Download How and Where Earthquakes Occur

How and Where
Earthquakes Occur
Key Idea: Most earthquakes result
from strain that builds up at plate
boundaries.
After completing this lesson, we’ll
know:
1.
2.
3.
4.
5.
The causes of earthquakes
Where earthquakes are most likely to occur
The elements of an earthquake
How the energy of an earthquake is released and
travels as waves
Types of waves generated by an earthquake
a) body waves (P waves, S waves)
b) surface waves (Love waves, Rayleigh waves)
What is an Earthquake?
An earthquake is a shaking of the earth
caused by a release of energy.
The release of energy could be generated
by many different causes, such as:
 The impact of a meteor
 The collapse of an cavern
 The eruption of a volcano
Most earthquakes however,
are generated by the strain that builds up
along faults at or near boundaries between
lithospheric plates.
 A fault is a break in the lithosphere along
which movement has occurred.
Examples of Faults
A Strike-Slip Fault
A Normal Fault
The Elastic Rebound Theory

When two plates are moving, friction occurs at
their contact. Due to friction, strain builds up,
causing plates to deform, or change shape.
 Eventually the strain becomes great enough to
overcome friction, and the plates move
suddenly, causing an earthquake.
 Then, the plates snap back to the shapes they
had before they were deformed, but at new
locations relative to each other.
(This model is the elastic-rebound theory)
The “Parts” of an Earthquake
The “Parts” of an Earthquake
The FOCUS
 is the point where the discharge of energy
takes place.
 It is where the earthquake originates and it
is usually many kilometers beneath the
surface.
 The EPICENTER is the point on the
Earth’s surface located directly above the
focus.
Focus and Epicenter
How deep do earthquakes form?
The depth at which an earthquake originates
depends upon the type of plate boundary
involved.
 At divergent boundaries, such as the Mid
Atlantic Ridge, the epicenter is around 30 Km
beneath the surface.
 Along transform boundaries, the depth is also
several km.
 At subduction boundaries, where a plate
plunges beneath its neighboring plate the focus
can be located as deep as 700 Km.
How does the energy released at
the focus travel?
 The
energy released at an earthquake
travels in waves.
 As the waves travel beneath the surface,
through the material of Earth’s body, they
are called body waves.
 There are two kinds of body waves:
1. P waves
2. S waves
P Waves
 The
P waves, also called Primary waves,
or compressional waves squeeze and
stretch the rock materials as they pass
through Earth. A slinky models the P
waves perfectly.
The S Waves
 The
S waves are also called sheer waves,
or secondary waves cause the rock
material to move perpendicular to the
direction in which the waves are traveling.
More on P and S Waves
The P waves
 travel faster than the S waves
 they can travel solids (rocks), and fluid
materials, such as magma, water, and air.
The S waves
 are slower
 they only travel through solid materials
 the denser the rock , the faster they travel.
Surface Waves

As the body waves reach the surface of the
Earth, they become surface waves. These are
the waves that produce damage during an
earthquake.
 The two types of surface waves are
1. Love waves, which cause particles of
material to move side to side;
2. Rayleigh waves cause particles of material
to move in elliptical patterns. These waves are
similar to the ripples at the surface of water.
Surface Waves
Let’s summarize:
1.
2.
3.
4.
5.
6.
7.
What are some of the cause of earthquakes?
Where are most earthquakes likely to occur?
What is the difference between focus and
epicenter?
What are body waves?
What is the difference between P waves and S
waves?
What are surface waves?
What is the difference between Love waves
and Rayleigh waves?