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Basic concepts for seismic source - Coulomb failure stress change

news 2025/5/23 15:41:26

In seismology, Coulomb stress (or Coulomb failure stress change, ΔCFS) is a fundamental concept used to understand how stress changes caused by one earthquake can promote or inhibit failure on nearby faults. It’s a key tool for modeling earthquake triggering, fault interactions, and seismic hazard assessment.

🔹 What Is Coulomb Stress?

Coulomb stress combines two components of stress acting on a fault:

$\Delta CFS = \Delta \tau + \mu' \Delta \sigma_n$

Where:

\Delta CFS: Coulomb failure stress change
\Delta \tau: Change in shear stress on the fault (in the direction of slip)
\Delta \sigma_n: Change in normal stress (positive = unclamping)
\mu’: Effective coefficient of friction (~0.4–0.6)
If ΔCFS > 0 → Fault is closer to failure → Promotes slip
If ΔCFS < 0 → Fault is stabilized → Inhibits slip

🔹 Why It Matters

1. Earthquake Triggering

When an earthquake occurs, it redistributes stress in the crust. Nearby faults can:

Experience an increase in ΔCFS (more likely to rupture)
Experience a decrease in ΔCFS (less likely to rupture)

This explains aftershock patterns and can forecast where stress has built up.

2. Fault Interaction

Earthquakes on one fault can transfer stress to another.
Helps assess multi-fault rupture potential, e.g., during earthquake sequences or doublets.

3. Aftershock Hazard Mapping

Aftershock zones often correlate with areas of positive ΔCFS.
Used to forecast aftershock likelihood.

4. Induced Seismicity

Fluid injection or extraction can change \Delta \sigma_n, affecting ΔCFS.
Important in geothermal, oil/gas, and CO₂ sequestration settings.

🔹 Real-World Example

After the 1992 Landers earthquake in California:

Seismologists observed increased Coulomb stress on nearby faults.
This helped explain the 1999 Hector Mine earthquake, which occurred in a region of positive ΔCFS.

In the 2023 Türkiye Earthquake Sequence:

Stress transfer analysis using Coulomb models can reveal how rupture on the East Anatolian Fault might have triggered subsequent events on nearby segments.

🔹 Visualization

Coulomb stress is typically shown in maps, where:

Red zones = positive ΔCFS (stress loading)
Blue zones = negative ΔCFS (stress unloading)

These maps are generated using fault geometry, slip models, and elastic dislocation theory.

🔹 Summary Table

Term	Meaning
ΔCFS	Change in Coulomb failure stress
Positive ΔCFS	Promotes fault slip
Negative ΔCFS	Inhibits fault slip
Applications	Aftershock forecasting, seismic hazard, fault interaction modeling