EARTHQUAKES
Chapter 5
What are Earthquakes?
• The shaking or
trembling caused by
the sudden release of
energy
• Usually associated
with faulting or
breaking of rocks
• Continuing
adjustment of position
results in aftershocks
• Near plate boundaries, there are many breaks in the rock
called faults
• Over time, as the plates gradually get pushed along,
more and more pressure builds up at the faults.
• Elastic rebound occurs when a fault can’t take any more
pressure and snaps.
• We experience this motion as an earthquake.
Seismology study of earthquakes
Seismologist scientist who studies
earthquakes and seismic waves
Keep in mind: Seismologists don’t
have to be right near an earthquake to
study it. Why is this?
World
Earthquake
Map
Fault  a break in the crust along
which plates shift and slip past
each other
Deformation  rocks changing shape due
to stress from tectonic
motion
Sometimes, rock
layers just
gradually crunch
together and
bend.
Plastic Deformation vs. Elastic Deformation
• Plastic Deformation - Stress causing a fold (ex: metal
bending)
• Elastic deformation - Stress causing a fold then a break or
earthquake (ex: rubber band breaking OR stick breaking)
Where Do Earthquakes Occur and
How Often?
~80% of all earthquakes
occur near the Pacific
Ocean (Ring of Fire)
• most of these result from
convergent activity
• more than 150,000 quakes
strong enough to be felt
are recorded each year
What causes an earthquake?
• The build up of stress along a fault line causing
elastic deformation & then an earthquake (elastic
rebound)
• Earthquake – The release of energy (seismic waves) from rock layers
moving along a fault
But if something can’t
bend, then it will
probably…
BREAK
Elastic rebound  stress builds up to the point
where the rock breaks and snaps
back
What is the Elastic Rebound?
• Explains how energy is
stored in rocks
• Rocks bend until the
strength of the rock is
exceeded
• Rupture occurs and the
rocks quickly rebound
to an undeformed
shape
• Energy is released in
waves that radiate
outward from the fault
Elastic Rebound
Animation
Focus vs. Epicenter
At your table, take one minute to discuss the following:
What are seismic waves?
How do they tell us about
the interior of the Earth?
Seismic waves
 Earthquake
vibrations that
travel through
the Earth.
Different types of
seismic waves
travel through the
Earth’s layers at
different speeds.
When an earthquake
occurs, seismic waves
are shot out in all
directions:
• Some travel
across the
Earth’s surface
(surface waves)
•
Some travel
through the Earth
(body waves)
P-waves
S-waves
Seismographs
Animation
Surface wave
What property determines
the speed at which seismic
waves travel?
Density of the rock
Seismic waves
travel best
through MORE /
LESS dense
layers.the
In rock
other words,
Surface wave
vibrations
unleashed by
earthquakes travel
best through…
Both can be
easily
demonstrated
with a Slinky!
Picture the molecules within the rock
to be like dominoes knocking into
Seismic Wave Motion with Surface Effects
Animation
Seismic waves
are like sound
waves.
They travel best
through solids
materials.
Full name
Where they
travel
P-waves
S-waves
L-waves
primary waves
Secondary
waves
surface waves
(lateral)
through the
entire earth
only through
solid
on the earth’s
surface
Relative
speed
fastest
slower
slowest, but
most
damaging
Type of
motion
in and out
side to side
up, down,
and around
Complete the
Earthquake Location
Activity
1. What does the red line in
the graph represent?
time/distance of S-waves
2. What does the purple line
represent?
time/distance of P-waves
3. Which location is the
farthest away from the
earthquake’s epicenter?
station C
4. What is the S-P time
interval (ΔT) for
location C?
23 – 12 = 11 minutes
5. Which location is the
closest to the earthquake’s
epicenter?
station A
6. How far is station A
from the epicenter?
~1,700 km
What is the measure of the strength of an earthquake
called?
Magnitude
Richter scale  measures the strength of the ground
•
1
Based onmotion
a factor of _____
for each level.
•
1 0 times as strong as a magnitude
A magnitude 3 is ____
0
1
0
100times as strong as a magnitude
A magnitude 4 is ____
2.
•
3.
•
4 4
5 times
6 
A magnitude
is _____
as strong as a magnitude
2.
•
7 x 10
x
10
x
10
times
= 1,000
stronger
How much stronger is a magnitude 7 than a magnitude
Haiti
earthquake
What is the difference
between this scale and the
Modified Mercalli Intensity
Scale (MMIS)?
List of greatest
magnitude
earthquakes
Tokyo, Japan:
1923
Japan: 2004
Magnitude = 6.6
New Zealand:
2010
Magnitude = 7.1
Mexico:
2010
Oakland, CA: 2007
Canterbury, England:
2010
Magnitude = 7.1
Anchorage, Alaska: 1964
Alaska: 1964
Goal 
Locate the epicenter of an earthquake
1.Earthquake Vocabulary Quiz
Homework 
TOMORROW
2. Crossword Puzzle
Warm-up Take out last night’s HW, Update TOC, copy
down HW
In your journal  Read the paragraph on
page 139 on the Modified Mercalli Intensity
Scale and write a brief explanation of what it is
an how its used.
What is an Epicenter?
• A point on the surface of the Earth directly above the
FOCUS of the earthquake.
•The point within the Earth from which
earthquake waves originate.
Use the data from the recording stations:
• Station A: San Francisco, California
P-Wave arrival 3:02:20
S-Wave arrival 3:06:30
What is the time
difference
between P and
S wave arrivals?
Use the data from the recording stations:
• Station B: Denver, Colorado
P-Wave arrival 3:01:40
S-Wave arrival 3:05:00
What is the time
difference
between P and
S wave arrivals?
Use the data from the recording stations:
• Station C: Missoula, Montana
P-Wave arrival 3:01:00
S-Wave arrival 3:03:00
What is the time
difference
between P and
S wave arrivals?
Difference in arrival times:
San Francisco: 4:10
Denver, Colorado: 3:20
Missoula, Montana: 2:00
TAKE A PIECE OF PAPER, AND MARK
OFF THE DIFFERENCE IN ARRIVAL TIME
MOVE THE PAPER UNTIL THE TWO
TICK MARKS LINE UP WITH THE P
AND S CURVES
WHEN TICK MARKS LINE UP, GO
STRAIGHT DOWN AND READ THE
EPICENTER DISTANCE
EPICENTER DISTANCE
OF 2800 KM
EPICENTER DISTANCES
San Francisco: 4:10
2,800km
Denver, Colorado: 3:20
2,000km
Missoula, Montana 2:00
1,100km
Recording Board
Difference in arrival times:
San Francisco: 4:10
=2,800km
5,000
4,000
3,000
2,000
1,000
Open your compass to
the EXACT distance on
the scale.
.
2,800km
.
.
2,000km
Goal  •
Review for Volcano/Earthquake Quest
Homework  Volcano/Earthquake Quest Review DUE
MONDAY
QUEST ON MONDAY! STUDY!
Warm-up
Please write down
homework(and
pick up from
counter), update
TOC and then
name the missing
processes on the
rock cycle
1
2
3
4