Video Related To: Indonesia: Earthquake magnitude 6.

A massive earthquake has struck off Indonesia's Sumatra island, US and Indonesian monitors report, prompting a tsunami alert across the Indian Ocean. The quake's magnitude was 8.7, according to a revised measurement from the US Geological Survey, and it struck 431 kilometres off the Indonesian city of Banda Aceh, on the island of Sumatra, on Wednesday at 2.38pm (1838 AEST) and at a depth of 33 kilometres. The US Geological Survey initially reported it as an 8.9-magnitude quake. Indonesian authorities measured the tremor at 8.5. The latest tremor was felt as far afield as Thailand and southern India. According to the Joint Australian Tsunami Warning Centre, there was no tsunami threat to the Australian mainland, islands or territories. There were no immediate reports of damage or casualties, said Prihyadi, an official at the Indonesian Meteorology, Geophysics and Climatology Agency. "We have issued tsunami warnings for Aceh, North Sumatra and West Sumatra," said Prihyadi, who like many Indonesians uses only one name. But residents across Sumatra fled their homes in panic, local media reported. Residents in Banda Aceh reported the ground shaking violently. "People are in a panic, and there are traffic jams everywhere in Banda Aceh," local Metro TV reported. "There is potential for a tsunami to hit five provinces in the region - Aceh, North Sumatra, West Sumatra, Bengkulu and Lampung," said Marzuki, an analyst at Indonesia's Geophysics and Meteorology agency. A tsunami watch <b>...</b>