Faults and Stresses

Faults of different types are charcteristic of the stress field in which they formed. Faults are nothing more than failure of the crust in shear. The crust is generally in a triaxial state of stress with three principal stress directions, one vertical and two horizontal (like a set of cartesian axes).

• Normal Fault

  In normal faulting, the largest (most compressive) stress is vertical. The smallest and intermediate stresses are horizontal. The shear fracture (fault) makes an angle of less than 45 degrees with the major (most compressive) principal stress direction, in this case the vertical. If the average stress is subtracted from all the principal stresses, the crust is in relative tension.
  

• Reverse Fault

  In reverse faulting, the smallest (least compressive) stress is vertical. The largest and intermediate stresses are horizontal. The shear fracture (fault) makes an angle of less than 45 degrees with the major (most compressive) principal stress direction, in this case the horizontal. If the average stress is subtracted from all the principal stresses, the crust is in relative compression.
  

• Strike-slip Fault

  In strike-slip faulting, the intermediate stress is vertical. The largest and smallest stresses are horizontal. The shear fracture (fault) makes an angle of less than 45 degrees with the major (most compressive) principal stress direction, in this case again the horizontal. There is no vertical movement.