The joint opens by displacements normal to the crack wall. Shear fractures - a fractures which form when the rock is subject to large compressive stresses near the surface of the earth. These fractures form by shear displacement parallel to the walls of the fracture. Shear displacement is usually considered to be less than one centimeter. Faults - shear fractures with displacements greater than one centimeter.
Most faults start as shear fractures.
Often deformation continues so that the fault slips a number of times. Large amounts of slip are associated with earthquakes. Fault Blocks - the fault of usually a planar surface which had a dip. The fault divides rocks above and below the fault into two blocks. Hanging wall block - The block above the fault plane.