Earthquakes generate compressional waves (p-waves) and shear waves (s-waves). Compared to the speed of shear waves in a given earth material, the speed of compressional waves is the fastest.
Because of this the compressional waves are the first waves to arrive on a complete record of ground shaking. They are also called P waves, which derives from this fact - P is an abbreviation for primary, first wave to arrive.They typically travel at speeds between ~1 and ~14 km/sec.
