EarthquakeEarthquake

•The shaking that results from the movement of rock beneath Earth’s surface

There are about _____ earthquakes per day, worldwide.

• 8,000

Where do Earthquakes Occur?Where do Earthquakes Occur?

• Earthquakes occur at tectonic plate tectonic plate boundariesboundaries, and along fracture zonesfracture zones (fault lines).

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Red lines = plate boundaries Red lines = plate boundaries Orange lines = Orange lines = fracture zonesfracture zones

The movement between plates and along faults is not smooth. The plates

move in jerks, giving rise to earthquakes. The locations of

earthquakes throughout the world mark the major tectonic boundaries.

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The red dots indicate areas where earthquakes have occurred.

The movement of Earth’s plates creates powerful forces that ___ or ___ the rock in the crust.

• squeeze• pull

StressStress• Stress: A force (push or pull) that acts

on rock to change its shape or volume

CompressionCompression

• Stress that squeezes rock until it folds or breaks

TensionTension

• Tension: Stress that stretches rock so that it becomes thinner in the middle

ShearingShearing

• Shearing: Stress that pushes a mass of a rock in opposite, horizontal directions

DeformationDeformation

• DeformationDeformation: A change in the volume or shape of Earth’s crust (which causes it to bend, stretch, break, tilt, fold or slide)

• Most changes in the crust occur so slowly that they cannot be observed directly

Review: How do stress Review: How do stress forces affect rock?forces affect rock?

• The three kinds of forces that affect rock are:• Shearing Shearing

– The rocks break and slip apart• TensionTension

– The rock stretches and becomes thin in the middle

• CompressionCompression– The rock squeezes until it folds or

breaks• These stresses work over millions of years to

change the shape and volume of rock

FaultsFaults

• A break in the Earth’s crust, no more than 10 miles deep, where slabs of rock slip past each other

• Faults occur when enough stress builds up in rock to make it move.

Why do faults form and Why do faults form and where do they occur?where do they occur?

• Faults usually occur along plate boundaries along plate boundaries or at fracture zonesat fracture zones, where the forces of plate motion compresscompress, pullpull, or shearshear the crust so much that the crust breaks (deforms)

• Rocks on both sides of the fault can move up or down, or sideways

Strike-Slip FaultsStrike-Slip Faults

• A type of fault where rocks on either side move past each other sidewaysmove past each other sideways with little up or down motion.

• ShearingShearing causes these types of faults

Normal Faults Normal Faults (sometimes called Thrust Thrust

FaultsFaults)

• A type of fault where the hanging wall slides downward

• Tension Tension forces cause normal faults

Hanging Wall & Hanging Wall & FootwallFootwall

• Hanging wall: The block of rock that forms the upper half of a fault

• Footwall: The block of rock that forms the lower half of a fault

Reverse FaultsReverse Faults

• A type of fault where the hanging wall slides up

• CompressionCompression forces cause reverse faults

Review:Review: What are the three types of fault? What force of deformation produces each type?• Strike-slip faultsStrike-slip faults

• Produced by shearingshearing

• Normal faultsNormal faults• Produced by tensiontension

• Reverse faultsReverse faults• Produced by compressioncompression

What is friction?What is friction?

• A force that opposes the motion of one surface as it moves across another surface

Friction exists because…

• surfaces are not perfectly smooth.

Describe what occurs when the friction along a fault line is low.

• The rocks on both sides of the fault slide by each other without much sticking

Describe what occurs when the friction along a fault line is moderate.

• The sides of the fault jam together

• From time to time they jerk free

• Small earthquakes occur

Describe what occurs when the friction along a fault line is high. • Both sides of the fault lock together and

do not move

• The stress increases until it is strong enough to overcome the force of friction

• Larger and/or more frequent earthquakes will occur

The San Andreas San Andreas faultfault in California is a transform boundary that contains ___ stress.

• high

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Fault-Block MountainFault-Block Mountain

• A mountain that forms where a normal (thrust) fault uplifts a block of rock

How does the process of a How does the process of a fault-block mountain fault-block mountain

begin?begin? • Where two plates move away from each other,

tension forces create many normal faults• When two of these normal faults form parallel

to each other, a block of rock is left lying between them

• As the hanging wall of each normal fault slips downward, the block in between moves upward

• When a block of rock lying between two normal faults slides downward, a valley forms

FocusFocus• The point beneath Earth’s surface

where rock breaks under stress and causes an earthquake

EpicenterEpicenter• The point on Earth’s surface

directly above an earthquake’s focus

Seismic WavesSeismic Waves

Seismic WaveSeismic Wave• A vibration that travels through

Earth, carrying the energy released during an earthquake

At what point do seismic waves first reach the surface?

• The epicenter

What determines how What determines how much the ground shakes much the ground shakes during an earthquake?during an earthquake?

• How close a location is to the epicenter

• The types of rock and soil surrounding the epicenter determines how much the ground shakes

There are three types of There are three types of seismic waves:seismic waves:

•Primary Waves (P waves)Primary Waves (P waves)•Secondary Waves (S waves)Secondary Waves (S waves)•Surface WavesSurface Waves

• An earthquake releases two types of waves: P waves and S waves

• When the waves reach Earth’s surface at the epicenter, surface waves develop.

Primary Waves (P Waves)Primary Waves (P Waves)

• A type of seismic wave that compresses and expandscompresses and expands the ground

• The first wave to arrive at epicenter

Secondary Waves (S Secondary Waves (S Waves)Waves)

• A type of seismic wave that moves the ground up and downup and down or side to sideside to side

• Second wave to arrive at epicenter

How are P waves different How are P waves different from S Waves?from S Waves?

• P waves compress and expand the P waves compress and expand the groundground

• P waves can travel through solids, P waves can travel through solids, liquids and gasesliquids and gases

• S waves move the ground up and down S waves move the ground up and down or side to side or side to side

• S waves can only travel through solidsS waves can only travel through solids

Surface WavesSurface Waves

• A type of seismic wave that forms when P waves and S waves reach Earth’s surface

• Surface waves are the most destructivemost destructive of the 3 types of waves, because they move both both sideways and up-and-down.

Why do you think surface Why do you think surface waves produce more severe waves produce more severe ground movements than P ground movements than P

waves and S waves?waves and S waves?

• Surface waves travel through Surface waves travel through loose soil, sand, gravel, mud, loose soil, sand, gravel, mud, and small rocks (not solid rock)and small rocks (not solid rock)

• These looser substances are These looser substances are more likely to shift and slidemore likely to shift and slide

How does the energy of How does the energy of an earthquake travel an earthquake travel

through Earth?through Earth?• Seismic waves carry the energy of an Seismic waves carry the energy of an

earthquake from the focus, through earthquake from the focus, through Earth’s interior, to the epicenter, and Earth’s interior, to the epicenter, and across the surfaceacross the surface

Detecting Seismic Detecting Seismic WavesWaves

Seismograph or Seismograph or SeismometerSeismometer

• A device that records ground movements caused by seismic waves as they move through Earth

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The old-fashioned kindiPhone seismometer-- yes, there’s an app for that!

Describe how a mechanical Describe how a mechanical seismograph records seismograph records ground movement.ground movement.

• A heavy weight attaches to a frame by spring or wire

• A pen connected to the weight rests its point on a rotating drum

• During an earthquake the seismic waves cause the drum to shake while the pen stays in place

• The pen records lines on the paper around the drum

Measuring EarthquakesMeasuring Earthquakes

MagnitudeMagnitude

• The measurement of an earthquake’s strengthstrength based on seismic waves and movement along faults

• the strength of ground ground movementmovement in a given place.

IntensityIntensity

Mercalli ScaleMercalli Scale

• A scale that rates earthquakes according to their intensityintensity and how much damage they causehow much damage they cause

• Developed in the early twentieth century

The Mercalli scale has ______ steps and describes how an earthquake affects ________, ________, and the ________.

• 12• People• Buildings • Land surface

How would you rate the How would you rate the damage to the Foligno city damage to the Foligno city hall on the Mercalli scale?hall on the Mercalli scale?

• The damage would probably rate VII - VIII

Richter ScaleRichter Scale

• A scale that rates the amplitude (height) of seismic waves measured by a seismograph

• It is a logarithmiclogarithmic scale, which means each level has 10 times the magnitude of the level below it.

Richter Magnitude

Comparable Energy Example (approx.)

-1.5 breaking a rock on a lab table1.0 large blast at construction site1.52.0 large quarry or mine blast2.53.03.54.0 small nuclear weapon4.5 average tornado (total energy)5.05.5 Little Skull Mtn, NV quake, 19926.0 Double Spring Flat, NV quake, 19946.5 Northridge, CA quake, 19947.0 Haiti Earthquake, Jan.12, 20107.5 Landers, CA quake 19928.0 SF, CA quake 19068.5 Anchorage, AK quake 19649.0 Chilean quake, 1960

10.0 (San-Andreas type fault circling Earth)12.0 (Fault through center of Earth)

What is a strength of the What is a strength of the Richter scale when Richter scale when

measuring earthquakes? measuring earthquakes? What is a weakness?What is a weakness?

• StrengthStrength: Provides accurate measurements for small, nearby earthquakes

• WeaknessWeakness: Does not measure larger and more distant earthquakes well

How are the Mercalli scale How are the Mercalli scale and the Richter scale and the Richter scale similar? How are they similar? How are they

different?different?

• The Mercalli scale measures the strength in terms of the amount of amount of damage causeddamage caused and the amount of amount of shakingshaking that people experienced

• The Richter scale measures the size of size of an earthquake’s seismic wavesan earthquake’s seismic waves

• Both measure the strengthstrength of an earthquake

Moment Magnitude Moment Magnitude ScaleScale

• A scale that rates earthquakes by estimating the total energy total energy released by an earthquakereleased by an earthquake

Why is the moment Why is the moment magnitude scale used magnitude scale used today by geologists to today by geologists to measure earthquakes?measure earthquakes?

• The moment magnitude scale determines the total energy releasedtotal energy released by an earthquake

• This scale uses a electronic seismograph that can measure earthquakes that are big or small, and near or far

• Geologist examine movement along the fault and the strength of broken rock

• These two measurements give a more more accurate measurementaccurate measurement of an earthquake

On which scale would an On which scale would an earthquake’s strength earthquake’s strength vary from one place to vary from one place to

another? Explain.another? Explain.

• The Mercalli scale, because the amount of shaking that people would feel and the damage to objects would be greater in a place closer to the earthquake’s epicenter

Review:Review: What are three scales for What are three scales for measuring earthquakes? Explain measuring earthquakes? Explain what each one measures.what each one measures.

• Mercalli ScaleMercalli Scale• Measures earthquakes on how much damage

they cause

• Richter ScaleRichter Scale• Measures seismic waves using a seismograph• Measures small and nearby earthquakes

• Moment Magnitude ScaleMoment Magnitude Scale• Looks at the total energy released• Measures large and distant earthquakes• Helps scientists predict how much fault

movement there was

Locating the EpicenterLocating the Epicenter

How do scientists How do scientists calculate how far a calculate how far a location is from the location is from the

epicenter of an epicenter of an earthquake?earthquake?

• Scientists calculate the calculate the difference between arrival difference between arrival times of the P waves and S times of the P waves and S waveswaves

• The further away an earthquake is, the greater the time between the arrival of the P waves and the S waves

Figure 17: Use the map scale to determine the distances from Savannah and Houston to the epicenter. Which one is closer?

• Houston • 800 Km

• Savannah• 900 km

Seismicity of the Caribbean plateSeismicity of the Caribbean plate

Haiti earthquake 1-12-2010Haiti earthquake 1-12-20107.07.0

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