Rescue officials in Jakarta have told Al Jazeera that with communications difficult and power cut in the quake area, a clear picture of the damage and numbers of casualties has yet to emerge.
A health ministry crisis centre official in Jakarta said rescue worker, medicine and food supplies were to begin deploying to the area on Thursday.
The military will also be mobilised for rescue and relief work, according to a presidential aide.
Officials have said the death toll from the quakes is expected to rise as rescue efforts get underway.
|Both quakes were centered near the |
city of Bengkulu in western Sumatra
On Wednesday evening hundreds of people fled their homes when the first quake struck just after 6pm local time (1100 GMT).
The epicentre of the first quake was about 100km southwest of Bengkulu.
Residents ran for higher ground as buildings collapsed in Padang, the capital of West Sumatra, and the town of Mukomuko, 300km from the quake's epicentre.
Officials said buildings along Sumatra's coast sustained extensive damage, some of them with people trapped inside.
Tsunami warnings were issued by countries around the Indian Ocean, and later withdrawn.
March 2007: A 6.4 magnitude earthquake and another measuring 6.3 in Sumatra kill more than 70 people.
May 2006: A magnitude 6.3 earthquake strikes south of Yogyakarta in central Java, killing more than 5,700 people.
July 2006: A magnitude 7.7 undersea earthquake hits Java's west coast, killing more than 700 and triggering a three-metre-high tsunami.
March 2005: A magnitude 8.7 earthquake strikes Nias and Simeulue islands off Sumatra's western coast, killing about 1,300 people.
December 2004: Undersea magnitude 9-9.3 earthquake triggers a tsunami, killing more than 280,000 in 12 Indian Ocean nations, including over 120,000 in Aceh province.
June 2000: A 7.9-magnitude earthquake hits Bengkulu province on Sumatra, killing more than 100 people.
December 1992: A magnitude 6.8 quake kills at least 2,200 people on islands in East Nusa Tenggara province.
Those warnings were reinstated following the latest quake on Thursday morning.
The alerts have triggered fears of a repeat of the December 2004 tsunami that killed more than 250,000 in 12 countries.
The latest earthquake was also felt in neighbouring Singapore where people ran out of high-rise office and apartment blocks.
Aldi, a member of staff at the M Yunus hospital in Bengkulu, told the Reuters news agency that patients were being treated in tents and more were needed.
"Glass was broken, ceilings collapsed, and the walls cracked in the hospital," he said.
A spokesman said Susilo Bambang Yudhoyono, the Indonesian president, had ordered the military to help in rescue efforts as the national emergency rescue system kicked in.
David Oppenheimer, a scientist with the US Geological Survey, said a magnitude eight earthquake usually leads to partial collapse and shifting of buildings off their foundations.
"That's the kind of stuff that causes death, especially in the Third World," he told Reuters.
Indonesia, the world's largest archipelago, is located on the so-called Pacific "Ring of Fire," an arc of volcanoes and fault lines encircling the Pacific Basin, and is prone to earthquakes.