A rule, derived experimentally in 1978 by the American geophysicist James Byerlee, that determines the relation between shear stress (τ) and normal stress (σ) at which rocks will fracture at a fault. Byerlee found that τ = 0.85σ for σ up to 200 MPa and τ = 50 + 0.6σ for σ above 200 MPa.