Questions: Fault Mechanics: Friction and Earthquake Rupture

Question 1

A previously stable fault sits near wastewater disposal wells that have been injecting fluid into the subsurface, raising pore pressure. According to the Coulomb failure criterion, why does seismic risk increase even though tectonic stresses haven't changed?

• A. Injected water chemically weakens fault minerals, reducing cohesion to zero
• B. Increased pore pressure reduces effective normal stress, lowering frictional resistance toward the pre-existing shear stress
• C. Fluid injection increases shear stress by altering regional tectonic forces
• D. Water raises subsurface temperatures, shifting the fault from brittle to ductile behavior

Answer: (B)

The Coulomb criterion states slip occurs when shear stress exceeds (cohesion + μ × effective normal stress), where effective normal stress = total normal stress − pore pressure. Raising pore pressure directly reduces the effective normal stress term, lowering frictional resistance. The fault approaches failure without any change in tectonic loading. This is the mechanism behind injection-induced seismicity — not altered tectonics, but a local reduction in the clamping force holding the fault closed.

Question 2

A magnitude 6 earthquake occurs on Fault A. Seismologists observe a cluster of aftershocks on nearby Fault B, which had been locked for decades. What best explains this pattern?

• A. The earthquake on Fault A reduced normal stress everywhere, weakening all faults in the region
• B. Fault A transferred positive Coulomb stress to Fault B, pushing it closer to its own failure threshold
• C. The seismic waves physically shook Fault B until it slipped from dynamic loading alone
• D. Fault B slipped sympathetically because it shares the same fault zone as Fault A

Answer: (B)

Coulomb stress transfer explains why a mainshock on one fault triggers aftershocks on other faults. The earthquake on Fault A rearranges the stress field: some nearby faults receive positive Coulomb stress changes (pushed toward failure) while others receive negative changes (stress shadows, temporarily stabilized). Fault B experienced a positive change — the combination of increased shear stress and/or reduced effective normal stress brought it closer to its own Coulomb threshold. This mechanism is predictive: the 1992 Landers earthquake's stress transfer correctly predicted elevated seismicity at the location of the 1999 Hector Mine earthquake.

Question 3

A fault with a low friction coefficient is generally more susceptible to slip than a fault with a high friction coefficient, most else being equal.

• T. True
• F. False

Answer: False

The Coulomb criterion compares shear stress to (cohesion + μ × effective normal stress). A low friction coefficient reduces the resistance term, but whether the fault slips depends on whether shear stress exceeds that resistance. A fault with low friction but also very low shear stress (e.g., oriented at an unfavorable angle to the regional stress field) may remain locked, while a high-friction fault under high shear stress may be closer to failure. Friction coefficient, normal stress, pore pressure, and shear stress all matter — no single parameter determines slip in isolation.

Question 4

Earthquake rupture propagates outward from the hypocenter because the slipping fault patch transfers stress to adjacent locked patches, which may then exceed their own failure threshold.

• T. True
• F. False

Answer: True

This is the core physical mechanism of rupture propagation. The hypocenter is the initial failure point; the rupture spreads because a slipping patch concentrates stress at its propagating edge, pushing adjacent locked patches toward their Coulomb threshold. Whether the rupture continues or stops depends on whether this transferred stress exceeds the adjacent patch's resistance. This cascading process determines earthquake magnitude — large earthquakes (M9) propagate hundreds of kilometers because stress transfer kept driving rupture forward; small earthquakes (M5) propagate only kilometers before stress transfer becomes insufficient.

Question 5

Explain why increasing pore fluid pressure can trigger slip on a fault that was previously stable, using the Coulomb failure criterion.

Model answer: Fault slip occurs when shear stress exceeds frictional resistance, which equals cohesion plus the friction coefficient times the effective normal stress. Effective normal stress equals total normal stress minus pore pressure. Increasing pore pressure directly reduces effective normal stress, which lowers frictional resistance. If tectonic loading has already brought shear stress close to the original resistance, even a modest pore pressure increase can push the fault past failure — without any change in tectonic forces.

This is the mechanism behind induced seismicity from wastewater disposal and geothermal operations. The Coulomb criterion has three adjustable quantities: shear stress (tectonically driven), normal stress (geometry and overburden), and pore pressure (manipulable by fluid injection). By raising pore pressure, engineers inadvertently 'unclamped' faults that were near failure. The key insight is that faults near populated areas may be tectonically loaded close to their threshold — only a small perturbation in any Coulomb term is needed to trigger slip.