A fracture zone, also referred to as a cluster or swarm, is a zone in which two or more fracture elements, initially of the same nature, are closely spaced as parallel, sub-parallel, or en echelon elements. If elements of the same nature exist outside the zone, the spacing of the elements within the zone is much closer than that outside of it. Fracture zones also have dimensions that are generally larger than single elements. Fracture zones represent important weaknesses in rocks, which predispose the initial structures to reactivation in a different mode or sense. Therefore, any fracture zone almost always involves elements other than the initial structure type. Each type of fracture zone has been described separately in the links below.