Tsunami

Introduction:

Tsunami is a Japanese word with the English translation, "harbor wave." Represented by two characters, the first character, "tsu," means harbor, while the second character, "nami," means "wave." It is a powerful series of waves caused by a disturbance under water. Waves travel through ocean and cause devastation when they reach land.

 Tsunamis are often referred as "tidal waves" by layman and as “seismic sea waves” by the scientific community. 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" is an earthquake-related, but a tsunami can also be caused by a non-seismic event, such as a landslide or meteorite impact.

As a result of their long wave lengths, tsunamis behave as shallow-water waves.  Shallow-water waves move at a great speed.

Let us see what this implies:

 In large water bodies like oceans, tsunamis not only propagate at high speeds, they can also travel great distances with limited energy losses.  The wave crests bend as the tsunami travels this is called refraction.

Tsunamis can be generated when the sea floor 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 sink to lower level, a tsunami can be created.  Earthquakes are particularly effective in generating 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, can also disturb the overlying water column. Similarly, a violent submarine volcanic eruption can create a great force that uplifts the water column and generates a tsunami. Conversely, supermarine landslides and cosmic-body like meteorite impacts disturb the water from above, as momentum from falling body is transferred to the water into which it falls. Tsunamis caused by some earthquakes in large water bodies like Pacific Ocean become weak 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 into giantwave. Tsunami travels at a speed that is related to the water depth hence, as the water depth decreases, the tsunami slows down. Consequently, as the tsunami's speed diminishes as it travels into shallower water, its height grows. Because of this effect, a tsunami, notnoticeable 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.

As a tsunami approaches shore, we've learned 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 becomes weak through bottom friction and turbulence. Despite these losses, tsunamis still reach the coast with tremendous amounts of energy.

 Tsunamis have great potential, stripping beaches of sand that may have taken years to accumulate and undermining trees and other coastal vegetation. It is Capable of flooding hundreds of meters inland past the typical high water level, the fast moving water associated with the tsunami can demolish homes and other coastal structures. Tsunamis may reach a maximum vertical height onshore above sea level, often called a run-up height, of 10, 20, and even 30 meters. These tsunamis cause severe damage to human lives and property.

-Prathibha Pandiri