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After the Tsunami
Nicholas School Professors Talk about the Distaster
by Monte Basgall
How tsunamis yield deadly waves
The massive death and destruction from tsunamis arise from
a tragic confluence of phenomena involving geology and the
physics of ocean waves, said Peter
Malin, a professor of seismology in the Nicholas
School’s Division of Earth and Ocean Sciences.
First of all, there is the fact that the magnitude 9.3 earthquake
off Indonesia significantly changed the depth of the ocean
floor over an area larger than the size of North Carolina
in a minute or two. Such a “megathrust” earthquake is caused
by one crustal plate abruptly slipping beneath another.
“You have to understand that it’s not just crust pushing
into the ground and diving,” Malin said. “There will be other
parts that get uplifted too. When you pick up the continental
shelf many feet this quickly, the water doesn’t have time
to adjust gradually. So suddenly you have an elevation difference
on the sea surface, creating great long waves—a tsunami—as
one might have from quickly tilting a bath tub a few inches.”
But in the deep ocean this vast wave stretches out over
many miles, he said. Being exceptionally long, such a wave
might be hardly noticeable to ships and boats passing over
it as it moves across the ocean at the speed of a jetliner
rather than the pace of a normal wind-driven wave.
“Think of the bathtub again,” Malin said. “If you make small
waves on top of the water it takes time for that wave to propagate
across the surface. But now slosh the tub. That sloshing moves
much faster because it has a much longer wavelength.”
Also, said Malin, the sequence of a tsunami is insidiously
deadly. When the spread-out waves finally reach beaches, they
can first suck the surf line far down from the shore, exposing
shallow stretches of ocean bottom and stranding sea life.
This tempting opportunity often lures unsuspecting tourists
and fish gatherers to venture out too far to escape when the
water returns with a vengeance.
At that tragic moment, the waves that were long and wide
in the depths of the sea consolidate in the shallow coastal
waters into a series of onrushing walls, with the later waves
being the tallest. This “wave group” behavior—well known to
surfers—is caused by “dispersion effects,” Malin said. Such
effects are caused by the interaction of waves of different
lengths, in which some lengths reinforce one another, combining
to cause large motions of the sea surface.
Residents of the East Coast of the United States should be
aware of tsunamis, said Malin. He cited, for example, the
possibility of waves generated in the Atlantic Ocean by large
landslides from the volcanic island of La Palma in the Canaries
off Africa.
La Palma’s Cumbre Vieja volcano is crumbling, with significant
portions of it having slipped into the Atlantic every few
thousand years. If that were to happen again, while different
from the Sumatra event, “it’s quite possible that a negligible
to significant tsunami could be set up and propagate across
the ocean to North and South America,” Malin said. “Not tomorrow
or next month, but just as frequently as in the past.”
Another warning against beachfront
development
The deadly tsunami is the latest—albeit most extreme—example
of the dangers of dwelling near the surf zone, said emeritus
Nicholas School Professor Orrin
Pilkey, who has long warned that hurricanes and
other storms will inevitably destroy seaside structures.
“I think this is yet another nail in the coffin of the rationality
of beachfront development,” Pilkey argued. “Maybe we should
forgo the view of the sea and the breeze from the sea. Set
the hotels way back, on the back sides of islands.”
Some areas of the tsunami-stricken region had already heeded
warnings about such damage. For example, according to The
New York Times, most new properties in Phuket, Thailand, conform
to a law establishing a 150-foot setback from the shoreline,
which limited damage from the tsunami. And the prime minister
has said that damaged coastal areas should be rebuilt to observe
such setback regulations.
Research by Pilkey has revealed that the North Carolina coast,
in fact, might have experienced a tsunami about 16,000 years
ago, when the sea level was some 300 to 400 feet lower than
today. He studied the seafloor seaward of Cape Hatteras.
“We found a massive deposit of sediment,” Pilkey said. “It
was more than 100 cubic kilometers in volume. By comparison,
Mt. St. Helens produced one cubic kilometer.”
Ecosystem destruction might have
contributed to damage
The scope of the tsunami destruction and its death toll might
have been increased by humans’ destruction of forested hillsides,
mangrove swamps and coral reefs, said Stuart
Pimm, the Nicholas School’s Doris Duke Professor
of Conservation Ecology.
“Throughout the world, and particularly in this region,
large areas of mangroves have been cleared, sometimes for
shrimp farming, sometimes for resorts and hotels,” he said,
stressing that he is talking in general without knowledge
of specific situations.
“I’m afraid a story that has been consistently missed, not
only with this disaster but also, for example, with the flooding
in Haiti last year, is that such natural ecosystems provide
an enormous amount of protection, he said.
“When you get a disaster like this, one thing you would
want to have between you and the ocean is a mangrove swamp
to absorb the impact of a storm surge or a tsunami.”
Read what other Duke professors have to say at http://www.dukenews.duke.edu/2005/01/aftertsunami_0105.html.
Monte Basgall, Duke News & Communications
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