Southern California is no stranger to earthquakes, with infamous faults like the San Andreas and Newport-Inglewood etched into public memory.

The latter triggered the 1933 Long Beach quake, the region’s deadliest in modern times, claiming 120 lives.

Yet, a lesser-known fault, the Elsinore, lurks in the region’s seismic web, largely silent but capable of catastrophic destruction, according to experts who urge greater public awareness.

On April 14, 2025, a magnitude 5.2 earthquake struck near Julian, San Diego County, rattling homes and nerves across Southern California.

Centered just south of the Elsinore fault zone, the quake caused no major damage or injuries but served as a stark reminder of the fault’s potential.

According to the California Institute of Technology (Caltech), the Elsinore is one of the largest fault systems in the region, stretching 190 miles from the U.S.-Mexico border through Riverside County’s Temecula, Murrieta, and Lake Elsinore, up to the Santa Ana Mountains.

Despite its size, it has remained historically quiet, with only one significant event—a magnitude 6 quake in 1910 near Temescal Valley—recorded in modern times.

This silence, however, masks a fearsome capability. Seismologist Lucy Jones, a Caltech research associate, warns that the Elsinore fault could unleash a magnitude 7.8 earthquake, a level of devastation unseen in Southern California’s recent history.

Such an event could produce “violent” shaking, potentially collapsing buildings and dislodging structures from their foundations across a wide area, including El Monte, Hacienda Heights, Rowland Heights, Whittier, La Habra, Brea, Yorba Linda, Placentia, Chino Hills, Corona, Lake Elsinore, Murrieta, and Temecula.

“Severe” shaking, capable of toppling chimneys and heavily damaging weaker structures, could extend to downtown Los Angeles, East L.A., Long Beach, Santa Ana, Anaheim, Irvine, and Riverside, according to U.S. Geological Survey models.

The Elsinore’s threat is amplified by its proximity to densely populated areas.

Unlike the San Andreas, which lies farther from urban centers, the Elsinore could channel seismic energy directly into the Los Angeles Basin, especially if it links with the Whittier fault to the north. Jones describes this scenario as “one of the scary earthquakes,” potentially surpassing the shaking intensity of a San Andreas rupture due to its closer proximity to Los Angeles.

California’s seismic risk stems from its position on the boundary between the Pacific and North American tectonic plates, which grind past each other, building strain over decades.

The Elsinore, a strike-slip fault, is long and capable of producing massive energy releases. “Think of it like a musical instrument,” says U.S. Geological Survey seismologist Katherine Scharer.

“A small kazoo can’t make much noise, but a larger oboe can produce a far louder sound.” The fault’s length allows it to generate more seismic energy, making a major rupture particularly destructive.

Historical comparisons underscore the potential scale of such an event. Southern California’s last magnitude 7.9 quake, in 1857, ruptured the San Andreas from Monterey to Los Angeles counties.

A similar-sized quake on the Elsinore could affect an even broader swath of the Southland, with “very strong” shaking felt in areas like the San Fernando Valley, the Westside, South Bay, and Orange County’s coast.

No modern quake in the region has matched this intensity or geographic reach.

The Elsinore’s relative inactivity makes it less likely to rupture in our lifetime compared to the San Andreas, which slips at 20 millimeters per year.

The Elsinore moves slower, but its risk surpasses that of the Newport-Inglewood fault, which creeps at just 1 millimeter annually. Around Corona, the Elsinore splits into the Chino and Whittier faults, the latter running beneath the Puente Hills and threatening the San Gabriel Valley and Orange County.

A rupture starting in San Diego County and extending north could funnel devastating energy into Los Angeles, Jones notes.

Monday’s 5.2 quake highlighted the effectiveness of California’s earthquake early warning system, with some residents receiving phone alerts before the shaking began.

While the quake’s remote epicenter and 8.9-mile depth limited damage, it underscored the region’s vulnerability.

Southern California has seen heightened seismic activity recently, with 15 seismic sequences of magnitude 4 or higher in 2024—the most in 65 years, per Jones. Yet, experts caution that these smaller quakes offer no clear predictor of a major event.

The region’s seismic history includes other reminders of its volatile geology.

The 1994 Northridge quake, at magnitude 6.7, killed 57 and caused widespread damage, while the 2008 Chino Hills quake (magnitude 5.4) caused minimal harm.

The Elsinore, however, poses a far greater threat due to its capacity for a larger rupture.

Despite progress in retrofitting vulnerable buildings in some cities, a Los Angeles Times investigation found that many Southern California suburbs lack plans to strengthen “soft-story” apartments, and unreinforced brick buildings remain a hazard in areas like the Inland Empire.

As California prepares for its annual ShakeOut drill, the Elsinore fault serves as a sobering reminder of the region’s ever-present seismic danger. While it may not be a household name, its potential for devastation demands attention and preparedness.