Seismic P-waves. Cutaway illustration of a section through the Earth, showing seismic waves propagating around the globe. Seismic waves (P-waves and S-waves) are waves of energy that travel through the Earth's layers as a result of an earthquake, explosion or volcano. P-waves (pressure waves or primary waves) are the first to arrive at a seismograph. They travel through the fluid layers of the Earth's interior, and are refracted slightly when they pass through the transition between the semi-solid mantle and the liquid outer core. As a result, there is a P-wave "shadow zone" between approximately 105 and 142 degrees from the earthquake's focus, where the initial P-waves are not registered.