When these stresses are applied slowly over time, the rocks will experience plastic deformation and become folded and bent. Along many places the stresses are not released slowly but build up over time until they are released all at once. This sudden release of energy results in the rocks breaking apart in a brittle fashion, fracturing and faulting. Earthquakes are the result of this sudden release of energy and take place along faults that cross the earth's surface and sub-surface. Before we can look at earthquakes in detail, we must first understand the different types of faults that can produce earthquakes.
Faults are the evidence of horizontal and vertical breaks in the earth's surface.
When different parts of the earth's surface are subjected to different stresses, they will either compress or pull-apart. These compressional and extensional stresses are what create the different faults we see. Faults occur along areas of weakened or brittle rock. As discussed in my earlier article on metamorphic rocks , when rocks are subjected to changes in pressure, they can be altered to form different rocks. Closer to the surface, or when the deformation occurs at pressures or temperatures lower than the alteration point of the rock, then a fracture will occur along a weakened or brittle zone in the rock.