Tsunami

[Utkars A <utkarsa at gmail dot com>]

What is a tsunami?

A tsunami (pronounced su-nah-me) is a wave train, or series of waves,
generated in a body of water by an impulsive disturbance that
vertically displaces the water column. Earthquakes, landslides,
volcanic eruptions, explosions, and even the impact of cosmic bodies,
such as meteorites, can generate tsunamis. Tsunamis can savagely
attack coastlines, causing devastating property damage and loss of
life.

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 non-seismic 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
characterised 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. The earthquake-generated 1960
Chilean tsunami, for instance, travelled across over 17,000 km across
the Pacific to hit Japan. The wave crests bend as the tsunami travels
-- this is called refraction. Wave refraction is caused by segments of
the wave moving at different speeds as the water depth along the crest
varies.

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.

How do landslides, volcanic eruptions, and cosmic collisions generate tsunamis?

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, tsuna-mis
generated from these mechanisms, unlike the Pacific-wide tsunamis
caused by some earthquakes, dissipate quickly and rarely affect
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
the shallower 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 slows.

The tsunami's energy flux, which is dependent on both its wave speed
and wave height, 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 typical high-water level, the fast-moving water
associated with the inundating tsunami can crush homes and other
coastal structures. Tsuna-mis may reach a maximum vertical height
onshore above sea level, often called a run-up height, of 10, 20, and
even 30 meters.