Goofy: “Tsunami is a water wave right? Water waves that I have surfed for at least 15 years now… What’s the difference?... The difference exists only in name?”
Me: “Not at all, as I have already told you Tsunami is a water wave that is formed due to displacement of a large amount of water. Such voluminous displacement is generally effected by an underwater earthquake, landslides or meteorite impacts.
The expected Tsunami for which we are on high alert is due to an underwater earthquake of magnitude 9. So let’s look into the details of Tsunamis generated by earthquakes.
Everything begin with energy, we walk, talk, jump, air flows, strings vibrate, waves occur… all are but manifestation of energy. Recall the string experiment you have performed with Jane… remember you had to use your energy to produce waves in the string?
Goofy: “Yep, I do.”
Me: “So if Tsunami is a wave there must be an energy source to let it happen. Yes, this energy is provided by the earthquake.
Cause of Earthquake
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| Fig. 1. Earth's Crust and Plates |
Earth has got hot magma inside and the solid crust where we live is actually composed of various “plates” which happen to float on the liquid magma ocean. The boundaries of plates are active volcanic activity sites. Activities of magma like contraction due to cooling, percolation of water into the hot zone etc, builds up of pressure within the rocks leading to movement of the various plates. The plates collide and pressure keeps on mounting in the rock strata. See the figure.
Now Earthquake takes place because of building up of such pressure (stress) within the rock strata inside the earth’s crust until they are deformed (strained) to a point (breaking stress) beyond which they would break causing a mass movement and release of a tremendous energy (shown by the blast symbol in the diagram).
The Point at which such break occurs is called the focus which is inside the crust and the point just above the focus on the surface of the earth is known as the epicenter. See the diagram.
Now Tsunami is a vast subject, so let us divide its study into two broad parts
1. Formation of Tsunami
1.1. Energy of Earthquake and Tsunami.
1.2. Comparative Energy Output Study.
2. Propagation of Tsunami
2.1. The Restoring force and Gravity waves
2.2. Tsunami in Deep, Intermediate and Shallow waters
2.3. Wave Shoaling Process and destruction.
2.4. Linear Wave Theory.
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| Fig. 2. Tsunami Formation |
1. Formation of Tsunami
When an earthquake occur underwater tremendous amount of energy is released between the plates, this energy (most intense at the epicenter) may be released in lot of forms like heat generated between the strata, sound energy, displacement of land mass etc, but a part of this energy is associated in giving a specific upthrust (force) to the water column above the sea bed causing a displacement of the water column. This initial lump of water displaced is sometimes called “Waterberg” in analogy with iceberg.
Since this actually is a mass of water raised to a height h (see the figure) against the gravitational force of earth, potential energy (mgh) is built up. This energy is transmitted via ocean waves known as Tsunami.”
Goofy: “How great is that energy? I mean it seems too low for a Waterberg to cause such a massive destruction to the coasts which is even impossible for some greatest storms too.”
Me: “Okay very nice question Goof
So you must be quite unaware of the amount of energy released by an earthquake. Let us see to it…
(To be continued…)
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