They are not as familiar as the freeways, but Southern California's major faults — such as the San Andreas, Newport-Inglewood and San Fernando — have become familiar markers on the local landscape.
But Monday's 4.4 earthquake in Encino is a reminder that the seismic danger extends well beyond those fault lines. The quake, which caused no damage but was the largest in the Los Angeles area in four years, erupted on a little-noticed fault deep under the Santa Monica Mountains.
The temblor surprised seismologists because it was the strongest to hit directly under the Santa Monica Mountains in the 80 years "since we started recording earthquakes in Southern California," Caltech seismologist Egill Hauksson said. Until now, experts recorded only magnitude 1 to 3 quakes there.
FOR THE RECORD:
Earthquake wake-up call: An article in the March 18 Section A about a 4.4 earthquake in Encino included a map that contained two errors regarding recent earthquakes in the Los Angeles region. A Pasadena earthquake that struck in 1988 had a magnitude of 5.0, not 4.4, and the location of a 5.9 quake near Alhambra was about three miles to the east of the site shown on the map. —
Monday's 6:25 a.m. temblor showed that for all that is understood about quakes, much remains unknown. Southern California's most destructive earthquakes in the last generation — the 5.9 Whittier Narrows in 1987 and the 6.7 Northridge in 1994 — occurred on faults unknown to scientists before the shaking began.
"Clearly, earthquakes happen in places you don't expect," said Thomas Heaton, director of Caltech's Earthquake Engineering Research Laboratory. "The bigger the earthquake, eventually, you really are surprised to find an earthquake on a fault you didn't know about."
Monday's earthquake was about 900 times weaker than the Northridge earthquake. The amount of earth that moved during the quake was probably the size of a football field; by contrast, blocks of earth 10 miles by 10 miles moved during Northridge, Hauksson said.
Still, the shaking was felt as far as Santa Barbara and San Clemente.
The temblor caused the fault to slip for only a fraction of a second, but it was enough to shake awake millions of Angelenos. The shaking lasted seconds longer.
"I've been through a lot of earthquakes. This one felt just more violent," said teacher Jennifer Graham, who was at her boyfriend's home near the epicenter.
The quake started underneath an upscale hillside neighborhood near Mulholland Drive and Sepulveda Boulevard.
At the home of Debbie Seidel, a 42-year-old mother of two, there was a fallen mirror resting on a chest that did not shatter. Her daughter's shoe rack had come unhinged and was spilling items to the ground.
Seidel's husband later texted her, "Our house is the epicenter."
"It was fast and hard," she said. "You felt that it was close. It was intense, but super short."
Seidel said she heard from some neighbors who reported "nothing bad, just scared nerves and curious kids who are living through this for the first time."
The shaking was actually worse farther from the epicenter on top of the soft, soil-filled Los Angeles Basin and the San Fernando and San Gabriel valleys.
That's because the quake's waves "bounce back and forth" within the basin and valleys, "so you get this bowl of Jello effect," Hauksson said.
A few miles away, in communities such as Westwood, Sherman Oaks and Beverlywood, the quake packed a punch.
"I kind of just grabbed a blanket and hid," Lilly Chang, 22, a UCLA psychobiology major, said after her boyfriend, Aaron Green, 28, grabbed her and jumped out of bed.
The quake caused Cristina Toth, 26 and Andresa Maia, 25, to flee UCLA's architecture building, where the two were working all night.
"We looked at each other," Maia said, "and we just sort of ran outside."
At the Sherman Oaks Beauty Center on Ventura Boulevard, bottles of shampoo and conditioner spilled to the floor. Elsewhere, residents reported picture frames falling.
The quake hit as KTLA-TV Channel 5 morning news was broadcasting live in Hollywood. The anchors ducked for cover.
"Earthquake, we're having an earthquake," anchor Chris Schauble said before he and co-anchor Megan Henderson ducked underneath the desk. Twitter erupted with Schauble memes, showing his face frozen in shock.
U.S. Geological Survey seismologist Lucy Jones applauded the KTLA anchors for doing the right thing — and had firm words for those snickering at them.
"You might feel a little silly doing it for the small one. When the Big One happens, it means you stay alive, as the lights come crashing down," Jones said.
The quake was the talk of social media, a testament in part to Los Angeles' quake drought in recent years. Between the late 1980s and early 1990s, a damaging and deadly earthquake hit California about every other year.
But even more moderate earthquakes have been rare. Normally, a 4.4 earthquake would be expected every year, but the last time such a quake hit the Los Angeles Basin was four years ago, centered in Pico Rivera, east of Los Angeles.
The U.S. Geological Survey and Caltech plan a closer examination of the fault system under the Santa Monica Mountains. In the past, discoveries of new faults have helped scientists better understand the seismic dangers Los Angeles faces.
The 1987 Whittier Narrows quake led to the discovery of an underground, invisible fault called the Puente Hills thrust fault, which could produce a catastrophic 7.5 shaker underneath downtown Los Angeles.
There was a 5% chance that Monday's quake was a prelude to a larger earthquake, but that probability falls to 1% by Tuesday morning, Jones said.