P-wave, or primary wave, is one of the two main types of seismic waves produced by an earthquake. It is the fastest seismic wave and usually the first to be detected by seismographs. P-waves are compressional waves that travel through solids, liquids, and gases. They cause particles in the material through which they are passing to move back and forth in the direction that the wave is traveling. P-waves are capable of traveling through the Earth's interior and can pass through both liquid and solid layers.
