Editor’s note: This article originally appeared as the cover story of the May 1994 Discover.
Like most people in Nicaragua, Chris Terry didn’t feel the mild earthquake that shook the country at about 8 p.m. on September 1, 1992. He didn’t notice anything out of the ordinary until some minutes later. Terry and his friend Scott Willson, both expatriate Americans, run a charter fishing business in San Juan del Sur, a sleepy village on Nicaragua's Pacific coast. On the evening of the earthquake they were aboard their boat in San Juan del Sur’s harbor.
”We were down below,” says Terry. “We heard a slam.” The sound came from the keel of their boat, which had just scraped bottom in a harbor normally more than 20 feet deep. Somehow the harbor had drained as abruptly as if someone had pulled a giant plug.
Terry and Willson didn’t have much time to contemplate the novelty of a waterless harbor. Within seconds they were lifted back up by a powerful wave. “Suddenly the boat whipped around very, very fast,” says Terry. “It was dark. We had no idea what had happened.”
The confusion was just beginning. As Willson and Terry struggled to their feet, the boat began dropping once again, this time into the trough of a large wave. Willson was the first to get out to the deck. There he found himself staring into the back side of a hill of water rushing toward the shore. “He was seeing the lights of the city through the water,” says Terry. “And then the swell hit, and the lights went out, and we could hear people screaming.”
One of those on the shore was Inez Ortega, the owner of a small beachfront restaurant. She hadn’t noticed the earthquake either. While preparing dinner she glanced out at the harbor and noticed that the water seemed unusually low. “I didn’t pay much attention at the time,” she says. But when she looked up again a swell of water at least five feet high was racing up the beach toward her restaurant.
“I started running, but I didn’t even get out of the restaurant when the wave hit,” she says. Ortega and several of her customers spent about half an hour swimming in a debris-filled stew before they managed to drag themselves out of the water.
Ortega and everyone else in San Juan del Sur looked about themselves in stunned silence. The waves had swept away restaurants and bars lining the beach, as well as homes and cars—and people—hundreds of yards inland. Terry and Willson managed to ride out the disaster on their boat. Still reeling, they witnessed the receding wake of the last wave.
“When the wave came back out, it was like being in a blender,” says Terry. Collapsed homes bobbed in the water around their boat.
Terry, Willson, and Ortega had survived a tsunami, a devastating wave triggered by an undersea earthquake. Although the waves that hit San Juan del Sur were extremely powerful, they rose only 5 to 6 feet high. Other parts of Nicaragua weren’t so lucky. All told, the offshore earthquake sent tsunamis crashing along a 200-mile stretch of the coast, and newspapers reported 65-foot waves in some places (though seismologists consider that figure unlikely; a more realistic wave height might be about 30 feet). The waves killed about 170 people, mostly children who were sleeping when the waves came. More than 13,000 Nicaraguans were left homeless.
Destructive tsunamis strike somewhere in the world an average of once a year. But the period from September 1992, the time of the Nicaraguan tsunami, through last July was unusually grim, with three major tsunamis. In December 1992 an earthquake off Flores Island in Indonesia hurled deadly waves against the shore, killing more than 1,000 people. Entire villages washed out to sea. And in July 1993 an earthquake in the Sea of Japan generated one of the largest tsunamis ever to hit Japan, with waves washing over areas 97 feet above sea level; 120 people drowned or were crushed to death.
In Japanese tsunami literally means “harbor wave.” In English the phenomenon is often called a tidal wave, but in truth tsunamis have nothing to do with the tame cycle of tides. While volcanic eruptions and undersea landslides can launch tsunamis, earthquakes are responsible for most of them. And most tsunami-spawning earthquakes occur around the Pacific rim in areas geologists call subduction zones, where the dense crust of the ocean floor dives beneath the edge of the lighter continental crust and sinks down into Earth’s mantle. The west coasts of North and South America and the coasts of Japan, East Asia, and many Pacific island chains border subduction zones. There is also a subduction zone in the Caribbean, and tsunamis have occurred there, but the Atlantic is seismically quiet compared with the restless Pacific.
More often than not, the ocean crust does not go gentle into that good mantle. As it descends, typically at a rate of a few inches a year, an oceanic plate can snag like a Velcro strip against the overlying continent. Strain builds, sometimes for centuries, until finally the plates spasmodically jerk free in an earthquake. As the two crustal plates lumber past each other into a new locked embrace, they sometimes permanently raise or lower parts of the seafloor above. A 1960 earthquake off Chile, for example, took only minutes to elevate a California-size chunk of real estate by about 30 feet. In some earthquakes, one stretch of the sea bottom may rise while an adjoining piece drops. Generally, only earthquakes that directly raise or lower the seafloor cause tsunamis. Along other types of faults—for example, the San Andreas, which runs under California and into the ocean—crustal plates don't move up and down but instead scrape horizontally past each other, usually without ruffling the ocean.
Seismologists believe the sudden change in the seafloor terrain is what triggers a tsunami. When the seafloor rapidly sinks—or jumps—during an earthquake, it lowers (or raises) an enormous mountain of water, stretching from the seafloor all the way to the surface. “Whatever happens on the seafloor is reflected on the surface,” says Eddie Bernard, an oceanographer with the National Oceanic and Atmospheric Administration (NOAA). “So if you imagine the kind of deformation where a portion of the ocean floor is uplifted and a portion subsides, then you’d have—on the ocean surface—a hump and a valley of water simultaneously, because the water follows the seafloor changes.”
One major difference between the seafloor and the ocean surface, however, is that when the seafloor shifts, it stays put, at least until the next earthquake. But the mound of water thrust above normal sea level quickly succumbs to the downward pull of gravity. The vast swell, which may cover up to 10,000 square miles depending on the area uplifted on the ocean floor, collapses. Then the water all around the sinking mound gets pushed up, just as a balloon bulges out around a point where it’s pressed. This alternating swell and collapse spreads out in concentric rings, like the ripples in a pond disturbed by a tossed stone.
Although you might think a tsunami spreading across the ocean would be about as inconspicuous as a tarantula walking on your pillow, the wave is, in fact, essentially invisible in deep ocean water. On the open sea, a tsunami might be only ten feet high, while its wavelength—the distance from one tsunami crest to another—can be up to 600 miles. The tsunami slopes very gently, becoming steeper only by an inch or so every mile. The waves so feared on land are at sea much flatter than the most innocuous bunny-run ski slope; they wouldn’t disturb a cruise ship’s shuffleboard game. Normal surface waves hide tsunamis. But that placid surface belies the power surging through the water. Unlike wind-driven waves, which wrinkle only the upper few feet of the ocean, a tsunami extends for thousands of fathoms, all the way to the ocean bottom.
Tsunamis and surface waves differ in another crucial respect: tsunamis can cross oceans, traveling for thousands of miles without dissipating, whereas normal waves run out of steam after a few miles at most. Tsunamis are so persistent that they can reverberate through an ocean for days, bouncing back and forth between continents. The 1960 Chilean earthquake created tsunamis that registered on tide gauges around the Pacific for more than a week.
”You’ve got to remember how much energy is involved here,” says Bernard. “Look at the size of these earthquakes. The generating mechanism is like a huge number of atomic bombs going off simultaneously, and a good portion of that energy is transferred into the water column.”
The reason for tsunamis’ remarkable endurance lies in their unusually long wavelengths—a reflection of the vast quantity of water set in motion. Normal surface waves typically crest every few feet and move up and down every few seconds. Spanning an ocean thus involves millions of wavelengths. In a tsunami, on the other hand, each watery surge and collapse occurs over perhaps 100 miles in a matter of minutes. For a large subduction-zone earthquake—magnitude 8 or more—the earthquake’s impulse can be powerful enough to send tsunamis traveling across the Pacific—from the Chilean coast to Japan, Australia, Alaska, and all the islands en route as well.
For much the same reason, tsunamis can race through the ocean at jetliner speeds—typically 500 miles an hour. To span a sea, they need travel a distance equal to just a few dozen of their own wavelengths, a few swells and collapses. That means the wave only has to rise and fall a handful of times before the surge reaches its destination. The outsize scale of a tsunami makes an ocean seem like a pond.
As a tsunami speeds on its covert way, undersea mountains and valleys may alter its course. During the 1992 Indonesian earthquake, villages on the south side of Babi Island were the hardest hit, even though the source of the tsunami was to the north of the island. Seismologists believe that the underwater terrain sluiced the tsunami around and back toward the island's south coast.
Only when a tsunami nears land does it reveal its true, terrible nature. When the wave reaches the shallow water above a continental shelf, friction with the shelf slows the front of the wave. As the tsunami approaches shore, the trailing waves in the train pile onto the waves in front of them, like a rug crumpled against a wall. The resulting wave may rear up to 30 feet before hitting the shore. Although greatly slowed, a tsunami still bursts onto land at freeway speeds, with enough momentum to flatten buildings and trees and to carry ships miles inland. For every five-foot stretch of coastline, a large tsunami can deliver more than 100,000 tons of water. Chances are if you are close enough to see a tsunami, you won't be able to outrun it.
As Inez Ortega and Chris Terry witnessed in San Juan del Sur, the first sign of a tsunami’s approach is often not an immense wave but the sudden emptying of a harbor. This strange phenomenon results from a tremendous magnification of normal wave motion. In most waves, the water within the crest is actually moving in a circular path; a wave is like a wheel rolling toward the shore, with only the top half of the wheel visible. When that wave is 100 miles long, the water in the crest moves in long, squashed ellipses rather than in circles. Near the front and bottom of the wave, water is actually on the part of the elliptical “wheel” moving backward—toward the wave and out to sea. If you’ve ever floated in front of a wave, you’ve probably felt the pull of the wave as water sloshes back toward the crest. With a tsunami, that seaward pull reaches out over tens of miles, sometimes with tragic results: when an earthquake and tsunami struck Lisbon in 1755, exposing the bottom of the city’s harbor, the bizarre sight drew curious crowds who drowned when the tsunami rushed in a few minutes later. Many people died in the same way when a tsunami hit Hawaii in 1946.
Although seismologists and oceanographers understand in broad terms how tsunamis form and speed across oceans, they are still grappling with some nagging fundamental questions. One of the major mysteries is why sometimes relatively small earthquakes generate outlandishly large waves. Such deceptive earthquakes can be particularly devastating because they may be ignored by civil agencies that are charged with issuing tsunami warnings.
The Nicaraguan earthquake is a case in point. By conventional measures, it shouldn’t have produced a tsunami at all. The earthquake registered magnitude 7.0 on the Richter scale, not puny by any means, but not large enough, seismologists believed, to pose much of a tsunami risk. The quake’s epicenter was 60 miles offshore, distant enough to dampen the tremors on land. Yet people who had not even felt the quake found themselves swept out to sea minutes later.
Hiroo Kanamori, a seismologist at Caltech, has made a point of studying the earthquakes that spring these unexpected tsunamis. Such earthquakes, he says, are responsible for some of the most damaging tsunamis on record. In 1896, for example, an earthquake in Japan was followed by a tsunami that drowned 22,000 people, even though survivors reported only mild shaking before the wave. And a relatively moderate 1946 quake in the Aleutian Islands sent a huge tsunami tearing across the north Pacific and into Hawaii, where it inundated much of the city of Hilo.
Twenty years ago Kanamori proposed an explanation for these surprise tsunamis. But to test his ideas he needed seismometers sensitive to a broad spectrum of ground movement, and the instruments of the 1970s just weren’t up to the job. Only in the past few years, in fact, have seismometers become sophisticated enough for his purposes. And the 1992 Nicaraguan tsunami proved an ideal test case.
Kanamori thinks some earthquakes may release their energy very slowly, over a minute or more, rather than in a brief, spastic lurch. This could happen, he says, if soft ocean sediments were sandwiched between two interlocked crustal plates. The lubricated plates would slide past each other smoothly, without sharp, building-shaking convulsions. “If you have two blocks of hard rock,” says Kanamori, “usually the friction between them is very high, so you can accumulate large amounts of stress. And when it slips, it slips very fast. But if you have lubricating materials in between, it can slip at relatively low stress, and when it slips, it goes slowly.”
