Chapter 19: Earthquakes - Buncombe County Schools … · • What causes earthquakes ... such as...

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What You’ll Learn• What causes earthquakes

and how they affectEarth’s surface.

• How earthquakes andthe destruction theycause are measured.

• What factors determineseismic risk.

Why It’s ImportantEarthquakes are naturalphenomena that cancause vast amounts ofdamage as well as manydeaths. Understandingwhat causes earthquakesis essential to our beingprepared for these naturaldisasters.

EarthquakesEarthquakes1919

Earthquake damage in Taiwan, 1999Earthquake damage in Taiwan, 1999

To learn more about earthquakes, visit the Earth Science Web Site at earthgeu.com

http://earthgeu.com

19.1 Forces Within Ear th 495

Earthquakes are natural vibrationsof the ground. Most quakes are causedby movement along enormous frac-tures in Earth’s crust. In this activity,you will model how movements alongthese fractures can cause earthquakes.

1. Slide the largest surfaces oftwo smooth wooden blocks against each other. Describe themovement.

2. Cut two pieces of coarse-grainedsandpaper so that they are about 1 cm larger than the largest surfaceof each block.

3. Place the sandpaper, coarse side up,against the largest surface of each

block. Wrap the paper over theedges of the blocks and secure itwith thumbtacks.

4. Slide the sandpaper-covered sidesof the blocks against each other.What happens?

Observe and InferIn your science journal,compare the two move-ments. Infer which ofthe two scenarios modelswhat happens during an earthquake.

Model An EarthquakeDiscovery LabDiscovery Lab

OBJECTIVES

• Define stress and strainas they apply to rocks.

• Distinguish among thethree types of faults.

• Contrast three types ofseismic waves.

VOCABULARY

stress focusstrain epicenterfaultprimary wavesecondary wavesurface wave

Earthquakes are natural vibrations of the ground caused by move-ment along gigantic fractures in Earth’s crust, or sometimes, by vol-canic eruptions. If you’ve experienced an earthquake or heard aboutquakes in the news, you know that they can be extremely destructive.There are some instances in which a single earthquake has killedmore than 100 000 people. Earthquakes have even destroyed entirecities. Anyone living in an area prone to earthquakes should be awareof the potential danger posed by these events and how to minimizethe damage that they cause.

STRESS AND STRAINMost earthquakes occur when rocks fracture, or break, deep withinEarth. Fractures form when stress, the forces per unit area acting ona material, exceeds the strength of the rocks involved. There are threekinds of stress that act on Earth’s rocks: compression, tension, andshear. Compression is stress that decreases the volume of a material,tension is stress that pulls a material apart, and shear is stress that

Forces Within Earth19.119.1

causes a material to twist. The deformation of materials in responseto stress is called strain. Figure 19-1 illustrates the strain caused bycompression, tension, and shear.

Laboratory experiments on rock samples show a distinct relation-ship between stress and strain. When the stress applied to a rock is plot-ted against strain, a stress-strain curve, like the one shown in Figure19-2, is produced. A stress-strain curve usually has two segments: astraight segment and a curved segment. Low stresses produce thestraight segment, which represents the elastic strain of a material. Elasticstrain causes a material to bend and stretch, and can be demonstratedby gently applying tension to a rubber band. When this tensional stressis released, the rubber band returns to its original size and shape. InFigure 19-2, note that elastic strain is proportional to stress, and thus, ifthe stress is reduced to zero, the strain, or deformation, disappears.

Ductile Deformation When stress exceeds a certain value, how-ever, a material undergoes ductile deformation, shown by the curvedsegment of the graph in Figure 19-2. Unlike elastic strain, this type ofstrain produces permanent deformation, which means that the mate-rial stays deformed even if the stress is reduced to zero. A rubber bandundergoes ductile deformation when it is stretched beyond its elasticlimit. This permanent deformation results in an increase in size andproduces slight tears or holes in the band. When stress exceeds thestrength of a material, the material breaks, or fails, as designated bythe X on the graph. From experience, you probably know that exert-ing too much tension on a rubber band will cause it to snap.

Most materials exhibit both elastic and ductile behavior. Brittlematerials, such as glass, certain plastics, and dry wood, fail beforemuch ductile deformation occurs. Ductile materials such as rubber,

496 CHAPTER 19 Ear thquakes

Typical Stress-Strain Curve

Stre

ss

Strain

Ductile deformationFailure

Elastic deformation

Elastic limit

Undeformedmaterial

Compressionalstrain

Tensionalstrain

Shearstrain

Figure 19-1 Compressioncauses a material to shorten(A). Tension causes a mater-ial to lengthen (B). Shearcauses distortion of a material (C).

Figure 19-2 A typicalstress–strain curve has twoparts. Elastic deformationoccurs as a result of low stress;ductile deformation occurswhen stress is high. Whendoes failure occur?

A

C

B

silicon putty, and metals, on the other hand, can undergo a great dealof ductile deformation before failure occurs, or, they may not fail atall. Most rocks are brittle under the relatively low temperatures thatexist in Earth’s crust but become ductile at the higher temperaturespresent at greater depths.

FAULTSMany kinds of rocks that make up Earth’s crust fail when stress isapplied too quickly, or when stress is great. The resulting fracture orsystem of fractures, along which movement occurs, is called a fault.The surface along which the movement takes places is called the faultplane. The orientation of the fault plane can vary from nearly hori-zontal to almost vertical. In diagrams, small arrows along the faultplane indicate the direction of movement of the rocks involved.

Types of Faults There are three basic types of faults, as shown in Figure 19-3. Reverse faults are fractures that form as a result ofhorizontal compression. Note that the compressional force results in a horizontal shortening of the crust involved. What evidence inFigure 19-3A indicates this shortening? Normal faults are fracturescaused by horizontal tension. Movement along a normal fault ispartly horizontal and partly vertical. The horizontal movementalong a normal fault occurs in such a way as to extend the crust. Notein Figure 19-3B that the two trees separated by the normal fault arefarther apart than they were before the faulting.

Strike-slip faults are fractures caused by horizontal shear. Themovement along a strike-slip fault is mainly horizontal, as shownin Figure 19-3C. The San Andreas Fault, which runs throughCalifornia, is a strike-slip fault. This fault is one of thousands offaults responsible for many of the state’s earthquakes. Motion alongthis fault has offset features that were originally continuous acrossthe fault, as shown in Figure 19-4.


Figure 19-4 The orangetrees in the backgroundhave moved to the right relative to those in the foreground as the result of the 1940 Imperial Valleyearthquake along the SanAndreas Fault.

Reverse fault Normal fault Strike-slip faultBA C

Figure 19-3 Reverse faultsform when horizontal stressis exerted on a rock bodyfrom opposite sides (A).Normal faults form whenbodies of rock are pulledfrom opposite sides (B).Strike-slip faults are causedby horizontal shear stress (C).

EARTHQUAKE WAVESMost earthquakes are caused by movements along faults. Recallfrom the Discovery Lab that some slippage along faults is relativelysmooth. Other movements, modeled by the sandpaper-coveredblocks, show that irregular surfaces in rocks can snag and lock. Asstress continues to build in these rocks, they reach their elastic limit,break, and produce an earthquake.

Types of Seismic Waves The vibrations of the ground duringan earthquake are called seismic waves. Every earthquake generatesthree types of seismic waves. Primary waves, or P-waves, squeezeand pull rocks in the same direction along which the waves aretraveling, as shown in Figure 19-5A. Note how a volume of rock,which is represented by the small red square, changes shape as a P-wave passes through it. Secondary waves, or S-waves, cause rocksto move at right angles in relation to the direction of the waves, asshown in Figure 19-5B. Surface waves are a third type of seismicwave that move in two directions as they pass through rock. An up-and-down movement similar to that of an ocean wave occurs asa surface wave travels through a rock. A surface wave also causesrocks to move from side to side as it passes, as shown in Figure 19-5C.

As you might guess from the name, surface waves travel alongEarth’s surface. P-waves and S-waves, on the other hand, passthrough Earth’s interior. For this reason, P-waves and S-waves arealso called body waves. The first body waves generated by a quake

P-wave

Particlemovement

Particlemovement

Particlemovement

Wavedirection

Wavedirection

S-wave Surface wave

Wavedirection

A B C


Figure 19-5 A P-wavecauses rock particles to moveback and forth as it passes(A). An S-wave causes rockparticles to move at rightangles to the direction ofthe wave (B). A surfacewave causes rock particles tomove both up and downand from side to side (C).

spread out from the point of failure of rocks at depth. This point,where an earthquake originates, is the focus of the earthquake. Thefocus is usually at least several kilometers below Earth’s surface. Thepoint on Earth’s surface directly above the focus is the earthquake’sepicenter. Locate the focus and the epicenter in the diagram shownin Figure 19-6.

You’ve just learned that Earth’s rocks deform when stress isexerted on them. If stress exceeds a certain limit, the rocks fracture toform faults. Movement along faults causes most earthquakes. Howare these earth-shattering events measured and what do they tell usabout Earth’s interior? You’ll find out the answer to this question inthe next section.

Figure 19-6, The focus ofan earthquake is the pointof initial fault rupture. Thesurface point directly abovethe focus is the epicenter.


Epicenter

Fault

Wave fronts

Focus

1. What are stress and strain?

2. Describe a typical stress-strain curve andthe relationship between the segments ofthe curve and stress and strain.

3. Compare and contrast the three types offaults.

4. What causes most earthquakes?

5. Thinking Critically Most earthquakes areshallow. Use the concepts of elastic andductile deformation to explain this fact.

SKILL REVIEW

6. Concept Mapping Use the following termsto complete the concept map to organize

some of the major ideas in this section. Formore help, refer to the Skill Handbook.

seismic waves

andmovemove

can be

body waves surface waves

P-waves S-waves

up and down

up and down

back and forth

back and forth

move

earthgeu.com/self_check_quiz

http://earthgeu.com/self_check_quiz

19.219.2

Figure 19-7 Which ofthese seismometers recordshorizontal motion duringan earthquake? Whichdetects vertical motion?

Seismic Waves and Earth’s Interior

The study of earthquake waves is called seismology. In view of thepotential for major disaster, it should come as no surprise to you thatmany scientists study earthquakes. Some seismologists, however,study earthquakes for another reason. The seismic waves that shakethe ground during a quake also penetrate Earth’s interior. This hasprovided information that has enabled Earth scientists to constructmodels of Earth’s internal structure. Thus, even though seismicwaves can wreak havoc on the surface, they are invaluable for theircontribution to our understanding of Earth’s interior.

SEISMOMETERS AND SEISMOGRAMSVibrations sent out by earthquakes shake the entire globe. Althoughmost of the vibrations can’t be felt great distances from a quake’s epi-center, they can be detected and recorded by sensitive instrumentscalled seismographs, or seismometers. Some seismometers consistof a rotating drum covered with a sheet of paper, a pen or other suchrecording tool, and a mass. Seismometers vary in design, as shown inFigure 19-7, but all include a frame that is anchored to the groundand a mass that is suspended from a spring or wire. Because of iner-tia, the mass tends to stay at rest as the ground and, consequently, theframe, vibrate during an earthquake. The relative motion of the massin relation to the frame is then registered on the paper with therecording tool, or is directly recorded onto a computer disk. Therecord produced by a seismometer is called a seismogram, a portionof which is shown in Figure 19-8.

OBJECTIVES

• Describe how a seismo-meter works.

• Explain how seismicwaves have been used todetermine the structureand composition ofEarth’s interior.

VOCABULARY

seismometerseismogram


Frame

FrameSpring

Mass Mass

WirePen

Pen

Rotatingdrum

Rotatingdrum

Travel-Time Curves Seismic waves that travel from the epicenterof an earthquake are recorded by seismometers housed in distantfacilities. Over many years, the arrival times of seismic waves fromcountless earthquakes at seismic facilities all over the globe have beencollected. Using these data, seismologists have been able to constructglobal travel-time curves for the initial P-waves and S-waves of anearthquake, as shown in Figure 19-9. These general curves have pro-vided the average travel times of all seismic waves for different dis-tances, no matter where on Earth an earthquake occurs. You canmake and use a travel-time curve by doing the Problem-Solving Labon the next page.

Look at Figure 19-9. Note that for any distancefrom the epicenter, the P-waves always arrive firstat a seismic facility. Note, too, that with increasingtravel distance, the time separation between thecurves for the P-waves and S-waves increases. Thismeans that waves recorded on seismograms frommore distant facilities are farther apart than wavesrecorded on seismograms at stations closer to theepicenter. This separation of seismic waves on seis-mograms can be used to determine the distancefrom the epicenter of a quake to the seismic facilitythat recorded the seismogram. Can you guess how?This method of precisely locating an earthquake’sepicenter will be discussed later in this chapter.

Figure 19-8 Use this section of a seismogram,which contains coloredlines to make it easier toread, to determine whichof the three types of seis-mic waves is the fastest.Which type is the slowest?

19.2 Seismic Waves and Ear th’s Interior 501

10:00:00P-wave

11:00:00

Tim

e o

f d

ay(G

reen

wic

h M

ean

Tim

e)

12:00:00

90 180 270 360 450

Seconds

540 630 720 810

S-wave Surface wave

1

10000 2000 3000

S

P

Epicenter distance (km)

4000 5000

2

3

4

5

6

7

8

9

Trav

el t

ime

(min

ute

s)

10

11

12

13

14

15

16

17

18Typical Travel-Time Curves

Figure 19-9 Travel-time curves show the time it takes for P-wavesand S-waves to travel to seismic sta-tions located at different distancesfrom an earthquake’s epicenter.

CLUES TO EARTH’S INTERIORMost of the knowledge of Earth’s interior comes from the study ofseismic waves, which change speed and direction when theyencounter different materials. Note in Figure 19-10 that P-waves andS-waves traveling through the mantle follow fairly direct paths.P-waves that strike the core, however, are refracted, or bent, so thatbeyond a distance of about 11 000 km from the quake’s epicenter,they disappear. The P-waves refracted into the core reemerge at a dis-tance of about 16 000 km from the epicenter. The region betweenthese two distances doesn’t receive direct P-waves and is known asthe P-wave shadow zone.

What happens to the S-waves generated by an earthquake? S-waves do not enter Earth’s core because they cannot travel throughliquids. Thus, like P-waves, they also do not reappear beyond the


Model seismic-wave travel timesSeismic waves can be compared to run-ners. Suppose that Pam and Sam are run-ning the 100-m dash. Pam runs the 100 min 12 s, and Sam runs it in 14.5 s.

Procedure1. Copy the graph shown. Plot two points

to show the running times for Pamand Sam. Draw a straight line fromeach point to the origin. These arePam’s and Sam’s travel-time curves.

2. Measure the separation of the twolines at a distance of 50 m. Is it moreor less than 1 s?

3. Slide a ruler, parallel to the verticalaxis, along the curves until you findthe distance at which the separation is1 s. What is that distance?

Analysis4. Is there any other distance with the

same separation?5. What is the average speed, in m/s, of

each runner?

Thinking Critically6. Double-check your answer by dividing

the distance in step 3 by each runner’sspeed to get each runner’s time to thatpoint. Do these times differ by 1 s?

7. What do Pam and Sam represent interms of seismic waves? Discuss someways in which a seismic travel-timecurve differs from yours.

Making and Using Graphs

Distance (m)

0 50 100

Tim

e (s

)

0

5

10

15

Travel-Time Graph

11 000-km distance. This disappearance of S-waves has allowed seis-mologists to reason that Earth’s outer core must be liquid. Detailedstudies of how other seismic waves reflect deep within Earth showthat Earth’s inner core is solid.

Earth’s Internal Structure The travel times and behavior ofseismic waves provide a detailed picture of Earth’s internal structure.These waves also provide clues about the composition of the variousparts of Earth. By studying seismic-wave travel times, scientists have

19.2 Seismic Waves and Ear th’s Interior 503

Figure 19–10 Refraction of P-waves into the outer core generates a P-wave shadowzone on Earth’s surface where no direct P-waves appear on seismograms. Other P-wavesare reflected and refracted by the inner core. S-waves cannot travel through liquids, andthus, don’t reappear beyond the P-wave shadow zone.

No

direct

P-waves N

odi

rect

P-w

aves

No direct

S-waves

P- andS-w

aves

P-an

dS-

wav

es

Earthquake focus

P-waveshadow zone

P-waveshadow zone

11 000 km11 000 km

16 000 km16 000 km

P-waves

S-waves

Outer core

Inner core

Mantle


Figure 19-11 Observationsof seismic wave velocitieshave enabled scientists tosubdivide Earth’s interiorinto various layers.

determined that the lithosphere, which includes the crust and the topof the upper mantle, is made up primarily of the igneous rocks gran-ite, basalt, and peridotite as shown in Figure 19-11. The crustal partof the lithosphere is composed of either granite or basalt. The man-tle section of the lithosphere is made of a dense, coarse-grained,intrusive rock called peridotite, which is made mostly of the mineralolivine. Much of the partially melted mantle, or asthenosphere, isalso thought to be peridotite. Earth’s lower mantle is solid and isprobably composed of simple oxides containing iron, silicon, andmagnesium. Seismic waves traveling through the core, together withgravity studies, indicate that this inner part of the planet is verydense and is probably made of a mixture of iron and nickel.

Earth’s Composition The composition data obtained from seis-mic waves is supported by studies of meteorites, which are solid,interplanetary bodies that enter Earth’s atmosphere. You might bethinking, but how can objects from space help to determine the com-position of Earth’s interior? Meteorites are pieces of asteroids, whichare rocky bodies that orbit the Sun. Asteroids are thought to haveformed in much the same way and at the same time as the planets inour solar system. Meteorites consist mostly of iron, nickel, andchunks of rock similar to peridotite in roughly the same proportionsas the rocks thought to make up Earth’s core and mantle.

Meteorites, together with the data provided by the travel times ofseismic waves, have enabled scientists to indirectly probe Earth’sinterior. What other kinds of information can these waves provide?As you’ll find out in the next section, seismic waves are used to deter-mine the strength of an earthquake as well as the precise location ofits epicenter.

1. Explain how a seismometer works.

2. What is a seismogram?

3. What is a seismic travel-time curve, andhow is it used to study earthquakes?

4. What is the P-wave shadow zone, andwhat causes it?

5. How have scientists determined the com-position of Earth’s mantle and core?

6. Thinking Critically As shown in Figure19-8, on page 501, surface waves are thelast to arrive at a seismic station. Whythen do they cause so much damage?

SKILL REVIEW

7. Recognizing Cause and Effect Is there anyway for P-waves to appear in the shadowzone? Explain. For more help, refer to theSkill Handbook.

Lithosphere: granite,basalt or peridotite;

solid

100

a

b

c

4.03.0 5.0

S-wave velocity (km/s)

6.0

200

300

Dep

th (

km

)

400

500

600

700

Asthenosphere:peridotite;

partly melted

Lower Mantle:simple oxides;

solid

To core:iron and nickel

Velocity vs Depth



OBJECTIVES

• Compare and contrastearthquake magnitudeand intensity and thescales used to measureeach.

• Explain why data fromat least three seismic sta-tions are needed to locatean earthquake’s epicenter.

• Describe Earth’s seismicbelts.

VOCABULARY

magnitudeRichter scalemoment magnitude scalemodified Mercalli scale

19.319.3 Measuring and Locating Earthquakes

How many earthquakes do you suppose occur each year? If you wereto rely only on news accounts of these events, you might guess adozen or so, at most. It may surprise you to learn that more than onemillion earthquakes occur each year! However, more than 90 percentof these are not felt and cause little, if any, damage. The earthquakesthat make the news are major seismic events that cause much dam-age, such as the one that occurred in Bam, Iran in December 2003.

EARTHQUAKE MAGNITUDE AND INTENSITYThe amount of energy released during an earthquake is measured byits magnitude. Many news accounts describe the magnitude of anearthquake on a numerical scale called the Richter scale, which wasdevised by an American seismologist named Charles Richter.An earthquake’s rating on the Richter scale is based on the size ofthe largest seismic waves generated by the quake. Each successivenumber in the scale represents an increase in seismic-wave size, oramplitude, of a factor of 10. For example, the seismic waves of a mag-nitude-8 earthquake on the Richter scale are ten times larger thanthose of a magnitude-7 earthquake, and 100 times larger than thoseof a magnitude-6 earthquake. The differences in the amounts ofenergy released by earthquakes are even greater than the differencesbetween the amplitudes of their waves. Each increase in magnitudecorresponds to about a 32-fold increase in seismic energy. Thus, anearthquake of magnitude-8 releases about 32 times the energy of amagnitude-7 earthquake, and over 1000 times the energy of a mag-nitude-6 earthquake. Some of the damage caused by a quake mea-suring 8.6 on the Richter scale is shown in Figure 19-12.

Figure 19-12 The damageshown here was caused by an 8.6-magnitude earthquake that struckAnchorage, Alaska in 1964.

505

Figure 19-13 Use the modi-fied Mercalli scale to deter-mine the intensity of theearthquake that caused thedamage to this grocery store in Washington State.

Moment Magnitude Scale While the Richter scale can still beused to describe the magnitude of an earthquake, most seismologiststoday use a scale called the moment magnitude scale to measureearthquake magnitude. The moment magnitude scale takes intoaccount the size of the fault rupture, the amount of movement alongthe fault, and the rocks’ stiffness. Unlike Richter-scale values, whichare based on the largest seismic waves generated by a quake, momentmagnitude values are estimated from the size of several types of seis-mic waves produced by an earthquake.

Modified Mercalli Scale Another way to assess earthquakes isto measure the amount of damage done to the structures involved.This measure, called the intensity of an earthquake, is determinedusing the modified Mercalli scale, which rates the types of damageand other effects of an earthquake as noted by observers during andafter its occurrence. This scale uses the Roman numerals I to XII todesignate the degree of intensity. Specific effects or damage corre-spond to specific numerals; the higher the numeral, the worse thedamage. A simplified version of the modified Mercalli intensity scaleis shown in Table 19-1. Use the information given in this scale to tryto rate the intensity of the earthquake that caused the damage shownin Figure 19-13.


Earthquake intensity depends primarily on the amplitude of thesurface waves generated. Because surface waves, like body waves,gradually decrease in size with increasing distance from the focus ofan earthquake, the intensity also decreases as the distance from aquake’s epicenter increases. Maximum intensity values are observedin the region near the epicenter; Mercalli values decrease to I at dis-tances very far from the epicenter.

Modified Mercalli scale intensity values of places affected by anearthquake can be compiled to make a seismic-intensity map.Contour lines join points that experienced the same intensity. Themaximum intensity is usually, but not always, found at the quake’sepicenter. You will generate a seismic-intensity map when you do theMiniLab on the next page.

Depth of Focus Earthquake intensity is related to earthquakemagnitude. Both measurements reflect the size of the seismic wavesgenerated by the quake. Another factor that determines the intensityof an earthquake is the depth of the quake’s focus. An earthquake canbe classified as shallow, intermediate, or deep depending on the loca-tion of the quake’s focus. Because a deep-focus earthquake producessmaller vibrations at the epicenter than a shallow-focus quake, ashallow-focus, moderate quake of magnitude-6, for example, maygenerate a greater maximum intensity than a deep-focus quake of

19.3 Measuring and Locating Ear thquakes 507

Table 19-1 Modified Mercalli Intensity Scale

I. Not felt except under unusual conditions.

II. Felt only by a few persons. Suspended objects may swing.

III. Quite noticeable indoors. Vibrations are like the passing of a truck.

IV. Felt indoors by many, outdoors by few. Dishes and windows rattle. Standing cars rock noticeably.

V. Felt by nearly everyone. Some dishes and windows break, and some plaster cracks.

VI. Felt by all. Furniture moves. Some plaster falls and some chimneys are damaged.

VII. Everybody runs outdoors. Some chimneys break. Damage is slight in well-built structures but considerable in weak structures.

VIII. Chimneys, smokestacks, and walls fall. Heavy furniture is overturned. Partial collapse of ordinary buildings occurs.

IX. Great general damage occurs. Buildings shift off foundations. Ground cracks. Underground pipes break.

X. Most ordinary structures are destroyed. Rails are bent. Landslides are common.

XI. Few structures remain standing. Bridges are destroyed. Railroad ties are greatly bent. Broad fissures form in the ground.

XII. Damage is total. Objects are thrown upward into the air.

Topic: Bam, IranTo learn more about damage caused by earth-quakes, visit the EarthScience Web Site at earthgeu.com

Activity: Research theDecember 2003 earth-quake in Bam, Iran.Describe what an eyewit-ness may have seen.

http://earthgeu.com

magnitude-8. Catastrophic quakes withhigh intensity values are almost always shal-low-focus events.

LOCATING AN EARTHQUAKEThe exact location of an earthquake’s epi-center and the time of the quake’s occur-rence are initially unknown. All epicenterlocations, as well as times of occurrence,however, can be easily determined usingseismograms and travel-time curves.

Distance to an Earthquake Lookagain at Figure 19-9 on page 501. Supposethe separation time for the P-waves and S-waves is six minutes. Based on knowntravel times of seismic waves, the distancebetween the earthquake’s epicenter and theseismic station that recorded the waves canonly be 4500 km—no more, no less. This isbecause the known travel time over thatdistance is eight minutes for P-waves and 14 minutes for S-waves. At greater distancesfrom the epicenter, the travel times for bothtypes of waves increase. This results in alarger P-S separation because S-waves lagbehind the faster P-waves. The distance to aquake’s epicenter, then, is determined bythe P-S separation. By measuring this sepa-ration on any seismogram as well as the dis-tance on a travel-time graph at which theP-curve and S-curve have the same separa-tion, the distance to a quake’s epicenter canbe determined. This distance is called theepicentral distance.

The distance between an earthquake’sepicenter and a single seismic station doesnot provide sufficient information to deter-mine the location of that epicenter—itcould be located in any direction from theseismic station. The only thing that is cer-tain is that the epicenter is located some-where on a circle centered on the seismicstation. The radius of this circle is equal to


How is a seismic–intensity map made?

Make a Map using seismic–intensity data.

Procedure1. Trace the map onto paper. Mark the loca-

tions indicated by the letters on the map.2. Plot these Mercalli intensity values on the

map next to the correct letter: A, I; B, III;C, II; D, III; E, IV; F, IV; G, IV; H, V; I, V; J, V;K, VI; L, VIII; M, VII; N, VIII; O, III.

3. Draw contours on the map to separate theintensity values.

Analyze and Conclude1. What is the maximum intensity value?2. Where is the maximum intensity value

located?3. Where is the earthquake’s epicenter?

MI

PA

Lake Erie

OHM

K E

N

D

IN

KY

WV

H

I

A

LG

FJ

C O

B

Intensity Values of a Quake

the epicentral distance. If the distances between three or more seismicstations and an earthquake’s epicenter are known, the exact location ofthe epicenter can be determined, as shown in Figure 19-14. You willlocate an actual epicenter in the GeoLab at the end of this chapter.

Time of an Earthquake The time of occurrence of an earth-quake can be easily calculated, again by using the travel-time graphshown in Figure 19-9. The exact arrival times of the P-waves and S-waves at a seismic station can be read from the seismogram. Thetravel time of either wave at the epicentral distance of that stationcan be read from the travel-time graph. The time of occurrence ofthe earthquake is then determined by subtracting the appropriatetravel time from the known arrival time of the wave.

SEISMIC BELTSOver the years, seismologists have collected and plotted the locationsof numerous earthquake epicenters. The global distribution of theseepicenters, shown in Figure 19-15 on the next page, reveals an inter-esting pattern. Earthquake locations are not randomly distributed.The majority of the world’s earthquakes occur in relatively narrowseismic belts that separate large regions with little or no seismicactivity. What causes this pattern of seismic activity?

Look back at Figure 17-13 on page 455, which shows Earth’s tec-tonic plates and the boundaries between them. How does this mapcompare with the map shown in Figure 19-15? By comparing thetwo maps, you can see that most earthquakes are associated with tec-tonic plate boundaries. Almost 80 percent of all earthquakes occur inthe Circum-Pacific Belt. Another zone of significant seismic activitystretches across southern Europe and Asia. About 15 percent of the

19.3 Measuring and Locating Ear thquakes 509

Epicenter

Station A

Station B

Station C

Epice

ntral

distan

ce A

Epice

ntral

distan

ce C

Epicentral

distance B

Figure 19-14 To determine the location ofan earthquake’s epicenter, the locations ofthree seismic stations are plotted on a map.A circle whose radius is equal to the corre-sponding epicentral distance is plottedaround each station. The point of intersec-tion of these circles is the earthquake’s epicenter.

Update For an onlineupdate of recent earth-quakes, visit earthgeu.comand select the appropriatechapter.

http://earthgeu.com

world’s earthquakes take place in this region, which is sometimescalled the Mediterranean-Asian Belt. Most of the remaining earth-quakes occur in narrow bands that run along the crests of oceanridges. A very small percentage of earthquakes happen far from tec-tonic plate boundaries and are distributed more or less at random.

Look again at Figure 19-15. Do you live in an area prone toearthquakes? If so, how can you minimize the damage done by theseseismic events?


1. What is earthquake magnitude and howis it measured?

2. What is earthquake intensity and how isit measured?

3. Explain why earthquake data from atleast three seismic stations are needed tolocate an earthquake’s epicenter.

4. Thinking Critically The separation of P-waves and S-waves on a seismogramrecorded 4500 km from the epicenter of anearthquake is six minutes. On another seis-mogram that separation is seven minutes.

Is the second station closer to or more dis-tant from the epicenter? Explain.

SKILL REVIEW

5. Interpreting Scientific Diagrams UseFigure 19-15 above to determine which ofthe following countries has the mostearthquakes: Ireland, Pakistan, SouthAfrica, or Australia. Why does this countryhave more earthquakes than the otherthree countries? For more help, refer tothe Skill Handbook.

Earthquake epicenter

Global Earthquake Epicenter Locations

Figure 19-15 This mapshows the locations ofearthquake epicenters.



19.419.4 Earthquakes and Society

Environmental ConnectionEnvironmental Connection

19.4 Ear thquakes and Society 511

Most earthquake damage results from the prolonged shaking of the ground by surface waves. During major quakes, this shakingcan last longer than a minute. Many structures cannot withstandsuch violent motion. Collapsing buildings are responsible for manyearthquake-related deaths. What other types of damage are causedby earthquakes? What kinds of factors affect the damage done dur-ing a quake? Is it possible to predict earthquakes?

SOME EARTHQUAKE HAZARDSThe damage produced by an earthquake is directly related to thestrength or quality of the structures involved. The most severe dam-age occurs to unreinforced buildings made of stone, concrete, orother brittle building materials. Wooden structures, on the otherhand, are remarkably resilient and generally sustain significantly lessdamage. Many high-rise, steel-frame buildings also sustain littledamage during an earthquake because they are reinforced to makethem earthquake resistant. Some buildings in earthquake-proneareas, such as California, even rest on large rubber structures thatabsorb most of the vibrations generated during a quake.

Structural Failure In many earthquake-prone areas, buildingsare destroyed as the ground beneath them shakes. In some cases, thesupporting walls of the ground floor fail and cause theupper floors, which initially remain intact, to fall and col-lapse as they hit the ground or lower floors. The resultingdebris resembles a stack of pancakes, and thus, the processhas been called “pancaking”. This type of structural failureis shown in the photograph on page 494, and was a com-mon result of the quake that rocked Turkey in 1999. You’lllearn more about this quake and the damage it caused inthe Science in the News feature at the end of this chapter.

Another type of structural failure is related to theheight of a building. During the 1985 Mexico City earth-quake, for example, most buildings between 5 and 15 sto-ries tall collapsed or were otherwise completely destroyedas shown in Figure 19-16. Similar structures that wereeither shorter or taller, however, sustained only minor

OBJECTIVES

• Discuss factors thataffect the amount ofdamage done by anearthquake.

• Explain some of the fac-tors considered in earth-quake probability studies.

• Define seismic gaps.

VOCABULARY

tsunamiseismic gap

Figure 19-16 The buildingsdamaged or destroyed dur-ing the 1985 Mexico Cityquake vibrated with thesame period as the seismicwaves.


damage. Can you guess why? The shaking caused by the quake hadthe same period of vibration as the natural sway of the intermediatebuildings, which caused them to sway violently during the quake.The ground vibrations, however, were too rapid to affect taller build-ings, whose periods of vibration were longer than the earthquakewaves, and too slow to affect shorter buildings, whose periods ofvibration were shorter.

Land and Soil Failure In addition to their effects on structuresmade by humans, earthquakes can wreak havoc on Earth itself. Insloping areas, earthquakes may trigger massive landslides. Most ofthe estimated 30 000 deaths caused by the 7.8-magnitude earthquakethat struck in Peru in 1970 resulted from a landslide that buried sev-eral towns. In areas with fluid-saturated sand, seismic vibrations maycause subsurface materials to liquefy and behave like quicksand, gen-erating landslides even in areas of low relief. Soil liquefaction canalso cause trees and houses to fall over or to sink into the ground andcan cause underground pipes and tanks to rise to the surface.

In addition to causing soil liquefaction, earthquake waves can beamplified as they travel through a soil. Because soft materials havelittle resistance to deformation, seismic waves are amplified in suchmaterials but are muted in more-resistant materials. Consequently,wave size and earthquake intensity are greatest in soft, unconsoli-dated sediments and relatively small in hard, resistant rocks such as

granite. The severe damage to structuresin Mexico City during the 1985 earth-quake is attributed to the fact that MexicoCity is built on soft sediments. The thick-ness of the sediments caused them to res-onate with the same frequency as that ofthe surface waves generated by the quake.This produced reverberations that greatlyenhanced the ground motion and theresulting damage.

Fault Scarps Fault movements associ-ated with earthquakes can produce areasof great vertical offset where the faultintersects the ground surface. These off-sets are called fault scarps. As shown inFigure 19-17, a distinct fault scarpformed as the result of an earthquake thatstruck central Idaho in 1983. The magni-tude-7.6 quake that rocked Taiwan in

Figure 19-17 This faultscarp was produced byfaulting associated with an earthquake that struckMount Borah, Idaho, in1983.


Figure 19-18 Vertical offsetalong a fault caused thiswaterfall to form during the1999 Taiwan earthquake.

Using Numbers Thespeed of tsunamis inthe open ocean canreach 800 km/h. Atthis speed, how long,will it take a tsunamito travel from Japanto California, a dis-tance of about 9000km? If the waves are160 km long, how far apart in time arethe wave crests?

1999 produced vertical offsets of up to 10 m, the greatest fault move-ment observed in recent history. An 8-m-high waterfall that formedwhere the fault crosses a river is shown in Figure 19-18.

Tsunami Another type of earthquake hazard is a tsunami, a largeocean wave generated by vertical motions of the seafloor during anearthquake. These motions displace the entire column of wateroverlying the fault, creating bulges and depressions in the water. Thedisturbance then spreads out from the epicenter in the form ofextremely long waves. While these waves are in the open ocean, theirheight is generally less than 1 m. When the waves enter shallowwater, however, they may form huge breakers with heights occa-sionally exceeding 30 m! These enormous wave heights, togetherwith open-ocean speeds between 500 and 800 km/h, make tsunamisdangerous threats to coastal areas both near and far from thequake’s epicenter. The magnitude-9.5 earthquake that struck Chilein 1960 generated a tsunami that destroyed many villages along an800-km section of the South American coast. The wave then spreadacross the entire Pacific Ocean and struck Japan 23 hours later,killing 200 people. Even several days after the quake had struck, sig-nificant changes in water levels were observed in many coastal cities.

SEISMIC RISKRecall that most earthquakes occur in areas called seismic belts. Theprobability of future quakes is much greater in these belts than else-where around the globe. The past seismic activity in any region isalso a reliable indicator of future earthquakes and can be used togenerate seismic-risk maps. A seismic-risk map of the United Statesis shown in Figure 19-19 on the next page. Can you locate the areasof highest seismic risk on the map? In addition to Alaska, Hawaii,and some western states, there are several regions of relatively high

seismic risk in the central and eastern United States. These regionshave suffered disastrous earthquakes in the past and probably willexperience significant seismic activity in the future. Locate your statein Figure 19-19. What is the seismic risk of your area?

EARTHQUAKE PREDICTIONTo minimize the damage and deaths caused by quakes, seismologistsare searching for ways to predict these events. Earthquake predictionresearch is largely based on probability studies. The probability of anearthquake’s occurring is based on two factors: the history of earth-quakes in an area and the rate at which strain builds up in the rocks.

Earthquake History Earthquake recurrence rates can indicatethat the fault involved ruptures repeatedly at regular intervals to gen-erate similar quakes. The earthquake-recurrence rate at Parkfield,California, for example, shows that a sequence of quakes of approx-imately magnitude-6 shook the area about every 22 years from 1857until 1966. This record indicates a 90-percent probability that amajor quake will rock the area within the next few decades. Severalkinds of instruments, including the lasers shown in Figure 19-20, arein place around Parkfield in an attempt to predict future quakes.


None

Damage expected

Minor

Moderate

Major

Seismic Risk Map of the United States

Figure 19-19 Areas of high seismic risk in the United Statesinclude Alaska, Hawaii, and some of the western states.


Figure 19-20 Lasers arejust one technique beingused to study earthquakeprobability in the areaaround Parkfield, California.Lasers can record very smallmovements along a fault.

Probability forecasts are also based onthe location of seismic gaps. Seismic gapsare sections of active faults that haven’texperienced significant earthquakes for a long period of time. One seismic gap inthe San Andreas Fault cuts through SanFrancisco. This section of the fault hasn’truptured since the devastating earthquakethat struck the city in 1906. Because of thisinactivity, seismologists currently predictthat there is a 67-percent probability thatthe San Francisco area will experience amagnitude-7 or higher quake within thenext 30 years.

Strain Accumulation The rate atwhich strain builds up in rocks is anotherfactor used to determine the earthquakeprobability along a section of a fault. To predict when a quake mightoccur, scientists make several measurements. The strain accumulatedin a particular part of the fault, together with how much strain wasreleased during the last quake along that section of the fault, are twoimportant factors in earthquake probability studies. Another factoris how much time has passed since an earthquake has struck that sec-tion of the fault.

Earthquake prediction is still a relatively new branch of geology.Being able to predict these destructive events can prevent damage toproperty, possibly reduce the number of injuries as a result of thequake, and, most importantly, save many lives.

1. Describe structural damage caused byearthquakes.

2. What are some of the effects caused bysoil liquefaction?

3. How are tsunamis generated?

4. What are some of the factors consideredin earthquake probability studies?

5. Thinking Critically Which structure is lesslikely to suffer severe damage during an

earthquake: a high-rise, steel-frame hotelbuilt on sediments, or a wood-framehouse built on bedrock? Explain.

SKILL REVIEW

6. Comparing and Contrasting Compare andcontrast the seismic risk of your state toat least three neighboring states. Formore help, refer to the Skill Handbook.




Locating an Epicenter

The separation of P- and S-waves on a seismogram allowsyou to estimate the distance between the seismic station

that recorded the data and the epicenter of that earthquake.If the epicentral distance from three or more seismic stations isknown, then the exact location of the quake’s epicenter can bedetermined.

ProblemDetermine the epicenter location andthe time of occurrence of an actualearthquake, using the travel times ofP- and S-waves recorded at three seis-mic stations.

MaterialsFigures 17-13, 19-8, and 19-9map on facing page calculatordrafting compass metric rulertracing paper

ObjectivesIn this GeoLab, you will: • Determine the arrival times of P- and

S-waves from a seismogram.• Interpret travel-time curves.• Plot an epicenter location on a map.• Relate seismic data to plate tectonics.

Safety Precaution

Preparation

1. The seismogram in Figure 19-8shows the arrival time of the first P-wave at 10 h, 50 min, 32 s GMT,Greenwich mean time. Estimate thearrival time of the first S-wave to thenearest tenth of a minute.

2. Subtract this S-wave time from theinitial P-wave time. What is the P-Sseparation on the seismogram, inminutes and tenth of minutes? Enterthis value in the data table for theBerkeley seismic station.

Procedure

Seismic Station P-S Separation Epicenter Distance Map Distance(min) (km) (cm)

Berkeley, CA 3.9 2300 4.6Boulder, CO 3.6 1900 3.8Knoxville, TN 4.9 2600 5.2

GEOLAB DATA TABLE

Seismic Station

GeoLab 517

1. Where is this epicenter located? 2. In which major seismic belt did this

earthquake occur?

3. Use Figure 17-13 to determine whichplates form the boundary associatedwith this earthquake.

3. The P-S separation observed ontwo other seismograms, whichare not shown, are also listed inthe table. Use the travel-timecurves in Figure 19-9 to deter-mine the distances at which theP- and S-curves are separatedby the time intervals listed inthe table. Enter these distancesin the table under the EpicenterDistance.

4. Carefully trace the map on thispage. Accurately mark the threeseismic station locations.

5. Determine the epicentral dis-tances on your tracing, using ascale of about 0.9 cm = 500 km.Enter your values in the tableunder the Map Distance.

6. Use the compass to draw circlesaround each station on the mapwith the radius of each circleequal to the map distance, incm, for that station.

7. Mark the point of intersection.This is the epicenter of theearthquake.

8. Determine the time of occurrence ofthis earthquake by reading the P-wavetravel time from Figure 19-9 for theepicentral distance for Berkeley.

Subtract this from the initial P-wavearrival time at Berkeley, which was 10 h, 50.5 min. Express this time interms of hours, minutes, and seconds.

1. What type of plate boundary is this? 2. Briefly describe the relative motion of

the plates involved.

3. Describe the tectonic motions thatcaused the earthquake.

Analyze

Conclude & Apply

MEXICO

UNITED STATES

CENTRALAMERICA

Mazatlán Tampico

Mexico City

Acapulco

Knoxville

NewYorkCity

Miami

Houston

Oklahoma City

Chihuahua

Berkeley

Phoenix

Boulder

500 km0

N

United States and Mexico

The AftermathImmediately following the quake, rescue

efforts and humanitarian aid were launched byinternational organizations such as the UnitedNations and the Red Cross. Food, water, shelter,and medical services were provided to victimsthrough relief efforts and donations. Searchteams found one man alive beneath rubble 13days after the quake struck. Initial estimates forrecovery and rebuilding of the city of Bam werebetween $70 million and $1 billion dollars. TheWorld Health Organization estimated that 30 mil-lion dollars would be needed to reestablish Bam’shealth services. Two hospitals were destroyed andhalf of Bam’s health workers were killed in theearthquake. A minimum estimate of two years wasgiven by the United Nations Office of Coordinationof Human Affairs for rebuilding the fallen city.


It is estimated that as many as 40,000 peoplewere killed in the quake, up to 30,000 more wereinjured, and more than 75,000 people were leftwithout homes. Eighty-five percent of the buildingsin the city were damaged or destroyed, and thewater supply system was damaged. A 2,000-year-old citadel, Arg-e Bam, thought to be the world’sgreatest mud fortress, was reduced to rubble as aresult of the quake. The historic fortress was thecity’s most popular tourist attraction.

Tectonic Factors The southeastern portion of Iran is a seismi-

cally active area. Although no earthquakes havepreviously been reported in Bam, major earth-quakes have occurred in the region northwest ofBam. Between 1981 and 1998, four earthquakeswith magnitudes greater than 5.6 occurred in thisarea. The origin of the December 26 quake isthought to be the Bam fault, a fault line that runsnorth-south through the region. According to theU.S. Geological Survey, the quake occurred as aresult of stresses created by the motion of theArabian plate northward against the Eurasianplate at a rate of approximately 3 cm/yr. The hori-zontal and vertical movement along a strike-slipfault resulted in the destruction of homes andbuildings throughout Bam.

Earthquake in IranAt 5:26 am local time on December 26, 2003, an earthquake rockedthe city of Bam, located in southeastern Iran. Some residents werealready sleeping outside due to a foreshock that occurred the previousevening. The earthquake had a magnitude of 6.6 and caused extensivedamage in terms of human life and property, including leveling an ancientfortress that had been a World Heritage Site.

Dealing with disasters of the magnitude ofthe Bam earthquake can seem overwhelm-ing. Contact emergency preparedness agen-cies in your area to find out what stepshave been taken to locally prepare for nat-ural disasters. What kinds of disasters dothese agencies primarily prepare for? Makea pamphlet to summarize your findings.

Activity

Summary

Vocabularyepicenter (p. 499)fault (p. 497)focus (p. 499)primary wave (p. 498)secondary wave

(p. 498)strain (p. 496)stress (p. 495)surface wave (p. 498)

Vocabularyseismogram (p. 500)seismometer (p. 500)

Vocabularymagnitude (p. 505)modified Mercalli

scale (p. 506)moment magnitude

scale (p. 506)Richter scale (p. 505)

Vocabularyseismic gap (p. 515)tsunami (p. 513)

Main Ideas• Stress is a force per unit area that acts on a material. The defor-

mation of materials in response to stress is called strain.• Reverse faults form as a result of horizontal compression; nor-

mal faults, horizontal tension; strike-slip faults, horizontal shear.• P-waves squeeze and pull rocks in the same direction along

which the waves travel. S-waves cause rocks to move at rightangles to the direction of the waves. Surface waves cause both an up-and-down and a side-to-side motion as they passthrough rocks.

Main Ideas• A seismometer has a frame that is anchored to the ground and

a suspended mass. Because of inertia, the mass tends to stay atrest as the ground and, thus, the frame vibrate during a quake.The motion of the mass in relation to the frame is registered andrecorded.

• Seismic waves are reflected and refracted as they strike differentmaterials. Analysis of these waves has enabled scientists todetermine the structure and composition of Earth’s interior.

Main Ideas• Earthquake magnitude is a measure of the energy released dur-

ing a quake and can be measured on the Richter scale. Intensityis a measure of the damage caused by a quake and is measuredwith the modified Mercalli scale.

• Data from at least three seismic stations are needed to locate anearthquake’s epicenter.

• Most earthquakes occur in areas associated with plate bound-aries called seismic belts.

Main Ideas• Earthquakes cause structural collapse, landslides, soil liquefac-

tion, fissures, fault scarps, uplift or subsidence, and tsunamis.Factors that affect the extent of damage done by a quakeinclude the type of subsurface as well as the quality, height,and structure of buildings and other structures involved.

• The probability of an earthquake is based on the history of quakesin an area and the rate at which strain builds in the rocks.

• Seismic gaps are places along an active fault that haven’t expe-rienced significant earthquakes for a long period of time.

Frame

Spring

MassPen

Rotatingdrum

SECTION 19.1

Forces WithinEarth

SECTION 19.2

Seismic Wavesand Earth’sInterior

SECTION 19.3

Measuringand LocatingEarthquakes

SECTION 19.4

Earthquakesand Society

Study Guide 519earthgeu.com/vocabulary_puzzlemaker

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1. What is stress?a. movement of waves parallel to rock particlesb. deformation of a material caused by applied

forcesc. forces per unit area acting on a materiald. unit of measure on the Richter scale

2. What is strain?a. forces per unit area acting on a materialb. deformation of a material caused by

applied forcesc. unit of measure on the Mercalli scaled. travel time of seismic waves

3. Which type of seismic wave causes rock particlesto move in the same direction as the wave movement?a. P-wave c. tension waveb. S-wave d. shear wave

4. What part of Earth doesn’t receive direct P-wavesfrom a quake?a. epicenter c. shadow zoneb. focus d. mantle

5. Which is used to measure magnitude?a. Richter scale c. shadow zoneb. Mercalli scale d. seismic gap

6. What is earthquake intensity?a. a measure of the energy releasedb. a measure of seismic riskc. a measure of damage doned. a measure of the quake’s focus

7. What is a seismic gap?a. a large fault scarpb. a part of an active fault that hasn’t recently

experienced seismic activityc. the time separation between P- and S-wavesd. the liquefaction of soil during a quake

Understanding Main Ideas 8. Compare and contrast the three types of faults.

9. Draw three diagrams to show how each type ofseismic wave moves through rocks.

10. Explain how a seismometer works.

11. How have seismic waves been used to determineEarth’s structure and composition?

12. Why are data from at least three seismic stationsneeded to locate an epicenter?

Use this figure to answer questions 13–15.

13. What type of fault is shown?

14. What type of force caused this fault to form?

15. Where is the fault plane?

16. Explain the relationship between worldwideearthquake distribution and tectonic boundaries.

17. What factors affect the damage done by anearthquake?

MORE THAN ONE GRAPHIC If a test question refers to more than one table, graph,diagram, or drawing, use them all. If you answerbased on just one graphic, you’ll probably missan important piece of information.

Test-Taking Tip

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Assessment 521

1. What happens to a material when it under-goes stress that exceeds its strength?a. The material undergoes ductile formation.b. The material is deformed permanently.c. The material returns to its original state.d. The material breaks or fails.

2. What is the order in which seismic waves arerecorded by a seismometer?a. S-wave, P-wave, surface waveb. surface wave, P-wave, S-wavec. P-wave, S-wave, surface waved. S-wave, surface wave, P-wave

INTERPRETING DATA Use the table below toanswer questions 3 and 4.

3. Approximately how much more energy wasreleased by the Chilean quake than theTaiwan earthquake?a. twice as muchb. ten times as muchc. thirty two times as muchd. one thousand times as much

4. Approximately how much larger was theamplitude of the waves generated by theAlaskan quake than the Taiwan quake?a. about twice as largeb. about ten times as large c. about one hundred times as larged. about one thousand times as large

18. What factors are studied in the field of earth-quake probability?

19. Explain why a stress-strain curve usually has twosegments.

20. Why do surface waves cause so much destruction?

21. Why are two types of seismometers generallyused to record the same earthquake?

22. How were P-waves and S-waves used to deter-mine the physical state of Earth’s core?

23. Compare and contrast the Richter scale and themoment magnitude scale.

24. Explain how shear stress is different from tensionand compression.

25. Compare and contrast the composition of Earth’s mantle and core with the composition of meteorites.

26. How do you think a thin, plastic, ruler wouldreact to a small amount of stress? What wouldhappen if the stress applied exceeded the elasticlimit of the ruler?

27. Describe several reasons why an earthquake ofmagnitude-3 can cause more damage than aquake of magnitude-6.

28. Why are tsunamis so destructive?

29. If rocks below the lithosphere are too hot toundergo brittle fracture, how is it possible to havedeep-focus earthquakes beneath island arcs?

30. Refer to Figure 19-19 on page 514. Explainwhy some areas in the eastern part of the UnitedStates are prone to major earthquake damageeven though these places are far from presenttectonic plate boundaries.

Thinking Critically

Applying Main Ideas

Standardized Test Practice

Location Year Richter Magnitude

Chile 1960 8.5 California 1906 7.9

Alaska 1964 8.6 Columbia 1994 6.8 Taiwan 1999 7.6

Some Earthquakes In Recent History

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