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. 1 to 12 We can plot earthquake intensity by gathering reports from observers. Although the reports will be subjective, and vary somewhat, most observers will agree on the intensity criteria, for example, feeling the quake while driving. For very strong quakes, damage provides fairly objective measures of intensity. Soils here are very shallow - usually less than a meter to bedrock.