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

## Resources

Background
Activities
• Determine Shallow Earth Structure
Vocabulary
Related Animations
• Travel Time Curves
• 3 Component Seismograph
• Multi-Station Seismograph Station

### Refraction

• For background on this animation series, download Background from the Resources box.

• Animations are available for preview in embedded YouTube.

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## Travel times through different media

This animation shows what happens to seismic waves as they encounter a boundary between a slower and faster layer in the Earth with introduction to Snell’s Law. (narrated)

Quicktime Animation (4 MB)

## Seismic Sprint—race to the seismometer

The animation on the left describes the relative speeds of the direct, critically refracted and head waves. It highlights how the wave that arrives first changes depending on station distance from the source. (narrated)

Quicktime Animation (1 MB)

## Seismic Sprint—graphing the seismograms

Using the same seismic ray paths, the animation below adds a graph to see how data recorded show the distance transition of first arrival wave paths. This process is explained in more depth in activity,How Shallow Earth Structure is Determined. (no audio)

Quicktime Animation (3 MB)

## Direct ray races 5 different refracted rays

This animation expands Refraction 3 to include 4 more seismic paths that migrate downward to be critically refracted off of increasingly higher-velocity boundaries showing that even though the deepest path is the longest, the transit time is less than the direct wave that travels entirely in the slow layer. AUDIO ARRIVAL SOUNDS.

Quicktime Animation (2 MB)

## Curving seismic paths through the Earth

Here we offer a simplified answer to the question posed in the title. The processes of refraction and reflection are applied to the whole earth. (narrated)

Quicktime Animation (3 MB)

## Comparative ray paths in two-layer model

The animation shows wave behavior and relative velocities in a layered model that has a low-velocity layer overlying a high velocity layer. Based on animations by Dr. Thomas Boyd. (audio- music)

Quicktime Animation (1 MB)

## Comparative ray paths in 2-layer model with wave front

This animation uses the wave fronts animated by Dr. Thomas Boyd, Colorado School of Mines, and superimposes ray paths seen in the previous animation. Images used in activity, How Shallow Earth Structure is Determined.

Quicktime Animation (1 MB)

Animations and videos are made in partnership with Earthscope, USGS, and Volcano Video & Graphics.

Please send feedback to Jenda Johnson.