Waves in the Earth

Any Alaskan who lives south of the Brooks Range may encounter a singularly odd happening perhaps once every few years, or far too often in a single year. My experience last fall was typical: sound asleep, near three in the morning, when suddenly Whazzat? I'm wide awake, all systems flooded with adrenaline. Shortly came a familiar rattle-rattle-shake--oh, yeah, just another earthquake.

But why did I wake up before the shaking began?

Many people can report similar experiences. Something catches their attention, but it's not the heaving or quivering of an earthquake. Local residents in a more alert state than I was for last fall's local tremblor spoke of hearing a loud thump or thud-- "Thought a tree fell on the house," as one put it. "Then everything began to shake."

It turns out that what apparently warns us of an impending earthquake is actually the earthquake itself. More properly, our attention is caught by the arrival underfoot of the first of the different types of waves an earthquake can generate.

Nearly all earthquakes large enough for people to feel start when the stress on some portion of the earth's crust exceeds the strength of the rock. When the rock breaks, snapping into a new position, its movement generates vibrations. These vibrations are the seismic waves that put zigzags on seismograms, make dishes dance on shelves, and sometimes collapse buildings.

Not all seismic waves are created equal. First, seismologists divide them into surface waves and body waves. As the names imply, surface waves travel along the earth's surface while body waves go through the earth.

Body waves are further divided into two types, compressional and shear. Compressional waves cause displacements within the earth parallel to the direction in which the wave is traveling. Because they are the swiftest of the seismic waves, they are usually the first ones to reach an observer or an instrument. Thus they are known as primary waves, or P waves for short. (The velocity of body waves depends mostly on depth within the earth. Measured at about ten kilometers--a bit over six miles--below the surface, typical P-wave velocities would lie between 6.0 and 6.5 kilometers (3.6 to 3.9 miles) a second.)

Shear waves displace material at right angles to their path--the way a dog's wagging tail disturbs things to the left and right of the animal's track. They usually have a larger amplitude but travel more slowly than P waves, and thus are likely to reach the surface later. Chronically second in any seismic-wave race, they are called secondary or S waves.

Surface waves arrive after the P and S waves. Their effect can be a rolling motion like that of swells at sea, or a very strong transverse shaking similar to that caused by S waves. Beyond a certain distance from the earthquake source, surface waves cause the strongest vibrations and worst damage in an earthquake.

So what does this description of the one-two-three punch of seismic waves have to do with an apparent wake-up call before an earthquake? Think for a moment again about the nature of those speedy P waves. Even though they usually have the smallest amplitude of the waves generated by a given seismic event, in a strong earthquake they could shake a structure enough to wake sleepers several seconds before the stronger S and surface waves arrived. The difference in amplitude and arrival times between the P and later waves could easily produce that odd effect of waking up for no apparent reason well before an earthquake arrives.

In a smaller earthquake, P waves might pass without being felt--but that doesn't necessarily mean they would go unnoticed. Even if humans can't directly sense the compression waves hastening through, they can hear some of the shiftings and shufflings they cause. Buildings, for example, can creak and groan noticeably in reaction to P-wave vibrations people don't notice.

This effect is what leads people to claim they heard an earthquake coming, because they did indeed hear something before they felt any shaking. But what they heard was only an echo of the speedy P wave--in effect, what triggers their awareness was really the sound of an earthquake going.