Tsunamis: When Tectonics and Water Combine

A sign warning of a tsunami.

LAUPAHOEHOE, Hawaii — Almost 70 years ago, in this one-time waterfront village, students sat in a classroom when the waves came and went with only destruction left in the wake.

An earthquake near Alaska on April 1, 1946, sent big, rolling walls of water to Hawaii. The disaster killed 159 people in the islands, including the 19 at the small village school. It was nowhere near the deadliest tsunami in history, but a poignant reminder that an earthquake can cause damage thousands of miles away.

Hawaii holds these memories close. In Hilo, the biggest city near Laupahoehoe, some 30 miles away, the Pacific Tsunami Museum provides residents and visitors with almost all the information they could want about the powerful waves that have come ashore in the state’s history.

What is a Tsunami?

Tsunamis are large waves caused by tectonic activity. When earthquakes occur or volcanoes erupt near water, the strong vibrations can pass through the water causing massive waves known as tsunamis.

Generally, most tsunamis are never felt on land. They are either slightly larger than normal waves, or the energy from the event is shifted off into uninhabited areas of the ocean, and dissipates along the way.

Sometimes, however, a powerful tsunami is directed toward land and waves have been known to reach 10 meters (33 feet) high. These huge waves sweep over beaches and travel inland, unleashing their powerful force on towns and cities.

What Causes a Tsunami?

So what exactly causes these giant waves? Well, like the Hawaii disaster of 1946, an earthquake is the culprit.

The planet has more than a dozen tectonic plates, pieces of the earth that at times shift and move because below them the mantle — and nearer the core, the molten — are not nearly as solid as the plates. Convection currents, or heat transfers, cause the plates to smash into each other, pull apart, or reposition themselves atop of one another, and the impact sends powerful shock waves through the earth.

Now add water to the equation. Think about sitting in a bathtub and moving your legs back and forth as if you were making a snow angel. The water begins to ripple, and the faster you move your legs, the bigger and stronger these ripples get.

This type of force, during an earthquake, is tens of millions of times stronger, thus the waves they generate can become much bigger. They might start as long waves only inches high, however as they get closer to shore, the shallow water forces the water into a high wave.

Volcanoes are another source of tsunamis, however, this is a much more rare occurance.

Historic Tsunamis

The 2004 Indian Ocean earthquake showed us how horrifying these waves can be. The disaster killed more than 230,000 people in 14 countries, about 70 percent of those casualties in Indonesia. There was about $15 billion in damage. The earthquake had a magnitude of 9.0 and the waves were felt as far away as South Africa. Hollywood even made a movie about it.

There are more than a thousand years worth of data about tsunamis, something you easily peruse at the National Oceanic and Atmospheric Administration website. You’ll see a lot of them have occurred in the Pacific Ocean, as the Ring of Fire is ripe for earthquakes and therefore tsunamis.

In 2011, a large tsunami occurred in Japan, after a 9.0 earthquake. It was the most costly disaster in history, at more than $300 billion. Entire towns were wiped off the map. Fukushima, a nuclear power plant was hit, spreading nuclear debris across the Pacific. But the death toll, at about 18,000, was much lower than the Indian Ocean disaster, as people in Japan had better monitoring and were therefore more prepared for it.

Learning from History

In 2011, Japanese meteorologists issued the most severe tsunami warning possible, prompting evacuations throughout low-lying areas of the country.

With this amount of warning, why was there still a significant death toll? The size of the waves, almost 8 meters in some places, exceeded the height of some tsunami walls, while thousands of residents did not realize they had not escaped to high-enough grounds.

At least they were warned. At the time of the Indian Ocean earthquake, there was no tsunami warning system, a problem in poorer countries. A system is in place today.

Perhaps most remarkable are the stories of survival. The death toll would have been higher had it not been for people who recognized the receding water levels at several beaches. They immediately alerted everyone around them and they all retreated to higher ground.

Tsunami Memorials

A lot of places have tsunami memorials. They can serve as a reminder to avoid future disasters or just a place to pay respects to those lost.

Japan has them, and you’ll find a handful in the countries along the Indian Ocean. In Khao Lak, Thailand, you’ll find the Police Boat 813 memorial, a reminder of the Navy boat that later washed ashore. In Hikkaduwa, Sri Lanka, there is the Train Disaster Memorial, for the 1,700 people that died when waves inundated an overcrowded train traveling between Colombo and Galle.

In Banda Aceh, Indonesia, they even built a museum, the Aceh Tsunami Museum, to memorialize the victims, much like what was done in Hilo, Hawaii. Outside Hilo, people can get a more intimate feel for history at Laupahoehoe Point Beach Park, where a memorial reminds everyone of the tragedy. The names of the victims are engraved in a rock at the former site of the school.


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