Overdue on the southern branch.
I grew up within the San Andreas Fault Zone in the foothills of the San Bernardino Mountains (see video). I’ve always been fascinated by things like plate tectonics and geological time. I don’t live within the zone now, but I’m close (on the Pacific Plate).
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Parts of the City of San Bernardino, California, lie within the San Andreas Fault Zone, though the precise relationship depends on how you define “within” and the fault’s broader zone of influence. The San Andreas Fault itself is a narrow rupture in the Earth’s crust, but its “fault zone” is typically considered a wider swath—often 1–2 km across—where associated fractures, deformation, and seismic hazards are significant.
The main trace of the San Andreas Fault runs just north of San Bernardino. It cuts through the San Bernardino Mountains, passing near communities like Cajon Pass and Devore, which are within the city’s northern boundaries or its immediate sphere of influence. Specifically, the fault’s surface trace is mapped about 5–10 km north of downtown San Bernardino, trending northwest-southeast. For example, the U.S. Geological Survey (USGS) and California Geological Survey (CGS) fault maps place it along the base of the mountains, near where Interstate 15 and State Route 210 meet the rugged terrain.
San Bernardino’s city limits extend northward into these foothills, encompassing areas like the Shandin Hills and parts of the Cajon Valley. While the exact fault trace doesn’t slice through the city’s urban core (e.g., downtown around E Street or Baseline Avenue), some northern neighborhoods and annexed regions are close enough—within 1–3 km—to be considered part of the broader San Andreas Fault Zone. The Alquist-Priolo Earthquake Fault Zoning Act, which restricts building directly on active faults, designates special study zones around the fault, and portions of these overlap with San Bernardino’s northern edges. For instance, the fault’s mapped trace near Devore Heights (a community within city limits) places it squarely in this hazard zone.
Beyond the main strand, the San Andreas Fault Zone includes subsidiary faults and splays that further complicate the picture. The San Bernardino area is also influenced by nearby faults like the San Jacinto Fault, which branches off the San Andreas and runs closer to the city’s western side, about 5 km from downtown. Some geologists argue the San Jacinto is now the more active boundary locally, but the San Andreas remains the dominant feature. During a major rupture (like the anticipated M8+ "Big One" on the southern segment), shaking would be intense across all of San Bernardino due to its proximity—within 10–15 km of the fault—regardless of whether the trace itself crosses city streets.
So, technically, the primary San Andreas Fault trace doesn’t bisect San Bernardino’s urban center, but parts of the city’s northern extent fall within the fault’s broader zone of deformation and seismic risk. Residents in those areas, and indeed the whole city, face significant hazards from a future rupture due to this close proximity.
Video source: https://t.co/l3CuePQGFB
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