INDEPTH: FORCES OF NATURE: Tsunamis

CBC News Online July 17, 2006

What is a tsunami?

A tsunami is a series of very long ocean waves created when a large body of water is displaced. A tsunami can hit shore with devastating impact, as one did on Dec. 26, 2004, when a series of waves pounded the coastlines of Southeast Asia, levelling whole villages and killing around 150,000 people.

Tsunami, pronounced soo-NAH-mee, comes from a Japanese word that means "harbour wave." It's often incorrectly called a tidal wave, which is a periodic movement of water produced by the gravitational pull of the sun and moon. Tsunamis are not connected with the weather or tides

How are tsunamis created?

Tsunamis can be generated by any disturbance that displaces a large amount of water, including earthquakes, volcanic eruptions, meteorites or landslides into the water or below its surface.

The tsunamis that hit the shorelines of eleven countries on Dec. 26, 2004, were triggered by a megathrust earthquake. Megathrust earthquakes are a potentially very destructive type caused when a tectonic plate in the Earth's crust slips under another one. In this case, a 1,000-km section of the India plate moved sideways and downward under the Burma plate just off the west coast of the Indonesian island of Sumatra, according to the U.S. government's Earthquake Hazards Program. The resulting earthquake measured a magnitude of 9.0 on the Richter scale, making it the most powerful tremor in 40 years. The collision caused the seabed under the Indian Ocean to rise by as much as 10 metres and possibly even 30. The vertical movement of the ocean floor triggered the tsunamis.

How big do tsunamis get?

In the deep ocean, tsunamis might have wavelengths as long as several hundred kilometres and reach speeds of up to 720 kilometres per hour. Yet the waves may be less than a metre tall, letting them pass unnoticed beneath ships at sea. When these waves enter the shallower water approaching shore, their speeds drop and their heights increase dramatically. They tend to get bigger if they roll over gentling sloping shores and underwater ridges, towering as high as 30 metres. The highest recorded tsunami occurred in Lituya Bay, Alaska, on July 9, 1958. The wave, triggered by a landslide in a narrow bay, reached a height of 518 metres by the time it hit the opposite slope. When tsunamis slam into shore, they can flood up to two kilometres inland, sweeping people out to sea, flattening buildings and toppling trees. Between five minutes and an hour can pass between a tsunami's waves, amplifying its destruction. For example, after an initial tsunami wave swept over Thai resorts in December 2004, people flocked onto the beaches to help the injured. Then a second wave struck and claimed even more victims.

What was the most destructive tsunami?

The most devastating recorded tsunami demolished parts of the East Indies on Aug. 27, 1883, after the volcano Krakatoa exploded. More than 36,000 people died because of the waves, which reached heights of 30 metres and speeds of 724 km/h.

Is there any warning?

The killer waves usually strike with little warning. When an earthquake rumbled off the coast of Hokkaido in Japan in July of 1993, the resulting tsunami hit just three to five minutes later, killing 202 people who were trying to flee for higher ground. Often a sharp swell gives the first sign as a series of tsunami waves approach the coastline. Then the water suddenly rushes outward, often exposing offshore areas for a few minutes. Then the first massive wave hits. Usually, the third to eighth waves are the biggest.

About 80 per cent of all tsunamis occur in the Pacific and many cities around the ocean – mostly in Japan, but also in Hawaii – have warning systems and evacuation procedures for serious tsunamis. One of the best ways to predict tsunamis is to monitor earthquakes, which set off most of the waves. Seismograph networks, wave gauges (such as those operated by international Tsunami Warning System) and satellite measurements of sea level changes can help warn of tsunamis.

How often do they occur?

There are an average of two tsunamis each year that cause damage somewhere in the world. About every 15 years, a destructive, Pacific-wide tsunami occurs, according to the U.S. government's West Coast and Alaska Tsunami Warning Center.

Can tsunamis hit Canada?

They have. On March 27, 1964, a large earthquake in Alaska triggered a tsunami that caused damage all the way to California. It pounded Vancouver Island shorelines, causing several million dollars of destruction to the community of Port Alberni. A warning system allowed thousands of people to flee their homes, so no lives were lost. B.C.'s Provincial Emergency Program warns that several areas are vulnerable to tsunamis, including the Queen Charlotte Islands, the West Coast north of Vancouver Island and the western shorelines of the island itself.

What do you do to survive a tsunami?

If you are near the ocean and feel a large earthquake, you should go inland or to higher ground immediately. If a tsunami were to be generated close to British Columbia, waves could reach shore within a few minutes – not enough time for officials to issue a warning. On land, know the community's suggested evacuation routes to safe areas. Prepare an emergency supplies kit for your home, car and work. Stay away from the coast because waves can roll in for hours. Getting to higher ground is the best bet. Otherwise, climb to an upper floor or roof. As a last resort, climb a tree. If you are on a boat, you should leave the harbour for open water, where tsunami effects aren't as damaging. If you're swept up in the waves, climb onto something that floats.

On land, know the community's suggested evacuation routes to safe areas. Prepare an emergency supplies kit for your home, car and work. Stay away from the coast because waves can roll in for hours.

How can low-lying regions protect themselves?

Asian countries have devised some low-tech ways to survive natural disasters such as cyclones. Unlike tsunamis, cyclones often come with advance notice. In Bangladesh, storm shelters are built on stilts, and emergency preparedness volunteers have radios and megaphones to warn of cyclones. In the Philippines and in India, car tires are placed on top of huts as anchors. In southern India, a series of loosely packed boulders form structures with sloping surfaces that can channel water into the sea. Mangrove forests are planted in Vietnam, Thailand, the Philippines and southern India to help filter wind and water during cyclones.