Tsunami is a Japanese word that is made of two
characters: tsu and nami. The character tsu means
harbor, while the character nami means wave.
Therefore, the original word tsunami describes large
wave oscillations inside a harbor during a ‘tsunami’
event. In the past, tsunami is often referred to as
‘tidal wave’, which is a misnomer. Tides, featuring
the rising and falling of water level in the ocean in a
daily, monthly, and yearly cycle, are caused by
gravitational influences of the moon, sun, and planets.
Tsunamis are not generated by this kind of
gravitational forces and are unrelated to the tides,
although the tidal level does influence a tsunami
striking a coastal area.
The phenomenon we call a tsunami is a series of
water waves of extremely long wavelength and long
period, generated in an ocean by a geophysical disturbance
that displaces the water within a short
period of time. Waves are formed as the displaced
water mass, which acts under the influence of gravity,
attempts to regain its equilibrium. Tsunamis are
primarily associated with submarine earthquakes
in oceanic and coastal regions. However, landslides,
volcanic eruptions, and even impacts of objects from
outer space (such as meteorites, asteroids, and
comets) can also trigger tsunamis.
Tsunamis are usually characterized as shallowwater
waves or long waves, which are different from
wind-generated waves, the waves many of us have
observed on a beach. Wind waves of 5–20-s period
(T¼time interval between two successive wave
crests or troughs) have wavelengths (l¼T2(g/2p)
distance between two successive wave crests or
troughs) of c. 40–620 m. On the other hand, a tsunami
can have a wave period in the range of 10 min
to 1 h and a wavelength in excess of 200km in a deep
ocean basin. A wave is characterized as a shallowwater
wave when the water depth is less than 5% of
the wavelength. The forward and backward water
motion under the shallow-water wave is felted
throughout the entire water column. The shallow
water wave is also sensitive to the change of water
depth. For instance, the speed (celerity) of a shallowwater
wave is equal to the square root of the product
of the gravitational acceleration (9.81ms2) and the
water depth. Since the average water depth in the
Pacific Ocean is 5 km, a tsunami can travel at a speed
of about 800kmh1 (500 mi h1), which is almost
the same as the speed of a jet airplane. A tsunami can
move from the West Coast of South America to the
East Coast of Japan in less than 1 day.
The initial amplitude of a tsunami in the vicinity of
a source region is usually quite small, typically only a
meter or less, in comparison with the wavelength. In
general, as the tsunami propagates into the open
ocean, the amplitude of tsunami will decrease for the
wave energy is spread over a much larger area. In the
open ocean, it is very difficult to detect a tsunami
from aboard a ship because the water level will rise
only slightly over a period of 10 min to hours. Since
the rate at which a wave loses its energy is inversely
proportional to its wavelength, a tsunami will lose
little energy as it propagates. Hence in the open
ocean, a tsunami will travel at high speeds and over
great transoceanic distances with little energy loss.
As a tsunami propagates into shallower waters
near the coast, it undergoes a rapid transformation.
Because the energy loss remains insignificant, the
total energy flux of the tsunami, which is proportional
to the product of the square of the wave
amplitude and the speed of the tsunami, remains
constant. Therefore, the speed of the tsunami decreases
as it enters shallower water and the height of
the tsunami grows. Because of this ‘shoaling’ effect, a
tsunami that was imperceptible in the open ocean
may grow to be several meters or more in height.
When a tsunami finally reaches the shore, it may
appear as a rapid rising or falling water, a series of
breaking waves, or even a bore. Reefs, bays, entrances
to rivers, undersea features, including vegetations,
and the slope of the beach all play a role
modifying the tsunami as it approaches the shore.
Tsunamis rarely become great, towering breaking
waves. Sometimes the tsunami may break far offshore.
Or it may form into a bore, which is a steplike
wave with a steep breaking front, as the tsunami
moves into a shallow bay or river.
The water level on shore can rise by several
meters. In extreme cases, water level can rise to more
than 20m for tsunamis of distant origin and over
30m for tsunami close to the earthquake’s epicenter.
The first wave may not always be the largest in the
series of waves. In some cases, the water level will
fall significantly first, exposing the bottom of a bay
or a beach, and then a large positive wave follows.
The destructive pattern of a tsunami is also difficult
to predict. One coastal area may see no damaging
wave activity, while in a neighboring area destructive
waves can be large and violent. The flooding of an
area can extend inland by 500m or more, covering
large expanses of land with water and debris. Tsunamis
may reach a maximum vertical height onshore
above sea level, called a runup height, of 30 m.
Since scientists still cannot predict accurately when
earthquakes, landslides, or volcano eruptions will
occur, they cannot determine exactly when a tsunami
will be generated. But, with the aid of historical records
of tsunamis and numerical models, scientists
can get an idea as to where they are most likely to be
generated. Past tsunami height measurements and
computer modeling can also help to forecast future
tsunami impact and flooding limits at specific coastal
areas.
Historical and Recent Tsunamis
Tsunamis have been observed and recorded since
ancient times, especially in Japan and the Mediterranean
areas. The earliest recorded tsunami occurred
in 2000 BC off the coast of Syria. The oldest
reference of tsunami record can be traced back to the
sixteenth century in the United States.
During the last century, more than 100 tsunamis
have been observed in the United States alone. Among
them, the 1946 Alaskan tsunami, the 1960 Chilean
tsunami, and the 1964 Alaskan tsunami were the
three most destructive tsunamis in the US history. The
1946 Aleutian earthquake (Mw¼7.3) generated
catastrophic tsunamis that attacked the Hawaiian Islands
after traveling about 5 h and killed 159 people.
(The magnitude of an earthquake is defined by the
seismic moment, M0 (dyncm), which is determined
from the seismic data recorded worldwide. Converting
the seismic moment into a logarithmic scale,
we define Mw¼(1/1.5)log10M010.7.) The reported
property damage reached $26 million. The 1960
Chilean tsunami waves struck the Hawaiian Islands
after 14 h, traveling across the Pacific Ocean from the
Chilean coast. They caused devastating damage not
only along the Chilean coast (more than 1000 people
were killed and the total property damage from the
combined effects of the earthquake and tsunami was
estimated as $417 million) but also at Hilo, Hawaii,
where 61 deaths and $23.5 million in property
damage occurred. The 1964 Alaskan
tsunami triggered by the Prince William Sound
earthquake (Mw¼8.4), which was recorded as one of
the largest earthquakes in the North American continent,
caused the most destructive damage in Alaska’s
history. The tsunami killed 106 people and the total
damage amounted to $84 million in Alaska.
Within less than a year between September 1992
and July 1993, three large undersea earthquakes
strike the Pacific Ocean area, causing devastating
tsunamis. On 2 September 1992, an earthquake of
magnitude 7.0 occurred c. 100km off the Nicaraguan
coast. The maximum runup height was recorded
as 10m and 168 people died in this event. A
few months later, another strong earthquake
(Mw¼7.5) attacked the Flores Island and surrounding
area in Indonesia on 12 December 1992. It
was reported that more than 1000 people were killed
in the town of Maumere alone and two-thirds of the
population of Babi Island were swept away by the
tsunami. The maximum runup was estimated as
26 m. The final toll of this Flores earthquake stood
at 1712 deaths and more than 2000 injures. Exactly
7 months later, on 12 July 1993, the third strong
earthquake (Mw¼7.8) occurred near the Hokkaido
Island in Japan (Hokkaido Tsunami Survey Group
1993). Within 3–5 min, a large tsunami engulfed the
Okushiri coastline and the central west of Hokkaido,
impinging extensive property damages, especially on
the southern tip of Okushiri Island in the town of
Aonae. The runup heights on the Okushiri Island
were thoroughly surveyed and they varied between
15 and 30m over a 20-km stretch of the southern
part of the island, with several 10-m spots on the
northern part of the island. It was also reported that
although the runup heights on the west coast of
Hokkaido are not large (less than 10 m), damage was
extensive in several towns. The epicenters of these
three earthquakes were all located near residential
coastal areas. Therefore, the damage caused by
subsequent tsunamis was unusually large.
On 17 July 1998, an earthquake occurred in the
Sandaun Province of northwestern Papua New
Guinea, about 65 km northwest of the port city of
Aitape. The earthquake magnitude was estimated as
Mw¼7.0. About 20min after the first shock, Warapo
and Arop villages were completely destroyed by tsunamis.
The death toll was at over 2000 and many of
them drowned in the Sissano Lagoon behind the Arop
villages. The surveyed maximum runup height was
15 m, which is much higher than the predicted value
based on the seismic information. It has been suggested
that the Papua New Guinea tsunami could be
caused by a submarine landslide.
The most devastating tsunamis in recent history
occurred in the Indian Ocean on 26 December 2004.
An earthquake of Mw¼9.0 occurred off the west
coast of northern Sumatra. Large tsunamis were
generated, severely damaging coastal communities in
countries around the Indian Ocean, including Indonesia,
Thailand, Sri Lanka, and India. The estimated
tsunami death toll ranged from 156 000 to 178 000
across 11 nations, with additional 26 500–142 000
missing, most of them presumed dead.
