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In this section we will review two methods to calculate normal and shear stresses on fractures and faults. The first part explains the conventions to measure the orientation of planar geological structures with respect to the “geographical coordinate system” composed of North, East, and vertical Depth. We will review the Mohr circle method in order to have a conceptual understanding of stress projection on faults and maximum ratio between shear stress and effective normal stress. Then, this section discusses the tensor method, which requires the definition of three coordinate systems and matrix multiplication. The tensor method can be easily implemented in a computer script but is difficult to work out manually.
The first step we need in order to find the stress projection on a plane is to define the orientation of such plane with respect to a reference coordinate system. Geological structures are usually referred to a geographical coordinate system North-East-Depth.