BAChapter 2: Earthquakes 51

VOCABULARYseismic wave p. 51focus p. 52epicenter p. 52seismograph p. 56

BEFORE, you learned

• Most earthquakes occur alongtectonic plate boundaries

• Different directions of stresscause normal, reverse, andstrike-slip faults

NOW, you will learn

• How energy from an earth-quake travels through Earth

• How an earthquake’s locationis determined

KEY CONCEPT

Earthquakes releaseenergy.

MAIN IDEA AND DETAILSRecord information aboutthe energy released byearthquakes.

Energy from earthquakes travels through Earth.

When you throw a rock into a pond, waves ripple outward from the spotwhere the rock hits the water. The energy released by an earthquaketravels in a similar way through Earth. Unlike the pond ripples, though,earthquake energy travels outward in all directions—up, down, and to the sides. The energy travels as (SYZ-mihk) whichare vibrations caused by earthquakes. Seismic waves from even smallearthquakes can be recorded by sensitive instruments around the world.

seismic waves,

EXPLORE Movement of Energy

How does energy travel?

PROCEDURE

On a flat surface, hold one end of a spring toy while a partner holds the other end. Stretch the spring, then squeeze some coils together and release them.

Again, hold one end of the spring while your partner holds the other end. Shake your end of the spring back and forth.

WHAT DO YOU THINK?• How did energy travel along the spring when

you gathered and released some coils?• How did energy travel along the spring when

you shook one end back and forth?

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1

MATERIALSspring toy

BA 52 Unit: The Changing Earth

All earthquakes start beneath Earth’s surface. The ofan earthquake is the point underground where rocks first begin tomove. Seismic waves travel outward from the earthquake’s focus.The (EHP-ih-SEHN-tuhr) is the point on Earth’s surfacedirectly above the focus. Scientists often name an earthquake after the city that is closest to its epicenter.

In general, if two earthquakes of equal strength have the same epicenter, the one with the shallower focus causes more damage.Seismic waves from a deep-focus earthquake lose more of their energy as they travel farther up to Earth’s surface.

The depths of earthquakes along tectonic plate boundaries arerelated to the directions in which the plates move. For example, anearthquake along a mid-ocean spreading center has a shallow focus.There, the plates are pulling apart, and the new crust that forms is thin.Subduction zones have a wide range of earthquake depths, from shallowto very deep. Earthquakes can occur anywhere along the sinking plates.

epicenter

focus

Focus and Epicenter

FPO

reading tip

The prefix epi- comes froma Greek word meaning “ontop of.” An earthquake’sepicenter is directly over its focus.

What is the difference between the focus and theepicenter of an earthquake?

Seismic waves spread out from the focus of an earthquake.

Structures fartherfrom the epicenterexperience lessshaking and lessdamage.

Structures near theepicenter experiencemore shaking andmore damage.

The earthquake beginsat the focus.

Seismic waves spreadout in all directions fromthe focus.

The epicenteris the point on thesurface directlyabove the focus.

fault

BAChapter 2: Earthquakes 53

Waves and EnergyWaves are part of your everyday life. For example, music reaches yourears as sound waves. All waves, including seismic waves, carry energyfrom place to place. As a wave moves through a material, particles ofthe material move out of position temporarily, causing the particlesnext to them to move. After each particle moves, it returns to its original position. In this way, energy moves through the material,but matter does not.

On October 17, 1989, an earthquake stopped baseball’s World Seriesat Candlestick Park in San Francisco. As the seismic waves arrived,fans heard a low rumble; then for about 15 seconds the stadium shookfrom side to side and up and down. About 20 minutes after the earth-quake was felt at the stadium, the seismic waves had traveled to theother side of Earth. There, the waves did not shake the ground hardenough for people to notice. The waves could be detected only by scientific instruments.

Why are some earthquakes deeper than others?PROCEDURE

Cut the first string into 4 pieces that are 4 cm long. Cut the second stringinto 3 pieces that are 8 cm long, and the third string into 4 pieces that are 15 cm long.

Use the key on the Earthquake Map to match string lengths with earthquake depths.

Tape one end of the pieces of string to the map at the earthquake locations, asshown in the photograph. Always cover the same amount of string with tape.

Hold the map upside down, with the strings hanging down. Observe the patterns of earthquake locations and depths.

WHAT DO YOU THINK?• What patterns among the strings do you observe? How do you

explain them?

• How might the earthquake depths relate to the sinking of atectonic plate in a subduction zone?

CHALLENGE Draw a line on the map, showing where thesubduction zone might be at Earth’s surface. How might thedepths of the earthquakes be different if the subduction zonewere on the other side of the island?

4

3

2

1

Subduction-Zone EarthquakesSubduction-Zone EarthquakesSKILL FOCUSAnalyzing

MATERIALS• different colors

of string• ruler• scissors• Earthquake Map• tape

TIME20 minutes

BA 54 Unit: The Changing Earth

Earthquakes produce three types of seismic waves: primary waves,secondary waves, and surface waves. Each type moves through materialsdifferently. In addition, the waves can reflect, or bounce, off boundariesbetween different layers. The waves can also bend as they pass from onelayer into another. Scientists learn about Earth’s layers by studying thepaths and speeds of seismic waves traveling through Earth.

Primary WavesThe fastest seismic waves are called primary waves, or P waves. Thesewaves are the first to reach any particular location after an earthquakeoccurs. Primary waves travel through Earth’s crust at an average speedof about 5 kilometers per second (3 mi/s). Primary waves can travelthrough solids, liquids, and gases. As they pass through a material, theparticles of the material are slightly pushed together and pulled apart.Buildings also experience this push and pull as primary waves passthrough the ground they are built on.

Secondary WavesSecondary waves are the second seismic waves to arrive at any particularlocation after an earthquake, though they start at the same time as primary waves. Secondary waves travel through Earth’s interior atabout half the speed of primary waves. Secondary waves are also calledS waves. As they pass through a material, the material’s particles areshaken up and down or from side to side. Secondary waves rock smallbuildings back and forth as they pass.

Secondary waves can travel through rock, but unlike primary wavesthey cannot travel through liquids or gases. Look at the illustrations onpage 55. As a primary wave passes through a material, the volume anddensity of the material change slightly. But as a secondary wave passes,the material changes slightly in shape. Liquids and gases do not havedefinite shapes. These materials flow—that is, particles in them do notreturn to their original positions after being moved. When scientistslearned that secondary waves cannot pass through Earth’s outer core,they realized that the outer core is not solid.

check your reading Why can’t secondary waves travel through liquids or gases?

Surface WavesSurface waves are seismic waves that move along Earth’s surface, notthrough its interior. They make the ground roll up and down or shakefrom side to side. Surface waves cause the largest ground movementsand the most damage. Surface waves travel more slowly than the othertypes of seismic waves.

VISUALIZATIONCLASSZONE.COM

Explore primary-waveand secondary-wavemotion.

reading tip

One meaning of primary is“first.” Primary waves arrivebefore secondary waves.

Earthquakes produce three types of seismic waves.

Seismic Waves

Primary Waves

In primary waves, the particles of materialsare slightly pushed together and pulled apartin the direction of the waves’ travel.

wave directionparticle motion

surfacewave

primarywave

secondarywave

Chapter 2: Earthquakes 55

Surface Waves

wave direction

Surface waves are seismic waves trappednear Earth’s surface. As depth within Earthincreases, motion due to surface wavesdecreases, then stops.

particle motion

Secondary Waves

wave direction

In secondary waves, the particles of materialsmove at a right angle to the direction of thewaves’ travel.

particle motion

How do particles move as primary waves and secondary waves pass through materials?

BA

BA 56 Unit: The Changing Earth

Seismic waves can be measured.Without listening to the news, scientists at seismic stations all over theworld know when an earthquake occurs. Seismic stations are placeswhere ground movements are measured. A (SYZ-muh-GRAF) is an instrument that constantly records ground movements.The recording of an earthquake looks like a group of wiggles in a line.The height of the wiggles indicates the amount of ground movementproduced by seismic waves at the seismograph’s location.

Using SeismographsSeparate seismographs are needed to record side-to-side movements andup-and-down movements. A seismograph that measures side-to-sidemovements has a heavy weight hanging from a wire. The weight remainsalmost still as the ground moves back and forth beneath it. A penattached to the weight records the movements. A seismograph thatrecords up-and-down movements has a heavy weight hanging from a spring. As the ground moves, the weight stays almost still as thespring absorbs the movement by getting longer or shorter. A penattached to the weight records the changes in distance between theground and the weight.

Check your reading Why is more than one kind of seismograph needed to record allthe movements of the ground during an earthquake?

Scientists use seismographs to measure thousands of earthquakes,large and small, every year. Some seismographs can detect groundmovements as small as one hundred-millionth of a centimeter.The recording produced by a seismograph is called a seismogram.By studying seismograms, scientists can determine the locations andstrengths of earthquakes.

seismographVOCABULARYAdd a magnet word diagram for seismographto your notebook.

This seismograph recordsside-to-side movements.

This seismograph recordsup-and-down movements.

weightspring

weightpen

pen

SIMULATIONCLASSZONE.COM

Learn more about seismology.

BAChapter 2: Earthquakes 57

Locating an EarthquakeTo locate the epicenter of an earthquake, scientists must have seismograms from at least three seismic stations. The procedure forlocating an epicenter has three steps:

Scientists find the difference between the arrival times of the primary and the secondary waves at each of the three stations.

The time difference is used to determine the distance of theepicenter from each station. The greater the difference in time, thefarther away the epicenter is.

A circle is drawn around each station, with a radius correspondingto the epicenter’s distance from that station. The point where thethree circles meet is the epicenter.

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0 250 500 kilometers

0 250 500 miles

Charleston, SC

epicenter

Detroit, MI

Minneapolis, MN

677 km

Seismograms provide data used to find an earthquake’s epicenter.

Finding an Epicenter

Determining Arrival Times1

10:50 10:51 10:52 10:53

Calculating Distance2

The arrival-time difference is used to determinethe distance of the epicenter from the station.

Distance from epicenter (km)

Tim

e di

ffer

ence

(s)

30

60

90

120

150

180

100 200 300 400 500 600 700 800 900 1000

79 s

677 kmPlotting Distance3

The distance from the station is used to plot acircle on a map. At least three circles are neededto locate the epicenter.

The time difference between the arrival ofprimary and secondary waves is recorded on a seismogram at each location.

79 seconds

primary wave arrival

secondary wave arrival

time difference

BA 58 Unit: The Changing Earth

Scientists can also use seismograph data to locate the focus of anearthquake. They study seismograms to identify waves that havereflected off boundaries inside Earth. Some of these waves help thescientists to determine the earthquake’s depth.

A seismogram records the time when the first primary wave arrives.This wave travels by a direct path. The data also show when the firstreflected primary wave arrives. After leaving the focus, this wave reflectsfrom Earth’s surface and then travels to the seismic station. The reflectedwave takes a longer path, so it arrives slightly later. The difference inarrival times indicates the depth of the focus. Scientists can make thenecessary calculations, but more commonly a computer is used to calculate the location of an earthquake’s epicenter and focus.

Scientists also use seismograms to determine earthquakes’magnitudes, or strengths. The more energy an earthquake releases,the greater the ground movement recorded. The greatest movementdetermines the earthquake’s strength on a magnitude scale.Stronger earthquakes get higher numbers. You will read more about earthquake magnitude scales in the next section.

KEY CONCEPTS1. Why does the greatest shaking

of the ground occur near anearthquake’s epicenter?

2. What information do you needto completely describe wherean earthquake started?

3. What types of information can a scientist get by studyingseismograms?

CRITICAL THINKING4. Compare and Contrast

How are primary and secondary waves similar? How are they different?

5. Apply What informationcould you get about an earthquake’s location fromonly two seismic stations’data? Explain.

CHALLENGE6. Apply Why might an earth-

quake’s primary waves, but not its secondary waves, reacha location on the other side ofthe world from the epicenter?

reading tip

The word magnitude comesfrom the Latin word magni-tudo, meaning “greatness.”

direct wavefocus

seismic stationreflected wave

fault

An earthquake’s depth is determined from the differencein arrival times of direct and reflected seismic waves.