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Earth Quakes chapter 19
Section 19.1 Forces within Earth
-Earthquakes occur when rocks fracture or break deep
within the Earth’s crust.
-These fractures are caused by stress (the amount of
force per unit area acting on a material)
3 types of stress
1. Compression: decreases volume of a material.
2. Tension: pulls a material apart
3. Shear: causes a material to twist
Strain: The deformation of materials in response to
Section 19.1 page 497
Strain: The deformation of materials in response to stress.
-As stress is added to rocks they deform (strain)
As this stress is removed, the rocks can return to their
original size and shape. (ex stretch rubber band and
release) This is elastic strain.
-Ductile Deformation: If stress
exceeds a certain value, the
material is permanently deformed.
Section 19.1 page 497
Fault: Fracture or system of fractures in Earth’s crust
that occurs when stress is applied too quickly or too
great;can form as a result of horizontal compression
(reverse fault), horizontal shear (strike slip fault), or
horizontal tension (normal fault)
Section 19.1 page 497
Normal Fault In a normal fault the two involved blocks are (by gravity)
pulling away from one another causing one of the fault blocks to slip
upward and the other downward with respect to the fault plane (it
is hard to determine whether both or just one block has moved.)
<------ ----->
Section 19.1 page 497
Reverse Fault
The reverse fault is a normal fault except the general movement
of the fault blocks is toward each other, not away from each other
as in the normal fault.
-----> <-----
Section 19.1 page 497
Strike slip Fault Probably the most well known and well studied fault is
the transcurrent (strike-slip) fault known as the San Andreas fault of
California. This fault marks the margin line between the Pacific and
North American Plates. Movement on a strike strip fault is
generally horizontal.
Section 19.1 page 497
Section 19.1 page 498
3 Types of seismic waves
1. Primary or P waves: Squeeze and pull rocks
together in the direction of travel. This is the fastest
wave and can travel through liquids or solids.
Section 19.1 page 498
2. Secondary or S waves: cause rocks to move at
right angles in relation to the direction of the
waves. S waves are slower than P waves and can
only travel through solids.
Section 19.1 page 498
3. Surface (L) waves : Up-and-down (rolling) or side-toside motion of the earth surface. Surface waves, the
slowest earthquake waves, travel along the surface of the
earth rather than down into the earth. Although they are the
slowest of all earthquake waves, L waves usually cause
more damage to society than P or S waves.
Section 19.1 page 498
Make a simple chart:
P wave
Solids + liquids
Particles move
S wave
Solids only
Particles move
up and down
L wave
Parts move
Section 19.1 page 499
Focus: Point where earthquake originates.
Focal depth:How far below Earth’s surface the focus is.
The closer the focus is to the surface the more damage it
Epicenter: Point on Earth’s surface directly above the
Section 19.2 page 500
Seismometers (seismograph)-instrument that
detects and records vibrations in the Earth’s crust.
Seismogram: The recorded data produced by the
Travel time curves: show the time it takes for the P
waves and S-waves to travel to a given seismic station.
Lab Skills:
Follow these steps to calculate epicenter distance.
Step 1. Find the arrival times for the P and S waves on
the seismogram. Ex: the P wave arrived at 4:00 and the
S wave came at 4:04 and 20 seconds.
Step 2. Find the difference in arrival time between the P
and S waves.
Ex: If the P wave arrives at 4:00, and the S wave arrives
at 4:04 and 20 seconds; the difference would then be 4
minutes and 20 seconds.
Step 3. Finding the difference of travel time between the
Look at the graph in your reference tables on page 11.
Each box going up the Y axis (side) is worth 20 seconds.
Therefore every 3 boxes = a minute
Going straight up and down between the two curves find
4 minutes and 20 seconds. (Hint: how many little boxes
would this be?)
Breaking step 3 down:
First off how many boxes are in a minute again?
There are 3.
So if we want to figure out 4 minutes and 20 seconds lets
multiply the number of minutes which is 4, by how many
little boxes equal one minute which is three.
4 minutes X 3 little boxes/minute = 12 little boxes
Did we have any seconds to deal with?
Yes we have 20 seconds.
How many boxes is 20 seconds worth?
20 seconds = 1 little box.
So how many little boxes do we have all together?
12 from the minutes + 1 from the seconds = 13 little boxes.
Finishing step 3:
So what do we do with this 13 boxes?
It is your job to find where there are 13 boxes going up and
down between the P and S wave curve.
If you count too many, move towards the left.
If you count too few, look more towards the right.
Don’t worry, the more you practice, the easier this gets.
Step 4. Finding the distance
Once you find the correct line, follow it straight down
and find the distance to the epicenter.
Here you should have come across the number 3.
This is in thousands of Km’s. 3 = 3,000 Km away from
the epicenter
So how much are the little boxes on the X axis (bottom)
If you count from the origin to 1, you find 5 boxes.
Now divide 1000 by 5 and you get 200.
So each little box is worth 200 Km.
Earthquakes II
The Sequel !!!
Rated E
Step 5. Using a compass on the map
a) Once you have the distance, use the scale on your
map and set the compass to the correct distance.
b) Place the point of the compass on the city where the
station is located.
c) Draw the circle
Step 6. Triangulation.
You will need to do this 3 times to find the epicenter.
If you only draw 2 circles, there will be 2 intersect
points representing 2 possible locations for the
When you draw 3 circles (correctly) there will only be
one point where all three circles intersect.
3 stations are needed to find the location of an epicenter.
Notice where all three circles intersect.
This is triangulation.
Section 19.2 Clues to the Earth’s Interior page 502-503
Most of our knowledge of the Earth’s interior comes from the
study of seismic waves.
These waves change speed and direction when they
encounter different materials.
This change in the waves direction is called refraction
The denser the material, the faster the waves will go.
-Liquids will block S waves.
From this information we can determine the density and
composition of the Earth’s interior.
Other evidence of Earth’s interior
Meteorites. But how is this so???
Meteorites that have orbited the Sun are thought to
have formed the same way the Earth has formed.
By this rational, the inside of a meteorite will have
a similar composition to that of the Earth.
19.3 Measuring and Locating Earthquakes page
How many earthquakes occur each year?
Over one million, you just can’t feel most of them!
Magnitude: The amount of energy released by an
Richter scale: Numerical scale used to measure the
magnitude of an earthquake, using values based on the size
of the earthquake’s largest seismic waves.
An earthquake at a number 7 is 10 times larger than a 6 and
releases 32 times more energy.
19.3 Measuring and Locating Earthquakes page
Moment magnitude scale: Takes into account the
size of the fault rupture, the amount of movement
along the fault, and the rocks’ stiffness.
19.3 Measuring and Locating Earthquakes page
Modified Mercalli Scale: Rates the type and extent
of damage and other effects of the earthquake as
noted by observers. (Listed on page 507). This
would be around a 9 on a scale of 12
19.3 Measuring and Locating Earthquakes page
Key points about damage:
-The closer a city is to the epicenter, the more damage
it will sustain.
-The closer the focal point is to the surface, the more
damage will occur.
-Earthquakes occur most often at plate boundaries
-80 percent at the Circum-Pacific belt (a seismic
19.4 Earthquakes
and Society
Earthquake Hazards
19.4 Earthquakes and Society
Earthquake Hazards
-Fault Scarps
Fault Scarps
19.4 Earthquakes and Society
Earthquake Hazards
-Structural failure
Look at the map. What kind of risk do we have where
we live?
Earthquake predictions are based on probability. This
is based on history of an area and the rate at which
strain builds up.
19.4 Earthquakes and Society
page 515
Seismic Gaps: Sections of active faults that haven’t
experienced significant earthquakes for a long period
of time. (Meaning it will happen sooner than later)
Ch 19 Quiz
1. Which of the following is not a type of stress?
A. Compression
C. Thrust
B. Tension
D. Shear
2. Where do most volcanoes and earthquakes occur
world wide?
Hot spots
Plate boundaries
The Canadian shield
The East Coast of North and South America
Ch 19 Quiz
3. Which wave can not pass through the outer
core because it is a liquid?
A. P Waves
B. S Waves
C. L Waves
D. R waves
4. What is the distance to the epicenter if the P
wave arrived at 3:00 and the S wave arrived at
3:05 and 40 seconds?
A. 1,000 Km
B. 2,000 Km
C. 3,000 Km
D. 4,000 Km
Ch 19 Quiz
5. If the epicenter was 2,000 Km away, how
long did the p wave travel for?
A. 1 minute
B. 2 minutes
C. 3 minutes
D. 4 minutes
Bonus: How many stations do you need to find
the epicenter of an earthquake?