Re: S waves - why don't they travel through liquid?

Why don’t S-waves travel through liquids?

There are two types of seismic waves, S-waves and P-waves.  The P-waves are 
compressional waves and are also called "pressure waves".  The S-waves are 
transverse waves and are also called "shear waves".  A key to understanding 
why S-waves don’t travel through liquids is the difference between 
compressional and transverse waves.  You can create compressional waves on a 
slinky by laying the slinky in a straight line on the ground and then 
pushing your end of the slinky forward and back.  A wave of compressed 
slinky coils will travel down the length of the slinky.  A compressional 
wave propagates in the same direction as its molecules (or slinky coils in 
this case) are displaced (or moved).  You can create a transverse wave on a 
slinky by again laying the slinky in a straight line on the ground and then 
moving your end of the slinky from left to right.  Now the coils of the 
slinky are displaced from side to side, but the wave still propagates 
forward down the length of the slinky.  A great animation of compressional 
vs. transverse waves can be found at http://www.matter.org.uk/
schools/Content/Seismology/pandswaves.html as well 
as a nice illustration at http://www.exploratorium.edu/faultline/
earthquakescience/pswaves.html 

So, why don’t S-waves travel through liquids?  It’s because liquids don’t 
have any shear strength and so a shear wave cannot propagate through a 
liquid.  Think of a solid material, like a rock.  That rock is made up of 
atoms and molecules bound to each other.  When you set the rock down, its 
molecules and atoms stay in place and the rock holds it shape.  This is 
because those bonds between the atoms and molecules have a certain amount of 
shear strength and they can resist the pull of gravity and stay together.  
Now imagine that you have a glass of water.  If you put the glass of water 
on the table and then somehow remove the glass, the water will not hold its 
shape.  Instead it will flow away along the surface of the table.  This is 
because water (and most other liquids) do not have shear strength – there 
are no bonds holding the water molecules together that will resist a shear 
force. 

In your question, you mentioned other transverse waves that can travel 
through liquids and I’d like to address them both:  water waves and light 
waves.  Water waves, in fact are not transverse waves, they only appear to 
be transverse because the top surface of the water moves up and down while 
the wave propagates forward.  Actually, the water molecules are not moving 
strictly up and down.  Rather, the individual water molecules move in 
circles beneath the wave.  For a good illustration of this, check out:
  /cgi-bin/circR?/posts/
archives/aug98/899474171.Ph.r.html
So, water waves aren’t actually transverse waves because the individual 
water molecules do not move only perpendicular to the direction the wave is 
propagating.

As for light waves, you are correct in saying that they are transverse.  But 
now this is a mathematical description meaning that the amplitude of the 
waveform is perpendicular to the direction the wave is propagating.  (This 
is the same as saying the slinky coils are moving from side to side while 
the wave is propagating down the length of the slinky.)  Even though the 
light waves are transverse, they are not the same as P- and S-waves, which 
are seismic waves.  One big difference between seismic waves and light waves 
is the speed at which the wave moves.  Light waves travel at the speed of 
light, very much faster than seismic waves travel through the Earth.  Thus, 
light waves interact with materials differently than P- or S-waves and so it 
is possible for light waves to travel through liquids, such as water.