[meteorite-list] Norway Meteorite Impact Site Believed to be Found

From: Chris Peterson <clp_at_meteoritecentral.com>
Date: Tue Jun 13 12:22:29 2006
Message-ID: <01c701c68eee$a33e0830$fd01a8c0_at_bellatrix>

Hi Sterling-

You are quite mistaken in this regard. Seismic events are routine with
moderate and large fireballs. I've optically recorded five in the last few
years that produced signals on seismometers in northern New Mexico (there
may well be others for which I didn't check). The fireball over the Pacific
Northwest a couple of years ago was actually tracked by its effect on an
array of seismometers. Even sonic booms from airplanes are recorded on
seismometers. The effect of a large mass of air on the ground is
significant.

On the other hand, I'm not aware of any actual impacts producing measurable
seismic signals. It would require extraordinary circumstances for a 1m iron
to reach the ground at anything other than ordinary terminal velocity, and
an ordinary fall will not generate a significant seismic signal unless it
nearly hits the seismometer! And it doesn't make much difference whether
it's iron or stone.

There is nothing about the recorded seismic signal in Norway that makes me
think this was anything other than an ordinary (large) fireball, or that
anything reached the ground with hypersonic velocity.

Chris

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


----- Original Message -----
From: "Sterling K. Webb" <sterling_k_webb_at_sbcglobal.net>
To: "Meteorite List" <meteorite-list_at_meteoritecentral.com>
Cc: "Chris Peterson" <clp_at_alumni.caltech.edu>
Sent: Tuesday, June 13, 2006 2:32 AM
Subject: Re: [meteorite-list] Norway Meteorite Impact Site Believed to be
Found


Hi, Chris, List

    I don't want it to seem like I'm jumping on anyone
in particular, and I'm NOT jumping on any one in particular,
but folks are just not paying attention here.

    You don't get a seismic event from an airburst unless it
really big or very close to the ground (and to get close to
the ground, you have to be big, so it's the same thing).

    Even assuming a very hard dense stone (achondrite)
50 meters in diameter that produces a 1,000,000 tons
of TNT airburst, you get ZERO seismic signal. This,
when even a lousy 1 meter iron will reach the ground
and achieve a Richter 0.2 reading. Duh. Just not paying
attention, you guys.

    I've fiddled with sizes, materials, entry speeds, impact
angles, for hours. You can't get a small seismic signal
from a stone. You get the threshhold of seismiticity when
the stone gets big enough -- 70 meters (varies with speed
and angle of entry, naturally) -- to punch down below
the stratosphere and produce a 30 megaTon airburst,
not before. The transition across that threshhold is very
steep, from the first tremble to broken windows. The
airburst too small to produce a seismic signal WILL
smash forests and snap off small trees, which did not
happen in Norway. Such an airburst would leave a
huge strewn field with thousands of fragments. No
indication of that in Norway. It would also produce
a much louder sound signal than indicted by the reports.

    The ONLY way you get a smallish seismic signal is
with a small object that actually reaches the ground,
and only a small iron will do that -- NO small stone can.
Yes, an airburst will produce a LOCAL seismic signal,
but a seismometer 50 miles away will record nothing.
Remember the case of the airburst in England that
triggered a seismic signal when seismometers in London
only thirty miles away registered nothing? Can't remember
the date, but I'll search for it.) The seismometers that
measured this signal were hundreds of miles away.

    Yes, I'm using a mathematical model, but Melosh has
the rep as a top man, if not the top man. Do you suppose
anyone went and checked its assumptions and methods?
http://www.lpl.arizona.edu/~marcus/CollinsEtAl2005.pdf
Did you? You work this all out numerically with real-world
parameters, using his or your own model, and come up
those precise figures? "High" altitude, and "a lot" of energy?
How many joules is "a lot"? How high is "high" altitude?
Am I unreasonable in prefering his well-thought-out model
to an off hand opinion?

    I'm sure all the people who just blow this off are fine and
wonderful people, the Swede who doesn't like the Norwegian
and says so (nothing surprising there), our model of temperamce,
Mike Farmer, and you, and Marco, all fine people, I'm sure, but
does it sound like anyone is actually bothering to work this out,
or are you just assuming your instantaneous impression is sufficient
and superior to actual investigation, however crudely done using an
on-line modeling program? Disagree? Do a better job of
quantifying it.

    I mean no one has to work this out to see if an event was
actually possible. We all have better things to do. Me too.
I agree the "crater" is not definitive of impact, could be a
rock fall; newspapers and webnews is often wrong and
careless. It appears to be a poor crater candidate. Not
the point. There was an impact (not an airburst)
somewhere in that area.

    You and Marco are meteor/fireball experts, I know, but
the event that brings an object to earth is different, no matter
how similar in appearance. No recovered object has ever
been from a meteor stream. The very few fireballs that bring
a substantial object hide among countless fireballs that don't,
and they ALL LOOK ALIKE. Power Law: for every 1 meter
lump, 30 10 cm lumps, 900 1 cm lumps, 2700 1 mm lumps,
all bright and flashy. For every 1 meter iron (0.7% of falls),
140 1 meter stones that won't reach ground, 4200 10 meter
stones that won't reach ground... and 120,000 bright 1 cm
body fireballs for every 1 meter iron. THEY ALL LOOK
THE SAME, until the last minute of the outcome... So you're
arguing the odds from what you know best, but...

1. A seismic signal means an impact or an airburst..
2. There was no airburst large enough to produce this seismic signal.
Hence...

     Excuse me for being more candid than usual, and I have no
intention of provoking bad feelings, but you know that people
are not working this through, but responding in an off-hand way,
thoughtlessly. No one has to agree with my approximations (or
my wild guesses), but I don't see anybody presenting a calculation
that shows a differing outcome, that explains the few facts in a
quantitative way Show me numeric values for size, density, speed,
entry angle that produce the object you believe it was and a seismic
signal such as was reported. (That's a generic "you," referring
to anybody, like all the "you's" in this post.) Use the Melosh model.
Use your own model. Work it out as an individual case. Whatever.
Demonstrate a little rigor. Shove a digit or two around. Make it
a reasoned argument, people, even in this forum.


Sterling K. Webb
Received on Tue 13 Jun 2006 09:38:21 AM PDT