Earthquake Waves Outrace Sound

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.