# Re: Extinction scenarios

```Message text written by "James R. Cunningham" with regard to my comments on
refraction of seismic waves reducing forces at the antipode of a bolide
impact:

"This is a subject worth more attention.  Would you elaborate please?

It is difficult to elaborate on the subject verbally, since
pictures make it so much clearer!  I think that the best I can do is
summarize with analogies.  The same way that any wave (light, sound, etc.)
is refracted when entering a medium of different density, seismic wave
paths are refracted (bent) at numerous points throughout the diameter of
the Earth, since at various points, the temperature, pressure, and
compositions change, thus changing the densities.  An earthquake at any
given point on the globe is notoriously difficult to detect at the
antiopodal point because the bulk of the seismic waves are refracted
towards other points, creating a "shadow zone."  This refraction (and, in
some cases, depending on the angle of incedence, reflection) is what has
(a) allowed us to determine the densities and then propose compositions of
various layers within the Earth, and (b) detect earthquakes at numerous
points around the globe after they occur (with the caveats that different
eaves arrive at different times -- this allows us to pinpoint the _exact_
point of origin of the quake as well as the precise time it occurred).

For some good pictures and some detailed text, check out any good
geophysics and/or seismology text.  The one I have on hand from my course
on this is:

Fowler, C.M.R.  1990.  _The Solid Earth:  An Introduction to Global
Geophysics_.  Cambridge:  Cambridge University Press, 472 pp.

Specifically, check out sections 4.3.1 ("Seismic Velocities for the Whole
Earth") and, for the math surrounding a simplistic two-layered "Earth"
model, section 4.4.2 ("A Two-Layered Model")

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Jerry D. Harris
Fossil Preparation Lab
New Mexico Museum of Natural History
1801 Mountain Rd NW
Albuquerque  NM  87104-1375
Phone:  (505) 899-2809
Fax:  (505) 841-2866
102354.2222@compuserve.com

```