A 7.7 magnitude earthquake has jolted the Indonesian island of Sumatra, triggering a brief tsunami watch but no immediate reports of serious damage or casualties.

The U.S. Geological Survey says the quake struck before dawn Wednesday. It was centered about 205 kilometers northwest of the island, 31 kilometers under the Indian Ocean.

The USGS initially measured the quake at 7.8, but now says it was a 7.7 magnitude.

The quake triggered a local tsunami warning which was canceled two hours later.

There is no word yet on damage or casualties. But witnesses report power blackouts in several parts of Aceh province and on Simeulue Island, west of Aceh.

Sumatra is the largest Indonesian island. A 9.1 magnitude quake struck off Aceh in December, 2004, triggering a historic tsunami that killed 226,000 people.

Indonesia is located in the Pacific Ocean's so-called Ring of Fire, where the continental plates meet. This creates conditions for frequent earthquake and volcanoes from Asia to the American Pacific coasts.