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The fault that most worries Sieh, though, is the Sunda megathrust. He had been studying it for a decade before it caused the 2004 tsunami; a few years ago he gave up a tenured professorship at Caltech and moved to Singapore in part to be closer to the fault. It stretches 3,700 miles from Myanmar to Australia. The 2004 quake happened near the northern end. "That particular stretch, from northern Sumatra up to the Andaman Islands, was on nobody's radar screen," says Sieh.

He had been working off Sumatra but several hundred miles to the south, measuring the ages of dead coral reefs. When the seafloor rises during an earthquake, it can thrust a reef above water, killing the corals; radiometric dating reveals when that happened. By 2003 Sieh and his colleagues had reconstructed a disturbing seismic history for west central Sumatra.

"We found what we call supercycles—clusters of big earthquakes occurring at regular intervals," he says. For at least the past 700 years pairs of large earthquakes had occurred about every 200 years on that segment of the Sunda megathrust, with the earthquakes in each pair separated by roughly 30 years. There had been a pair around 1350 and 1380, another in the early to mid 1600s, and a third in 1797 and 1833—two centuries ago. It looked like another pair of quakes was due.

The discovery worried Sieh so much that in July 2004 he and his colleagues began distributing posters and brochures on the Mentawai Islands, where they were doing their research, warning people about tsunamis. Five months later northern Sumatra was devastated, and Sieh's group received a lot of publicity. "We got credit we didn't deserve," he says. "Our forecast was for a different part of the fault." But that forecast still stands—in fact, says Sieh, the first of the anticipated pair of quakes already happened, in September 2007. A magnitude 8.4, it did comparatively minor damage. At Padang, capital of the province of West Sumatra, the tsunami was only around three feet high. Padang is a low-lying city of more than 800,000. Sieh fears it may not fare as well the next time.

"There's never been a more precise forecast of a giant earthquake, period," he says. "Our forecast is for an 8.8 magnitude earthquake in the next 30 years. Nobody can say whether it will be 30 seconds from now or 30 months. But we can say it's very likely to happen within 30 years.

"What are you going to do?" he goes on. "Move the whole city for something that happens once every 200 years? That for me is the quintessential human predicament in regard to these very unlikely but very consequential events. The fundamental problem is not that scientists don't know enough, and it's not that engineers don't engineer enough. The fundamental problem is that there are seven billion of us, and too many of us are living in places that are dangerous. We've built ourselves into situations where we simply can't get away. And I think this will be a century of paying the consequences."

When the tsunami hits Padang, most people will have no high ground to run to and no more than 20 minutes to run. Much of the city stands less than 15 feet above sea level. The waves could inundate nearly everything within roughly a mile of the waterfront. The open-air restaurants that line the harbor will be swept away first; dark water will surge down streets clogged with motorbikes; thousands of flimsy one- and two-story homes and shops will vanish. The death toll is likely to be much higher than in Japan last March—probably closer to the 90,000 lost in Banda Aceh.

Life in Banda Aceh these days blends the horrific and the miraculous. The cataclysm that left the city strewn with contorted corpses, stripped naked by the waves, also brought peace, ending decades of violent conflict between Acehnese secessionists and the Indonesian government. "During the war you would also see bodies in the streets," says Syarifah Marlina Al Mazhir, program coordinator for the American Red Cross in Indonesia and a Banda Aceh resident. "The tsunami changed everything. And now we can go out at night!" A massive infusion of aid has helped rebuild the city, and young people pack its innumerable cafés late into the night. But everyone knows someone who died on December 26, 2004. "Sometimes when I close my eyes, I can still hear people screaming," one woman told me. In a small park children too young to remember play on a slide in the shadow of a 200-foot-long, 2,600-ton ship, preserved where the tsunami dropped it, on top of some houses, more than a mile inland.

On a sultry July morning in Padang, an elementary school about a half mile from the beach is drilling for the inevitable. At about 10 a.m. an alarm bell rings, and children erupt from their classrooms into the small, sandy courtyard—boys in white shirts and red pants, girls in white blouses and head scarves and ankle-length red skirts. Squatting in circles, they hold their small backpacks over their heads to protect them from debris that might fall during an earthquake. They all chant in unison. "They're repeating the 99 names of Allah," says Patra Rina Dewi. " ‘The Merciful, the Compassionate, the Guardian.' It's to keep them calm during a real emergency."

Patra, 39, is the energetic head of a small nonprofit tsunami-awareness organization called Kogami, which she and a few friends founded after seeing reports from Banda Aceh. Under pressure from Kogami, Padang has already marked 32 evacuation routes, and nine of a planned hundred multistory shelters are under construction to allow some people to escape the waves. Meanwhile Patra and her staff of 16 have started tsunami drills in schools like this one. Because there is no high ground nearby, the 567 students here have been drilled to run about two miles inland. But the 80 or so first graders can't run fast enough. "The first graders need 40 minutes to reach the safe area," says Elivia Murni, one of the teachers. "They will disappear if the tsunami comes. We won't be able to save them."

There are about a thousand schools along the coast of West Sumatra, and Kogami has started training programs in 232. It won't even try in some of the fishing villages that dot the coast northwest of Padang. "Sometimes I can't sleep at night," Patra says as we leave one of those villages. Lush hills rise to the east, but broad, muddy rice fields would make it impossible to reach the hills in time. "There are no escape routes for them here," Patra says. "If we told them about the tsunami danger, we would only leave them feeling hopeless."

When March 12 finally dawned in Minamisanriku, Jin Sato and his diminished band on the roof were cold, sodden, and utterly exhausted. They climbed down fishing nets that had washed up against the red steel skeleton of the gutted building and made their way up a nearby hill, where other survivors were gathering. Sato's office is now in a prefab building on that hill. He's 60 and trimly built, with thick black hair and glasses and a level, serious gaze. His hands are scarred from gripping the radio antenna. Buddhist prayer beads encircle his left wrist.

The town Sato grew up in is gone, but he is still responsible for many of its people, who are living in shelters or temporary housing. The land here dropped more than two feet after the earthquake, so large parts of the former town flood at high tide. Resurrecting Minamisanriku may prove impossible, and that is a source of anxiety for the survivors. "People want to stay here, where their ancestors lived and died," says Sato. "They don't want to move."

Tim Folger's article on rare earths appeared last June. His family hails from Nantucket, but he lives now in Gallup, New Mexico, far from the sea.
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