An earthquake happens when a sudden movement of two blocks of rock occurs along a break (fault) deep within the earth's crust. Earthquakes are caused when stress, building up within the rocks of the earth's crust, is suddenly released. The rocks split and slip past each other causing the ground to vibrate. Cracks along which the rocks slip are called faults. These faults may come through to the surface of the earth, or be deep within the crust.

Fault Motion

Normal Fault  (Dip-Slip Fault)
In a normal fault, the block above the fault moves down relative to the block below the fault. This fault motion is caused by tensional forces and results in extension. [Other names: normal-slip fault, tensional fault or gravity fault]

Reverse Fault  (Dip-Slip Fault)
In a reverse fault, the block above the fault moves up relative to the block below the fault. This fault motion is caused by compression forces and results in shortening. A reverse fault is called a thrust fault if the dip of the fault plane is small. [Other names: thrust fault, reverse-slip fault or compression fault]

Strike-Slip Fault

In a strike-slip fault, the movement of blocks along a fault is horizontal. If the block on the far side of the fault moves to the left, as shown in this animation, the fault is called left-lateral. If the block on the far side moves to the right, the fault is called right-lateral. The fault motion of a strike-slip fault is caused by shearing forces. [Other names: lateral fault, tear fault or wrench fault]

Oblique-Slip Fault

Oblique-slip faulting suggests both dip-slip faulting and strike-slip faulting. It is caused by a combination of shearing and tension of compression forces.

Earthquakes rarely happen at the surface of the ground, usually they are well below the surface. Most of the time the depth of the focus is shallow, however earthquakes have been detected as deep as 720 km. in the crust. The place on a fault where the slip first occurs is called the focus, while the position directly above the focus on the ground surface is called the epicenter. Earthquakes can be classified into three groups according to their depth. Shallow: The focus is less than 60 km., Intermediate: The focus is between 60 and 300 km., Deep: The focus is greater than 300 km.