|Robert.Nicolson at Sub...
|Posted: Mon Dec 27, 2004 4:40 pm Post subject: The physics of Tsunami's
|I thought some of you might be interested to read this :-
The Physics of Tsunamis
Published on December 27, 2004
The Island-Asia News Network
A tsunami (pronounced tsoo-nah-mee )is a wave train, or series of waves,
generated in a
body of water by an impulsive disturbance that vertically displaces the
Earthquakes, landslides, volcanic eruptions, explosions, and even the
impact of cosmic
bodies, such as meteorites, can generate tsunamis. Tsunamis can savagely
coastlines, causing devastating property damage and loss of life.
What does "tsunami" mean?
Tsunami is a Japanese word with the English translation, "harbor wave."
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
public, and as "seismic sea waves" by the scientific community.
The term "tidal wave" is a misnomer; although a tsunami's impact upon a
dependent upon the tidal level at the time a tsunami strikes, tsunamis
are unrelated to
Tides result from the imbalanced, extraterrestrial, gravitational
influences of the
moon, sun, and planets. The term "seismic sea wave" is also misleading.
implies an earthquake-related generation mechanism, but a tsunami can
also be caused by
a nonseismic event, such as a landslide or meteorite impact.
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
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
transoceanic distances with limited energy losses
How do earthquakes generate tsunamis?
Tsunamis can be generated when the sea floor abruptly deforms and
the overlying water. Tectonic earthquakes are a particular kind of
earthquake that are
associated with the earth's crustal deformation; when these earthquakes
the sea, the water above the deformed area is displaced from its
Waves are formed as the displaced water mass, which acts under the
gravity, attempts to regain its equilibrium.
When large areas of the sea floor elevate or subside, a tsunami can be
Large vertical movements of the earth's crust can occur at plate
interact along these boundaries called faults. Around the margins of the
for example, denser oceanic plates slip under continental plates in a
process known as
subduction. Subduction earthquakes are particularly effective in
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,
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
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
above, as momentum from falling debris is transferred to the water into
debris falls. Generally speaking, tsunamis generated from these
mechanisms, unlike the
Pacific-wide tsunamis caused by some earthquakes, dissipate quickly and
coastlines distant from the source area.
What happens to a tsunami as it approaches land?
As a tsunami leaves the deep water of the open ocean and travels into
water near the coast, it transforms. If you read the "How do tsunamis
differ from other
water waves?" section, you discovered that a tsunami travels at a speed
that is related
to the water depth - hence, as the water depth decreases, the tsunami
The tsunami's energy flux, which is dependent on both its wave speed and
remains nearly constant. Consequently, as the tsunami's speed diminishes
as it travels
into shallower water, its height grows.
Because of this shoaling effect, a tsunami, imperceptible at sea, may
grow to be
several meters or more in height near the coast. When it finally reaches
the coast, a
tsunami may appear as a rapidly rising or falling tide, a series of
breaking waves, or
even a bore.
What happens when a tsunami encounters land?
As a tsunami approaches shore, we've learned in the "What happens to a
tsunami as it
approaches land?" section that it begins to slow and grow in height.
Just like other water waves, tsunamis begin to lose energy as they rush
onshore - part
of the wave energy is reflected offshore, while the
shoreward-propagating wave energy
is dissipated through bottom friction and turbulence.
Despite these losses, tsunamis still reach the coast with tremendous
amounts of energy.
Tsunamis have great erosional potential, stripping beaches of sand that
may have taken
years to accumulate and undermining trees and other coastal vegetation.
Capable of inundating, or flooding, hundreds of meters inland past the
water level, the fast-moving water associated with the inundating
tsunami can crush
homes and other coastal structures. Tsunamis may reach a maximum
onshore above sea level, often called a runup height, of 10, 20, and
even 30 meters.
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