Map from US Geological Survey shows where the 7.9 R quake has struck off the Sumatran coast on Wednesday. USGS

A tsunami alert for west and north Sumatra and Aceh has been issued after a powerful and shallow earthquake of magnitude 7.9 struck off the west coast of the Indonesian island of Sumatra late on Wednesday.

The epicenter was 808 km (502 miles) southwest of Padang, USGS said. It was 10 km deep, compared to the 8 km depth of the devastating Nepal quake in 2015.

There were no immediate reports of damage or casualties expected with shallow quakes. "So far there have been no reports (of damage) yet," an official of the National Meteorological Agency said, according to Reuters. "In Bengkulu (on southwest coast of Sumatra) they didn't feel it at all."

The US Geological Survey initially put the magnitude at 8.2 and then lowered it to 7.9.

Indonesia, badly hit by the quake and tsunami in 2004, sits on the highly seismically active zone "Pacific Ring of Fire", where many continental plates on the earth's crust keep colliding.

In fact, the plate boundary southwest of Sumatra is part of a long tectonic collision zone that extends over 8,000 km from Papua in the east to the Himalayan front in the west, says the USGS.

Indonesian President Joko Widodo, who was in North Sumatra at the time of the quake, is safe, the government said. 

Following Indonesia, neighbouring Australia also issued a tsunami warning for its western coast.