Speed of tsunami thru water

[TK]

I've heard that the "wave" travels at 500 mph (or 600 mph? Does it vary or is this a diff. of opinion?). Is it a shock wave, and what determines the speed of "effect" thru water (ripple from a pebble, from an explosion, etc.)?

 

[damonpip]

http://www.newton.dep.anl.gov/askasci/env99/env157.htm

 

[CycloWizard]

The speed of the tsunami will depend on the magnitude of the quake. The magnitude of the quake will dictate the impulse pressure. The tsunami is the surrounding water dissipating this pressure impulse. It's pretty easy to get an approximate analytical solution for the velocity as a function of depth. I can do it for you if you're really interested, though not tonight. I have a test on similar things tomorrow, so I'm a little burnt out. 🙁

 

[TK]

Thanks for the link damonpip. Looks like an interesting site.
Cyclo Wizard - thanks for the response. What I am interested in is not the specific speed, but rather what determines that speed, and why it varies. The speed of sound is constant (temp., pressure, etc. considered) and not affected by the energy of the event that causes it (I think), so this seems to be different (and slower - and varying). What I want to know is where to find out about this and similar phenomena (speed of ripples/waves from a pebble, a boulder, a nuclear blast).

TK

 

[Mday]

what determines the speed:
initial force of the earthquake (dependent on magnitude obviously)
the amount of water available
the surrounding geography
surface winds (at water surface) *
existing waves *
the geography as the wave propogates


* these can be negligible with respect to tsunamis because they are of LARGER magnitudes. of course if there's a huge storm the wave can be affected and vice versa.

I would not equate them to any bomb since a bomb will do equal damage from ground zero while a wave takes on characteristics given the terrain it travels across the water. not to mention the tsunami can pick up an island and leave much of it intact and unharmed while a bomb with equal magnitude will wipe it out. what you can equate is the force of the earthquake to a bomb set off at the epicenter.

tsunamis have much higher dependence on fluid dynamics than a bomb even though air is considered a fluid.

 

[damonpip]

Are you certain that the force of the earthquake has an effect on the speed of the tsunami? I thought that the speed was only a function of the water depth and gravity.

 

[TK]

The question I had in mind was -- if a bomb, erupting volcano, or earthquake occur at a given point, near the origin of the event (deep in the middle of the ocean for instance) what is moving, shortly thereafter, and how fast is it going? If something goes off in the atmosphere, sound waves radiate out at about 1100 fps, regardless of strength of the event. Do shock waves travel at different speeds than sound? If so, what is moving? In the water, sound travels at about 4800 fps (over 3200 mph), so, what is moving here - a pressure wave, or a bunch of water?
Discounting wind, etc., does a ripple (wave) in the water, travel at different speeds depending on the cause of the disturbance?

 

[TK]

damonpip - I also question whether or not the starting force affects the speed of the tsunami!

TK

 

[CycloWizard]

Originally posted by: Mday
what determines the speed:
initial force of the earthquake (dependent on magnitude obviously)
the amount of water available
the surrounding geography
surface winds (at water surface) *
existing waves *
the geography as the wave propogates


* these can be negligible with respect to tsunamis because they are of LARGER magnitudes. of course if there's a huge storm the wave can be affected and vice versa.

I would not equate them to any bomb since a bomb will do equal damage from ground zero while a wave takes on characteristics given the terrain it travels across the water. not to mention the tsunami can pick up an island and leave much of it intact and unharmed while a bomb with equal magnitude will wipe it out. what you can equate is the force of the earthquake to a bomb set off at the epicenter.

tsunamis have much higher dependence on fluid dynamics than a bomb even though air is considered a fluid.

:thumbsup:

I'd neglect surface winds for tsunamis in a similar manner to existing waves since the shear stresses created by air on water are very small relative to the stresses within the water. For engineering calculations, free surfaces like this are usually considered to have zero shear stress, though as you mentioned, this may change for very large wind velocities.

If you're talking about setting off a bomb underwater, it would act just as an earthquake. Basically, in both cases, the disturbance (pressure waves) are caused by an impulse of pressure. You have a large concentration of pressure at a point and the fluid medium acts to counteract that gradient by creating waves. The speed of waves created in air is usually considered to be near the speed of sound because, unless special conditions are met, the speed of sound is the limiting velocity in a fluid. It's complicated as to why that is the case (i.e. I can't remember 😀), but really the speed of sound is the maximum and would occur very close to the blast, if at all, with the velocity decreasing as distance from the source increases as 1/r^2, where r is the distance from the impulse. You'll note that sound travels in the same way - the source creates pressure waves that travel through the air/water. Any pressure source is the same.

Originally posted by: damonpip
Are you certain that the force of the earthquake has an effect on the speed of the tsunami? I thought that the speed was only a function of the water depth and gravity.

The magnitude may or may not. Once a certain magnitude is reached, the speed near to the source will be the speed of sound (the limiting speed). At higher magnitudes, then, the speed cannot increase so the velocity will be unaffected. At lower magnitudes, the velocity will be less.

 

[TK]

Cyclo Wizard - you refer to the speed of a disturbance from an impulse in a medium (water) as slowing per 1/r^2, but then you say that sound travels the same way (I think). But sound does not slow down, it just gets weaker with distance. This is part of my problem! The magnitude of the impulse that causes sound does not affect the velocity of the sound wave - just it's strength. So what is different about the "impulse" wave (and what is that wave?) that determines some speed, (speed of impulse? - magnitude of impulse? - both? - other?)

Any good books on the subject?

Thanks so far.

TK

 

[CycloWizard]

Originally posted by: TK
Cyclo Wizard - you refer to the speed of a disturbance from an impulse in a medium (water) as slowing per 1/r^2, but then you say that sound travels the same way (I think). But sound does not slow down, it just gets weaker with distance. This is part of my problem! The magnitude of the impulse that causes sound does not affect the velocity of the sound wave - just it's strength. So what is different about the "impulse" wave (and what is that wave?) that determines some speed, (speed of impulse? - magnitude of impulse? - both? - other?)

Any good books on the subject?

Thanks so far.

TK

Yeah, I think you're right. I think I'm just thinking about this too hard when I could just solve the equations and figure it out. But this week is my vacation from school-related things for once, so I'll have to get back to you on that one. 😉

 

[ahurtt]

I would think that the amount of water displaced by an event such as a tsunami would determine how fast and how far the actual wave of water that we see travels. The force from the event is what caused the displacement and passes very rapidly out in all directions until it dissipates. Now what you have left is X amount of water that has to relocate because something else took up the same space it used to occupy. Think back to the old lesson about what happens when an unstoppable force hits an immovable object? The force goes through the object. The force going through the object does not really hurt the object or move it. But if you changed the wording of that question slightly to read "what happens when an unstoppable object meets an immovable object. . .?" Well most likely then both objects are destroyed but the force still continues on. So if you throw a little pebble in a pond, the resulting force is weak and little displacement occurs. And keep in mind that even though it might be by an imperceptible amount, you have just caused the water line of the body of water you threw the stone into to rise or spread wider.

 

[TK]

ahurtt,

The amount of water displaced by an event probably has an effect on how far away the the disturbance is felt, but I can't see why it would affect the speed of propagation. The velocity of the event might determine the speed of propagation (I don't see that either - that is why I asked).

The part you said about immovable objects and unstoppable forces lost me a bit! Obviously " there ain't no such". But, if there were, and there was an immovable object and an ALMOST unstoppable force, it would seem to me that the force would be completely absorbed by the object without affecting the object.
If the force were unstoppable, then by definition - there could be no immovable object. If a force could move thru the object, it would seem that something in the object would have to "carry" the effect - it would be the medium wouldn't it?

TK

 

[itachi]

tsunamis are phenomenons caused by waves, they are not waves themselves.. the speed of a tsunami is a function of wavelength- frequency. the amount of water displaced by the event has an effect on how far away the distance can be felt.. as a wave propagates, the wave loses energy.. after some time the wave will "disappear."