A major 7.8-magnitude quake has hit Indonesia's northern Sumatra, US seismologists say, triggering a local tsunami watch.

The quake struck at a depth of 31 kilometres, just off northern Sumatra, at 5.15am today (8.15am AEST), according to the US Geological Survey (USGS).

The Pacific Tsunami Warning Centre issued a watch for local tsunamis but said a widespread destructive tsunami was not expected.

Dale Grant, a geophysicist at the USGS, said the quake was considered a major event, but that there were no reports yet of a tsunami hitting the Sumatran mainland.

Thailand's National Disaster Warning Centre also issued a tsunami alert, warning people in coastal areas to evacuate to a safe place.

The Joint Australian tsunami warning centre said there was no tsunami threat to the Australian mainland or to Australian islands or territories. Mr Grant said the depth of the earthquake would affect how severely it would impact its surroundings.

"The deeper the event, the less energy is radiated at the surface, so a shallow event - something that occurs right at the surface - all that energy is then right there.

"So shallow events typically are very damaging and dangerously and the place where this event occurred, the better."

Indonesia sits on the Pacific "Ring of Fire", where the meeting of continental plates causes high volcanic and seismic activity.

A massive tsunami hit Indonesia and other countries in the Indian Ocean rim on Boxing Day in 2004, killing about 220,000 people, most of them in Aceh province in northern Sumatra.

A 7.6-magnitude quake in West Sumatra province in September last year killed about 1000 people, according to official figures.

About 5400 people in Thailand were killed in the 2004 tsunami, and Thailand has since installed a high-tech warning system designed to reassure tourists and businesses that the country's beaches are safe.