2 Where and How are Fractures Formed?‌

This section presents basic theories about conditions that cause fracture formation and propagation. Fractures are produced by brittle deformation—permanent change in rock or soil due to cracks—that takes place at relatively shallow depths, where the rock mass is subjected to stresses that exceed certain magnitudes. We briefly describe the sources of these stresses and present a widely-accepted criterion that determines the types of fractures that will form. Finally, we explain how fluid pressure can trigger fracture formation, even when the magnitude of stresses is not high enough to cause fracture formation.