Weight hingedto allow movement

Rotating drumrecords motion

Pen

Bedrock

Supportmoves with

EarthWeight does

not movewith groundmotion due

to inertia

Earth moves

Bedrock

222 Chapter 8

8.2 Measuring Earthquakes

Reading StrategyOutlining As you read, make an outline ofthe important ideas in this section. Use thegreen headings as the main topics and theblue headings as subtopics.

Key ConceptsWhat are the types ofseismic waves?

How is an earthquakeepicenter located?

How is the size of anearthquake measured?

Vocabulary◆ seismograph◆ seismogram◆ surface wave◆ P wave◆ S wave◆ moment

magnitude

The study of earthquake waves, or seismology, dates back almost2000 years. The first attempts to discover the direction of earthquakeswere made by the Chinese. Seismographs are instruments that recordearthquake waves. The idea behind seismographs can be demonstratedwith a weight suspended from a support attached to bedrock as shownin Figure 5. When waves from an earthquake reach the instrument, theinertia of the weight keeps it stationary, while Earth and the support

vibrate. Because the weight stays almostmotionless, it provides a reference pointto measure the amount of movementthat occurs as waves pass through theground below. The movement of Earthcompared to the stationary weight canbe recorded on a rotating drum, shownin Figure 5.

Modern seismographs amplify andelectronically record ground motion,producing a trace, called a seismogram.A typical seismogram (seismos � shake,gramma � what is written) is shown inFigure 6.

Measuring Earthquakes

I. Earthquake Waves

A. Surface Waves

B.

II.

A. ?

?

?

Figure 5 The seismograph(seismos � shake, graph � write)amplifies and records groundmotion.

222 Chapter 8

FOCUS

Section Objectives8.4 Identify the three types of

seismic waves.8.5 Explain how to locate the

epicenter of an earthquake.8.6 Describe the different ways

earthquakes are measured.

Build VocabularyWord Parts Tell students that theprefix seismo- is Greek for “shaking.” Askthem to infer what a seismograph and aseismogram are. Challenge them tocome up with other terms that beginwith the prefix seismo- (seismology,seismologist).

Reading StrategyB. Body Waves

II. Locating an EarthquakeA. Earthquake DistanceB. Earthquake DirectionC. Earthquake Zones

III. Measuring EarthquakesA. Richter ScaleB. Moment Magnitude

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Reading Focus

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Section 8.2

Customize for Inclusion Students

Learning Disabled Place the palms of yourhands together and slide one quickly againstthe other. This movement represents two rocksurfaces slipping against each other. The finger-tips of the hand that moves forward are likethe rock that moves forward. This is called a“push wave” or P wave. The wave travels atabout 8 km/s.

Earthquakes also send out a second kind ofwave. This time put your hands together with

a pencil between them. Slide one handforward to represent the slipping rock surfaces.The pencil rotates, or twists, as you move yourpalm. In the same way, rocks twist betweenslipping surfaces. The twisting rocks send a“twist wave,” or S wave, throughout Earth.A twist wave travels more slowly than a pushwave, moving through Earth at about 5 km/s.

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Earthquakes and Earth’s Interior 223

Earthquake WavesThe energy from an earthquake spreads outward aswaves in all directions from the focus. Seismogramsshow that two main types of seismic waves are pro-duced by an earthquake—surface waves and bodywaves.

Surface Waves Surface waves are seismic wavesthat travel along Earth’s outer layer. The motion ofsurface waves is complex. Surface waves travel alongthe ground and cause the ground and anything rest-ing upon it to move. This movement is like oceanwaves that toss a ship. Surface waves move in an up-and-down motion as well as a side-to-side motion, asshown in Figures 7E and 7F. The side-to-side motionis especially damaging to the foundations of build-ings. These movements make surface waves the mostdestructive earthquake waves.

Body Waves The other waves that travel through Earth’s interiorare called body waves. Body waves are identified as either P waves or S waves, depending on how they travel through the materials withinEarth. Figures 7B and 7D shows differences between the two kinds ofwaves. P waves are push-pull waves—they push (compress) and pull(expand) rocks in the direction the waves travel. P waves are alsoknown as compression waves. In contrast, S waves shake the particlesat right angles to their direction of travel. This can be shown by fas-tening one end of a rope and shaking the other end, as in Figure 7C.S waves are transverse waves. P waves temporarily change the volumeof the material they pass through by alternately compressing andexpanding it, as in Figure 7A. S waves temporarily change the shape ofthe material they pass through. Gases and liquids will not transmit S waves because they do not rebound elastically to their original shape.

A seismogram shows all three types of seismic waves—surface waves, P waves, and S waves. By observing a typical seismicrecord, as shown in Figure 8 on page 225, you can see that the first P wave arrives at the recording station, then the first S wave, and thensurface waves. The waves arrive at different times because they travel atdifferent speeds. Generally, in any solid material, P waves travel about1.7 times faster than S waves. Surface waves travel the slowest at about 90 percent of the speed of the S waves.

Which seismic wave travels fastest?

First P wave First S wave

Surface waves

1 minute

T I M E(Earlier) (Later)

Figure 6 Typical SeismogramThe first wave to arrive is the Pwave, followed later by S waves.The last waves recorded are thesurface waves.Measuring What is the timeinterval in minutes between thestart of the first P wave and the start of the first S wave?

INSTRUCT

Earthquake Waves

Seismic WavesPurpose Students will see the waysthat the three different seismic wavesmove though substances with the useof a coiled spring toy.

Materials coiled spring toy

Procedure Have a student hold oneend of the spring toy. Hold the otherend of the toy and step away from thestudent to stretch the spring out. Gatherapproximately one fifth of the spring inyour hand and let go. Ask students toexplain what they observed as thebunch of coils moves down theextending spring. Explain to studentsthat this is how a P wave travels througha medium. Again, with the springstretched out, gently move the toyfrom side to side in a snake-like motion.Students should observe how thetoy moves as a result of such motion.Explain that this is how an S wavemoves. Finally, move your end of the toyin a rolling motion (like winding afishing reel) creating waves. Studentsshould realize they are observing themotion of surface waves.Kinesthetic, Visual

IntegrateLanguage ArtsAncient cultures had their own ways ofexplaining earthquakes. For example,an ancient Indian legend explains howelephants carried Earth on their backs.When the elephants grew tired andlowered their heads, an earthquakeoccurred. An ancient Siberian legendsays that Earth was pulled on a sledby dogs. Whenever one of the dogsstopped to scratch its fleas, an earth-quake resulted. Challenge students todiscover more ancient legends aboutearthquakes, and then to make up theirown myth or legend that might serve asan explanation for earthquakes.Verbal

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Earthquakes and Earth’s Interior 223

Answer to . . .

Figure 6 approximately 5 minutes

P waves

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Slinky at rest

Push slinky

Compress

Expand CompressWave direction

Particle motion

Wave direction Particle motion

Rope at rest

Shake rope

Wave direction

Particle motion

Wave direction

P waves are compression waves that alternately compressand expand the material through which they pass.

S waves are transverse waves which cause material to shakeat right angles to the direction of wave motion. The length ofthe red arrow is the displacement, or amplitude, of the S wave.

One type of surface wave moves the ground from side toside and can damage the foundations of buildings.

Another type of surface wave travels along Earth’s surfacemuch like rolling ocean waves. The arrows show themovement of rock as the wave passes. The motion followsthe shape of an ellipse.

The back-and-forth motion produced as P waves travelalong the surface can cause the ground to buckle and fracture.

S waves cause the ground to shake up-and-downand sideways.

Locating an EarthquakeThe difference in velocities of P and S waves provides a way to locatethe epicenter. You can compare this difference to a race between twocars. The winning car is faster than the losing car. The P wave alwayswins the race, arriving ahead of the S wave. The longer the race, thegreater will be the difference in arrival times of the P and S waves at thefinish line (the seismic station). The greater the interval measured ona seismogram between the arrival of the first P wave and the first Swave, the greater the distance to the earthquake source.

Figure 7 Each type of seismicwave has characteristic motions.

224 Chapter 8

A

C

E

B

D

F

Seismic Waves

224 Chapter 8

Locating anEarthquakeBuild Reading LiteracyRefer to p. 156D in Chapter 6, whichprovides the guidelines for reciprocalteaching.

Reciprocal Teaching Have studentsread the section with a partner. Onepartner reads a paragraph out loud.Then the other partner summarizes theparagraph’s contents and explains themain concepts. The partners continue toswitch roles with each new paragraphuntil they have finished the section.Intrapersonal

Build Reading LiteracyRefer to p. 216D which provides theguidelines for this reading strategy.

Compare and Contrast Ask studentsto use the visuals and the captions inFigure 7 to describe how the three typesof waves are different. (P waves compressand expand material in the same horizon-tal direction of the waves’ energy. S wavesare transverse waves that cause the groundto shake up and down, perpendicular tothe waves’ direction. Surface waves travelalong the outer layer and can move inboth up-and-down motions and side-to-side motions.)

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Section 8.2 (continued)

Customize for Inclusion Learners

Learning Disabled Ask students to simulateP waves and S waves with a slinky and jumprope. Instruct them to recreate the scenarios

pictured in Figure 7A and 7C if they requirehelp getting started.

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Earthquakes and Earth’s Interior 225

Figure 8 Locating an Earthquake A A travel-time graph is used to determine the distance tothe epicenter. The difference in arrival times of the first P wave and the first S wave in the graph is5 minutes. So the epicenter is roughly 3800 kilometers away. B The epicenter is located using thedistance obtained from three seismic stations. The place the circles intersect is the epicenter.

Trav

el t

ime

(min

utes

)

16

15

14

13

12

11

10

9

8

7

6

5

4

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2

1

0

1000 2000 3000 mi

Distance to epicenter

5 min.

S-wav

e cu

rve

P-wave

curve

First S wave

First P wave

1000 2000 3000 4000 km

Montréal

São Paulo

Epicenter

Paris

6700

km

5500 km

8400km

Earthquake Distance A system for locating earthquake epicen-ters was developed by using seismograms from earthquakes whoseepicenters could be easily pinpointed from physical evidence. Travel-time graphs are constructed from these seismograms, as shown in Figure8A. Using the sample seismogram in Figure 6 and the travel-time curvesin Figure 8A, we can determine the distance from the recording stationto the earthquake in two steps. First, find the time interval between thearrival of the first P wave and the first S wave on the seismogram. Second,find on the travel-time graph the equivalent time spread between the Pand S wave curves. From this information, you can see that this earth-quake occurred 3800 kilometers from the seismograph.

Earthquake Direction Now we know the distance, but whatabout the direction? The epicenter could be in any direction from theseismic station. As shown in Figure 8B, the precise location can befound when the distance is known from three or more different seis-mic stations. On a globe, we draw a circle around each seismic station.Each circle represents the distance of the epicenter from each station.The point where the three circles intersect is the epicenter of the quake.

Travel-time graphs from three or more seismographs can beused to find the exact location of an earthquake epicenter.

A B

Integrate MathThe epicenter of an earthquake is locatedusing information on the arrival times ofthe P and S waves at three seismographstations. Ask: Why must threeseismograph stations be used? Whyaren’t two enough to locate theepicenter? (The intersection of threecircles will yield a more exact location. Ifonly two stations were used, these twocircles would most likely intersect at twopoints. This would give two possiblelocations for the same earthquake.)Logical, Visual

Use VisualsFigure 8 Make sure studentsunderstand how to read the travel-timegraph. Explain that distance is expressedin both kilometers (bottom) and miles(top), and tell them that one kilometerequals approximately 0.6 miles. Ask:If a station records 2 minutes elapsedtime between the arrival of the firstP wave and the arrival of the firstS wave, how far in kilometers is thatstation from the epicenter? (about1000 km) A good way to help studentscomprehend all the informationcompiled in a travel-time graph is tocreate one on the board. Plot datapoints to simulate how seismicrecordings are used to create Pand S wave curves.

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Earthquakes and Earth’s Interior 225

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Earthquake Zones About 95 percent of the majorearthquakes occur in a few narrow zones, as shown inFigure 9. Most of these earthquakes occur around theouter edge of the Pacific Ocean. This zone is known as thecircum-Pacific belt. Active earthquake areas in this zoneinclude Japan, the Philippines, Chile, and Alaska’s AleutianIslands. A second zone of earthquake activity occurs alongthe Mediterranean Sea. This is the Mediterranean-Asianbelt. Another continuous belt extends for thousands ofkilometers through the world’s oceans. This zone coin-cides with the oceanic ridge system.

Measuring EarthquakesHistorically, scientists have used two different types of

measurements to describe the size of an earthquake—intensity and magnitude. Intensity is a measure of theamount of earthquake shaking at a given location based onthe amount of damage. Intensity is not a quantitative meas-urement because it is based on uncertain personal damageestimates. Quantitative measurements, called magnitudes,were developed that rely on calculations using seismograms.Magnitudes are a measure of the size of seismic waves or theamount of energy released at the source of the earthquake.

Where do most earthquakes occur?

Measuring the Distanceto Epicenters

Procedure1. Look at Figures 6 and 8A. Figure 6 is a

seismogram and Figure 8A is a travel-time graph. Use the graph to answerthe Analyze and Conclude questions.

2. Make sure to use only the bottom scaleon the x-axis, measured in kilometers,to answer the questions.

Analyze and Conclude1. Reading Graphs What is the difference

in arrival times in minutes between thefirst P wave and first S wave for stationsthat are the following distances from anepicenter: 1000 km, 2000 km, 2400 km,and 3000 km?

2. Inferring How does the difference inarrival times of the first P wave and firstS wave on a seismogram change? Howdoes it change if the station is fartherfrom the epicenter?

3. Predicting How do you think thevibrations recorded on a seismogramwould change as the distance to theepicenter increases?

Circum-Pacificbelt

Mediterranean-Asianbelt

Figure 9 Distribution of the 14,229 earthquakes with magnitudes equal to or greater than 5 from 1980 to 1990.Observing Where do you find most of the earthquakes—in the interiors of the continents or at the edges?

226 Chapter 8

226 Chapter 8

Section 8.2 (continued)

MeasuringEarthquakes

Measuring the Distanceto EpicentersObjectiveAfter completing this activity, studentsshould be able to use a travel-timegraph and data from a seismogram todetermine information about theepicenter of an earthquake.

Skills Focus Inferring, Predicting

Prep Time 5 minutes

Class Time 20 minutes

Expected Outcome Students willsuccessfully use the travel-time graphand seismogram to answer thequestions.

Analyze and Conclude1. 1000 km � approximately 2 minutes2000 km � approximately 3.5 minutes2400 km � approximately 4 minutes3000 km � approximately 4.4 minutes2. The farther away from the epicenter,the greater the time between the arrivalof the first P wave and the first S wave.3. The vibrations recorded on theseismogram would probably becomeless pronounced, with peaks that werenot as high, or in some other waydifferent, as the distance to theepicenter increases.

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The circum-Pacific belt, or the Ring of Fire,accounts for approximately 75 percent of theworld’s earthquake activity. It also containssome of the major volcanic mountains andmountain ranges on Earth. That is why it iscalled the Ring of Fire. The following is a list ofthe mountain ranges and volcanoes that are

part of the Ring of Fire: the Andes Mountainsand the volcanoes Cotopaxi and Azul, theMexican volcanoes Popocatepetl andParicutin, the Cascade Mountains andMount Saint Helens, the Aleutian Islands,and Mount Fuji volcano in Japan.

Facts and Figures

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Earthquakes and Earth’s Interior 227

Richter Scale A familiar but outdated scale for measuring themagnitude of earthquakes is the Richter scale. The Richter scale isbased on the amplitude of the largest seismic wave (P, S, or surfacewave) recorded on a seismogram. Earthquakes vary greatly in strength,so Richter used a logarithmic scale. A tenfold increase in wave ampli-tude equals an increase of 1 on the magnitude scale. For example, theamount of ground shaking for a 5.0 earthquake is 10 times greater thanthe shaking produced by an earthquake of 4.0 on the Richter scale.

Seismic waves weaken as the distance between the earthquake focusand the seismograph increases. The Richter scale is only useful forsmall, shallow earthquakes within about 500 kilometers of the epicenter.Most of the earthquake measurements you hear on news reports usethe Richter scale. Scientists, however, no longer use it.

Moment Magnitude In recent years, scientists have been usinga more precise means of measuring earthquakes. It is called themoment magnitude scale. The moment magnitude is derived fromthe amount of displacement that occurs along a fault zone. It doesn’tmeasure the ground motion at some distant point. The moment mag-nitude is calculated using several factors. These factors include theaverage amount of movement along the fault, the area of the surfacebreak, and the strength of the broken rock: (surface area of fault) �(average displacement along fault) � (rigidity of rock). Together thesefactors provide a measure of how much energy rock can store beforeit suddenly slips and releases this energy during an earthquake.Moment magnitude is the most widely used measurement for earth-quakes because it is the only magnitude scale that estimates theenergy released by earthquakes.

Table 1 describes the damage and incidence of earthquakes of dif-ferent magnitudes. Compare this information to the earthquakes listedin Table 2 on page 228.

Table 1 Earthquake Magnitudes and Expected World Incidence

Moment Magnitudes Effects Near EpicenterEstimated Number

per Year

� 2.0 Generally not felt, but can be recorded � 600,000

2.0–2.9 Potentially perceptible � 300,000

3.0–3.9 Rarely felt � 100,000

4.0–4.9 Can be strongly felt 13,500

5.0–5.9 Can be damaging shocks 1,400

6.0–6.9 Destructive in populous regions 110

7.0–7.9 Major earthquakes; inflict serious damage 12

8.0 and above Great earthquakes; destroy communities 0–1near epicenter

Many students may think that small- tomedium-sized earthquakes in an areawill reduce the chances of a majorearthquake in the same region becausethe smaller earthquakes will release all ofthe built-up energy. To challenge thismisconception, ask students to considerthe amount of energy released in anearthquake. Ask: How much moreground shaking does an earthquakewith a measure of 8.0 on the Richterscale have compared with an earth-quake with a measure of 3.0 on theRichter scale? (about 100,000 timesmore ground shaking) How manysmaller earthquakes measuring 3.0 onthe Richter scale would need to occurto equal the same amount of groundshaking of an 8.0 earthquake? (about100,000 smaller earthquakes)Logical

Build Reading LiteracyRefer to p. 474D in Chapter 17, whichprovides guidelines for monitor yourunderstanding.

Monitor Your UnderstandingDisplay a world map with the names ofmajor cities. Help students find thelocations where the earthquakes listed inTable 2 occurred. Then ask studentswhether the earthquake’s location is inany of the earthquake zones describedon page 226. (Most are in the circum-Pacific belt. Armenia—1988; Iran—1990;Latur, India—1993; and Izmit, Turkey—1999 are in the Mediterranean-Asian belt.The Charleston, SC, earthquake of 1886 isnot in any of the zones described in thetext.)

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Earthquakes and Earth’s Interior 227

Answer to . . .

Figure 9 at the edges

along the edge of thePacific Ocean

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228 Chapter 8

Section 8.2 Assessment

Reviewing Concepts1. List the two categories of seismic waves.

2. Briefly describe how the epicenter of anearthquake is located.

3. Describe the two different ways tomeasure the size of an earthquake.

4. In what order do the basic types of seismicwaves reach a seismograph?

Critical Thinking5. Comparing and Contrasting Describe the

differences in speed and mode of travelbetween primary waves and secondary waves.

6. Applying Concepts How does a seismographmeasure an earthquake?

Descriptive Paragraph Write a para-graph describing in your own words whatwould occur in an earthquake that has beenmeasured as a moment magnitude of 6.0.

*1886 Charleston, South Carolina 60 Greatest historical earthquake in the easternUnited States

*1906 San Francisco, California 1500 7.8 Fires caused extensive damage.

1923 Tokyo, Japan 143,000 7.9 Fire caused extensive destruction.

1960 Southern Chile 5700 9.6 Possibly the largest-magnitude earthquake everrecorded

*1964 Alaska 131 9.2 Greatest North American earthquake

1970 Peru 66,000 7.8 Large rockslide

*1971 San Fernando, California 65 6.5 Damages exceeded $1 billion.

1985 Mexico City 9500 8.1 Major damage occurred 400 km from epicenter.

1988 Armenia 25,000 6.9 Poor construction practices caused greatdamage.

*1989 Loma Prieta, California 62 6.9 Damages exceeded $6 billion.

1990 Iran 50,000 7.3 Landslides and poor construction practicescaused great damage.

1993 Latur, India 10,000 6.4 Located in stable continental interior

*1994 Northridge, California 57 6.7 Damages exceeded $40 billion.

1995 Kobe, Japan 5472 6.9 Damages estimated to exceed $100 billion.

1999 Izmit, Turkey 17,127 7.4 Nearly 44,000 injured and more than 250,000displaced.

1999 Chi Chi, Taiwan 2300 7.6 Severe destruction; 8700 injuries

2001 El Salvador 1000 7.6 Triggered many landslides

2001 Bhuj, India 20,000† 7.9 1 million or more homeless

*U.S. earthquakes†Widely differing magnitudes have been estimated for some earthquakes. When available, moment magnitudes are used.SOURCE: U.S. Geological Survey

Table 2 Some Notable EarthquakesYear Location Deaths (est.) CommentsMagnitude†

228 Chapter 8

ASSESSEvaluateUnderstandingHave students write three reviewquestions for this section. Studentsshould then break into groups ofthree or four and ask each othertheir questions.

ReteachReview the types of seismic waves fromearthquakes by asking students toexplain what they see in Figure 7.

An earthquake measuring a momentmagnitude of 6 could prove to bepotentially devastating to structures notbuilt to new earthquake standards.Poorly built structures would suffersignificant damage. However, structuresthat were constructed with earthquakesafety in mind would most likely farewell. Students should use Tables 1 and 2from the text as a reference in looking atthe damage caused by earthquakes witha moment magnitude of 6 or more.

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Section 8.2 (continued)

station. A circle is drawn around each seismo-graph station and the circles meet where theearthquake epicenter is likely to be found.3. Earthquakes can be measured by theirintensity (or level of damage done) or by themagnitude (amplitude of seismic waves).4. P wave, S wave, surface wave5. P waves push and pull rocks in the direc-tion of travel. Their velocity is greater thanthe velocity of S waves. S waves shake theparticles of material at right angles to theirdirection of travel.

Section 8.2 Assessment

1. The two categories of seismic waves arebody waves (P and S) and surface waves.2. The epicenter of an earthquake is locatedusing data taken from at least three differentseismograph stations. The time that the firstP wave arrives at the station is then subtractedfrom the time that the first S wave arrives.This value can then be turned into a distanceusing a travel-time diagram. This distancemeans that the epicenter is that far from the

6. In concept, a seismograph has a weightwhich is suspended from a support that isattached to bedrock. When the bedrockshakes, the weight remains stationary whichallows it to act as a reference point. Themovement of Earth can then be comparedto the weight and recorded on a stationarydrum.

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