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Blowin' in the Wind, Issue #014 Tsunami and Earth Quake Information - January 1, 2005
December 30, 2004
Earth Quake Information for youPeople need more earth quake information. What happened in those photos and tsunami pictures seen on TV and the newspapers. Earthquakes measuring up to 9.0 on the Richter scale occurred that fateful Sunday Morning. December 26, 2004.
Tsunami Facts and ScienceAs the tsunami involves the entire depth of the ocean water, its speed depends on that depth. The speed is actually proportional to the square root of the depth, as this relation shows:
Speed = Square Root of (depth * acceleration due to gravity)
Acceleration due to gravity is about 32 feet, or just about 10 metres per second per second.
That means if you drop a rock off a bridge, one second later it will be going 32 feet per second if it doesnt hit the ground yet.
Anyhow, take this number and multiply by the depth, for example 30000 feet (very deep water) to get 960000. Then take the square root and you have almost 1000 feet per second. Or about 675 miles per hour.
If metric is your game, 4 km is a good typical depth. Thats 4000 m. Now multiply by 10 to get 40000 and take the square root and you have 200 m/s, which is 720 km/hr, or 450 miles per hour
What happens as the wave approaches a shore? Lets say the swimming area is only 100 feet deep. A wave of this sort would travel SQRT(100*32) = about 55 feet/second, or 40 mph.
Imagine a freeway full of cars going 400 mph and one in front hits the brakes and slows down to 40. Think the ones behind would pile up? Thats exactly what the water does. Thats what makes the waves are high. Scientists call this shoaling.
Ordinary waves, caused by wind, are simply moving wrinkles on the ocean surface. They do not normally disturb water more than a few meters down.
Other waves are caused by tides (the moons and suns gravity pulling on the ocean water against the planets rotation) coming in and going out. We call them tidal waves. They are not tsunamis.
Tsunamis result when the earth moves below the ocean. Look at this earth quake information. If a new trench were to drop 5 metres, for instance, then a column of water, perhaps thousands of metres high, drops about the same distance. And then a big space needs to be filled. How many tons is that?
Lets see, if it is 100 km long and 20 m wide, 5000 m high, and one tonne per cubic metre means 10 billion tonnes instantly set in motion, and this volume increases rapidly as the shock waves radiate outward towards land.
Now what about a simple wavelength calculation? Ordinary waves might come every 10 seconds, for example, and travel about 15 m/s, or 30 mph. Meaning each one is about 150 m (close to 500 feet sometimes) apart.
Tsunamis might strike every 20 minutes, and if they travel 200 metres per second, the space between each in this case is 240 km or 150 miles. Remember the actual speeds depend on the ocean depth.
You may have heard of them referred to as shallow waves, even though they originate at the bottom. Shallow water waves means that the wavelength (240 km) is much greater that the depth (5 km). By looking at these numbers, you can see that certainly is so.
By the way, not much can dampen a wave with such a huge wavelength, even beaches and hotels have trouble. Another strange thing is, in the deep water, these waves go by nearly unnoticed.
What are long gravity waves?
Waves that travel in density stratified fluids, such as the wide open sea, under the effects of buoyancy forces. They need the restoring force of gravity, and are bounded by sharp density changes, such as the ocean surface and floor.
In weather, similar buoyancy waves result in various types of alto cumulus patterns, such as ACSL. Other meteorological concepts such as Coriolis force and Rossby waves come into the equations in special circumstances.
Tsunamis are fast, but...
Seismic waves are much faster – so are electromagnetic waves. With the right equipment people should have been able to send information from which the pending disaster could be deduced to all countries to be affected. But alas, the correct apparatus is probably too expensive for these places. India, Thailand and Indonesia to name a few on the map.
They will be working to improve this shortfall.
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