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Re: Chicxulub's Antipode (Re: cause of death at KT)

I know it's been a while since Physical Geology, but it is S waves (aka "Shear" waves) that cannot travel through a liquid. S waves rely upon elastic properties in a solid to transimit the classic sinusoidal waves. Only pressure waves (P waves) can travel through a liquid (or a gas for that matter...which is important to us!).

P waves help geophysicists find alterations in the rock because they get refracted at boundary layers (between liquid and solid layers, or between solid layers of differing density). The Moho layer was found because P waves travel through it more quickly, so some P waves arrive at a given distance away from an earthquake more quickly than those that travel through the upper crust.

There is both a P and S wave shadow on the far side of the Earth. The S wave shadow is created by the liquid portion of the core, which prevents S waves from traveling throug it. The P wave shadow is created by differential refraction of the inner layers of the Earth, which causes some portions of the far side of the planet (from a seismic event) to get no P waves, while other parts get the P waves "focused" on them. There's a good picture of that at Wikipedia if you search "P waves".

Scott Hartman
Science Director
Wyoming Dinosaur Center
110 Carter Ranch Rd.
Thermopolis, WY 82443
(800) 455-3466 ext. 230
Cell: (307) 921-8333


P.S. One interesting misconception that used all the time in education (even college physics) is that sound waves travelling rhough the air are drawn as S waves for ease of representation, even though (by necessity!) we hear P waves.

-----Original Message-----
From: bigelowp@juno.com
To: dinosaur@usc.edu
Sent: Sat, 2 Sep 2006 10:09 AM
Subject: Re: Chicxulub's Antipode (Re: cause of death at KT)

On Sat, 02 Sep 2006 15:32:09 -0700 "James R. Cunningham"
<jrccea@bellsouth.net> writes:
What is the mechanism by which it [P-waves] is absorbed in a liquid?

It has something to do with the differences between the compressability
of liquid rock vs solid rock.

I'm no physicist (trust me!). But I vaguely remember a Geology 101
instructor bringing into class one of those desktop "dynamic art
displays". You know, that contraption with the 3 steel balls on strings?
The impinging ball whacks the middle ball (which doesn't move), and the
middle ball transfers the energy of the impact to the ball on the
opposite side, which swings outward. That's purportedly how P-waves
work; they need a solid medium in order to propogate. To model how
liquid attenuates the energy of a P-wave, just wrap some polyurethane
foam around the middle ball and run the experiment again.

The MOHO layer (a layer of mushy rock at the base of the crust) was
discovered because P-waves from earthquakes got attenuated within this
zone.  Seismic recorders picked up strong S-waves but very weak P-waves.


Phillip Bigelow wrote:

> Well, maybe not.  P-waves ("compression waves") tend to get
absorbed by
> liquid,

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