Earthquakes Forces in Earth’s Crust Earthquakes and Seismic Waves Monitoring Earthquakes Earthquake Safety Table of Contents

Earthquakes

Forces in Earth’s Crust

Earthquakes and Seismic Waves

Monitoring Earthquakes

Earthquake Safety

Table of Contents

Earthquakes

Forces in the Earth’s Crust• The movement of Earth’s plates creates

enormous forces that squeeze or pull the rock in the crust.– Stress – force that acts on rock to change its

shape or volume

• Three different kinds of stress can occur in the crust: tension, compression, and shearing. – work over millions of years to change the

shape and volume of rock

- Forces in Earth’s Crust

Earthquakes - Forces in Earth’s Crust

Types of Stress

• Tension – pulls on the crust, stretching rock so that it becomes thinner in the middle.

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Types of Stress

• Compression – stress force that squeezes rock until it folds or breaks.


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Types of Stress• Shearing – stress that pushes a mass of

rock in two opposite directions


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Kinds of Faults• Most faults occur along plate

boundaries, where the forces of plate motion push or pull the crust so much that the crust breaks. – Fault – a break in the rock of the

crust

• There are three main types of faults: normal faults, reverse faults, and strike-slip faults.


Earthquakes

Kinds of Faults


• Tension in Earth’s crust pulls rock apart, causing normal faults.

• Normal fault – hanging wall falls down below foot wall. Ex: Rio Grande rift valley, New Mexico

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Kinds of Faults• A reverse fault has the same structure as

a normal fault, but the blocks move in the opposite direction.

• Reverse fault – hanging wall move up above foot wall Ex: Northern Rocky Mountains


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Kinds of Faults


• In a strike-slip fault, the rocks on either side of the fault slip past each other sideways, with little up and down motion.

• Strike-slip fault – two sides slide past each other– Ex: San Andreas Fault

Earthquakes

Changing Earth’s Surface• Over millions of years, the forces of plate

movement can change a flat plain into landforms, such as: – Anticlines and synclines, folded mountains,

fault-block mountains, and plateaus.


Earthquakes

Changing Earth’s Surface• Over millions of years, the forces of plate

movement can change a flat plain into landforms, such as: – Anticlines and synclines, folded mountains,

fault-block mountains, and plateaus.


Earthquakes

Folding Earth’s Crust• Anticline – upward fold in

rock formed by compression of Earth’s crust.

• Syncline – downward fold in rock formed by compression in Earth’s crust.

• Folding produced some of the world’s largest mountain ranges – Ex: Himalayas in Asia and

Alps in Europe


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Stretching Earth’s Crust• Fault-Block Mountains – form from two

normal faults, running parallel to each other– Two hanging walls fall leaving a raised area in

between.


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Uplifting Earth’s Crust • Plateau – large area of flat land elevated

high above sea level.– Form when forces in Earth’s crust push up a

large, flat block of rock – Ex. Colorado Plateau


Earthquakes

Links on Faults

• Click the SciLinks button for links on faults.


Earthquakes

Earthquakes and Seismic Activity• Earthquake – shaking that results from the

movement of rock beneath Earth’s surface.– Occur all the time, usually too small to notice– Most begin in the lithosphere (100 km below

surface)• Focus – point beneath Earth’s surface where rock

breaks under stress and causes an earthquake.• Epicenter – point on Earth’s surface directly above an

earthquake’s focus.

- Earthquakes and Seismic Waves

Earthquakes

Types of Seismic Waves• Seismic waves – carry energy from an

earthquake away from the focus, throughout the Earth


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Types of Seismic Waves• There are 3 main categories of seismic waves:

P waves, S waves, and surface waves. • P waves - seismic waves that compress and

expand the ground like an accordion.


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Types of Seismic Waves• S waves – seismic waves that vibrate from side

to side as well as up and down.


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Types of Seismic Waves• Surface waves - move more slowly than

P waves and S waves, but they produce the most severe ground movements.


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Seismic Waves Activity

• Click the Active Art button to open a browser window and access Active Art

about seismic waves.


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Measuring Earthquakes

• There are three commonly used scales of measuring earthquakes:– Mercalli scale, Richter scale, and the moment

magnitude scale.


Earthquakes

Measuring Earthquakes• Mercalli scale – developed to rate earthquakes

according to the amount of damage at a given place.


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Measuring Earthquakes• Richter scale – rates an

earthquake’s magnitude based on the size of its seismic waves.

• Magnitude – measure of an earthquake’s strength based on seismic waves and movement along faults.

• Seismograph – device that records ground movements caused by seismic waves


Earthquakes

Measuring Earthquakes

• Moment magnitude scale – rates earthquakes by estimating the total energy released by an earthquake.– Determined by studying data from

seismographs

• Each one-point increase in magnitude represents the release of roughly 32 times more energy.


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Measuring Earthquakes• Magnitude below 3 = Scarcely

noticed• Between 3-5 = little damage• Between 5-6 = Moderate damage• Above 6 = Great damage• Largest Earthquakes ever to be

recorded had a magnitude measuring above 9– Chile 1960– Alaska 1964


Earthquakes

It’s Your Turn!!

Create an Earthquake!!

Earthquakes

Seismic Wave Speeds

• Seismographs at five observation stations recorded the arrival times of the P and S waves produced by an earthquake. These data are shown in the graph.


Earthquakes

Seismic Wave Speeds

X-axis––distance from the epicenter; y-axis––arrival time.

• Reading Graphs:• What variable is

shown on the x-axis of the graph? The y-axis?


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Seismic Wave Speeds

7 minutes

• Reading Graphs:• How long did it take

the S waves to travel 2,000 km?


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Seismic Wave Speeds

4 minutes

• Estimating:• How long did it take

the P waves to travel 2,000 km?


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Seismic Wave Speeds

2,000 = 3.5 minutes

4,000 = 4.5 minutes

• Calculating:• What is the difference in

the arrival times of the P waves and the S waves at 2,000 km? At 4,000 km?


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Locating the Epicenter• Geologists use seismic waves to locate an

earthquake’s epicenter.


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Locating the Epicenter• Seismic waves travel at different speeds.

– P waves travel faster than S waves– To tell how far the epicenter is from the

seismograph, scientists measure the difference between the arrival times of the P waves and S waves.

– The longer the time difference, the further the epicenter is from the seismograph station.

Earthquakes

Locating the Epicenter

• For scientists to figure out the location of the epicenter they need P and S wave information from at least three different locations– Once the distance is calculated for each

station a circle is drawn on a map (with a radius = the distance calculated) around each of the three seismograph stations

– The place where the three circles intercept should be the earthquake’s epicenter

Earthquakes

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Seismic Waves in the Earth

• Click the Video button to watch a movie about seismic waves in the earth.


Earthquakes

The Modern Seismograph• Seismic waves cause the seismograph’s drum to

vibrate. But the suspended weight with the pen attached moves very little. Therefore, the pen stays in place and records the drum’s vibrations.

- Monitoring Earthquakes

Earthquakes

Reading a Seismogram

• Seismogram – record of an earthquake’s seismic waves produced by a seismograph.– The height of the jagged lines are greater for a

more severe earthquake.

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Instruments That Monitor Faults• Geologists have developed instruments to measure:

– changes in elevation, tilting of the land surface, and ground movements along faults.


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Instruments That Monitor Faults• Tiltmeter – measures tilting or raising of the

ground

• Creep meter – uses a wire stretched across a fault to measure horizontal movement of the ground

• Laser-ranging device – uses a laser beam to detect horizontal fault movements

• GPS – (Global Positioning System) measure tiny movements of markers set up on the opposite sides of a fault.

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Using Seismograph Data

• Seismographs and fault-monitoring devices provide data used to map faults and detect changes along faults.

• Geologists are also trying to use these data to develop a method of predicting earthquakes.

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Monitoring Changes Along Faults

• How rocks move along a fault depends on how much friction there is between the sides of the fault

• Friction – force that opposes the motion of one surface as it moves across another surface.– Ex: San Andreas Fault, California

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Monitoring Changes Along Faults• The map shows the probability of a strong

earthquake along the San Andreas fault. A high percent probability means that a quake is more likely to occur.


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Links on Earthquake Measurement

• Click the SciLinks button for links onearthquake measurement.


Earthquakes

Earthquake Risk• Geologists can determine earthquake risk

by locating where faults are active and where past earthquakes have occurred.

- Earthquake Safety

Earthquakes

How Earthquakes Cause Damage • Causes of earthquake damage

include:– shaking, liquefaction,

aftershocks, and tsunamis. • Shaking - triggers landslides and

avalanches– Destroys buildings and bridges,

topples utility poles, and fractures gas and water mains

• Liquefaction – violent movements suddenly turn loose soil into liquid mud.

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How Earthquakes Cause Damage• Aftershock – earthquake that occurs

after a larger earthquake in the same area.– Can sometimes cause the most

damage• Tsunami – large wave produced by an

earthquake on the ocean floor.– Ex: December 26, 2004 –

earthquake on the seafloor of the Indian Ocean caused a tsunami to hit the shores of Indonesia killing 200,000 people

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How Earthquakes Cause Damage

• Tsunamis spread out from an earthquake's epicenter and speeds across the ocean.– The waves are amplified as they approach the

shore.

- Earthquake Safety

Earthquakes

Steps to Earthquake Safety

• The best way to protect yourself during an earthquake is to drop, cover, and hold.

• Before an earthquake occurs be prepared– Keep a supply of

canned food, bottled water, flashlights, batteries and a portable radio

Earthquakes

Designing Safer Buildings

• To reduce earthquake damage, new buildings must be made stronger and more flexible.

- Earthquake Safety

Earthquakes

Designing Safer Buildings• Base-isolated building –

buildings mounted on bearings designed to absorb the energy of an earthquake.– rests on shock-absorbing

rubber pads or springs – Flexible joints can be

installed in gas and water lines to keep them from breaking.

Earthquakes

Earthquake Damage

• Click the Video button to watch a movieabout earthquake damage.

- Earthquake Safety

Earthquakes

More on Earthquake Risk

• Click the PHSchool.com button for an activity about earthquake risk.

- Earthquake Safety

Documents

Earthquakes Forces in Earth’s Crust Earthquakes and Seismic Waves Monitoring Earthquakes Earthquake Safety Table of Contents