Chapter 9. Earthquakes

9.1 Introduction

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Author: Jordan Raddick, Johns Hopkins University

Editor: Beth Hufnagel, Anne Arundel Community College

The goals of this module: At the end, you should be able to:

Explain how scientists use data from earthquake waves to learn about the Earth's interior.
Identify and describe Earth's layers: crust, mantle, outer core, inner core.

In this module you will explore:

How earthquake waves reveal Earth's structure
Types of earthquake waves
Earth's layers

Why you are doing it: Earth's interior is hidden from direct observation, but earthquake waves allow scientists to understand its structure.

9.2 Background

The interior of the Earth is hidden from our eyes, just as surely as the most distant galaxy. But we have still learned a great deal about the interior structure of the Earth, thanks to the technique of sensing earthquake waves.

Whenever there is an earthquake anywhere in the world, a system of seismometers all over the world is ready to record data. The data is plotted into a graph called a seismograph. The main source of data shown on seismographs is the arrival times and intensities of earthquake waves. Earthquake waves come in three types: surface waves, primary (P) waves, and secondary (S) waves. The P and S waves reveal information about the interior of the Earth, since P waves can move through solids and liquids, but S waves can only move through solids. There is also an abrupt change in the direction of the P wave when it moves from one type of material to another.

In this activity, you will look at simulated seismograph data to see what you can learn about the inside of the Earth from P and S waves.

9.3 Propagation Of Waves Caused By Earthquakes

The animation below shows an imaginary earthquake in Asmara, Eretria - Asmara is located in a fault zone near the Red Sea, so it really does get earthquakes!

This animation imagines that the Asmara earthquake can be recorded by seismometers around the world. The animation shows three such seismometers: Riyadh, Saudi Arabia; Boston, Massachusetts, USA; and Fortaleza, Brazil. Click "Collect data from seismic stations" to simulate the earthquake and start collecting data. As the animation runs, you will see each seismometer record waves arriving from the earthquake. At the end of the animation, the waves will be labeled.

Run the animation and pay attention to which waves arrive at which stations, and when the waves arrive.

Question Sequence

Question 9.1

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Try again. Watch the animation for the Riyadh station again, and pay attention to which waves arrive when. Remember that time is the horizontal axis in the seismographs, so the first waves to arrive will be the ones to the left.

Correct. P waves always arrive first!

Incorrect. P waves always arrive first!

Question 9.2

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Try again. Watch the animation again and look at the labels on the seismographs at each seismic station.

Correct. The seismometer in Boston records only P waves.

Incorrect. The seismometer in Boston records only P waves.

Question 9.3

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Try again. Watch the animation again and look at the labels on the seismographs at each seismic station.

Correct. Fortaleza is in the "shadow zone" of this earthquake.

Incorrect. Fortaleza is in the "shadow zone" of this earthquake.

9.4 S Waves And P Waves

On the last page, you saw seismographs from three seismic stations for the Asmara earthquake. Here is the animation again:

Now, think about why the seismic stations recorded the data that they did. Look at this image for some hints.

Question Sequence

Question 9.4

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Try again.

Correct. To get from the Asmara earthquake to the Boston station, earthquake waves must pass through the Earth's outer core. S waves are back-and-forth vibrations, and these types of transverse waves cannot pass through liquid. This implies that the outer core is liquid, and therefore S waves cannot pass through it.

Incorrect. To get from the Asmara earthquake to the Boston station, earthquake waves must pass through the Earth's outer core. S waves are back-and-forth vibrations, and these types of transverse waves cannot pass through liquid. This implies that the outer core is liquid, and therefore S waves cannot pass through it.

Question 9.5

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Try again.

Correct. There is an abrupt change in the density and chemical composition between the mantle and the core. At this boundary, seismic waves bend - both P and S waves. Waves that were heading toward Fortaleza bend back up toward the surface and end up away from Fortaleza. Therefore, no S or P waves arrive at that station; Fortaleza is located in the "shadow zone" of the Asmara earthquake.

Incorrect. There is an abrupt change in the density and chemical composition between the mantle and the core. At this boundary, seismic waves bend - both P and S waves. Waves that were heading toward Fortaleza bend back up toward the surface and end up away from Fortaleza. Therefore, no S or P waves arrive at that station; Fortaleza is located in the "shadow zone" of the Asmara earthquake.

9.5 Interior Structure of the Earth

Choose the corresponding label for each structure of the Earth.

Question 9.6

Liquid Outer Core	a/7BmRqVZ5NYbTl/N6kH17dEd5T79k/P
Solid Crust	chfsPCC82uyd33/DAhY9LyJWADtaDODX
Inner Core	+xi1Jsh4IMVcChYl1OrygyCbrv1ZBQuC
Denser, Solid Mantle	inGUChaEIM8v6G7z4RwP017nwpuSnKfy

Correct.

Try again.

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Correct. The S waves cannot propagate through liquids.

Incorrect. The S waves cannot propagate through liquids.