Where rock is brittle, stress makes it break. A fault is a break along which the rock on one side has moved past the other. Faults are named by which way the blocks moved, and that tells you which stress made them.
Why it works
On a sloping fault, the block above the fault plane is the hanging wall and the block below is the footwall (miners named them: you stand on the footwall and hang your lantern from the hanging wall). Pull the crust apart and the hanging wall slides down the slope; the crust gets longer. Push the crust together and the hanging wall is shoved up the slope; the crust gets shorter and thicker. Shear the crust sideways and the blocks slide past each other horizontally.
| Fault | Motion | Stress | Example |
|---|---|---|---|
| Normal | hanging wall down | tension | Basin and Range, Nevada |
| Reverse | hanging wall up (steep fault) | compression | many mountain fronts |
| Thrust | hanging wall up, fault dipping under about 30° | compression | Himalayas, Canadian Rockies |
| Strike-slip | blocks slide sideways | shear | San Andreas Fault (right-lateral) |
Key idea
Name a fault by how the hanging wall moved relative to the footwall: down for normal (tension), up for reverse and thrust (compression). Strike-slip faults move sideways; looking across one, if the far side moved right, it is right-lateral.
Worked examples
Worked example: Throw and heave
A normal fault dips $60^\circ$. The hanging wall slid 40 m down the fault plane. How far did it drop vertically, and how far did the crust stretch?
- The slip is the hypotenuse of a right triangle whose angle is the dip.
- Vertical (throw): $40 \sin 60^\circ = 40 \times 0.866 \approx 34.6\,\text{m}$.
- Horizontal (heave): $40 \cos 60^\circ = 40 \times 0.5 = 20\,\text{m}$ of stretching.
Answer: About 34.6 m down and 20 m of stretching.
Worked example: Which side moved?
Standing on one side of a strike-slip fault, you see a fence on the far side offset 3 m to your left. What kind of fault?
- Looking across the fault, the far side moved left.
- That is left-lateral, and a person on the other side would also see your side moved to their left.
Answer: Left-lateral.
Common mistake
A common slip is to decide a fault's type by which side stands higher today. Erosion can wear the raised side down, or the land may be flat. The type is set by how the hanging wall moved compared with the footwall: follow a matching layer across the fault. Check: if the same layer is lower on the hanging wall side, it is a normal fault, whatever the landscape looks like.