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Overview of Seismic Waves
Seismic waves are caused most often by movements in the Earth's crust, but explosions, asteroid impacts and other geological or human-caused events can also cause seismic waves. Seismic waves are classified into two categories: body waves and surface waves. Body waves are seismic waves that travel deep through the interior of the Earth. Surface waves are seismic waves that travel through oceans and the first few kilometers of the Earth's crust. There are two types of body waves: p-waves and s-waves. There are also two types of surface waves: Love waves and Rayleigh waves.
P-Waves
P-waves, or primary waves, are longitudinal waves (meaning that they move material -- earth and stone -- in the same direction as they travel, the direction of propagation). They are also called compression waves because they push and pull material as they move. They are the fastest type of seismic waves and are, therefore, the first ones to be detected by seismographs. When p-waves travel through air they become sound waves; however, p-waves actually travel faster through solids (earth and stone) than they do through fluids (water and air). These waves are also less destructive, usually only felt as a bump or rattle during an earthquake.
S-Waves
S-waves, or secondary waves, are transverse waves (meaning that they move material -- earth and stone -- perpendicular to the direction that they travel). So, these waves move earth up and down and side-to-side as they move. They are slower than p-waves and so are detected second. However, these waves cause more damage than p-waves because they are shearing waves; they cause the ground to rapidly split and move in different directions. Also, s-waves only move through solids because fluids (water and air) do not respond to shearing forces.
Rayleigh Waves
Rayleigh waves, also called lamb waves or ground roll, are the first type of surface wave. These waves move earth in a rippling motion, meaning they radiate outward from the epicenter and lift material up and down and side-to-side as they move, similar to the way water ripples. They are named for John William Strutt, Lord Rayleigh, the mathematician who first developed the formula describing the waves in 1885. Rayleigh waves move slower than body waves, but because they are much more shallow and have larger amplitudes, they cause more damage than body waves.
Love Waves
Love waves, or L-waves, are the second type of surface wave. These are surface shearing waves that radiate outward from the epicenter and cause material to rapidly split and move in different directions, similar to s-waves. They are named for A.E.H. Love, the British mathematician who first predicted the waves and described them mathematically in 1911. Love waves are slower than body waves, but faster than Rayleigh waves and have larger amplitudes. They are the most destructive to structures because they cause large splits and shears in the ground that destroy buildings and bridges.