Fault Types

Why This Matters

When you're studying earthquake engineering, understanding fault types isn't just about memorizing names—it's about recognizing how tectonic stress translates into ground motion and what that means for structural design. The type of fault determines the direction of rupture, the characteristics of seismic waves produced, and ultimately the kinds of forces your buildings and infrastructure will need to withstand. You're being tested on your ability to connect plate boundary dynamics, stress orientation, and fault geometry to real engineering decisions.

Each fault type produces distinct ground motion patterns that engineers must anticipate. A strike-slip fault generates different shaking characteristics than a thrust fault, which means different design considerations for foundations, lateral force-resisting systems, and site selection. Don't just memorize that normal faults involve extension—know why extensional stress produces downward hanging wall movement and what that implies for structures built across or near the fault trace.

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Dip-Slip Faults: Vertical Movement Dominates

Dip-slip faults involve movement primarily along the fault's dip angle, meaning the blocks move up or down relative to each other. The direction of movement depends entirely on whether the crust is being pulled apart (extension) or pushed together (compression). Understanding dip-slip mechanics is foundational because it connects directly to regional stress regimes.

Normal Fault

• Extensional stress causes the hanging wall to drop—this occurs when tectonic plates diverge or when the crust stretches and thins
• Common at divergent boundaries like mid-ocean ridges and continental rift zones (East African Rift is a classic example)
• Moderate to large magnitude earthquakes result, with ground rupture patterns that engineers must account for in foundation design

Reverse Fault

• Compressional stress pushes the hanging wall upward relative to the footwall—the opposite motion of normal faults
• Found at convergent plate boundaries where plates collide and crust shortens horizontally
• Generates highly destructive earthquakes because compression stores enormous elastic strain energy before sudden release

Thrust Fault

• Low-angle reverse fault (typically <45°) where the hanging wall overrides the footwall along a gently dipping plane
• Responsible for major mountain-building events—the Himalayas and Andes formed largely through thrust faulting
• Produces the largest recorded earthquakes (2011 Tōhoku, 2004 Sumatra), demanding conservative engineering design in subduction zones

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Strike-Slip Faults: Horizontal Movement Dominates

Strike-slip faults accommodate lateral motion where blocks slide horizontally past each other with minimal vertical displacement. The fault plane is typically near-vertical, and movement occurs along the strike direction. These faults are critical in earthquake engineering because they often run through populated areas and produce intense, localized shaking.

Strike-Slip Fault

• Horizontal block movement with negligible vertical offset—classified as right-lateral or left-lateral depending on the direction of motion
• Associated with transform plate boundaries where plates slide past one another (San Andreas Fault is the textbook example)
• Sudden rupture produces sharp, high-frequency ground motion that can devastate structures aligned parallel to the fault trace

Transform Fault

• Specific strike-slip fault connecting offset plate boundaries—accommodates differential motion between spreading ridges or other tectonic features
• Critical role in plate tectonics by allowing rigid plates to move across Earth's curved surface
• Generates powerful shallow earthquakes because rupture occurs in brittle upper crust, requiring stringent seismic design in affected regions

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Complex Fault Motion: Combined Mechanisms

Some faults don't fit neatly into pure dip-slip or strike-slip categories. When multiple stress directions act simultaneously, faults can exhibit combined vertical and horizontal movement. Engineers working in tectonically complex regions must account for these multi-directional ground motions.

Oblique-Slip Fault

• Combines vertical and horizontal displacement in a single fault motion—neither pure dip-slip nor pure strike-slip
• Develops in complex tectonic settings where extensional, compressional, and shear stresses interact simultaneously
• Variable earthquake characteristics make hazard assessment challenging; engineers must consider multiple ground motion directions in structural design

Dip-Slip Fault (General Category)

• Umbrella term for faults with dominant vertical motion—includes both normal and reverse fault types
• Essential for understanding crustal deformation mechanics and predicting how strain accumulates and releases
• Engineering implications vary by subtype—normal faults may create grabens affecting foundations, while reverse faults can cause surface uplift and folding

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Quick Reference Table

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Self-Check Questions

1. Which two fault types both involve compressional stress, and what geometric feature distinguishes them from each other?

2. A site investigation reveals a fault with both significant vertical and horizontal displacement. What fault type is this, and why does it complicate seismic design compared to simpler fault types?

3. Compare and contrast normal faults and reverse faults in terms of stress regime, hanging wall motion, and typical tectonic setting.

4. An FRQ asks you to explain why subduction zones produce the world's largest earthquakes. Which fault type would you discuss, and what characteristics make it capable of storing enormous strain energy?

5. If you're designing a structure near the San Andreas Fault, what type of ground motion should you primarily design for, and which fault classification explains this motion pattern?