Faults are underground tectonic ruptures along which rocky masses move in relation to one another. This movement can be convergent (towards each other, as in the Andes), divergent (away from each other, e.g. mid-ocean ridges) and/or transformational (horizontal sideways movement, like in the San Andreas fault).
The fault plane, which may be horizontal, vertical, slightly inclined, or some other variant, can range from a few centimetres to several hundred kilometres in size and involve rock masses or entire crustal sections grinding against each other.
Any fault along which one or more earthquakes have taken place within the past 10,000 years is said to be active (USGS, 2011).
When an earthquake occurs, usually only a section of the fault shifts. This is referred to as the fracture surface, and within it lies the hypocentre of an earthquake (Weidmann, 2002). In principle, the larger the fracture surface underground and the more it shifts, the stronger the earthquake will be.