Elastic strain is recoverable whereas deformation produced by plastic flow is not - a permanent change in shape results. The yield point is the point where elastic behavior stops and plastic behavior begins. When we talk about structural geology we will look in more detail at elastic and plastic behavior of Earth materials. In this section we are primarily concerned with what happens when a rock fails and energy is released. The Dilatancy Model predicts that rocks undergo an increase in volume just before they rupture.
Small cracks begin to form.
Water may enter these cracks and increase the fluid pressure which further increases the volume of the rock and decreases its strength until rupture takes place. The depth to the focal point is an important piece of information about the quake. Recall, for example, that Earthquakes along mid-ocean ridges are quite shallow whereas those along a subduction zone may be quite deep. When you throw a stone into a quiet body of water waves are created which move out from the point of impact. Energy is being propagated along these paths and as it moves some of the energy is lost.