A geologist looks at a screen showing the seismograph reading of the powerful earthquake that rocked Sumatra island at the office of Indonesia's meteorological agency in Jakarta on Wednesday. (Irwin Fedriansyah/Associated Press)

A second earthquake struck off the coast of western Indonesia on Thursday, a day after the region was hit by a more powerful quake that toppled buildings, trapping thousands of people under rubble and killing at least 200, officials said.

The shallow inland 6.8 magnitude earthquake hit 240 km south of Padang at a depth of just 24 kilometres, the U.S. Geological Survey said. There were reports the second quake badly damaged dozens of additional buildings.

The first quake, with a magnitude of 7.6, struck at 5:15 p.m. local time Wednesday. It had an epicentre about 50 kilometres off the coast of Sumatra, according to Indonesia's meteorological agency.

A higher death toll is expected once officials tally casualties in other areas of West Sumatra province, where communications and roads have been severed.

The massive underwater quake, which had a depth of 85 kilometres, briefly prompted the Pacific Tsunami Warning Center to issue a tsunami alert for Indonesia, Malaysia, India and Thailand. It was later lifted when there were no immediate reports of high waves.

Priyadi Kardono, a spokesman for the National Disaster Management Agency, said Thursday that about 500 buildings collapsed in the coastal city of Padang in the Southern Sumatra province as well as in other town and cities.

The 7.6-magnitude quake had an epicentre about 50 kilometres off the coast of Sumatra. (CBC)

The hospital in Padang is among the collapsed buildings, said Rustam Pakya, head of the country's Health Ministry.

Other collapsed buildings include homes, hotels, schools and shops, Indonesian Vice-President Jusuf Kalla told a news conference Wednesday.

The rubble of other collapsed buildings are thought to have trapped thousands of people, Pakya said.

A field hospital is being prepared to assist the injured and medical teams were on the way from neighbouring provinces, he said.

Piles of rubble

Witnesses said people ran out of homes and buildings in fear when the quake hit.

Television footage showed devastation, with piles of rubble and smashed houses in Sumatra.

The quake has also caused widespread power and phone line outages, according to officials, making communication difficult. Several injuries are being reported by witnesses.

An earthquake survivor receives medical treatment in Padang Panjang, West Sumatra, on Wednesday. (Tundra Laksamana/Associated Press)Bridges collapsed in Sumatra, according to reports, and landslides also cut off the coastal area.

There were also reports that the tremor caused several water pipelines to break, causing flooding in streets.

It is not yet clear if the quake had any impact on the country's large oilfields and liquefied natural gas terminal, which are also based on Sumatra.

Wednesday's tremor was felt in the capital of Jakarta, and in Singapore and Malaysia.

Geologists have long said Padang, with a population of 900,000, may one day be destroyed by a huge earthquake because of its location.

Indonesia, a vast archipelago, straddles continental plates and is prone to seismic activity along what is known as the Pacific Ring of Fire. The quake was along the same fault line that spawned the massive December 2004 Indian Ocean tsunami that killed more than 230,000 people in a dozen countries.

Hours earlier, a tsunami had swept into Samoa, American Samoa and Tonga, killing at least 99 people after a strong underwater quake in the South Pacific.

With files from The Associated Press