Re: is about seismic waves (S waves)

     Hi Carolyn,

     At a depth of aprox. 2900 km (CMB - Core Mantle Boundary) there
is the most notable feature of the velocity-depth curves. This is a 
discontinuous drop in the velocity of P waves across the boundary and
below it S waves have never been observed. We know that S waves don't 
propagate in liquid material so this is the main reason for believing
that outer core is liquid. This way, all the energy of S waves that
reaches CMB is reflected to the mantle.
     S waves would reach a point immediately opposite the focus by 
reflection, at the mantle/crust boundary if the mantle was homogeneous.
As all the layers of the Earth are not completely homogeneous, difficultly
the reflected S wave would reach the point exactly simultaneously opposite
to the focus. But, in certain cases, for practical ends, we can consider
the homogeneous crust (a model) and to set up equations that take into
account this fact. This way, we are "forcing" the reflected wave
to reach the point simultaneously opposite to the focus, what in fact 
doesn't happen.

     I hope to have helped you. If you need further explanations, please
send me a message.

     Emilson P. Leite
     IAG/USP - Dept. of Geophysics.
     São Paulo - Brazil