Fault Earthquakes always occur on faults. Faults are places in the earth where the rocks are broken and the rocks on one side have moved in some direction relative to the other. Faults are planes, not lines. The line seen on a map is the intersection of two planes - the plane of the fault with the plane of the earth s surface. A thrust occurs on a plane that is at an angle to the surface such that one side slides up over the other.
Normal faults are also on angled fault planes, but one side moves down and away from the other.
Strike-slip events occur on vertical faults and one side slides by the other. Strike-slip and thrust are the most common types in southern California. Every point on the fault plane releases energy so bigger faults produce bigger earthquakes. We can thus estimate how big an earthquake to expect by mapping the length of the fault. The longest fault in southern California is the San Andreas fault that is long enough to produce a magnitude 8 earthquake.