Scientists believe the recent significant earthquake in Southern California could have an unexpected cause, though it has reignited concerns about the potential ‘Big One.’

Stretching over 100 miles across Southern California, from the US-Mexico border through San Diego and Riverside Counties, and up to Los Angeles, lies the lesser-known Elsinore fault, which is part of the infamous San Andreas fault system.

Located just 15 miles from San Diego County, the Elsinore fault ranks as one of the largest fault zones in Southern California, according to the California Institute of Technology.

Seismologist Lucy Jones warned that although the Elsinore fault has been relatively quiet throughout history, it still has the potential to produce an earthquake with a magnitude up to 7.8.

Data from the Southern California Earthquake Center and the US Geological Survey (USGS) shows that major earthquakes exceeding a magnitude of 6.0 have occurred on the Elsinore fault every 100 to 200 years, with the last one recorded in 1910.

“The Elsinore fault poses one of the major risks in Southern California,” Jones remarked after the 5.2-magnitude earthquake in San Diego on Monday. She further noted that in recent years, the region has experienced more noticeable earthquakes than in previous decades.

The USGS has formulated estimates for the possible devastation if the ‘Big One’ were to impact the Elsinore fault instead of the San Andreas, particularly as seismic activity seems to be on the rise.

If such a catastrophe were to occur, the resulting destruction could potentially span from San Diego to Los Angeles, and possibly even further.

Though distinct from the San Andreas fault, the Elsinore fault is part of the larger San Andreas fault system, a network of faults that collectively manage the tectonic movements in California.

In Southern California, the Elsinore fault runs parallel to the San Andreas, lying to the east. It is approximately 110 to 150 miles long and extends from San Diego County, right up to the US-Mexico border.

Dr. Jones confirmed that the seismic activity observed earlier this week was linked to the Elsinore fault.

“The Elsinore fault is essentially a smaller counterpart to the San Andreas,” Jones explained to KCAL News. “The San Andreas fault splits into four parallel faults as it reaches Southern California – the San Andreas, San Jacinto, Elsinore, and Newport-Inglewood.”

Despite the relative quiet of the Elsinore fault, Jones cautioned that when earthquakes do occur there, they can have far-reaching consequences, potentially affecting Los Angeles, which has a population of about four million.

In 2017, the USGS simulated a 7.8-magnitude earthquake along the Elsinore fault, extending northwest into the Whittier fault, closer to Los Angeles.

Such a disaster would cause severe shaking and significant damage to structures in Los Angeles.

According to USGS researchers, the Modified Mercalli Intensity (MMI) in Los Angeles could reach levels of 7.5 to 9.0, indicating high seismic activity, with total destruction occurring at MMI levels of 10 to 12.

In San Diego, the simulated earthquake would result in an MMI of 4.0 to 6.5, leading to severe shaking, cracks in walls, chimney collapses, and minor damage to buildings, though injuries would be minimal.

However, a large 7.8-magnitude earthquake shifting from the Elsinore fault to the Whittier fault would result in significant fatalities, injuries, and financial damage in Los Angeles.

At an MMI of 4.5, cities like Las Vegas (about 300 miles from Los Angeles) could experience mild shaking with some minor damage. The seismic energy would also reach other areas such as Fresno, Bakersfield, and Santa Barbara, impacting roughly 23 million people.

Jones explained that major earthquakes like the 7.3 magnitude Landers earthquake in 1992 and the 6.7 magnitude Northridge earthquake in 1994 helped relieve local tectonic stress in Southern California, leading to a quieter period. But now, it seems the region is experiencing a rise in seismic activity, reminiscent of the 1980s.

Earlier predictions for a major earthquake along the San Andreas fault, with a magnitude close to 8.0, suggested that it could result in approximately 1,800 fatalities and 50,000 injuries throughout California.

The San Andreas fault runs through major cities like San Francisco, San Bernardino, and Palmdale, as well as regions surrounding Los Angeles and Palm Springs. The infamous 1906 San Francisco earthquake, thought to have had a magnitude of 7.9, was caused by this fault, which stretches over 800 miles.