Seismic waves are elastic waves that travel through Earth. Surface Waves P waves travel fastest and thus are the first waves to arrive. They are sound waves that travel through Earth. They travel slower than P waves but are usually larger than P waves. Surface waves travel along Earth's surface and are the largest waves observed at distance from shallow earthquakes.
P waves transmit a strain that squeezes or expands a rock in the direction that the wave is moving.
S waves transmit a strain that shears the rock in a direction perpendicular to the direction that the wave is moving. To record seismic waves, seismologists operate a global network of seismic recording systems. The recording system records the vibrations picked up by the seismometer. It may save the record to a sheet of paper or directly to a computer disk. A plot of a ground motion with time varying along the horizontal axis is called a seismogram .