concept

P wave

The fastest seismic wave, and therefore the first to reach a station — the P stands for primary. It travels by compressing and rarefying material along its direction of travel, exactly as sound does, so all it needs is something that resists being squeezed. That lets a P wave cross solid rock, molten rock, water and air alike. Its speed depends on both the bulk and shear moduli divided by density, so every internal boundary it crosses bends it a little.