California's Plate Boundary: The San Andreas Transform Fault

Transform Boundary: Pacific vs. North American Plates

California sits on a transform plate boundary, where the Pacific Plate and North American Plate slide horizontally past each other.

The San Andreas Fault System

• Primary boundary: ~1,200 km (800 mi) long.
• Right-lateral strike-slip motion: Pacific Plate moves NW relative to North America at ~35–50 mm/yr.
• Creates linear valleys, offset streams, sag ponds, and fault scarps.

Seismic Consequences

Stress accumulates along locked segments, releasing in major earthquakes (1906 San Francisco M7.9, 1857 Fort Tejon M7.9). The fault system includes many parallel strands (Hayward, Calaveras, San Jacinto) distributing deformation.

Not Subduction or Spreading

Unlike the Cascadia Subduction Zone (north) or East Pacific Rise (south), California's boundary is predominantly transform—horizontal slip, not collision or divergence.

This tectonic setting defines California's earthquake hazard and dramatic topography.