Subject: Re: What's new on the tsunami front?
From: David Morrison
Submitted: Fri, 20 Dec 2002 09:40:04 -0800
Message number: 41
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Michael and LSST mailing list:
Al Harris (who of course is on both the LSST and the NASA teams) is
taking the lead in contacting Melosh and other experts to try to
establish the sources of the apparent differences in various tsunami
calculations and publications. What is essential for this exercise is
that at we all try to focus on the same magnitude events. Most
previous impacts hazard work has dealt primarily with impacts in the
global catastrophe (million megaton) to mass extinction (100 million
megaton) class. In contrast, empirical data from nuclear explosions
is down at least 10^7 in energy from the KT mass extinction event.
The energy release from large earthquakes may approach the scale of
event we are interested in, but the linear scale of the displaced
region is much greater than the transient water cavity produced by an
impact explosion. We can hardly expect simple scaling over such broad
ranges of energy and dimension.
The point I want to make, and what Al Harris and I both advocate, is
to focus on a single case: the impact of a 300 m diameter asteroid
into deep ocean. The energy of such an explosion would be about 1000
megatons and the transient cavity about 4 km in diameter and of order
1 km deep. It would be very helpful to see whether, and by how much,
the estimates for such an event vary. We might further specify a
distance from the shore (perhaps of order 1000 km) for the test case.
The reasons for selecting this size are: (1) the larger (D>1 km) NEOs
are already being taken care of by the current Spaceguard Survey; (2)
past claims of maximum risk from tsunami have been for NEO impacts in
the few-hundred-meter range; and (3) it is the 300 m threshold that
is important for LSST justification and that was used by the two NRC
panels that recommended a LSST NEO survey. I believe that until we
focus on such a nominal case we will be unable to clarify why
different investigators seem to be producing widely different
estimates about the tsunami risk from NEOs.
David Morrison
At 4:10 PM -0500 12/19/02, Michael Strauss wrote:
>Hello all,
> We discussed the physics of tsunami propagation at the last LSST SWG
>meeting, and a recent talk by Jay Melosh was quoted, saying that the
>dangers of these events has been overestimated. I just finished
>reading the papers by Hills and Mader, and by Ward and Asphaug; I
>would very much like to understand what, if anything, they did
>incorrectly in their calculations. Ward and Asphaug use simple energy
>arguments to predict the size of water waves from an impact, and
>propagate them with linear theory (and thus no run-up when the wave
>hits the continental shelf). Hills and Mader use empirical data
>from what I understood to be nuclear tests to come to similar
>conclusions, but push this further to estimate the effects of run-up
>and breaking; they claim that historical tsunamis show an increase of
>height of a factor 10-25 from the deep-water tsunami.
>
> The statement at the SWG meeting was that Melosh claims that the
>propagation of relatively short-wavelength waves from an impact is
>qualitatively different from these. I poked a bit around on the web,
>and wasn't able to find anything by him on this subject. This is of
>tremendous importance; does anyone have suggestions on how we can gain
>further understanding of this? Would it make sense to write to Melosh
>directly?
>
> Thanks, Michael
>
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--
+++++++++++++++++++++++++++++++++++++++++++
David Morrison, NASA Ames Research Center 240-1
Tel 650 604 5094; Fax 650 604 4251
david.morrison@arc.nasa.gov or dmorrison@arc.nasa.gov
website: http://astrobiology.arc.nasa.gov
website: http://nai.arc.nasa.gov
website: http://impact.arc.nasa.gov
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