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29 May 2006 10:52
The powerful earthquake that hit Indonesia was just the latest display of violent seismic activity on the archipelago, which stretches across one of the most unstable parts of the Earth’s surface.
The country’s position on the planet’s crust means it will continue to experience such catastrophes, just as it has done for the past 50-million years or so, according to seismologists.
“The problem with Indonesia is that you have an area of intense seismic activity coinciding with a very densely populated part of the world,” said Gary Gibson, professor of seismology at the RMIT University in Melbourne, Australia.
“It means there will always be some terrible loss to earthquakes in Indonesia.”
The plates of the planet’s crust that float on the molten core of the Earth smash against each other constantly, but while those plates usually move only a little bit each year, those that meet at Indonesia move more quickly.
“These are probably the most active plates in the world—one is moving at around seven centimetres a year,” said Mark Leonard, seismologist at Geoscience Australia.
“That’s incredibly fast and as a result it produces a lot of energy that has to be dissipated somehow,” Leonard said. “And that is usually through earthquakes.”
Indonesia lies on a fault line, a huge crack in the Earth’s surface, where the hardened crust that forms the outer layer of the planet is at its thinnest.
The Indonesian fault is what seismologists call a subduction zone, an area where one plate buckles under another, causing the land above to crumple.
The area has been hard hit with quakes in recent years.
The 9,2-magnitude quake that caused the 2004 Asian tsunami disaster, which killed around 168 000 people in Indonesia, was off the coast of Sumatra.
The March 2005 quake that hit the Indonesian island of Nias killed around 600 people, while Saturday’s quake killed nearly 5 000.
“Essentially you have the Asian Plate, on which you find South-east Asia, sitting stationary, while the neighbouring Australian Plate is hitting it at a rate of seven centimetres a year,” said Leonard.
This collision creates stress on the surrounding rock, which bears the tension until it reaches the breaking point—when it releases the strain via violent earthquakes.
That sudden energy release can also cause cracks to appear in the Earth’s surface, allowing the molten rock below to spew upwards in volcanic eruptions.
The infamous explosion at Krakatoa, an island group near Java almost blown out of the sea in 1883 by a volcanic eruption, was one such episode.
As the Earth’s crust buckles, the affected layers have nowhere to move but up, and so mountains are created.
Many of the world’s great mountain ranges, such as the Alps in Europe, the Himalayas in Asia and the Rockies in North America, were created in this way.
Using sophisticated measuring equipment, scientists have discovered that the plates that meet near Indonesia are clashing at different rates along the fault line, producing earthquakes of differing magnitudes.
Near the main island of Java, where a 6,3 magnitude earthquake hit early on Saturday, the rate of crust lost in the subduction zone is high, around seven to eight centimetres a year.
Further west in Sumatra it is lower, at about six centimetres.
“You find that there are far fewer earthquakes in the sxi-seven magnitude range the further west you go,” said Leonard.
“There are far more powerful quakes further east but there are fewer people living on those islands, so the effects seem lessened,” he said.
The clashing of continents that happens at Indonesia has gone on since prehistoric times and scientists do not expect it to end, meaning the region can expect to be continually hit by earthquakes.
“Most of the world’s fault lines are under the sea so you don’t get to feel most of them—the line that runs near New Zealand, for instance, is almost as active as the one near Indonesia,” said Melbourne’s seismology professor Gibson. - AFP
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