Seismic Waves: On Exploring Earth's Interior

Summary

This educational video script delves into the use of seismic waves for exploring Earth's interior. It explains the difference between the focus and epicenter of an earthquake, and how geophysicists use the speed of seismic waves to infer the density and composition of Earth's layers. The script details the properties of P and S waves, their interaction with Earth's layers, and how their travel times help determine earthquake locations. It also discusses how seismic waves refract and reflect within the Earth, and the significance of different wave paths like PKP and PP in understanding Earth's structure.

Takeaways

• 🌏 Seismic waves are crucial for exploring Earth's interior, helping us understand its composition and structure.
• 📍 The focus is the point within Earth where an earthquake originates, while the epicenter is the point on the surface directly above it.
• 🌊 Seismic waves propagate in all directions from the focus, with their speed influenced by the density of the materials they travel through.
• 🔍 Geophysicists use the speed of seismic waves to infer the properties of Earth's interior, including density and composition.
• 📊 Seismographs record seismic activity, with P waves (compressional) arriving first, followed by S waves (shearing), and finally surface waves.
• 🏗 Surface waves, although the slowest, are the most damaging during earthquakes due to their extensive ground shaking.
• 🔄 P waves cause particles to move back and forth in the direction of wave propagation, similar to a slinky's compression and dilation.
• 📉 S waves involve side-to-side particle movement, perpendicular to the direction of wave propagation, and cannot travel through liquids.
• 📈 The velocity of P and S waves changes with depth due to variations in density, with significant changes at the mantle's transitions.
• 🔮 Geophysicists analyze wave paths, such as PP, PCP, and PKP waves, to understand how seismic waves travel through Earth's layers.
• 🌐 Snell's law explains how seismic waves refract as they move through materials of different densities, leading to curved paths within Earth.

Q & A

What is the focus of an earthquake?

The focus of an earthquake is the location where the earthquake originates below the Earth's surface.

What is the difference between the focus and the epicenter of an earthquake?

The focus is the point within the Earth where the earthquake originates, while the epicenter is the point on the Earth's surface directly above the focus.

How do geophysicists use seismic waves to explore Earth's interior?

Geoscientists use seismic waves by measuring their speed and direction to infer the density and composition of the Earth's interior.

What is a seismograph and what does it measure?

A seismograph is an instrument that measures seismic activity, specifically the shaking of the ground caused by earthquakes.

What are the three main types of seismic waves and in what order do they typically arrive at a seismic station?

The three main types of seismic waves are P waves, S waves, and surface waves. They typically arrive in the order of P waves first, followed by S waves, and then surface waves.

Why are surface waves significant in terms of earthquake damage?

Surface waves are significant for earthquake damage because they cause the most ground shaking and can be the primary cause of damage to buildings.

How does the motion of a P wave differ from that of an S wave?

P waves are compressional waves that move particles back and forth in the direction of wave propagation, while S waves are shearing waves that move particles perpendicular to the direction of propagation.

What causes the abrupt changes in seismic wave velocity at certain depths within the Earth?

Abrupt changes in seismic wave velocity at certain depths are caused by changes in the Earth's composition, which leads to a sudden increase in density, rather than a gradual increase due to pressure.

Why do seismic waves curve as they travel through the Earth?

Seismic waves curve as they travel through the Earth due to refraction, which occurs when waves encounter changes in density or material boundaries, bending towards or away from the perpendicular based on Snell's law.

How do geophysicists determine the location of an earthquake using seismic waves?

Geoscientists determine the location of an earthquake by analyzing the difference in arrival times between P waves and S waves, which is a function of the distance from the seismic station to the earthquake.

What is the significance of the PKP wave in studying the Earth's interior?

The PKP wave is significant because it travels through the Earth's core, providing information about the core's density and composition, which helps scientists understand the Earth's interior structure.

Outlines

00:00

🌏 Seismic Waves and Earth's Interior Exploration

This paragraph introduces the use of seismic waves to explore the Earth's interior. It explains the difference between the focus and epicenter of an earthquake and how geophysicists use the speed of seismic waves, which is dependent on the density of the material they travel through, to learn about the Earth's interior. The paragraph also discusses how geophysicists create artificial seismic waves to study the Earth's surface layers. A seismograph is described as a tool that measures seismic activity, with a focus on P waves, S waves, and surface waves, each with different speeds and effects on the Earth's surface. The paragraph concludes with an explanation of how P waves are compressional waves that cause particles to move forward and backward, and how S waves are shearing waves that move particles perpendicular to the direction of propagation.

05:03

🌋 Understanding Seismic Wave Paths and Refraction

This paragraph delves into how seismic waves refract and reflect within the Earth, altering their paths due to changes in density and encountering boundaries. It describes the simple and complex pathways of P and S waves, including primary waves and those that interact with the Earth's core, such as PCP and PKP waves. The concept of Snell's law is introduced to explain how the angle of refraction is influenced by density differences, leading to the curving paths of earthquake waves. The paragraph also discusses how scientists use the travel times of these waves to determine the location of an earthquake by analyzing the difference between the arrival times of P and S waves at various seismic stations.

10:08

📈 Calculating Earth's Density and Composition Through Seismic Wave Analysis

The final paragraph summarizes the importance of seismic wave travel times in determining the density of the Earth's interior. It emphasizes that by knowing the travel time and distance of seismic waves, geophysicists can calculate the density of the materials through which the waves pass. This information is crucial for understanding the composition and structure of the Earth's interior. The paragraph also touches on the concept that the first arrival of seismic waves at distant stations is often a P wave that has traveled through the Earth's core, highlighting the significance of these observations in seismological studies.

Keywords

💡Seismic Waves

Seismic waves are vibrations that travel through the Earth's layers, typically caused by earthquakes or other geological events. They are crucial for understanding the Earth's interior as they can reveal information about the composition and structure of the Earth's layers. In the script, seismic waves are discussed in the context of how they are used to explore the Earth's interior, with different types of waves, such as P and S waves, providing different insights.

💡Focus

The focus is the point within the Earth where an earthquake originates. It is a key concept in seismology as it helps scientists pinpoint the source of seismic activity. The script mentions that seismic waves emanate from the focus and move in all directions, which is essential for understanding how these waves can be used to study the Earth's interior.

💡Epicenter

The epicenter is the point on the Earth's surface directly above the focus of an earthquake. It is an important term in the study of earthquakes as it helps to determine the location of the seismic activity. In the script, the epicenter is used to distinguish between the point of origin (focus) and the point on the surface where the effects are most felt.

💡P Waves

P waves, or primary waves, are the first type of seismic wave to arrive at a seismic station after an earthquake. They are compressional waves that can travel through both solid and liquid, making them the fastest type of seismic wave. The script explains that P waves are important for determining the distance to an earthquake's epicenter because they are the first to arrive.

💡S Waves

S waves, or secondary waves, are shearing waves that follow P waves and are slower. They can only travel through solids, not liquids, which is why they provide information about the Earth's solid layers. The script discusses how S waves are used to understand the Earth's interior and how their velocity changes with depth.

💡Surface Waves

Surface waves are seismic waves that travel along the Earth's surface and are the slowest of the seismic waves. They are responsible for much of the damage caused by earthquakes due to their intense shaking. The script highlights the importance of surface waves in causing ground shaking and damage to buildings.

💡Seismograph

A seismograph is an instrument used to measure and record seismic waves. It is essential for studying earthquakes and the Earth's interior. The script describes how a seismograph measures the shaking of the ground caused by earthquakes and how it records the arrival of different types of seismic waves.

💡Density

Density refers to the mass per unit volume of a material. In the context of the script, density is a critical factor affecting the speed of seismic waves as they travel through the Earth. The denser the material, the faster the waves can move. This property is used by geophysicists to infer the composition of the Earth's interior based on the speed of seismic waves.

💡Refraction

Refraction is the bending of seismic waves as they pass from one medium to another with a different density. It is governed by Snell's law, which relates the angle of incidence and refraction to the densities of the two media. The script explains how refraction causes seismic waves to follow curved paths as they travel through the Earth's layers with varying densities.

💡Travel Time

Travel time refers to the time it takes for a seismic wave to travel from its source to a recording station. It is a crucial parameter in seismology for determining the distance to an earthquake and for studying the Earth's interior. The script discusses how the difference in travel times between P and S waves can be used to calculate the distance from a seismic station to an earthquake's epicenter.

💡Velocity

Velocity is the speed at which seismic waves travel through different materials. The script mentions that velocity varies with depth in the Earth and is affected by the density of the material. Understanding these variations in velocity is essential for interpreting seismograph readings and for exploring the Earth's interior.

Highlights

Seismic waves are used to explore Earth's interior.

An earthquake's point of origin is called the focus, and the point directly above it on the surface is the epicenter.

Waves from an earthquake move in all directions through the Earth's layers at varying speeds based on material density.

Geophysicists use the speed of earthquake waves to learn about Earth's interior structure.

Artificial waves created by vibrating the ground can reflect off underground surfaces, revealing information about Earth's layers.

Seismographs measure seismic activity, including ground shaking caused by earthquakes.

P waves are the first to arrive at a seismograph, being compressional waves that move quickly through solids.

S waves follow P waves and are shearing waves that move at a slower speed through solids.

Surface waves are the slowest and cause significant ground shaking, often leading to earthquake damage.

P and S waves provide insights into Earth's interior, with P waves causing compression and dilation in rock particles.

S waves are shearing waves with particle motion perpendicular to the direction of wave propagation.

Seismic velocity varies with depth due to changes in material density within the Earth.

There are abrupt changes in wave velocity at specific depths, indicating compositional changes in the mantle.

At the core, S wave velocity drops to zero, and P wave velocity slows due to the transition from solid to liquid.

Waves refract and reflect within the Earth, altering their paths based on density changes and boundaries.

Seismic waves can travel through the core (PKP waves) or reflect off the core (PCP waves), affecting their arrival times.

The curvature of wave paths is explained by Snell's law, which relates refraction to density differences.

Scientists use the travel times of P and S waves to determine the distance of seismic stations from an earthquake's epicenter.

The first arrival of waves at distant stations can be P waves that have traversed the Earth's core.

Geophysicists calculate Earth's interior density by analyzing wave travel times and paths.

Transcripts

I've added punctuation and capitalization  to the text without changing the words:

Following up on our activity with seismic waves,  we're going to talk about how seismic waves are  

used to explore Earth's interior. Now, just  as a reminder to get all on the same page with  

terminology: an earthquake happens somewhere below  the surface, and the place that the earthquake  

happens is called the focus. Just above it on  the Earth's surface is the epicenter. Waves,  

of course, emanate from the focus,  and they move in all directions.

Now, a wave moving through something solid will  move at a speed that is a function of the density  

of the material that it is moving through. So,  geophysicists can exploit this property of waves  

to learn something about the Earth's interior  by measuring earthquake waves. By the way,  

geophysicists also create their own ways traveling  through the earth and use those to explore the  

Earth's interior, like this diagram shows. In  this case, they're using a vibrator to pound the  

ground and send waves down, and those waves will  reflect off of surfaces underneath and back up,  

and their travel time is measured, and that tells  you something about the surface layers of the  

earth. What lives at reasonable distances below,  and this is used for exploration all the time.  

We'll talk about that a little bit later.  Meanwhile, let's get back to earthquakes.

A seismograph measures seismic activity.  It measures the shaking of the ground  

caused by almost anything, but in this  case, we're talking about earthquakes,  

and in this case, the time starts at the  left and moves to the right. At first,  

we just have background with wiggles, and then  an earthquake wave arrives. The first wave to  

arrive is called a P wave. It's a compressional  wave and it moves pretty quickly through a solid  

medium. Then there is a secondary wave called  an S wave, and that one is a shearing wave  

and it moves at a reasonable speed also  through a solid medium. And then finally,  

surface waves will arrive. Surface waves are the  slowest to get there, and as the name implies,  

they move along the surface. Surface waves are  really important for earthquake damage. They  

tend to cause a lot of ground shaking and can  be the primary cause of damage to buildings.

P and S waves tell us something about the  interior of the Earth, so let's look at both  

of them a little bit more closely. P waves are  compressional waves, and if you are wanting to  

imagine yourself as a small bit of rock as the P  wave passes, that little bit of rock is squeezed  

and then it's stretched. A slinky is a really  nice example of this. If you pull and release  

a slinky from one end while the other is held  in place, you'll see the individual metal bands  

move closer together and farther apart as the wave  passes, and that's exactly what happens in rock.  

There is compression and there is dilation, and  individual particles move forward and backward,  

squeezing and stretching as the wave passes. This  is another diagram showing the same thing. The  

direction of propagation is along the length of  the block, and the individual particles also move  

parallel to the length of the block, squeezing  and stretching with the material around them.

Now, as you might imagine, if wave propagation  requires being able to push against and stretch  

out from a little bit of rock next to you, then  the more dense the material, the faster this is  

going to be able to happen. If the pieces of  rock were quite spread apart, it would move  

pretty slowly, but when they're crushed closely  together, the wave can move pretty quickly.

An S wave is a shearing wave, and the motion is  perpendicular to the direction of propagation. So,  

in this case, the direction of propagation  is still along the length of the block,  

but the individual particles are moving up  and down. Seismic velocity varies with depth,  

and you've seen this diagram before as part of  your exercise. Let's look at it again a little  

more closely and notice a couple of things.  Within the mantle, both the S wave and the P  

wave velocity increases with depth, and that's  because both of them increase with density,  

and so you can see that clearly in the mantle.  But look a little bit closer at the top. There  

are two transitions where the philosophy  changes abruptly at 410 kilometers depth  

and 660 kilometers depth. This is where the  composition in the upper mantle is changing,  

and that causes the density to increase suddenly,  rather than gradually as increasing pressure would  

do. There are, of course, other transitions at  the outer core, and again at the inner core,  

where you go from a solid to a liquid, and  therefore the S wave velocity drops to zero,  

and the P wave velocity slows considerably  from the solid mantle to the liquid outer core.

Now, remember from physics that waves will  both refract, which means they'll bend, and  

they will reflect when they encounter changes in  density or boundaries in a system. And, of course,  

they do this within the Earth, and we're going to  take a look at exactly how to see what pathways  

earthquakes waves travel. So, in this case,  we're looking at an earthquake at the little star  

somewhat below the surface. And if you look on the  right-hand side of the diagram, this diagram shows  

the primary P waves and S waves. And there are  very simple patterns in this case. They're showing  

you two waves that have gone down and have been  bent back towards the surface and are registered  

at a seismic station at the end of the arrows.  And of course, the surface waves are shown on  

this side too. These are the simplest wave paths  that there are, but there are many other ways that  

happen, and on the left-hand side, it's showing  you some of the other more complicated wave paths.  

For example, the very first one is a P-P wave.  This one from the earthquake, the wave emanated  

up towards the surface, bounced off the surface,  went back down again, and then emerged at the  

location of the P-P. So it was a P wave the whole  way. There's also an S wave shown there. But let's  

take a look at some of these ones that get down  to the core. This wave traveled down, bounced off  

the core, reflected off the core, and came back up  to the surface. And we call that one a PCP wave,  

with a lowercase P, because it was a P wave  (compressional wave) in both of its pathways. That  

wave could also have hit the core and refracted  and traveled through the core to the other side  

and then refracted again. The density changes  abruptly and come back up to the surface way  

over here, and we call that a PKP, the K meaning  it traveled through the core. Scientists use these  

designations to look at a seismograph recording  of an earthquake and tell what its pathway was.

Let's talk about why these paths curve. You  might remember from physics class Snell's law,  

which says that as a wave passes from  one material to another, it will refract,  

and that angle of refraction is a function  of the difference in density between the two  

materials. When the wave is traveling from  material one to a more dense material two,  

the refraction is away from the perpendicular,  like it's shown here. And of course,  

this is how the earth works. The layers are more  dense as you go deeper, and so the refraction is  

always towards a shallower angle as you  move towards more dense materials in the

Earth, now, as you go through layer upon layer  upon layer or even just gradual change to more and  

more dense material, the refraction will continue  to move towards shallower and shallower angles,  

and that creates, in the long run,  a curved path for earthquake waves.

So, back to a cross-section of the Earth. You've  seen this before; also, this is from IRIS, and  

it's showing you the different wave paths and a  seismograph, or actually a bunch of seismographs,  

measuring this particular earthquake at various  places around the Earth. The way that you look at  

these, this stacked seismograph pattern, is by  picking out any one location. So, for example,  

this seismograph that is 90 degrees away from  the earthquake epicenter, and at this particular  

location, the P wave arrives first, and then a  P P wave, one that has bounced off the surface  

and returned as a P wave, and finally, the  third major arrival is an S wave. The P waves  

are generally the first to arrive, especially  in shorter distances from the earthquake. The  

S waves arrive later, and take a look at the  difference between these two; the P wave and  

S waves have different travel times. The P wave  is faster than the S wave in all conditions,  

and as a consequence, the difference between the  P wave arrival time and the S wave arrival time  

is a function of how far this recording station is  from the earthquake. It's one of the primary ways  

that scientists determine where an earthquake  happened; they can look at the difference in  

arrival time and figure out how far away that  particular seismic station was from an earthquake.

There is another wave that is the first  arrival, and look down here at the bottom,  

the peak AP wave. There is an area in the  Earth far away from the earthquake epicenter  

where the P wave has to travel through  the core in order to get there, and so  

the peak AP P wave is the first wave to arrive  at far-distant earthquake recording stations.

So, in summary, travel time of a wave through  a solid medium is a function of its density,  

and since we know time, and since we  know distance, we can calculate density,  

and geophysicists use this to figure out what the  interior of the Earth looks like and is made of.