How are seismic waves used to study the Earth's interior?

• A. By analyzing arrival times, speeds, and paths of P-waves and S-waves.
• B. By measuring the Earth's magnetic field.
• C. By observing earthquake aftershocks only.
• D. By analyzing only surface waves.

Answer: (D)

Seismic waves reveal the Earth’s interior because their motion records how the planet’s different layers affect them. By looking at when waves arrive at seismograph stations, how fast they travel, and how their paths bend, scientists can infer changes in material properties with depth. The most informative waves are body waves: P-waves, which compress and expand the material, and S-waves, which shear it. P-waves can move through both solids and liquids, while S-waves cannot travel through liquids, so their presence or absence in a region tells us about liquid layers like the outer core. Changes in speed and direction as these waves cross boundaries map out crust, mantle, outer core, and inner core. Surface waves stay near the Earth’s exterior and mainly reflect near-surface properties, giving limited insight into deep interior. That’s why using arrival times, speeds, and paths of P-waves and S-waves is the best approach to study the interior.