Why Can't Transverse Waves Go Through Liquid?(유체에서 파동의 성질)

At the start of this year, I learned that scientists(geologists) were able to find out the Earth has a liquid core by using two body waves: P-waves and S-waves. These two waves are different in characteristics. There are two types of waves. One is a transverse wave, like the S-wave, and the other is called a longitudinal wave(P-wave).

 

A transverse wave is a type of wave that is known to a lot of people. In the case of a transverse wave, the direction of the wave is perpendicular to the direction of the movement of the medium. Examples of this kind of wave are ripples on the surface of water, vibration of a string instrument, seismic S-waves, and electromagnetic waves such as light waves, microwaves, and radio waves, As a result, the wave looks like this:

 

https://cdn.kastatic.org/ka-perseus-images/bdee68f794185f6348574d22bc68fc0fcfa6bb7c.png

On the other hand, longitudinal waves move differently. When a longitudinal wave moves, the medium's direction of movement is parallel to the one of the wave. Example of this kind of wave are sound waves, ultrasound waves, and seismic P-waves. In this case, the wave will look like this:

 

http://ffden-2.phys.uaf.edu/webproj/212_spring_2015/Dylan_Sanders/dylan_sanders/longitudinalwave.jpg

Then, why can't transverse waves(seismic S-wave) go through liquid(and gas)? The answer is the shear stress of the medium the wave is traveling.

 

If the direction of the wave is towards the right, then the medium will have to travel up and down. In order for the wave to be continued, the medium has to come back down(or up) after it is moved upwards(or downwards) by the wave. The type of force that enables these situations to happen is shear stress. However, liquids and gases don't have shear stress. Imagine a tank full of water, and you wave your hand very slowly in it,. Would the water particles that were removed from their earlier position try to return to it? Probably not. However, if liquid such as water had shear force, it would have. Because liquid and gas don't have shear stress, the moved medium won't return to it's previous position, therefore failing to continue the transvers wave. The same is the case for seismic S-waves.

 

Here I had another question: then how come longitudinal waves can travel trough liguid and gas, that lack shear stress? How is it possible? In the case of longitudinal waves. the direction of the medium is parallel to the direction of the wave. Therefore, even when the medium lacks shear force, the energy of the wave can move the medium and propel itself forward. The only difference from when it moves in a solid medium is that it becomes slower.

 

In conclusion, transverse waves such as the seismic S-wave can't travel through liquid or gas because the medium lacks shear force, and cannot continue the wave.