The size of the end zone and the propagation velocity of a displacement jump

Asymptotic expressions are obtained for the end zone of a propagating displacement jump. A formula is presented for the size of the end zone when there is linear weakening in it which, when used in Novozhilov's structural criterion, provides an answer to the question as to why the propagation velocity of normal or shear displacement jump, which is conventionally observed under mechanical loading in experiments and during earthquakes, is less than the Rayleigh wave velocity.