Killer Waves—Myths and Realities
THE sun had set just a few minutes earlier. On this tranquil Friday, July 17, 1998, the men, women, and children of several small villages on the northern coast of Papua New Guinea were suddenly shaken by a magnitude-7.1 earthquake. “The main shock,” says Scientific American, “rocked 30 kilometers (nearly 19 miles) of coastline . . . and suddenly deformed the offshore ocean bottom. The normally flat sea surface lurched upward in response, giving birth to a fearsome tsunami.”
An observer says that he heard what sounded like distant thunder, which gradually faded as the sea slowly receded below the normal low-water mark. A few minutes later, he spotted the first wave, which was about ten feet [3 m] high. It overtook him as he was trying to run away from it. A second, larger wave flattened his village and swept him along for nearly a mile [1 km], into a nearby mangrove forest. “Debris hanging from the tops of palm trees indicated that the waves reached heights of 14 meters [46 feet],” reports Science News.
That evening giant waves took the lives of at least 2,500 people. As a twist of irony, a lumber company later donated timber for new schools, but there were virtually no children left to go to school. Almost all—more than 230—had been killed by the tsunami.
What Are Tsunamis?
Tsunami is a Japanese word that means “harbor wave.” This is “a fitting term,” says the book Tsunami!, “as these giant waves have frequently brought death and destruction to Japanese harbors and coastal villages.” What gives these freak waves their awesome power and size?
Tsunamis are sometimes called tidal waves. Strictly speaking, however, tidal waves are simply the surging and waning swells that we call tides and are caused by the gravitational pull of the sun and the moon. Even the massive waves—sometimes over 90 feet [25 m] high—that are whipped up by gale-force winds cannot be compared with tsunamis. If you were to dive beneath these tidal waves, you would find that their influence weakens the deeper you go. At a certain depth, the water is hardly disturbed. But not so with tsunamis. Their influence reaches from the surface right to the ocean floor, even though the water may be miles deep!
A tsunami runs deep because it is generally caused by violent geologic activity on the seafloor. For this reason, scientists sometimes refer to tsunamis as seismic waves. The seafloor may rise, lifting the column of water above it and creating a gentle swell, which may cover 10,000 square miles [25,000 km2]. Or the ocean floor may sink, briefly creating a hollow on the ocean surface.
Either way, gravity causes the affected water to oscillate up and down—a motion that spawns a series of concentric waves, like those formed when a stone hits a pond. This phenomenon shatters the popular myth that tsunamis are just single rogue waves. Instead, they usually fan out in what is called a tsunami wave train. Tsunamis may also be triggered by volcanic eruptions or undersea landslides.
One of the most devastating series of tsunamis in recorded history was formed by the August 1883 explosion of Krakatau, a volcano in Indonesia. Some of the resulting waves reached the incredible height of 135 feet [41 m] above sea level and swept away some 300 coastal towns and villages. The death toll probably exceeded 40,000.
The Tsunami’s Dual Personality
Wind-generated waves never go faster than 60 miles [100 km] per hour, and they are usually much slower. “Tsunami waves, on the other hand,” says the book Tsunami!, “may travel as fast as a jetliner, an astonishing 500 miles [800 km] per hour or more in the deep waters of an ocean basin.” Yet, despite their speed, they are not dangerous in deep water. Why?
First, because on the open sea, a single wave is usually less than ten feet [3 m] high; and second, because the wave can be hundreds of miles from crest to crest, giving it a gentle slope. Hence, tsunamis can pass under ships without even being noticed. The master of a ship lying off the coast of one of the Hawaiian Islands was not even aware that a tsunami had passed by until he saw huge waves pounding the distant shore. The general rule for safety at sea is for ships to reach water that is a depth of at least 100 fathoms, or 600 feet [180 m].
Tsunamis change character when they approach land and come into shallower water. Here, friction with the seafloor slows the wave down—but not evenly. The back of the wave is always in deeper water than the front and so travels slightly faster. In effect, the wave compresses, transforming its decreasing velocity into increased wave height. Meantime, trailing waves in the wave train catch up, piling into the waves in front.
In their final stage, tsunamis may descend on a section of coast as a breaking wave or as a wall of water called a bore, but more commonly, they appear as a fast-rising tidelike flood that surges well above the normal high-water mark. Water has been known to surge more than 170 feet [50 m] above the normal sea level and carry debris, fish, and even chunks of coral thousands of feet inland, obliterating everything within its path.
Deceptively, the first sign of an approaching tsunami is not always the appearance of a growing swell racing toward shore. It may be quite the opposite—an abnormal outgoing tide that drains beaches, bays, and harbors dry and leaves fish flapping on the sand or mud. What determines the initial conditions is which part of the wave train reaches shore first—the rise or the trough.a
When the Beach Runs Dry
It was a calm evening on November 7, 1837, on the Hawaiian island of Maui. About seven o’clock that evening, explains the book Tsunami!, the water began to recede from the beach, leaving the reef exposed and fish stranded. Many excited islanders ran out to pick up the fish, but a few individuals, who were more alert, ran for high ground, possibly knowing from past experience what was about to happen. Soon, a terrifying surge of water rushed in and carried the entire village of 26 grass houses, along with the inhabitants and livestock, 800 feet [200 m] inland and dumped them into a small lake.
On that same evening, thousands of people were assembled at a beach on another island for a religious service. Once again, the sudden withdrawal of water caused curious Hawaiians to rush down to the beach in crowds. Then, a gigantic wave, rising 20 feet [6 m] above the normal high-water mark, appeared as if out of nowhere and rushed ashore “with the rapidity of a racehorse,” according to one observer. Retreating water washed even strong swimmers out to sea, where some drowned because of exhaustion.
How Often Do They Strike?
“Since 1990,” says Scientific American, “10 tsunamis have taken more than 4,000 lives. In all, 82 were reported worldwide—a rate much higher than the historical average of 57 a decade.” However, this reported increase, the magazine adds, is largely attributed to improved communications, while the high death tolls are due in part to increases in coastal populations.
The Pacific Ocean is especially noted for tsunamis because its basin is seismically the most active. In fact, “hardly a year goes by without at least one destructive tsunami striking somewhere in the Pacific,” says one reference, which also states that “over the past fifty years, 62 percent of all earthquake-related deaths in the United States have been caused by tsunamis.”
Can They Be Predicted?
Between 1948 and 1998, about 75 percent of tsunami warnings given in Hawaii were false alarms. Understandably, such a record invites complacency. However, a much better system of detection, incorporating modern technology, is now being deployed. At the heart of the improved detection system is a bottom pressure recorder (BPR), which, as its name suggests, is placed thousands of feet down, at the bottom of the ocean.
This highly sensitive instrument is able to register the difference in water pressure as a tsunami travels overhead—even one no higher than a single centimeter. Using sound waves, the BPR transmits data to a special buoy, which then forwards it to a satellite. In turn, the satellite relays the signal to the tsunami-warning center. Scientists are confident that this more precise early-warning system will curb the number of false alarms.
Perhaps the most important factors in promoting safety are public awareness and education. Even the best warning system is useless if people ignore it. So if you live in a tsunami-prone, low-lying coastal region and local authorities announce a tsunami warning or you sense an earthquake or you see an unusual out-going tide, be sure to seek high ground immediately. Remember, in the open sea, tsunamis can travel at the speed of a jet plane and may barrel in at highway speed near shore. So once you see the wave, chances are you will not be able to outrun it. However, if you meet up with a tsunami when you are out at sea enjoying a cruise or fishing, you can relax—your cup of coffee or glass of wine resting on the table will likely remain undisturbed.
a According to Discover magazine, the circular or elliptic motion of water that exists within all waves is also a factor in the receding water. People swimming in the ocean tend to feel an outward pull of water just before a wave reaches them. This effect is greater with tsunamis and hence is a factor in the draining of beaches or harbors in advance of the first wave.
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Tsunamis are often generated by a seismic disturbance on the ocean floor
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New technology, using deep-ocean detectors, attempts to predict tsunamis
16,000 feet [5,000 meters]
Karen Birchfield/NOAA/Pacific Marine Environmental Laboratory
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A tsunami drove a board through this truck tire
U.S. Geological Survey
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The Scotch Cap lighthouse in Alaska before the 1946 tsunami hit (left)
The total destruction afterward (top)
U.S. Coast Guard photo
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U.S. Department of the Interior