Types of Faults: Normal, Reverse, Thrust, Strike-Slip
The San Andreas Fault cuts California for 1 200 km. But not all faults move sideways — some drop blocks down, others shove blocks over each other. The motion reveals the stress that created it.
Stand on one side of the San Andreas Fault and look across: the land on the far side has slid hundreds of kilometres relative to where you stand. That is a <strong>strike-slip fault</strong>. But travel to the Basin and Range of Nevada and you see a different story: mountain blocks tilted up and valleys dropped down along <strong>normal faults</strong>. In the Himalayas, India is being shoved over Asia along <strong>thrust faults</strong>. Every fault type is a signature of the stress that made it.
What is a fault?
A fault is a fracture in Earth's crust along which rock on one side has moved relative to rock on the other. Faults form when stress exceeds the strength of the rock. Once a fault exists, it can become a weak zone that moves again and again, accumulating large total displacements over millions of years.
The two sides of a fault are named by imagining yourself standing on the fault and looking down into it. The block beneath your feet is the footwall; the block hanging over your head is the hanging wall. These terms are essential for describing the direction of motion.
Normal faults — tension pulls the hanging wall down
In a normal fault, the hanging wall moves down relative to the footwall. This happens under tensional stress, where the crust is being stretched and pulled apart. The Basin and Range Province of the western United States is a classic example: a broad region of tilted mountain blocks and dropped valleys formed as the crust stretched.
Normal faults are common at divergent plate boundaries, in continental rifts, and in zones of post-orogenic collapse where mountains spread sideways under their own weight after the compressional force ends.
Reverse and thrust faults — compression pushes the hanging wall up
In a reverse fault, the hanging wall moves up relative to the footwall. This happens under compressional stress. Reverse faults have steep dip angles (typically greater than 45°). When the fault plane is gentler — less than about 45° — the fault is called a thrust fault.
Thrust faults are especially important in mountain building. The Himalayas, the Appalachians, and the European Alps all contain major thrust faults that have shoved one sheet of crust over another for hundreds of kilometres. The low angle allows large horizontal displacements with relatively modest vertical uplift.
Strike-slip faults — shear slides blocks sideways
In a strike-slip fault, the motion is predominantly horizontal, parallel to the fault plane. The two blocks slide past each other like two cars sideswiping in a parking lot. The San Andreas Fault in California is the world's most famous example: the Pacific Plate slides northwest past the North American Plate at a few centimetres per year.
Strike-slip faults are classified by the relative motion of an observer standing on one block. If the opposite block moves to the right, it is a dextral (right-lateral) fault. If it moves to the left, it is sinistral (left-lateral). The San Andreas is dextral.
- Total slip = 3.0 m = 3000 mm.
- Time interval = 150 years.
- Average rate = 3000 mm ÷ 150 yr = 20 mm/yr.
- Total slip = 2.4 m = 2400 mm.
- Time = 1200 yr.
- Rate = 2400 ÷ 1200 = 2.0 mm/yr.
- Rate = 4.5 mm ÷ 50 yr = 0.09 mm/yr.
- Total uplift = 0.09 mm/yr × 10 000 yr = 900 mm = 0.9 m.
Check your understanding
- A fault is a fracture along which rock has moved. The hanging wall is above the fault; the footwall is below.
- Normal faults form under tension (hanging wall down). Reverse and thrust faults form under compression (hanging wall up).
- Thrust faults are low-angle reverse faults that allow large horizontal displacements during mountain building.
- Strike-slip faults form under shear and show horizontal offset (dextral or sinistral).
- Fault type directly reveals the stress regime that created it.
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