A massive earthquake struck off Indonesia's Sumatra island on Wednesday,
US and Indonesian monitors reported, prompting a Indian Ocean-wide
tsunami alert.

The quake's magnitude was 8.7, according to a
revised measurement from the US Geological Survey, and it struck 431
kilometres (268 miles) off the Indonesian city of Banda Aceh.

The
city was near the epicentre of a devastating 9.1-magnitude quake in
2004 that triggered a tsunami across the Indian Ocean, which killed
220,000 people, including 170,000 in Aceh.

The latest tremor was felt as far afield as Thailand and southern India.
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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.
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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.