IN PAST TSUNAMIS, TANTALIZING CLUES TO FUTURE ONES
By Kenneth Chang
New York Times
January 4, 2005
The Cascadia fault, a 600-mile-long collision between two chunks of the
earth's crust off the Pacific Northwest coast, has been quiet for a long
time, and that is not a comforting fact.
Major earthquakes occur somewhere in the world every year or two.
Catastrophic tsunamis -- giant waves generated by undersea earthquakes or
landslides -- strike less often, and some of the largest of tsunamis
originate in places that do not, at first glance, appear particularly
The devastating tsunamis created Dec. 26 by a magnitude 9.0 earthquake that
killed as many as 150,000 people on the shores of the Indian Ocean caught
the world off guard, including most tsunami experts.
"Here is something that we didn't foresee," said Dr. Costas E. Synolakis, a
professor of civil engineering at the University of Southern California.
Dr. Synolakis said that experts would now take a closer look at the Indian
Ocean and that he expected that they would find geological evidence of
earlier tsunamis, which would allow scientists to estimate how often they
occur. "We're going to have a much better idea of what the hazard is," he
said, "because right now we don't know."
Observations of the destruction will also provide a check to their computer
models and could provide insight into the earthquake, the first magnitude
9.0 in 40 years, that produced them. Like the Cascadia fault, the Indian
Ocean fault is also at a place, known as a subduction zone, where one
tectonic plate plunges beneath another, and up to 600 miles of the fault
ruptured. The overlying plate jumped upward more than 15 feet, lifting the
water above it and setting off the tsunami.
An earthquake about as strong as last week's struck Cascadia in 1700,
sending tsunamis across the Pacific. Seismologists foresee a repeat. The
question is, When?
There are warning systems in the Pacific and plans up and down the coast to
cope with such a disaster, which is a situation far different from that on
the shores of the Indian Ocean. Only in the last year or so did Dr. Phil
Cummins, an Australian seismologist who was at the vanguard of concern,
conclude that a disastrous tsunami was possible. When it occurred, he was
still preparing a position paper that marshaled evidence of past tsunamis
and pressed for an international warning system. He had already been urging
awareness of the danger in presentations to scientists in Japan and Hawaii.
Tsunamis seem to be among the most mysterious of natural disasters, but
scientists know a good deal about how they occur and are working to
understand them better. Tsunamis follow the same laws of physics as ordinary
surf waves generated by wind. The difference is size. For wind-driven waves,
the distance between wave crests -- the wavelength -- is at most a few
hundred yards. For tsunamis, that wavelength can be a hundred miles or more.
Because the wavelength is so much greater than the ocean depth, the speed of
the wave depends on that depth. In water 2.5 miles deep, the average depth
in the Pacific, a tsunami travels almost as fast as a jetliner, 440 miles an
Ships at sea notice nothing. As a tsunami races past, the ocean surface
rises and falls slightly, a few feet at most, over a period of several
minutes to a couple of hours. Underwater, the effects are more pronounced.
The downward pressure of a surf wave dissipates a few hundred yards below
the surface, while the pressure force of a tsunami extends to the ocean
That led the National Oceanic and Atmospheric Administration to develop
instruments it has named tsunameters. With six deployed in middle of the
Pacific since 2001 in waters 2.5 to 4 miles deep, the tsunameters can detect
the perturbations in water pressure as a tsunami passes above.
When it detects something, it sends a signal by sound waves to a buoy on the
surface. The signal is relayed to a satellite and then back to earth to
tsunami warning centers in Hawaii and Alaska. "That whole thing only takes
two minutes to get the signal through," said Christian Meinig, engineering
division leader at the N.O.A.A.'s Pacific Marine Environmental Laboratory,
which designed and built the tsunameters.
No significant tsunamis have yet occurred in the Pacific for the tsunameters
to detect, but they have prevented a false alarm. In November 2003, a
magnitude 7.8 undersea earthquake occurred near the Aleutian Islands,
spurring officials to issue a tsunami warning. When the wave passed over a
tsunameter, they saw it was small and canceled the warning.
While there is a good understanding of how tsunamis travel in the deep
ocean, less is known about how they crash ashore.
Dr. Harry Yeh, professor of ocean engineering at Oregon State University,
leaves tomorrow for India, one of several scientists who will survey the
wounds of the tsunami on the coast. In particular, the scientists want to
record the height of the tsunami waves as they crashed onto land. "Sometimes
you can see by the scar marks on the tree," he said. Elsewhere, they might
find mud marks on building walls or collect accounts from survivors.
"Those data are very perishable," Dr. Yeh said. As cleanup begins, the
telltale signs are wiped away, and survivors' stories often change, as their
personal memories are affected by what they have heard and read.
Videos captured of the tsunami seemed to pale next to the cataclysmic
imaginings of Hollywood movies, but "looking at the videos, you would be
fooled," said Dr. Synolakis of U.S.C.
For one, those who tried to videotape more imposing waves might not have
survived. But also, unlike an ordinary wave, which quickly dissipates and
rolls back out, a tsunami is a long sheet of water. "Behind the wave is a
change in sea level coming in," Dr. Synolakis said. "The wave is coming and
coming and coming. A three- or four-meter tsunami can be quite devastating."
One cubic yard of water weighs nearly a ton, and a tsunamis come ashore at
speeds of about 30 miles an hour. An oncoming tsunami can hit a building
with millions of pounds of force, said Dr. Peter E. Raad, a professor of
mechanical engineering at Southern Methodist University in Dallas.
"And that's before you put anything in the water," he said.
Trees, automobiles and pieces of concrete all become lethal projectiles as
they are swept along by the rushing water.
Computer simulations by Dr. Raad aim to improve the understanding of the
rushing waters in order to construct buildings that better withstand
tsunamis. For example, the lower floors may have walls that break away when
hit, but the support columns may survive and hold up the upper stories, he
said. A parking garage may be placed away from the beach to prevent cars
from being swept away, he said.
Tsunamis caused by underwater landslides can be even more destructive. In
1998, seismologists were surprised when a modest magnitude 7.0 earthquake
off Papua New Guinea was followed by a 30-foot-high tsunami that killed more
than 2,100 people. The earthquake, it turned out, had caused nearly a cubic
mile of sediment to give way.
Three-dimensional maps of the bottom of California's Monterey Bay show
several sections that have given way -- and others that have cracked and may
collapse in the future. Some scientists have suggested that the outer edge
of the East Coast's continental shelf is also prone to cave-ins.
Others, including Dr. Steven N. Ward of the University of California, Santa
Cruz, have warned that the volcano Cumbre Vieja in the Canary Islands off
northwestern Africa could be nearing one of its periodic collapses. As the
volcano grows through eruptions, the sides become unstable and eventually
fall into the ocean. During the last eruption in 1949, a two-mile-long crack
opened up and one side of the volcano slid 10 feet.
"Geologically, we're getting close to the end," Dr. Ward said. "It's really
the cycle of life for these volcanoes. They grow too big, they collapse."
In Dr. Ward's computer models, when Cumbre Vieja collapses -- and that may
not happen for hundreds of thousands of years -- about 100 cubic miles of
rock will slide into the ocean at speeds greater than 200 miles per hour,
and the splash will generate tsunamis 300 feet high crashing into the
northwestern coast of Africa. Waves 40 feet high will reach New York. Other
scientists have pointed out that such catastrophic landslides are very rare
-- Cumbre Vieja last collapsed 500,000 years ago -- and that there is no
geologic evidence of such mega-tsunami in the past. They suggest that the
landslides will not accelerate quickly enough to produce the waves Dr. Ward
Even more catastrophically -- and even more rarely -- may be tsunamis caused
by asteroids' crashing into an ocean. In 1998, researchers at Los Alamos
National Laboratory reported that a three-mile-wide asteroid hitting the
middle of the Atlantic Ocean at 40,000 miles per hour would send tsunamis
that would crash tens of miles inland.
Such asteroid impacts, fortunately, occur only every 10 million years or so.
The tsunami that hit Washington State in 1700 was not of such gigantic
proportions, but it was big. And evidence of it was recognized only two
Until a couple of decades ago, the Cascadia fault, where a piece of ocean
floor the size of Oregon and known as the Juan de Fuca plate disappears
beneath North America, was also considered a low seismic threat. Oregon and
Washington have far fewer earthquakes than California, and few in the
northern states have caused much damage.
In 1987, Dr. Brian F. Atwater, a geologist with the United States Geological
Survey, discovered near the mouth of the Columbia River and in several other
estuaries in Washington the scars of a large tsunami, including spruce tree
forests that had suddenly turned into salt water tidal flats when the land
elevation dropped several feet. "There must be tens of thousands of stumps
in the estuaries," he said.
Dr. Atwater also found layers of sand that had been washed in. Radiocarbon
dating and examination of the tree rings placed the trees' death in 1700.
Historical records from Japan pinned down an exact date and almost an exact
time for the earthquake -- 9 p.m. Jan. 26, 1700. The Japanese documents
described 10-foot waves that washed away about a dozen houses in a fishing
village on the east coast of Japan nine hours later.
Computer simulations by Dr. Kenji Satake of the Active Fault Research Center
in Japan indicate that for 10-foot waves to reach Japan, the Cascadia
earthquake that generated them was huge, between magnitude 8.7 and 9.2,
rupturing the entire 600-mile fault. The 1700 earthquake was not a singular
event in the Cascadia subduction zone. Dr. Atwater's sediments show evidence
of seven earthquakes there in the past 3,500 years ago. For now, the Juan de
Fuca and North American tectonic plates are locked tight against each other,
accounting for the seismic lull.
But as the Juan de Fuca plate pushes inexorably to the northeast, pressure
along the Cascadia is building, seismologists are certain that it will break
again, most likely unleashing another volley of tsunamis. The interval
between earthquakes has been roughly 300 to 1,000 years. It has been 305
years since the 1700 earthquake. Scientists will not be surprised if one
occurs tomorrow, or several centuries from now.
"Geologically soon," said Dr. Yeh of Oregon State, "is 1,000 years or 100
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