What kind of waves travel at a velocity slightly less than shear waves and have a propagation mode that is both longitudinal and transverse with respect to the surface?

• A. Rayleigh waves
• B. Transverse waves
• C. L waves
• D. Longitudinal waves

Answer: (A)

Rayleigh waves are surface waves that travel along the material’s surface with particle motion in a vertical plane that contains the direction of travel. This motion is elliptical and decays with depth, so near the surface the displacement has both longitudinal (along the surface) and transverse (normal to the surface) components. Because of this mixed motion, the wave’s propagation mode involves both P and S type motion relative to the surface, and its speed is slightly less than the shear (S) wave velocity in the material—typically about 0.9 of Vs. That combination—surface confinement, mixed longitudinal/transverse motion at the surface, and a velocity just below Vs—fits Rayleigh waves exactly.

The other options don’t match: transverse (shear) waves are bulk waves with motion perpendicular to the direction of travel and don’t travel along the surface; Love waves (often labeled as L waves) are surface waves with horizontal shear polarization and do not exhibit the same vertical–horizontal elliptical motion; longitudinal (P) waves move the particles parallel to the direction of travel and travel faster than S waves, not as a surface-confined mode.