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

.*Focus: Point beneath the earth’s surface where the earth moves

*Epicenter: The point on the surface directly above the focus. It is where the strength of the earthquake is strongest.

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Earthquakes

Types of Seismic WavesP 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

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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Earthquakes

Seismic Waves Activity

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- Earthquakes and Seismic Waves

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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

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 Richter Scale

The Richter Scale for measures the 'size' or 'strength' of an earthquake. This scale is quantitative and based on the amount of energy released by an earthquake.

The energy of a quake is a function of both the amplitude and the duration of a single wave.

The seismogram below shows waves with a wide range of amplitude and duration.

This would be magnitude 8 quake!

Earthquakes - 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

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?

- Earthquakes and Seismic Waves

Earthquakes

Seismic Wave Speeds

7 minutes

Reading Graphs:

How long did it take the S waves to travel 2,000 km?

- Earthquakes and Seismic Waves

Earthquakes

Seismic Wave Speeds

4 minutes

Estimating:

How long did it take the P waves to travel 2,000 km?

- Earthquakes and Seismic Waves

Earthquakes

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?

- Earthquakes and Seismic Waves

Earthquakes

Locating the Epicenter

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

- Earthquakes and Seismic Waves

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

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