The fastest type of seismic wave that travels through Earth’s interior is the P-wave.
“P” means primary because these waves usually arrive first at a seismograph after an earthquake. It can also refer to pressure: P-waves move rock particles back and forth in the same direction the wave travels, producing alternating compressions and rarefactions.
Unlike S-waves, P-waves can pass through solids, liquids, and gases. That property is especially important in studying Earth’s interior: S-waves cannot travel through the liquid outer core, but P-waves can cross it, although they refract and change speed. Their paths helped seismologists infer Earth’s layered structure.
P-wave speed depends on the material’s stiffness, compressibility, and density. Typical earthquake P-waves travel at about 5–8 km/s, with speeds reaching roughly 13.5 km/s in the lower mantle. S-waves are slower and involve shearing motion, while Rayleigh and Love waves travel along the surface and generally arrive later, though surface waves can produce stronger shaking and damage.