SEISMIC SLINKY

A Slinky makes a handy model of earthquake waves. See for yourself which waves may send you rocking and rolling, and which ones may bounce you out of your chair.

Before this activity, you may want to read about waves in the earth. You can also see illustrations of the motion of P and S waves and the motion of Rayleigh and Love waves.

What do I need?
• A Slinky (two if you have them)
• A partner

Assembly
Stretch the Slinky 6 feet (2 meters) or more between your partner and yourself. Or, tie one end of the Slinky to a chair or other solid object 6 feet away.

What do I do?
1. Pull the Slinky toward you a bit and then push it away. Notice that a wave travels along the Slinky from you to your partner.

Photo: Lily Rodriguez

The vibrating parts of the Slinky move back and forth in the same direction as the wave is traveling. This type of wave is called a longitudinal wave, or a compression wave, and it’s a model for seismic primary waves, or P waves. They’re known as primary waves because they’re the fastest of the earthquake waves, arriving first at distant points.

2. Shake one end of the Slinky up and down. Notice that a different type of wave travels along the Slinky.

This time, sections of the Slinky move up and down, perpendicular to the direction in which the wave is traveling. This type of wave is called a transverse wave. It provides a model for S waves, the secondary seismic waves that travel through solid rock. (S waves can’t travel through liquid or gas.) More specifically, this models SV waves—S waves in which the vibratory motion is vertical.

Photo: Lily Rodriguez

You can also shake the Slinky from side to side. In this case, sections of the Slinky will move horizontally—but, as above, the movement of the Slinky is at right angles to the direction of the progressing wave. S waves that behave in this manner are called horizontal S waves or SH waves.

S waves and P waves are known as body waves. They transmit energy through the earth in all directions from the earthquake’s focus—the place underground where the temblor began.

Photo: Lily Rodriguez

3. Push and pull the Slinky at the same time that you move it up and down; your hand should move in a circle.

This circular-motion wave models an earthquake wave called a Rayleigh wave. Rayleigh waves transmit energy from the epicenter—the point on the surface that’s above the focus—along the earth’s surface.

4. Hold two Slinkys, one above the other. Move them both from side to side at the same time—but move the top Slinky further in each direction than the bottom Slinky. This is a model of a Love wave. Like the S wave, a Love wave is a transverse wave, but like the Rayleigh wave, it’s confined to the earth’s surface. The bottom Slinky illustrates that the energy of a surface wave quickly diminishes with depth.

What's going on?
An earthquake begins when stress on large blocks of rock becomes greater than the strength of the rock. The rock fractures, releasing vast amounts of energy. This energy is carried outward in all directions by various seismic waves, some of which can reach the opposite side of the earth in about twenty minutes. The further the waves travel from the focus of the earthquake, the weaker they become.

P waves push and pull the underground rocks, causing structures on the surface to move back and forth. SH waves move the rocks beneath the earth’s surface from side to side, giving buildings on the surface a good shaking, often with very damaging effects. With SV waves, the shaking is in a vertical direction—which sometimes can be enough to launch you out of your seat.

Surface waves are long, slow waves. Love waves shake things from side to side. The slowest seismic waves, Rayleigh waves, are rolling waves that make you feel as if you’re struggling to keep your balance on a ship in the open ocean.