Our treatment to this point has been limited to body waves, solutions to the seismic wave equation that exist in whole spaces. However, when free surfaces exist in a medium, other solutions are possible and are given the name surface waves. There are two types of surface waves that propagate along Earth's surface: Rayleigh waves and Love waves. For laterally homogeneous models, Rayleigh waves are radially polarized (P/SV) and exist at any free surface, whereas Love waves are transversely polarized and require some velocity increase with depth (or a spherical geometry). Surface waves are generally the strongest arrivals recorded at teleseismic distances and they provide some of the best constraints on Earth's shallow structure and low-frequency source properties. They differ from body waves in many respects – they travel more slowly, their amplitude decay with range is generally much less, and their velocities are strongly frequency dependent. Surface waves from large earthquakes are observable for many hours, during which time they circle the Earth multiple times. Constructive interference among these orbiting surface waves, together with analogous reverberations of body waves, form the normal modes, or free oscillations of the Earth. Surface waves and normal modes are generally observed at periods longer than about 10 s, in contrast to the much shorter periods seen in many body wave observations.

Love waves

Love waves are formed through the constructive interference of high-order SH surface multiples (i.e., SSS, SSSS, SSSSS, etc.).