In seismology, several types of surface waves are encountered. Surface waves, in this mechanical sense, are commonly known as either "Love waves" (L waves) or "Rayleigh waves". A seismic wave is a wave that "travels through the Earth, often as the result of an earthquake or explosion." Love waves have transverse motion (movement is perpendicular to the direction of travel, like light waves), whereas Rayleigh waves have both longitudinal (movement parallel to the direction of travel, like sound waves) and transverse motion. Seismic waves are studied by seismologists and measured by a seismograph or seismometer. Surface waves span a wide frequency range, and the period of waves that are most damaging is usually 10 seconds or longer. Surface waves can travel around the globe many times from the largest earthquakes. Surface waves are caused when P waves and S waves come to the surface.