The 4.9-magnitude earthquake that shook Southern California on Monday raised some classic questions: Why did it feel so long to some and so short to others, and why did some not feel it?
Not surprisingly, people in the epicenter near Barstow felt the strongest and longest tremors.
But in the Los Angeles Basin, some residents felt nothing, while others reported shaking for up to 15 seconds.
In Pasadena, about 95 miles southwest of the epicenter, US Geological Survey geophysicist Morgan Page felt nothing as he walked on the first floor, but his colleague, who was sitting in a chair on the second floor, felt the shaking for several seconds.
And next to Los Angeles International Airport, on the sixth and seventh floors of The Times’ El Segundo headquarters, about 115 miles southwest of the epicenter, reporters felt 15 to 20 seconds of relatively gentle swaying.
The ground beneath you is important
For people in the epicenter of the earthquake, the shaking may last longer because you feel the ground motion as well as the stronger movement caused by the earthquake, Page said. By contrast, people further away can feel only a few major ground motions.
What kind of rock and soil is under your feet is also important.
If you’re in a basin – whether it’s the Mojave Basin or the Los Angeles Basin – “you have a basically shakier sediment” below you that can produce “a longer duration of shaking, and it also amplifies the shaking,” Page said.
The Los Angeles Basin is a 6-mile-wide bathtub hole in the bedrock filled with loose sand and gravel eroded from the mountains and forming the flat land where millions of people in Southern California live. When the earthquake energy is sent to this sedimentary basin, it amplifies the intensity of the shaking – perhaps 10 times worse than if a person is on the bedrock – and also causes the shaking to resonate like a bowl of Jell-O, prolonging its duration.
“You get waves that are trapped in the basin, and it just makes the shaking last longer. But you also get amplification, so you feel waves of higher amplitude, and therefore stronger shaking, without amplification, you wouldn’t feel it. So two- both contributed to this duration,” Page said.
People who sit are more likely to sway than those who walk or move. “When you’re doing something versus when you’re sitting down, there’s a difference,” he said.
A higher floor amplifies the shaking
How high up you are on the building also matters.
Shaking will be felt more strongly by people on higher floors than on the ground floor, Page said.
Imagine that you have a table and put a spring up, and have a weight on it – like an inverted pendulum. “If you shake a table, the top of the spring will shake more than the bottom,” Page said, explaining how people on the top floor of the same building will feel more movement than those on the bottom level. .
“On the bottom floor, you’re attached to the ground. And on the top floor, you’re swinging around in this part-flexible, part-rigid structure,” Page said.
Lessons from Ridgecrest
The difference in shaking was immediately apparent during the 2019 magnitude 7.1 Ridgecrest earthquake, centered 125 miles northeast of downtown LA.
During the quake, someone sitting in the lobby of a downtown skyscraper may have felt a light jolt for a few seconds. But on the 50th floor, the experience was terrifying, with the sensors on that floor going back and forth like feet in every direction for two to three minutes.
On the 48th floor of another downtown building, one woman felt vertigo and her husband felt dizzy after what seemed like minutes.
“Seasickness is what happens when you’re at this high altitude” during certain earthquakes, Caltech civil engineering research professor Monica Kohler said in 2019.
Even across central LA during Monday’s quake, which occurred at 1 p.m., people reported different shaking experiences. In Silver Lake, a 6-year-old tuxedo cat, Lucas, was sleeping through rocking, when his owner, on the third floor of an apartment building, felt a rolling movement for four to six seconds.
In the Los Feliz-East Hollywood area, one person sitting at a table felt two waves of shaking – an initial wave of about three to four seconds, then an easing of shaking, then a second wave in which the walls shook for five seconds. up to six seconds. But others nearby felt nothing.
Monday’s magnitude 4.9 quake was centered in California’s Mojave Desert, about halfway between Los Angeles and Las Vegas.
P waves and S waves
Los Angeles is likely to be very far from the epicenter for anyone to feel the earthquake’s “P” wave – the first type of shaking wave generated by the earthquake, in which the rock vibrates in the direction of the wave. But people in LA can feel “S” waves, where bedrock oscillates perpendicular to the direction of wave propagation, according to Page.
The last tremors felt here may be due to longer surface waves, Page said. Surface waves are the longest waves that enter and pass through the Earth’s surface, arriving even after S waves.
Surface waves are “often waves that feel like being on a boat. They’re almost like the waves that make you sick, against the horrible P and S waves,” Page said.
Times staff writers Jim Buzinski, Brittny Mejia, Joel Rubin, Nathan Solis and Terry Tang contributed to this report.
