Chapter 8: Three Prototype Waves

Abstract

Seismic waves are mechanical waves, meaning that these waves involve the actual motion of the rock particles. Seismic waves that travel through the entire body of the rock are called body waves. In contrast, seismic surface waves travel close to the surface rather than through the rock. Seismic body waves are of two fundamental types: longitudinal and transverse. In a longitudinal wave, the oscillating particles of the medium are displaced parallel to the direction of propagation (i.e., the direction of energy transmission) of the wave. In a transverse wave, the particles are displaced in a direction perpendicular to the propagation direction. When a steel rod is struck on one end by a hammer, a wave pulse in the form of a longitudinal compression of the rod travels down its length. If the rod is struck periodically, a succession of such pulses, known as a wave train, travels down the rod. Sound propagates through the air as longitudinal waves. In contrast, the familiar water waves that come from the point where a stone is dropped into a quiet pond are transverse waves. Furthermore, because these waves are confined to the water layer close to the surface, they are surface waves rather than body waves. Importantly, various types of seismic surface waves also can be identified on seismograms. Surface waves usually travel more slowly and have larger amplitudes and longer wavelengths than body waves.