Subject: tsunami front: concurrence and a correction
From: Alan W Harris
Submitted: Fri, 20 Dec 2002 10:07:58 -0800
Message number: 43
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Dear Michael and Dave,
First I want to confirm my concurrence with Dave's technical points
below. We need to focus on the size events that are of primary interest to
the next generation of surveying. I get the feeling that much of the
dispersion in results among different tsunami modelers is due to the size
of events they emphasize. for example, Ward and Asphaug concentrate on 250
m diameter, but also consider 1 km. Hills & Goda seem hung up on 10 km
diameter "dinosaur killers", of which none are coming our way for the next
million years at least.
The small correction is that Steve Chesley is charged with leading the NASA
study and report on tsunami hazard. I am charged with the overall chapter
on impact risk, including both land and water impacts. Bill Bottke has
also taken a strong interest, and in fact is the one who uncovered the
mysterious Van Dorn report and brought Jay Melosh into the action. In any
case, I think all of us agree on the strategy outlined by Dave in his message.
Cheers,
Al
At 09:40 AM 12/20/2002 -0800, David Morrison wrote:
>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
*******************************************************************
Alan W. Harris
Senior Research Scientist
Space Science Institute
4603 Orange Knoll Ave. Phone: 818-790-8291
La Canada, CA 91011-3364 email: harrisaw@colorado.edu
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