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Earthquakes: What are they and
what causes them to happen?
What is an Earthquake?
An earthquake is a shaking of the ground caused
by a sudden release of energy in the Earth’s crust.
More than a million of them occur each year!
However, most are too small to be felt or to cause
damage.
What are Earth’s
Earth’s crust is the outer layer and is
Layers?  The
made of rock. It is very thin compared to
Outer core
Mantle
the other layers.
 The mantle is the very hot layer of rock
beneath the crust. No one has ever been to
the mantle, but rock from the mantle
sometimes reaches the Earth’s surface
through volcanoes.
 The outer core is made of liquid, or molten
iron.
 The inner core is made of solid iron. Even
though the core is the hottest layer, great
pressure at the center of the Earth keeps
the inner core solid.
Inner Core
Crust
What are Plate Tectonics?
 Earth’s surface is made up of
many plates that float on the soft
rock of the mantle. As the
mantle moves, the plates also
move.
 Plates move only a few
centimeters each year.
 When one plate moves, it affects
the other plates.
 As plates move around, they
cause great changes in the
Earth’s surface, such as
mountains, valleys, volcanoes,
and earthquakes!
What Happens When Plates
Move?
Plate Tectonics
Convergent Plate
Boundaries
plates crunch
together and release
energy
Divergent Plate
Boundaries
plates pull apart and
new crust is formed
mountains develop,
volcanoes erupt, and
earthquakes can happen
Transform
Boundaries
plates slide past
each other
valleys and volcanoes
develop, earthquakes
can occur
Lots of earthquakes!
Types of Boundaries
 Many earthquakes occur along faults in the Earth’s
crust. A fault is a place where pieces of the plates
move.
 A normal fault is where tension weakens the crust
until the rock fractures, and one rock moves
downward from another. This occurs when two plates
are pulling apart, as in a divergent boundary.
 A strike-slip fault is where two blocks of rock are
moving past each other horizontally, as in a transform
boundary. The famous San Andreas Fault is a strikeslip fault.
 A reverse fault is where a fault block is forced
upward, usually during a collision with
another block, as in a convergent boundary.
What is a
Fault?
What Happens When an
Earthquake Starts?
The sudden release of energy from an earthquake sends out
several different shaking movements, or seismic waves.
Earthquakes generate two types of waves: Primary (P)
waves and Secondary (S) waves.
What Happens When an Earthquake
Starts?
 Surface waves are ripples of energy that
spread outward when rocks slip past each
other along a fault, just like throwing a
stone into a calm pond.
 Body waves are seismic waves that travel
through material rather than over its
surface. There are two types of body waves:
P-waves and S-waves.
 The P-wave is also known as the sound
wave. It travels through the interior of the
Earth rather than over the surface as a series
of squeezes and stretches. P-waves reach
everywhere around the Earth after about 20
minutes.
 The S-wave, or shear wave, produces a
shaking motion, like if you tied a rope to a
poll and shook the other end side to side.
S-waves can only travel in solid material and
travel slower than P- waves.
How Do Scientists Know Where an
Earthquake Has Happened?
•
The location directly above the hypocenter on the Earth’s surface is called the
epicenter.
 It take three seismographs to locate an earthquake. Scientists draw a circle on a
map around the three seismographs where the radius of each is the distance
from the station to the earthquake, the intersection of those three circles is the
epicenter.
The focus is the area beneath Earth’s surface
where rock that is under stress breaks,
triggering an earthquake.
How are Earthquakes Measured?
 Earthquakes are measured by instruments
called seismographs. It has a base that sets
firmly in the ground, and a heavy weight that
hangs free. When an earthquake causes the
ground to shake, the base of the seismograph
shakes too, but the heavy weight does not. The
spring that it is hanging from absorbs all the
movement. The seismograph records the
difference in position between the shaking part
and the motionless part.
 The recording is called a seismogram. It is used
to determine how large the earthquake was. A
short wiggly line that wiggles very little means
a small earthquake. A long wiggly line that
wiggles a lot means a large earthquake.
The Richter Magnitude Scale
Description
Richter
Magnitudes
Micro
Less than 2.0
Very Minor
2.0-2.9
Usually not felt, but recorded.
Minor
3.0-3.9
Often felt, but rarely causes damage.
Light
4.0-4.9
Noticeable shaking of indoor items,
rattling noises. Significant damage
unlikely.
5.0-5.9
Can cause major damage to poorly
constructed buildings over small regions.
Slight damage to well-designed
buildings.
Strong
6.0-6.9
Can be destructive in areas up to about
100 miles across in populated areas.
Major
7.0-7.9
Can cause serious damage over larger
areas.
Great
8.0-8.9
Can cause serious damage in areas
several hundred miles across.
Rare Great
9.0 or greater
Moderate
Earthquake Effects
Microearthquakes, not felt.
Devastating in areas several thousand
miles across.
How Long Does an Earthquake Last?
 Sometimes an earthquake has
foreshocks. These are smaller
earthquakes that happen in the same
place as the larger earthquake.
 The largest, main earthquake is
called the mainshock.
 Mainshocks always have aftershocks
that follow. These are smaller
earthquakes that occur afterwards
in the same place as the mainshock.
Depending on the size of the
mainshock, aftershocks can
continue for weeks, months, or even
years after the mainshock!
Famous Earthquakes Around the World
 San Francisco, California: April 18, 1906
(Magnitude: About 8)
 Tokyo, Japan: September 1, 1923
(Magnitude: about 8.25)
 Chile: May 22, 1960
(Magnitude: About 9)
 Anchorage, Alaska: March 27, 1964
(Magnitude: About 8.5)
Number of Earthquakes
How Many Earthquakes Happen
Each Year?
1,000,000
100,000
10,000
1,000
100
10
1
1
2
3
4
5
6
7
Magnitude of Earthquakes
8
Where do Earthquakes Occur
Most Often in the United States?
Earthquake Damage