## Seismic Wave Behavior: Curving paths through the Earth

Novice Chinese

### Why do seismic waves travel a curving path through the Earth?

If there were no changes with depth, seismic waves would travel a straight path to the other side of the Earth. But wave arrivals to distant seismic stations have taught us that there are layers. Seismic waves travel through the Earth and follow the same laws of refraction and reflection as any other wave at interfaces. When they encounter boundaries between different media, the waves react according to Snell’s law, and the angle of refraction across the boundary will depend on the velocity of the second media relative to the first.

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#### Keypoints:

• The Earth has boundary changes deep within
• Seismic waves are refracted and reflected at the boundaries and return to the surface
• The boundaries were determined by seismic wave arrivals to distant stations

## Related Animations

Seismic shadow zones have taught us much about the inside of the earth. This shows how P waves travel through solids and liquids, but S waves are stopped by the liquid outer core.

Animation Novice

The wave properties of light are used as an analogy to help us understand seismic-wave behavior.

Animation Novice

The shadow zone is the area of the earth from angular distances of 104 to 140 degrees from a given earthquake that does not receive any direct P waves. The different phases show how the initial P wave changes when encountering boundaries in the Earth.

Animation Novice

The shadow zone results from S waves being stopped entirely by the liquid core. Three different S-wave phases show how the initial S wave is stopped (damped), or how it changes when encountering boundaries in the Earth.

Animation Novice

Seismic waves travel a curving path through the earth due to changes in composition, pressure, and temperature within the layers of the Earth.

Animation Novice

Seismic waves travel at different speeds through different materials. In this 2-layer model two wave fronts leave an impact at the same time but the lower layer is faster.

Animation Novice

Seismic waves travel a curving path through the earth due to changes in composition, pressure, and temperature within the layers of the Earth.

Animation Novice

Animation shows the race between the direct seismic wave vs. the deeper, longer-path critically refracted seismic wave. Graph records the arrival times.

Animation Novice

In this model of increasing velocity with depth, the critically refracted seismic rays speed up with depth as they pass 5 different velocity boundaries.

Animation Intermediate

In this model of increasing velocity with depth, the critically refracted seismic rays speed up with depth as they pass 5 different velocity boundaries.

Animation Intermediate
Animation Intermediate

## Related Lessons

Students work first in small groups, and then as a whole class to compare predicted seismic wave travel times, generated by students from a scaled Earth model, to observed seismic data from a recent earthquakes. This activity uses models, real data and emphasizes the process of science.

Lesson Novice

Like other waves, seismic waves obey the laws of physics. In this activity Physics students have the opportunity to apply their understanding of the basic concepts of waves (e.g. reflection, refraction and transmission of energy) as they examine seismic data to determine how far it is from the surface to the bedrock.

Lesson Intermediate

## Related Software-Web-Apps

Easily view seismograms from stations around the world for large earthquakes. Plots can be used for a variety of activities including to determine the diameter of Earth’s outer core as part of a classroom exercise.
Software-Web-App Novice
Easily view seismograms from stations around the world for large earthquakes. Plots can be used for a variety of activities including to determine the diameter of Earth’s outer core as part of a classroom exercise.
Software-Web-App Novice

Seismic Waves is a browser-based tool to visualize the propagation of seismic waves from historic earthquakes through Earth’s interior and around its surface. Easy-to-use controls speed-up, slow-down, or reverse the wave propagation. By carefully examining these seismic wave fronts and their propagation, the Seismic Waves tool illustrates how earthquakes can provide evidence that allows us to infer Earth’s interior structure.

Software-Web-App Novice

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