Quaking Earth, Racing Waves

In July 2004 the village school on Tello Island, Indonesia, had a visitor with a startling story to tell. As the students in their red and white uniforms sat quietly listening, geologist Kerry Sieh explained that under the ocean, 60 miles from their island, was a ticking time bomb. For hundreds of years, the Sunda Megathrust Fault had been storing energy that would be released in massive undersea earthquakes. The powerful quakes would likely cause tsunamis, fast-moving waves that could wipe out the entire seaside village.

The students and their teachers were surprised by Sieh's warnings. They'd never felt giant earthquakes or seen tsunami waves. How did he know that the earth was going to shake?

Sieh explained that, for more than a decade, scientists from the California Institute of Technology had been studying a section of the fault just to the south. They'd figured out that major earthquakes shook the region about every 200 years. The last big quake was in the early 1800s, which meant another could come at any time. Though Sieh couldn't say exactly when it would happen, he was almost certain there would be at least one major earthquake in the students' lifetimes.

But no one could have known that the next big quake would hit just a few months later.

Rising Corals

Scientists know a lot about earthquakes after they happen, but they can't predict what hour, day, year, or even decade an earthquake will hit. So how did Kerry Sieh know to warn the Tello islanders that an earthquake might happen soon? He read the corals.

In the Indian Ocean, big corals called Pontes grow from the sea floor to the water's surface, then outward. The ocean floor sinks slowly between earthquakes, dragging the coral down, then rises quickly during a quake, raising the coral up again. Over hundreds of years, all this up and down causes the coral to grow outward in doughnut-shaped rings. Sieh discovered that by looking at the growth patterns of Porites coral heads near the fault, he could pinpoint the dates of past earthquakes, and maybe find a pattern that would help predict future quakes.

Using underwater chainsaws, Sieh and other scientists sliced off slabs of coral heads that were hundreds of years old. Sure enough, they found that, on a section of the fault just to the north of the Mentawai Islands and just to the south of Tello, earthquakes occurred in pairs about every 200 years. One pair of quakes hit in the 1300s, another in the 1500s, and a third in 1797 and 1833-almost 200 years ago. According to the corals, it was time for another big quake.

Sinking Islands

The corals weren't the only evidence of underground rumblings in Indonesia. The Sunda Megathrust Fault at the bottom of the Indian Ocean marks the collision between two of the plates that make up the earth's surface, one oceanic, the other continental. Between earthquakes, the plates are stuck together. As the oceanic plate slips slowly downward, it squeezes the continental plate sideways about half an inch a year, and drags it down a few inches a year as well. The islands on top of the continental plate are dragged down too, as much as half an inch a year. The more years between earthquakes, the more the islands sink-and the more stress builds up at the fault.

The islanders could tell that the water line was shifting. "They can see their boardwalks and harbors sinking," Sieh said. Trees that once grew tall on shore were now underwater, and wells that once gave freshwater were full of salty seawater instead. But no one thought that this had anything to do with earthquakes or tsunamis.

Evidence from Global Positioning System, or GPS, stations they'd set up to measure island sinking also had convinced the scientists that a big quake could rock the area at any time. "As we came to realize what we were learning, and how much at risk people were," said Sieh, "we couldn't keep quiet."

In July 2004, Sieh visited five islands and gave presentations at schools, churches, mosques, and village squares. Sieh and his colleagues planned to return the following year to visit more islands and teach more people about their research.

Then, six months later, a quake struck.

December 26, 2004

The ground shook so violently that people were knocked off their feet. Dishes fell from shelves, roofs collapsed, trees toppled. Two minutes after it began, the shaking stopped. It had been the biggest earthquake anywhere on the planet in 40 years.

Like a twig you bend and bend until it breaks, pressure that had been building along the Sunda Megathrust Fault for hundreds of years had finally given way. Along a section of the fault longer than the state of California, the oceanic and the continental plates suddenly, violently separated, sending out earth-shaking waves. But the worst was still to come.

The continental plate sprang up as much as 20 feet, pushing up the water above it. Tsunami waves rippled out in all directions, gaining power as they raced across the open ocean as fast as jet airplanes. The first wave, 100 feet tall in some places, hit the Indonesian island of Sumatra 15 minutes after the earthquake. Waves swamped the coast of Thailand 75 minutes later, then India and Sri Lanka, and even Africa, 3,000 miles from the quake's center.

The deadly waves kept flooding beaches for hours. Hundreds of thousands of people were killed, and millions were left homeless.

Kerry Sieh was at home in California when he heard the news. Immediately, Sieh thought of his friends on the islands he'd visited. Had they escaped the quaking ground and giant waves? Had their homes and villages been destroyed? Communication by phone or email was impossible. On January 1, he flew back to Indonesia, uncertain of what he'd find.

Safe for Now

People in Tello were lucky. Their island was more than 200 miles from the epicenter of the quake, the most powerful point. On Tello during the quake, the earth shook, but not violently. Later, a small tsunami, three to six feet high, swept through the village, flooding houses. People were shaken and scared, but unharmed.

As he traveled to the other islands he'd visited in July, Sieh was relieved to hear the same story. Few homes had been destroyed, and no lives were lost. But danger still lurked. An earthquake on one section of a fault can increase stress along the rest of the fault. And the thousands of miles of the Sunda Megathrust Fault that hadn't ruptured in December were still ripe for another quake.

Sure enough, another earthquake shook an area to the south on March 28. This quake was 10 times less powerful than the one in December, yet it was still the second-biggest quake to rock the world in 40 years.

Again, Sieh's friends escaped harm. But the quakes were proof that what the scientist had said was true, and they convinced some islanders to take action. Today, on the island of Simuk, people are leaving their homes near the shore and rebuilding their town at the island's highest point, the hill where Sieh erected his GPS station.

The quakes also provided Sieh with a lot of work to do. On Sumatra, the rising continental plate pushed up vast stretches of beach that had been underwater. "We saw thousands of dead corals," Sieh said. He is looking at data from the GPS stations to find out exactly how the nearby islands moved during the quakes.

As they travel the islands by boat and helicopter, Sieh and his colleagues will explain why earthquakes and tsunamis happen and what people can do to prepare. They can build their houses out of lightweight wood or bamboo rather than heavy concrete, which would cause more damage if it toppled during a quake. They can move their villages away from the beach, or build pathways to higher ground.

Sieh doesn't know exactly when or where it will hit, but he's certain another big quake is coming along the section of the fault south of Tello. Until it does, he'll try to understand as much as he can about why and how the earth moves, and he'll teach the people who live nearby about the danger that lurks under the waves.

Copyright Carus Publishing Company Oct 2005
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