Earthquakes - Forces in Earth’s Crust

Types of Stress

The stress force called tension pulls on the crust, stretching rock so that it becomes thinner in the middle.

Earthquakes

Types of Stress

The stress force called compression squeezes rock until it folds or breaks.

- Forces in Earth’s Crust

Earthquakes

Types of Stress

Stress that pushes a mass of rock in two opposite directions is called shearing.

- Forces in Earth’s Crust

Earthquakes

Kinds of Faults

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

- Forces in Earth’s Crust

Earthquakes

Kinds of Faults

A reverse fault has the same structure as a normal fault, but the blocks move in the opposite direction.

- Forces in Earth’s Crust

Earthquakes

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.

- Forces in Earth’s Crust

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.

- Forces in Earth’s Crust

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.

- Forces in Earth’s Crust

Earthquakes

Building VocabularyA definition states the meaning of a word or phrase. As you read, write a definition of each Key Term in your own words.

Key Terms: Examples:

- Forces in Earth’s Crust

stress Stress is a force that acts on rock to change its shape or volume.

tension The stress force called tension pulls on the crust, stretching rock so that it becomes thinner in the middle.

compression The stress force called compression squeezes rock until it folds or breaks.

shearing Stress that pushes a mass of rock in two opposite directions is called shearing.

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

reverse fault A reverse fault has the same structure as a normal fault, but the blocks move in the opposite direction.

Key Terms: Examples:hanging wall

footwall

strike-slip fault

anticline

syncline

plateau

The block of rock that lies above a normal fault is called the hanging wall.

The rock that lies below is called the footwall.

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

A fold in rock that bends upward into an arch is an anticline.

A fold in rock that bends downward to form a valley is a syncline.

A plateau is a large area of flat land elevated high above sea level.

Earthquakes

Links on Faults

Click the SciLinks button for links on faults.

- Forces in Earth’s Crust

Earthquakes

Graphic Organizer

faultCompression produces

Effect

Normal fault produces

Cause of Fault

produces fault

Reverse

Tension

Shearing Strike-slip

Earthquakes

Types of Seismic Waves

Seismic waves carry energy from an earthquake away from the focus, through Earth’s interior, and across the surface.

- Earthquakes and Seismic Waves

Earthquakes

Types of Seismic Waves

P waves are seismic waves that compress and expand the ground like an accordion. S waves are seismic waves that vibrate from side to side as well as up and down.

- Earthquakes and Seismic Waves

Earthquakes

Types of Seismic Waves

Surface waves move more slowly than P waves and S waves, but they produce the most severe ground movements.

- Earthquakes and Seismic Waves

Earthquakes

Seismic Waves Activity

Click the Active Art button to open a browser window and access Active Art about seismic waves.

- Earthquakes and Seismic Waves

Earthquakes

Measuring Earthquakes

The Mercalli scale was developed to rate earthquakes according to the amount of damage at a given place.

- Earthquakes and Seismic Waves

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 and Seismic Waves

Earthquakes

Locating the Epicenter

Geologists use seismic waves to locate an earthquake’s epicenter.

- Earthquakes and Seismic Waves

Earthquakes

Main Idea

Detail Detail Detail

Identifying Main IdeasAs you read the section “Types of Seismic Waves,” write the main idea in a graphic organizer like the one below. Then write three supporting details. The supporting details further explain the main idea.

Seismic waves carry the energy of an earthquake.

P waves compress and expand the

ground.

S waves vibrate from side to side as well as up and

down.

Surface waves produce the most

severe ground movements.

- Earthquakes and Seismic Waves

Earthquakes

Seismic Waves in the Earth

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

- Earthquakes and Seismic Waves

Earthquakes

End of Section Two

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

Instruments That Monitor Faults

In trying to predict earthquakes, geologists have developed instruments to measure changes in elevation, tilting of the land surface, and ground movements along faults.

- Monitoring Earthquakes

Earthquakes

Using Seismographic Data

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.

- Monitoring Earthquakes

Earthquakes

SequencingAs you read, make a flowchart like the one below that shows how a seismograph produces a seismogram. Write each step of the process in a separate box in the order in which it occurs. How a Seismograph Works

Incoming seismic waves

Vibrate the rotating drum

The suspended pen remains motionless and records the drum’s vibration.

- Monitoring Earthquakes

Earthquakes

Links on Earthquake Measurement

Click the SciLinks button for links onearthquake measurement.

- Monitoring Earthquakes

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End of SectionThree

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

A tsunami spreads out from an earthquake's epicenter and speeds across the ocean.

- Earthquake Safety

Earthquakes

Designing Safer Buildings

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

- Earthquake Safety

Earthquakes

Asking QuestionsBefore you read, preview the red headings and ask a what, how, or where question for each heading. As you read, write answers to your questions.

Where is the quake risk highest? Earthquake risk is the highest along faults and where past earthquakes have occurred.

How do earthquakes cause damage? Earthquake damage occurs as a result of shaking, liquefaction, aftershocks, and tsunamis.

How can you stay safe during an earthquake?

The best way to stay safe during an earthquake is to drop, cover, and hold.

What makes buildings safe from earthquakes?

Buildings can be made safer by being built stronger and with greater flexibility.

- Earthquake Safety

Question Answer

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

Earthquakes

End of Section Four