Destructive power of tsunamis and its reasons

 Tsunami is a seismic sea wave or tidal wave, catastrophic ocean wave, usually caused by a submarine earthquake, an underwater or coastal landslide, or a volcanic eruption. The term tidal waveis frequently used for such a wave, but it is a misnomer, for the wave has no connection with the tides.
Sea-level rise can significantly increase the tsunami hazard, which means that smaller tsunamis in the future can have the same adverse impacts as big tsunamis would today,

Topography of the seafloor near the shore affects the power of Tsunamis:-

• Tsunami waves become dangerous only when they get close to the coast: the height of a Tsunami wave grows larger as the water becomes more and more shallow in a wave shoaling process
• An increase in wave amplitude results in “shoaling” when waves, including tsunamis, run from deep to shallow water. This is significant in coastal regions. This phenomena occurs because of the force from the seabed as it becomes shallower. This slows down the wave: the shallower the water, the slower the wave.
• Even when tsunamis have only a small amplitude (less than a meter) they can shoal up to many meters high as they hit shallow water. When a tsunami hits shallower coastal waters, the trough or base of the wave contacts the beach floor. As a consequence, the leading edge of the tsunami slows dramatically due to the shallower water, but the trailing part of the wave is still moving rapidly in the deeper water.
• The wave is compressed and its velocity slows below 80 kilometers per hour. Its wavelength diminishes to less than 20 kilometers and its amplitude is magnified many times. This piling up of tsunami energy results in growth of the wave height.
• The form of the adjacent geography to deep water (open bays and coastline), can shape the tsunami into a step-like wave with a steep braking face. The wave height as it crashes upon a shore depends almost entirely upon the submarine topography offshore. Steeper shorelines produce higher tsunami waves.
• Because of the factors of low amplitude in deep water and large wavelength, tsunamis are often not noticed in mid-ocean. As the tsunami hits shallower water, the velocity slows, wavelength decreases and the waves height (amplitude) increases.
• Small islands with steep slopes usually experience little runup – wave heights there are only slightly greater than on the open ocean. This is the reason that islands with steep-sided fringing or barrier reefs are only at moderate risk from tsunamis.

However, this is not the case for islands such as the Hawaiian or the Marquesas. Both of these island chains do not have extensive barrier reefs and have broad bays exposed to the open ocean

By: SONAM SHEORAN