There is some evidence that shallow earthquakes produce greater seafloor vibrations than deeper ones do, he explained.
(Watch a video about how tsunamis form.)
But this does not explain why other regions appear to have been spared the tsunami's effects. Tsunamis generally propagate most strongly in a direction perpendicular to the fault zone, which in this case should have sent a wave toward Australia, Okal said.
Although the Australian government closed beaches for many hours, the tsunami did not strike there.
Okal suspects that Australia was protected by the Great Barrier Reef, a ridge at the tip of Papua New Guinea.
But it will take some time to determine why this happened, he said, because the interaction between tsunamis and shallow underwater landforms is very difficult to calculate.
Although full casualty reports have yet to be released, Okal noted that, given the size of the wave and the number of houses destroyed, the death toll appears to be remarkably light.
"If [the death toll] stays within this order of magnitude, this is nothing short of a miracle," he said.
"There are reports saying that the people felt the earthquake and ran away from the beach, which is exactly what they are supposed to do," he added.
Overall, however, there is a need for continued tsunami awareness, Okal said.
"We get a tsunami like this pretty much every year, and occasionally two a year or so," he said.
Free Email News Updates
Best Online Newsletter, 2006 Codie Awards
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES