This little-known earthquake fault has been quiet. But it can unleash devastation across SoCal

Los Angeles Times

15-04-2025

Below California's famed beaches, mountains and metropolitan areas lies a sinister web of earthquake faults — some so infamous that their names are burned into the state's collective consciousness.
There is, of course, the mighty San Andreas, whose massive slip caused the great 1906 San Francisco earthquake and whose notoriety has sparked multiple movies, video games, books, T-shirts and collectibles.
Also well known in L.A. is the Newport-Inglewood fault, which unleashed the 1933 Long Beach earthquake — the deadliest temblor in Southern California's modern history.
The large earthquake fault close to the one that moved Monday morning in the mountains of San Diego County, however, is comparatively obscure. But the Elsinore fault is part of a larger seismic zone that experts fear and believe more people should know about.
The Elsinore fault zone is actually one of the largest in Southern California, according to Caltech, but 'in historical times, has been one of the quietest.'
However, that inactivity belies a devastating potency. The fault is capable of generating a magnitude 7.8 earthquake, said seismologist Lucy Jones, a Caltech research associate.
'The Elsinore fault is one of the major risks in Southern California,' Jones said.
The Elsinore fault zone runs from the Sonoran Desert in Imperial County through the western edge of Riverside County communities like Temecula, Murrieta and Lake Elsinore.
By the time it reaches Corona, it splits into two segments — the Chino fault, which heads toward Chino Hills; and the Whittier fault, which is near or bisects the L.A. County suburbs of Whittier, La Habra Heights, Hacienda Heights and Rowland Heights, and La Habra, Brea and Yorba Linda in Orange County.
A particularly frightening, and plausible, prospect would be an earthquake that races up the Elsinore fault northwest onto the Whittier fault. That would 'pour all the energy straight into the L.A. Basin. It's one of the scary earthquakes,' Jones said.
According to one hypothetical scenario published by the U.S. Geological Survey, a magnitude 7.8 earthquake on the Elsinore fault zone, including the Whittier fault, could bring 'violent' shaking — enough to heavily damage buildings or even jolt them off their foundations — over a relatively large area of the Southland, including El Monte, Hacienda Heights, Rowland Heights, Pico Rivera, Whittier, La Habra, Brea, Yorba Linda, Placentia, Chino Hills, Corona, Lake Elsinore, Murrieta and Temecula.
That's a much larger area than the part of the San Fernando Valley that saw 'violent' shaking during the 1994 Northridge earthquake, which was a magnitude 6.7.
'Severe' shaking — enough to topple chimneys and greatly damage poorly built buildings — may be felt a bit farther away from the ruptured fault, including in downtown L.A., East L.A., Long Beach, Alhambra, West Covina, Pomona, Ontario, Riverside, Downey, Norwalk, Santa Ana, Garden Grove, Anaheim, Orange, Irvine and Lake Forest.
And 'very strong' shaking could be felt farther out, including the San Fernando Valley, the Westside, South Bay, the ports of Los Angeles and Long Beach, the Orange County coast, Rancho Cucamonga, Fontana and San Bernardino.
No earthquake in modern times has been this powerful, or triggered such substantial, damaging shaking across such a wide swath of California.
Southern California's last magnitude 7.9 earthquake occurred in 1857, when the San Andreas ruptured between Monterey and Los Angeles counties. The last equivalent temblor in Northern California was the great 1906 earthquake, which ruptured the San Andreas between Humboldt and Santa Cruz counties and destroyed much of San Francisco.
The Elsinore fault can be thought of as a sibling of the San Andreas, along with the sizable San Jacinto fault, said Kate Scharer, research geologist with the U.S. Geological Survey. The San Jacinto fault zone starts at the Cajon Pass and moves southeast through San Bernardino and Riverside County before heading into Imperial County.
The three faults all move at relatively fast rates on average — meaning each is, generally speaking, more likely to rupture during the span of a human lifetime. The San Andreas and San Jacinto move at an average rate of 20 millimeters a year, while the Elsinore moves at a more modest clip of about 5 millimeters a year. By contrast, the notorious Newport-Inglewood fault moves far slower, at a rate of 1 millimeter a year.
'There's some risk that it could be the next one,' Scharer said of the risk of a Big One on the Elsinore fault zone. 'We focus so much on the San Andreas, but we have this whole suite of active faults that are accumulating strain.'
Monday's magnitude 5.2 temblor was an uneasy reminder of the fault's power. While the quake, which was centered near Julian, sent shaking across the region, there were no reported injuries or major damage. But that might not be the case next time, especially if a quake strikes closer to major cities.
Monday's earthquake resulted in at least nine aftershocks of magnitude 2.5 or greater, including a magnitude 4 a little more than an hour later.
Monday's magnitude 5.2 earthquake was preceded a day earlier by a magnitude 3.3 earthquake, which is now considered a foreshock.
The San Jacinto fault zone is quite dangerous in its own right — cutting right through the heart of the Inland Empire, underneath many highly populated areas. It's possible the San Jacinto and San Andreas faults could rupture together in a magnitude 7.5 earthquake.
California is at major risk of significant earthquakes because it sits on the edge of a tectonic plate boundary, where the Pacific plate — upon which sits San Diego, Los Angeles and Santa Barbara — is slowly moving northwest relative to the North American plate, upon which sits San Francisco, the Central Valley and Big Bear Lake.
That strain accumulates over decades and centuries, and is eventually unleashed in the form of earthquakes around that tectonic plate boundary.
The Elsinore is a very long fault. The longer the rupture goes in an earthquake, the more total seismic energy is produced.
'The analogy is like an instrument — a little, small kazoo, you can't make it very loud,' Scharer said. 'But if you get to an oboe ... you can get a much louder sound because it's such a bigger instrument, so more energy can basically be pumped through that system.'
Some of California's biggest cities have made strides in requiring seismically vulnerable buildings to be retrofitted, but vulnerabilities remain. A Times investigation published in November found that a number of Southern California's suburbs had no active plans to require flimsy apartment buildings, known as 'soft-story,' to be retrofitted against earthquakes.
One particular dangerous spot is the Inland Empire, where brick buildings — a hazard seemed so dangerous that L.A. ordered them fixed or demolished decades ago — are still a threat, despite the fault zones lying underneath the region.
Another risk is possibly defective steel frame buildings, which are required to be retrofitted in a few cities, like Torrance, Santa Monica and West Hollywood, but not in Los Angeles.
Los Angeles and Orange counties have had little experience with severely damaging earthquakes during the last generation.
Since 1998, there has been only one earthquake of magnitude 5 or greater under Los Angeles and Orange counties. That was a 2014 magnitude 5.1 earthquake centered in Brea, which caused more than $2.5 million in damage in that city, Fullerton and La Habra.
There was also the magnitude 5.4 Chino Hills earthquake of 2008. Centered in San Bernardino County, but just east of Los Angeles and Orange counties, it caused little damage.