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 Living on a Fault Line
A year after the tsunami, scientists fear that another monster earthquake might one day strike Sumatra. How frightened should we be? |
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Krakatau
The Son Also Rises |
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Tsunami
In the Wake of Tragedy
[01/10/2005] |
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Given Sieh's past record, his warnings have to be taken seriously. Sieh didn't foresee the scale of destruction and death wrought by the Aceh earthquake and tsunami. But he and a group of Indonesian and American colleagues had long been warning of the dangers of a massive shock off the coast of Sumatra. They concentrated most of their efforts on the communities inhabiting the string of islands that lie off the coast, an archipelago that sits virtually on top of the epicenter of both the Dec. 26 earthquake and the Nias event. After the December temblor, Sieh warned that a second one was coming. Just three weeks before the Nias shock, he told a reporter that "other parts within the section of this fault should be considered dangerous," and that the possibility of an imminent tremor was a "major issue." He had just finished preparing a new version of the poster he and his colleagues had been using to alert the islanders to the signs of a tsunami. "The poster was going to press the morning we heard news of the second quake," Sieh says ruefully.
The two shocks were triggered by ruptures in what geologists call the Sunda megathrust, a 3,000-km-long "subduction zone" where two tectonic plates—the vast interlocking sections of the earth's crust which together form the planet's surface—have been butting heads for millions of years. That produces the forces making Sumatra the most active seismic and volcanic region in the world. Under the Indian Ocean 200 km off the west coast of Sumatra, the leading edge of the Australian plate is pushing itself into and under the heavier, higher Sunda plate, an area that encompasses Sumatra.
Both the Aceh earthquake, which at a magnitude of 9.3 was the third most powerful ever recorded, and the one near Nias island, resulted from the sudden release of built-up pressure along the Sunda megathrust, scientists say. But each of those earthquakes released pressure only in one segment. And each increased pressure on nearby segments that remained locked. In scientific language, British geologist John McCloskey spelled out the consequences in the journal Nature in April: "Calculations show that stresses imposed by the second rupture have brought the megathrust closer to failure."
How can such definite forecasts be made about earthquakes, whose occurrences have been notoriously impossible to predict? "Because of the research that has been done in Sumatra over the last 12 years we now have data for three full cycles of seismic activity in this segment dating back a thousand years," Sieh explains. "That is unique. In no other comparable zone have we been able to assemble anything like such detailed data." Three different strands of research have been woven together. First, the skimpy (sometimes non-existent) historical record was consulted for evidence of past seismic activity. Then current seismic activity in the region was observed from global-position-system (GPS) monitors that Sieh and his team have been installing on the islands since 2001. The sensors, which are capable of measuring earth movements as small as a few millimeters, have allowed scientists to build up a detailed picture of the state of the megathrust under the string of islands on which they are positioned. The data collected during and after the two recent earthquakes was particularly revealing. "One of the monitors we put in place just before the Nias quake caught the biggest vertical movement caused by an earthquake ever recorded—a 15-meter uplift," Sieh says. By the time the whole system is up and running in 2007, it will have cost about $1 million, all raised from research grants, and will boast a string of 40 GPS monitors.
Lastly, and most importantly, Sieh and his Indonesian colleague Danny Natawidjaja have spent more than a decade examining coral reefs at the islands off the west coast of Sumatra. Using hydraulic chainsaws, the men hacked out cross-sections of huge coral heads that, like the rings on a tree trunk, can provide a remarkably accurate chronological record of past seismic events. The islands lie at the edge of the Sunda plate and are pulled downward 5 cm a year by the force exerted by the Indo-Australia plate. When the built-up pressure is suddenly released in a huge earthquake, the islands—and the corals growing on them—spring back upward by as much as five or ten meters. That usually pushes the corals out of the water, causing them to die and marking for scientists the moment of the tremor. Using this coral calendar, Sieh and Natawidjaja were able to detail a surprisingly consistent cycle of quakes. The segment of the megathrust facing the coast's biggest city, Padang, has recorded three mega-earthquakes in the last seven centuries, they say, first in about 1360, then around 1605 and finally in 1797 and 1833, a pair of tremors that are classed together as one because the second quake was triggered by the first. "That gives us a cycle that averages out at 220 years," says Sieh. "If we calculate from 1797, that would mean that we are due for another quake any time now. And then there's the added pressure on the fault from the two quakes in Aceh and Nias."
Sieh is careful to avoid making any exact prediction about when a new disaster might occur. And some other geologists have raised questions about the sense of urgency and imminent danger that Sieh conveys. Even Sieh himself is quick to say that earthquake prediction is "voodoo science." But the quiet Californian believes that both the historical evidence—"look how quickly Nias came after Aceh: five of eight gigantic quakes that occurred in the last century have been followed by earthquakes of similar or larger magnitude"—and the momentum of the research data points to something happening sooner rather than later.
Who would be most at risk? Sieh and his colleagues are principally concerned about the effect of a quake and tsunami on Padang, a city of nearly 1 million inhabitants strung along a beach halfway down Sumatra's west coast. Several factors add to the worry. One is population size. Banda Aceh, where the tsunami penetrated 8 km inland at some points, killing an estimated 90,000 people, has only a third of Padang's population. Then there's geography: 50% of Padang's population lives within 5 km of the shore. The death toll wreaked by a tsunami hitting Padang could dwarf the 169,000 dead and missing in Aceh, Sieh warns.
A group of Indonesian and German scientists are in the process of installing an early-warning system off the coast of Sumatra consisting of pressure sensors on the seabed and sonar buoys to detect changing wave heights. The system is expected to be fully functioning by 2008 and scientists say that they will be able to tell within 15 minutes of an earthquake whether a tsunami is imminent. When it is finally in place, the system will provide an invaluable extra warning mechanism. But Sieh and other scientists point out that a tsunami would take half an hour or less to reach Sumatra, making preparations for a speedy and orderly evacuation critical. As yet, though, there doesn't seem to be any sense of urgency in Padang. The city government has set up an Earthquake and Tsunami Command Post. But despite the grand name, the reality is a small shack on the beach that features a hastily drawn map of the city with arrows representing escape routes. Budiman, the 60-something retired civil servant appointed to man the post, says the danger has been exaggerated. "People in Padang should not worry too much," he says with a reassuring smile, as he sits in the post smoking a clove cigarette. "The possibility that a tsunami will hit is very low."
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