Earthquake Illusions

One of the most perplexing aspects of observational seismology is trying to reconcile eyewitness reports of earthquake phenomena with physical factors which seem to render them impossible.

The most common example is the "I heard it coming" report. Most often, these take a variation of the theme: "I heard the earthquake coming several seconds before it hit the house" or "I heard it go by" (words to that effect).

Reports of this type are so common, and rendered with such conviction, that they cannot be dismissed as mere illusion. The problem is, they cannot be rationally explained. The fastest (and therefore, first arriving) earthquake waves travel in the earth at about 5 miles (8 km) per second. Sound waves in the air travel at only about 0.2 miles per second. Therefore, the earthquake waves must reach the observer before any sound wave it might create. Further, it would be almost impossible for anyone to track, by means of sound alone, a wave which is traveling at 5 miles per second.

There are two types of waves which are discernible in most felt earthquakes. The first to arrive is almost always the weaker and may not be felt, but may provide the "sound" for the larger wave which follows seconds later. This appears to be reasonable explanation of why people sometimes think they hear earthquakes coming. It still does not explain the illusion of hearing them move across fields, city blocks, etc., but the total "wave train" is a few to many miles long, and the successive arrivals of different wave fronts may give this impression.

Even seasoned professionals may be fooled. A noted California seismologist was inspecting equipment in his seismographic vault several years ago when a fairly strong earthquake occurred. He later reported that, while the shaking was going on, he plainly observed waves an inch or so high spread several inches apart traveling across the floor of the vault. The floors of seismographic vaults are massive affairs of reinforced concrete, plainly not adapted to this kind of flexibility. After the earthquake had passed, he closely inspected the floor and could find no evidence of cracking of any kind, but he still maintained that he saw what he saw.

While surface waves are commonly seen in the open during large earthquakes (these are not the primary earthquake waves, but a secondary effect rather like the shaking of bowl full of jello), waves such as those reported by the seismologist are clearly impossible.

Or are they?