Questions: Earthquake Generation and Stress Release Mechanisms

Question 1

A seismically quiet fault has not ruptured in 200 years. A geologist argues this means the fault is now low-risk. What does the elastic rebound theory actually predict?

• A. The geologist is right — long periods of quiet indicate the fault is releasing stress slowly through aseismic creep
• B. The long quiet period likely means more elastic strain energy has accumulated — locked faults build stress continuously, so silence often means greater stored energy and higher risk
• C. Earthquake risk is independent of time since the last rupture
• D. The fault is safer because rock strength increases when faults are not disturbed

Answer: (B)

Elastic rebound theory describes faults as stress accumulators: tectonic forces steadily increase strain on locked fault surfaces, like compressing a spring. The longer since the last rupture, the more energy stored — not the less. A 200-year gap without rupture on an active fault boundary is cause for concern, not reassurance. The exception (aseismic creep releasing stress gradually) is real in some fault zones but must be confirmed by geodetic data, not assumed.

Question 2

A 'beach ball' focal mechanism diagram shows compressional quadrants on the top and bottom with dilatational quadrants on the left and right sides. What type of faulting does this indicate?

• A. Strike-slip faulting with horizontal motion along a transform boundary
• B. Normal faulting due to extensional stress, typical of rift zones
• C. Thrust faulting due to compressional stress, typical of subduction zones
• D. Oblique-slip faulting with both vertical and horizontal components

Answer: (B)

Normal faulting (extension) produces a specific focal mechanism pattern: the hanging wall drops, generating compression above and below the fault and dilation on the sides. This 'T-shaped' compression pattern in the beach ball corresponds to a vertical maximum stress axis — exactly the extensional regime of rift zones. Contrast with thrust faulting (compression horizontal), which shows compression on the sides and dilation top-and-bottom, and strike-slip, which shows alternating compressional and dilatational quadrants.

Question 3

The elastic rebound theory explains why earthquakes are sudden and violent: tectonic forces build elastic strain in locked rock over years or decades, then release it in seconds when the fault slips.

• T. True
• F. False

Answer: True

This is the core physical explanation for earthquake mechanics. Plates do not slide smoothly — friction locks faults, and elastic strain energy accumulates in surrounding rock. When shear stress exceeds frictional strength, the locked fault ruptures and rock snaps to a new configuration, releasing decades of stored energy in a few seconds. This explains both the sudden onset and the large magnitude of major earthquakes.

Question 4

After a major earthquake, neighboring faults are typically moved further from failure because stress has been released throughout the region.

• T. True
• F. False

Answer: False

Coulomb stress transfer is more complex than uniform stress relief. Slip on one fault changes the stress field in specific geometric patterns — some nearby faults are brought CLOSER to failure (increased Coulomb stress), others are moved further away. This explains why aftershock sequences follow predictable spatial patterns and why one large earthquake can trigger others on neighboring faults. Stress transfer is directional, not a blanket release.

Question 5

Explain the elastic rebound theory. Why do faults remain locked for long periods and then rupture suddenly, rather than allowing continuous gradual sliding?

Model answer: Faults are locked by friction between fault surfaces. Tectonic plate motion continuously loads the surrounding rock with elastic strain energy — like compressing a spring. As long as the applied shear stress stays below the frictional strength, the fault holds. When stress finally exceeds friction, the fault ruptures suddenly: rock on either side snaps to a new, lower-strain position, releasing the accumulated energy as seismic waves. Gradual sliding (aseismic creep) occurs in some faults where friction is low, but high-friction faults alternate between locked (stress building) and ruptured (energy released) states.

The key is the threshold behavior of friction: faults don't yield gradually — they hold until they fail. This threshold behavior, combined with continuous tectonic loading, is what produces the episodic, high-energy ruptures we call earthquakes. Understanding this explains why the most dangerous faults are often those that have been quiet longest — they may have accumulated the most elastic strain.