BRITTLE FAULTING

53 Faulting jpb, 2020 BRITTLE FAULTING Because most rocks are brittle at low temperature and low confining (lithostatic) pressure, virtually every rock at or near the Earth’s surface exhibits evidence of brittle failure, i.e. deformation-induced loss of cohesion. Brittle failure results from the irreversible and very rapid propagation and connection of cracks, a process called fracturing. Cracks are grain-scale planes that pre-exist or nucleate under stress on some microscopic defect of the material. Fracturing initiates at stress levels near the yield strength (or maximal differential stress 1 3 σ -σ ), marking the elastic limit of the material. Tensile stress cannot act across the resulting physical discontinuity that has no cohesion. By reference to laboratory observations, a broad mechanical classification of fractures recognizes two fracture modes: - Extension fractures, resulting from the initial separation of two formerly contiguous surfaces; displacement is parallel to the minimum principal stress (i.e. maximum tension), which is orthogonal to the fracture plane. - Shear fractures resulting from initial displacement along the fracture plane; fractures and displacement are oblique to the maximum principal stress (maximum compression). This lecture discusses qualitatively the relationships of the earthquake source to faulting. Brittle faulting is a process producing localized offset along a shear fracture. This lecture discusses how and qualitatively comments the relationships of the earthquake source to brittle faulting. Fault orientation relative to principal stress axes Theoretical considerations A force F that acts on P can be resolved into components normal (F N ) and parallel (F S ) to the plane P. The components have magnitudes: N F Fcos = θ and S F Fsin = θ (1) with θ the angle between the direction of the force and the normal to the plane (see lecture on mechanical aspects of deformation; forces and stresses).

BRITTLE FAULTING

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brittle deformation

fracturing

brittle failure

cohesion

elastic limit