Stress - Fault Mechanism

Principal Axes of Stress
Material behavior science, through its detailed analysis of stress conditions, has derived an important conclusion that has proven very useful in structural geology: for any point in a homogeneous stress field, there exist three mutually orthogonal planes along which all shear stresses vanish and only the components of normal stress exist. These three planes are known as the principal planes of stress, and the axes of their intersection are thus the principal axes of stress. These axes are used to describe what are referred to as the three principal stresses. This ideal triaxial system makes everything simpler, since it allows us to speak in terms of only normal stresses, i.e., compression, or "squeezing," and extension, or "pulling apart" ( Figure 3 ).
In geology, principal stress is usually spoken of in terms of compression, which is taken as positive, and tension as negative. (For materials and engineering science, the opposite is true-i.e., tension is positive, compression is negative.) The three principal axes, or stress directions, are correspondingly written as s1 (maximum principal stress), s2 (intermediate principal stress), and s3 (least, or minimum, principal stress). For our purposes, it is useful to understand four special states of stress:
1. uniaxial stress, where two principal stresses are zero and the other is nonzero
2. biaxial stress, where two principal stresses are nonzero and the other is zero
3. triaxial stress, where all three principal stresses are nonzero
4. pure shear stress, where s1 equals s3 and is nonzero, while s2 is zero. This is actually a special case of biaxial stress.Within the earth's crust, the most common stress situation is triaxial, with s1 > s2> s3> 0.