Virginia Germino asks, "Can anyone tell us the source of the noise of our earthquake? The sound here was very like that of tons of snow sliding off a steep roof, but that didn't happen, so what did cause the noise?"

Al Weed says, "Actually, I thought it was the snow sliding off the roof, but realized as it was happening that the snow had already melted so it must be something else. The window in my office overlooks some lightweight cables that hold up grapes on the adjoining trellis and they vibrated like guitar strings for about 30 seconds after the building settled down."

Well, the sound that Virginia and Al heard was much like the sound I heard, so I thought I would inquire. Here is what Virginia Tech's Martin Chapman has to add to Larry Gedney's description back in 1986:

George:

The article by Dr. Gedney does a good job of explaining earthquake sounds [See below]. We often hear a sound wave that has traveled most of the way from the focus of the earthquake to the listener through the solid rock deep in the Earth, and the remaining tiny fraction of the entire path as an ordinary sound wave in the air. The seismic P wave (sound wave in rock) converts to an ordinary acoustic wave (sound wave in air) at the ground surface near the listener. These waves are followed by the slower traveling, but more powerful seismic shear waves in the ground that are more strongly felt and may result in various types of earthquake damage.

I can add that geological circumstances and the distance from the earthquake to the listener play a large role in determining the nature of the sounds. Small, shallow focus earthquakes in regions with very hard rock near the ground surface are often experienced more by their noise than by their shaking. This is the case with many of the smaller earthquakes that have occurred in central Virginia, as well as in other regions of eastern North America. The closer the listener is to the earthquake epicenter, the louder and sharper the noise will be. Very near the epicenter, the earthquake may be heard as a very loud, sharp, explosive sound. At larger distances, the sound may resemble the rumbling of thunder. As the shock becomes larger in magnitude, the shaking and rattling of a building and its contents adds to the noise.

Martin Chapman (electronic mail, December 14, 2003)

"Probably the most common report of earthquake phenomena that the Geophysical Institute receives is, "I heard it coming," or "I heard it before it hit." This is also the most difficult phenomenon to explain, because it is physically impossible.

Sound travels in air at about 1100 feet per second. The fastest waves to leave the source of an earthquake travel at a speed nearly twenty times that. The earthquake waves must therefore reach an observer before any air wave that might have been generated at the source. The catch is that the first-arriving earthquake wave is a type that is often too small to be felt. In other words, the earthquake has already arrived when the observer thinks he or she "hears it coming. The larger waves that are felt follow.

There are two types of waves that radiate outward from the source of an earthquake and travel within the earth (there are also types that travel along the surface, but we won't be concerned with those). The waves that travel the fastest and so arrive first are called P-waves. Particle motion in a P-wave is back-and-forth along the direction of propagation. It might be likened to striking a long iron bar on the end with a hammer and picking up the vibration on the other end. This is the same type of particle motion that occurs in a sound wave traveling through the air. Thus, the P-wave is actually a sound wave that travels within the earth.

The second wave to arrive, and nearly always the stronger one, is called the S-wave. Particle motion in an S-wave is like that along a rope that has been tied to a post at one end and given a shake at the other. The vibration is sideways, or transverse to the direction of propagation of the wave along the rope.

In very strong earthquakes, both waves can be felt, but during smaller events, commonly only the S-wave is felt. Observers think that they have "heard it coming," when the P-wave has already passed them by.

A test of this perception was recently performed in southern California by a team of U.S. Geological Survey personnel headed by David Hill. In a very active segment of the Imperial fault system near the Mexican border, they suspended a microphone several feet in the air and recorded its signals on a tape recorder simultaneously with those from a seismometer buried in the ground.

At night, when it was very still, they recorded three small earthquakes between magnitudes 2.0 and 3.0. An operator at the site reported that he did, indeed, hear the earthquakes before he felt them.

On playing back the data, it was found that the noise recorded by the microphone did coincide with the arrival of the P-wave recorded by the seismometer. (In fact, it actually followed it by a few hundredths of a second, but this was because the sound still had to travel upward from the ground to the microphone.) Strangely, there was no sound recorded when the stronger S-wave arrived a couple of seconds later, but this was probably because the S-wave vibrates more slowly, and its frequency was out of the audio range.

This test was significant because it was the first recording of earthquake sounds that came solely from the earth. Prior to this, existing sound recordings of earthquakes had all been made by happenstance inside buildings, which are the main source of noise during earthquakes. To my knowledge, this is the only documented case to explain the "heard-before- felt" phenomenon which demonstrates something that seismologists have been saying all along, but few people who have been through earthquakes are willing to accept." (Larry Gedney, Earthquake Waves Outrace Sound, Article # 761, Alaska Science Forum, March 24, 1986).