# How and where Earthquakes Occur

```How and Where Do
Earthquakes Occur?
Causes of Earthquakes
How Do Earthquakes Occur?
Vocabulary
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stress
strain
fault
primary wave
secondary wave
surface wave
Focus
epicenter
How Do Earthquakes Occur?
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.
How Do Earthquakes Occur?
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.
How Do Earthquakes Occur?
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.
How Do Earthquakes Occur?
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.
How Do Earthquakes Occur?
How Do Earthquakes Occur?
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.
How Do Earthquakes Occur?
Seismic Body Waves
How Do Earthquakes Occur?
Seismic Surface Waves
• Surface waves are seismic waves that
travel along Earth’s surface. When Pwaves and S-waves reach Earth’s surface,
they become surface waves called Love
waves and Rayleigh waves.
How Do Earthquakes Occur?
Seismic Surface Waves
10.1 Section Review
1. Explain where earthquakes are most likely to originate and why they
originate in these places.
2. Describe the difference between the focus of an earthquake and the
epicenter of an earthquake.
3. Draw and label a diagram illustrating two types of surface waves.
CRITICAL THINKING
4. 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.
5. 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?
How Do Earthquakes Occur?
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.
How Do Earthquakes Occur?
How Do Earthquakes Occur?
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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