The San Andreas Fault System
The San Andreas Fault is a 750-mile-long strike-slip fault that extends from the Salton Sea to Cape Mendocino. It represents the boundary between the Pacific Plate and North American Plate, shifting approximately 1.6 inches per year.
Earthquake Probability
The U.S. Geological Survey estimates a 72% probability of at least one magnitude 6.7+ earthquake striking California within the next 20 years, with the San Andreas Fault responsible for approximately 25% of this risk. The fault ruptured last in 1906, producing a magnitude 7.9 earthquake that devastated San Francisco.
Segmentation and Risk
The San Andreas Fault divides into three major segments: the Northern, Central, and Southern segments. The Southern Segment has not ruptured in over 300 years, indicating a high probability of rupture. A magnitude 7.5 earthquake on this segment could cause $400+ billion in economic damage.
Stress Accumulation
Stress modeling indicates the Southern Segment has accumulated sufficient stress for a major earthquake. The Central Segment ruptured in 1989 (Loma Prieta, M6.9), temporarily reducing stress, but continues accumulating strain.
Preparation Challenges
Despite known risks, approximately 2.6 million people live directly in high-risk earthquake zones along the fault. Building code improvements and seismic retrofitting remain incomplete.