Which method is used to study the Earth's deep interior?

• A. Seismic tomography
• B. Gravity measurements
• C. Magnetic sounding
• D. Seismic waves

Answer: (D)

Studying Earth's deep interior hinges on how waves move through its layers. Seismic waves produced by earthquakes travel through all the planet’s layers, and their speeds, paths, and how they split or fade depend on the material they traverse. By recording when these waves arrive at seismometers around the world, scientists infer the layering and properties of the interior, from crust to mantle to core. The behavior of different wave types is key: S-waves cannot travel through liquids, while P-waves slow, refract, or change direction in different materials, creating shadow zones that reveal a liquid outer core and a solid inner core. Seismic tomography builds detailed 3D images from many seismic-wave measurements, but it fundamentally relies on the information carried by seismic waves themselves. Other methods like gravity measurements or magnetic sounding provide additional clues, but seismic waves are the primary tool for mapping the deep interior.