Susan Holmes, age 12, of Phoenix, Arizona, for her question:

Is it true that earthquake. waves cannot pass through fluids?

The answer to this question is a double one. Some earthquake vibrations cannot pass through fluids, but others can. This double answer may sound complex. In fact, it is almost as complex as the earthquake waves themselves.

Beginning from an older word for earthquake, which is borrowed from another older word meaning shake. Seismology, naturally, is the science of. earthquakes and a seismologist uses a seismograph instrument to gather information on the shuddering shivers that go on in the earth. A major earthquake sends shivering vibrations in and through and around the earth. These are called seismic waves. And seismic waves behave in very strange ways. Besides, there are several different kinds and each type of seismic wave has a behavior pattern of its own.

Sound waves are pulses of energy passing through surrounding particles. Seismic waves, like sound waves, also vibrate through surrounding particles. Sound waves spread out in all directions from a center, such as a beating drum. They travel at about 1,100 feet a second through the skimpy particles of air, four times faster through the denser particles of water and still faster through densely packed particles of steel. Seismic waves are started by massive earth movements at the core of the earthquake, which may be miles underground. Some types behave like the sound waves, radiating out from the core and passing through solids, liquids and air. But there are other types of seismic waves which do not follow these basic rules.

The three main types of earthquake vibration are called P waves, S waves and L waves.'. Each has its own story to tell the seismologist. The P stands for the primary waves that push out in all directions from the core of disturbance. They move through solids, liquids and gases. The first P wave to reach the surface pinpoints the epicenter of the earthquake at ground level. These P waves bring enormous amounts of energy to the surface. Some of this energy starts L waves that fan out around the earth's crust. L stands for the long wave lengths of these leisurely vibrations. There are at least three types of L waves that spread outward, on or just below ground level. seismic vibrations are the S, alias the secondary or transverse waves. Like the P waves, they start at the deep core of the quake. But S waves shake and vibrate the surrounding particles from side to side. You can demonstrate this motion with a skipping rope. Tie one end and shake the other end up and down. The rope forms waves at right angles to a line between its two ends. These strange transverse waves can pass through rigid solids but not through liquids or gases. When they reach the surface, their energy joins the L waves already spreading through the curved crust.

The different seismic waves travel at different speeds, some in different directions and some through different substances. The seismograph keeps a continuing written record of them all. Each can give a bit of evidence about the kind of material through which it passed. The evidence from different seismic waves is pieced together to give a picture of the inner, unreachable depths of our planet.