Resolved fault traction and the Mohr construction
LetΣ be the symmetric compression-positive stress tensor, the negative of tensile-positive Cauchy stress.
Key knowledge
LetΣ be the symmetric compression-positive stress tensor, the negative of tensile-positive Cauchy stress. On a plane with outward unit normaln, physical traction is−Σn, normal compression isσ_n=nᵀΣn and shear magnitude is‖(I−nnᵀ)Σn‖. In a principalσ₁–σ₃ section, a normal making angleθ toσ₁ givesσ_n=m+dcos2θ and signedτ=dsin2θ, withm=(σ₁+σ₃)/2 andd=(σ₁−σ₃)/2. The points lie on a Mohr circle of centrem and radiusd.
Values and equations
σ_n=nᵀΣn; |τ|=‖(I−nnᵀ)Σn‖; σ_n=m+dcos2θ; τ=dsin2θ; (σ_n−m)²+τ²=d²
- Σ,σ_n,τ,m,d,σ₁,σ₃
- compression-positive tensor, normal/shear tractions, circle centre/radius and principal stresses Pa
- n,I
- unit plane normal and identity matrix 1
- θ
- normal angle to the maximum principal axis in the stated section rad
Conditions and scope
- σ₁≥σ₂≥σ₃, an ordinary symmetric stress tensor without couple stresses, Cartesian orthonormal coordinates and‖n‖=1. The signedτ convention uses tangentq=−sinθe₁+cosθe₃ and physical traction−Σn; the general vector magnitude does not depend on tangent orientation.
- The displayed circle uses normals in theσ₁–σ₃ principal plane; general3D normals need the full tensor. A stress state does not uniquely identify a fault plane, history or strength. Isotropic compression has no resolved shear.
Sources · 1
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Read actual traction versus tensor, geometric stress/force projection, positive-compression convention, principal axes and§2.5.1 double-angle/circle formulas. Full3D projection is a disclosed Cartesian tensor/vector derivation, with physical traction sign made explicit. Excluded flawed blanket static-force/third-law argument for stress symmetry, purported ellipsoid encoding every traction norm and fixed depth-dependent stress values.
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Entry details
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