A massive earthquake registering a magnitude up to 7.7 rocked Indonesia's Sumatra island early Saturday, injuring at least 48 people, damaging houses and causing panicked residents to flee buildings and run into the streets.

Seismologists in Singapore said there was a high risk of tsunami — powerful waves caused by seismic activity — in western Sumatra.

There were contradictory reports of the quake's magnitude. Australian scientists said it registered a magnitude 7.7, making it a major earthquake capable of widespread, heavy damage.

The U.S. Geological Survey and the Hong Kong Observatory said the quake registered 7.5, while Indonesia's Meteorology and Geophysics Agency said it registered 5.3.

Another earthquake measuring 4.8 followed on Saturday evening in the same region, said Indonesian seismologists, adding that more aftershocks were expected.

At least 48 people, including a 6-year-old girl, were hospitalized on Simeulue island, 250 miles from Sumatra's northern tip, Simeulue hospital chief Dr. Hanif said.

"We fear that there are dozens more with even worse injuries. There are no roads on this island. It is very hard to get to the casualties," said Hanif, who, like many Indonesians, uses only one name.

Four government buildings and dozens of shops were damaged, said Ibnu Aban, the island's deputy government leader.

Hundreds of residents on Simeulue island pitched tents after the second earthquake as they were too afraid to stay in their houses for the night, Aban said, adding that others set up soup kitchens.

Mark Leonard of Geoscience Australia said, "It's about the same magnitude of the very destructive Gujarat quake in India two years ago," which killed 13,000 people.

He said the temblor was not likely to be as destructive as that quake because it was offshore. The USGS Web site said the quake was located 13 miles off the coast of northern Sumatra and was nearly 21 miles deep.

However, Indonesian seismologist Karyono said the 8:26 a.m. quake was centered onshore, about 20 miles north of Singkil town in Aceh province. Aceh is about 1,100 miles northwest of Jakarta.

Karyono said the quake struck in marshland with no inhabitants.

Suprapto Siswapranoto, a retired lawyer in Medan, a large city in northern Sumatra, said, "This is the strongest earthquake I have felt in the 60 years I have lived here. My house is made of strong concrete and the pillars shook strongly."

Tremors lasting several seconds were felt up to 300 miles away at the provincial capital, Banda Aceh, where people panicked and ran out of buildings, witnesses said.

Indonesia, the world's largest archipelago nation, is prone to seismic upheaval because of its location on the so-called Pacific "Ring of Fire."