Earthquakes result when the subducting crust gets stuck, then lurches back into motion. Volcanoes are formed when subducted rock melts and returns to the surface as magma. (Learn more about how and why earthquakes happen.)
The large Indonesia earthquakes of December 2004 and March 2005 were caused by a similar plate collision off the island of Sumatra.
But the Java earthquake has some important differences, Mark Leonard, senior seismologist with the government organization Geoscience Australia in Canberra, wrote in an email.
Not only was the epicenter of the Java quake several hundred miles away from Sumatra, but the motion was of a type geologists refer to as strike-slip, Leonard says.
Strike-slip earthquakes involve sideways motion along a fault.
The 2004 and 2005 earthquakes were "thrust" earthquakes, in which the two sides of the fault were rammed more directly toward each other.
Also, Leonard says, this weekend's earthquake originated only 18.6 miles (30 kilometers) beneath the surface.
Usually subduction-zone earthquakes in this region occur 40 to 60 miles (70 to 100 kilometers) deep.
"This all suggests that this earthquake was possibly not on the main subduction zone," he wrote, "but on a shallower [unmapped] strike-slip fault [in the overlying crust]."
"I am speculating," he added, "but if this is the case, [the shallow nature of the quake] would explain why the damage is greater than for other magnitude 6.0 to 6.5 earthquakes in the last couple of decades."
The damage has been significant, with many villages destroyed and two of Indonesia's major cultural sites affected.
Borobudur, the largest Buddhist temple on Earth (see photo), was built 1,200 years ago about 25 miles (40 kilometers) north of the royal capital of Yogyakarta (pronounced JOG-jakarta), the major city in the affected region.
The temple lay abandoned for centuries before being rediscovered and restored in the early 1900s. The recent quake damaged the structure, but it appears to be mostly intact.
Prambanan, the oldest Hindu temple in Indonesia, was not so fortunate. This complex, built about 1,150 years ago, appears to have taken significant damage, according to newspaper reports.
The epicenter for Saturday's earthquake is also only a few dozen miles from the erupting Merapi volcano. But this doesn't mean that the eruption triggered the quake.
In a statement posted to its Web site, the U.S. Geological Survey notes that magma movements at volcanoes can produce shallow earthquakes.
"In the cases of many earthquakes that occur in the general vicinity of volcanoes, however, there are not obvious links to volcanic eruptions," the agency said.
Leonard, of Geoscience Australia, was more certain.
Though Merapi and the recent earthquake were produced by the same large-scale tectonic forcesthe slow collision of the Sunda and Australian plates.
The fact that the quake occurred during the eruption "is a coincidence," he wrote.
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