Fault propagation as shear fractures in an elastic medium for a static or quasi static case is considered under the link, 'Fracture Mechanics - Basic Premises.' Conceptually, shear fractures (Mode II) can propagate in-plane if the Stress Intensity Factor reaches a critical value, KIIC. However, under upper crustal loading conditions, in-plane shear fracture propagation in pristine rock is believed to be highly rare if it exists at all for some special cases. Rather, the extension of a fault occurs in the form of opening mode fracture (Mode I) at an angle to the main shear fracture, which is usually known as kinking or splaying (please see the link, 'Splay Fractures' and the references therein).

Seismological observations indicate that rupture propagation at Rayleigh wave velocity occurred, at least locally, during some large strike-slip events such as the 1999 Izmit earthquake (Bouchon et al. 2001) and the 2001 Kunlunshan earthquake (Bouchon and Vallee, 2003). Laboratory studies by Rosakis et al. (1999) and Xia et al. (2004) provided experimental verification of the so-called supershear rupture propagation. Fault propagation with Rayleigh wave velocity under dynamic loading always occurs along existing mature faults. In fact, the authors of a recent study (Perrin et al., 2015) were able to correlate the location of the largest earthquake slip and fast rupture velocity with along strike change in fault structural maturity associated with fault growth.

One of the consequences of high velocity fault propagation is fault branching (please see the link, 'Splay Faults,' and the references therein). Fault branching generally occurs at an acute angle relative to the propagation direction. The branch-fault angle increases with the main fault propagation velocity. Another complex process associated with high velocity rupture propagation under dynamic loading is off fault tensile fracturing which occurs commonly on one side of the fault (Griffith et al., 2009) as opposed to the tensile quadrants on either sides of the fault associated with the static or quasi static cases.