S waves or secondary waves can travel only through solid materials, not P waves or primary waves. P waves (or Primary waves) are the first waves to arrive at a seismograph. The P waves are the fastest seismic waves and can move via solid, liquid, or gas. They are similar to sound waves. They leave behind a trail of rarefactions and compressions on the medium they move through. The P waves are also known as pressure waves for this reason. Certain animals, like dogs, can feel the P waves much before an earthquake hits the crust (i.e. surface waves arrive). The humans can only feel the ramifications it has on the crust.