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

From: Sterling K. Webb <kelly_at_meteoritecentral.com>
Date: Wed Jan 12 12:43:58 2005
Message-ID: <41E561BB.2550DDD9_at_bhil.com>

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...


Sterling K. Webb
--------------------------------------
Chris Peterson wrote:

> >From 1994? And Russian, too? Might as well be the middle ages! <g> Like I
> said, I think I'll wait for the simulations to improve. Personally, I would
> be no more surprised by 1000m waves than by 10m waves. And I wouldn't make
> fun of the work at Los Alamos that led to the smaller value (some of the
> best work in modeling impacts is coming out of Los Alamos) or of the
> newspapers reporting on the research.
>
> Chris
>
> *****************************************
> Chris L Peterson
> Cloudbait Observatory
> http://www.cloudbait.com
Received on Wed 12 Jan 2005 12:43:23 PM PST