# Seismic Waves

## Discover how scientists learn about Earth's interior using the different properties of different types of seismic waves.

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Seismic Waves

License: CC BY-NC 3.0

[Figure1]

Are seismic waves like ocean waves?

Yes, in some ways. Ocean waves travel at the interface between the sea surface and the atmosphere. They have all the features that all waves have. Some seismic waves also travel at an interface. Which ones?

### Waves

Energy is transmitted in waves. Every wave has a high point called a crest and a low point called a trough. The height of a wave from the center line to its crest is its amplitude. The distance between waves from crest to crest (or trough to trough) is its wavelength. The parts of a wave are illustrated in Figure below.

The crest, trough, and amplitude are illustrated in this diagram.

### Earthquake Waves

The energy from earthquakes travels in waves. The study of seismic waves is known as seismology. Seismologists use seismic waves to learn about earthquakes and also to learn about the Earth’s interior.

One ingenious way scientists learn about Earth’s interior is by looking at earthquake waves. Seismic waves travel outward in all directions from where the ground breaks and are picked up by seismographs around the world. Two types of seismic waves are most useful for learning about Earth’s interior.

#### Body Waves

P-waves and S-waves are known as body waves because they move through the solid body of the Earth. P-waves travel through solids, liquids, and gases. S-waves only move through solids (Figure below). Surface waves only travel along Earth's surface. In an earthquake, body waves produce sharp jolts. They do not do as much damage as surface waves.

• P-waves (primary waves) are fastest, traveling at about 6 to 7 kilometers (about 4 miles) per second, so they arrive first at the seismometer. P-waves move in a compression/expansion type motion, squeezing and unsqueezing Earth materials as they travel. This produces a change in volume for the material. P-waves bend slightly when they travel from one layer into another. Seismic waves move faster through denser or more rigid material. As P-waves encounter the liquid outer core, which is less rigid than the mantle, they slow down. This makes the P-waves arrive later and further away than would be expected. The result is a P-wave shadow zone. No P-waves are picked up at seismographs 104o to 140o from the earthquakes focus.

License: CC BY-NC 3.0

[Figure2]
• S-waves (secondary waves) are about half as fast as P-waves, traveling at about 3.5 km (2 miles) per second, and arrive second at seismographs. S-waves move in an up and down motion perpendicular to the direction of wave travel. This produces a change in shape for the Earth materials they move through. Only solids resist a change in shape, so S-waves are only able to propagate through solids. S-waves cannot travel through liquid.

#### Earth's Interior

By tracking seismic waves, scientists have learned what makes up the planet’s interior.

• P-waves slow down at the mantle core boundary, so we know the outer core is less rigid than the mantle.
• S-waves disappear at the mantle core boundary, so we know the outer core is liquid.

License: CC BY-NC 3.0

[Figure3]

The graph above shows how P and S waves move through the interior of the Earth.

Surface Waves

Interesting earthquake videos are seen at National Geographic Videos, Environment Video, Natural Disasters, Earthquakes: http://video.nationalgeographic.com/video/player/environment/. Titles include:Surface waves travel along the ground, outward from an earthquake’s epicenter. Surface waves are the slowest of all seismic waves, traveling at 2.5 km (1.5 miles) per second. There are two types of surface waves. The rolling motions of surface waves do most of the damage in an earthquake.

• “Earthquake 101.”
• “Inside Earthquakes” looks at this sudden natural disaster.

### Summary

• P-waves arrive first to a seismograph because they are faster. They travel through solids, liquids, and gases.
• S-waves arrive second to a seismograph, and they only travel through solids.
• The behavior of P- and S-waves indicates that the outer core is liquid.

### Practice

Use this resource to answer the questions that follow.

1. What types of waves do earthquakes produce?

2. What are the fastest body waves?

3. What is the shadow zone?

4. What do S-waves do?

5. List and explain the two types of surface waves.

### Review

1. What are the properties of P-waves?

2. What are the properties of S-waves?

3. How do scientists use seismic waves to learn about Earth's interior?

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