Earthquakes Rock SoCal: Could 'Something Bigger' Be Coming?

SOUTHERN CALIFORNIA — Several magnitude 4.0 or greater earthquakes have rattled Southern California in the past week — and there’s a chance more could be on the way.

There is a small chance, about 5 percent, that an earthquake will be followed by a larger quake, with the likelihood decreasing over time, according to Gabrielle Tepp, a staff seismologist at California Institute of Technology’s Seismological Laboratory.

“When something like this happens, there is a slightly elevated chance that something bigger could be coming,” Tepp told Patch.

Southern California has experienced what Tepp called “clusters” of earthquake activity not only this week, but since the start of the year.

“We have been having a lot of magnitude 4s lately since the start of 2024 in Southern California. That’s just a result of the randomness of earthquakes,” Tepp said. “If earthquakes followed a specific pattern, we’d be able to predict them, but we can’t.”

Earthquakes aren’t entirely random, however.

“They have to happen at fault and when stress builds up, but when exactly they happen is somewhat random,” she explained.

“Because of that, sometimes you’ll go quiet with very few earthquakes,” she added. “Other times you’ll get clusters, like this, with a bunch of them that are unrelated. Other times there will be a more steady rate to them. We just happen to be in one of the clusters, right now, with a lot of activity.”

Since the start of the year, there have been five “significant earthquakes” in California, according to the United States Geological Survey. All of them have happened in Southern California and two of them struck this past week.

The quakes also struck near different faults.

“It’s not confined to just one specific area,” Tepp said.

According to the USGS, earthquakes are considered “significant events” due to a combination of magnitude, the number of “Did You Feel It” responses, and the PAGER (Prompt Assessment of Global Earthquakes for Response) alert level.

A 4.1-magnitude quake hit New Year’s Day near Rancho Palos Verdes in Los Angeles County. Four days later, a 4.2-magnitude quake was recorded near Lytle Creek in the San Gabriel Mountains in San Bernardino County. On Jan. 24, a 4.2-magnitude quake struck near San Bernardino.

This past week, a magnitude 4.6 earthquake hit at 1:47 p.m. Friday, Feb. 9, near Malibu, which was followed by a series of smaller quakes in the area. The quake struck near the Malibu Coast fault and Santa Monica Bay fault, which is an area that is known to be seismically active.

Since record-keeping began in 1932, there have been six magnitude 4 or greater earthquakes within about 6 miles of the quake, according to the Southern California Seismic Network. The largest was a magnitude 5.3 on Feb. 21, 1973.

Three days after the Malibu quake, an earthquake swarm rattled east of San Diego in the El Centro and Imperial areas of Southern California.

“For a normal mainshock tectonic sequence, you have one big earthquake and a bunch of aftershocks that are usually smaller magnitudes,” Tepp said. “When you get a bunch of earthquakes that are all very similar magnitudes, we consider that a swarm because there’s not really a clear mainshock.”

Of the earthquakes, the first and largest was a 4.8-magnitude quake recorded at 12:36 a.m. near El Centro, according to the USGS. A 4.6-magnitude quake struck six minutes later.

The earthquake swarm continued through the morning and into Tuesday. As of 12:35 p.m. Tuesday, the Southern California Seismic Network had recorded 232 “events” in the swarm, with the smallest being a 0.8-magnitude quake.

“The Imperial Valley is known for earthquake swarms,” Tepp said. “These happen pretty regularly in that region, so it’s the type of activity that we’d expect for that region. It’s a swarm-prone area.”

Although the region is known for earthquake activity, swarms are typically linked to the San Andreas fault, which ends near Bombay Beach in the Salton Sea. This swarm is believed to be linked to the Weinert-El Centro fault, a branch of the San Jacinto fault system, which is one of the most active fault zones in Southern California.

“I don’t recall a swarm of aftershocks like this ever occurring on the Weinert,” Tom Rockwell, a San Diego State University geologist, told the San Diego Union-Tribune. “Is it a foreshock to something bigger? No one knows.”

The earthquake swarm was one of the strongest to hit Southern California in years.

Four of the earthquakes between 12:36 and 12:59 a.m. activated the USGS ShakeAlert Earthquake Early Warning System, which detects significant quakes early enough so that alerts can be delivered to residents and automated systems potentially seconds before shaking arrives.

The MyShake early-warning app sent more than 79,000 alerts for the 4.8-magnitude quake and more than 87,000 alerts for the 4.6-magnitude quake, according to Robert-Michael de Groot, a coordinator at ShakeAlert Earthquake Early Warning System for the USGS Earthquake Science Center.

De Groot explained that MyShake and other partner apps send alerts when the estimated magnitude is 4.5 or greater to phones in the Modified Mercalli Intensity (MMI) III or greater zone. The Wireless Emergency Alert sends alerts to WEA-capable devices when the estimated magnitude is 5.0 or greater to phones in the MMI IV or greater zone.

According to the intensity scale, MMI III is “weak” shaking that is felt by people indoors, especially on upper floors of buildings. MMI IV is “light” shaking and felt indoors by many and outdoors by few people.

“We use about one second of data from the earthquake to make a decision about how big it’s going to be,” de Groot told Patch. “With earthquake early warning, it’s got to be fast.”

Following the swarm, another 4.1-magnitude earthquake struck late Tuesday night in Imperial County. The quake was recorded at 11:53 p.m. about 6.2 miles north of Westmorland, which is part of the El Centro Metropolitan Area, according to the USGS.

The quake was considered a “separate event” from the swarm.

“It was far enough away and on a different set of faults,” Tepp explained. “I would consider it something different, but it’s a complicated tectonic area.”

The quake struck near the Westmorland fault. There have been 109 magnitude 4 or greater earthquakes within about 6 miles of the quake since the start of record-keeping, according to the Southern California Seismic Network. The largest was a magnitude 6.2 on Nov. 24, 1987.

De Groot said that earthquake watchers are studying the recent quake activity.

“Whenever these things happen, we watch them very carefully,” de Groot said. “We want to make sure that we watch the trends and compare it to other activity in the past to see if this might lead to something. We’re always thinking about what could happen next.”

Experts agree that it’s not a matter of if the “Big One” is coming but when.

Tepp said there are several faults in Southern California that are “capable of producing damaging earthquakes.

“There’s going to be another damaging earthquake at some point,” she said. “The best thing you can do is be prepared.”

Most of California has at least a 75% chance of a damaging earthquake in the next century, according to a newly released USGS map. A large portion of the state has over a 95% chance of a damaging earthquake.

The latest USGS National Seismic Hazard Model released in January shows where damaging earthquakes are most likely to occur based on seismic studies, historical geologic data, and the latest data-collection technologies. The model updated a previous version released in 2018.

This year marked the 30th anniversary of the destructive 1994 Northridge earthquake that killed at least 57 people, injured thousands and caused billions of dollars in damage in Southern California.

There’s a 60% chance that a magnitude 6.7 quake will hit the Los Angeles area again within 30 years, according to the USGS. There’s a 46% chance of a magnitude 7.0 quake and a 31% probability of a magnitude 7.5.

“We know that it’s going to happen, it’s just hard to put our finger on exactly when,” de Groot said. “Our best attempt though is that they happen about every 30 years — meaning a Northridge-sized earthquake in the Los Angeles area.”

The Northridge quake happened on a previously undiscovered fault. Experts are even more concerned about the San Andreas fault. The fault, which runs more than 800 miles long, has been responsible for some of the state’s largest quakes.

Seismologists have warned the public for years that Southern California is “overdue” for an 8.0-magnitude earthquake courtesy of the state’s longest fault. To put that into perspective, a quake that size is 60 times more powerful and six times longer than the Northridge earthquake.

“When someone says we are overdue, what they are usually saying is that the time since the last major earthquake is greater than that historic recurrence time,” Tepp explained. “It’s an average. So sometimes it will be less and sometimes it will be more.

“We’re past that average now,” she noted. “We’re within the window that we are kind of waiting and expecting something to happen.”

She and de Groot both encouraged residents to be prepared for earthquakes. Create an emergency kit and plan, and also download the MyShake early-warning app, which helped alert residents near the most recent quake swarm.

The creation of the earthquake early warning system has been one of the biggest advancements in the three decades since the Northridge quake.

“We live in earthquake country. Earthquakes are going to continue to happen,” he said. “Now there’s a system in place that detects the earthquake as soon as it reaches the surface, moves that information quickly to where it needs to go, and then gets alerts out to people who need them.”