The Deadliest Tsunami in History?

National Geographic News

Updated January 7, 2005

Tsunamis: Facts About Killer Waves

The earthquake that generated the great Indian Ocean tsunami of 2004 is estimated to have released the energy of 23,000 Hiroshima-type atomic bombs, according to the U.S. Geological Survey (USGS).

Giant forces that had been building up deep in the Earth for hundreds of years were released suddenly on December 26, shaking the ground violently and unleashing a series of killer waves that sped across the Indian Ocean at the speed of a jet airliner.

By the end of the day more than 150,000 people were dead or missing and millions more were homeless in 11 countries, making it perhaps the most destructive tsunami in history.

The epicenter of the 9.0 magnitude quake was under the Indian Ocean near the west coast of the Indonesian island of Sumatra, according to the USGS, which monitors earthquakes worldwide. The violent movement of sections of the Earth's crust, known as tectonic plates, displaced an enormous amount of water, sending powerful shock waves in every direction.

The earthquake was the result of the sliding of the portion of the Earth's crust known as the India plate under the section called the Burma plate. The process has been going on for millennia, one plate pushing against the other until something has to give. The result on December 26 was a rupture the USGS estimates was more than 600 miles (1,000 kilometers) long, displacing the seafloor above the rupture by perhaps 10 yards (about 10 meters) horizontally and several yards vertically. That doesn't sound like much, but the trillions of tons of rock that were moved along hundreds of miles caused the planet to shudder with the largest magnitude earthquake in 40 years.

Above the disturbed seafloor the great volume of the ocean was displaced along the line of the rupture, creating one of nature's most deadly phenomena: a tsunami. Within hours killer waves radiating from the earthquake zone slammed into the coastline of 11 Indian Ocean countries, snatching people out to sea, drowning others in their homes or on beaches, and demolishing property from Africa to Thailand.

Tsunamis have been relatively rare in the Indian Ocean, at least in human memory. They are most prevalent in the Pacific. But every ocean has generated the scourges. Many countries are at risk. (Read "Tsunami: Facts About Killer Waves" for more about killer waves' causes and warning signs—information that can be a lifesaver in a tsunami zone.)

For more about the Indian Ocean tsunami of 2004 scroll down the page.

The Indian Ocean tsunami traveled as much as 3,000 miles (nearly 5,000 kilometers) to Africa, arriving with sufficient force to kill people and destroy property.

A tsunami may be less than a foot (30 centimeters) in height on the surface of the open ocean, which is why they are not noticed by sailors. But the powerful pulse of energy travels rapidly through the ocean at hundreds of miles per hour. Once a tsunami reaches shallow water near the coast it is slowed down. The top of the wave moves faster than the bottom, causing the sea to rise dramatically.

The Indian Ocean tsunami caused waves as high as 50 feet (15 meters) in some places, according to news reports. But in many other places witnesses described a rapid surging of the ocean, more like an extremely powerful river or a flood than the advance and retreat of giant waves.

Tsunamis can extend inland by a thousand feet (300 meters) or more. The enormous force and weight of so much water sweeps away almost everything in its path. As many as a third of the people who died in the Indian Ocean tsunami were children; many of them would not have been strong enough to resist the force of the water. Many people were crushed by debris or when the sea hurled them against structures.

Witnesses said the approaching tsunami sounded like three freight trains or the roar of a jet. In some places the tsunami advanced as a torrent of foaming water.

In several places the tsunami announced itself in the form of a rapidly receding ocean. Many reports quoted survivors saying how they had never seen the sea withdraw such a distance, exposing seafloor never seen before, stranding fish and boats on the sand. Tragically the novelty of the sight apparently stoked the curiosity of the people who ran out onto the exposed seafloor. Tourists in Thailand were seen wandering around photographing the scene.

Geographic Knowledge Saved Lives

People who knew geography knew what the receding ocean meant. Survivors who knew it meant trouble reported how they ran for high ground, rounded up family and friends, and tried to warn people who were drawn to the water's edge. Experts say that a receding ocean may give people as much as five minutes' warning to escape to high ground. That may have been enough time for many of the people who were killed by the 2004 tsunami to save themselves, if only they knew what to do.

A British newspaper reported that a school student, on vacation in Thailand, recalled a geography lesson about tsunamis and what the withdrawal of the ocean meant. She warned her family and they saved themselves.

In India a man told the Associated Press how he saved his village of some 1,500 people because he recalled watching a National Geographic television documentary about tsunamis [Killer Wave], and remembered that when the ocean receded it was a sign of danger. He sounded the alarm and led the people to high ground, saving almost the entire village.

Somehow the animals also seemed to know that disaster was imminent. Many people reported that they saw animals fleeing for high ground minutes before the tsunami arrived. Very few animal bodies were found afterwards.

When the ocean started to return on December 26 it was in the form of the tsunami—a series of crashing waves in some places and a sudden flood hundreds of yards inland in others. Reports quoted survivors saying they could not run away fast enough, although many people did manage to escape.

Death struck randomly. People who were together when the tsunami struck were separated in the torrent. Some survived; others succumbed or disappeared. A baby was found floating safely on a mattress.

Survivors of the Indian Ocean tsunami reported that the sea surged out as fast and as powerfully as it came ashore. Many people who had survived the wall of water rushing inland were seen being swept out to sea when the ocean retreated.

A tsunami is a series of waves, and the first wave may not be the most dangerous. A tsunami "wave train" may come as surges five minutes to an hour apart. The cycle may be marked by repeated retreat and advance of the ocean. Some people did not know this on December 26. Once the first wave had gone, they thought it was safe to go down to the beach.

The Indian Ocean tsunami destroyed thousands of miles of coastline and even submerged entire islands permanently. The island country Maldives rises only a few feet above sea level, but it is largely protected by outlying coral reefs. Even so, the tsunami swept across the reefs and was reported to have washed over some islands entirely. The capital and many tourist resorts in the Maldives were flooded. Astonishingly, relatively few people were killed. The country was likely protected from the full force of the tsunami by its reefs.

Rotting Corpses

As the day of horror drew to a close the ocean calmed. But where at the start of the day people were going about their normal lives or relaxing at exotic beach resorts now millions of people were struggling with the reality of tens of thousands of dead or missing relatives, destroyed homes, and shattered lives. The thousands of corpses, many hanging in trees or washed up on beaches, immediately started to rot in the tropical heat. With no food or clean water and open wounds, the risk of famine and epidemic diseases was high. Health authorities feared that the death toll might double to 300,000.

Across the world the magnitude of the disaster and the scale of the suffering prompted a new wave—one of sympathy, support, and assistance for the people affected by the tsunami.

Deep beneath the ocean, at the source of the great earthquake and tsunami, the Earth's tectonic plates continued their relentless pressing against one another. Pressure was already building for the day when pent-up energy will once again be released violently—but hopefully not for hundreds of years.

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