Previous Animations of the Month
Previous Animations of the Month can be found on the archive page.
If you cannot view the YouTube videos below, click the links next to them labeled “Direct Link to Animation” to view the videos using Quicktime Player.
Introduction
What is a fault?
A fault is a rock fracture where the two sides have been displaced relative to each other. An earthquake is what happens when these two blocks of the earth, seemingly stuck together, suddenly slip past one another. The surface where they slip is called the fault or fault plane. The slip causes a sudden shaking or vibration in the Earth due to the sudden release of energy from within the Earth. Because of pressure and friction at depth, plates rarely slide quietly past each other. Stress builds up between them until it reaches a critical strain, and the accumulated potential energy is released as the earthquake. The higher the stress, the greater the energy released. (Remember when you first learned about the energy release of a rubber band under stress?) The relative motion between the plates controls what type of a fault results.
The fault model lecture after the animations further describes the concepts above.
Quick Demo
It’s a Snap! An earthquake is caused by a sudden slip on a fault, much like what happens when you snap your fingers. Put your thumb and middle finger together. With mild pressure, or with greasy fingers they slide past each other easily with little stress. With dry fingers and higher stress, you can affect an audible response. Before allowing your fingers to snap, push them together and sideways. Allow friction to keep them from slipping. When you apply enough stress to overcome this friction, your fingers move suddenly, releasing energy. The same “stick-slip” process goes on in the earth. Stresses in the earth’s outer layer push the sides of the fault together. The friction across the surface of the fault holds the rocks together so they do not slip immediately when pushed sideways. Eventually enough stress builds up and the rocks slip suddenly, releasing energy in waves that travel through the rock to cause the shaking that we feel during an earthquake. Snapping fingers can’t generate an earthquake, but they do generate energy in the form of sound waves that travel from fingers to ear. Sound waves are compressive waves; the P waves generated from an earthquake are also compressive waves. [modified from http://earthquake.usgs.gov/learning/topics/megaquakes.php]
About the Animations
These animations of four faults are elementary examples of fault motion intended for simple demonstrations. Faults are categorized into three general groups based on the sense of slip or movement. If the main sense of movement on the fault plane is up or down, the fault is known as a dip-slip fault. Where the main sense of slip is horizontal the fault is known as a strike-slip fault. Oblique-slip faults have significant components of both strike and dip slip.
When you snap your fingers (see box above) the whole “fault plane” moves due to uniform friction release; an earthquake happens on a discrete area of the fault plane that becomes un-stuck.
Dip-Slip Faults
Normal Fault
In a normal fault, the block above the fault moves down relative to the block below the fault. This fault motion is caused by tensional forces and results in extension. [Other names: normal-slip fault, tensional fault or gravity fault] EX., Sierra Nevada/Owens Valley; Basin & Range faults
Direct Link to Normal Fault (Small 1.4mb)
Direct Link to Normal Fault (Larger 2.2mb)
Reverse Faults
In a reverse fault, the block above the fault moves up relative to the block below the fault. This fault motion is caused by compressional forces and results in shortening. A reverse fault is called a thrust fault if the dip of the fault plane is small. [Other names: thrust fault, reverse-slip fault or compressional fault] EX., Rocky Mountains, Himalayas
Direct Link to Reverse Fault (Small 1.34mb)
Direct Link to Reverse Fault (Larger 2.11mb)
Strike-Slip Fault
In a strike-slip fault, the movement of blocks along a fault is horizontal. If the block on the far side of the fault moves to the left, as shown in this animation, the fault is called left-lateral. If the block on the far side moves to the right, the fault is called right-lateral. The fault motion of a strike-slip fault is caused by shearing forces. Examples: San Andreas Fault, California; Anatolian Fault, Turkey [Other names: transcurrent fault, lateral fault, tear fault or wrench fault.]
Direct Link to Strike-Slip Fault (Small 1.55mb)
Direct Link to Strike-Slip Fault (Larger 2.2mb)
Oblique Fault
Oblique-slip faulting suggests both dip-slip faulting and strike-slip faulting. It is caused by a combination of shearing and tension or compressional forces. Nearly all faults will have some component of both dip-slip (normal or reverse) and strike-slip, so defining a fault as oblique requires both dip and strike components to be measurable and significant.
Direct Link to Oblique Fault (Small 1.32mb)
Direct Link to Oblique Fault (Larger 1.99mb)
Fault Models Lecture
Dr. Robert Butler, University of Portland, discusses Faults and Folds
Direct Link to Fault Models Lecture (13mb)
Fault Activities
USGS “A Model of Three Faults”
http://interactive2.usgs.gov/learningweb/textonly/teachers/faults.htm
“Teaching about Plate Tectonics and Faulting Using Foam Models”, Dr. Larry Braile
http://web.ics.purdue.edu/~braile/edumod/foammod/foammod.htm
Additional Resources
For images and more descriptions about faults see the NOAA slide show and information page - a rich source of information.
http://www.iris.washington.edu/gifs/slides/faults/slideshow/index.htm
For an interactive map of the youngest faults and folds of the continental United States, go to: http://gldims.cr.usgs.gov/qfault/viewer.htm This extensive database summarizes geologic, geomorphic, and geographic information for about 2,000 Quaternary faults in the United States
