8.3 Destruction fromEarthquakes

Reading StrategyMonitoring Your Understanding Previewthe Key Concepts, topic headings, vocabulary,and figures in this section. List two things youexpect to learn. After reading, state what youlearned about each item you listed.

Key ConceptsWhat destructive eventscan be triggered byearthquakes?

Can earthquakes bepredicted?

Vocabulary◆ liquefaction◆ tsunami◆ seismic gap

What I ExpectTo Learn What I Learned

a. b.

c. d. ??

??

The Good Friday Alaskan Earthquake in 1964 was the most violentearthquake to jar North America in the 20th century. The earthquakewas felt throughout Alaska. It had a moment magnitude of 9.2 and lasted3 to 4 minutes. The quake left 131 people dead and thousands home-less. The state’s economy was also badly damaged because the quakeaffected major ports and towns. Had the schools and businesses beenopen on this holiday, the death toll would surely have been much higher.

Seismic VibrationsThe 1964 Alaskan earthquake gave geologists new insights into the roleof ground shaking as a destructive force. The damage to build-ings and other structures from earthquake waves depends on severalfactors. These factors include the intensity and duration ofthe vibrations, the nature of the material on which thestructure is built, and the design of the structure.

Building Design All multistory buildings in Anchorage,Alaska, were damaged by the vibrations. However, the more flex-ible wood-frame buildings, such as homes, were less damaged.Figure 10 offers an example of how differences in constructioncan affect earthquake damage. You can see that the steel-framebuilding on the left withstood the vibrations. However, thepoorly designed building on the right was badly damaged.Engineers have learned that unreinforced stone or brick build-ings are the most serious safety threats during earthquakes.

Figure 10 Earthquake DamageThis five-story building inAnchorage, Alaska, collapsedfrom the great earthquake of1964. Very little structural damagewas incurred by the steel-framedbuilding to the left. Inferring Why do some buildingsundergo little damage, whilenearby buildings are nearlydestroyed?

Earthquakes and Earth’s Interior 229

FOCUS

Section Objectives8.7 Describe the factors

contributing to earthquakedamage.

8.8 Identify other dangersassociated with earthquakes.

8.9 Explain the potential forearthquake prediction.

Build VocabularyParaphrase Ask students to write thevocabulary words on a sheet of paper.Instruct students to write a definition, intheir own words, for each term as theyencounter the term while going throughthe chapter. After writing their owndefinition, they should also write acomplete sentence with the term.

Reading StrategySample answers:a. how seismic vibrations can causedamageb. Damage depends on the buildingdesign, intensity and length of time ofthe vibrations, and the material that thebuilding was constructed on.c. dangers associated with earthquakesd. These include tsunamis, landslides,and fire.

INSTRUCT

Seismic VibrationsReading StrategyInvite a structural engineer to speakto the class about the construction ofearthquake-safe buildings. Havestudents ask about specific regulationsfor building codes in your area.Interpersonal

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Earthquakes and Earth’s Interior 229

Section 8.3

Answer to . . .

Figure 10 Damage to buildings oftendepends on the construction anddesign of the building. For example,buildings made of wood often are moreflexible than buildings made of concrete.

Customize for Inclusion Students

Gifted When we consider how manyearthquakes there are in one year, the numberof earthquakes that cause terrible damage isactually very small. The amount of damage anearthquake causes depends on many conditions.For example, if a building is well constructedand built on solid ground, it may survive anearthquake. Most injuries and deaths duringearthquakes are because of poor construction

or substandard building sites. Another seriousproblem is not knowing how to respond to anearthquake. When people panic and rush outof buildings there is a danger of being trampled,suffocated, or injured by falling debris. Go tothe Red Cross website at http://www.redcross.org/services/disaster and read aboutearthquake safety.

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Sea level

Tsunami speed:835 km/h

Tsunami speed:340 km/h

Tsunami speed:50 km/h

Water depth:5500 meters

Displacement

Water depth:900 meters Water depth:

20 meters

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Figure 11 Movement of a Tsunami A tsunami is generated by movementof the ocean floor. The speed of a wave moving across the ocean is related tothe ocean depth. Waves moving in deep water travel more than 800kilometers per hour. Speed gradually slows to 50 kilometers per hour atdepths of 20 meters. As waves slow down in shallow water, they grow inheight until they topple and hit shore with tremendous force.

Liquefaction Where loosely consolidated sediments are saturatedwith water, earthquakes can cause a process known as liquefaction.Under these conditions, what had been stable soil turns into a liquidthat is not able to support buildings or other structures. Buildings andbridges may settle and collapse. Underground storage tanks and sewerlines may float toward the surface.

TsunamisMost deaths associated with the 1964 Alaskan quake were caused byseismic sea waves, or tsunamis. These destructive waves often are calledtidal waves by news reporters. However, this name is incorrect becausethese waves are not produced by the tidal effect of the moon or sun.

Causes of Tsunamis A tsunami triggered by an earthquakeoccurs where a slab of the ocean floor is displaced vertically along afault. A tsunami also can occur when the vibration of a quake setsan underwater landslide into motion. Once formed, a tsunamiresembles the ripples created when a pebble is dropped into a pond.A tsunami travels across the ocean at speeds of 500 to 950 kilometersper hour. Despite this speed, a tsunami in the open ocean can passwithout notice because its height is usually less than 1 meter, and thedistance between wave crests can range from 100 to 700 kilometers.However, when the wave enters shallower coastal water, the waves areslowed and the water begins to pile up to heights that sometimes aregreater than 30 meters, as shown in Figure 11.

When does liquefaction occur?

Q What is the largest wave triggered by an earthquake?

A The largest wave everrecorded occurred in LituyaBay, about 200 kilometers westof Juneau, Alaska. On July 9,1958, an earthquake triggeredan enormous rockslide thatdumped 90 million tons of rockinto the upper part of the bay.The rockslide created a hugesplash wave that swept over theridge facing the rockslide. Thesplash uprooted or snapped offtrees 522 meters above the bay.Even larger splash waves mayhave occurred 65 million yearsago when an estimated 900-meter wave is thought to haveresulted from a meteoriteimpact in the Gulf of Mexico.

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Many students may have heard thatthe safest place in a house during anearthquake is in a doorway. Challengethis misconception by pointing out thatmodern doorways are no stronger thanother sections of a house and usuallyhave doors that could swing and injuresomeone. Encourage students to comeup with another plan for earthquakesafety. This should involve duckingunder a sturdy table or desk and stayingclear of objects that could tip over, suchas file cabinets and bookcases.

Build Reading LiteracyRefer to p. 334D in Chapter 12, forguidelines on outlining content.

Outline Have students read thesection. Then have students use theheadings as major divisions in anoutline. Allow students to refer to theiroutlines when answering the questionsin Section 8.3 Assessment.Visual

TsunamisBuild Science SkillsObserving Have pairs of studentsinvestigate recent or historicallysignificant tsunamis. (They may uselibrary resources or conduct a Websearch.) After students have had time toobtain information, have them comparetheir findings with another group.Interpersonal, Verbal

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Tsunami Warning System The destruction from a largetsunami in the Hawaiian Islands led to the creation of a tsunami warn-ing system for coastal areas of the Pacific. Large earthquakes are reportedto the Tsunami Warning Center in Honolulu from seismic stationsaround the Pacific. Scientists use water levels in tidal gauges to determinewhether a tsunami has formed. Within an hour of the reports, a warningis issued. Although tsunamis travel very rapidly, there is sufficient timeto evacuate all but the area closest to the epicenter. Fortunately, mostearthquakes do not generate tsunamis. On the average, only one or twodestructive tsunamis are generated worldwide every year. Only about onetsunami in every 10 years causes major damage and loss of life.

Other DangersThe vibrations from earthquakescause other dangers, including land-slides, ground subsidence, and fires.

Landslides With manyearthquakes, the greatest damageto structures is from landslidesand ground subsidence, or thesinking of the ground triggered bythe vibrations. The violent shakingof an earthquake can cause the soiland rock on slopes to fail, resultingin landslides. Figure 12 shows someof the damage landslides can cause.Earthquake vibration can alsocause large sections of the ground to collapse, liquefy, or subside.Ground subsidence can cause foundations to collapse, as shown in Figure 12. It can also rupture gas and water pipelines.

Fire The 1906 San Francisco earthquake reminds us of the majorthreat of fire. The city contained mostly large wooden structures andbrick buildings. The greatest destruction was caused by fires thatstarted when gas and electrical lines were cut. Many of the city’s waterlines had also been broken by the quake, which meant that the firescouldn’t be stopped. A 1923 earthquake in Japan caused an estimated250 fires. They devastated the city of Yokohama and destroyed morethan half the homes in Tokyo. The fires spread quickly due to unusu-ally high winds. More than 100,000 people died in the fires.

What areas are protected by thetsunami warning system?

Earthquakes and Earth’s Interior 231

Figure 12 This landslide causedby the 1964 Alaskan earthquakedestroyed many homes. Morethan 200 acres of land slid towardthe ocean.Interpreting Photos Assumingthe land was originally horizontal,to what angle have the trees on the left side of the photo been tilted?

Other Dangers

Build Science SkillsUsing ModelsStudents should createa model of a house on asmall hill using sand,potting soil, and thin wire netting. Themodel should be no more than 30 cmhigh. Then, students will shake theirmodel in such a way that mimics anearthquake. They will then observe whathappens to the hill and the buildingsplaced on it. Ask: What happens to thebuildings on the slope? (Answers willvary but students should see that thebuildings slid down the slope.) Whatimpact could water have on themodel earthquake? (The damagewould probably be worse if the hillsidewas saturated with water.)Kinesthetic

Use CommunityResourcesInstruct students to ask their villageor city officials about local tsunamis,landslides, or fires that resulted from anearthquake. Some sources to contactmight be fire departments, city halls,and newspaper or media archives.Ask them to brainstorm appropriatequestions and ask the official they hopeto interview.

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Earthquakes and Earth’s Interior 231

Answer to . . .

Figure 12 45°

Liquefaction occurswhen loosely consoli-

dated soils saturated with water areshaken by earthquake waves.

coastal areas of thePacific

The 1906 earthquake in San Francisco was oneof the most devastating in the United States.The earthquake and resulting fires caused anestimated 3,000 deaths and $524 million inproperty loss. Damage in San Francisco alonewas estimated at $20 million; outside the city,it was estimated at $4 million. The duration ofthe shaking in San Francisco was about1 minute.

The earthquake damaged buildings andstructures in all parts of the city and county

of San Francisco. On the San Andreas fault,buildings were completely destroyed or tornapart; trees fell to the ground. The surface ofthe ground was torn and heaved into furrow-like ridges. Roads crossing the fault line wereimpassable. Pipelines were broken, shutting offthe water supply to the city. The fires thatignited soon after the earthquake quicklyraged through the city because of the lackof water to control them.

Facts and Figures

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Section 8.3 Assessment

Reviewing Concepts1. What destructive events can be triggered

by an earthquake?

2. What physical changes have been used inthe attempts to predict earthquakes?

3. What is a tsunami?

4. What is a seismic gap?

Critical Thinking5. Making Judgments Do you think scientists

are close to being able to accurately predictearthquakes? Explain your answer.

6. Drawing Conclusions Why is it incorrect torefer to tsunamis as tidal waves?

Predicting EarthquakesThe earthquake in Northridge, California, in 1994 caused 57 deathsand about $40 billion in damage. Scientists warn that quakes of simi-lar or greater strength will occur. But can earthquakes be predicted?

Short-Range Predictions The goal of short-range predictionis to provide an early warning of the location and magnitude of a largeearthquake. Researchers monitor possible precursors—things that pre-cede and may warn of a future earthquake. They measure uplift,subsidence, and strain in the rocks near active faults. They measurewater levels and pressures in wells. Radon gas emissions from fracturesand small changes in the electromagnetic properties of rocks are alsomonitored. So far, methods for short-range predictions of earth-quakes have not been successful.

Long-Range Forecasts Long-range forecasts give the probabil-ity of a certain magnitude earthquake occurring within 30 to 100-plusyears. These data are important for updating building codes, which havestandards for designing earthquake-resistant structures. Long-rangeforecasts are based on the idea that earthquakes are repetitive or cycli-cal. In other words, as soon as one earthquake is over, the forces in Earthwill begin to build strain in the rocks again. Eventually the rocks will slipagain, causing another earthquake. Scientists study historical recordsof earthquakes to see if there are any patterns of recurrence. They alsostudy seismic gaps. A seismic gap is an area along a fault where there hasnot been any earthquake activity for a long period of time. There hasbeen only limited success in long-term forecasting. Scientists don’tyet understand enough about how and where earthquakes will occurto make accurate long-term predictions.

Figure 13 Effects ofSubsidence Due to Liquefaction This tilted buildingrests on unconsolidated sedimentthat imitated quicksand duringthe 1985 earthquake in Mexico.

Earthquakes In Section 8.1, youlearned about the elastic energy storedin rocks before an earthquake and theelastic rebound hypothesis. How couldthis information be used to try topredict earthquakes?

For: Links on predictingearthquakes

Visit: www.SciLinks.org

Web Code: cjn-3082

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PredictingEarthquakesIntegrate BiologyCan Animals Predict Earthquakes?There is much speculation as to theability of animals to predict earthquakes.Documented cases have shown snakesand bees rapidly leaving their homes,excessive dog barking, and erraticbehavior in domesticated and wildanimals prior to major earthquakes.The US Geological Survey, however,is more skeptical. They acknowledgethe abundance of cases of reportedbehavioral changes prior to anearthquake but there aren’t enoughreproducible connections to conclusivelystate that animals are predicting theearthquakes. Have students researchspecific cases of odd animal behaviorprior to earthquakes and present theirfindings in a newspaper article.Verbal

ASSESSEvaluateUnderstandingHave students work in groups to developa short public service announcement onthe other dangers facing areas that haveexperienced an earthquake.

ReteachAsk students to use the diagram inFigure 11 to explain how tsunamis aregenerated and how they move to shore.

If there were some way to measure theamount of energy stored in rocks, thismight lead to the prediction ofearthquakes. If scientists could observeand measure the buildup of stress withinrocks, they might be able to determinethe amount of stress the rocks couldwithstand before the energy needed tobe released. This could provide anestimate of time for an earthquake.

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4. A seismic gap is an area along a fault thathas not had any earthquake activity for a longperiod of time.5. Answers will vary. Sample answer: Scientistsdon’t yet understand enough about how andwhere earthquakes occur to make accuratepredictions.6. Tidal waves are caused by the gravitationalpull of the moon and sun. Tsunamis are largewaves caused by earthquake movements.

Section 8.3 Assessment

1. Events such as landslides, tsunamis, andfires can be triggered by earthquakes.2. Physical changes such as uplift, subsidence,strain in rocks along faults, water levels inwells, and radon gas emissions from fractureshave been measured in hopes of predictingearthquakes.3. A tsunami is a seismic sea wave created byan underwater earthquake or a landslide underthe ocean floor generated by an earthquake.

Download a worksheet on predictingearthquakes for students tocomplete, and find additionalteacher support from NSTA SciLinks.

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