Figure 1. Principles of S-wave splitting in a fractured rock. The incident S wave splits into two components – S1 and S2 – that transmit through a fracture zone and reflect individually from fracture-bounding interfaces. S1 is the fast-S mode, and its particle-displacement vector is oriented parallel to the fracture planes. S2 is the slow-S mode, and its particle-displacement vector is oriented perpendicular to the fracture planes. Reflected modes are not shown.