Photograph by Heri Juanda, AP

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Residents of Banda Aceh, Indonesia, head for high ground after Wednesday's tsunami alert.

Photograph by Heri Juanda, AP

No Tsunami? Why Earthquake Spared Indonesia Today

Quake's odd location and size make it "something new," expert says.

The powerful earthquake that struck off Indonesia Wednesday sparked a short-lived tsunami alert for much of the Indian Ocean—and panic in the streets of Aceh Province, in which a 2004 tsunami off Indonesia killed some 170,000 people.

Scientists say Indonesia's good fortune Wednesday has everything to do with the size of the quake and the motion of the ocean floor. The surprising strength and location of the earthquake, they add, make it "something new."

According to the U.S. Geological Survey, the first earthquake had a magnitude of 8.6 and was quickly followed by an 8.2-magnitude aftershock. In Aceh hospitals were quickly evacuated, and residents fled to high ground in cars and motorcycles and poured into the streets to search for separated family members, according to the Associated Press.

The temblors also caused several countries in the region to issue tsunami watches, but the alarm was lifted after a few hours when no serious waves were observed.

"There was a tsunami, but the waves were just below 1 meter [3.3 feet]," said Emile Okal, a geophysicist at Northwestern University in Evanston, Illinois. "That is significant, but it's not going to do much damage."

This is a sharp contrast to the December 2004 tsunami, when waves reached heights of nearly ten stories.

(Read a National Geographic photographer's account of the 2004 Indonesian tsunami aftermath.)

Quake Hit in Right Place, at Right Size—For Indonesians

There are two main reasons Wednesday's earthquake off Indonesia did not spawn a giant tsunami, Okal said.

First, the quake was of a smaller magnitude—8.6, contrasted with 9.3 for the temblor that struck off the west coast of Sumatra, Indonesia, in 2004. (See pictures of the 2004 Indonesia tsunami aftermath.)

Second, the quake was what's known as a strike-slip earthquake, where the motion is primarily side-to-side—like two ships sideswiping each other. In the 2004 quake, vast swaths of the seafloor thrust sharply upward, drastically disturbing the waters above.

On Wednesday "the amplitude of the tsunami was less than what it would have been had it involved more vertical motion," Okal said.

Indonesia Earthquake "Rather Interesting"

Wednesday's quake is interesting, Okal added, because it did not occur at a so-called subduction zone, where tectonic plates collide and one dives beneath the other into Earth's mantle.

Rather, the earthquake happened about a hundred miles (150 kilometers) from the nearest subduction zone, making the temblor an "intraplate" quake. Intraplate quakes tend to be much smaller, on the order of magnitude 6 or so, Okal said.

The combination of these two factors—a high magnitude for an intraplate quake and its strike-slip nature—makes Wednesday's earthquake "rather intriguing," Okal added.

"This is in a sense something new," he said. "It's something we've never seen at this level of size in this particular area."