Oceanography 10, T. James Noyes, El Camino College
5B-1
Tsunami
Tsunami at the Shoreline
The word “tsunami” combines
the Japanese kanji (ideograms)
for “harbor” and “wave.” The
people most affected by tsunami live in harbors, and in
some cases harbors can funnel
the waves together in a harbor,
making them even larger.
Tsunami are – technically speaking – ocean waves,
but they are quite unlike ordinary ocean waves at
the shoreline. They do not come crashing down
upon the shoreline, because they never become
large enough to “break.” (At most, the front of the
wave collapses when it is pushed forward too fast
by the water behind it.) Instead, they are very
long-wavelength waves that just keep going
forward when they hit land, surging up the shoreline at 30 mph or more (faster than you can run).
The tsunami picks up people, smashing them into buildings and squishing them between other
objects (e.g., cars) it picks up. So much “junk” is picked up that people struggle to stay at the
surface to breathe. You may consider water to be soft, but it is quite heavy; just imagine how
much it would hurt to have several jugs of water dropped on top of your. Bodies and the debris
are sucked out into the sea when the water finals slides back into the ocean. Anyone still alive is
typically injured, so they cannot swim for long and drown.
Not a Tsunami. “The Great Wave Off Kanagawa” by Hokusai is a famous picture,
but does not accurately show what a tsunami looks like. The figure on the upper right
shows how a tsunami is actually water flooding up the shoreline, not a rising crest.
Oceanography 10, T. James Noyes, El Camino College
5B-2
Causes of Tsunami
Professional oceanographers call tsunami
Tsunami can be caused by any phenomenon which
“seismic sea waves,” because the word
makes a large amount of water to move up or down. “seismic” refers to earthquakes, the most
The most common cause of tsunami are underwater
common cause of tsunami.
earthquakes. Typically the seafloor only moves up
or down a small amount (a few feet) over a large area (hundreds or even thousands of miles)
during earthquakes, which creates a wave with a small height (few feet) and large wavelength
(hundreds of miles). All the water above the moving seafloor is set into motion, so the
earthquake injects a tremendous amount of energy into the ocean, but it is very spread out. Like
a “normal” wave, the energy of a tsunami becomes more and more concentrated as the tsunami
approaches the shoreline. Its crests grow higher and higher as the water becomes shallower and
shallower.
few feet
Not all earthquakes in the ocean
cause tsunami. For example, an
2.5 miles
earthquake deep inside the Earth
will not make the seafloor move
up or down very much. Even
300 miles
though we commonly have
earthquakes here in southern
few feet
California, we are not a highrisk area for a big, locallygenerated tsunami, because
2.5 miles
most of our earthquakes are on
land and the earth here usually
moves side-to-side (not up and
300 miles
down) during an earthquake.
Farther up the coast, though, north of San Francisco, a
huge tsunami like the one which struck Asia is not
only possible but likely one day. In both places, there
is a trench where the ocean crust is plunging down
beneath continental crust. The leading edge of the
continental crust gets pulled down by friction with the
oceanic crust going beneath it, but sooner or later it
will get pushed too far, overcome the friction, and
“snap back” into shape. (This is somewhat like
bending an eraser and then releasing it.) This does not
mean that we are completely safe here in southern California. Tsunami created on the other side
of the ocean can still be quite large when they get here, and local earthquakes could trigger an
underwater landslide that leads to a tsunami.
Oceanography 10, T. James Noyes, El Camino College
5B-3
Tsunami in Deep Water and Tsunami Warnings
Tsunami are small out in the middle of the ocean (no more than a few feet high), and grow larger
as they approach the shoreline like “normal” (wind-generated) waves. They’re very fast,
attaining speeds well over 400 mph (they speed of a jet aircraft), but slow down as the approach
the shoreline like ordinary waves. The difference is that tsunami have extremely long
wavelengths and therefore their wave orbitals which reach all the way down to the bottom of the
ocean, so they always “feel the bottom,” limiting their speed. Putting all these numbers together
shows that tsunami are harmless out in the middle of the ocean: it would take a 400-mph tsunami
with a height of 2 feet and a wavelength of 100 miles at least 15 minutes (100 miles / 400 mph =
0.25 hours = 15 minutes) to make a person or ship travel up 2 feet then down 2 feet and then
back up again, the distance you could step in a second or two. In fact, all the ordinary waves out
in the ocean would make it very hard to feel the tsunami at all.
Many kinds of waves travel “better”
Earthquake waves (seismic waves) travel through the
through solids than liquids. For example,
solid earth much faster than tsunami travel through
put your ear against your table or desk,
the ocean. So even though tsunami are very fast,
and “knock” or rap the tabletop with your
scientists can detect large underwater earthquakes and
fist. Then, lift your head off the tabletop
issue a tsunami warning before the tsunami arrives at
and “knock” again. In which case is the
a shoreline (unless the earthquake is very close to
sound louder? Is sound transmitted
shore). However, not all earthquakes cause tsunami.
“better” through solids or air?
To avoid issuing a false alarm which would cause
unnecessary panic, hurt the economy, and damage scientists’ credibility (for “crying wolf”),
scientists only issue a tsunami warning if they have additional observations which suggest that a
tsunami has been generated. For example, they look for higher or lower tides than those
predicted for the coastline, and unusually high or low pressure on the deep ocean floor. (Note:
This means that scientists must be able to predict the tides very accurately if they hope to detect
small changes caused by distant tsunami.)
Led by the United States and Japan, the nations bordering the Pacific Ocean created the Pacific
Tsunami Warning Center in 1949 to better coordinate information from all the countries of the
Pacific Ocean and thus issue fast and accurate warnings of potential tsunami. Unfortunately,
there was no such network in the Indian Ocean in December 2004, so even once tsunami experts
in the center realized what was happening, they had no way to quickly contact the proper
officials and emergency organizations in countries likely to be hit by the tsunami.
Oceanography 10, T. James Noyes, El Camino College
5B-4
The Asian Tsunami of 2004
On December 26, 2004, at about 8 am (local time), a large earthquake in the northeast corner
of the Indian Ocean created a massive tsunami which caused destruction as far away as Africa
(on the other side of the ocean). The earthquake had a magnitude of at least 9.0, making it one
of the largest earthquakes of the last 100 years. The earthquake took place deep beneath the
earth (about 30 km or 19 miles), but it resulted in major changes in the ocean floor. About
1200 km (750 miles) of ocean floor moved, in places rising several meters (well over 10 feet).
The earthquake and the resulting tsunami killed more than 230,000 people, making it one of
the worst natural disasters in modern history. Unfortunately, the countries bordering the
Indian Ocean did not have a warning system in place (unlike the Pacific Ocean). In addition,
many people were not familiar with tsunami warning signs (for example, sea level falls rapidly
in some cases before it rises again). Relief agencies reported that about 1/3 of the deaths were
children (children make up a large percentage of the population of many of the affected
countries, and would have had more difficulty resisting the surging waters). Oxfam noted that
in coastal fishing villages 4 times more women died than men since they were on the shoreline
while their husbands were at sea. (Tsunami are small out in the middle of the ocean, and only
grow large as they approach the shoreline.)
Sumatra, Indonesia.
Brown areas show
where vegetation
was stripped by
the tsunami.
Astronaut
Photograph
NASA
Sumatra, Indonesia.
Photo taken by
Philip A. McDaniel,
U.S. Navy.
(Public Domain)
Oceanography 10, T. James Noyes, El Camino College
5B-5
Map of the Indian Ocean showing wave height from satellite measurements and models.
Courtesy of the NOAA Laboratory for Satellite Altimetry (public domain).
During the Asian tsunami of 2004, several satellites were measuring the height of the surface of
the ocean as the tsunami passed underneath them. The data show a deep-water height of almost
4 feet and a wavelength of over 400 miles.
Oceanography 10, T. James Noyes, El Camino College
5B-6
Tsunami at the Shoreline
As a wave moves into shallow
water, its orbitals “feel the
bottom,” causing it to slow
down. The wave crests that
are closer to the shore (“in front”) are in shallower water, so they are moving slower than the
wave crests farther out in the ocean (“behind”). This allows the wave crests out in the ocean to
get closer to the wave crests near the shore, reducing the wavelength (the distance between the
crests). This “squeezes” the water in-between the two waves horizontally; it cannot go down
(the ocean is getting shallower as it approaches the shore!), so it goes in the only direction it can:
up! This is how a small tsunami crest out in the middle of the ocean becomes a large crest at the
shoreline.
Here is another way to think about
why waves grow at the coast: the
front part of a wave crest is in
slightly shallower water than the
back part of the wave crest, so it is
always going a little slower than
the back part of the crest. As the
back part of the crest catches up to
the front part of the crest, more and
more of the water that was spread out over a wide area gets concentrated in a narrower area. I
always hesitate to use this explanation because many students think that a wave crest that is
“behind” another wave crest can actually “catch up” to the wave crest that is “in front” of it. This
cannot happen, because the wave crest that is “behind” slows down more and more as it enters
shallower water, so it can never actually “catch up” to the wave crest “in front” of it. Instead, it is
always “catching up,” but never can quite do so. The key thing to remember to avoid confusion
is that the back part of a wave crest is “catching up” to the front part of the same wave crest; two
separate wave crests are not merging.
Tsunami are not the same height everywhere along a coast. One area may be hammered hard,
while a nearby area is relatively unhurt, because local conditions like the shape of the shoreline
and ocean bottom can have a large effect on the tsunami as it comes ashore. For example,
communities with coral reefs offshore were not hit as hard, apparently because the steep slopes
of coral reefs act as walls, causing tsunami to reflect (bounce back) and thus protecting the
shoreline. Some bays and harbors get narrower and narrower, funneling the surging water: there
is not enough space for all the water surging into the bay, so it piles up and gets forced forward
even faster to make room for the water behind it. Like ordinary waves, tsunami refract 1 and
interfere 2 as they come into the shore, so they may miss some locations in favor of others or two
crests may come together, producing one, even-larger crest. Using information about the nearby
ocean floor, oceanographers can make predictions about which places are likely to get hit the
worst by a tsunami 3.
1
“Refract” means “change direction.” Like ordinary waves, tsunami bend (“turn”) towards the shore.
“Interfere” means that wave crests can add together, or a wave and trough can “cancel” one another.
3
depending upon the direction and wavelength of a tsunami
2
Oceanography 10, T. James Noyes, El Camino College
5B-7
Tsunami Safety
The most important rule to survive a tsunami is the most obvious one: If you are ever caught
somewhere threatened by tsunami, get away from the shoreline and go uphill, as high as you can
as fast as you can. If you are near the shoreline and feel a large earthquake (e.g., big enough to
knock you off your feet), then you should get away from the shoreline just in case, even if a
warning has not been issued.
In addition, everyone should know that tsunami can
have several large crests, and that the first crest to hit
the shoreline may not be the largest. Many people
who survived the first crest of the Asian tsunami died
when the second crest hit the shoreline. Not only do
tsunami have crests, but they also have troughs.
Sometimes the trough of the tsunami arrives before
the first crest, causing the water along the shoreline
to recede and suddenly produce a very “low tide.”
Often, people are amazed at their good fortune and
go out to look at “tide pools” and animals caught
high and dry. They are the first to die when the crest
arrives, coming in far faster than they can run.
Courtesy of Mark Wallace
CC BY 2.0
The Japanese Tsunami of 2011
Around 20,000 people died in the earthquake and tsunami that struck the coast of Japan on
March 11, 2011, much less than the over 200,000 people who died in Indonesia and elsewhere in
the Indian Ocean in 2004. The earthquakes were of similar magnitude; why were far fewer
people killed in Japan? The Japanese were well aware of the dangers of earthquakes and tsunami,
and devoted resources and time to preparing for the disasters, including clearly-labeled
evacuation routes and large seawalls. Schoolchildren drill regularly for disasters, so everyone
learns early in life what do and practices regularly. For example, in Iwate, elementary and
middle school students gathered in the playground (the evacuation spot), made sure no one was
left behind, and sprinted nearly a mile to the safety zone; they knew what to do without the
teachers needing to tell them, saving precious time. Because the earthquake occurred at the
trench next to the coast and tsunami travel very fast, the tsunami reached the closest part of the
coast in minutes, before the Pacific Tsunami Warning system could issue a warning. Instead, it
was investment in preparation, education, and training that saved tens of thousands lives in Japan
that day.
This is not to say that the Japanese system worked perfectly. For example, some people died
owing to overconfidence in the country’s preparations and defenses. The earthquake was larger
than scientists thought was possible: the consensus view was that the old, dense sea floor sinking
at the trench would go down more easily due to its high density and not get “stuck” as much as it
did. Therefore, the tsunami was many times larger than Japan had prepared for, so the seawalls
were not high enough and the evacuation zones not far enough from the coast in some places.
Some people were so confident in the seawalls that they did not even run when warning sirens
went off.
Oceanography 10, T. James Noyes, El Camino College
5B-8