Video: Differentiating between P-Waves and S-Waves

The houses (a) and (b) shown in the diagram are each being vibrated by seismic waves. The waves are traveling perpendicularly to the Earth’s surface where the houses stand. Which house is being vibrated by a P-wave? Which house is being vibrated by an S-wave?

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Video Transcript

The houses a and b shown in the diagram are each being vibrated by seismic waves. The waves are traveling perpendicularly to the Earth’s surface where the houses stand. Which house is being vibrated by a P-wave? Which house is being vibrated by an S-wave?

Okay, so in this question, we’ve been given two different diagrams, one of a house a and the other of a house b. We’re being told that each one of these houses is being vibrated by seismic waves. These are the waves that travel through the Earth and cause earthquakes and tsunamis. Additionally, we’ve been told that the waves are travelling perpendicularly to the Earth’s surface where the houses stand.

Now, because seismic waves travel through the Earth, essentially, what’s happening is that, in each case, this is the surface where the houses stand. And because the waves are travelling perpendicular to the surface, they are moving in this direction, from bottom to top in the diagram in both cases. However, one of the houses is being vibrated by a P-wave. And the other is being vibrated by an S-wave. And we can see that this is resulting in slightly different vibrations for each house.

For example, for house a, from the diagram, we can see that it’s shaking side to side. This is what the diagram is trying to imply to us with these slightly less intensely coloured parts, showing us that the house is moving from left to right. However, for house b, we can see that it’s being vibrated up and down. That’s what the diagram is trying to tell us here.

And so here’s a simplification of what’s going on. If this is the surface on which house a is standing, then a seismic wave is moving through the Earth toward the surface of the Earth. And as it reaches the surface, it results in the house vibrating from left to right. However, if this is the surface on which house b is standing, then the seismic wave which is vibrating it is once again moving through the Earth toward the surface and resulting in the house vibrating in this direction, up and down.

Now, just to confirm, the reason that we know that the seismic waves are indeed travelling in this direction in both cases is because, once again, we’ve been told that they’re travelling perpendicularly to the Earth’s surface. And so, in other words, they’re travelling at 90 degrees to the Earth’s surface in both cases. The difference between the two waves though is that one is a P-wave. And the other is an S-wave. And we can tell these two apart by the direction of oscillation of each of the houses. Because the house that’s moving left to right, as a result of the wave hitting the surface of the Earth, is oscillating in a direction perpendicular to the direction in which the wave is travelling.

So in other words, we’re not fussed about the surface of the Earth anymore. What we’re looking for is the relationship between the direction in which the wave is travelling and the direction in which the house is oscillating. Whereas for house b, we see that the wave is travelling in this direction. And the house is oscillating in a direction parallel to this wave motion. This means that, in one case, we’re dealing with a transverse wave. And in the other, we’re dealing with a longitudinal wave. The wave that’s vibrating house a is transverse because the medium through which the wave itself is travelling is oscillating perpendicular to the direction of the waves travel.

In this case, the medium that we’re talking about is either the Earth itself, which is below the surface, or the house which is above the surface. And so if we raise the surface of the Earth slightly higher just to make the diagram clearer, what’s happening is that our transverse wave is coming in like this. So the medium, in this case, the Earth, is oscillating left to right as the wave travels from bottom to top. And that’s what’s resulting in the house on the surface of the Earth oscillating left to right.

However, in the case of house b, what we see is that the wave is travelling in this direction. But the medium through which it’s travelling, so that’s the Earth, is actually oscillating upward and downward. Each little part of the Earth is oscillating up and down and up and down. And the wave itself overall moves from bottom to top. And we can essentially just imagine this as the surface of the Earth bouncing up and down, which then causes the house on the surface of the Earth to oscillate up and down.

And so at this point, we’ve seen the difference between a transverse and a longitudinal wave. In a transverse wave, the medium oscillates in a direction that’s perpendicular to the direction in which the wave is moving. Whereas in a longitudinal wave, the medium oscillates in a direction parallel to the direction in which the wave is going.

Now, at this point, we can recall the difference between P-waves and S-waves. Both of these are kinds of seismic waves. But we can recall that P-waves are longitudinal waves whereas S-waves are transverse waves. And this means that, in both cases, for both houses a and b, as the seismic waves move from bottom to top towards the Earth’s surface perpendicular to the surface, the house that oscillates up and down is being hit by a P-wave because it’s longitudinal. And that is house b. Whereas the house that oscillates from left to right is being hit by an S-wave because that’s a transverse wave. And that is house a.

Therefore, at this point, we’ve answered both our questions. The house being vibrated by a P-wave is house b. And the house being vibrated by an S-wave is house a.

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