Earthquakes shake the ground because fault rupture releases vibrations that radiate in the form of seismic energy. These earthquake waves, also called body waves, come in two distinct forms: Primary or "P" waves and Secondary or "S" waves. When body waves reach the free surface of the earth some of their energy is converted into complex surface waves that are trapped near the surface of the earth and produce generally lower frequency ground motions.
P waves are compressive waves that do not produce much damage. They can move through any type of material and travel at almost twice the speed of S waves. High frequency P waves due not weaken or attenuate as rapidly as S waves so they retain higher frequencies when they arrive at seismic stations. In air, P waves take the form of sound waves and therefor move at the speed of sound, 330m/s at sea level. Some people even report hearing an earthquake (due to the higher frequency P waves vibrating or rustling objects) before they feel the S waves arrival. Typical speeds in Earth are faster: 1450m/s in water and 5000m/s in granite.
S waves are shear waves that deform the ground perpendicular to their direction of travel. Unlike P waves, S waves are unable to pass through air and liquids such as water and magma. This is how we know Earth’s outer core is molten – these waves cannot pass through it! When S waves deform the ground, it causes lateral or shear (back and forth) forces on structures. Older buildings were constructed primarily to withstand gravity (vertical forces); therefore they are more prone to fail due to the strong lateral loading experienced during a big earthquake.
- Both P and S waves are generated across a broad spectrum of frequencies. The higher the frequency, the faster the energy from the earthquake attenuates, or dissipates, with distance. Also, due to attenuation and geometrical spreading, locations close to the source of the rupture that caused the earthquake will receive more energy (and shaking) than more distant locations.
- When P and S waves arrive at the surface of the Earth, part of the energy is trapped and guided by Earth's surface. Their behavior is different than for body waves, so we say they are converted to Surface Waves. The two types are Love waves and Rayleigh waves.
Rayleigh waves produce a long rolling motion along the earth's surface much like the motion in a boat on the open sea. They travel a little slower than Love waves.