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.
When the subducting plate gets stuck, stress builds up until the plate lurches forward violently, causing an earthquake (interactive feature: how earthquakes work).
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."
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