Tsunami and its properties and propagation of waves for UPSC

Tsunami is a Japanese word for “Harbour wave”. A tsunami is a series of very long-wavelength waves in large water bodies like seas or large lakes caused by a major disturbance above or below the water surface or due to the displacement of a large volume of water.

They are sometimes referred to as tidal waves because of long wavelengths, although the attractions of the Moon and Sun play no role in their formation.

Earthquakes (e.g. 2004 Indian Ocean Tsunami), volcanic eruptions (e.g. tsunami caused by the violent eruption of Krakatoa in 1883), landslides (tsunami caused by the collapse of a section of Anak Krakatoa in 2018), underwater explosions, meteorite impacts, etc. have the potential to generate a tsunami.

Subduction zones off Chile, Nicaragua, Mexico and Indonesia have created killer tsunamis.

The Pacific among the oceans has witnessed the greatest number of tsunamis (over 790 since 1990).

# Mechanism of Tsunami Waves

Megathrust earthquakes cause a sudden displacement in a seabed sufficient to cause the sudden raising of a large body of water.

As the subducting plate plunges beneath the less dense plate, stresses build-up, the locked zone between the plates give way abruptly, and the parts of the oceanic crust is then upthrust resulting in the displacement of a large column of water vertically.

The tsunami on December 26, 2004, was caused after an earthquake displaced the seabed off the coast of Sumatra, Indonesia.
A marine volcanic eruption can generate an impulsive force that displaces the water column and gives birth to a tsunami.

During a submarine landslide, the equilibrium sea-level is altered by sediment moving along the floor of the sea. Gravitational forces then propagate a tsunami.

Most destructive tsunamis can be caused due to the fall of extra-terrestrial objects on to the earth.

# Properties of Tsunami Waves

Wave crest and trough: The highest and lowest points of a wave are called the crest and trough respectively.

Wave height: It is the vertical distance from the bottom of a trough to the top of a crest of a wave.

Wave amplitude: It is one-half of the wave height.

Wave period: It is the time interval between two successive wave crests or troughs.

Wavelength: It is the horizontal distance between two successive crests.

Wave frequency: It is the number of waves passing a given point during a one second time interval.

# Propagation of the waves

Gravity acts to return the sea surface to its original shape.

The ripples then race outward, and a tsunami is caused.

As a tsunami leaves deep waters and propagates into the shallow waters, it transforms. This is because as the depth of the water decreases, the speed of the tsunami reduces. But the change of total energy of the tsunami remains constant.

With the decrease in speed, the height of the tsunami wave grows. A tsunami which was imperceptible in deep water may grow to many metres high, and this is called the ‘shoaling’ effect.

Sometimes, the sea seems to at first draw a breath, but then this withdrawal is followed by the arrival of the crest of a tsunami wave. Tsunamis have been known to occur suddenly without warning.

In some cases, there are several great waves separated by intervals of several minutes or more.

The first of these waves is often preceded by an extraordinary recession of water from the shore, which may commence several minutes or even half an hour beforehand.