Putri Prameshwari, Jakarta Globe 2 Oct 09;
This week’s jolts off the west coast of Sumatra raise a critical question: was this the “big one” we’ve been waiting for?
Geology experts on Thursday said no, but it could nudge such an event a little closer.
Wednesday’s powerful quake that shook Padang came from a spot near — but not inside — the Sunda Trench, a long undersea crescent of seismic energy stretching from north of Aceh to the east of East Timor.
Tectonic plates from India and Australia are grinding slowly under plates that support Indonesia and Burma at a rate of up to six centimeters per year, causing explosive releases of force.
Quake expert Sri Widiyantoro, from the Bandung Institute of Technology, said the precise cause of the jolt in Padang did not come from the seam between plates, but was triggered by a break in the middle, a type known as an intraplate quake.
That means the colossal one some geologists say is a near certainty — is still to come.
He warned that if the subduction fault slipped in several places at once, it could set off a massive shock “with a magnitude of more than eight.”
Danny Hilman Natawidjaya, a geologist from the Indonesian Institute of Sciences (LIPI), said the 7.6-magnitude temblor disturbed the subduction zone, which runs directly under the Mentawai island chain off the west coast of Sumatra.
“The quake has affected the tectonic plates, increasing tension between them,” Danny said.
Geologists have long been watching the movement of those plates and measuring massive forces mounting between them. Some have predicted a huge jolt may be due over the next few decades, with the threat of an even greater humanitarian disaster hanging over densely populated Padang or Bengkuklu to the city’s south.
Predictions have been issued “since early 2004, before the great 2004 tsunami and earthquake,” California Institute of Technology geologist Kerry Sieh said. A series of big shakes — including a magnitude 8.2 that struck Bengkulu province in September 2007, and one off Aceh that triggered the 2004 tsunami — showed that all the segments had released their energy, except that of Padang.
But pressure on the fault means a vastly larger 8.8-magnitude quake, coupled with a five meter high wall of water, is sure to hit in the coming decades, Sieh said.
“This earthquake today is a flea compared to this tiger of a quake that is coming,” he said. “It’s 100 percent likely. The question is when is the date. The strain has been building off Padang for this 8.8 for 175 years.”
Experts have called on the government to invest in quake-resistant buildings and widen Padang’s roads — predicting an exodus of about 500,000 people in the event of a major quake. But little has been done. Geological Disaster Mitigation and Volcanology Center head Surono said there is a “perception that building such expensive infrastructure is not economically viable” because of the uncertainty of earthquake prediction.
“West Sumatra is like a supermarket for geological disasters. There are active volcanoes, landslides, land quakes caused by faults,” he said. “Being close to the faults means Padang is always prone to earthquakes. Every day, there is a tectonic quake there, but they may be too small for any effect to be felt.”
Danny said people in Padang, a city of about 900,000, live precariously close to the moving plates, and must always be vigilant for tremors or tsunamis.
Fauzi, a geophysicist from the Meteorological, Climatology and Geophysics Agency (BMKG), said the Padang quake could actually have released some of the energy built up between the plates. But he down played speculation that the quake could trigger an even more destructive calamity.
“No scientist can predict when an earthquake will happen,” Fauzi said, “so let’s not make the public more worried than they already are.”
Fauzi said the subduction zone could give birth to “the big one” as early as tomorrow, in the next 50 years or even over the next century.
Tectonic plates covering the earth, also known as the lithosphere, are constantly on the move. The shifting generates huge amounts of heat and force along the boundaries, which releases at breaking points in the form of quakes or volcanoes.
Fauzi said the earthquake that hit Jambi on Thursday morning was not related to the tremor near Padang.
The 7.0-magnitude quake had an inland epicenter, not under the ocean.
“Therefore, they are two totally different kinds of earthquakes,” Fauzi said, “they just happened to occur within hours” of each other. Some 18 hours before the Padang quake struck, another 8.0-magnitude tremor hit near Samoa in the Western Pacific, triggering a deadly tsunami that killed at least 150 people.
Danny said that the Samoa earthquake was not at all related to the Padang quake, and are located some 7,600 kilometers apart.
Danny said Wednesday’s tremor might be related to the one that hit Tasikmalaya in September, which could be felt in Jakarta.
“We don’t know for sure” if they are directly related, Danny said, “but the two epicenters are located in the same subduction zone.”
Additional reporting by AFP
Devastating Indonesian earthquake 'still to come'
Shanta Barley, New Scientist 2 Oct 09;
The earthquake which devastated the city of Padang in Sumatra, Indonesia, this week, killing more than 1100 people, may have been only a hint of worse to come. Since 2004, geologists have been predicting a far nastier earthquake in the region – a shallow tremor that will rip the sea floor apart, trigger a devastating tsunami and kill far more people.
"Another earthquake is on its way, and all it will take to trigger it is the pressure of a handshake," says John McCloskey, a seismologist at the Environmental Sciences Research Institute at the University of Ulster in Coleraine, Northern Ireland.
Padang experienced a magnitude-7.6 earthquake on 30 September, just after 5 pm local time. Images of terrified relatives waiting to identify dead bodies, their T-shirts clutched over their noses to mask the stench, military officials stalking between bright yellow, zipped-up body bags and centuries-old Dutch colonial mansions obliterated in an instant have flooded around the world.
At first, geologists assumed this was the earthquake they had predicting for many years. "Padang has bad geology," explains McCloskey. "It sits 40 kilometres above the most earthquake-prone stretch of the interface between the Indo-Australian and Eurasian plates."
This interface has not experienced the stress relief of an earthquake for over 200 years, according to McCloskey's analysis of historical coral growth rings, which show no sign of seafloor uplift. GPS measurements of the rate of plate motion suggest that there has been around 13 metres of movement in this area over the same period. "A shallow earthquake at the plate interface off Padang is long, long overdue," says McCloskey.
Freak event
Yet the earthquake which struck this week off Padang did not occur at the plate interface, which lies 500 kilometres offshore. The epicentre was just 45 kilometres from Padang, far away from the plate interface. What's more, it originated 80 kilometres underground, far deeper than the place at which the Indo-Australian and Eurasian plates crunch together.
Further evidence comes from the orientation of the rupture caused by this week's quake. "The rupture spread in a north-south orientation, rather than east-to-west, as we would expect along the plate interface," says McCloskey.
All the clues add up to the earthquake being a freak rupture of an ancient stressed fracture zone embedded deep within the Indo-Australian plate rather than slippage at the plate interface. "What we're looking at is probably a vestigial crack left over from some distant spreading centre," says McCloskey.
So, what kind of damage will the tsunami-triggering earthquake that the geologists have been predicting near Padang inflict? McCloskey has built computer models of over 125 scenarios in which shallow, powerful earthquakes at the interface off Padang jolt the sea floor, triggering tsunamis. In most, devastating tsunamis are generated. They will reach the city about 30 minutes after the earthquake hits.
His simulations suggest that 25 per cent of tsunamis would be over 5 metres tall as they reached the coast; the highest waves would be 12 metres tall. "In reality, of course, waves will gather height and become more turbulent as they power inland, which means they could be far higher over the city," says McCloskey.
Escape routes
If the people of Padang are well prepared, then most should survive, says McCloskey. Within 30 minutes, the young and the fit should be able to reach the 10-metre elevation contour that rises 2 kilometres back from the coast, he says, which would at least protect them from waves lower than 10 metres.
However, over 100,000 people – a seventh of the city's population – are blocked from running directly to higher ground by the barbed wire-laced, 10-metre-high walls of a huge military airport.
"Padang needs to build a tunnel under that airport, because if they don't these poor people will have to run parallel to the coast for several hundred metres while the tsunami is coming at them," says McCloskey. So far, no steps have been taken to build such an exit route. "Sometimes you despair," he says.
Journal references: Earth and Planetary Science Letters, DOI: 10.1016/j.epsl.2007.09.034; Nature, DOI: 10.1038/nature07572
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