Which statement correctly describes S waves generated by an earthquake?

• A. S waves travel slower than P waves and shear rock at right angles to the travel direction
• B. S waves travel faster than P waves and compress rock in the travel direction
• C. S waves travel at the same speed as P waves and do not deform rocks
• D. S waves are purely horizontal motion with no shear

Answer: (A)

S waves are shear, or transverse, seismic waves. They move rock by deforming it perpendicular to the direction the wave is traveling, producing motion that is sideways (or up-and-down) in the plane at right angles to the travel path. Because the material’s resistance to shear is typically less than its resistance to compression, S waves travel slower than P waves. They also can only move through solids, since liquids cannot support shear stresses.

The statement that best describes S waves is that they travel slower than P waves and shear rock at right angles to the travel direction. The other options mix up the motion and speed: P waves push and compress rocks along the travel direction, S waves are not as fast as P waves, S waves do deform rocks via shear, and their motion is not purely horizontal with no shear.