Science of Tsunami Waves
Tsunami Propogation Map from NOAA
Science behind Tsunami waves
I surf whenever I can and as I result I spend a lot more time than most people thinking about waves. The recent Tsunami that caused such a tragedy in Japan is worth understanding from a scientific standpoint.
First of all these waves are not large 100ft tall waves of water - they are small in height relatively speaking or amplitude (since we are going to get scientific). However, what makes them very dangerous is they are very very long period waves. The period is the amount of time that it takes for one wave (trough to trough) to pass or a wavelength. Normally, when surfing wave periods are ~10sec or less for wind swell and ~15sec for storm swell and if the swell is greater than ~20sec its a very powerful swell train that is going to pack a punch. This is when guys get on jetskis and head out to the outer reefs in Hawaii or head to Maverick's. A Tsunami period is on the order of minutes to hours. The result is a low amplitude, very long period wave that just pushes a wall of water onto land for minutes.
The quick math around velocity is:
Velocity =V Wavelength = l Period = P
V = l/P
To travel from Japan to California in 10 hours which is 9000 km means 900km/h or about 550mph.
Thus for this hypothetical example (which is a close approximation of the actual Japanese Tsunami) where the period is an hour and wavelength is 900km. 900km/h = l / P
All of this changes when the wave enters shallow water but the characteristics of these waves are remarkable.
These waves travel very fast in deep water and lose little energy which is why they travel so far and also helps to explain why surfers care so much about storms thousands of miles away from their own beaches.
For those interested in getting more detail there is a good scientific paper here http://www.tulane.edu/~sanelson/geol204/tsunami.htm
If anything here is confusing or incorrect please let me know. Also, if anyone has the actual data from the Japanese Tsunami or others we can add it to the comments section with the math. Finally - my prayers and thoughts go out to everyone in Japan who is suffering through this terrible natural disaster.
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