The answer to whether Los Angeles sits directly on the San Andreas Fault is no, but its proximity poses the single greatest seismic threat to the region. The common misconception that the fault runs directly beneath the city overlooks the actual, complex geographical relationship. The San Andreas Fault’s sheer scale creates a massive, long-term seismic hazard for the entire Los Angeles metropolitan area. The true danger comes from its powerful potential and the network of secondary faults triggered by the strain it imposes on the tectonic plate boundary.
The Geographical Relationship Between LA and the San Andreas Fault
The main trace of the San Andreas Fault (SAF) does not cut through the densely populated heart of the Los Angeles basin. It runs approximately 30 to 40 miles northeast of downtown Los Angeles, passing through the sparsely populated Antelope Valley. Communities near the fault include Palmdale and Wrightwood. This section is easily visible in roadcuts along Highway 14, where warped rock layers reveal the immense tectonic stress.
The fault traces a line along the northern edge of the San Gabriel Mountains, passing through the Cajon Pass before continuing northwestward. This proximity is close enough to guarantee severe and prolonged ground shaking in the metropolitan area during a major rupture. Los Angeles is situated on the Pacific Plate, constantly moving northwestward relative to the North American Plate. The fault extends for roughly 750 miles through California, acting as the boundary between these two major tectonic plates.
Understanding the Nature of the San Andreas Fault
The San Andreas Fault is classified as a right-lateral strike-slip fault, meaning the Pacific Plate is grinding horizontally past the North American Plate. The average rate of this movement is about 0.79 to 1.38 inches (20 to 35 millimeters) per year.
This movement is not smooth; the plates are locked in many places, allowing strain to accumulate over decades. The southern segment, closest to Los Angeles, has the potential to generate an earthquake up to magnitude 8.1. The 1857 Fort Tejon earthquake (magnitude 7.9) serves as a historical model for this seismic threat.
A unique geological feature is the “Big Bend,” where the fault changes its trajectory around the Transverse Ranges. This bend forces the horizontal motion to include a component of compression. This compression creates thrust forces directed southward toward the Los Angeles basin, intensifying stress on the crust.
Local Fault Systems Within the Los Angeles Basin
While the San Andreas Fault represents the regional, high-magnitude threat, the most immediate danger comes from a network of active faults running directly beneath the city. These local systems, created by compression from the San Andreas’s “Big Bend,” produce higher intensity, localized ground shaking. They are capable of generating major events much closer to densely populated areas.
One system is the Newport-Inglewood Fault Zone, a strike-slip fault responsible for the destructive 1933 Long Beach earthquake. Another highly hazardous system is the Puente Hills Thrust Fault, which is a “blind thrust fault” meaning it does not break the surface. This fault runs for over 25 miles directly under downtown Los Angeles.
The Puente Hills system is considered one of the most dangerous faults due to its position beneath major infrastructure, and is thought to have caused the 1987 Whittier Narrows earthquake. Modeling suggests a major rupture could have the same destructive impact as a larger magnitude earthquake on the distant San Andreas. This local fault network, including the Northridge Blind Thrust Fault (1994 Northridge earthquake), poses a catastrophic risk because energy is released immediately beneath the urban center.