Making Waves

Japanese quake gave scientists an unprecedented look at a big tsunami

By many measures, the magnitude 9.0 earthquake that shook Japan a year ago was a record-breaker. It was the largest quake in the country’s written history, the trigger for the worst nuclear accident in 25 years and the costliest natural disaster ever.

Amid such superlatives, it’s easy to forget one more: During the Tohoku-oki quake, the seafloor off Japan’s coast wrenched itself farther apart than scientists had ever measured along any seafloor. In places, chunks of ground slipped horizontally past their neighbors by more than 50 meters and vertically by 10 meters.

“The earthquake was a scofflaw,” says Emile Okal, a geophysicist at Northwestern University in Evanston, Ill. “It violated the scaling laws we’re used to.”

That deviant behavior is what made the quake so deadly, by producing a monster tsunami. When the seafloor moves by half the length of a football field, it displaces an awful lot of water. Of the approximately 20,000 people who died on March 11, 2011, more than 90 percent drowned, were washed away or were otherwise killed by water. So researchers have been studying what happened off Japan’s coast, seeking ways to better detect a lawless quake, track the resulting tsunami and ultimately save lives.

Some of the work, based on survivor videos, reveals how quickly the deadly water surged into and then drained from coastal villages. Other research, looking at ancient sand deposits and boulders tossed like pebbles, suggests that Pacific-wide tsunamis like Tohoku-oki may be more common than once thought.

There’s some good news among the bad. The Japan tsunami was the earliest and best-detected monster wave ever, thanks to warning buoys set up globally after the 2004 Indian Ocean tsunami killed a quarter of a million people. With new findings from the Japan disaster and data from the global buoys, scientists in the United States are working to develop a forecast system that will in principle give people a better warning by predicting areas most likely to flood rather than the heights of incoming waves.

Still, one year after the Tohoku-oki disaster, scientists are far from taming the tsunami hazard. When it comes to translating scientific know-how into reducing death tolls from disasters, says Caltech seismologist Hiroo Kanamori, “we are always one step behind.”

In the wake

Many types of geological disturbances, including underwater landslides and volcanic eruptions, can trigger tsunamis. Most tsunamis, however, are set off by earthquakes, such as those that strike off the east coast of Japan. Here, the western part of the Pacific crustal plate dives beneath a tendril of the North American plate, building up strain that’s released occasionally in earthquakes.

Scientists and emergency planners in Japan are well aware of the tsunami threat; in June 1896, the Sanriku earthquake triggered a massive wave that killed more than 27,000 people. But the March 2011 disaster was simply off the scale compared with what most people would have expected.

The Tohoku-oki tsunami got so large not only because of the sheer amount of slip, but also because of the way the ground moved during the earthquake. When the quake hit, part of the seafloor that had been sloping down at a steep angle quickly lurched toward the surface, displacing an unprecedented amount of water, Takeshi Tsuji, a marine geologist at Kyoto University, said in San Francisco in December at a meeting of the American Geophysical Union.

Moments after the rupture came the first sign a tsunami was on its way. One Russian and three U.S. tsunami buoys nearby detected a huge movement of water, up to 1.64 meters high. “We knew immediately, within 30 minutes, that this was gigantic,” says Eddie Bernard, former director of the National Oceanic and Atmospheric Administration’s Pacific Marine Environmental Laboratory in Seattle.

Nowhere was the tsunami felt more dramatically than in the narrow inlets that riddle Japan’s Sanriku coast, north of the city of Sendai. Fishing villages nestle within the inlets where they are protected from wind and everyday waves, but such locations are the worst place to be when a tsunami arrives, says Costas Synolakis, a tsunami expert at the University of Southern California in Los Angeles and at the Hellenic Center for Marine Research in Anavyssos, Greece.

In the open ocean, a tsunami typically appears as a few extra centimeters or tens of centimeters moving atop the water column. But once the wave starts to approach land, the energy that had been spread over the entire ocean’s depth becomes squeezed into a shallow layer. This compression ramps up the tsunami’s amplitude as high as meters or tens of meters, especially in inlets that funnel the water forward. The Japanese waves reached as high as 40 meters.

At the fishing port of Kesennuma Bay, where nearly 1,500 people died, scientists have gone back to the scene of dramatic videos by two survivors to re-create what happened. Aware of the local risks, Kesennuma’s emergency manager had sent out a tsunami alarm within two minutes of the earthquake — before either the Japan Meteorological Agency or the Pacific Tsunami Warning Center, the national and international agencies in charge of similar alerts. Within 30 minutes the tsunami arrived at the port, swamping the bay.

In June, a team led by Hermann Fritz of the Georgia Institute of Technology’s Savannah campus used lasers to scan the surroundings where many survivors had clustered: a Coast Guard building, a vertical evacuation platform at the local fish market and a hill marked as an evacuation route. From the laser data and photos of the port, Fritz and his colleagues generated a photorealistic three-dimensional rendering of the landscape. The team then calibrated survivor videos against this data, mapping precisely how water inundated the bay and receded — information that’s impossible to obtain by surveying after the fact.

By measuring how current flowed on the water’s surface, the scientists calculated that soon after the tsunami reached its maximum height of 9 meters in the bay, it receded at unsurvivable speeds. The outflow sped up from 3 meters per second to 11 meters per second within just two minutes — something no one caught in the water could navigate through. “These currents are very important because they cause a lot of damage,” Fritz says.

When it comes to building concrete breakwaters, seawalls and other coastal defenses, Sanriku is perhaps the best-protected coastline in the world. Stone tablets left by past generations often mark the high-water point of historic floods. In some places, such long memories help plan prevention: The village of Otanabe was devastated by 15-meter-high waves during the 1896 tsunami, so residents rebuilt with a 15.5-meter-high seawall. In March 2011, the barrier kept the sea back. But overall, Sanriku’s coastal defenses were built to withstand a tsunami an order of magnitude smaller than the one that arrived. One much-ballyhooed breakwater in Kamaishi Bay, completed three years earlier at a cost of $1.6 billion, mostly crumbled in the face of the Tohoku-oki tsunami.

Future forecasts

To help coastal residents better prepare, with or without concrete defenses, scientists are promoting new flooding forecasts instead of the usual reports of incoming wave heights. Few people inherently understand the concept of wave height, says Bernard: “They don’t know what a 3-meter or 6-meter tsunami means.” Another problem with wave-height forecasts is that coastlines are variable. A 1-meter tsunami might cause extensive flooding in one place, whereas a 3-meter tsunami that hits nearby might not lead to flooding at all.

Flooding forecasts could be particularly useful for countries that lie across an ocean basin from a massive quake, and hence have time to prepare for an oncoming wave. “An earthquake shakes for minutes, while a tsunami crashes for hours,” Bernard says.

The map above shows cumulative wave heights predicted following the March 2011 tsunami off Japan’s coast. A new forecast system would warn of flooding risk rather than wave height. NOAA

In Hawaii, the aftereffects of the Tohoku-oki earthquake continued to arrive throughout the night. Because of the way the Hawaiian Islands are arranged, a tsunami can become trapped “and just keep banging around with no time for the water to drain,” Bernard says. The city of Kahului, on the north side of Maui, flooded extensively not just from the initial wave but also from the second and third that arrived soon thereafter. Emergency officials had evacuated much of the coastline, but fine-tuning computer programs used to predict the areas that will flood could mean less overall disruption, Bernard says.

New data to improve such forecasts come thanks to the network of ocean buoys designed for tsunami warnings, called the Deep-ocean Assessment and Reporting of Tsunamis, or DART, array. NOAA started using six of these buoys in 2001, and ramped up its investment after the 2004 Indian Ocean disaster. Today dozens of DART buoys, run by countries from the United States to Russia to Australia, operate constantly. In each, a recorder on the seafloor monitors the pressure of water passing overhead; a buoy tethered on the surface can instantly transmit warnings when a tsunami arrives.

The Tohoku-oki tsunami was the first to be measured by multiple DARTs right near where the quake happened, and was also the first mega-tsunami — with wave heights more than 1 meter in the open ocean — ever detected in real time. Data from the buoys are giving scientists confidence to push their tsunami forecasts into new realms, says Vasily Titov of the NOAA lab, such as cranking out local forecasts within one hour for U.S. coastlines or creating specialized forecasts for crucial facilities such as nuclear power plants, oil and gas infrastructure, and ports and harbors.

Following the Japanese tsunami, NOAA scientists tested their forecasting potential by taking wave-height data from buoys along the Japanese coast and simulating where current programs say the flooding should be expected. The resulting prediction map matched well with flooding actually observed, Titov says.

Titov has produced similar simulations for the U.S. Pacific Northwest coast, which is thought to be at high risk of a large earthquake and tsunami. After a magnitude 9.1 quake, Titov has calculated, wave heights could reach as high as 10 meters at some places along the Oregon and Washington coasts, such as near the mouth of the Columbia River or north of Coos Bay, Ore.

The bottleneck to warnings may not be technology but human organization, or lack thereof. A full-scale test in October of the new Indian Ocean tsunami warning system, set up explicitly to prevent a repeat of the death toll in 2004, went relatively smoothly. But some countries, such as Somalia, have not implemented national plans to respond and pass the message to local residents when an alert from the oceanwide system comes in.

Detailed mapping is helping scientists understand how floodwaters inundate and then recede. Tsunami waters reached only partway up evacuation buildings at Kesennuma Bay (photorealistic depiction of aftermath at top, flooding in blue at bottom). from top: H. Fritz et al/Geophysical Research Letters 2012; H. Fritz/Georgia Tech and David Phillips/UNAVCO

Ring of Fire risk

Such warning systems may ultimately get more use in the Pacific than previously thought. “Paleotsunami” studies, which look for evidence of waves from centuries past, are beginning to show just how common these disasters are around the Pacific’s Ring of Fire.

Over the last decade, for instance, Japanese studies have revealed the scale of a tsunami that struck in July 869. An earthquake, probably around magnitude 8.6, sent sand and other debris flooding across the Sendai plain, Daisuke Sugawara of Tohoku University in Sendai said at the geophysics meeting. Eerily, these deposits match almost exactly the region that was inundated in March 2011.

Farther out in the Pacific, scientists are cobbling together the tsunami history of the small islands that dot the ocean’s vast expanses. In the Cook Islands, for instance, shells embedded into the sides of trees speak to the violent wave that swept over after a volcano erupted and collapsed cataclysmically near Vanuatu in the year 1452. Traces of the tsunami linger as high as 30 meters above sea level, yet tsunami assessments for the islands say residents there don’t need to worry about anything higher than 2.8 meters. “We are most definitely underestimating the hazard and risk,” James Goff, a tsunami expert at the University of New South Wales in Sydney, said at the geophysics meeting.

Other hints come from the traditional environmental knowledge of local residents. In New Zealand, 15th century Maori tales tell of people being thrown into the dunes by a nasty beast attacking from the sea. The tail of the beast broke off and became a small offshore island, a constant reminder of the ocean’s threat.

Past tsunamis may even have influenced how people settled islands across the Pacific. Early Polynesians had spread into the Samoan archipelago by 2,800 years ago but then stopped — quite possibly because that’s when a big tsunami washed across the Pacific. Similarly, the long-distance Pacific voyaging networks collapsed after the 15th century Vanuatu eruption, Goff said. At least three of four known massive Pacific tsunamis in the last 2,000 years coincided with big changes in human settlement, he said at the meeting.

For now at least, Japan seems to be recovering far more resiliently from its own wave disaster. Parts of the coast around the damaged Fukushima Daiichi nuclear reactors remain off-limits, but people have moved back into other areas to start rebuilding their lives. And government officials are already talking about one way to cope with the threat of future tsunamis.

Figuring the coast has gotten the worst it will get for quite some time, the minister for reconstruction suggested in January that the country should perhaps rebuild its concrete tsunami barriers — to the same height they were before.

Alexandra Witze is a contributing correspondent for Science News. Based in Boulder, Colo., Witze specializes in earth, planetary and astronomical sciences.

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