What are earthquakes
and where do they
Where do Earthquakes Occur?
Earthquakes can occur near the Earth’s
surface or far below the surface.
Most earthquakes occur at plate boundaries,
but some happen at faults located in the
middle of tectonic plates.
Earthquake Locations Around the World
What Causes Earthquakes?
• Earthquakes are caused by movement along
• When stress is placed on rocks it deforms, or
• This is called Elastic Deformation (remember
faults and folds, tension and compression)
• Rock is stretched and bent until it can no longer
take the stress.
• When enough stress builds up in the rock it, it
slips and energy is released.
• The rock then returns to its original shape. This
is called Elastic Rebound. Think of a rubber
band, you can only stretch it so far until it
breaks and return to its original shape.
• This energy is felt as an earthquake.
Earthquakes in Alabama??
Do we live near a plate boundary?
Do we live near a fault?
The New Madrid Fault
If there is no plate boundary in the middle of the
United States, why did these earthquakes take
Geologists are beginning to understand the
answer. The New Madrid Fault Zone is part of an
ancient plate boundary. In this area, the North
American Plate tried to form a divergent plate
boundary about 500 million years ago. The
splitting stopped before new plates could form.
The faults in the New Madrid Zone are remnants
of this old event. Earthquakes occur because the
North American Plate is still "settling down". The
faults in the New Madrid Zone do not reach the
Earth’s surface. They are buried beneath
thousands of feet of rock and sediment deposited
by the Mississippi River. Geologists have located
them by looking at the patterns of earthquakes in
Several of the
ever recorded in the
occurred in the
Midwest, far from
any plate boundary.
took place in an area
called the New
Madrid Fault Zone,
named after the
town of New Madrid,
Over a three-month period in the
winter of 1811 to 1812, the New
Madrid Fault Zone was struck by
three huge earthquakes estimated to
be greater than magnitude 8.0
The New Madrid Fault is what causes
Alabama to experience earthquakes.
The map shows
occur in Alabama
was on July 27,
of this year. It
measured a 2.6
on the Richter
This was not a
earthquake, but it
was recorded by
used to measure
3 Types of Faults associated
PLATE MOTION FAULT TYPE
Transform Strike –Slip Fault
Convergent Reverse Fault
Divergent Normal Fault
Strike-Slip Fault occurs at a
Reverse Fault occurs at a
Normal Faults occur at Divergent
Chapter 8: Sections 1: Earthquakes and Faults:
PLATE MOTION FAULT TYPE
Transform Plates move past Strike –Slip Fault
each other Fault blocks move past each other
Convergent Plates move together Reverse Fault
Fault blocks move together
Divergent Plates move apart Normal Fault
Fault blocks move apart
How do Earthquake Waves Travel?
Energy released from moving
plates and faults travels through
the Earth as waves.
These waves are called seismic
Types of Seismic Waves
1. BODY WAVES: Seismic waves that travel
through the Earth.
2. SURFACE WAVES: Seismic waves that travel
along the surface.
1.At different speeds and
2 In different ways
Depending on what kind of material
they are moving through.
• There are two types of body waves:
P-waves are pressure waves.
P-waves travel through solid, liquids and gases.
P-waves are the fastest waves.
P-waves are the first waves to be detected and the first waves to
P-waves are also called primary waves because they arrive first.
P-waves cause rock to squeeze and stretch. Imagine a slinky!
The area of the Earth that does not receive seismic energy is called
the shadow zone.
S-waves are the second fastest waves.
S-waves are also called secondary waves
because they arrive second.
S-waves are slower than P-waves.
S-waves move rock from side to side.
S-waves can’t travel through parts of the
Earth that are completely liquid.
3. Surface Waves
Surface waves move along the surface of
Surface waves can move up, down and
around or side to side.
Surface waves move the slowest and
cause the most destruction.
Seismic Wave Arrival
• First- P-waves
• Second- S-waves
• Third- Surface Waves
The shadow zone results from S waves being stopped entirely by the liquid
core and P waves being bent (refracted) by the liquid core.
The Moho is the boundary between the crust and the
mantle in the earth. This is a depth where seismic waves
change velocity, or speed. They tend to increase at the
• 1. Most Earthquakes occur along ________________.
• 2. The first seismic waves to arrive are______________.
• 3. The second seismic waves to arrive are _____________.
• 4. The last seismic waves to arrive are_______________.
• 5. Which seismic waves travel the fastest?___________
• 6. Which type of seismic wave can move through a solid,
liquid or a gas?________________
• 7. Which seismic wave cannot travel through material that
is completely liquid?______________
• 8. Which seismic waves are the slowest and the most
A seismograph is an instrument used by scientists to measure
Seismologists who study earthquakes can determine when an
earthquake started by noting the arrival times of P-waves and S-
A seismograph records vibrations in the Earth and determines the
strength and location of an earthquake.
ball would drop
from the dragon to
the frog. It told the
0 1 2 3 4 5 6 7 8
Time in Minutes
1. How many minutes did it take for the
P-Waves to arrive?
2. How many minutes did it take for the
S-waves to arrive?
3. How long did the surface waves last?
• The epicenter is the point on the
Earth’s surface directly above an
earthquake’s starting point.
• The focus is the point inside the
Earth where the earthquake begins.
• The epicenter is located directly
above the focus.
• There are two major ways
earthquakes are measured:
• 1. Richter Scale
• 2. Mercalli Scale
Richter Scale/Measures Magnitude
The Richter Scale measures magnitude.
The measurements are given in numbers.
Measures the energy released by an earthquake
Earthquake Severity-Energy released by an earthquake.
Richter Earthquake Magnitudes Effects
Less than 3.5 Generally not felt, but recorded.
3.5-5.4 Often felt, but rarely causes damage.
Under 6.0 At most slight damage to well-designed buildings.
Can cause major damage to poorly constructed buildings over
6.1-6.9 Can be destructive in areas up to about 100 kilometers
across where people live.
7.0-7.9 Major earthquake. Can cause serious damage over larger
8 or greater Great earthquake. Can cause serious damage in
areas several hundred kilometers across.
Mercalli Scale/Measures Intensity
In seismology a scale of seismic intensity is
a way of measuring or rating the effects of
an earthquake at different sites.
The Modified Mercalli Intensity Scale is
commonly used in the United States by
seismologists seeking information on the
severity of earthquake effects. Intensity
ratings are expressed as Roman numerals
between I at the low end and XII at the high
I. People do not feel any Earth movement.
II. A few people might notice movement if they
are at rest and/or on the upper floors of tall
III. Many people indoors feel movement. Hanging
objects swing back and forth. People outdoors
might not realize that an earthquake is occurring.
IV. Most people indoors feel movement. Hanging
objects swing. Dishes, windows, and doors
rattle. A few people outdoors may feel
movement. Parked cars rock.
V. Almost everyone feels movement. Doors swing
open or close. Dishes are broken. Pictures on
the wall move. Small objects move or are turned
VII. People have difficulty standing. Drivers feel their cars shaking. Some
furniture breaks. Loose bricks fall from buildings. Damage is slight to moderate
in well-built buildings; considerable in poorly built
VIII. Drivers have trouble steering. Houses that are not bolted down might shift
on their foundations. Tall structures such as towers and chimneys might twist
and fall. Well-built buildings suffer slight damage. Poorly built structures suffer
severe damage. Tree branches break. Hillsides might crack if the ground is wet.
IX. Well-built buildings suffer considerable damage. Houses that are not bolted
down move off their foundations. Some underground pipes are broken. The
X. Most buildings and their foundations are destroyed. Some bridges are
destroyed. Dams are seriously damaged. Large landslides occur. Water is
thrown on the banks of canals, rivers, lakes. The ground cracks in large areas.
Railroad tracks are bent slightly.
XI. Most buildings collapse. Some bridges are destroyed. Large cracks appear
in the ground.. Railroad tracks are badly bent.
XII. Almost everything is destroyed. Objects are thrown into the air. The ground
moves in waves or ripples. Large amounts of rock may move.
As you can see from the list above, rating the Intensity of an earthquake's
effects does not require any instrumental measurements. Thus seismologists
can use newspaper accounts, diaries, and other historical records to make
intensity ratings of past earthquakes.
1. What was the
magnitude of the
2. Where was the
Maui V-VI intensity the
Hawaii 3.According to the
6.7 Mercalli Scale on
VII what island did the
VI V most damage
Mercalli Intensity Scale
1. What is the intensity at
2. What is the intensity at
3. What is the intensity at
4. What is the intensity at
5. What is the intensity at
Earthquakes in Alabama?
Largest Earthquake in Alabama: 1916 South
of Birmingham, in Irondale
5.1 on the Richter Scale
VII (7) on Mercalli Scale