EQs occur on faults. When an EQ occurs on one of these faults, the rock on one side of the fault slips with respect to the other. The fault surface can be vertical, horizontal, or at some angle to the surface of the earth. The slip direction can also be at any angle. The fault is at an angle to the surface of the earth and the movement of the rock is up or down.
Surface rupture occurs when movement on a fault deep within the earth breaks through to the surface.
ALL EQs result in surface rupture. Fault rupture almost always follows preexisting faults which are zones of weakness. Rupture may occur suddenly during an EQ or slowly in the form of fault creep. Sudden displacements are more damaging to structures because they are accompanied by shaking. Fault creep is the slow movement of faults in the earth's crust.