A magnitude 5.2 earthquake, centered about 18 miles southwest of Bakersfield, was felt across a wide swath of Southern California Tuesday night.
The quake, initially estimated at magnitude 5.3, struck at 9:09 p.m., according to the U.S. Geological Survey. It was followed by dozens of aftershocks of magnitude 2.5 and greater, including a magnitude 4.5 quake less than a minute after the first, and another magnitude 4.1 quake at 9:17 p.m.
The epicenter was in a sparsely populated agricultural area about 14 miles northwest of the unincorporated community of Grapevine in Kern County, 60 miles northwest of Santa Clarita and about 88 miles northwest of downtown Los Angeles.
Two minutes after the quake, a large boulder — about the size of an SUV — was reported blocking several lanes of southbound traffic on Interstate 5, about a mile south of Grapevine Road, the California Highway Patrol said. The boulder was still blocking traffic lanes at least an hour after the quake.
The area closest to the epicenter felt “very strong” shaking on the modified Mercalli intensity scale; this area includes a section of the California Aqueduct, which carries water from Northern California to Southern California.
While the shaking was felt in more populated areas, including Bakersfield, Santa Clarita and Ventura, the USGS calculated that only “weak” shaking was felt, which can shake stationary cars and cause vibrations in a building similar to a passing truck.
Some residents affected by the quake reported a prolonged period of shaking. One person in Los Feliz felt movement for 45 seconds, with at least three different waves: a weak one, followed by a strong one, then another weak one. In South Pasadena and Whittier, residents felt about 20 seconds of shaking, contained in two separate waves.
In Pasadena, seismologist Lucy Jones said she felt about three seconds of shaking.
There were no immediate reports of damage. And not everyone felt the quake. Los Angeles County Sheriff’s Deputy Jose Gomez said he didn’t feel any shaking while driving to work at the Santa Clarita police station. No damage was reported.
The Los Angeles Fire Department said no significant damage was reported within city limits.
The USGS said the quake was felt throughout the Los Angeles Basin and interior valleys as well as in Santa Maria, Bakersfield and Fresno.
Many Southern Californians reported receiving alerts from the USGS’s earthquake early warning system, for example through the MyShake app or on their Android phones. (The earthquake early warning system is automatically installed on Android phones, but people with Apple iOS phones must install the MyShake app to receive the earliest alerts.)
One person reported receiving 30 to 45 seconds of warning before feeling the shaking. Another person, east of Anaheim, reported receiving 30 seconds of warning before the shaking arrived.
Jones, a Caltech research associate, said the duration of shaking can vary wildly in the Los Angeles area because the length of time the ground moves in a given location can depend on the soil and rocks beneath the location, whether a person is sitting still or moving around, and even whether a person is on the ground floor or at the top of a skyscraper — those on higher floors feel the shaking more strongly.
The reason some people were able to feel more than one wave of shaking is because the first aftershock occurred very early — less than a minute — after the main quake, Jones said.
Allen Husker, a geophysics professor and director of the Southern California Seismic Network at Caltech, said it was not surprising that so many people in the Los Angeles area felt significant shaking during a magnitude 5.2 earthquake north of Grapevine. The shaking occurred at night, when people are resting and are more likely to feel the tremors of a distant earthquake than if they were outside during the day and active.
Another reason many people felt significant movement is because of how the shaking is amplified in the Los Angeles Basin. The basin is a 6-mile-deep, bathtub-shaped hole in the underlying bedrock, filled with sand and gravel eroded by mountains that form the flat land where millions of people live. It stretches from Beverly Hills through southeast Los Angeles County and into northern Orange County.
“The pelvic effect… increases the shaking that you would normally have,” Husker said.
The effect occurs when waves from the quake arrive and hit the walls of the pool and then bounce off the walls of the pool, Jones said, resulting in a “prolonged duration.”
A major earthquake on the San Andreas fault would cause perhaps 50 seconds of strong shaking in downtown Los Angeles. “This earthquake was much smaller, of course,” Jones said, “but it was big enough to create some of those basin effects and bounce things around.”
As with all earthquakes, there was a 1 in 20 chance that Tuesday’s quake was a precursor to a larger one. The risk of another larger quake diminishes over time.
In the past 10 days, there have been no earthquakes of magnitude 3.0 or greater centered nearby.
According to a recent three-year data sample, an average of five earthquakes measuring 5.0 to 6.0 magnitude occur each year in California and Nevada.
Tuesday’s quake occurred about 12 miles northwest of the epicenter of the 7.5-magnitude earthquake that struck Kern County on July 21, 1952. That quake killed 12 people and damaged older and poorly constructed masonry buildings, according to the USGS. Some of those structures collapsed in communities including Tehachapi, Bakersfield and Arvin; heavy damage was reported at Kern County General Hospital.
The tremors from the 1952 earthquake were felt as far away as San Francisco and Las Vegas, and caused significant nonstructural damage to tall buildings in the Los Angeles area and damage to at least one building in San Diego, according to the USGS.
The 1952 earthquake occurred on the White Wolf fault. Tuesday’s earthquake was not associated with any previously mapped fault.
The earthquake occurred at a depth of 9 km. Did you feel this earthquake? Consider reporting what you felt to the USGS.
Find out what to do before and during an earthquake near you by signing up for our Unshaken newsletter, which breaks down emergency preparedness into bite-sized steps over six weeks. For more on earthquake kits, the apps you need, Lucy Jones’ top tips and more, visit latimes.com/Unshaken.
The first version of this article was generated automatically by Quakebot, a computer application that monitors the latest earthquakes detected by the USGS. A Times editor reviewed the article before publication. If you want to learn more about the system, see our list of frequently asked questions.
Times reporters Jon Healey, Ian James, Jason Neubert, Sandra McDonald and Raul Roa contributed to this report.
