Deadly Java Tsunami Caused by Slow-Moving Quake

Richard A. Lovett
for National Geographic News
July 18, 2006

A large 7.7 magnitude earthquake struck offshore of the Indonesian island of Java (map of Indonesia) yesterday, killing more than 300 people and forcing thousands of others to flee.

The temblor struck at 3:19 p.m. local time, when many people were on the beach. It generated a tsunami reported to have produced waves from 10 to 20 feet (3 to 6 meters)* high.

The waves were much smaller than the monster swells of the tsunami that struck on December 26, 2004, which left more than 200,000 dead across several Indian Ocean countries including Indonesia (Southeast Asia tsunami full coverage).

Puji Pujiono is the regional disaster response advisor for the United Nations Office for the Coordination of Humanitarian Affairs.

"Rows of houses along the coast were swept away, [but] the waves did not go far inland," Pujiono told South Africa's Business Day news service.

"There was damage, not devastation."

Many Java residents recognized the danger and fled before the waves struck.

Nonetheless, the initially low casualty figures have steadily mounted into the hundreds.

Indonesia's tsunami warning system is still under development and is not scheduled to be operational on Java until 2007.

Comparing Quakes

The earthquake that created yesterday's tsunami bears similarities to the mammoth 9.3 magnitude temblor that sparked the 2004 disaster, experts say.

The primary similarity is that both quakes were caused by the collision of tectonic plates offshore of Indonesia.

At the site of yesterday's quake, the Australian plate is subducting, or being thrust beneath, Indonesia's Sunda plate at a rate of 2.3 inches (59 millimeters) a year, the U.S. Geological Survey reports on its Earthquake Hazards Program Web site.

The 2004 quake was caused by a similar collision between the Indian Ocean plate and the Sunda plate near the island of Sumatra.

But there are substantial differences between the two events, says Emile Okal, a geophysics professor at Northwestern University in Evanston, Illinois.

First of all, yesterday's earthquake was much smaller, which means it didn't do much to relieve the area's tectonic strain.

"It's not something that will set the area free [of quakes] for the next hundred years or so," Okal told National Geographic News.

Nor, he said, is the recent quake directly linked to the 2004 event.

That's because the 2004 temblor, he says, involved the movement of a plate attached to India.

"In this case we're talking about Australia. It's not exactly the same block." Furthermore, the plates collide at different angles at Sumatra than they do at Java.

"So yes, it's the same country" experiencing the quakes, Okal said. "But it doesn't mean that the conditions are a carbon copy."

Slow Zipper

Yesterday's Java earthquake, Okal said, is of a rare type known as a "tsunami earthquake."

That's not simply an earthquake that generates a tsunami. It's one that produces a surprisingly large tsunami for its magnitude.

Such earthquakes, Okal says, are characterized by relatively slow propagation of the earthquake along the fault.

Traditionally, the energy released during an earthquake moves along the fault at speeds of about two miles (three to three and a half kilometers) a second, like the steady opening of a giant zipper.

But in tsunami earthquakes, the ground unzips at half-speed or even slower.

This causes the Earth to shake slowly, producing relatively little damage to buildings but causing the sea to slosh more heavily, producing tsunamis.

The slow, low-frequency vibrations also fool seismologists into underreporting the temblor's initial magnitude, because the scientists are looking for conventional, fast-paced vibrations.

In this case, initial reports said that yesterday's 7.7 magnitude quake had a 7.2 magnitude. It might seem like a small difference, but it's a major distinction on the magnitude scale. For example, a magnitude 8 quake is ten times stronger than a magnitude 7.

Also, Okal notes, a similar quake struck the region in 1994 about 250 miles (400 kilometers) away from yesterday's event.

"That's geologically interesting," he said, "because we can legitimately ask whether there are areas that are prone to these slow earthquakes. The tentative answer, since yesterday, would be, Perhaps."

* Note the tsunamis were inititally reported to be only 2m in height.