Rocks do not always bend; sometimes they break. A fault line is the surface trace of a fracture along which rock masses have slipped relative to each other. Movement may be sudden, producing earthquakes, or gradual creep.

Faults are classified by the direction of slip—normal, reverse, or strike-slip. The San Andreas Fault is a well-known strike-slip boundary between the Pacific and North American plates. Fault scarps, offset streams, and linear valleys often mark the location of active faults on the landscape.

Engineers avoid building critical structures directly on active fault lines when possible. Seismologists instrument faults to measure strain accumulation. Mapping fault lines is a fundamental task in assessing earthquake hazard for any region.

• Fracture where rock masses have moved
• Can produce earthquakes when movement is sudden
• Classified by slip direction
• Often visible as linear landscape features