tsumami crashes through the Thailand coast in 2004. (Photo
courtsey of David Rydevik)
On December 26th, 2004, a wall of water as high
as 30 feet crashed into shorelines along the Indian Ocean from
Africa to Australia washing away whole coastal towns and villages.
With a death toll of more than 200,000, it is perhaps the largest
natural disaster in the history of man. What caused these huge
waves and where will they strike next?
It was early in the morning, almost 2:18a.m., on
April 1st, 1946 at the Scotch Cap Lighthouse on Unimak Island,
Alaska. The five men that were on duty inside the five-story building
that stood on a bluff some 40 feet above the sea were nervous.
An earthquake had occurred less than an hour ago, shaking the
station violently for thirty seconds. Twenty-seven minutes later
a short, hard aftershock occurred. The officer on watch at the
lighthouse that night, Leonard Pickering, was trying to get news
about the quake from Dutch Harbor navy radio. Perhaps as he did
so he looked out the window and pondered why the ocean was so
amazingly low. So low that the sea bed was exposed.
Perhaps he didn't. We will never know for sure.
What we do know is that a few minutes later the sea returned in
a violent rush. Waves climbed up the 40-foot bluff and engulfed
the lighthouse. The water was so high that it would have topped
the tip of the light, if it had still been there. As it was, the
building probably collapsed when the wavefront hit it, killing
all five men inside. When rescuers from a nearby coast guard station
reached the site early the next morning, they found the area covered
with debris. Only the foundation of the structure still stood
to attest that the lighthouse had ever existed. As for the crew,
the rescuers that day found only an amputated human foot, a kneecap,
and bit of human intestines.
wall of water approaches the Scotch Cap Lighthouse (Copyright
Lee Krystek, 2005)
The Scotch Cap Lighthouse and its keepers were the
victims of a tsunami (tsoo-nah-mee), a series of violent waves
that can rush into coastal locations and sweep away cars, level
buildings and kill people. Typically, a tsunami can reach a height
of 30 feet above sea level, but in some extreme cases, as at the
Scotch Light, they can reach heights of over one-hundred feet.
of an Earthquake
The most deadly tsunamis are the result of earthquakes.
The sudden movement of the earth can raise or lower a vast portion
of the ocean bottom by as much as ten feet in just a few minutes.
When the sea bottom goes up, the column of water piled on top
of it is also pushed up, creating a bulge in the surface of the
ocean. As this mass of water spreads out, a series of waves are
created that fan out from the earthquake area. These waves make
up the tsunami.
The word tsunami is Japanese for "harbor
wave." Sometimes the English term tidal wave is used, but
it is misleading as a tsunami has little to do with the tides.
They are also, in most respects, unlike normal ocean waves. A
typical ocean wave is caused by wind sweeping across the water.
These waves may appear large when driven by a storm, but they
only involve the top few feet of the ocean. Normal ocean waves
also have a short "wave-length" (the distance from wave crest
to wave crest) that is usually less than a hundred feet.
In contrast, tsunamis involve the entire water column
from seabed to surface. Their wave lengths are also very long,
maybe as much as a hundred miles. Because there is so much water
moving in the tsunami, the energy involved is tremendous. Despite
this, a tsunami is practically invisible in deep water. To a ship
at sea it may appear as a rapidly-moving three-foot-high swell
that is easily lost among the normal ocean waves. Only as it approaches
the beach does its true size become apparent.
Scotch Cap lighthouse before and after the wave hit. (U.S.
The waves in a tsunami move very rapidly through
deep water reaching speeds of 500 miles per hour. As wave approaches
land the water grows increasingly shallow and friction with the
ocean bottom slows the wave. As other waves back up behind it,
the wavelength shortens and the top of the wave height increases
until it may be ten, twenty, thirty feet, or higher. The actual
height of a tsunami wave is hard to measure without risking life
and limb, so scientists usually gauge their size by a term called
the run up. This is the maximum vertical height above sea
level that the water reaches on the coast. In the case of Scotch
Cap the run up was measured at over 100 feet.
The amount of damage done by the waves will vary
widely depending not only on the size of the wave, but the configuration
of the shoreline and the sea bottom. In the 2004 Indian Ocean
tsunami, the Maldive Islands suffered tremendous damage, while
just a little to the south the island of Diego Garcia had very
minor damage because it was protected by an offshore underwater
Even after being slowed by the shallow bottom as
they approach land, tsunami waves still move faster than any human
being can run, often with velocities of 45 miles per hour or more.
As one scientist observed "by the time you see a tsunami approaching,
it is too late to get away."
graphic represents the propagation of the wave from the
original earthquake across the Indian Ocean in the 2004
Because it is too late to escape a tsunami by the
time you see it, tsunami warning systems are extremely important.
The deadly 1946 tsunami that destroyed Scott Cap Light station
and also ravaged much of Hawaii caused the United States to set
up the Pacific Tsunami Warning System (PTWS) in 1948. At
the headquarters in Ewa Beach, Hawaii, staff members monitor seismographic
information from around the world. If an earthquake occurs, they
combine the information from several seismograph stations at different
locations to get the position of the quake. If the quake occurs
below the sea floor and it is strong enough (7.5 or higher on
the Richter scale), it may have generated a tsunami and a Tsunami
Watch may be issued.
The center also operates a number of deep sea buoys
that are connected to sensors on the bottom of the sea. By measuring
pressure, the sensors can tell the height of the water above them
to within less than a half an inch. If the sensor detects the
passage of a wave that looks like a tsunami, it contacts the buoy
which in turn sends a signal via satellite to the PTWS. Confirmation
of a tsunami is also made by getting reports from each coastal
location along the wave's route as it arrives there. Once a tsunami
is confirmed, a warning is issued to affected areas.
Since the PTWS was set up, it has warned the public
about several significant tsunami events and saved hundreds of
lives. Unfortunately, because the Indian Ocean had not had a major
tsunami event in many years, no warning system was in place in
there in December 2004 when the tsunami struck and the result
was a huge loss of life.
Tidal Wave, Tidal Bore or Rogue Wave: What's the Difference?
is a large wave produced by the vertical movement of the
sea floor, a meteor strike or a landslide. While the term
tidal wave has been used interchangeably with tsunami,
a tidal wave is more properly synonmous with a tidal
bore. A tidal bore is created in a river when a rapidly-moving
tide is slowed by friction with the river bottom and the
water builds up into a large breaking wave. Tidal bores
are fairly predictable events as they occur at particular
locations at particular times. Occasionally, though, they
surprise observers as happened with some students who went
to see a bore in the Qiantangjiang River in China in 2002.
They found running for their lives as it unexpectedly topped
the sea wall and chased them down the street.
A rogue wave
occurs during heavy storms when two or more normal ocean
waves created by strong storm winds meet and reinforce each
other, giving the combined wave an exceptional height. These
waves can grow as tall as 90 feet and present a danger to
even the largest ships that cross their paths.
Even without a tsunami warning system there are
some signs that may warn coastal residents of an impending tsunami.
It is prudent to always evacuate a coastal area after a strong
earthquake. If the sea suddenly recedes and the seabed is exposed,
this may also be a warning that a tsunami is on the way. Unfortunately
in the past, events such a sudden drop in the sea level has drawn
the curious down to the beach, increasing the casualties when
the actual waves arrived. Needless deaths have also occurred when
victims failed to realize that a tsunami can be made up of multiple
waves and the first one to arrive often isn't the biggest. The
curious can wander down to the oceanfront to see the damage by
the first wave and be killed when the next wave arrives a few
The regions most likely to be affected by a tsunami
are those which are seismically active. The Pacific Ocean, where
two continental plates meet, is notorious for the creation of
tsunamis. Here, in what scientists call a subduction zone, the
Pacific Ocean plate is sliding under the North American plate.
When they move, the grinding between the plates creates shock
waves that run through the rock. We interpret these as an earthquake.
If the earthquake raises or lowers the seabed, a tsunami is likely
to result. The December 26th, 2004 disaster involved the Indian
plate and the Burma plate near Sumatra. Previously, this area
under the Indian Ocean has been quiet and no significant tsunamis
had occurred there in many years. This has scientists wondering
if other places not traditionally associated with tsunamis might
be under threat.
York City Under Eighty Foot Waves?
While the continental United States has been spared
tsunamis in recent years, anywhere along the Pacific coast might
be vulnerable. In 1964, Crescent City in northern California was
hit by a tsunami generated by an earthquake near Valdez, Alaska.
The four waves associated with that tsunami washed away 29 city
blocks and killed 11 people.
The Atlantic coast of North America has seen fewer
and smaller tsunamis, but there is still a chance that a major
one could occur. In 1929 a giant wave hit Newfoundland, Canada,
killing 50 people. Scientists are also concerned that a fault
zone in the Caribbean might generate a wave that would run up
the United States east coast. Subduction zones are the most frequent,
but not the only cause, of tsunamis. A submarine landslide, a
large meteor hitting the ocean or volcanic action can all cause
tsunamis. Some scientists are concerned about a volcano on the
Canary Islands in the Atlantic near Africa. A 2001 study predicted
that aneruption there could cause a gigantic landslide (a rock
about the size of Manhattan) that would end up in the ocean and
most likely cause a large tsunami. Computer models run by the
scientists show that such a wave could cross the Atlantic in nine
hours and engulf the east coast (including New York City) with
80 foot waves.
A more recent 2012 study, however, indicates that
such a nightmare scenario might only happen far in the future
when the volcano has grown another half mile in height. Smaller
slides creating 10 to 15 foot waves along the U.S. shore are more
likely. Even so, as the disaster along the Indian Ocean has shown
us, mankind needs to take the steps necessary to prepare for these
big waves. Failing to do so can have catastrophic results.
Tsunami!, Earth and Space Sciences University
of Washington, http://www.geophys.washington.edu/tsunami/intro.html
West Coast & Alaska Tsunami Warning Center,
West Coast & Alaska Tsunami Warning Center, http://wcatwc.arh.noaa.gov/subpage1.htm
Scotch Cap Lightstation Disaster by Alan
Yelvington, SEMper PARatus PACarea Emergency & Disaster Planning
Information Page, http://www.semparpac.org/ltsta.html
Deep-ocean Assessment and Reporting of Tsunamis
(DART) by Hugh B. Milburn, Alex I. Nakamura, Frank I. Gonzalez,
Could a tsunami hit the U.S.? by Edie Magnus,
MSNBC News, http://msnbc.msn.com/id/6798858
Waves of Destruction by Tim Folger, Discover
Magazine, May 1994.
Lee Krystek 2005. All Rights Reserved.