In an earthquake, the initial movement that causes seismic vibrations occurs when two sides of a fault suddenly slide past each other. A fault is a large fracture in rocks, across which the rocks have moved. The width of the fault is usually much smaller, on the order of a few millimeters to meters. The size of an earthquake depends on The area of the fault that ruptured. The distance that the rocks on the two sides of the fault slide past one another.
Small earthquakes rupture small faults or small sections of large faults.
Fault movement during such events is quick, small quakes last only a fraction of a second and the rocks on either side of the fault don't move very far. Large earthquakes rupture faults that are tens to thousands of kilometers long. Such ruptures can take minutes to complete, so strong shaking near the earthquakes can last several minutes and rocks across the fault can be offset tens of meters during very large earthquakes. The most commonly used quantification of earthquake size is the magnitude. Fortunately, large earthquakes are less frequent that small earthquakes.