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How and Where Do Earthquakes Occur?PowerPoint Presentation

How and Where Do Earthquakes Occur?

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### How and Where Do Earthquakes Occur?

Causes of Earthquakes

Vocabulary

- stress
- strain
- fault
- primary wave
- secondary wave
- surface wave
- Focus
- epicenter

What Is an Earthquake?

- Earthquakes are natural vibrations of the ground caused by movement along fractures in Earth’s crust, or sometimes, by volcanic eruptions.
- Most earthquakes occur along plate boundaries.
- More than a million earthquakes occur each year.

What Causes Earthquakes?

- Most earthquakes occur when rocks fracture, or break, deep within Earth.
- Fractures form when stress exceeds the strength of the rocks involved.
- Stress is the forces per unit area acting on a material.
- Strain is the deformation of materials in response to stress.

Stress and Strain

- There is a distinct relationship between stress and strain that can be plotted as a stress-strain curve.
- A stress-strain curve usually has two segments: a straight segment and a curved segment.
- Low stresses produce the straight segment, which represents the elastic strain of a material.
- If the elastic strain is reduced to zero, the deformation disappears.

Faults

- A fault is the fracture or system of fractures along which movement occurs.
- The surface along which the movement takes places is called the fault plane.

The Focus of the Earthquake

- The place underground where the break first occurs is the focus of the earthquake.
- The epicenter is the location at Earth’s surface just above the focus.
- When the vibrations reach the surface , we feel them as an earthquake, first at the epicenter and then at greater distances from the epicenter.

Earthquake Waves

- When an earthquake occurs, its energy radiates in waves away from the focus as shown in the previous figure.
- Seismic waves that travel from the focus through the Earth’s body are called body waves.
- Every earthquake produces two types of body waves, called P-waves and S-waves:
- P-waves or primary waves travel the fastest.
- S-waves or secondary waves travel more slowly than p-waves and vibrate in all directions perpendicular to the direction of travel.

Seismic Body Waves

Seismic Surface Waves

- Surface waves are seismic waves that travel along Earth’s surface. When P-waves and S-waves reach Earth’s surface, they become surface waves called Love waves and Rayleigh waves.

Seismic Surface Waves

10.1 Section Review

- Explain where earthquakes are most likely to originate and why they originate in these places.
- Describe the difference between the focus of an earthquake and the epicenter of an earthquake.
- Draw and label a diagram illustrating two types of surface waves.
CRITICAL THINKING

- Compare and contrast body waves and surface waves. Explain how the depth of an earthquake’s focus might determine the extent of the damage it causes.
- MATHEMATICS Suppose an earthquake’s P waves travel at an average speed of 6 kilometers per second, and its S waves travel at an average speed of 3.4 kilometers per second. How long will it take the P waves to reach a recording station that is 60 kilometers from the focus? How long after the P waves will the S waves reach the same station?

Locating the Epicenter

- A seismograph records the magnitude of the earthquake and the time the seismic waves arrive.
- Seismologist use the difference in the speeds of the P and S waves to locate the epicenter of earthquakes.

Types of Faults

- There are three basic types of faults:
- Reverse faults are fractures that form as a result of horizontal compression.
- Normal faults are fractures caused by horizontal tension.
- Strike-slip faults are fractures caused by horizontal shear.

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