Why was the 4.9 magnitude Los Angeles earthquake terrifying to some while others didn't feel it at all?

The 4.9-magnitude earthquake that struck Southern California on Monday raised some classic questions: Why did some people feel it last so long while others felt it last so short, and why didn't some people feel it at all?

Not surprisingly, people near the epicenter near Barstow felt the strongest and longest shaking.

But in the Los Angeles basin, some residents felt nothing at all, while others reported swaying for up to 15 seconds.

In Pasadena, about 95 miles southwest of the quake's epicenter, U.S. Geological Survey geophysicist Morgan Page didn't feel anything as she walked on the first floor, but her colleague, sitting in a chair on the second floor, felt a tremor for two seconds.

Next to Los Angeles International Airport, on the sixth and seventh floors of the New York Times headquarters in El Segundo, about 115 miles southwest of the epicenter, journalists felt relatively mild shaking for 15 to 20 seconds.

The ground beneath you is important.

People near the epicenter of an earthquake may feel the shaking last longer because they feel weaker ground motions in addition to the stronger motions generated by the earthquake, Page said. By contrast, people farther away may feel only some of the major ground motions.

The type of rocks and soil under your feet also matters.

If you're in a basin — whether it's the Mojave or the Los Angeles Basin — “you have basically vibrating sediments” underneath you that can produce “vibrations that last longer, and also amplify the vibration,” Page said.

The Los Angeles Basin is a six-mile-deep, bathtub-shaped hole in the bedrock filled with weak sand and gravel eroded from the mountains that forms the flat land where millions of people live in Southern California. When earthquake energy is sent into these sedimentary basins, it amplifies the intensity of the shaking—perhaps 10 times worse than if someone were on the bedrock—and causes the shaking to sway like a bowl of Jell-O, prolonging its duration.

“You get waves trapped in the basin, and that makes the vibration last longer. But you also get amplification, so you feel the waves at a higher amplitude, and therefore a stronger vibration, which without the amplification, you wouldn’t feel. So both contribute to that duration,” Page said.

People who are sitting are more likely to experience tremors than those who are walking or moving. “If you’re doing something versus if you’re sitting still, it makes a big difference,” she said.

Upper floors increase vibration.

Your height in the building also matters.

People on a higher floor will feel the vibration more strongly than people on the ground floor, Page added.

Imagine if you had a table and attached a spring to it, and a weight—like an inverted pendulum. “If you shake the table, the top of that spring is going to swing much more than the bottom,” Page said, explaining how people on the top floor of the same building would feel much more movement than those on the bottom floor.

“On the lower floor, you're connected to the ground. On the upper floor, you're swinging on this partly flexible, partly rigid structure,” Page said.

Lessons Learned from Ridgecrest

The difference in shaking was readily apparent during the 2019 magnitude 7.1 Ridgecrest earthquake, which was centered 125 miles northeast of downtown Los Angeles.

During that quake, someone sitting in the lobby of a downtown skyscraper might have felt mild vibrations for a few seconds. But on the 50th floor, the experience was terrifying, with sensors on that floor swinging back and forth about a foot in each direction for two to three minutes.

On the 48th floor of another downtown building, a woman felt dizzy and her husband felt motion sick after what seemed like minutes of swinging.

“Seasickness is something you feel when you’re in these tall buildings” during certain earthquakes, Monica Kohler, a research professor of civil engineering at the California Institute of Technology, said in 2019.

Even in downtown Los Angeles, during Monday’s quake, which struck at 1 p.m., people reported varying shaking experiences. In Silver Lake, Lucas, a 6-year-old cat, fell asleep during the shaking, while his owner, who was on the third floor of an apartment building, felt a rolling motion for four to six seconds.

In the Los Feliz-East Hollywood area, a person sitting at a table felt two waves of shaking — an initial wave that lasted about three to four seconds, followed by a wave in which the shaking subsided, and a second wave in which the walls trembled for five to six seconds. But another person standing nearby felt nothing.

The epicenter of Monday's 4.9-magnitude earthquake was in California's Mojave Desert, about halfway between Los Angeles and Las Vegas.

P waves and S waves

Los Angeles was likely too far from the epicenter of Monday’s quake for anyone to have felt a “P” wave — the first type of shock wave generated by an earthquake, in which rocks vibrate in the direction of the moving wave. But someone in Los Angeles may have felt an “S” wave, in which the underlying rocks vibrate perpendicular to the direction the wave is traveling, Page said.

The last tremor felt by residents in the area was probably caused by longer-lived surface waves that came later, Page said. Surface waves are the longest-lived waves that travel along the Earth's surface, arriving even later than S waves.

“Surface waves,” Page said, “are often the waves that make you feel like you’re on a boat. They’re almost the kind of waves that make you feel motion sick, as opposed to the jittery P and S waves that hit you earlier.”

Jim Buczynski, Brittney Mejia, Joel Rubin, Nathan Solis and Terry Tang of The New York Times contributed to this report.

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