Lesson Video: Seismic Waves Physics

In this video, we will learn how to describe the similarities and differences between infrasonic S-waves and P-waves and how they travel through the Earth’s interior.


Video Transcript

In this video, we will be learning about the mechanisms that allow energy from earthquakes to be transferred from one place on Earth to another. This energy is carried along by what are known as seismic waves. And to understand seismic waves, we first need to learn about the structure of the inside of the Earth. Why this is the case, we will see shortly. But firstly, let’s take our good old Earth and slice it up, just for this video, so that we can look inside it.

So here’s a diagram showing what we would see if we were to remove a chunk of the Earth. We can see that the inside of the Earth has a layered structure and that there are four very distinct layers. So let’s start by looking at the very thin layer that makes up the surface of the Earth. This layer is known as the crust. And the crust is a very thin layer. It’s only about eight kilometres thick, and it’s a solid layer. So far, it’s the only layer of the Earth that humans have managed to directly access. In other words, humans live on the surface of the crust, and we’ve never managed to dig all the way through the crust into the layer beneath it.

However, by gathering data on seismic waves, scientists have been able to determine what the inside of the Earth should look like. And moving further towards the centre of the Earth, the layer beneath the crust is known as the mantle. Now, as we can see, the mantle is a very, very thick layer, much thicker than the crust and much thicker than any of the other layers beneath it as well. The mantle, just like the crust, is solid. However, the mantle is much closer to a liquid than the crust is. And what we mean by that is the fact that the mantle can flow very, very slowly. Whereas the crust is hard, rocky material that’s a lot more solid and does not allow for any soil flowing.

Now, scientists have been able to gather what the mantle is actually made of because this is the stuff that comes up in volcanic eruptions. It’s parts of the mantle coming all the way through the crust up to the surface of the Earth. And so, all the humans have not been able to dig down to the mantle. We’ve been able to gather lots of bits of mantle when they come up to the surface of the Earth through volcanoes. Anyway, so moving on from the mantle, we then see that there are two layers beneath it. Both of these layers have a very similar composition, and so, collectively, the two layers combined, they’re known as the Earth’s core. Specifically, the outer layer, the outer part of the core is known as the outer core. And the inner part of the core is known as the inner core.

Such imaginative names, right? But the reason that scientists group these two layers together into what they call the core of the Earth is because both of these layers are made up of iron and nickel predominantly. And this is what generates the magnetic field of the Earth. This is what allows us to navigate on the surface of the Earth by using a compass. And the magnetic field generated mainly by the Earth’s core is what protects us from lots of harmful radiation coming from outer space. Because the magnetic field directs all of that radiation away from where it could harm us. So if the entire core, the inner and the outer core, is made up of lots of iron and nickel. Then why do we bother separating them into two layers? Why don’t we just call it the core entirely?

Well, this is because the outer core is actually liquid, whereas the inner core is solid. And this is why although they have very similar compositions, we have to treat them as two very distinct layers because one is a liquid and one is a solid. And by the way, we may as well label on our diagram that the mantle and the crust are also both solid. But for the mantle, we have to specify that it can actually flow very, very slowly. And so, these are the four main layers forming the structure of the Earth. So let’s now imagine that we zoom into this diagram onto the crust and the upper layers of the mantle. When we do, we see that these layers, the crust and the mantle, aren’t quite as smooth as we thought they might be, based on the previous diagram.

In fact, what we’re seeing here and here is one bit of the crust, this bit here, being subducted or going underneath this part of the crust. And so, from this we can gather that the crust isn’t one whole piece. It’s broken up into lots of little sections known as tectonic plates. And so, we can say that this is one tectonic plate in our diagram and this is another. And this diagram is just showing one example of what can happen when tectonic plates interact with each other. In fact, these tectonic plates can move relative to each other. Remember, we said earlier that some of the mantle can flow very, very slowly. Well, the flowing of the mantle makes these tectonic plates move with respect to each other.

In other words, let’s say that some of the mantle over here is flowing in this direction. In that case, that’s going to cause our tectonic plate, which we’ve labelled plate number one, to move further underneath plate number two. In other cases, plates will move past each other, one sliding in one direction, the other sliding in the other direction. And in lots of these situations, this can lead to a buildup of energy. Because, essentially, what we’ve got is a massive chunk of rock scraping against another massive chunk of rock. And so, there’s a huge amount of friction there, causing a lot of pent-up energy. Especially, because, in this case, the first plate is being bent as it gets subducted underneath the second plate. And eventually, as these plates move around, that energy can get released.

So, for example, in this case, let’s say that our first plate subducts so far underneath the second plate. That, eventually, the second plate, which currently we can see is being bent like a springboard, is allowed to unbend very quickly to become more flat. Just like how a springboard with a diver on it will be released and bowing back into place as soon as the diver jumps off. And this process releases lots of energy, which has to go somewhere. And so, it travels through the Earth in the form of seismic waves. In other words, waves that travel through the structure of the Earth carrying that energy away from the point at which it was released. In other words, as a short recap, when tectonic plates move past each other, or, in this particular case, when one moves beneath the other, then a lot of energy gets built up as the plates scrape and snag on each other. And all of that energy is released when the plates finally manage to break loose from their snags and move past each other.

This sudden release of energy causes an earthquake. And all of that energy is carried away from the point at which it was released by waves known a seismic waves. Now, it’s worth noting that seismic waves can be caused by earthquakes. But they can also be caused by volcanic eruptions or even by big explosions caused by humans, such as nuclear weapons’ tests. Essentially, any waves that result in some sort of wobbling of either the Earth’s crust or the mantle are seismic waves that are carrying energy through the Earth. And interestingly, there are two different kinds of seismic wave. So let’s take a look at what the two different kinds of seismic waves are.

The two different kinds of seismic waves are known as primary waves, or P-waves, and secondary waves, or S-waves. So first things first, why are they called primary and secondary waves? Well, let’s recall our internal structure of the Earth once again. And let’s now imagine that the tectonic plates moving past each other has resulted in a release of energy at this point here. In other words, an earthquake is occurring, and seismic waves are going to be released. Now, let’s also imagine that on the surface of the Earth, let’s say here, we’ve placed a set of detectors. These detectors detect vibrations in the surface of the Earth. Which means that as soon as a seismic wave arrives at the detector, the wave will cause the surface of the Earth to wobble and shake, which the detectors will be able to detect.

Now, coming back to the point at which all of this energy is released once again, we should know that this energy will be carried away from the point by both primary waves and secondary waves. And naturally, these waves will propagate away from that point as soon as all other energy is released. However, primary waves, which we’ll label in green here, will get to a detector faster than the secondary waves will, which we’ll label in orange. In other words, primary waves travel faster. And so, regardless of where on Earth our detector is placed, if there’s an energy release at some point in time over here. Then the primary waves will get to our detector before the secondary waves will. And that’s why primary waves are known as primary waves. They’re the first seismic waves to arrive at any detector because they travel faster than secondary waves.

So let’s now draw in a few more of the seismic waves travelling away from the point at which the energy was released. Having drawn this diagram, there’s a couple of things we can notice about both primary and secondary waves. Firstly, we can see that they moved in a sort of curved motion. In other words, they don’t move in straight lines through the Earth. Now, this actually happens because of the gradual change in composition of all of the layers of the Earth, depending on exactly how far into the Earth we are. In other words, even though everything we’ve labelled in pink is the mantle, the composition of the mantle nearer to the crust is going to be different to the composition of the mantle nearer to the core. And this change in composition occurs gradually as we get deeper and deeper. And it’s because of this gradual change in composition that seismic waves don’t actually travel in straight lines. They take curved paths.

Now, the second thing that we can notice is that if we look over here where we placed our first detector, then we see both primary waves in green and secondary waves in orange being picked up by the detector. And the same is true here, here, and here. However, if we were to place detectors on the Earth, for example here and here, then we would only detect primary waves, the green waves. We don’t see any orange waves there. And, specifically, we can see that the orange waves cannot actually travel through the core of the Earth. As soon as they get to the core, they stop propagating. And there’s a very specific reason for this. The reason is that primary waves are longitudinal waves whereas secondary waves are transverse waves. Let’s recall firstly that a longitudinal wave is one for which the medium in which the wave is moving moves back and forth in a direction parallel to which the wave itself is moving.

In other words, if we imagine that these are the atoms that make up one small part of the mantle and if a longitudinal wave comes along in this direction. Then the atoms themselves will move back and forth as the wave moves this way. For example, at some point in time, instead of these evenly spaced atoms. We might see some regions clumped together and then some regions where there’s not very many atoms. And then some regions where the atoms are clumped together again. That’s due to the backward and forward motion of these atoms as the wave moves from left to right. And that is what a longitudinal wave is. However, if we imagine the same particles in the mantle, and this time a transverse wave comes along in this direction. Then the particles themselves, the atoms, actually move up and down. They move in a direction perpendicular to or at right angles to the direction in which the wave is moving.

In other words, as the wave moves left to right, we see that these atoms are moving up and down, resulting in a wave that looks something like this. That’s a transverse wave. But what does the fact that P-waves are longitudinal, and S-waves are transverse have to do with the fact that S-waves cannot travel through the Earth’s core. Well, remember, we said earlier that the Earth’s outer core is liquid. Well, let’s imagine what happens now if we’ve got a longitudinal wave moving through the mantle. But then, we’ve got a boundary between the mantle and the liquid outer core. Well, in that situation, as the wave propagates from left to right, the liquid particles can also move back and forth and therefore allow the wave to propagate through it. In other words, the wave can not only move to the mantle, but it can also move through the outer core. And this is why the P-waves, the longitudinal waves, can be detected anywhere on the Earth’s surface.

However, if we now imagine a transverse wave moving from left to right, passing through the mantle, and arriving at the boundary between the mantle and the outer core. Then what we’ll realise is that because liquid particles are not so strongly bonded to each other as solid particles are, the transverse wave will not be able to move through it. Because the whole idea behind a transverse wave is that if we’ve got these particles initially stationary, then the upward motion of one particle kicks this second particle into an upward motion as well. And the reason for this is because there is a strong bond between all of these solid particles. However, there is no such strong bond between liquid particles. So even if one liquid particle is forced to move up due to a wave coming in this direction, there is no bond here between the first black particle and the second one to cause the second particle to move upward.

All that to say that longitudinal waves can travel through a liquid whereas transverse waves cannot travel through a liquid. And therefore P-waves will be detected all over the surface of the Earth whenever some event causes seismic waves to be released. However, the secondary transverse waves will only be detected in some locations and not in other locations. Because they cannot travel through the liquid outer core of the Earth. And in fact, this is how scientists determine how large the outer core actually is. Because if we were to take this detector here and move it along the surface of the Earth. Eventually, the detector would get to a point where it can no longer detect any secondary waves. Because, remember, secondary waves can travel all the way through the mantle at any point. But as soon as they hit the outer core of the Earth, they will not be able to travel. And this data allows us to gauge exactly how large the outer core of the Earth is.

But anyway, so coming back to our primary or P-waves and secondary or S-waves. We’ve seen that primary waves are longitudinal whereas secondary waves are transverse. Which means that the longitudinal primary waves can travel through both solids and liquids. Whereas the transverse secondary waves can only travel through solids not through liquids. And lastly, we can say from what we saw earlier that primary waves, or P-waves, travel faster than S-waves and, of course, vice versa. S-waves travel slower than P-waves, which is why they’re given the labels primary and secondary. When seismic waves are released from a point, a detector placed at any other point will first detect primary waves and then secondary waves. Assuming, of course, that the detector is placed somewhere such that secondary waves can actually get to it. Because, remember, they can’t travel through the outer core of the Earth.

But anyway, so now that we’ve learnt about the inner structure of the Earth, as well as what P- and S-waves are, let’s take a look at an example question.

Which of the following statements about P-waves is not correct? A) P-waves travel through liquids. B) P-waves travel through solids. C) P-waves are slower than S-waves. D) P-waves are longitudinal.

Now, this final statement, the fact that P-waves are longitudinal, is something that we should recall about P-waves. They are actually longitudinal waves. But, in this question, we’re looking for a statement about P-waves that is not correct. And, therefore, because this statement is correct regarding P-waves, that cannot be the answer to our question. So immediately, just by recalling the nature of P-waves, we’ve eliminated one of the four options. Now, let’s look in a little bit more detail at the consequences of the fact that P-waves are longitudinal waves. Let’s start by imagining that we’ve got a boundary between some solid object and some liquid object.

Let’s imagine, first of all, that these pink dots represent the atoms making up the solid object. They are very nicely ordered because they’re the atoms making up a solid object. And let’s also imagine that these black dots represent the atoms making up a liquid object. Now, in a liquid, atoms are a lot more free to flow around each other and have a lot more unrestricted movement. But for simplicity’s sake, we’ve drawn them to be equally as ordered as the particles in the solid here. But anyway, let’s now imagine that a longitudinal wave comes along in this direction. And let’s also remember that a longitudinal wave is the kind of wave that propagates because the medium through which the wave is moving will oscillate back and forth in the direction parallel to which the wave itself is moving.

In other words, the particles in the solid — which can slightly jiggle backwards and forwards because although they can’t move freely, they can at least move a little bit — will allow a longitudinal wave to pass through the solid. Because as the particles jiggle backwards and forwards, the wave moves from left to right. And, actually, the same is true for liquids. Because even though these liquid particles are free to move in whatever direction they want and they’re not restricted as to where they can go. The fact of the matter is that these particles within the liquid can still move back and forth as the wave moves, in this case, from left to right. And one particle moving forward will bump into the next particle, causing that want to move forward and causing the first one to lose energy. And then lots of collisions between lots of particles in the liquid will allow a longitudinal wave to pass through the liquid.

And so, as we’ve seen, longitudinal waves can pass through both solids and liquids. Therefore, looking at statements A and B — which say that P-waves, which are longitudinal, travel through liquids and through solids — well, these statements are correct statements about P-waves. Whereas we’re looking for a statement that is not correct. And, hence, we can eliminate these options as well, which only leaves us with option C. This one says that P-waves are slower than S-waves. However, we should recall that P-waves are called P-waves because the P actually stands for primary. And this refers to the fact that when seismic waves are released, the first kind of wave to be detected anywhere is the P-wave, the primary wave. And then sometime later, the secondary or S-waves come along.

And this means that P-waves are actually faster than S-waves, which means that we’ve found the incorrect statement about P-waves. That incorrect statement is that P-waves are slower than S-waves. Because they, actually, are faster than S-waves.

So now that we’ve taken a look at an example question, let’s quickly summarise what we’ve learned in this lesson.

We started off by seeing that the inside of the Earth is divided up into layers. The outermost layer is known as the crust. Then the next one down is known as the mantle. And then is the core of the Earth, divided up into the outer core and the inner core. And then we learned about two different kinds of seismic wave. Firstly, P-waves or primary waves are waves which travel fast. And they are longitudinal waves, which means that they can travel through both solids and liquids. And then there’s S-waves or secondary waves, which are slower than P-waves. And they are transverse waves, so they travel through solids only.

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