1. Introduction to
Faults

2. San Andreas Fault, California
What is a fault?
A fault is a break or crack in Earth’s crust along which rocks shift their position.
The San Andreas Fault accommodates the motion of the transform boundary separating the Pacific and North American plates. This photograph is an aerial shot of the fault. The raised topography along a fault is called a ‘fault scarp’.
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San Andreas Fault, California

3. Describing a fault: Fault Line Fault Plane Foot Wall Hanging Wall
How do earth scientists describe faults?
This diagram labels the parts of a fault on a divergent plate boundary. The two blocks are being pulled apart (as noted by the ‘Stress’ label and arrows above).
The ‘fault plane’ is the surface on which motion occurs.
The ‘fault line’ is the line along the surface that follows the fault plane (this is what we see in the San Andreas Fault picture on the previous slide – the fault plane is not visible but the fault line is visible).
The ‘fault scarp’ is the portion of the fault plane that is now exposed at the surface.
The ‘foot wall’ and ‘hanging wall’ describe the two sides of the fault when the fault plane is not vertical. These can be easily described by imagining a miner walking down a tunnel that follows the fault plane. The side of the fault that the miner walks on is called the ‘foot wall’. The side of the fault on which the miner hangs a lantern is called the ‘hanging wall’.
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4. Fault Type: Normal
A normal fault occurs when the two blocks are being pulled apart.
Question for students: What type of plate boundary has two plates being pulled apart? (Answer: divergent boundary)
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5. Plate Boundary: Divergent
Divergent plate boundaries tend to have normal faults.
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6. Fault Type: Reverse
A reverse fault occurs when the two blocks are being pushed together.
Question for students: What type of plate boundary has two plates being pushed together? (Answer: convergent boundary)
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Additional information: ‘Thrust faults’ are a specific type of reverse fault on which the angle is very shallow. Here, for example, the fault plane is almost vertical. In a thrust fault, the fault plane is close to horizontal.

7. Plate Boundary: Convergent
Convergent plate boundaries tend to have reverse faults.

8. Fault Type: Strike-slip
A strike-slip fault occurs when the two blocks slide past each other.
Question for students: What type of plate boundary has two plates sliding past each other? (Answer: transform boundary)
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Additional information: Strike-slip faults can be categorized as ‘left-lateral’ and ‘right-lateral’. The directions left and right correspond to the direction of motion of the block on the far side of the fault line. For example, if you imagined standing on top of the block on the right hand side and you looked across the fault line, then everything on the opposite side of the fault would be moving to your left. This is a ‘left-lateral’ fault. Conversely, if the direction of the arrows were reversed then it would be a ‘right-lateral’ fault.
The differences between left- and right-lateral can be taught to students by having them line up in two lines facing each other. The fault line is the space dividing the two lines of students. If you have all students take one step to their left, then it will appear that the students across the ‘fault line’ are moving to their right. This is the same regardless of which side of the line students stand on, and represents the motion on a RIGHT-lateral fault. If they instead each take a step to their right, then the opposite will occur and represent the motion on a LEFT-lateral fault.

9. Plate Boundary: Transform
Transform boundaries tend to have strike-slip faults.
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10. Type of Plate Boundary Type of Faulting
Each type of plate boundary has a type of faulting that it primarily accommodates.
Type of Plate Boundary
Type of Faulting
Divergent
Normal
Convergent
Reverse
Transform
Strike-Slip
The table matches up plate boundary types with corresponding faulting types.

11. Photos of faults.
Figure Sources:

12. Fault Zones Fault zone: a region of numerous, closely spaced faults
A fault zone is a region of numerous, closely spaced faults. In southern California, for example, the Pacific plate and North American plates slide past each other (transform boundary). The San Andreas fault is the most well-known of the faults accommodating this motion, but other faults help as well. In the southernmost part of California, the main faults are the San Andreas Fault, the San Jacinto Fault, and the Elsinore fault. Each of these could additionally be considered as being a ‘fault zone’ because the faults are made up of smaller faults. The San Jacinto Fault is commonly referred to as the San Jacinto Fault Zone because of the branching off of smaller faults.
Figure source: modified from the interactive SCEC fault map at

13. What’s happening in CA?
Review of plate motion in western North America:
Figure 1 (from ):
Northern-most portion of the cost, from Canada to the northern end of California: Here, the Juan de Fuca plate is being subducted beneath the North American plate.
Most of California, from north of San Francisco to just south of the border with Mexico: The Pacific plate and North American plate slide past each other on a transform plate boundary. Most of this motion occurs on the San Andreas Fault.
In the Sea of Cortez (separating Baja California from mainland Mexico): The Pacific plate and North American plates are pulling apart from each other in this region, called the East Pacific Rise (divergent plate boundary).
Figure 2: Map of earthquakes in CA in a one week period prior to 9/21/2009. Earthquakes are color-coded (red occurred in the previous hour, blue occurred in the previous day, and yellow occurred in the previous week). An updated version of this map can be obtained on the USGS website (link below). The size of the boxes correspond with the magnitude of the earthquakes. Red lines map known faults. Figure from
Main talking points:
- Earthquakes occur throughout California and not just on the San Andreas Fault. Several earthquakes occur in areas where faults are not on the map. Generally, students don’t realize that this many earthquakes occur (here, 547 in the previous week). Most earthquakes are small and are not felt many people.

14. Will CA break off and fall into the ocean?
A common question: Will California break along the San Andreas Fault and fall into the ocean?
The answer: No! Motion along the San Andreas fault is horizontal; the side containing Los Angeles and San Diego is sliding past the other side. At some point (### million years from now), the side containing Los Angeles and San Diego may no longer be attached to the rest of North America, but it won’t ‘fall into the ocean’. In addition, the thickness of the continental crust is thicker than the depth of the oceans, so it is not possible for this much land to ‘fall into the ocean’.
Animation from:

15. Faults in Southern California
Focus on faults in Southern California. Light blue lines show faults; yellow lines show major freeways for reference. The online version of this map (link given below) is interactive. Clicking on an individual fault leads to a page giving more information about that specific fault.
SCEC SoCal fault map - discuss that earthquakes tend to happen on faults (not randomly), and that there are more faults than just the major ones (SAF) -- point out nearby faults (SJFZ, Elsinore, Rose Canyon)