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Earthquakes
Forces and Faults
Think About It…
What happens to a rubber
band that is stretched past
its elastic limit?
A. Stress and Strain…
What are earthquakes?
•
natural vibrations of the ground caused by
movement along gigantic fractures in Earth’s crust
Stress and Strain…
1. When do most
earthquakes occur?
•
when rocks fracture
(break) deep within
Earth
2. When do fractures
form?
•
when stress exceeds
the strength of the
rocks involved
Stress and Strain…
What is stress?
… the forces per unit area
acting on a material
… There are three types of
stress.
3.
Stress
Compression decreases the volume of a
material – occurs at convergent
boundaries
http://rst.gsfc.nasa.gov/Sect2/05-01_reverse_fault-jyougashima_DSC8766.jpg
Stress!
Tension pulls a material apart –
at divergent boundaries
http://rpmedia.ask.com/ts?u=/wikipedia/commons/thumb/b/b1/Boudin_vein.jpg/180px-Boudin_vein.jpg
Stress!
Shear
(horizontal)
causes a
material to
twist – at
transform
boundaries.
Strain!
5.
What is strain?
• the deformation
of materials in
response to
stress
• It is represented
by the solid line
on your
diagrams.
Elastic Limit
a.
b.
A slab of rock’s elastic limit…
• … is the point at which it will
undergo permanent deformation
At failure…
• … a material breaks!
• We call this failure an
earthquake!
Faults…
1. Rocks will fail and form a fault…when
stress is applied too quickly or when
stress is great
2. A fault is the resulting fracture or system
of fractures along which movement
occurs.
Strike-Slip Faults vs Dip-Slip Faults
Normal Fault – The Sierra Nevadas
http://fela2fela.files.wordpress.com/2012/02/sierra-nevadas.jpg
http://www.pbs.org/weta/thewest/places/states/california/ca_sierra.jpg
http://geology1a-1.wikispaces.com/file/view/NormalFault.gif/90969209/NormalFault.gif
Normal Fault…
Is caused by…
 tension (at a divergent boundary)
 Movement of crust…
 block above fault moves down


http://www.iris.edu/gifs/animations/faults.htm
Reverse Fault – The Himalayas!
http://www.geographicguide.net/earth/pictures/himalayas-asia.jpg
http://img.wylio.com/flickr/500/494666903
Reverse Fault…
Is caused by…
 compression (at
a convergent
boundary)
 Movement of
crust…
 block above
fault moves up
and over

http://www.iris.edu/gifs/anim
ations/faults.htm
http://homepage.ufp.pt/biblioteca/GlossarySaltTectonics/PlatesJPG/Fig.R018-ReverseFault.jpg
http://7bcore3.wikispaces.com/file/view/StrikeSlipLLFault%5B1%5D.gif/193098468/StrikeSlipLLFault%5B1%5D.gif
Strike-slip Fault…


Is caused by…
 horizontal shear (at a transform boundary)
Movement of crust…
 blocks slide past each other
 horizontal (no vertical movement)
http://www.iris.edu/gifs/animations/faults.htm
The San Andreas Fault…

http://www.sanandreasfault.org/
A Video About Earthquakes

https://www.youtube.com/watch?v=Yukp
0bPkQxs
Earthquakes
Seismic Waves and Earth’s
Interior
The Focus of an Earthquake…
1.
2.
The focus of an earthquake is where the rocks break and it
is located at least several miles beneath Earth’s surface.
The epicenter is on Earth’s surface just above it.
http://www.em.gov.bc.ca/Mining/Geoscience/SurficialGeologyandHazards/Earthquakes/PublishingImages/eq-2b.gif
Earthquake Waves…
1. What are seismic waves?
• vibrations of the ground during an earthquake
2. How do the following types of seismic waves
move rocks?
a. Primary (P) waves – squeeze and pull rocks
in the same direction along which the waves
are traveling
b. Secondary (S) waves – cause rocks to
move at right angles in relation to direction of
waves
c. Surface waves – move in 2 directions – up
and down and side to side (slowest and most
damaging!)
d. See next slide…
http://gomyclass.com/geology10/files/lecture9/html/web_data/file22.htm
Earthquake Waves…
http://aspire.cosmic-ray.org/Labs/SeismicWaves/
Seismogram…

http://www.geo.mtu.edu/UPSeis/reading.html
“So which wiggles are the earthquake? The P wave will be the first
wiggle that is bigger than the rest of the little ones (the
microseisms). Because P waves are the fastest seismic waves, they
will usually be the first ones that your seismograph records. The
next set of seismic waves on your seismogram will be the S waves.
These are usually bigger than the P waves.
The surface waves are the other, often larger, waves marked on
the seismogram. Surface waves travel a little slower than S waves
(which, in turn, are slower than P waves) so they tend to arrive at
the seismograph just after the S waves.”
Travel-Time Graph (not time travel!)
1. A travel-time graph shows…
The average
travel time of P
and S waves for
different distances
2. One can calculate
the distance to
the epicenter
using the arrival
times of the p and
s waves.
•
http://mtweb.mtsu.edu/cribb/100eq.html
Travel-Time Curves
Suppose the waves
recorded on a
seismogram at
Station A are
farther apart than
they are at Station
B…
The epicenter is
farther from Station
A.
Focus Question…
How are earthquakes
measured and located?
Earthquake Magnitude and Intensity
How is the amount of energy released by
an earthquake measured?
•
by its magnitude
Haiti, 1/2010 (7)
The Richter Scale…
a.
The Richter scale is based on…
The size of the largest seismic waves
generated by the quake
b.
Each successive number in the
represents…
An increase in seismic-wave size of a factor of
10
c.
Each increase in magnitude corresponds
to…
A 32-fold increase in seismic energy
d.
A magnitude 8 EQ releases 32 times
more energy than a magnitude 7 EQ.
The Richter Scale
Haiti, 1/2010 (7)
Japan, 3/2011 (9)
http://sahanasingh.wordpress.com/2011/05/17/failing-to-prepare/
The Modified Mercalli Scale…
a.
The intensity of an earthquake can be
determined by…
The amount of damage done to the structures
involved
b.
The modified Mercalli scale is used to
determine the intensity of an EQ by…
Rating the types of damage and other effects of
an earthquake as noted by observers
This scale uses Roman numerals from I to
XII. It was developed by Giuseppe
Mercalli in 1902.
http://www.geo.mtu.edu/UPSeis/intensity.html
Modified
Mercalli
Scale…
http://scearthquakes.cofc.edu/images/MercalRichtscale.gif
Modified Mercalli Scale…
d. A seismic intensity map is made
by…
• … compiling the MMsI values
(Modified Mercalli scale intensity
values), plotting them on a map,
& joining points of similar
intensity.
• Mercalli values at distances very
far from the epicenter…
Decrease to I
Time to put the Mercalli Scale into
practice!
Think About It…
What is the strongest
earthquake that has
happened in your lifetime?
Depth of Focus…
e. Another factor that
determines the
intensity of an
earthquake is…
• The depth of an
EQ’s focus
Shallow-focus events
will produce
catastrophic quakes
with high intensity
values.
Seismic
Intensity
Map…
Northridge,
California
Earthquake
http://seismo.berkeley.edu/blogs/seismoblog.php/2009/01/17/today-in-earthquake-history-northridge-1994
Damage in Northridge, Ca…
http://www.weldreality.com/northridge-earthquake-4.jpg
More Damage in Northridge, Ca…
Locating an Earthquake…
1.
The distance to an
earthquake’s epicenter is
determined by…
1.
the P-S wave separation
(the time difference b/tw
the arrival of the P-wave
and the S-wave)
2.
This distance is called…
1. Epicentral distance
3.
Three seismograph
stations are needed to
determine an EQ’s
epicenter. See next slide…
Locating an Earthquake…
5. A travel-time graph determines the exact time of an EQ.
http://www.geo.mtu.edu/UPSeis/images/triangle.gif
Seismic Belts…
1.
2.
Most earthquakes are associated with…
1. Tectonic plate boundaries
80% of earthquakes occur in…
1.
3.
Another 15% of earthquakes occur at the
1.
4.
The Circum-Pacific Belt
Mediterranean-Asian Belt
Most of the remaining earthquakes occur in
narrow bands that run along the crests of ocean
ridges.
http://earthquake.usgs.gov/
http://oceanworld.tamu.edu/resources/oceanography-book/earthquakes.htm
Lab 9 USA
Map…
http://www.worldatlas.com/geoquiz/mississi.gif
Think About It…
Why do developing
countries such as Haiti
suffer such devastation
from earthquakes?
Focus Question…
How do earthquakes affect
the four spheres of Earth?
Structural Failure…
1. The types of
buildings that sustain
the most severe
damage from
earthquakes are…
•
Typical building in Iraq
unreinforced
made of stone,
concrete, or
other brittle
materials
Church, Armenia, 1988
http://www.siampinoy.net/forum/55-ano-ba-ang-latest-post-it-here/88292-chile-earthquake-may-have-shortened-days-on-earth.html
Structural Failure…
2. The types of buildings that sustain the least
amount of damage from earthquakes are…
• wooden
• high-rise, steel-framed
Kobe,
Japan
1995
Structural Failure…
3.
•
Pancaking is…
when supporting
walls of the
ground floor
collapse and
cause the upper
floors to fall and
collapse
Pancaked Building in Turkey
Structural Failure…
4. Building with 515 stories may
collapse because…
• their natural sway
has the same
period of vibration
as the EQ
http://www.wbdg.org/resources/seismic_design.php
Land and Soil
Failure…
5.
Earthquakes in
sloping areas
can trigger…
• landslides!
El Salvador, 2001, mag 7.6
Land and Soil Failure…
Soil liquefaction
is…
• when subsurface
materials liquefy
and behave like
quicksand due to
the seismic
vibrations of an
EQ
7. Seismic waves are
amplified in soft
materials because…
• they have little
resistance to
deformation
6.
http://en.wikipedia.org/wiki/Soil_liquefaction
Land and Soil Failure…
7. Because Mexico
City is built on
soft sediments,
there was much
damage to
structures there
during the 1985
EQ.
http://seismo.berkeley.edu/blogs/seismoblog.php/2008/09/19/title
Fault Scarps…
8. A fault scarp is…
•
9.
an area of great
vertical offset
where the fault
intersects the
ground surface.
The fault scarp
shown created a…
•
waterfall
Tsunamis…
10. A tsunami is caused
by…
•
the vertical motions
of the seafloor
displacing the entire
column of water
over the fault
11. A tsunami in the
open ocean is
usually…
• 1 meter high
• You probably
wouldn’t notice it
if you were there!
Tsunami strikes Ao Nang, Thailand
http://en.wikipedia.org/wiki/2004_Indian_Ocean_earthquake_and_tsunami
Tsunamis…
12. A tsunami
is dangerous
when it
enters
shallow water
because…
• it moves
very quickly
• it can be
very high –
up to 30m!
Sri Lanka, 2004
Tsunamis…
13. The most recent devastating tsunami
was in…
• Japan, March 2011
Seismic Risk…p. 510
Most earthquakes occur at…
• tectonic plate boundaries
Seismic Risk…
14a. Five states with the
greatest seismic risk…
•
•
•
•
•
Alaska
California
Hawaii
Utah
Nevada
The San Andreas
Fault is a
transform boundary.
http://pubs.usgs.gov/fs/2005/3052/images/map.jpg
Seismic Risk…
14c. The seismic risk
of the NC Piedmont
and mountains…
• moderate
14d. The seismic risk
of the coastal plains
of NC…
• minor
Earthquake Prediction…
15. Earthquake
recurrence rates
indicate …
• that the fault
ruptures
repeatedly at
regular intervals
to generate similar
quakes
http://upload.wikimedia.org/wikipedia/commons/8/8d/San_Andreas.jpg
Earthquake Prediction…
21. Seismic gaps are
• sections of active
faults that haven’t
experienced
significant EQs for
a while
• An earthquake is
likely to happen
here!
The San Andreas Fault
Earthquake Prediction…
17. Why is earthquake prediction important?
… to save lives and
prevent property
damage!!
EQ Shaking Hazards in the USA…
F. Clues to Earth’s Interior (p. 503)
1. Where does the knowledge of Earth’s interior
come from?
• The study of seismic waves
2. How do P-waves and S-waves move through the
mantle?
• They follow fairly direct paths
3. What do P-waves do when they strike the core?
• They bend
4. Why don’t S-waves enter Earth’s core?
• They cannot travel through liquids
Clues to Earth’s Interior
5.
What does the
disappearance of Swaves tell us about
Earth’s outer core?
• That it must be
liquid
6.
What shows us that
Earth’s inner core is
solid?
• Studies of how P
waves are
deflected deep
within Earth
http://www.columbia.edu/~vjd1/ray_paths.gif
G. Earth’s Internal Structure…
Crust of lithosphere – granite (continental
crust), basalt (oceanic crust)
 Upper mantle – peridotite (intrusive,
bumpy, igneous)
 Asthenosphere – peridotite
 Lower mantle – oxides of iron, silicon, and
magnesium
 Core – iron and nickel (magnetic field is
generated here!

G. Earth’s Internal Structure …
http://astronomy.nju.edu.cn/~lixd/GA/AT4/AT407/HTML/AT40703.htm
Let’s pause for a moment. What does all of
this have to do with North Carolina?
Have you heard of “fracking”?
What does this have to do with
earthquakes?
http://www.youtube.com/watch?v=qm7e5
53S7fg
 http://www.youtube.com/watch?v=btRIH
GKKKes
 http://www.youtube.com/watch?v=NW_xJ
qPjE_I
