There are many ways scientists can gather evidence to learn about Earth's interior including direct evidence from rock samples and indirect evidence from seismic waves.
Evidence #4 - Seismic Waves from Earthquakes
The structure of Earth's deep interior cannot be studied directly. But geologists use seismic (earthquake) waves to determine the depths of Earth's layers. Earthquakes happen to create different kinds of waves when they occur and they behave differently when they interact with different layers of Earth that scientists can measure. Seismic stations located all around on Earth's surface detect and record the strengths of these different seismic waves from earthquakes. Geologists use this data to understand the structure of Earth's much like how an X-ray can "see" the structure of your bones without physically going inside your body.
Refraction
All types of waves change speed and direction when they pass through different materials including seismic waves. This phenomenon is called refraction. That is what makes light focus in a magnifying glass, and refraction is also what makes seismic waves travel in curved paths through the Earth. The result of this behavior, in combination with the fact that S-waves cannot travel through liquids help scientists make reliable inferences about the the internal structure and layers of the Earth.
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Summary of Seismic Waves and Earth's Internal Structure
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Topic Tips
- Seismic waves provide indirect evidence about the internal structure of the Earth. As P-waves and S-waves are created by earthquakes they travel through the Earth (or not) and they change speed and direction (refract). This information provides valuable clues about the various layers of the Earth.