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This Won’t Hurt a
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Killer
Surf!
Scientists
hasten to predict and prepare for a monster wave
threatening California’s coast.
By: Krista Conger
By anyone’s
standards, the north-central coast of Papua New Guinea would
qualify as a tropical paradise. White sand beaches slope
gently past nodding palm trees into the blue waters of the
southwestern Pacific, embracing the ocean in a subtle arc as
far as the eye can see. The bright yellow sun warms the
waves lapping gently at the shore. But the waters of Papua
New Guinea have not always been so tranquil and
inviting.
On the evening of July 17, 1998, the villagers in the
town of Arop were celebrating the arrival of a three-day
national holiday. Feasting was well under way on the narrow
spit of land separating the open ocean from Sissano lagoon.
When a magnitude 7.0 earthquake struck the area, the
villagers were startled but unhurt. After all, their huts
were built on stilts, capable of swaying with the shaking
ground. But elders in the village were worried. Remembering
native folklore, they urged the revelers to move to the
other side of the lagoon, away from the ocean. Although a
few people did take heed, piling into boats to cross the
inland water, most people ignored the warnings.
About 10 minutes later, a loud booming noise coming from
the ocean drew many people to the beach to investigate. To
their horror, they saw a wall of water nearly 50 feet high
rushing towards them at about 40 miles per hour. There was
no escape. Within minutes, more than 2,000 people had either
drowned or been killed by the force of the water and its
accumulated debris. All traces of habitation between ocean
and lagoon had vanished—swept bare by the scouring action
of millions of tons of water. Older palm trees unable to
bend with the force of the wave snapped like matchsticks.
Farther inland, the water lifted sturdy buildings and moved
them yards away from their concrete foundations.
The devastation at Sissano lagoon was complete. In all,
three villages along about six miles of beach were nearly
wiped out. Frightened survivors refused for days to descend
from the highest inland hills, and demanded that visiting
scientists tell them when the next wave would come. The
teams of international researchers arriving to investigate
the huge wave, or tsunami, could not answer their frantic
questions.
Tsunamis,
or tidal waves, don’t happen just in Papua New Guinea—they
can occur at nearly any point along the ”Ring of Fire,” the
tectonically active boundaries of the Pacific Ocean. The
word tsunami means ”harbor wave” in Japan, a country all too
familiar with the destructive power of these waves. Written
records document hundreds of tsunamis that have struck the
country’s coast over the centuries. Tsunami is a more
scientifically accurate term than the commonly used ”tidal
wave,” because tsunamis are cataclysmic events unrelated to
the tidal movements of normal waves.
Tsunamis are usually triggered when earthquakes move huge
chunks of the ocean floor up or down, quickly displacing a
massive volume of water. It’s somewhat like striking the
side of a child’s plastic wading pool—the shaking of the
plastic moves the water and creates a wave that can slosh
over the side of the pool. But in the real world, the wading
pool is the entire Pacific Ocean, and the sides of the pool
are the footholds of some of the most heavily populated
areas in the world.
In 1960, a magnitude 8.6 earthquake off the coast of
Chile spawned a tsunami that traveled across the Pacific at
more than 500 miles per hour, hitting first Hawaii and then
23 hours later, Japan. More than 1,000 people died in Chile,
61 in Hawaii, and nearly 200 in Japan. The destruction in
Chile alone totaled $550 million.
Coastal towns in Washington, Oregon, and California are
not immune to the threat of the unpredictable waves. Within
the past century, the northern coast of California,
including Santa Cruz, has been hit more than five times by
the powerful waves, causing millions of dollars worth of
damage and several deaths (see sidebar).
It’s a threat worth considering. A tsunami similar in
height and power to the one in New Guinea would easily
engulf the Santa Cruz boardwalk, submerging souvenir shops,
arcades, and unwary tourists alike. The Giant Dipper, the
wooden roller coaster that has thrilled adventure seekers
for half a century, would be taken on a wild ride of its own
as the mighty wave first toppled it, then swept it out to
sea. Much of downtown Santa Cruz would be underwater within
seconds.
Without an effective public warning system, the
devastating property damage would seem insignificant in the
face of catastrophic loss of life. To prevent this scenario,
geologists, seismologists, engineers, and public planning
officials have joined forces to create the National Tsunami
Hazard Mitigation Project in an effort to prepare for these
uncontrollable natural disasters.
Tsunami
preparedness is necessarily two-pronged. First, a tsunami
wave must be reliably detected in the open ocean in time for
people living in threatened areas to be warned, and second,
plans must exist for alerting and evacuating the affected
residents. Until now, it has been fairly difficult to
predict tsunamis, and frequent false alarms in Hawaii have
served only to make people less responsive to the threat.
But scientists at the Pacific Marine Environmental
Laboratory (PMEL) based in Seattle are working to make
tsunami detection more accurate.
The fact that tsunamis come in more than just one flavor
complicates the scientists’ job. In the last century, the
tsunamis that did the most damage to the West Coast and
Hawaii were generated by earthquakes hundreds or thousands
of miles away, near the Aleutian Islands in Alaska. These
large underwater earthquakes spawn waves that can take
several hours to reach the Lower 48. If the waves of these
distantly generated tsunamis can be detected while in the
open ocean, there may still be time to warn residents of
low-lying coastal areas.
In contrast to their traveling cousins, the second type
of tsunami is generated near the coast. As in the New Guinea
tragedy, such tsunamis are caused by earthquakes just
offshore and result in a wave that reaches land very quickly
with no time for warning. But even for a locally generated
tsunami, the New Guinea event was special. Seismologists
from around the world converged on the site shortly after
the tsunami to try to discover how a relatively moderate
earthquake could cause such an unusually huge wave. Their
understanding and reconstruction of the event may be vitally
important to Californians trying to assess the risks of
tsunamis along their coastline.
”The north coast of Papua New Guinea is probably the most
tectonically active place in the world,” says Eric Geist, a
seismologist with the United States Geological Survey. A
short distance off the northern New Guinea coast, two
continental plates collide, rubbing together and pushing
against one another in a slow geological lambada. But based
on the scientists’ calculations, even a 7.0 earthquake
originating 60 miles offshore couldn’t displace enough water
to fully explain the giant wave.
International teams of scientists are currently mapping
the ocean floor off the New Guinea coast to determine
whether the earthquake destabilized a patch of sediment,
sending it sliding down a slope into deeper water. The
moving land may have pushed the water ahead of it to form
the wave. While this would not be the first time a landslide
has been identified as a tsunami generator, it would be one
of the largest and most devastating of these events.
The landslide theory for New Guinea has disturbing
implications for other parts of the world. It suggests that
even a relatively small earthquake is capable of causing a
huge tsunami, and that the hills and valleys of the ocean
floor near a populated beach may be time bombs simply
waiting for the appropriate trigger.
Some scientists think that Californians should take note.
The Monterey Bay Canyon, less than a mile off the shore of
Moss Landing between Santa Cruz and Monterey, has sides
sloping sharply down nearly two miles. Such steep drop-offs
create ample opportunities for underwater landslides.
Additionally, Monterey Bay itself is bisected by two faults,
the Monterey Bay fault and the San Gregorio fault.
In 1998, the California Division of Mines and Geology
upgraded the San Gregorio to a Class A fault after comparing
seismographic history in the region with recent movements
detected along the fault line. The upgrade underscores the
potential of the fault to rupture and cause an earthquake
with a magnitude of 7.0 or greater. With its new
classification, the San Gregorio may be on its way to join
its illustrious sisters the San Andreas and Hayward faults
as household names for seismically savvy California
residents.
This information has scientists worried. A tsunami caused
by an earthquake and subsequent landslide in Monterey Bay
Canyon would occur much too quickly for a public warning
system to be effective. Like the elders at the Sissano
lagoon, people must take responsibility for getting
themselves away from the shore after an earthquake. To help
people protect themselves, scientists from the National
Oceanic and Atmospheric Administration (NOAA) have teamed up
with local emergency preparedness officials to create
tsunami inundation maps for several coastal communities. By
identifying low-lying coastal regions that are likely to
flood during a local tsunami, and by mapping out safe
evacuation routes in populated areas, the two groups hope to
enable local residents to respond appropriately to a tsunami
threat by moving away from the water even if they don’t hear
an official warning. Even if people can’t get completely out
of the wave’s path, ”every step you take inland reduces the
forces your body has to deal with,” says Eddie Bernard,
director of PMEL.
In contrast to locally generated tsunamis, long-distance
tsunamis offer the greatest opportunity for detection and
warning before they strike land. Tsunami experts are working
to perfect a deep-water monitoring system that would allow
them to warn people hours in advance of an approaching
wave.
Together, the two approaches may help coastal residents
in Washington, Oregon, and California feel more secure. And
that’s a good thing because recent scientific discoveries
indicate that the chance of a disastrous tsunami slamming
into the West Coast is more real than the Hollywood scenario
it calls to mind.
Less than
200 miles off the shore of Washington and Oregon lies the
largest active fault outside of Alaska, the Cascadia
subduction zone. Stretching about 1,000 miles from Eureka,
California, to Vancouver Island in Canada, it is exactly the
kind of fault scientists expect to cause a tsunami.
Scientists estimate that a wave generated from a Cascadia
rupture would reach the shore within 20 minutes. Bruce
Jaffe, a seismologist with the U.S.G.S., believes he has
found evidence of a huge tsunami which devastated the
northwest coast of the United States 300 years ago, as a
result of a rupture in the Cascadia fault.
Jaffe studies the sediment left in the wake of a tsunami
wave. He looks at cross-sections of the muck, analyzing the
size of the particles in the different layers to discover
how large the wave may have been. Close to the shore, where
the water is moving inland quickly, the wave deposits only
larger particles of dirt and debris. When the wave has lost
some of its energy further inland, it leaves behind smaller
grains of sand.
In a bit of geological detective work, Jaffe put together
evidence from his sedimentation studies, several drowned
pine forests, and local Indian legends to conclude that a
massive tsunami hit Washington, Oregon, and northern
California on January 27, 1700. He was able to gauge the day
and time of arrival by comparing dates deduced by counting
the rings of the drowned trees to written records in Japan
of a large tsunami that hit its coast the same day.
While seismologists can’t predict for sure when the
Cascadia subduction zone will slip again, they agree that
the ruptures probably occur in 400- to 500-year cycles. If
this is true, the danger to Washington, Oregon, and
California has been building for the past 300 years.
Evidence suggests that when the stress on the fault is
finally released it could trigger an earthquake of magnitude
8 or 9.
Until now,
the best indicator of an approaching tsunami has been the
occurrence of a large earthquake somewhere under the
Pacific. But the fundamental uncertainty as to which
earthquakes may cause a tsunami and where the wave may be
headed makes accurate prediction extremely difficult.
Finding a tsunami is not simply a matter of watching the
ocean and waiting for a large wave to pass.
The speed and height of a tsunami are proportional to the
depth of water through which the wave is traveling. As the
ocean bottom becomes shallower near the shore, the pull on
incoming currents increases. This slows the water movement
and causes the water to pile on top of itself, creating
waves. But in the deep ocean, with less friction, a tsunami
can travel at up to 500 miles per hour, nearly the speed of
a jetliner. At the same time, the height of the wave in the
open ocean can be less than three feet. A bird looking down
on the ocean as a tsunami passes underneath would be unable
to detect anything different from normal wave patterns, and
a boat would continue to bob up and down without noticing
anything amiss.
The main difference between regular ocean waves and
tsunami waves is reflected in measurements of wave period
and wavelength. Wave period is the time elapsing between one
wave and the next as they pass a fixed point, and wavelength
is the measurement of the distance between the peaks or
troughs of two consecutive waves. Normal waves have periods
on the order of seconds, and lengths that vary from 15 to
200 feet. Tsunami waves are much more spread out. The
periods between them can be over an hour, and their
wavelengths can exceed 400 miles.
The strange patterns of tsunami waves can spell disaster
for the unwary. People unaware of the length of time between
tsunami waves have been killed when they ventured onto the
beach to clean up debris left by the first wave. ”The
initial wave can be small, but the second wave (hours or
minutes later) can be huge,” says Rich Eisner, the regional
administrator of the California Office of Emergency
Services. Additionally, an unexpected drop in ocean levels
is sometimes the first warning of an impending tsunami.
Curious people exploring the inviting expanse of ocean
bottom and coral reefs revealed by the receding water are
exceptionally vulnerable when the wave strikes.
According to the laws of physics, a wave loses energy at
a rate inversely proportional to its wavelength. So tsunami
waves, with their long wavelengths, lose energy very slowly.
That is why they can travel huge distances across the ocean
and travel far up on land when they hit shore. PMEL
scientists have exploited the differences in wavelength
between normal waves and tsunami waves in the development of
their ”Deep-ocean Assessment and Reporting of Tsunamis,” or
DART project.
The project is a joint effort by PMEL and NOAA to detect
tsunamis when they are still far out in the ocean, giving
time to warn coastal communities of incoming waves. The
success of the plan hinges on sophisticated pressure
recorders placed on the floor of the north Pacific which
detect changes in the height of the water above the
monitors. Originally developed as transportable units to
accurately measure water depth when drilling for oil, the
devices were modified by researchers to function as
stationary recorders far beneath the waves.
The scientists plant the rectangular, box-like devices on
the sea floor, sometimes as far as three miles below the
ocean’s surface. A small, hollow tube filled with mineral
oil provides an outlet to the sea. The tube connects to a
sophisticated mechanism that resembles a curled party
noisemaker at a child’s birthday party. Just as the
noisemaker’s paper tube extends when one blows into its
mouthpiece, the increase in ocean pressure caused by the
passage of a wave overhead unfurls a piece of curved metal
in the mechanism. But unlike the party favor, the metal
cannot straighten completely because its curled tip is
anchored to a solid support by a thin quartz crystal.
The crystal serves as a translator, converting increased
water pressure into numerical measurements that scientists
can analyze. It accomplishes this task by vibrating in
response to a constant electric current. When external
pressure increases, the metal tube tugs against the solid
support in its attempts to unfurl, stretching the crystal
from end to end. Like a tightening guitar string, the
vibrational frequency of the crystal increases as it is
stretched. The corresponding passage of a tsunami trough
reduces the pressure and decreases the vibrational frequency
of the quartz. The detector monitors these changes in
frequency, and transmits the data acoustically through the
water to a buoy on the ocean surface. The buoy then beams
the information to a satellite and on to tsunami warning
centers in Washington and Alaska.
As one might expect, the differences in water pressure
between wave peaks and troughs are nearly infinitesimal
miles beneath the surface. But according to Eddie Bernard,
the recorders can detect pressure changes as small as one
part per million, making them extremely sensitive.
The detectors report the normal pressure changes caused
by everyday waves every 15 minutes, and the surface buoy
beams the information to the satellite once an hour. But if
the pressure differences are separated by more time than
expected for a normal wave, indicating the characteristic
long wavelength of a tsunami, the detector breaks its
pattern to signal to the buoy immediately, and continues
signaling every 15 seconds.
In preliminary tests, the detectors have worked well.
While testing one of the first prototypes in 1988, the
researchers were able to detect a tsunami wave that was only
a few millimeters high in the open ocean, demonstrating the
detector’s potential usefulness and accuracy. But it has
taken 10 years to begin to overcome the technical
difficulties of keeping the sensitive instruments functional
in the hostile ocean environment. Frequent lapses in
transmission remain unexplained; they may be the result of
acoustical interference in conversation between detector and
buoy. Physical hazards exist in the open ocean as well. On
Christmas morning 1998, a ship struck the buoy of the team’s
most successful prototype so far and destroyed it. ”We test
them in the lab, but it’s not the same as putting them in
the ocean,” says Hugh Milburn, an engineer involved in the
project. ”We learned that the hard way.”
Although team members have had difficulty making the
detectors reliable enough for use far from shore, their
prospects are brightening. They have redesigned the
signaling mechanism to be less sensitive to acoustical
interference and are hopeful that this will make the
detection system more robust. They have also included
back-up acoustic signalers, just in case.
In October 1999, the researchers plan to replace the
damaged buoy and plant six more new ones off the coasts of
Washington and Oregon. This spring, the team also plans to
begin testing a detector in the deep waters of the Monterey
Bay Canyon. Milburn explains that the proximity of such deep
water so close to shore will make it easy to monitor and
check on the detector. Additionally, the San Gregorio fault
running under Monterey Bay, as well as the steep canyon
sides, make it a potential tsunami site and a practical
place to test the equipment, Milburn says.
Even if the
DART project is successful, simply predicting a coming
tsunami is useless unless people know how to respond to the
threat. Most people don’t realize the potential dangers of
the big waves. ”Human memory is short,” says David
Oppenheimer, a seismologist with the U.S.G.S. ”Santa Cruz
really got whacked in the 1964 tsunami.” And yet hundreds of
Santa Cruz residents lined the beaches and boardwalk in 1986
when officials issued a tsunami warning after an earthquake
in the Aleutian Islands. The warning was called off and no
local damage was reported, but the event highlighted public
misconceptions about the large waves.
”It’s a very big but not well understood risk,” says Rich
Eisner. Eisner is working with the National Tsunami Hazard
Mitigation Program and Costas Synolakis, a civil engineer at
the University of Southern California, to design ”Tsunami
Inundation Maps” for regions of the California coastline.
Synolakis is using sophisticated computer programs to map
which coastal areas and portions of communities may be
flooded during tsunamis of varying sizes. Eisner and the
National Tsunami Hazard Mitigation Program will use the data
to plan evacuation routes for local residents. Eventually
they plan to produce maps for the entire California
coast.
Within the next year, says Eisner, signs will be posted
on local beaches warning people to immediately move away
from the water to higher ground whenever they feel an
earthquake. Other warning signals include a sudden
withdrawing of the ocean levels to below the low tide mark,
or a roaring noise coming from the ocean as debris picked up
by a wave is pushed along the ocean bottom. Signs explaining
the danger are already springing up on the Washington and
Oregon coastline.
Officials hope that the signs will educate coastal
residents in time to prepare them for the next tsunami. But
while they want people to be prepared, they don’t want them
to panic. ”Tsunamis are only hazardous to you if you live in
a low-lying area along the coast,” says David
Oppenheimer.
So although it’s not worth lying awake at night worrying
about being swept away by a giant wave, it might pay to be
prudent. The Santa Cruz boardwalk may not look at all like a
South Sea island, but ignoring the signs of a potential
tsunami can be as deadly here as it was in New Guinea.
Director of PMEL Eddie Bernard agrees that preparedness
is better than panic. ”A tsunami is going to happen, but it
doesn’t have to be a disaster,” he says. Reflecting on the
terrible loss of life in Papua New Guinea, he says, ”We
don’t want to be telling these kinds of stories about our
own coastline.”
TSUNAMIS
1896:
An earthquake off the Japanese coast triggered a
10-foot-high tsunami in Santa Cruz. No tsunami-related
deaths reported.
1906:
The great San Francisco earthquake,
magnitude approximately 8.3, caused a small tsunami,
as measured by local tide gauges. No tsunami-related
deaths reported.
1946:
A 7.4 magnitude earthquake off the Aleutian Islands in
Alaska generated a 15-foot-high tsunami in Santa Cruz.
The wave swept a man walking on Cowell Beach out to
sea where he drowned. The same earthquake caused a
tsunami in Hawaii that killed 173 people. This
destructive tsunami was the incentive for the
development of the Pacific Tsunami Warning Center.
Since 1946 the center has issued 20 tsunami warnings,
five of which correctly predicted Pacific-wide
tsunamis.
1964:
The 8.4 magnitude ”Good Friday” earthquake in Prince
William Sound spawned a deadly wave that killed 115
people in Alaska before moving down the Pacific coast.
This event was the most recent Pacific-wide tsunami.
Four people were killed on a beach in Oregon, and 11
people were killed in Crescent City, California.
Crescent City suffered $7 million in damage due to the
30-foot wave. All told, the 1964 earthquake caused $10
million in damage along the California coast.
1986:
A tsunami warning was posted for the West
Coast in response to a series of earthquakes in the
Aleutian Islands ranging in magnitude from 4.4 to 7.7.
Wave height reached 10 feet in Hawaii, but only a few
inches in Santa Cruz.
1989:
The magnitude 7.1 Loma Prieta earthquake generated a
small local tsunami along the Pacific coast. Water
levels in Monterey Bay rose between 1 and 4 feet after
the earthquake. Water continued to slosh back and
forth in the bay for at least a day, rising and
falling by as much as a foot each cycle. An underwater
landslide of a 2- by 5mile section of the Monterey Bay
Canyon wall was also recorded after the quake.
1992:
A 7.2 earthquake off the coast of Humboldt County in
California produced a small tsunami about 1 to 3 feet
high. The quake occurred at the southern boundary of
the Cascadia zone, and was the first well-documented
California example of a subduction zone earthquake
resulting in a tsunami.
1994:
A tsunami warning was issued for Santa Cruz and the
rest of the West Coast in response to a magnitude 7.9
earthquake off the coast of Japan. The warning was
called off when the waves reaching Santa Cruz were
less than 12 inches high. However, huge waves were
recorded in the Kuril Islands off Russia.
-
- BIO
-
- WRITER
Krista Conger
- B.A., biochemistry,
University of California, Berkeley; Ph.D., cancer biology, Stanford
University.
Internship: postponed owing to baby.
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1999 Krista Conger
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