A powerful 7.8-magnitude earthquake hit Indonesia's Sumatra island early this morning, triggering a tsunami alert in nearby Thailand. According to the US Geological Survey, the quake struck at a depth of 46 kilometres off the northwest coast of Sumatra at 5:15 am (0345 IST), but there were no reports of damage. Indonesian geologists said it had a magnitude of 7.8 and the epicentre was 60 kilometres southeast of Sinabang, onSimeulue Island of Aceh province, which was devastated by a massive quake and tsunami in 2004.The Pacific Tsunami Warning Center issued a watch for tsunamis in Sumatra but said a destructive ocean surge was no expected.The National Disaster Warning Centre in Thailand said there was a high risk of a tsunami on the Andaman Coast, which was battered by the Indian Ocean tsunami in 2004 that killed an estimated 5,400 people in Thailand alone.Officials in Sinabang and the Indonesian capital of Jakarta said there are no immediate reports of damage.Electricity was down in the Acehnese capital of Banda Aceh but mobile phones are working. Indonesia was hit by the tsunami, in 2004 in which at least 168,000 people killed when the sea surged over the northern tip of Sumatra after a 9.3-magnitude quake split the seabed to the island's west.