1. Part 2: Earthquake faults and waves

2. What is an Earthquake? Simple answer: Why should the ground shake?
Shaking ground
Why should the ground shake?
Movement along a fault
Something hits the ground:
explosion
landslide
building collapse
meteorite impact
What is a fault, and why should ground move along it?

3. Stress
Stress is a force (push, pull or twist) applied to an area (to a surface)
There are three types of stress: compression, tension, and shear
Rocks respond to stress by deforming. Strain is the amount of
deformation experienced by a rock.

4. Stress deforms rocks Stress changes the shape of an object.
Tension lengthens an object,
making it wider in the direction
of maximum stress.
Compression shortens an object,
making it thinner in the direction
Shear stress doesn’t lengthen or
shorten, but changes the angles
of an object – shear rotates the

5. Strain = deformation caused by stress
Strain is elastic if deformed rocks
return to original shape
Strain is plastic or ductile if deformed
rocks do not recover original shape
Strain is brittle if rocks break

6. Compression Experiments
Stress applied to rock cylinders
Initially, a rock deforms elastically
(strain increases linearly with
stress).
Elastic deformation ends at the
yield point.
Past the yield point, a rock may
deform plastically (permanent
deformation), up to a point (failure
point) where it breaks.
For brittle rocks, there may be no
plastic deformation, and the yield
point is also the failure point
(straight from elastic deformation
to breaking).
Permanent
deformation

7. Faults result from stress
A fault is a fracture in a rock resulting from brittle failure due to stress

8. An Earthquake Model
An earthquake is a sudden movement of the material on either side of a fault.

9. Elastic Rebound Theory
Based on observations
of 1906 San Francisco
earthquake
Slow and steady
application of stress on
opposite sides of a fault
cause bending, storing
energy as elastic strain
Eventually, yield point of
rocks on fault is
reached, and energy is
released suddenly
(breaking rocks along
fault and causing ground
motion
Cycle begins to repeat
again.

10. Fault Terminology A fault is a planar fracture, so we
can talk about a fault plane
(shown in red).
The fault’s strike is defined as the
compass direction (e.g.,
northeast) of the line that is
created when the fault plane
intersects a flat surface.
The fault’s dip is defined as the
angle of the fault plane relative to
a horizontal surface.
The hanging wall is all of the
material that is above the fault
plane.
The footwall is all of the material
that is below the fault plane.

11. Faults and Stress
Fault plane in red
Movement shown by red arrow
Tension and compression cause movement along the dip of the fault plane (hanging wall moves down or up relative to footwall).
Shear stress causes motion along the strike of the fault plane.

12. Fault Types – Dip Slip Tension causes hanging wall to drop relative to
footwall
Section of land along fault
widens, but also thins
This type of fault is called a
normal fault
A low-angle normal fault is
called a detachment
Thinner crust, notice lack of flesh-colored layer along fault – widened crust

13. Examples of Normal Faults

14. Horst and Graben
horst
graben
horst
Grabens (structural valleys) are created when a block of crust (hanging wall) drops along two normal faults. The footwalls remain high and are called horsts.
Above: parallel mountain ranges and down-dropped valleys of the Basin and Range in the Western U.S.

15. Fault Types – Dip Slip Compression causes
hanging wall to rise relative
to footwall
Section of land along fault
narrows, but also thickens
This type of fault is called a
reverse fault
A low-angle normal fault is
called a thrust

16. Reverse and Thrust Faults
Left: Providence Mountains of the Mojave National Scenic Preserve of California
Right: Chilean Andes

17. Fault Types – Strike Slip
Shear stress causes motion
along the strike. The
hanging wall does not move
up or down relative to the
footwall
This type of fault is called a
strike-slip fault
Faults are either left-lateral
strike-slip faults or right-
lateral strike-slip faults,
depending on the relative
sense of motion.
Left-lateral strike-slip fault
Right-lateral strike-slip fault

18. Examples – Strike-Slip Faults

19. Seismic Waves
An earthquake occurs when stored energy is released suddenly.
The fault breaks, and one side of the fault moves relative to the other side of the fault.
The release of energy produces vibrations of the ground known as seismic waves, which move away from the point of rupture (hypocenter).

20. Body vs. Surface Waves
There are two different types of body waves (P waves and S waves) and two different types of surface waves (Rayleigh waves and Love waves)

21. P waves
P waves are also called a compressional waves, as individual particles experience both compression and then dilation (stretching) as the wave passes.
P waves are also called primary waves, as they have the fastest velocity of all of the seismic waves
P waves can travel through both solids and liquids inside the body of the Earth.

22. S waves
S waves are also called a shear waves, as individual particles experience shearing as the wave passes.
S waves are also called secondary waves, as they have the second fastest velocity of all of the seismic waves
S waves can travel only through solids inside the body of the Earth.

23. Rayleigh waves
Rayleigh waves can travel only along the surface of the Earth
The rolling motion of a Rayleigh wave is similar to that of water waves in the ocean.

24. Love waves Love waves can travel only along the surface of the Earth
Love waves have a jerky sideways motion relative to the direction of propogation

25. Comparing Seismic Waves
Video showing motions and relative velocities of P, S, and surface waves:

26. References
Title page: Normal fault, Arches NP, Utah. Photo from Geotrippers at
Title page: earthquake recorded by a seismometer in Germany. Image from USGS at
What is an Earthquake: Clip art from file:///C:/Documents%20and%20Settings/Owner/Desktop/G148/images/Clip%20Art%20Earthquake%20Fault%20Clipart.html
Stress: Force diagram from clip art at Stress diagram from Bruce Railsback at
Stress deforms rocks: Image drawn by Melinda Hutson, Portland Community College
Strain = deformation caused by stress: Image from USGS at
Compression experiments: top image from Bottom image drawn by Melinda Hutson, Portland Community Colllege.

27. References
Faults result from stress: Image from
An Earthquake Model: Image from
Elastic Rebound Theory: Left image from Right image from
Fault Terminology: Image drawn by Melinda Hutson, Portland Community College
Faults and Stress: Left image drawn by Melinda Hutson, Portland Community College. Right image from
Fault Types – Dip Slip (normal faults): Image from University of Arizona, media resources at
Examples of Normal Faults: Images from geotripper site at

28. References
Horst and Graben: Left image from geotrippers at Right image from Teacher features, California Geological Survery at
Fault Types – Dip Slip (reverse): Image from Teacher features, California Geological Survery at
Reverse and Thrust Faults: Left image from geotrippers at Right image from This site credits “Photo credit Constantino Mpodozis. Image from website: but the cornell link is broken.
Fault Types – Strike Slip: Images from USGS at and and credited to
Steven Dutch, University of Wisconsin.

29. References
Examples – Strike-Slip Faults: Images from Geology photos page says free to use for noncommercial purposes. Left image just says left-lateral in siltstone. Right image says Oregon Coast.
Seismic Waves: Image from a resources for educators page from CatlTech at
Body vs. Surface Waves: Large image from Small image from South Carolina Earthquake Education Site at
P waves; S waves; Rayleigh wave; Love waves: Image and animations from