A seismic wave is an elastic wave generated by an impulse such as an earthquake or an explosion. Seismic waves may travel either along or near the Earth's surface (Rayleigh and Love waves) or through the Earth's interior (P waves and S waves). The velocity (speed and direction) of seismic waves varies according to the material (type of rock, molten core, or oil) through which they pass. Primary (P) and secondary (S) waves are transmitted by the solid Earth. P waves vibrate in the direction that they are advancing; S waves vibrate at right-angles to the direction in which they are advancing. Only P waves are transmitted through fluid (liquid or gas) zones. See also seismology.