The Earth’s lithosphere is extremely active, as continental and oceanic plates constantly pull apart, collide and scrape alongside each other. When they do, they form faults. There are different types of faults: reverse faults, strike-slip faults, oblique faults, and normal faults.
In essence, faults are large cracks in the Earth’s surface where parts of the crust move in relation to one another. The crack itself does not make it a fault, but rather the movement of the plates on either side is what designates it as a fault. These movements prove that the Earth has powerful forces that are always working beneath the surface.
Faults come in all sizes; some are tiny with offsets of only a few meters, while others are large enough to be seen from space. Their size does, however, limit the potential for earthquake magnitude. The San Andreas fault’s size (around 800 miles long and 10 to 12 miles deep), for example, makes anything above an 8.3 magnitude quake virtually impossible.
Parts of a Fault
The main components of a fault are (1) the fault plane, (2) the fault trace, (3) the hanging wall, and (4) the footwall. The fault plane is where the action is. It is a flat surface that may be vertical or sloping. The line it makes on the Earth’s surface is the fault trace.
Where the fault plane is sloping, as with normal and reverse faults, the upper side is the hanging wall and the lower side is the footwall. When the fault plane is vertical, there is no hanging wall or footwall.
Any fault plane can be completely described with two measurements: its strike and its dip. The strike is the direction of the fault trace on the Earth’s surface. The dip is the measurement of how steeply the fault plane slopes. For example, if you dropped a marble on the fault plane, it would roll exactly down the direction of dip.
Normal faults form when the hanging wall drops down in relation to the footwall. Extensional forces, those that pull the plates apart, and gravity are the forces that create normal faults. They are most common at divergent boundaries.
These faults are “normal” because they follow the gravitational pull of the fault plane, not because they are the most common type.
The Sierra Nevada of California and the East African Rift are two examples of normal faults.
Reverse faults form when the hanging wall moves up. The forces creating reverse faults are compressional, pushing the sides together. They are common at convergent boundaries.
Together, normal and reverse faults are called dip-slip faults, because the movement on them occurs along the dip direction — either down or up, respectively.
Reverse faults create some of the world’s highest mountain chains, including the Himalaya Mountains and the Rocky Mountains.
Strike-slip faults have walls that move sideways, not up or down. That is, the slip occurs along the strike, not up or down the dip. In these faults, the fault plane is usually vertical so there is no hanging wall or footwall. The forces creating these faults are lateral or horizontal, carrying the sides past each other.
Strike-slip faults are either right-lateral or left-lateral. That means someone standing near the fault trace and looking across it would see the far side move to the right or to the left, respectively. The one in the picture is left-lateral.
While strike-slip faults occur across the world, the most famous is the San Andreas fault. The southwestern portion of California is moving northwestward towards Alaska. Contrary to popular belief, California will not suddenly “fall into the ocean.” It will just continue moving at about 2 inches per year until, 15 million years from now, Los Angeles will be located right next to San Francisco.
Although many faults have components of both dip-slip and strike-slip, their overall movement is usually dominated by one or the other. Those that experience considerable amounts of both are called oblique faults. A fault with 300 meters of vertical offset and 5 meters of left-lateral offset, for example, would not normally be considered an oblique fault. A fault with 300 meters of both, on the other hand, would.
It’s important to know a fault’s type — it reflects the kind of tectonic forces that are acting on a specific area. Because many faults show a combination of dip-slip and strike-slip motion, geologists use more sophisticated measurements to analyze their specifics.
You can judge a fault’s type by looking at the focal mechanism diagrams of earthquakes that occur on it — those are the “beachball” symbols you’ll often see on earthquake sites.