Tsunami: The killer wave

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Devastated by tsunamis, Japan faces multiple emergencies

Japan, a world leader in earthquake engineering, has been paralyzed by a series of giant waves that followed one of the most violent earthquakes in a century.

Residents of the port town of Kamaishi in Iwate prefecture watch in horror as the first huge tsunami waves sweep away cars and buildings.

Although the magnitude 9.0 quake on Mar. 11, 2011, apparently did not collapse high-rise buildings, the ensuing tsunamis flattened vast areas along the northeast coast. The death toll is swelling steadily as bodies wash in on the surf, and citizens and Japan’s Self Defense Forces scour a landscape turned upside down by inconceivably powerful waves.

The news recalls the estimated 250,000 people who perished, mainly on the Indonesian island of Sumatra, in the 2004 “Christmas tsunami” that followed a huge, offshore quake. (Both Japan and Indonesia are volcanic lands in the Ring of Fire, which partly surrounds the Pacific Ocean in a giant series of subduction zones and volcanoes.)

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Shortly after Japan stopped shaking at 2:46 pm local time on Friday, March 11, we began hearing about troubles at a series of nuclear plants. After the reactors automatically shut down during the quake, emergency systems for removing heat still being generated in the reactors were routinely switched on.

But because the electric grid was down and the standby generators were damaged — perhaps by seawater — the emergency cooling failed. By Tuesday, March 15, three reactors had exploded, a fourth was burning, radioactive material was airborne, reactor workers were being evacuated, electricity was growing short in Tokyo, and the crucial containment vessels were under severe threat if not already breached.

With the first nuclear meltdowns since Chernobyl, in 1986, under way, global stock markets were crashing.
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Aerial view from helicopter of flooded town and large plume of smoke in air.

Photo: U.S. Navy
A helicopter flies over the city of Sendai, as it delivers more than 1,500 pounds of food donated by citizens of Ebina City, Japan, to survivors of the earthquake and tsunami.

What causes tsunamis?

As Japan licks its wounds, The Why Files wants to know what causes tsunamis. How do they travel across the ocean? How they have impacted coastal people through history? Can we reduce our vulnerability to nature at its most cataclysmic?

As plates shift and sink, disturbance causes development of high speed waves that hit coasts.

Movement of the sea floor translates into waves at the surface.

Tsunamis — once slangily called tidal waves — are extremely powerful waves caused by large undersea disturbances. (“Tsunami” derives from Japanese for “harbor wave,” reflecting the fact that harbors can concentrate their energy. True tidal waves are the slow oscillations that drive ocean tides in response to solar and lunar gravity.)

Although landslides and volcanoes cause some tsunamis, probably 95 percent result from underwater earthquakes that contain a strong vertical motion. Such quakes often occur where one of Earth’s tectonic plates dives, or “subducts,” beneath another.

Like the Sunda trench near Sumatra, the subduction zone in the Japan trench is notorious for large earthquakes, says Timothy Masterlark, an associate professor of geological science at the University of Alabama. Although the timing is always uncertain, he says, “The history was known, big earthquakes were known, and even though the people and government went to great lengths to prepare, at some level … there is simply nothing they can do.”

Lessons from Sumatra

Masterlark, who has studied the giant, 2004 earthquake and tsunami in Sumatra, says the magnitude 9.0 earthquake in Japan likely broke a fault stretching at a shallow angle from the sea floor roughly 150 kilometers beneath Japan, along a trench several hundred kilometers in length.

We asked Masterlark how, if the slip was mainly horizontal, the rocks had enough vertical movement to cause a tsunami. “In Sumatra, we found a shallow slip created some vertical movement because the rock at the surface was softer, so the fault became more vertical, which changed the slip from mostly horizontal to mostly vertical.”

To imagine how vertical movement of the seafloor causes a tsunami, imagine making waves by throwing a stone in a pond. Even though earthquakes disturb the bottom of the water, the analogy works: just as a larger stone, thrown faster, makes a larger wave, the size of the tsunami depends on extent and speed of the ocean-floor movement.

The tsunami is usually most intense close to the earthquake: as waves spread from the epicenter in a typical arc-shaped pattern, their energy also spreads out.

ENLARGE

Aerial view of flooded village with debris strewn throughout, mountains surround village.

A ruined village near the coast of Sumatra after the 2004 tsunami.

Spread out, but still powerful

One factor that distinguishes tsunamis from more familiar waves is their extreme wavelength. On the open ocean, the peaks of waves may be 300 kilometers apart, and they may travel at 500 to 600 miles per hour. Even though they can keep pace with a jetliner, you wouldn’t see a tsunami from the cockpit of a jet. A killer tsunami may be only 2 feet tall in mid-ocean — far too small to be noticed from an airplane or even a ship, yet it can carry huge amounts of energy across the Pacific.

In some earthquakes, the biggest killer is not the shaking, but the walls of water created by undersea earth movement.

By Tuesday, tsunami damage had caused three reactors to explode. A fourth was burning, and stock markets were reeling.

All that kinetic energy can hide in waves we can barely see because long-wavelength waves are extremely deep, and the massive amount of water moving beneath the surface contains enormous energy.

In deep water, boats can ride the worst tsunamis without noticing them; but when they reach shallow water and “run aground,” these waves become dangerous.

Like all waves, tsunamis slow when the lower part of the wave encounters the upward-sloping ocean floor. But while the front of the wave slows, the wave behind is still moving faster, causing a giant pile-up at the front, and the kinetic energy that was spread through the ocean depth concentrates in a towering wave at the surface.

Wild waves

It is these surface waves — which can be 10 meters high or taller as they cross the beach — that cause the utter destruction of tsunamis. Like all waves, tsunamis have both a rising and a falling motion, says Masterlark. “Depending on where you are with respect to the earthquake, you may first see a wall of water, or the opposite, the sea retreating.” In 2005, during a research cruise to Sumatra, “We were told that the tourists had heard that the ocean was retreating, and saw this as a great holiday, ‘Let’s walk on the seashore,” and this wall of water came in and killed them. This was a great warning, when they saw the water retreat, they should have headed away from the shore.”

Tsunamis have other quirks. They can be spaced as much as one hour apart, so subsequent waves can kill those who return to help victims of earlier waves.

In 1998, Harry Yeh, a civil engineering professor now at the University of Oregon, told us that tsunamis can have decidedly unconventional behavior. In one case, he said, a tsunami destroyed houses in a cove without damaging a house on an unprotected headland: “It’s the exact opposite of what a storm wave would do.”

Brown house floating in open ocean.

Photo: U.S. Navy
More of the tsunami’s aftermath…

A GRIM LIST

Tsunamis have been attacking coastal people throughout recorded history:

Nov. 1, 1755: Lisbon, Portugal

A series of massive earthquakes levels Lisbon during the celebration of All Saints’ Day. Collapsing stone buildings kill thousands. As fires ignited by overturned candles ravage the city, residents seek relief from the heat near the waterfront. About an hour after the quake, a tsunami estimated at 50 feet tall sweeps in from the sea. The combined cataclysm kills about 60,000 people; only 15 percent of Lisbon’s houses remain standing.

August 27, 1883: Indonesia

Krakatau, a volcano in the Sunda Straits, explodes with a gigantic roar audible 3,000 miles away. The explosions blow 20 cubic kilometers of rock into the sky. Undersea cracks allow massive amounts of seawater into a white-hot magma chamber. When the water turns to steam, the explosion causes tsunamis that cause most of the 37,000 deaths on nearby Sumatra and Java. Ironically, history’s most deadly tsunami is caused by a volcano, not an earthquake.

1896: Japan

The Sanriku tsunami starts, as many do, when the sea withdraws with a great sucking and hissing sound. Striking a totally unprepared town during a festival, the wave kills 27,000 and destroys more than 10,000 houses. Fishermen at sea don’t notice the deadly wave and return to an ocean strewn with the corpses of loved ones and the wreckage of their homes.

April 1, 1946: Alaska and Hawaii

A large earthquake on Unimak, an island in the Aleutian chain, shakes the remote, steel-reinforced concrete Scotch Cap lighthouse, which stands about 100 feet above the North Pacific. Minutes later, a huge wave obliterates the lighthouse, leaving practically no trace of the five Coast Guardsmen inside. Five hours later, the tsunami slams into Hilo, Hawaii, obliterating the waterfront and killing 159.

May 21-22, 1960: Chile and Hawaii

An astonishingly strong series of earthquakes in Chile — culminating in one of the three largest quakes in the 20th century (magnitude 8.9) sinks 300 miles of coastline into the sea, activates one volcano, devastates five provinces, and causes tsunamis that kill an estimated 1,000 to 1,500 people. Fourteen hours later, the tsunami arrives in Hilo. Ignoring warnings, many residents stay in homes near the bay, increasing the death toll by 61.

December 26, 2004: Indonesia, Thailand, Sri Lanka and India

Following a 9.0 quake off the west coast of Northern Sumatra, over 230,000 people perished in the Indian Ocean tsunami, which struck 15 countries. At the time, Indian Ocean nations lacked an ocean-wide warning system, causing the tragedy to strike without warning. Even a warning system would have had limited utility to close-in coastal communities, given the jet-like speed of the waves.

A warning

Map of Japan, circles indicate earthquakes, largest off east coast at 9.0, Sendai largest nearest town.

Modified from original image by NASA
Location of foreshocks, aftershocks and the March 11 Japan earthquake (M 9.0). Circle size represents quake magnitude. Dotted lines = foreshocks; solid lines = aftershocks

The Pacific Tsunami Warning Center, established in Hawaii in the wake of the deadly 1946 tsunami, is a nexus in the global warning network. Since almost all tsunamis originate in earthquakes, the warning centers rely on data from seismographs, many of them located on the unstable ring of fire.

Tsunami warnings are now triggered automatically, says Masterlark, based on measurements of earth movement. “Seismographs are excellent because in seconds they can tell that a quake of some magnitude, big enough to trigger a tsunami, has occurred. This information can automatically trigger a warning in seconds.”

In tsunamis, seconds saved can translate into lives saved.

Researchers are working to use global positioning system (GPS) data to refine size estimates, Masterlark adds, to give “a more refined view of the potential risk, but this takes a little longer and is still in a research mode.”

Further confirmation of the size of the wave may come from special purpose ocean buoys, if they are in the right place, Masterlark says. “But they only work once the tsunami has already arrived, so they can only confirm or help refine the warning.”

Tricks of the tsunami trade

In terms of generating tsunamis, not all underwater earthquakes are created equal, says Andrew Newman, assistant professor of earth and atmospheric sciences at Georgia Tech. “A few times a decade, we have what we call ‘tsunami earthquakes’ that create a tsunami that’s much larger than would be expected for the magnitude of the earthquake,” largely due to a shallow rupture. “Usually a magnitude 7.8 earthquake would create a tsunami that might rise only 20 centimeters to 1 meter [when it reaches land], but one in Sumatra last year created a 17-meter tsunami.”

ENLARGE

Aerial view of coastline stripped of vegetation and structures, debris scattered about.

Photo: U.S. Navy
Tsunami damage north of Sendai, Japan.

These large tsunamis come from a smaller break in the ocean floor, and so contain relatively little energy and do not travel well across the ocean, Newman says. But they also offer less warning because local people do not feel the massive shaking associated with a major tsunami.

Newman and colleagues have developed software to detect the peculiar signature of the tsunami earthquake, and are now running it on a research basis. “We get an earthquake or tsunami warning within four or five minutes, our algorithm starts processing, and a few minutes after that, the system sends email to the Pacific Tsunami Warning Center and the U.S.G.S. [Geological Survey],” Newman says.

Although the Japanese had little time between the earthquake and the tsunami, Newman says the national warning system did work. “In some ways, you have to look at real success in Japan. They have developed a substantial tsunami warning system, and it worked in as quickly as three minutes. People did evacuate, for the large part. Much of the video you see is from helicopters, or people watching from two or three stories up in buildings. There is only so much you can do with these events; this is a massive force.”

But the rising casualty counts highlights the deadly role of proximity to the quake, says Masterlark. “The very sad part is that because the quake was so close to the coast, they had very little warning; the time between the earthquake and the tsunami was minutes.”

More distant regions had adequate warning, Masterlark adds. “We had several hours before the wave reached Hawaii, and so were prepared. But Japan, unfortunately, even if you knew it was coming, you had only minutes, and that’s not enough time for many people to get to higher ground.”

Triangular yellow sign with wave symbol in black and Japanese text below.
Photo: Sarah Ruth

Basic tsunami safety

Public education and quick personal action remain the only ways to reduce the tsunami death toll:

1. Be on guard for strong earthquakes, which can spark a tsunami. If you feel one near the water, run inland.

2. Heed the warnings, and stay tuned to emergency radio stations.

3. Never go down to the beach to watch for tsunamis — they move much faster than you can run. People die doing this.

4. Most structures in the danger zone provide no protection. However, the upper stories of tall, reinforced concrete hotels can provide refuge if you have no time to move inland or to higher ground.

5. A tsunami is a series of waves. Don’t go near the water until you hear the all-clear from emergency authorities.

Following fatal footsteps?

Seismologists are loathe to predict earthquakes, but in the past decade or two, they have recognized that earthquakes occur in series along major faults in Turkey and Sumatra, as big quakes place extra stress on the adjacent fault. In Sumatra, a violent series of quakes began in 2004 with a magnitude 9.1, a magnitude 8.7 in 2005, a magnitude 7.6 in 2009, and a magnitude 7.7 in 2010.

The large quake in 2005 did not cause a major tsunami, but its timing, just three months after the Dec. 26 monster, suggests a compelling reason to focus intensively on the earthquake zone in the Japan trench, says Masterlark. “I am not trying to be alarmist, but I’m trying to look at where earthquakes have occurred along nearby faults to identify faults at risk. We’ll bring in numerical modeling and try to predict this as fast as possible. Time is of the essence, as we saw in Sumatra.”

– David Tenenbaum

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Terry Devitt, editor; S.V. Medaris, designer/illustrator; Jenny Seifert, project assistant; David J. Tenenbaum, feature writer; Amy Toburen, content development executive

Bibliography

  1. Google crisis response.
  2. NOAA: tsunami info.
  3. NOAA: Honshu tsunami graphics.
  4. Pacific tsunami warning center.
  5. USGS tsunami research.
  6. Surviving a tsunami.
  7. National Geographic: tsunamis.
  8. Science behind the disaster.
  9. Before and after satellite pictures.
  10. Tsunami footage.
  11. Japan tsunami news.
  12. Earthquake FAQs.
  13. USGS earthquake info.
  14. Physics of tsunamis.