location on the so-called "Ring of Fire," an arc of volcanoes and fault lines encircling the Pacific Basin.

Indonesia was the nation worst-hit when a powerful undersea earthquake triggered a tsunami in the Indian Ocean in December 2004.

The tsunami killed more than 168,000 people in north Sumatra's Aceh province alone.

 PHOTO CAPTION

Map locates magnitude 7.5 quake off Sumatra, Indonesia.