Earthquakes produce two types of potentially destructive waves that move through the earth from the point of the fault: primary, or pressure waves and secondary, or shear waves. Primary waves, also called P waves, exert a force of compression and travel through rock at speeds that can exceed 225 mph. Secondary waves, also called S waves, exert a shearing force and travel only half as fast as P waves, but are capable of causing much greater damage when they reach the surface.
When secondary seismic waves reach the surface, they can have different characteristics and will move the ground in a direction perpendicular to their direction. Named after A.E.H. Love, the British mathematician who first described them, Love waves are S waves that cause horizontal shearing, which results in the ground moving from side to side. Rayleigh waves, named after John William Strutt, Lord Rayleigh, who first predicted them, move the ground in the manner of an ocean wave. These are S waves that cause the ground to roll over and under when they reach the surface.
An earthquake occurring under the sea floor can cause a potentially destructive ocean wave called a tsunami. A shift in the ocean floor causes a displacement of water and a transfer of energy that can produce a potentially forceful and highly destructive tidal wave when it reaches a shoreline. As a tsunami approaches the shallower depths near land, the horizontal force is transferred upward and can cause the wave to reach heights of 100 feet or more.