Why is the wave height of tsunami not so big in open seas?
Dr. Sagar Chahar, Delhi
Tsunami is a Japanese word meaning harbour wave. Tsunami waves are somewhat similar to the wind generated waves we see at a beach. Wind generated waves are restricted to a thin layer below the water surface and the up-down oscillations of the water particles (typically 0.1 – 3 m at the surface) quickly decrease as one moves down from the water surface. We can create such waves by a paddle.
Tsunami waves, on the other hand, are created by a large motion of the earth’s crust, for example in an earthquake. A large plate (hence large wavelength) of the earth’s crust creates these waves by displacing water and the energy liberated is huge. Wave amplitudes are small (0.1 – 1 m) but the wavelength is large, typically 100 to 1,000 km. Oscillations of water particles are not just restricted to the water surface but are distributed in the entire depth. We cannot create such waves by a paddle. Because of the large volume of water involved, energy of tsunami waves is large even though the amplitudes are small. This is why it is difficult to observe the height of tsunami waves in the open sea.
But the destructive power of the tsunami waves is felt when they reach the seashore. Here the same energy gets concentrated into a smaller depth and hence amplitude becomes very large and gets noticed.
Thus, the tsunami waves correspond to large amount of energy liberated in a large volume with relatively small amplitude becoming noticeable only when the waves reach shallow waters.
Prof. Mohan D. Deshpande
M.S. Ramaiah University of Applied Sciences, Bengaluru
This week’s question
How does an ATM work on touching the glass on the machine?
K Ananthanarayanan, Kanyakumari, Tamil Nadu
