Which of the following causes particles to move back and forth in the direction the waves are traveling?

• A. S waves
• B. P waves
• C. Surface waves
• D. T waves

Answer: (B)

When a wave causes the medium to be compressed and stretched along the path it travels, the particles move back and forth in the same direction as the wave. That compressional, longitudinal motion is characteristic of P waves. They push and pull particles in the direction the wave is moving, creating alternating regions of compression and rarefaction that carry the wave forward.

S waves move particles perpendicular to the direction of travel, so they don’t show this back-and-forth motion along the travel direction. Surface waves involve motion near the surface and are typically more complex, often elliptical. T waves aren’t a standard seismic category. So the description matches P waves.