Scientists think a significant earthquake in Southern California this week may have an unexpected cause, but it has rekindled fears of the ‘Big One’.

Stretching more than 100 miles across Southern California, from the US-Mexico border through San Diego and Riverside Counties, and up towards Los Angeles, is the little-known Elsinore fault, a branch of the notorious San Andreas fault.

Only 15 miles from San Diego County, the Elsinore fault is one of the biggest fault zones in Southern California, according to the California Institute of Technology.

Seismologist Lucy Jones cautioned that although Elsinore has been among the quietest in recorded history, it is still capable of produce an earthquake with a magnitude of up to 7.8.

According to statistics from the Southern California Earthquake Center and the US Geological Survey (USGS), large earthquakes with a magnitude of greater than 6.0 have occurred on the Elsinore fault every 100 to 200 years, with the last one being in 1910.

“The Elsinore fault is one of the major risks in Southern California,” Jones said after the 5.2 magnitude quake in San Diego on Monday.

“The last couple years, we’ve been having more felt-earthquakes than we had for several decades,” she added.

The USGS has developed its own estimates of what a devastating earthquake may look like if the “Big One” impacted the Elsinore fault rather than the San Andreas, since seismic activity appears to be increasing.

Devastation from that enormous disaster would probably extend from San Diego to Los Angeles and perhaps even farther.

Despite being a distinct fault line, the Elsinore fault is a component of the greater San Andreas fault system, a collection of faults that cooperate to manage the tectonic plate movement in California.

In Southern California, Elsinore runs east of and parallel to the San Andreas. It is roughly 110 to 150 miles long and stretches through San Diego County from the US-Mexico border.

Dr. Jones attested that the Elsinore fault line’s seismic activity was linked to Monday’s seismic event.

“The Elsinore is sort of a junior sister to the San Andreas. This San Andreas breaks up when it comes into Southern California and splits into four parallel faults – the San Andreas, the San Jacinto, the Elsinore, and the Newport-Inglewood,” Jones explained to KCAL News.

Even though there aren’t many earthquakes along the Elsinore, the seismologist cautioned that when they do occur, the devastation can reach Los Angeles, which is home to about four million people.

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

That ‘Big One’ would have caused severe shaking and extensive structural damage to buildings and their foundations in Los Angeles.

USGS researchers projected that the Modified Mercalli Intensity (MMI) in Los Angeles would rise to levels 7.5 to 9.0, which are close to the top of the seismic activity scale, after mapping the devastation on the MMI.

Only levels 10 through 12 are more severe, and “total destruction” would result from that degree of earthquake activity.

The fictitious earthquake in San Diego would have an MMI of 4.0 to 6.5, resulting in severe shaking, wall cracks, chimney collapses, and minor damage to other buildings all around the city.

Overall, the forecast indicated that there would be little chance of widespread injuries and that the damage to San Diego would be controllable.

However, there would be a significant chance of fatalities, serious injuries, and financial damage in Los Angeles if a large 7.8- magnitude earthquake moved from the Elsinore fault to the nearby Whittier fault.

At an MMI of 4.5, cities as far east as Las Vegas (300 miles from LA) would experience mild shaking that might still result in modest damage.

Additionally, the seismic force would move northward, affecting places like as Fresno, Bakersfield, and Santa Barbara. There are almost 23 million people living in that region.

“The magnitude 7.3 Landers earthquake in 1992 and the magnitude 6.7 Northridge earthquake in 1994 were two significant earthquakes in the 1990s that ‘depressed’ the local tectonic stress in South California for decades,” Jones said.

According to Jones, this kept things relatively quiet for years, but it now seems that the area is beginning to see the same spike in seismic activity as it did in the 1980s.

According to earlier predictions, there might be 1,800 fatalities and 50,000 injuries throughout California from an earthquake near the San Andreas fault that is close to 8.0 on the Richter scale.

The San Andreas fault traverses major cities like San Francisco, San Bernardino, and Palmdale, as well as areas around Los Angeles and Palm Springs.

The destructive 1906 San Francisco earthquake, which is thought to have had a magnitude of 7.9, was caused by the fault, which is more than 800 miles long.

That earthquake claimed more than 3,000 lives and destroyed 80% of the city.