U.S. Department of Commerce,
Tsunamis, also called seismic sea waves or, incorrectly,
tidal waves, generally are caused by earthquakes, less commonly by
submarine landslides, infrequently by submarine volcanic eruptions and
very rarely by a large meteorite impact in the ocean. Submarine volcanic
eruptions have the potential to produce truly awesome tsunami waves. The
Great Krakatau Volcanic Eruption of 1883 generated giant waves reaching
heights of 125 feet above sea-level, killing thousands of people and
wiping out numerous coastal villages.
The 1992 Nicaragua tsunami may have been the result of a
"slow" earthquake comprised of very long-period movement occurring
beneath the sea floor. This earthquake generated a devastating tsunami
with localized damage to coastal communities in Nicaragua.
Not all earthquakes generate tsunamis. To generate tsunamis,
earthquakes must occur underneath or near the ocean, be large and create
movements in the sea floor. All oceanic regions of the world can
experience tsunamis, but in the Pacific Ocean there is a much more
frequent occurrence of large, destructive tsunamis because of the many
large earthquakes along the margins of the Pacific Ocean.
How Often do Tsunamis Occur?
Tsunami Hits the Coast
As a tsunami wave
approaches the coast (where the sea becomes shallow), the trough
(bottom) of a wave hits the beach floor, causing the wave to
slow down, to increase in height (the amplitude is magnified
many times) and to decrease in wavelength (the distance from
crest to crest). At landfall, a tsunami wave can be hundreds of
meters tall. Steeper shorelines produce higher tsunami waves.
In addition to large tsunami waves that crash onto shore,
the waves push a large amount of water onto the shore above the
regular sea level (this is called runup). The runup can cause
tremendous damage inland and is much more common than huge,
thundering tsunami waves.
Tsunamis are very rare. There are roughly six major tsunamis each
Tiny Model of a Tsunami:
You can make a tiny model of a tsunami by dropping a rock into a bowl of
water, causing ripples to propagate (travel) outwards from the site of
impact. Another way is to slightly jolt the bowl of water and watch it
slosh over the rim on one side.
Ring of Fire
About two-thirds of the earth is covered by the
waters of the four oceans. The Pacific Ocean is the
world's largest, covering more than one third of the
total surface area of our planet. The Pacific Ocean
is surrounded by a series of mountain chains, deep
ocean trenches and island arcs, sometimes called a
"ring of fire." The great size of the Pacific Ocean
and the large earthquakes associated with the "ring
of fire" combine to produce deadly tsunamis.
In less than a day, these tsunamis can travel from
one side of the Pacific to the other. However,
people living near areas where large earthquakes
occur may find that the tsunami waves will reach
their shores within minutes of the earthquake. For
these reasons, the tsunami threat to many areas
(Alaska, the Philippines, Japan or the U.S. West
Coast) can be immediate (for tsunamis from nearby
earthquakes taking only a few minutes to reach
coastal areas) or less urgent (for tsunamis from
distant earthquakes taking from 3 to 22 hours to
reach coastal areas).
Earth and Earthquakes
and sea floor that cover the earth's surface are part of a
world-wide system of plates that are in motion. These
motions are very slow, only an inch or two per year.
Earthquakes occur where the edges of plates run into one
another. Such edges are called fault lines or faults.
Sometimes the forces along faults can build-up over long
periods of time so that when the rocks finally break an
earthquake occurs. Examples of features produced by forces
released along plate edge faults are the Andes Mountains in
South America (on land) and the Aleutian Trench near Alaska
(under water). When powerful, rapid faulting occurs
underneath or near the ocean, a large earthquake is produced
and, possibly, a tsunami.
The deep ocean trenches off
the coasts of Alaska, the Kuril Islands, Russia,, and South
America are well known for their violent underwater
earthquakes and as the source area for destructive
The tsunami generating process
is more complicated than a sudden push against the column of
ocean water. The earthquake's magnitude and depth, water
depth in the region of tsunami generation, the amount of
vertical motion of the sea floor, the velocity of such
motion, whether there is coincident slumping of sediments
and the efficiency with which energy is transferred from the
earth's crust to ocean water are all part of the generation
Height and Water Depth
In the open
ocean a tsunami is less than a few feet high at the surface,
but its wave height increases rapidly in shallow water.
Tsunamis wave energy extends from the surface to the bottom
in the deepest waters.
As the tsunami attacks the coastline,
the wave energy is compressed into a much shorter distance
creating destructive, live-threatening waves.
deep ocean, destructive tsunamis can be small--often only a
few feet or less in height--and cannot be seen nor can they
be felt by ships. But, as the tsunami reaches shallower
coastal waters, wave height can increase rapidly. Sometimes,
coastal waters are drawn out into the ocean just before the
tsunami strikes. When this occurs, more shoreline may be
exposed than even at the lowest tide. This major withdrawal
of the sea should be taken as a warning of the tsunami waves
that will follow.
Pacific-Wide and Local Tsunamis
last large tsunami that caused widespread death and
destruction throughout the Pacific was generated by an
earthquake located off the coast of Chile in 1960. It caused
loss of life and property damage not only along the Chile
coast but in Hawaii and as far away as Japan. The Great
Alaskan Earthquake of 1964 produced deadly tsunami waves in
Alaska, Oregon and California.
In July 1993, a
tsunami generated in the Sea of Japan killed over 120
peoples in Japan. Damage also occurred in Korea and Russia
but not in other countries since the tsunami wave energy was
confined within the Sea of Japan. The 1993 Sea of Japan
tsunami is known as a "local event" since its impact was
confined to the nearby regional area in the proximity of the
earthquake that generated the tsunami. For people living
along the northwestern coast of Japan, the tsunami waves
followed the earthquake within a few minutes. Local tsunamis
also occurred in Nicaragua (1992), Indonesia (1992, 1994)
and the Philippines (1994) killing thousands of people.
Scientific studies indicate that local tsunamis generated
off the northern California, Oregon and Washington coast can
arrive within five to 30 minutes after the earthquake is
Wave animation showing the initial "drawback" of
If the first
part of a tsunami to reach land is a trough�called a
drawback�rather than a wave crest, the water along
the shoreline recedes dramatically, exposing
normally submerged areas.
|A drawback occurs because the water
propagates outwards with the trough of the wave at
its front. Drawback begins before the wave arrives
at an interval equal to half of the wave's period.
Drawback can exceed hundreds of metres, and people
unaware of the danger sometimes remain near the
shore to satisfy their curiosity or to collect fish
from the exposed seabed.
Where the ocean is over 20,000 feet deep, unnoticed
tsunami waves can travel at the speed of a commercial jet
plane, nearly 600 miles per hour. They can move from one
side of the Pacific Ocean to the other in less than a day.
This great speed makes it important to be aware of the
tsunami as soon as it is generated. Scientists can predict
when a tsunami will arrive since the speed of the waves
varies with the square root of the water depth. Tsunamis
travel much slower in shallower coastal waters where their
wave heights begin to increase dramatically.
Offshore and coastal
features can determine the size and impact of
tsunami waves. Reefs, bays, entrances to rivers,
undersea features and the slop of the beach all help
to modify the tsunami as it attacks the coastline.
When the tsunami reaches the coast and moves inland,
the water level can rise many feet. In extreme
cases, water level has risen to more than 50 feet
for tsunamis of distant origin and over 100 feet for
tsunami waves generated near the earthquake's
epicenter. The first wave may not be the largest in
the series of waves. One coastal community may see
no damaging wave activity while in another community
destructive waves can be large and violent. The
flooding can extend inland by 1000 feet or more,
covering large expanses of land with water and
When the wave enters
it slows down and its amplitude
Since scientists cannot predict when earthquakes will
occur, they cannot determine exactly when a tsunami will be
generated. However, by looking at past historical tsunamis,
scientists know where tsunamis are most likely to be
generated. Past tsunami height measurements are useful in
predicting future tsunami impact and flooding limits at
specific coastal locations and communities. Historical
tsunami research may prove helpful in analyzing the
frequency of occurrence of tsunamis and their relationship
to large earthquakes.
What You Should Do
Be aware of tsunami facts. This knowledge could save
your life! Share this knowledge with your relatives and
friends. It could save their lives!
If you are in school and you hear there is a tsunami
warning, you should follow the advice of teachers and other
If you are at home and hear there is a tsunami warning, you
should make sure you entire family is aware of the warning.
Your family should evacuate your house if you live in a
tsunami evacuation. Move in an orderly, calm and safe manner
to the evacuation site or to any safe place outside your
evacuation zone. Follow the advice of local emergency and
law enforcement authorities.
If you are at the beach or near the ocean and you feel the
earth shake, move immediately to higher ground. DO NOT wait
for a tsunami warning to be announced. Stay away from rivers
and streams that lead to the ocean as you would stay away
from the beach and ocean if there is a tsunami. A regional
tsunami from a local earthquake could strike some areas
before a tsunami warning could be announced.
Tsunamis generated in distant locations will generally give
people enough time to move to higher ground. For locally
generated tsunamis, where you might feel the ground shake,
you may only have a few minutes to move to higher ground.
High, multi-story, reinforced concrete hotels are located in
many low-lying coastal areas. The upper floors of these
hotels can provide a safe place to find refuge should there
be a tsunami warning and you cannot move quickly inland to
higher ground. Local Civil Defense procedures may, however,
not allow this type of evacuation in your area. Homes and
small buildings located in low lying coastal areas are not
designed to withstand tsunami impacts. Do not stay in these
structures should there be a tsunami warning.
Offshore reefs and shallow areas may help break the force of
tsunami waves, but large and dangerous waves can still be
threat to coastal residents in these areas. Staying away fro
all low-lying coastal areas is the safest advice when there
is a tsunami warning.
If You Are on a Boat or
Since tsunami wave activity is imperceptible in the open
ocean, do not return to port if you are at sea and a tsunami
warning has been issued for your area. Tsunamis can cause
rapid changes in water level and unpredictable dangerous
currents in harbors and ports.
If there is time to move your boat or ship from port to deep
water (after you know a tsunami warning has been issued),
you should weigh the following considerations:
Most large harbors and ports are under the control of a
harbor authority and/or a vessel traffic system. These
authorities direct operations during periods of increased
readiness (should a tsunami be expected), including the
forced movement of vessels if deemed necessary. Keep in
contact with the authorities should a forced movement of
vessels be directed.
Smaller ports may not be under the control of a harbor
authority. If you are aware there is a tsunami warning and
you have time to move your vessel to deep water, then you
may want to do so in an orderly manner, in consideration of
other vessels. Owners of small boats may find it safest to
leave their boat at the pier and physically move to higher
ground, particularly in the event of a locally generated
tsunami. Concurrent severe weather conditions (rough seas
outside of safe harbor) could present a greater hazardous
situation to small boats, so physically moving yourself to
higher ground may be the only option.
Damaging wave activity and unpredictable currents can effect
harbors for a period of time following the initial tsunami
impact on the coast. Contact the harbor authority before
returning to port making sure to verify that conditions in
the harbor are safe for navigation and berthing.
As dangerous as tsunamis are, they do not happen very often.
You should not let this natural hazard diminish your
enjoyment of the beach and ocean. But, if you think a
tsunami may be coming, the ground shakes under your feet or
you hear there is a warning, tell your relatives and
friends, and move quickly to higher ground.