Re: the Deccan Antipode

[awrobinson@amoco.com (Andrew W. Robinson)]

> 
>  A couple of years ago, I posted a similar hypothesis (on a BBS)  that an
> impact of Chicxulub size might have resulted in effects on the antipode side
> of the planet. A geophysicist quite convincingly showed me that it would be
> impossible because the seismic waves generated _cannot_ pass directly through
> the core of the earth! They are deflected at (I believe) 120 degree angles.
> Even if the Deccan plateau were antipode to the impact this is shear
> coincidence. BTW the Deccan floods began BEFORE the impact and it is now
> believed that they are associated with a hotspot in the Indian Ocean.
> 
>                                               Regards,
>                                               Thomas R. Lipka

As a seismologist in a former life (graduate school), I remember that
the antipode is one place that seismic energy will be received.

The shadow zone depends on two factors, the angle of incidence of the
seismic rays and the fact that the seismic velocity of the outer core
is less than that for the mantle above it. At certain angles, the
seismic energy gets deflected into the outer core instead of reflected
back up to the earth's surface; thus it is not recorded on
seismographs. The shadow zone has a beginning and an end. (I forget the
angles, but I could look them up if there is enough interest.)

The math is fairly straightforward. The deflection angle is calculated
by Snell's Law:

     sin a1   sin a2
     ------ = ------
       v1       v2

In this case, a1 would be the angle the seismic ray hits the core -
mantle boundary, v1 is the seismic velocity of the lower mantle, v2 is
the velocity of the outer core, and a2 is the deflection angle.

For a ray travelling straight down (toward the antipode), the angle a1
will be 0:

     v2
     -- sin 0 = 0 = sin a2; a2 = 0
     v1

The deflection angle will also be 0 degrees, meaning the ray keeps
traveling straight through. The upshot is that seismic energy will be
recorded at the antipode.

Two notes on this "derivation". First, the derivation says nothing
about energy being focused at the antipode. That may happen, but I
haven't seen the math to indicate that it does. Second, I am not
suggesting that the Deccan Traps resulted from the Chicxulub bolide. As
Thomas and others have noted, the Deccan Traps preceded the Chicxulub
bolide by millions to tens of millions of years.

Andrew Robinson