Questions: Coulomb Stress Transfer and Fault Interaction

Question 1

A magnitude 7.0 earthquake occurs on a right-lateral strike-slip fault. Where would you expect the highest concentration of aftershocks relative to the mainshock rupture zone?

• A. Directly on the ruptured fault segment, because that is where stress was highest before the mainshock
• B. In the stress shadow adjacent to the ruptured fault, where the mainshock released the most accumulated stress
• C. In lobes extending off the ends of the ruptured fault and obliquely away from it, where Coulomb stress increased
• D. Randomly distributed around the epicenter within a radius proportional to magnitude

Answer: (C)

Coulomb stress transfer produces a specific spatial pattern: lobes of increased stress extending off the ends of the ruptured fault (roughly in the direction of slip) and at angles of ~30–45° from the fault plane. Aftershocks cluster in these positive ΔCFS regions — studies show 85% or more of aftershocks fall where the mainshock increased Coulomb stress. The ruptured fault segment itself is in a stress shadow immediately after the earthquake (the shear stress there was released by the rupture). The distribution is far from random; it reflects the mechanical stress redistribution predicted by elastic dislocation theory.

Question 2

The Coulomb failure stress change (ΔCFS) formula includes both a shear stress term and a normal stress term (ΔCFS = Δτ + μ′Δσₙ). Why does a change in normal stress on a receiver fault affect how close it is to failure?

• A. Normal stress increases the fault's temperature, causing thermal weakening that promotes failure
• B. Normal stress perpendicular to the fault clamps it shut (resists slip) or unclamps it (promotes slip) — reducing normal stress allows the fault to slide at lower shear stress
• C. Normal stress only matters for thrust faults; for strike-slip faults only shear stress matters
• D. Normal stress affects the speed of rupture but not whether the fault will fail

Answer: (B)

The Coulomb failure criterion (like frictional sliding) involves both shear stress (driving slip) and normal stress (clamping the fault faces together). Friction resists slip with a force proportional to the normal force clamping the surfaces. Reducing normal stress (unclamping the fault) lowers the friction that must be overcome, so the fault can fail at lower shear stress — it has been brought closer to failure. Increasing normal stress does the opposite: it clamps the fault more tightly and moves it away from failure. This is why the complete ΔCFS formula must include both terms; an earthquake can trigger a nearby fault by reducing normal stress even without adding shear stress.

Question 3

A large earthquake can place nearby faults in a stress shadow, reducing the probability of earthquakes on those faults for years or even decades.

• T. True
• F. False

Answer: True

Stress shadows are real and seismically detectable. When a mainshock redistributes stress, some fault segments experience decreased Coulomb stress — they are clamped more tightly or have reduced shear stress in the slip direction. Seismicity rates in stress shadows typically decline below the long-term background rate for years after the mainshock. The 1906 San Francisco earthquake placed much of the surrounding fault network in shadow, and seismicity in shadow zones remained suppressed for decades. This has practical implications for hazard assessment: a major earthquake both increases risk on some faults and decreases it on others, not simply raises risk everywhere.

Question 4

After a large earthquake, most stress in the surrounding region is released equally — the ruptured fault and nearby faults are most equally stable because the mainshock reduced stress throughout the area.

• T. True
• F. False

Answer: False

An earthquake does not release stress uniformly — it redistributes it. While the ruptured segment itself experiences a large reduction in shear stress, adjacent regions and faults receive stress transferred from the earthquake. The characteristic Coulomb stress pattern has lobes of increased stress extending off the fault ends and diagonally, while stress shadows flank the sides of the rupture zone. This asymmetric redistribution is why aftershocks cluster in specific spatial patterns rather than being distributed evenly around the mainshock. Assuming uniform stress release would predict random aftershock locations, which contradicts the well-documented clustering in positive ΔCFS lobes.

Question 5

Why do aftershock locations cluster in regions of positive Coulomb stress change rather than being randomly distributed around the mainshock?

Model answer: A mainshock redistributes stress by elastic deformation of the surrounding crust — relaxing stress on the ruptured fault but loading adjacent regions. The Coulomb stress change (ΔCFS = Δτ + μ′Δσₙ) quantifies how much closer to failure each point in the surrounding rock has moved. Faults or fault segments where ΔCFS > 0 have been brought closer to their failure threshold; those in regions where pre-existing stress was already near critical will rupture as aftershocks. The spatial pattern of positive ΔCFS lobes reflects the geometry of the mainshock's slip — specifically the direction of stress load transfer from the ends of a strike-slip fault. That ~85% of aftershocks fall in positive ΔCFS regions is strong evidence that stress redistribution, not random chance, governs their locations.

This understanding has direct practical applications: after a large earthquake, Coulomb stress calculations can be performed within hours using source parameters from the focal mechanism, allowing seismologists to identify which fault segments face elevated risk. These rapid assessments inform emergency response and public communication about ongoing hazard. The framework also applies over longer timescales, explaining how sequences of large earthquakes can progressively load unruptured fault segments in a region.