[meteorite-list] LANL: Meteor Could Cause Big Tsunami

[Chris Peterson]

No argument that we should have tsunami monitoring systems in every ocean. 
And such systems might even provide data about ocean impacts that happen 
fairly often and are sub-tsunami producers.

BTW, the last estimate I read suggested that the actual plate movement for 
the Indian Ocean event may have involved a vertical shift of 8 meters over 
an area 1000 km long by 100-200 km wide. That is a massive volume of water, 
and in terms of energy alone is much greater than that produced by a 
mid-velocity, 1 km diameter impactor. And the earthquake coupled 100% of its 
energy into the water, whereas an impactor would couple only a small 
percentage.

Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com


----- Original Message ----- 
From: "Sterling K. Webb" <kelly at bhil.com>
To: "Meteorite-List" <meteorite-list at meteoritecentral.com>
Cc: "Chris Peterson" <clp at alumni.caltech.edu>
Sent: Wednesday, January 12, 2005 10:43 AM
Subject: Re: [meteorite-list] LANL: Meteor Could Cause Big Tsunami


> Hi,
>
>    I don't see how I could be making fun of Los Alamos by citing a work by 
> J.
> G. Hills of the Los Alamos National Laboratory! I wasn't making fun of 
> Gisler's
> work, either. I was being querulous about the newspapers.
>    Of course we could wait years? decades? for the perfect simulation. Or
> better still, since they're just simulations, wait for the impact to occur 
> and
> then just measure the devastation. But I thought the purpose was to 
> evaluate
> risks, and you can't do that without an estimation of the risks.
>    Only the Pacific ocean and the nations surrounding it have a working 
> tsunami
> sensor and warning system. We have seen the result, in the Indian ocean, 
> of not
> having such a system, which might have cut the number of deaths in half.
>    It's inconceivable to me, considering the very low cost of such a 
> system,
> that there should be none for the north or south Atlantic ocean basin 
> either.
> This is because the seismic risk is considered negligible (just as it was 
> in the
> Indian ocean).
>    But I think it is necessary to try to assess impact as one of the 
> possible
> tsunami risks. You're quite right that the calculation is a very murky 
> one,
> though.
>    As for the water displacement, Chris earlier wrote: "There is some 
> question
> about the dynamics of the water displacement- that is, most of it goes up, 
> not
> out. And that total volume of water is somewhere between a few tens and 
> few
> hundreds of cubic kilometers. Contrast that with the recent Indian Ocean 
> event.
> The shift in the ocean floor resulted in the displacement of over 1000 
> cubic
> kilometers of water, and produced waves in most locations of 3-5 meters."
>    Water waves do not consist of water moving in the direction of wave 
> motion.
> They consist of water oscillating at a right angle to the direction of 
> wave
> motion. So water in a wave is never forced "out," only up-and-down.
>    As every ten-year-old intuitive physicist knows, the best way to make a
> water wave is with a good ker-plunck!
>    In an "ideal" water wave (infinitesimal particles, frictionless fluid) 
> ALL
> motion is vertical, i.e., transverse to the vector of the wave's 
> propagation.
> The wave propagates; the water does not.
>    In a "real" water wave, the surface is composed of small cyclindrical 
> cells
> which revolve as they go up and down, producing a small surface motion in 
> the
> direction of propagation. This produces a small frictional loss which will
> eventually cause the wave to die out (after many thousands of kilmoters).
>    It's only when a water wave interacts with a boundary (shoreline) that 
> the
> kinetic energy of that vertically oscillating mass is suddenly transformed 
> into
> horizontal motions, with devastating effects.
>    An impact that produces up-and-down motion is the perfect way to create 
> a
> wave. The seismic event in the Indian ocean was a thrust slip, in which 
> one
> plate of the earth's crust was forced upward by a few meters (less than 
> ten)
> over a broad area (few hundred sq. km.).
>    This up-and-not-down motion seems to have translated perfectly into a 
> water
> wave of about the same height as the plate displacement. (Since water is
> imcompressible, this is pretty much inevitable.)
>    Personally, I think a low-altitude airburst of an small incoming object
> would more efficiently produce water waves than is generally appreciated, 
> and
> represents an underestimated risk.
>    Now, to find a simulation to prove it...