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.
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.
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.
