Scooby96

DarkStar - no the worlds worst tsunami/tidal wave killed 3,700,000 lives!

Leslie

As I said before, the Tsunami is a travelling shock wave through the water until it approaches shallower water at the coastline. The water at the coast is dragged out to sea initially until the shock wave coincides with the mass of water which is piling up and then the energy from the shock wave is converted into pushing the mass of water inland with the tragic results we heard about.

Les

messiah

Wasn't the 1883 Tsunami from Java triggered by Krakatoa? And didn't the wave from that circle the earth 3 times?

I heard that when the Tsunami hit Sri Lanka, the water was travelling at around 300mph, and that it covered the 1200miles from source in 2 hours - so to begin with this shockwave must have travelling beyond the sound barrier.

I also found it interesting that the rise in water level (even if it was "only" 1m) was 200 miles from front to back...

Just goes to show how scary Mother Nature really can be.

OllyK

The speed of sound in air (circa 760 mph, but temperature, humidity and altitude all affect it) is not the same as the speed of sound through water, it travels faster through water (approx 3200 mph)

Drunken Bungle Whore

For anyone that's vaguely interested in these things am currently reading a FABULOUS book called Krakatoa by Simon Winchester - it's all about the scientific and social causes and effects of the Krakatoa erruption of 1883 - fabulous descriptions around the development of plate tectonic theory and the impacts of the spice traders on the local population etc.


(Should I get my anorak now?)

AvalancheS8

That fact right there is why they cause so much damage, it's not a traditional "wave shaped" wave with a peak and then dropping away behind, it's more that the sea level suddenly drops and then comes back up at high speed, 10m higher than it was before. Because of that and the relatively high speed of approach to the beach it can put the whole coast under 10m of water up to 100s of meters inland. At the point that the wave front is reaching 300m inland, the normal coast line will still be deep under water.

ajm

It IS a normal wave, albeit one with a very long wavelength, it is this that allows it to travel at the speed it does in deep water.

AvalancheS8

Yes, but it's tempting to think of this thing as having looked like a normal wave like people surf on, but 30 ft high, quite thin from back to front, when in fact it looked more like a big band of water, with a front running the entire length of the coast, 10m high and 10s of times thicker from front surface to back than you would expect for the height of it.

Basically it's going to have a similar effect to dropping the whole country 10m deeper into the sea for a minute or so and then pulling it out.

ajm

Not exactly because the waveform changes when it gets into shallower water. As I said in my earlier post, the speed of the wave is restricted by the depth of the water:-

v = sqrt(g * d) where v is velocity, g is acceleration due to gravity and d the depth of the water.

When depth decreases the speed of the wave decreases. As speed decreases, wavelength decreases and amplitude INCREASES (in order to keep total energy the same). This is effectively bunching or "shoaling" of the waves. Thus the waves that hit the shoreline look much more like typically shaped surf waves and are much higher than the wave was when it was in deeper water.

BexTait

This what I've understood from looking through a couple of my text books:
The tsunami (or seismic sea wave) is caused by displacement of the sea floor (eg volcanic eruptions, submarine mass movements, earthquakes). The displacement of the water generates waves that have small amplitudes, long lengths and high velocities, the latter being related to sea depth. The waves are transmitting a large amount of energy, especially as the whole depth of water is involved (with normal waves, the displacement is only a small amount of the depth in deep water)

As the wave approaches shallowing water (along continental shelves) velocity decreases and amplitude increases, making the waves 'taller'. The energy of the wave is still the same, which is why it travels inland and is destructive.

AvalancheS8

This is what I was talking about:



Taken from here

ajm

The wavelength at impact would depend on the profile of the shore. If it was steeply shelving then the picture above may be more of an accurate representation, if it was a shallow sloping shoreline then the waves could resemble surf from hell!

The point I am trying to make is that in either instance the wave WILL increase its amplitude and reduce its wavelength drastically compared to when it is out in deep sea. Those diagrams are misleading in that they don't seem to acknowledge this. They seem to suggest the wave will hold its shape and just roll over the land and this is not the case. Yes the waves will be nothing like wind generated waves but we knew this already!

Drunken Bungle Whore

Just got my hands on this weeks New Scientist and they have some really interesting stuff on the quake/ Tsunami. To quote a couple of bits that are relevant to the original questions:

"So far the best evidence suggests that a swathe of the sea floor lifted by up to 5 meters while other parts dropped by 2.5 metres. The body of water above, 5 kilometers high, instantly moved with it, and from that moment a tsunami was inevitable.

The speed of a tsunami in the ocean is directly proportional to the depth of the water - so at the deepest points of the Indian Ocean the water raced forward by upto 900KM per hour. ( ) The first models of the wave show that at this stage it was broad but shallow - 100KM from front to back but less than 0.5m high. When the displaced water reached the shallower continental shelf it was forced to slow, allowing the back to catch up with the front to create the destructive tsunami wave."

That's the basics - but their webseite has some other really good stuff on it:

www.newscientist.com