section_path: Occurrence/Rupture dynamics
section_name: Rupture dynamics
paragraph_text: Rupture dynamics
  
  A tectonic earthquake begins as an area of initial slip on the fault surface that forms the focus. Once the rupture has been initiated, it begins to propagate away from the focus, spreading out along the fault surface. Lateral propagation will continue until either the rupture reaches a barrier, such as the end of a fault segment, or a region on the fault where there is insufficient stress to allow continued rupture. For larger earthquakes, the depth extent of rupture will be constrained downwards by the brittle-ductile transition zone and upwards by the ground surface. The mechanics of this process are poorly understood because it is difficult either to recreate such rapid movements in a laboratory or to record seismic waves close to a nucleation zone due to strong ground motion.
  
  In most cases, the rupture speed approaches, but does not exceed, the shear wave (S-wave) velocity of the surrounding rock. There are a few exceptions to this: