SoCal's String Of Strong Quakes May Mean More Big Ones Are Likely

LOS ANGELES, CA — This year, Southern California has endured more sizeable quakes than it has in decades, and the shaking has people on edge.

The jarring trend continued Thursday with a magnitude 3.9 quake centered in Lake Elsinore.

Monday's 4.4 magnitude quake centered near Highland Park was the region's 13th quake of a magnitude of 4.0 or greater, said Dr. Lucy Jones, a seismologist with the California Institute of Technology in Pasadena. The last time there were that many unrelated quakes of a magnitude 4.0 or larger in the Southland was in 1988, according to Jones.

Since then, there have been about five earthquakes per year of 4.0 or greater in Southern California over the past 20 years.

“So, this is an active year, much like we used to see,” Jones told KTLA. “It does seem like we should expect this to continue at a higher rate.”

Since 1932, the average number of earthquakes with magnitudes of 4.0 or greater in Southern California is between 10 and 12, Jones posted on X. That would make the quake drought of recent years the anomaly.

The number of earthquakes greater than 4.0 in 2024 is closer to the average recorded in Southern California over the past 90 years.

The sudden flurry of quakes follows a period of decades in which Southern California experienced fewer than average large quakes. It's clear that seismic activity is picking up.

Unfortunately, it's not likely to mean fewer quakes nor reduce the chance of the Big One.

“The most constant feature of earthquakes is the relative number of large to small,” Jones told KTLA. “For every magnitude seven, you have 10 magnitude sixes, 100 magnitude fives, 1,000 magnitude fours, etc. So, if your rate of [magnitude] fours goes up, your chance of having a bigger one would go up by about the same amount.”

Jones told reporters Monday's earthquake appeared to be based in the same group of faults that created the 1987 Whittier Narrows quake. She noted that it occurred in roughly the same location as a 3.4-magnitude quake that struck on June 2.

"There have been others in the past," Jones said. "In 1989, we had a pair of four-and-a-halfs there were also in the system that were just a half- hour apart. There have been plenty of small ones over the years that are also within this."

Specifically, Monday's quake struck on the Puente Hills Thrust Fault that runs directly underneath Los Angeles County and is capable of causing more death and destruction than the famed San Andreas Fault, which may produce larger temblors but centered in more remote areas.

According to researchers at San Diego State University and the Scripps Institution of Oceanography in La Jolla, the San Andreas fault has a history of unleashing massive earthquakes every 180 years, give or take 40 years. As of now, it's been about 300 years since the last really 'Big One' shook Southern California.

Monday's temblor hit at 12:20 p.m., centered 2 miles south-southeast of Highland Park, according to the U.S. Geological Survey. The magnitude was initially reported at 4.6, but was later reduced to 4.4. It struck at a depth of about 7.5 miles.

Many residents reported a rolling sensation that lasted for several seconds. In Pasadena, close to the quake's epicenter, a water line ruptured as a result of the quake at Pasadena City Hall, and water could be seen pouring out of a pipe on the building's exterior. Employees mostly evacuated the building, but began going back inside around 1:15 p.m.

The quake had few aftershocks larger than a magnitude 2.5.

The quake came on the heels of a 5.2-magnitude earthquake on Aug. 6 centered in the Bakersfield area that was also felt across most of the Southland.

On Wednesday a magnitude 3.4 quake struck on a different fault in the Salton Seal area.

Local officials urged residents to use the quake as a reminder to be prepared for a shaker at any time.

"Having lived through the Northridge earthquake, today's tremor made me flashback to what we know are life saving rules during an earthquake: drop, cover, and hold on," county Supervisor Kathryn Barger said in a statement Monday. "It was also a reminder to us all that we live in earthquake country and we need to be prepared."

City News Service