Sumatra rattled by new aftershock
U.N. official: At least 500 confirmed dead after Monday's quake
CNN's Hugh Riminton repors on devastation on Nias Island.
Aid workers are finding those affected live in inaccessible areas.
Scientists try to find out why two quakes had such different results.
(CNN) -- The west coast of northern Sumatra in Indonesia was hit Wednesday by another strong jolt, 48 hours after an 8.7 magnitude earthquake stirred memories of December's earthquake and tsunamis that killed more than 174,000 people.
Wednesday's aftershock measured 6.3 when it struck at 11:19 p.m. local time (11:19 a.m. ET; 4:19 p.m. GMT) along the same fault line and at the same depth -- 20 miles (30 kilometers) -- as the earlier quake, which struck at 11:09 p.m. Monday.
At least 500 people are confirmed dead following Monday's quake, said deputy U.N. humanitarian coordinator Masood Hyder. He said the number is expected to rise.
Most of the deaths were on the island of Nias, off Sumatra's west coast. Monday's epicenter was about 60 miles (100 kilometers) south of the December 26 quake.
Multiple smaller quakes followed Monday's, including two others 6.0 or higher. Both of those occurred in the early hours after the initial quake, which itself was considered an aftershock of December 26. More than 300,000 people are listed as dead or missing from that event.
Although many towns and villages on Nias were badly damaged, a feared tsunami did not materialize Monday.
As military helicopters landed with relief supplies, survivors of Monday's great earthquake used their hands to dig through rubble searching for loved ones.
The scene in the main town of Gunungsitoli was one of destruction. Numerous buildings in the city's commercial district were flattened.
The coastal road linking the town with the airport 12 miles (20 kilometers) away was impassable due to a landslide that sent massive boulders cascading down onto the road. The airport runway sustained cracks, although it was open to light traffic.
Indonesian troops could be seen guarding official buildings in Gunungsitoli, but the overwhelming impression on the island was one in need of shelter, earth-moving equipment and emergency medical support.
The World Food Program estimated it would deliver 350 tons of food to the area Wednesday.
Helicopters from the Singapore air force landed in a sports stadium in Gunungsitoli, but a crowd looted supplies soon after they were unloaded. This prompted the U.N.'s operations officer to order no more landings in the stadium.
Three Chinook helicopters ferried out the most critically injured people Wednesday morning, although more casualties continued to arrive for evacuation.
In Washington, U.S. officials said the USNS Mercy hospital ship has been ordered to remain in the Indonesia area to help with earthquake relief.
The ship, which had been traveling for the past several weeks through the region on various humanitarian relief missions after the December tsunami, is expected to reach Nias in about six days from its current station near East Timor.
In some areas of Gunungsitoli, survivors used their hands to sift through rubble in the absence of large equipment.
Many people slept in churches and mosques overnight, but most residents opted to sleep outside in the rain, rather than inside, CNN's Hugh Riminton reported from the scene.
Dino Patti Djalal, an Indonesian government spokesman, said about 150 buildings had collapsed.
"Many of the victims have fled to the hills they are so traumatized. Many have not returned to their homes yet," he said.
He said they are in most need of food, medicine and heavy equipment. "Any medical teams would be helpful. We are already also employing the assistance of friendly countries to help the victims."
A geologist said Tuesday that it was luck that spared southern Asia from widespread devastation like the one last year.
CNN's Barbara Starr contributed to this report.