Definition of Tsunami

Tsunami is the giant waves that are caused by earthquakes or volcanic eruptions under the sea. Tsunami’s are also called seismic sea wave or tidal wave. Out in the depths of the ocean or sea, Tsunami waves do not dramatically increase in higher heights as the depth of the ocean decreases. The speed of tsunami waves depends on ocean depths rather than the distance from the source of the wave.

sea tidal wave ocean
The aftermath of the 2004 Tsunami struck India killing off 230,000 people. It was generated by a 9.1 magnitude earthquake that struck in the Indian ocean

Origin of a Tsunami

After an earthquake or other generating impulse occurs. A train of simple, progressive oscillatory waves is propagated great distances over the ocean surface in ever-widening circles. Much like the waves produced by a pebble falling into a shallow pool. In deep water, a tsunami can travel as fast as 800 km (500 miles) per hour. The principal generation of a tsunami is the displacement of a substantial volume of water or perturbation of the sea. This displacement of water is usually attributed to either earthquake, landslides, volcanic eruptions, glacier calving, or more rarely by meteorites and nuclear tests. However, The possibility of a meteorite causing a tsunami is debated.

Seismic of tsunami

Tsunamis can be generated when the sea floor abruptly deforms and it vertically displaces the overlying water. Tectonic earthquakes are a particular kind of earthquake that is associated with the Earth’s crustal deformation. When these earthquakes occur beneath the sea, The water is above the deformed area is displaced from its equilibrium position.

Landslides caused my tsunami

In the 1950s, It was discovered that tsunamis larger than had previously been believed possible can be caused by giant submarine landslides. These rapidly displace large water volumes, As energy transfers to the water at a rate faster than the water can absorb. Their existence was confirmed in 1958, When a giant landslide in Ligula Bay, Alaska, caused the highest wave ever recorded, which had a height of 524 meters (1,719 ft.). The wave did not travel far, as it struck land almost immediately. The wave struck three boats– each with two people abroad-anchored in the bay. One boat rode out the wave, but the wave sank the other two, killing both people abroad one of them.

Meteorological

Some meteorological conditions especially rapid changes in barometric pressure as seen with the passing of a front can displace bodies of water enough to cause trains of waves with wavelengths comparable to seismic tsunamis, but usually with lower energies.

Man-made tsunamis

There has been the potential of the induction of at least one actual attempt to create tsunami waves as a tectonic weapon. In world war II, The New Zealand Military Forces initiated Project Seal, Which attempted to create small tsunamis with explosives in the area of today’s Shakespear Regional Park; The attempt failed. There has been speculation on the possibility of using nuclear weapons to cause tsunamis near an enemy coastline. Even during World War II Considerations of the idea of using conventional explosives were explored.

Characteristics of a tsunami

Tsunamis cause damage by two mechanisms: the smashing force of a wall of water traveling at high speed, and the destructive power of a large volume of water draining off the land and carrying a large amount of debris with it, even with waves that do not appear to be large.

Scale and Intensity and magnitude

As with tsunamis, Several attempts have been made to set up scales of tsunami intensity or magnitude to allow comparison between different events. To calculate the tsunami intensity “i” according to the formula:

\,{\mathit  {I}}={\frac  {1}{2}}+\log _{{2}}{\mathit  {H}}_{{av}}

where {\mathit  {H}}_{{av}} is the “tsunami height”. The formula yields:

  • I = 2 for {\mathit  {H}}_{{av}} = 2.8 meters
  • I = 3 for {\mathit  {H}}_{{av}} = 5.5 meters
  • I = 4 for {\mathit  {H}}_{{av}} = 11 meters
  • I = 5 for {\mathit  {H}}_{{av}} = 22.5 meters
  • etc.

Magnitude scales

The first scale that genuinely calculated a magnitude for a tsunami, Rather than an intensity at a particular location was the ML scale proposed by Murty & Loomis based on the potential energy.

tsunami magnitude scale Mt calculated from,

{\displaystyle \,{\mathit {M}}_{t}={a}\log h+{b}\log R+{\mathit {D}}}

Where h is the maximum tsunami wave amplitude (in m) measured by a tide gauge at a distance R from the epicenter, a, b and D are constants used to make the Mt scale match as closely as possible with the moment magnitude scale.

Warning and Predictions

sea ocean tidal wave
warnings chart for regional, local and distant

Drawbacks can serve as a brief warning. People who observe drawback can survive only if they immediately run for high ground and seek the upper floors of nearby buildings. A tsunami cannot be precisely predicted, even if the magnitude and location of an earthquake are known. Geologists, oceanographers, and seismologists analyze each earthquake and based on many factors may or may not issue a tsunami warning. Other warning and predictions are also done by animals

Mitigation for tsunamis

In some tsunami-prone countries, Earthquake engineering measures have been taken to reduce the damage caused by onshore. Japan where tsunami science and response measures first began following a disaster in 1896, has produced ever-more elaborate countermeasures and response plans. The country has built many tsunami walls up to 12 meters (39 ft.) high to protect populated coastal areas. Other localities have built floodgates of up to 15.5 meters (51 ft.) high and channels to redirect the water from an incoming tsunami.

Nikshan~


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