Which statement about S waves is true?

• A. They shear rock perpendicular to the travel direction and travel slower than P waves
• B. They compress rocks along the travel direction and travel faster than P waves
• C. They travel faster than P waves in all materials
• D. They move only through liquids

Answer: (A)

S waves are the transverse, or shear, component of seismic energy. They move ground motion perpendicular to the direction the wave is traveling, because they propagate by twisting the rock rather than compressing it along the travel path. This shear motion is possible only in solids, where the material has shear rigidity. Since P waves are compressional and move by pushing and pulling along the travel direction, they ride on the stiffer response of the material and thus travel faster. The relation vp = sqrt((K + 4/3 G)/ρ) and vs = sqrt(G/ρ) shows that the P-wave speed is greater than the S-wave speed in real materials, because K + 4/3 G > G. Additionally, liquids do not have shear rigidity, so S waves cannot propagate through liquids. Therefore, the statement that S waves shear rock perpendicular to the travel direction and travel slower than P waves is correct.