What is Riedel shear?

In a zone of strike-slip simple shear, Riedel shears are the short shear fractures that form first, in two sets: the R shears, at a small angle to the zone and with its sense of slip, and the R$'$ shears, at a large angle and with the opposite sense. Later P shears, symmetric to the R shears about the zone, and tension fractures T complete the pattern.