Seismic energy is transmitted by two different types of waves - body waves that travel through the Earth and surface waves that travel along or just below the ground surface. There are several varieties of each of these types that behave differently and have very different effects on geological materials and manmade structures.
P-waves or primary waves are the fastest seismic waves and can travel through solids, liquids and gases. This type of wave is a compressional wave (like a sound wave) and the motion of an individual particle, while the wave passes through a material, is forward and back in the same direction as the wave is traveling.
S-waves
or secondary waves, are slower than P-waves and can only travel through
solids. This type of wave is a shear wave (another
's' word like the name of the wave type) where the motion of a particle is perpendicular
to the direction of travel of the wave itself. Here is how to create a shear
wave:
Take a 6 foot piece of rope and have a friend hold one end while you stand 5-6 feet away holding the other end. With a little slack in the rope, wiggle your wrist back and forth fairly rapidly. You should see a wave form move down the rope toward your friend as long as you are moving your hand. This S-wave that you are creating is moving through the solid material (the rope) and is transmitting energy from you to your friend. HOWEVER, any individual segment of the rope is only really moving left and right (or maybe up and down depending on which way you were wiggling).
Eventually the body waves reach the surface and both 'interact' with manmade structures and generate surface waves.
Rayleigh (R) waves are very similar to water waves - a piece of the ground surface moves in a vertical, elliptical path parallel to the direction of wave movement.
Love (L) waves are shear waves where the shearing (back and forth) motion, is confined to a horizontal plane at the Earth's surface.
OK, look up from your computer screen at the room you are sitting in. How big is it? Fifteen feet square? Whatever. Now imagine each of the above wave types with a wave length of say 10-20 feet passing through your room.
The P-wave would try to have the room oscillate in size from 10 to 20 feet with the mean 15 feet.
The S-wave would have the back wall moving down while the front wall was moving up.
The R-wave would have the floor rolling like a big wave out on Lake Mendota during a storm.
The L-wave would have the back wall going left while the front wall was going right.
OUCH!! Close to the epicenter of a quake you might experience several of these types of motion simultaneously!
Is it any wonder that buildings fall down under this kind of assault?