Contrary to intuition, an earthquake does not make the pendulum swing. Instead, the pendulum remains fixed as the ground moves beneath it. The boundary between the two types of behavior is the natural period of the pendulum. Since earthquake vibrations can have periods of many seconds, we need a pendulum with a very long period. We can construct a pendulum with a very long arm, or we can build a compact instrument by building a horizontal pendulum.
Material expands and contracts in volume and particles move back and forth in the path of the wave.
P-waves are essentially sound waves and travel through solids, liquids or gases. Material does not change volume but shears out of shape and snaps back. Particle motion is at right angles to the path of the wave. Surface Waves Several types, travel along the earth's surface or on layer boundaries in the earth. The slowest waves but the ones that do the damage in large earthquakes.