How Does a Tsunami Work?


(Photo: Grolier)

Somewhere along the vast ocean floor, a colossal plate of oceanic crust strains to dip under another. It hits a snag, and the pressure builds. Then, with a tremendous jolt, the plates slip. The force quakes Earth and jostles the ocean water above it. A tsunami is born.

Tsunamis are violent ocean waves that may be truly gigantic. They can travel through the ocean with the speed of a jetliner, then rise to heights of 30 feet (9 meters) or more before crashing onto shore and rushing far inland.

The awesome power of tsunamis can have deadly consequences. In the past century, tsunamis have killed more than 50,000 people. In 1992 and 1993 alone, major tsunamis hit Nicaragua, Indonesia, and Japan—with a combined death toll of 2,000. On July 17, 1998, an earthquake off the coast of Papua New Guinea triggered three devastating tsunamis, each about 30 to 40 feet (9 to 12 meters) high. The massive waves annihilated several of the island's villages and killed about 2,200 people; thousands more were injured.

What is a Tsunami?

Tsunamis are popularly known as "tidal waves," even though they have nothing to do with tides. Somewhat more accurate is the translation of the Japanese term tsunami: "harbor wave." The name refers to the way that these waves can fill and overwhelm an entire harbor, as they have many times in Japan's history. Indeed, the islands of Japan have been hit by more tsunamis than any other country.

Many scientists use the term seismic wave to describe tsunamis, since these waves are often created by seismic, or earthquake, activity along the ocean floor. Tsunamis can also arise through a sudden and massive undersea movement, such as a landslide or volcanic eruption.

Tsunamis can occur in any ocean. However, they are most likely to arise in the Pacific Ocean's so-called "Ring of Fire." This "ring," circling much of the Pacific, marks an area of great instability in Earth's crust.

Anatomy of a Killer

Be it an undersea earthquake, landslide, or volcanic eruption, the event that triggers a tsunami typically lasts a few seconds or, at most, a minute or two. Yet the tsunamis that result can continue to pound nearby shores for hours, and distant shores for days. Indeed, a tsunami is rarely a one-hit phenomenon.

Just as a single rock thrown into a lake creates many ripples spreading out from the impact, the force that spawns tsunamis sends out a series of underwater shock waves. But this force is not a single point, like the rock. The fault line on which an earthquake is centered can extend for hundreds of miles.

In a tsunami-forming earthquake, the ground on one side of a fault line suddenly lifts or sinks. All of the water above it also rises or falls, forming a high point (called a wave crest) or low point (a wave trough) in the water. In this way, each shudder or jolt of the earthquake sends out another tsunami front.

In the very deep water of the open ocean, tsunamis travel quickly and spread out from one another. The spacing within a series of seismic waves can be as much as 100 miles (160 kilometers).

A newborn tsunami traveling in open water is virtually invisible. The surface of the ocean generally rises and falls only a few inches—or at most a few feet—between the wave's crest and trough. As a consequence, tsunamis cannot be reliably spotted by airplane. Even observers in a ship would hardly feel the wave's passage beneath them because the bulk of a tsunami lies beneath the surface. The rise and fall of its wave reaches all the way down to the seafloor.

Importantly, tsunamis do not die out after traveling a short distance (as a windblown surface wave might). The seismic wave's energy is greatest at its source and diminishes as it travels. But that initial burst of energy is often so great that even a so-called "diminished" seismic wave can do a great deal of damage.

Consider, for instance, a 1960 earthquake off the coast of Chile, registering 8.9 on the Richter scale. It created tsunamis that reached heights of up to 35 feet (10.7 meters) along much of the Chilean coast. The waves also headed eastward across the open ocean. Fifteen hours later, they hit Hilo, Hawaii, killing 61 people and injuring 282. Eight hours after that, this same series of waves reached Hokkaido and Honshu, Japan. At that point, they reached "just" 12 to 20 feet (3.7 to 6 meters) in height—enough to kill more than 180 people, leave 500,000 more homeless, and cause more than $400 million in property damage. Indeed, the energy of this set of tsunamis was so great that measurable waves continued to reverberate back and forth across the Pacific for longer than a week.

The speed of tsunamis can be amazing. In open ocean, they generally travel more than 500 miles (800 kilometers) per hour. In the very deepest parts, that speed increases to nearly 600 miles (960 kilometers) per hour. As a result, they can traverse the entire Pacific Ocean in less than 24 hours.

A tsunami is forced to slow down only when it approaches the coast. Unfortunately, this is when it becomes dangerous! Near to shore, the deep-ocean floor rises to meet the beach. When the front of the tsunami reaches this shallow area, the friction created by the water climbing over the rising ocean floor slows the water in the leading edge of the wave. Meanwhile, the back of the wave may be many miles behind in deep water—still traveling at great speed, as are the waves to come.

This slowing of the leading edge of the first wave triggers a "pileup"—not unlike that seen when a car suddenly slows on a crowded highway. The rest of the wave, and perhaps the wave behind it, bunch up. The water has nowhere to go but up.

Some of the most massive tsunamis rear 30 feet (9 meters) or more above the surface of the water. There have even been historical reports of tsunamis more than 100 feet (30 meters) high—as tall as a 10-story building!

By the time it hits shore, a tsunami may have slowed to as little as 30 miles (48 kilometers) per hour. What it lacks in speed, it makes up for in power. A typical tsunami may dump more than 100,000 tons of water per 5 feet (1.5 meters) of coastline. It can spread its water arms 1 mile (1.6 kilometers) or more across—depending on the features found underwater and onshore, which can help squeeze or widen the wave—and reach more than 1,000 feet (300 meters) inland, smashing buildings and hurling boats, cars, boulders, and other objects with ease. This vulnerable coastal area is called the inundation zone.

Tsunamis typically do the most damage in the coastal zones that lie closest to their place of origin—within 30 to 60 minutes of tsunami-travel time. If the epicenter of the earthquake is close enough to shore that shaking is actually felt, any resulting tsunamis will hit within seconds or minutes.

The moments just before a tsunami hits can be a time of eerie calm. In some places, the water begins to slowly but surely rise. In others, the water actually pulls back from the coast. In some instances, harbors and bays are entirely emptied of their water. This happens if the trough of the wave is traveling ahead of the crest. The temptation to explore such a bared ocean floor can be deadly. Such was the case when crowds of curious onlookers walked into the emptied harbor of Lisbon, Portugal, before a tsunami hit in 1755; and again off the shore of Hilo, Hawaii, in 1946.

A much wiser move would have been to run to high ground as far inland as possible. But escape is not always possible. In 1992, tsunami waves crashed into the front of an Indonesian island, split in two to wrap around the island, and then recombined to destroy two villages on the supposedly safe leeward side.

Tsunami Safety

The power of a tsunami is the stuff of legend. Some people believe that a tsunami wiped out the mythical city of Atlantis. Another theory holds that it was the initial harbor-emptying phase of a tsunami that was the real force behind Moses' parting of the Red Sea. It has even been suggested that giant tsunamis—spawned from a gigantic asteroid impact—helped to kill off the world's dinosaurs.

Despite modern technology, humans are not much better off than the dinosaurs when it comes to avoiding tsunamis. The only thing people can do is get out of their way, and fast! Many coastal communities—especially those along the Ring of Fire—have developed tsunami evacuation plans. Their aim is to shepherd people off beaches and low-lying coastal areas and onto higher ground. Crucial to any such plan is an early-warning system.

The Pacific Tsunami Warning Center was built in 1948. It collects seismic data from 26 member countries and a number of other nonmember participants via satellite, and keeps watch 24 hours a day for any suspicious shaking that might trigger tsunamis. A second warning center, in Palmer, Alaska, studies local seismic activity that could trigger tsunamis heading toward the coasts of British Columbia, Washington, Oregon, and California.

When seismometers detect a quake of magnitude 7.5 or greater near or under the ocean, the warning centers send out an initial alert to all local authorities within three hours of tsunami travel time. Once the warning goes out, civil-defense or local authorities must decide whether and how to evacuate coastal areas.

Meanwhile, the warning centers begin monitoring water levels, using the U.S. National Ocean Survey's system of tide-gauge stations, found along coasts and in harbors and bays. Changes in local tides enable the warning centers to determine if a tsunami has indeed been generated, and how big it might be.

Scientists are also working on developing methods to detect tsunami movement within the ocean. Deep-sea sensors that each measure the weight of the column of water above them are now being tested. Changes in this weight could indicate that a tsunami crest has passed by.

Fortunately, there has not been a Pacific-wide, tsunami-causing quake since 1960. This is not to say there have not been any recent tsunamis. Indeed, since 1992, some 3,000 people have died in local tsunamis. In July 1993, for instance, a magnitude 7.8 earthquake off the coast of Japan produced a tsunami 35 feet (10.7 meters) high, which all but destroyed the fishing and resort island of Okushiri off the west coast of Hokkaido. The waves killed at least 120 residents, toppled a concrete lighthouse, and left kelp hanging from power lines.

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