A magnitude 4.7 earthquake north of Malibu rocked Southern California on Thursday, followed by a magnitude 3.4 aftershock about an hour later.
The earthquake struck at 7:28 a.m. and was felt in the area. No damage was reported. There were at least three aftershocks of magnitude 2. At 8:40 a.m., there was a magnitude 3.4 aftershock.
Residents should prepare for more aftershocks. “Earthquakes like to cluster with other earthquakes in space and time, and this earthquake is no exception,” said Morgan Page, a geophysicist with the US Geological Survey. In some cases, aftershocks are smaller and fade over time, but there is a 1 in 20 chance that, in the next week, there will be another earthquake of magnitude 4.7 or greater.
Earthquakes can reach magnitude 8 in this area, Morgan said, which is actually “a reasonable standard for anywhere in California.” This is because individual named faults can connect with others in the same seismic event to form larger earthquakes.
There were more real earthquakes felt in the more populated areas of Southern California. But that fact doesn’t help predict whether larger, more destructive earthquakes will happen sooner, seismologists say.
Some scientists have concluded that earthquake activity rises in the region before a large earthquake; Other scientists say seismic activity actually decreases before large earthquakes, said USGS seismologist Susan Hough. So the seismic activity we see doesn’t give us a clue as to when a big, destructive earthquake will happen.
The epicenter of Thursday’s quake was closest to the Malibu fault, said seismologist Lucy Jones, a Caltech research associate. Preliminary analysis shows that the earthquake has a 40% chance of being associated with the Malibu fault and a 46% chance of being associated with the Anacapa fault.
An earthquake of this magnitude ruptures only a small part of the fault, perhaps only a few hundred meters. So, these small earthquakes often occur on small faults that are not related to larger faults and are mapped on the earth’s surface.
“Moderate” and “light” shaking, as defined by the Modified Mercalli Intensity Scale, was felt in parts of Malibu, Thousand Oaks, Agoura Hills, Calabasas and Westlake Village. Moderate shaking is felt by almost everyone and can break windows and plates; light shaking can rattle dishes, windows and doors; makes a cracking sound on the wall, and can feel like a heavy truck has hit the building.
In Thousand Oaks, one resident reported the shaking lasted a few seconds, while another resident elsewhere felt it for 12 seconds. There are also many people in Southern California who did not feel the earthquake.
Some of the strongest tremors were felt in western L.A. County — not surprising, Jones said, because of the area’s proximity to the epicenter but also because of deep silt deposits on the surface in the area, which magnify shaking from earthquakes.
“Ground” shaking was felt over most of the Los Angeles metropolitan area, including downtown LA; Santa Monica; Long Beach; San Fernando, San Gabriel and Antelope valleys; Orange County and the Inland Empire. There were crowdsourced reports indicating the tremors were felt as far away as San Diego and Bakersfield.
People on the shores of LA County felt the tremors. Some people in Redondo Beach and Long Beach felt shaking for 10 seconds. In Redondo Beach, there was a person who felt the shock from a young age and then it got stronger, but no one fell off the shelf. In Long Beach, residents felt shaking and spinning.
Near Los Angeles International Airport, an apartment building in El Segundo shook and swayed.
The earthquake shook an anchor broadcasting live at KTTV-TV, the Fox affiliate in Los Angeles, which has a studio east of Santa Monica. “We’re having an earthquake right now,” the anchor said, followed by a roar and shouts of “Whoa!”
Emil’s Bake House is a favorite morning stop for commuters streaming from the 101 Freeway to Malibu via Kanan Road in Agoura Hills.
Julius Speck is at the counter serving customers lattes, scones and vegetable juices when the windows and screen cases rattle.
“I think someone put something in the back,” Speck said, pointing to the kitchen. “I’m confused.”
Shake only a few seconds and Speck said take a deep breath and ask the next customer what they want to order.
Over the last 65 years, Jones said, there was an average of eight to 10 independent sequences of earthquakes in Southern California with at least one magnitude 4 earthquake or more. In some years, there are only one or two earthquakes; The highest number was 13 earthquakes in 1988. There were 14 earthquakes of magnitude 4 and above in Southern California until this year, Jones said.
“So yes, this is a more active year than last year,” Jones said. But, he said, “we can’t say yet that it’s actually statistically significant to see this.”
However, the latest quake is “a very good reminder that the quiet of the last few decades is not the long-term picture, and we need to be prepared,” Jones said.
Another earthquake in Malibu, magnitude 4.6, was felt on February 9. The quake’s epicenter was about six miles southwest of Thursday’s earthquake, and was probably unrelated to Thursday’s quake, Page said.
Another cluster of earthquakes has been reported this summer in the El Sereno neighborhood, on the Eastside of Los Angeles, which occurred on the Puente Hills thrust fault system. Most recently there was an earthquake of magnitude 4.4 that happened on August 12, which was enough to cause shampoo bottles to fall from the shelves in the Doel store in Alhambra, and beat the “shredded cheese” sign from the refrigerated aisle.
They were preceded by a pair of earthquakes in early June – magnitude 3.4 on June 2 and magnitude 2.8 on June 4, and a magnitude 2.9 earthquake in the same area on June 24.
The Puente Hills thrust fault system is the same overall fault network that produced the 1987 Whittier Narrows earthquake of magnitude 5.9 – which killed eight people and caused some $358 million in damage. The Puente Hills thrust fault system can produce earthquakes of magnitude 7.5, which occur in heavily populated areas of LA and Orange counties and can kill 3,000 to 18,000 people.
The largest earthquake to hit California so far this year was a magnitude 5.2 earthquake that struck on August 6, centered in rural farmland about 15 miles northwest of the unincorporated Kern County community of Grapevine on Interstate 5 and about 19 miles southwest of Bakersfield. .
The area near Malibu has experienced stronger earthquakes in the past. On January 18, 1989, a magnitude 5 earthquake occurred 8 miles southeast of Malibu Point, at the bottom of Santa Monica Bay; several people were injured and items fell from store shelves and several windows were broken, according to the Southern California Earthquake Data Center.
On New Year’s Day in 1979, a magnitude 5.2 earthquake struck about 8 miles south of Malibu Point, famous for occurring during the Rose Bowl game between USC and Michigan. “Some fans in the stadium were shocked by the shaking, but the game went on,” the Southern California Earthquake Data Center said.
Thursday’s earthquake moved in a horizontal, side-to-side motion, also known as “strike-slip motion,” Jones said. Another type of movement that can be felt in other earthquakes in Southern California is upward movement on a dipping fault.
Some residents were alerted by the earthquake early warning system, powered by the USGS ShakeAlert system. In Koreatown, residents get a two-second warning before shaking. The free earthquake early warning system app, MyShake, can be downloaded on the iOS and Google Play app stores. Android also has an earthquake early warning system built into the operating system.
The scientists also asked people to fill out the “Do You Feel?” crowdsourcing form on the USGS website to let officials know what the intensity of shaking felt at that location.
The ShakeAlert system is already operational statewide in California, Oregon and Washington, with plans underway to extend it to Nevada, said geophysics professor Allen Husker, head of the Southern California Seismic Network at Caltech.
On the C Line train (formerly the Green Line) heading west toward Crenshaw station, cars blared on cell phone alarms as phones lit up because of earthquake warnings. Passengers reached for their phones or looked around in alarm, but the train moved forward, moving as usual. Despite the instructions on the phone, no one fell on the floor or looked for cover in the chair. The shaking of the earthquake is not seen in the train, which sways when it goes down the track.
It is not uncommon for people to feel varying degrees of shaking, and for some to feel none at all. If you are sitting still, the duration of the shaking you feel may be longer than if someone is moving. And being on top of soft sediments in valleys and basins, where shocks are amplified as they bounce, can cause shocks to last longer than if you were on top of bedrock.
The Los Angeles Fire Department, as well as officials with Ventura County, did not report any damage. The LA County Fire Department did not receive calls, or respond to emergency calls, about the earthquake.
The Los Angeles County Sheriff’s Department locked down all jail facilities after the earthquake to conduct a damage assessment. No damage or injuries were reported and just before 10am, a spokesman confirmed the facility was all back to normal operations.
In general, an earthquake must be at least magnitude 5 to see damage, if it’s a relatively shallow event, Jones said.
The quake was centered in the Malibu Hills on Kanan Dume Road around Ramirez Canyon.
“This is a good reminder to people that we live in earthquake country. And we need to take steps that will help us all be safe in the event of a larger earthquake,” said Hough.
Times staff writers Luke Money, Iliana Limon Romero, Ruben Vives, Richard Winton, Jon Healey and Keri Blakinger contributed to this report.
