Which fault primarily caused the 1906 San Francisco earthquake?

The 1906 San Francisco earthquake was primarily caused by movement along the San Andreas Fault. This strike-slip fault marks the boundary between the Pacific Plate and the North American Plate, which slide laterally past one another.

The earthquake struck Northern California on April 18, 1906, at about 5:12 a.m. Its modern estimated magnitude is 7.9, and the rupture propagated for roughly 476 kilometers along the San Andreas system. Surface displacement reached several meters in places, dramatically reshaping roads, fences, and the landscape.

A common mix-up is the Hayward Fault, which also runs through the Bay Area and remains a major earthquake hazard, but it did not cause the 1906 main shock. The disaster became especially destructive because fires followed the shaking; ruptured gas mains and other damage allowed fires to burn for several days. More than 80% of San Francisco was destroyed, and the event helped transform modern earthquake science and building practices.