When the 4.4-magnitude earthquake struck the Los Angeles area Monday afternoon, Angelenos reported a lengthy quake with significant shaking.
But the number attached to an earthquake only tells you so much about it. A variety of other factors influence how a quake is felt on the Earth’s surface, according to University of California, Berkeley Seismology Lab director Richard Allen.
“Even if it’s the same magnitude, of course the further you are away from the earthquake will have an effect, but also the kind of sediments or rocks that you’re sitting on will affect the amount of shaking,” he says. “For that matter, even the building you’re in can cause you to feel more shaking or less shaking.”
Magnitude describes a quake’s strength
Magnitude is the measurement of the strength of an earthquake, and it’s usually expressed using the Richter Scale. Larger, longer-lasting earthquakes have higher magnitudes than smaller, shorter ones.
Your proximity to the epicenter of an earthquake will likely determine how strongly you feel it: the nearer you are, the more intense it will be.
That’s not always the case though. For some higher magnitude earthquakes, the most intense shaking is felt along the fault line where the earthquake occurs, Allen says. That means that two people equally far from the epicenter may feel shaking at different intensities if only one of them is along the fault line.
If you’re in a building when a quake strikes, the type of structure also matters. Allen says one- or two-story buildings only amplify seismic shaking a little bit, while taller buildings and skyscrapers absorb much more of a quake’s energy and can sway back and forth.
Soil plays a major role in earthquake intensity
The makeup of the soil and rocks beneath you can also affect how strong an earthquake feels.
People living in sedimentary basins with soft soils experience more powerful shaking, because the soil effectively amplifies the seismic waves, while those located on harder rocky surfaces may feel a weaker quake.
That’s because a wave entering soil is forced to slow down, “but the number of peaks and troughs per second has to stay the same,” Allen said. “The frequency can’t change. The velocity has changed. And that means that all the energy gets squished into a shorter wavelength, and that’s what gives you the larger amplitude.”
Soft soils can also cause quakes to last longer by trapping energy and making it bounce around a sedimentary basin, increasing the duration of the tremors.
“So if you are in a sedimentary basin, you typically get larger amplitudes and you get longer duration of the shaking,” he said. “So it’s a bit of a double whammy.”
Jolting shakes or a rolling sensation? It’s a matter of waves
Intensity aside, waves can also feel different.
P waves — the first ones that emanate from an earthquake — are often higher-frequency and can result in jerky shaking.
“And then the S waves that come later,” Allen said, “they typically are much more gradual sort of oscillations, and so it feels like it’s that sort of rolling sensation.”
People close to the epicenter of an earthquake typically feel the more intense P waves, even though the S waves come right after them, according to the U.S. Geological Survey. Those located further away from the quake may only feel the S waves that travel more slowly.
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