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Another tragedy strikes Indonesia

May 30, 2006 By Philip Ball This article courtesy of Nature News.

On 27 May an earthquake struck Java, Indonesia, about 25 kilometres south-southwest of the city of Yogyakarta, claiming more than 5,000 lives. takes a look at the situation.

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How big was the quake?

The US Geological Survey has characterized this as a magnitude 6.3 event. That's strong, but not huge; in fact, earthquakes of this size typically happen somewhere in the world every two or three days.

Is it the quake size that determines how bad things get?

The amount of energy released by a quake is important, but the casualties in this case may be disproportionate to the quake's size. "I'm surprised there was that much damage," says geologist James Jackson of the University of Oxford, UK.

The number of people injured and killed often tends to depend more on the nearby population density and on the quality of the buildings than on the quake magnitude. In wealthy countries such as the United States and Japan, earthquake-proofing of buildings and other civic structures tend to keep casualties low for medium-sized quakes such as this one. The most recent quake at Parkfield, California, in 2004 was magnitude 6.0 but killed no one. In contrast, the magnitude-6.6 quake in Bam, Iran, in 2003 caused more than 40,000 deaths, partly because of poor building techniques.

Local geology can also make a difference. At Bam, the destruction was compounded by the city's fatal location, right at the end of the fault, where much of the seismic energy was focused. And at Northridge in California in 1994, damage caused by a magnitude-6.7 quake was exacerbated by sandy soil, which was rendered fluid (liquefaction) by the ground shaking. That was the costliest earthquake in US history, but only 51 people died.

Is this the same fault line as the one that triggered the tsunami?

No, but they're loosely related. The Indian Ocean near the east coast of Indonesia is a patchwork of three major tectonic plates: the Sunda plate to the east, the India plate to the northwest, and the Australia plate to the southwest. The tsunami was caused by a rupture at the convergence of the India plate and the Burma microplate. The Java quake last weekend happened further south, where the Australia plate dips northeastwards beneath the Sunda plate (and thus beneath Java) in a process known as subduction.

It's as bad as you see on television.
Andrew Jeremijenko,
doctor working in the Yogyakarta region
Earthquakes are often triggered at subduction zones by slippage of the sinking lower plate. But the Yogyakarta quake was caused by a secondary effect of subduction. The compression of the two plates as they move together put pressure on small, shallow faults behind the actual line of subduction. It was the sideways rupture of one of these that caused the quake. The same process was responsible for the devastating quake in Kobe, Japan, in 1995.

What effect will the quake have on the nearby Merapi volcano, which has been murmuring this past month?

The two are related: the volcanic activity at Merapi, about 80 kilometres to the north of the earthquake focus, is also due to the subduction process. Melting of rock above a subducting plate produces magma that rises up to the surface, which is why there are commonly volcanic islands behind subduction zones.

Some earthquakes have been triggered by magmatic processes linked to volcanic eruptions nearby. Such a link seems unlikely on Java. But the quake itself could affect the volcano. Brian Baptie of the British Geological Survey in Edinburgh, UK, says that there has been a slight increase in Merapi's activity since Saturday. "A lot of shaking could destabilize the lava dome," he says (see ' Indonesian volcano poses unpredictable threat'). It is not clear whether the quake could also affect the deeper magma plumbing system of the volcano.

Andrew Jeremijenko, a doctor who has been working with victims of the quake, says that there was another eruption from Merapi on Monday morning. He fears that a really large one could create many casualties, putting further strain on the relief effort.

Was an event like this expected?

It certainly wasn't surprising. "The whole of Java is seismically active," says Baptie, because it sits over a subduction zone. "There's an earthquake of this size every five years or so in the region." Jackson adds that shallow-fault quakes "aren't uncommon". But Yogyakarta itself has never experienced anything as catastrophic as this before, and seems to have been unprepared for it.

What is the biggest challenge faced by the local people now?

"It's as bad as you see on television," says Jeremijenko. "It's basically a disaster zone." He says there are people with multiple injuries, many needing amputations, and "people are dying who probably shouldn't die." There is a shortage of nurses and equipment, and to make matters worse, it has been raining quite a lot, with many casualties protected only by tents because of a lack of hospital beds. Jeremijenko fears that it is probably only a matter of time before there are epidemic outbreaks of tetanus and possibly cholera, measles and other infectious diseases.

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