Electromagnetism, Properties Of Waves, Motion, Gases & The Kinetic Theory Worksheet With Answers Page 13
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36Refraction of seismic waves.
Both the density and stiffness increase with depth in the mantle, but the rigidity wins and
so the speed of both S- and P-waves increases with depth. If the speed of the waves
changes then the waves will refract and so will change direction.
Refraction in the Mantle
Over a few hundred km refraction has the following effect
– ignoring the curvature of the Earth:
F = earthquake focus
S = Seismometer
The waves curve because the bottom edge travels faster than the top edge and so it
overtakes the top edge. This makes it bend upwards. Note that both P- and S-waves curve
like this. They both travel faster the deeper they go into the mantle.
Inside the core.
The waves refract/bend at the core-mantle
boundary because they slow down. Inside the
core, the waves curve gradually, just like in
the mantle, because the deeper they get, the
faster they become - because the core is
more rigid at greater depths. They don’t
refract/bend very much though because the
speed
doesn’t
change
very
much
– see the graph. (The dotted lines represent
the normal which is always at 90˚ to the
boundary).
If the waves pass through the inner core, they refract again. They also refract as they
pass back into the mantle.
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