The Feel of a Quake: Why the Difference? : Seismology: Whether it’s a jolt or a rolling motion depends on the distance from the epicenter.

Why does one earthquake feel so much different from another?

The earthquake that rattled through Southern California on Wednesday may have felt quite different to people who experienced the Montebello quake last June 12, even if they were in the same place both times.

Workers in office buildings in downtown Los Angeles, for example, reported feeling a sharp jolt from the 4.5 magnitude Montebello quake, whereas Wednesday’s 5.5 temblor produced a long rolling motion.

Why should one earthquake produce sharp jolts and another make the Earth roll back and forth like a ship at sea?

The short answer is that one quake was closer than the other.

Wednesday’s quake felt “different” to many Los Angeles residents because “it was farther away,” said seismologist Lucile Jones of the U.S. Geological Survey’s Pasadena office. “That’s the main thing.”

Wednesday’s quake was centered three miles northwest of Upland, about 40 miles east of downtown Los Angeles. The Montebello quake, by contrast, was only about six miles from the Central City area.

An earthquake produces energy that moves out in all directions in the form of seismic waves. The waves are emitted over a wide range of frequencies, from low frequencies that cause a rolling motion to high frequencies that cause sudden jolts.

As the seismic waves radiate out from the quake, they are “attenuated,” or dampened, by the soils and rocks through which they travel. Thus the ground absorbs the energy of the shock waves, causing them eventually to die out, but “it doesn’t do it evenly over all frequencies,” Jones added.

An earthquake centered nearby “feels jerky because you are feeling all the frequencies,” she said.

The higher frequency waves--the ones that cause the sudden jolts--dampen out quickly as the distance from the epicenter grows.

But the low-frequency waves roll along, and “just as you can hear a low-pitched sound far away, you can feel the low-frequency waves at a greater distance,” Jones said.

The waves also travel through the Earth at different speeds. The fastest wave, called the “P” (primary) wave, arrives first and it usually registers a sharp jolt.

“The P wave tends to be a higher frequency,” Jones said. “It feels more abrupt, but it attenuates very quickly, so if you are far away you often won’t feel the P wave.”

The second waves to arrive are called “S” (secondary) waves, and while these high-frequency waves are slower they are also larger, so they produce a much stronger jolt.

Finally, the low-frequency waves rumble through, causing the ground to roil.

When the Upland quake struck, Jones and her colleagues in Pasadena knew “it had to be quite a distance because it was a long, low rumble and it lasted quite a while.” The fact that it lasted “quite a while” also told the seismologists that it was a substantial quake.

She said she guessed the magnitude, 5.5, and the location “right away.” But she admits she had a little help on the location because a smaller quake had struck Upland three hours earlier and she concluded correctly that it had been a foreshock to the larger quake.

Seismologists can also estimate the distance to the epicenter fairly accurately just by noting how much longer it takes the S waves to arrive after the P waves.

“If you feel the P wave and count the seconds until the S wave,” you can tell how far away the quake is, Jones said. The S wave travels 5 miles per second slower than the P wave, so if the difference is 5 seconds the epicenter is 25 miles away, she added.

“Of course, you have to recognize the P wave,” she said.

As the waves travel through the Earth they interact dramatically with the medium through which they travel. Sand, loose soil and even clay actually amplify the waves.

Someone standing on such soils would feel the energy from the quake far more intensely than someone standing on rock, which transmits the wave very efficiently and thus is jolted less.

The same holds true for structures, which tend to function far better if they are built on rock than if they are constructed on loose soils that are so common throughout the Los Angeles Basin.

Thus people, or structures, the same distance from an earthquake would have a very different experience if they were on different types of soils.

But for someone who happened to be at the same place for more than one quake--such as at home or at work--the primary factor determining how the earthquake “feels” is the distance and the magnitude of the temblor.