Earthquakes shake SoCal: Could 'something bigger' be coming?

SOUTHERN CALIFORNIA — Several earthquakes measuring 4.0 or greater have rocked Southern California in the past week — and there are likely more on the way.

There is a small chance, about 5%, that the quake will be followed by a larger quake, with the probability decreasing over time, according to Gabrielle Tipp, a seismologist at the California Institute of Technology's Seismic Laboratory.

“When something like this happens, there's a little bit more chance that something bigger could happen,” Tip told Patch.

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Southern California has seen what Tibb called “clusters” of seismic activity not just in the past week, but since the beginning of the year.

“We've recently been seeing a lot of magnitude 4 earthquakes since the beginning of 2024 in Southern California,” Tip said. “This is just a result of the randomness of the earthquakes.” “If earthquakes followed a specific pattern, we would be able to predict them, but we can't.”

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However, earthquakes are not completely random.

“These things should happen accidentally and as tension builds up, but exactly when they happen is somewhat random,” she explained.

“For this reason, sometimes it is quiet with very few earthquakes,” she added. “Other times, you'll get clusters, like this one, with a bunch of them that aren't related to each other. Other times, there'll be a more constant rate of them. We just happen to be in one of the clusters, right now, with a lot of activity.”

Since the beginning of the year, five “major earthquakes” have occurred in California, according to the USGS. They have all occurred in Southern California and two of them occurred in the past week.

Earthquakes also struck near various faults.

“It's not limited to just one specific area,” Tipp said.

According to the USGS, earthquakes are considered “significant events” due to a combination of magnitude, number of “did you feel it” responses, and PAGER (Prompt Assessment of Global Earthquake Response) alert level.

A 4.1 magnitude earthquake struck on New Year's Day near Rancho Palos Verdes in Los Angeles County. Four days later, a 4.2 magnitude earthquake was recorded near Little Creek in the San Gabriel Mountains in San Bernardino County. On January 24, a 4.2 magnitude earthquake struck near San Bernardino.

Last week, a 4.6 magnitude earthquake struck at 1:47 pm on Friday, February 9, near Malibu, followed by a series of smaller earthquakes in the area. The earthquake occurred near the Malibu Coast Fault and Santa Monica Bay Fault, an area known to be seismically active.

Since record keeping began in 1932, six earthquakes of magnitude 4 or greater have occurred within about 6 miles of the quake, according to the Southern California Earthquake Network. The largest was 5.3 magnitude on February 21, 1973.

Three days after the Malibu earthquake, a cluster of earthquakes shook east of San Diego in the El Centro and Empire areas of Southern California.

“For a normal mainshock tectonic sequence, you have a large earthquake and a bunch of aftershocks that are usually of smaller magnitude,” Tipp said. “When you get a group of earthquakes of very similar magnitude, we consider that a swarm because there isn't really a clear mainshock.”

Among the earthquakes, the first and largest was a 4.8 magnitude quake recorded at 12:36 a.m. Monday, February 12, near El Centro, according to the USGS. A 4.6 magnitude earthquake occurred six minutes later.

The earthquake swarm continued through the morning and into Tuesday, February 13. As of 12:35 p.m. on February 13, the Southern California Seismic Network recorded 232 “events” in the swarm, the smallest of which was a magnitude 0.8 earthquake.

“The Imperial Valley is known for its earthquake swarms,” Tip said. “These things happen regularly in that area, so that's the type of activity we would expect in that area. It's an area that's prone to swarms.”

Although the area is known for earthquake activity, the swarms are usually associated with the San Andreas Fault, which ends near Bombay Beach in the Salton Sea. This swarm is believed to be associated with 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 remember a set of aftershocks like this happening on the Weinert River,” Tom Rockwell, a geologist at San Diego State University, told the San Diego Union-Tribune. “Is it a harbinger of something bigger? No one knows.”

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

Four of the quakes that occurred between 12:36 and 12:59 a.m. activated the USGS Earthquake Early Warning System, which detects large quakes early enough that alerts can be delivered to residents and automated systems seconds before tremors arrive.

The MyShake early warning app sent more than 79,000 alerts for a magnitude 4.8 earthquake and more than 87,000 alerts for a magnitude 4.6 earthquake, according to Robert Michael DeGroot, coordinator of the ShakeAlert earthquake early warning system for seismic science at the USGS. center.

De Groot explained that MyShake and other partner apps send alerts when the estimated volume is 4.5 or more for phones in a Modified Mercalli Intensity (MMI) III or larger area. Wireless Emergency Alert sends alerts to WEA-enabled devices when the rated volume is 5.0 or greater for phones in an MMI IV or larger area.

According to the intensity scale, MMI III is a “weak” shaking felt by people indoors, especially on the upper floors of buildings. MMI IV is a “mild” vibration and is felt by many at home and abroad by a few people.

“We use about one second of earthquake data to make a decision about the magnitude of the earthquake,” De Groot told Patch. “With the early warning of the earthquake, things have to be quick.”

Credit: Vibrate Alert

In the wake of the swarm, another 4.1-magnitude earthquake struck late Tuesday night in Imperial County. The quake was recorded at 11:53 p.m. on February 13, about 6.2 miles north of Westmoreland, part of the El Centro Metropolitan Area, according to the USGS.

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

“It was far enough away and with a different set of errors,” Tip explained. “I would consider it something different, but it is a complex tectonic area.”

The earthquake occurred near the Westmoreland Fault. There have been quakes with a magnitude of 109 or greater within about 6 miles of the quake since record keeping began, according to the Southern California Earthquake Network. The largest was 6.2 magnitude on November 24, 1987.

De Groot said seismic monitors are studying recent earthquake activity.

“When these things happen, we monitor them very carefully,” De Groot said. “We want to make sure we're monitoring trends and comparing them to other activities 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 whether or not the “big event” will come, but when it will come.

There are many faults in Southern California “capable of producing devastating earthquakes,” Tebb said.

“There will be another devastating earthquake at some point,” she added. “The best thing you can do is prepare.”

Most of California has at least a 75% chance of a devastating earthquake in the next century, according to a newly released USGS map. Much of the state has more than a 95% chance of a devastating earthquake.

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

Credit: USGS

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

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

“We know it will happen, but it is difficult to say exactly when,” De Groot said. “Our best attempt is to have these earthquakes occur about every 30 years — that means a Northridge-sized earthquake in the Los Angeles area.”

The Northridge earthquake occurred on a fault that had never been discovered before. Experts are more concerned about the San Andreas fault. The fault, which extends more than 800 miles, has been responsible for some of the state's largest earthquakes.

Seismologists have warned the public for years that Southern California was “overdue” for an 8.0 magnitude earthquake on the state's longest fault. To put that into perspective, an earthquake of this size is 60 times stronger and six times longer than the Northridge earthquake.

“When someone says we're late, what they're usually saying is that the time since the last major earthquake is greater than the time of this historical recurrence,” Tip explained. “It's average. So sometimes it will be less and sometimes it will be more.”

“We are beyond that average now,” she noted. “We are inside the window where we are waiting and anticipating something to happen.”

She and De Groot encouraged residents to prepare for earthquakes. Create an emergency kit and plan, and also download the MyShake early warning app, which helped alert residents near the recent earthquake swarm.

The establishment of an earthquake early warning system has been one of the biggest developments in the three decades since the Northridge earthquake.

“We live in an earthquake country. Earthquakes will continue to happen,” he said. “There is now a system that detects an earthquake as soon as it reaches the surface, quickly transmits that information to where it needs to go, and then sends alerts to the people who need it.”

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