Oceanography (2+1)

Tsunamis

Tsunamis are triggered by earthquake, underwater volcanoes, land slides, and meltingof icebergs from glacier .  Sudden movement of the earth’s crust may  also produce seismic sea waves, or tsunamis. These waves are often incorrectly called tidal waves. Since a seismic sea wave has nothing to do with tides. Most tsunamis are in the pacific ocean and are not so common in the atlantic and Indian oceans. The Hawaiian islands experience a tsunami every two years.

If a large area of several hundred square kilometers, of the earth’s crust below the sea surface is suddenly displaced, it causes a sudden rise or fall in the sea surface level above it. In the case of a rise, gravity causes the suddenly elevated water to return to the equilibrium surface level: if a depression is produced, gravity forces cause the surrounding water to flow into it. Both cases result in the production of waves with extremely long wave lengths (100 to 200 km) and long periods as (10-20 minutes). Since the average depth of the oceans is about 4000m ( or 4 km or 13,000 ft,) this depth is less than one-twentieth the wavelength of these waves, and tsunamis are shallow-water waves. These seismic waves radiate from the point of the seismic disturbance at gD and move across the oceans at a speed  determined by the ocean’s depth ( C=  gD) and about 200 m/sec (400mbp). Since they are shallow-water waves, tsunamis may be refracted, diffracted, or reflected in midocean.

When a tsunami leaves its point of origin, it may have a height of 1 to 2 m, but this height is distributed over its many-kilometer wavelength. It is not easily seen or felt when superimposed on the other distortions of the sea’s surface, and a vessel in the open ocean is in little or no danger if a tsunami passes. The danger occurs only if the vessel has the misfortune to be directly above the area  the original seismic disturbance.

The energy of a tsunami is distributed from the ocean surface to the ocean floor and over the length of the wave. When the path of the wave is blocked by a coast or island, the wave behaves like any other shallow-water wave, and the energy is compressed into a smaller water volume as the depth rapidly decreases. This rapid and sudden increase in energy density causes the wave height to build rapidly, and the loss of energy is also rapid when the wave breaks. A tremendous surge of moving water races up over the land, destroying buildings, docks, and trees. 

A tsunami often affects an area with a radius of more than 150km in exceptional circumstance and it can have a wave length as high as 1000km .when a tsunami hits the coast. It comes in waves after waves and in most cases the third to eighth waves are the most dangerous as they are the largest.

The leading edge of the tsunami wave group may be either a crest or a trough. If the initial crustal disturbance was an upward motion, a crest is formed first; if the crustal motion was downward, a trough is formed. If a tough arrives before the first crest, sea level drops rapidly, exposing sea plants and animals.

Tsunami prediction is an important and difficult task. The detection of under water seismic activity somewhere in the world does not mean that a tsunami has been generated. Because the waves are so long and low. They cannot be detected until they come into shore somewhere. Even though a tsunami is detected on some coast it is not possible to predict what other coastal area might be hit. Due to refraction by sea floor topography, these seismic waves will go some direction and not other. As they move at a speed of about 700km/hour across deep water it may not be possible to warn or evacuate  coastall population even if one did know where the waves are headed.