adapted to HTML from the course notes of Prof. Stephen A. Nelson Tulane
What is a Tsunami?
What does "tsunami" mean?
Tsunami is a Japanese word with the English translation, "harbor wave."
Represented by two characters, the top character, "tsu," means harbor,
while the bottom character, "nami," means "wave." In the past, tsunamis
were sometimes referred to as "tidal waves" by the general public, and as
"seismic sea waves" by the scientific community.
The term "tidal wave" is a misnomer; although a tsunami's impact upon a
coastline is dependent upon the tidal level at the time a tsunami strikes,
tsunamis are unrelated to the tides. Tides result from the imbalanced,
extraterrestrial, gravitational influences of the moon, sun, and planets.
The term "seismic sea wave" is also misleading. "Seismic" implies an
earthquake-related generation mechanism, but a tsunami can also be caused
by a nonseismic event, such as a landslide or meteorite impact.
A tsunami is a very long-wavelength wave of water that is generated by
sudden displacement of the seafloor or disruption of any body of standing
Tsunamis are sometimes called "seismic sea waves", although, as we will
see, they can be generated by other mechanisms than earthquakes.
Tsunamis have also been called "tidal waves", but this term should not be
used because they are not in any way related to the tides of the
Earth. Because tsunamis occur suddenly, often without warning, they
are extremely dangerous to coastal communities.
Physical Characteristics of Tsunamis
All types of waves, including tsunamis, have a wavelength, a wave height,
an amplitude, a frequency or period, and a velocity.
Wavelength is defined as the distance between two identical points on a
wave (i.e. between wave crests or wave troughs).
Normal ocean waves have wavelengths of about 100 meters.
Tsunamis have much longer wavelengths, usually measured in kilometers and
up to 200 kilometers
Wave height refers to the distance between the trough of the wave
and the crest or peak of the wave.
Wave amplitude- refers to the height of the wave above the still
water line, usually this is equal to 1/2 the wave height.
Tsunamis can have variable wave height and amplitude that depends on
water depth as we shall see in a moment
Wave frequency or period - is the amount of time it takes for one
full wavelength to pass a stationary point.
Wave velocity is the speed of the wave. Velocities of normal ocean
waves are about 90 km/hr while tsunamis have velocities up to 950
km/hr (about as fast as jet airplanes), and thus move much more
rapidly across ocean basins. The velocity of any wave is equal to the
wavelength divided by the wave period.
V = l/P
Tsunamis are characterized as shallow-water waves. These are different
from the waves most of us have observed on a the beach, which are caused
by the wind blowing across the ocean's surface. Wind-generated waves
usually have period (time between two successive waves) of five to twenty
seconds and a wavelength of 100 to 200 meters.
A tsunami can have a period in the range of ten minutes to two hours
and wavelengths greater than 500 km.
A wave is characterized as a shallow-water wave when the ratio of the
water depth and wavelength is very small. The velocity of a shallow-water
wave is also equal to the square root of the product of the acceleration
(980cm/sec/sec) and the depth of the water,
V=Ög * d
The rate at which a wave loses its energy is inversely related to its
wavelength. Since a tsunami has a very large wavelength, it will lose
little energy as it propagates. Thus, in very deep water, a tsunami will
travel at high speeds with little loss of energy. For example, when the
ocean is 6100 m deep, a tsunami will travel about 890 km/hr, and thus can
travel across the Pacific Ocean in less than one day.
As a tsunami leaves the deep water of the open sea and arrives at the
shallow waters near the coast, it undergoes a transformation. Since the
velocity of the tsunami is also related to the water depth, as the depth
of the water decreases, the velocity of the tsunami decreases. The change
of total energy of the tsunami, however, remains constant
Furthermore, the period of the wave remains the same, and thus more water
is forced between the wave crests causing the height of the wave to
Because of this "shoaling" effect, a tsunami that was imperceptible in
deep water may grow to have wave heights of several meters or more.
If the trough of the tsunami wave reaches the coast first, this causes a
phenomenon called drawdown, where it appears that sea level has dropped
Drawdown is followed immediately by the crest of the wave which can catch
people observing the drawdown off guard. When the crest of the wave hits,
sea level rises (called run-up ).
Run-up is usually expressed in meters above normal high tide.
Run-ups from the same tsunami can be variable because of the influence of
the shapes of coastlines. One coastal area may see no damaging wave
activity while in another area destructive waves can be large and violent.
The flooding of an area can extend inland by 300 m or more, covering large
areas of land with water and debris. Flooding tsunami waves tend to carry
loose objects and people out to sea when they retreat.
Tsunamis may reach a maximum vertical height onshore above sea level,
called a run-up height, of 30 meters. A notable exception is the landslide
generated tsunami in Lituya Bay, Alaska in 1958 which produced a 60 meter
Because the wavelengths and velocities of tsunamis are so large, the
period of such waves is also large, and larger than normal ocean
waves. Thus it may take several hours for successive crests to reach
the shore. (For a tsunami with a wavelength of 200 km traveling at
750 km/hr, the wave period is about 16 minutes).
Thus people are not safe after the passage of the first large wave, but
must wait several hours for all waves to pass. The first wave may not be
the largest in the series of waves. For example, in several different
recent tsunamis the first, third, and fifth waves were the largest.
How do earthquakes generate tsunamis?
Tsunamis can be generated when the sea floor abruptly deforms and
vertically displaces the overlying water. Tectonic earthquakes are a
particular kind of earthquake that are associated with the earth's crustal
deformation; when these earthquakes occur beneath the sea, the water above
the deformed area is displaced from its equilibrium position. Waves are
formed as the displaced water mass, which acts under the influence of
gravity, attempts to regain its equilibrium. When large areas of the sea
floor elevate or subside, a tsunami can be created.
Large vertical movements of the earth's crust can occur at plate
boundaries. Plates interact along these boundaries called faults. Around
the margins of the Pacific Ocean, for example, denser oceanic plates slip
under continental plates in a process known as subduction. Subduction
earthquakes are particularly effective in generating tsunamis.
4 Stages in Tsunami Formation
1--Initiation: Earthquakes are commonly associated with ground
shaking that is a result of elastic waves traveling through the solid
earth. However, near the source of submarine earthquakes, the seafloor
is "permanently" uplifted and down-dropped, pushing the entire water
column up and down. The potential energy that results from pushing
water above mean sea level is then transferred to horizontal
propagation of the tsunami wave (kinetic energy). For the case shown
above, the earthquake rupture occurred at the base of the continental
slope in relatively deep water. Situations can also arise where the
earthquake rupture occurs beneath the continental shelf in much
shallower water. Note: In the figure the waves are greatly exaggerated
compared to water depth! In the open ocean, the waves are at most,
several meters high spread over many tens to hundreds of kilometers in
2--Split: Within several minutes of the earthquake, the initial
tsunami (Panel 1) is split into a tsunami that travels out to the deep
ocean (distant tsunami) and another tsunami that travels towards the
nearby coast (local tsunami). The height above mean sea level of the
two oppositely traveling tsunamis is approximately half that of the
original tsunami (Panel 1). (This is somewhat modified in three
dimensions, but the same idea holds.) The speed at which both tsunamis
travel varies as the square root of the water depth. Therefore the
deep-ocean tsunami travels faster than the local tsunami near shore.
3--Amplification: Several things happen as the local tsunami
travels over the continental slope. Most obvious is that the amplitude
increases. In addition, the wavelength decreases. This results in
steepening of the leading wave--an important control of wave runup at
the coast (next panel). Note also that the deep ocean tsunami has
traveled much farther than the local tsunami because of the higher
propagation speed. As the deep ocean tsunami approaches a distant
shore, amplification and shortening of the wave will occur, just as
with the local tsunami shown above.
4--Runup: As the tsunami wave travels from the deep-water,
continental slope region to the near-shore region, tsunami runup
occurs. Runup is a measurement of the height of the water onshore
observed above a reference sea level. Contrary to many artistic images
of tsunamis, most tsunamis do not result in giant breaking waves (like
normal surf waves at the beach that curl over as they approach shore).
Rather, they come in much like very strong and very fast tides (i.e.,
a rapid, local rise in sea level). Much of the damage inflicted by
tsunamis is caused by strong currents and floating debris. The small
number of tsunamis that do break often form vertical walls of
turbulent water called bores. Tsunamis will often travel much farther
inland than normal waves. Do tsunamis stop once on land? After runup,
part of the tsunami energy is reflected back to the open ocean. In
addition, a tsunami can generate a particular type of wave called edge
waves that travel back-and forth, parallel to shore. These effects
result in many arrivals of the tsunami at a particular point on the
coast rather than a single wave suggested by Panel 3. Because of the
complicated behavior of tsunami waves near the coast, the first runup
of a tsunami is often not the largest, emphasizing the importance of
not returning to a beach several hours after a tsunami hits.
How do tsunamis differ from other water waves?
Tsunamis are unlike wind-generated waves, which many of us may have
observed on a local lake or at a coastal beach, in that they are
characterized as shallow-water waves, with long periods and wave lengths.
The wind-generated swell one sees at a California beach, for example,
spawned by a storm out in the Pacific and rhythmically rolling in, one
wave after another, might have a period of about 10 seconds and a wave
length of 150 m. A tsunami, on the other hand, can have a wavelength in
excess of 100 km and period on the order of one hour.
As a result of their long wave lengths, tsunamis behave as shallow-water
waves. A wave becomes a shallow-water wave when the ratio between the
water depth and its wave length gets very small. Shallow-water waves move
at a speed that is equal to the square root of the product of the
acceleration of gravity (9.8 m/s/s) and the water depth - let's see what
this implies: In the Pacific Ocean, where the typical water depth is about
4000 m, a tsunami travels at about 200 m/s, or over 700 km/hr. Because the
rate at which a wave loses its energy is inversely related to its wave
length, tsunamis not only propagate at high speeds, they can also travel
great, transoceanic distances with limited energy losses.
How do landslides, volcanic eruptions, and cosmic collisions generate
A tsunami can be generated by any disturbance that displaces a large water
mass from its equilibrium position. In the case of earthquake-generated
tsunamis, the water column is disturbed by the uplift or subsidence of the
sea floor. Submarine landslides, which often accompany large earthquakes,
as well as collapses of volcanic edifices, can also disturb the overlying
water column as sediment and rock slump downslope and are redistributed
across the sea floor. Similarly, a violent submarine volcanic eruption can
create an impulsive force that uplifts the water column and generates a
tsunami. Conversely, supermarine landslides and cosmic-body impacts
disturb the water from above, as momentum from falling debris is
transferred to the water into which the debris falls. Generally speaking,
tsunamis generated from these mechanisms, unlike the Pacific-wide tsunamis
caused by some earthquakes, dissipate quickly and rarely affect coastlines
distant from the source area.
This image shows Lituya Bay, Alaska, after a huge, landslide-generated
tsunami occurred on July 9, 1958. The earthquake-induced rockslide, shown
in upper right-hand corner of this image, generated a 525 m splash-up
immediately across the bay, and razed trees along the bay and across
LaChausse Spit before leaving the bay and dissipating in the open waters
of the Gulf of Alaska
How Tsunamis are Generated
There is an average of two destructive tsunamis per year in the Pacific
basin. Pacific wide tsunamis are a rare phenomenon, occurring every 10 -
12 years on the average.
Most of these tsunamis are generated by earthquakes that cause
displacement of the seafloor, but, as we shall see, tsunamis can be
generated by volcanic eruptions, landslides, underwater explosions, and
Earthquakes cause tsunamis by causing a disturbance of the seafloor. Thus,
earthquakes that occur along coastlines or anywhere beneath the oceans can
The size of the tsunami is usually related to the size of the earthquake,
with larger tsunamis generated by larger earthquakes. But the sense
of displacement is also important. Tsunamis are generally only
formed when an earthquake causes vertical displacement of the seafloor.
The 1906 earthquake near San Francisco California had a Richter Magnitude
of about 7.1, yet no tsunami was generated because the motion on the fault
was strike-slip motion with no vertical displacement.
Thus tsunamis only occur if the fault generating the earthquake has normal
or reverse displacement.
Because of this, most tsunamis are generated by earthquakes that occur
along the subduction boundaries of plates, along the oceanic trenches.
Since the Pacific Ocean is surrounded by plate boundaries of this type,
tsunamis are frequently generated by earthquakes around the margins of the
Examples of Tsunamis generated by Earthquakes
April 1, 1946 - A magnitude 7.3 earthquake occurred near Unimak Island
in the Aleutian Islands west of Alaska, near the Alaska Trench.
Sediment accumulating in the trench slumped into the trench and
generated a tsunami. A lighthouse at Scotch Gap built of steel
reinforced concrete was located on shore at an elevation of 14 m above
mean low water.
The first wave of the tsunami hit the Scotch Gap area 20 minutes after
the earthquake, had a run-up 30 m and completely destroyed the
4.5 hours later the same tsunami reached the Hawaiian Islands after
traveling at an average speed of 659 km/hr.
As it approached the city of Hilo on the Big Island, it slowed to about
47 km/hr (note that even the fastest human cannot run faster than about
35 km/hr) and had a run-up of 18 m above normal high tide. It killed 159
people (90 in Hilo) and caused $25 million in property damage. Hilo Tsunami Simulation(.mpg)
May 22, 1960 - A magnitude 8.6 earthquake occurred along the subduction
zone off South America.
Because the population of Chile (movie clip) is familiar with
earthquakes and potential tsunamis, most people along the coast moved to
15 minutes after the earthquake, a tsunami with a run-up of 4.5 m hit
The first wave then retreated, dragging broken houses and boats back
into the ocean.
Many people saw this smooth retreat of the sea as a sign they could ride
their boats out to sea and recover some of the property swept away by
the first wave.
But, about 1 hour later, the second wave traveling at a velocity of 166
km/hr crashed in with a run-up of 8 m.
This wave crushed boats along the coast and destroyed coastal buildings.
This was followed by a third wave traveling at only 83 km/hr that
crashed in later with a run-up of 11 m, destroying all that was left of
The resulting causalities listed 909 dead with 834 missing.
In Hawaii, a tsunami warning system was in place and the tsunami was
expected to arrive at 9:57 AM.
It hit at 9:58 AM and 61 people died, mostly sightseers that wanted to
watch the wave roll in at close range (obviously they were too close).
The tsunami continued across the Pacific Ocean, eventually reaching
Japan where it killed an additional 185 people. Chile Tsunami Simulation(.mpg)
March 27, 1964 - The Good Friday Earthquake in Alaska had a magnitude of
8.5 on the Richter Scale.
This earthquake also occurred along the subduction zone, and as we saw
in our study of earthquakes, caused deformation of the crust where huge
blocks where dropped down as much as 2.3
Because the coastline of Alaska is sparsely populated, only 122 people
died from the tsunami in Alaska.
With a tsunami warning system in place in Crescent City, California, all
the townspeople moved to higher ground.
After watching four successive waves destroy their town, many people
returned to the low lying areas to assess the damage to their property.
The fifth wave had the largest run-up of 6.3 m and killed 12 people.
September 2, 1992 - A magnitude 7 earthquake off the coast of Nicaragua
in Central America occurred along the subduction zone below the Middle
America Trench. The earthquake was barely felt by the residents of
Nicaragua and was somewhat unusual.
A 100 km-long segment of the oceanic lithosphere moved 1 m further below
the over riding plate over a period of two minutes.
Much energy was released but the ground did not shake very much.
Seawater apparently absorbed some of the energy and sent a tsunami onto
Residents had little warning, 150 people died and 13,000 people were
Volcanoes that occur along coastal zones, like in Japan and island arcs
throughout the world, can cause several effects that might generate a
Explosive eruptions can rapidly emplace pyroclastic flows into the water,
landslides and debris avalanches produced by eruptions can rapidly move
into water, and
collapse of volcanoes to form calderas can suddenly displace the water.
The eruption of Krakatau in the Straights of Sunda, between Java and
Sumatra, in 1883 generated at least three tsunamis that killed 36,417
It is still uncertain exactly what caused the tsunamis, but it is known
that several events that occurred during the eruption could have caused
A large Plinian eruption column blasted pumice and ash up to 40 km into
This Plinian eruption column likely collapsed several times to produce
pyroclastic flows, any of which could have generated a tsunami. A loud
explosive blast was heard as far away as Australia.
This blast was likely caused by a phreatic explosion that occurred as a
result of seawater coming in contact with the magma.
The explosion could have generated at least one of the tsunamis.
At some point during the eruption a caldera formed by collapse of the
Areas that were once more than 300 m above sea level were found 300 m
below sea level after the eruption.
The sudden collapse of the volcano to form this caldera could have
caused one or more tsunamis.
Earthquakes were felt throughout the eruption.
Any one of these submarine earthquakes could have caused a tsunami.
One of the tsunamis had a run-up of about 40 m above normal sea level.
A large block of coral weighing about 600 tons was ripped off the
seafloor and deposited 100 m inland.
One ship was carried 2.5 km inland and was left 24 meters above sea level,
with all of its crew swept into the ocean. We have previously discussed
the likelihood that tsunamis accompanied the eruptions of Mount Pelée,
Martinique in 1902 and Vesuvius in 79 A.D.
Landslides moving into oceans, bays, or lakes can also generate tsunamis.
Most such landslides are generated by earthquakes or volcanic eruptions.
As previously mentioned, a large landslide or debris avalanche fell into
Lituya Bay, Alaska in 1958 causing a wave with a run-up of about 60 m as
measured by a zone completely stripped of vegetation.
Nuclear testing by the United States in the Marshall Islands in the 1940s
and 1950s generated tsunamis.
While no historic examples of meteorite impacts are known to have produced
a tsunami, the apparent impact of a meteorite at the end of the Cretaceous
Period, about 65 million years ago near the tip of what is now the Yucatan
Peninsula of Mexico, produced tsunamis that left deposits all along the
Gulf coast of Mexico and the United States.
Mitigation of Risks and Hazards
The main damage from tsunamis comes from the destructive nature of the
waves themselves. Secondary effects include the debris acting as
projectiles which then run into other objects, erosion that can undermine
the foundations of structures built along coastlines, and fires that
result from disruption of gas and electrical lines.
Tertiary effects include loss of crops and water and electrical systems
which can lead to famine and disease.
Within the last century there have been 94 destructive tsunamis which have
resulted in 51,000 deaths.
Despite the fact that tsunami warning systems have been in place since
1950, deaths still result from tsunamis, especially when the source of the
earthquake is so close to a coast that there is little time for a warning,
or when people do not heed the warning or follow instructions associated
with the warning.
Prediction and Early Warning
For areas located at great distances from earthquakes that could
potentially generate a tsunami there is usually plenty of time for
warnings to be sent and coastal areas evacuated, even though tsunamis
travel at high velocities across the oceans.
Hawaii is good example of an area located far from most of the sources of
tsunamis, where early warning is possible and has saved lives.
For earthquakes occurring anywhere on the subduction margins of the
Pacific Ocean there is a minimum of 4 hours of warning before a tsunami
would strike any of the Hawaiian Islands.
The National Oceanic and Atmospheric Administration (NOAA)
has set up a Pacific warning system for areas in the Pacific Ocean, called
Pacific Tsunami Warning Center.
It consists of an international network of seismographic stations, and
tidal stations around the Pacific basin that can all send information via
satellite to the Center located in Hawaii.
When an earthquake occurs somewhere in the region, the Center immediately
begins to analyze the data looking for signs that the earthquake could
have generated a tsunami.
The tidal stations are also monitored, and if a tsunami is detected, a
warning is sent out to all areas on the Pacific coast.
It takes at least 1 hour to assimilate all of the information and issue a
Thus for an average velocity of a tsunami of 750 km/hr, the regional
system can provide a warning sufficient for adequate evacuation of coastal
areas within 750 km of the earthquake.
In order to be able to issue warnings about tsunamis generated within 100
to 750 km of an earthquake, several regional warning centers have been set
up in areas prone to tsunami generating earthquakes.
These include centers in Japan, Kamchatka, Alaska, Hawaii, French
Polynesia, and Chile.
These systems have been very successful at saving lives.
For example, before the Japanese warning system was established, 14
tsunamis killed over 6000 people in Japan.
Since the establishment of the warning system 20 tsunamis have killed 215
people in Japan.
Like all warning systems, the effectiveness of tsunami early warning
depends strongly on local authority's ability to determine that their is a
danger, their ability to disseminate the information to those potentially
affected, and on the education of the public to heed the warnings and
remove themselves from the area.
Tsunami Safety Rules
In case you are ever in an area where there is a threat of tsunami, I have
downloaded the following tsunami safety rules from the
West Coast & Alaska Tsunami Warning Center Home Page: http://www.alaska.net/~atwc/
A strong earthquake felt in a low-lying coastal area is a natural
warning of possible, immediate danger. Keep calm and quickly move to
higher ground away from the coast. All large earthquakes do not cause
tsunamis, but many do. If the quake is located near or directly under
the ocean, the probability of a tsunami increases. When you hear that
an earthquake has occurred in the ocean or coastline regions, prepare
for a tsunami emergency.
Tsunamis can occur at any time, day or night. They can travel up
rivers and streams that lead to the ocean.
A tsunami is not a single wave, but a series of waves. Stay out of
danger until an "ALL CLEAR" is issued by a competent authority.
Approaching tsunamis are sometimes heralded by noticeable rise or fall
of coastal waters. This is nature's tsunami warning and should be
A small tsunami at one beach can be a giant a few miles away. Do
not let modest size of one make you lose respect for all.
Sooner or later, tsunamis visit every coastline in the Pacific. All
tsunamis - like hurricanes - are potentially dangerous even though
they may not damage every coastline they strike.
Never go down to the beach to watch for a tsunami! WHEN YOU CAN SEE
THE WAVE YOU ARE TOO CLOSE TO ESCAPE. Tsunamis can move faster than a
person can run!
During a tsunami emergency, your local emergency management office,
police, fire and other emergency organizations will try to save your
life. Give them your fullest cooperation.
Homes and other buildings located in low lying coastal areas are not
safe. Do NOT stay in such buildings if there is a tsunami warning.
The upper floors of high, multi-story, reinforced concrete hotels can
provide refuge if there is no time to quickly move inland or to higher
If you are on a boat or ship and there is time, move your vessel to
deeper water (at least 100 fathoms). If it is the case that there is
concurrent severe weather, it may safer to leave the boat at the pier
and physically move to higher ground.
Damaging wave activity and unpredictable currents can effect harbor
conditions for a period of time after the tsunami's initial impact. Be
sure conditions are safe before you return your boat or ship to the
Stay tuned to your local radio, marine radio, NOAA Weather Radio, or
television stations during a tsunami emergency - bulletins issued
through your local emergency management office and NationalWeather
Service offices can save your life