## Determining Shallow Earth Structure

1h 30min Intermediate

• Supporting Slides
• Student Handouts
• InstructorGuide.pdf

This activity is intended to extend students' learning of fundamental physics concepts (e.g. reflection, refraction and transmission of energy) through a real-world application.

Energy from an explosion travels through the Earth as seismic waves. Like other waves, seismic waves obey the laws of physics. For example, seismic waves have difficulty passing from one medium into another, hence they reflect (partially) from such a boundary. Waves also have different speeds in different media because each medium has its own rigidity. Different speeds in different rock layers give rise to refraction effects, including critical angle. Using basic physics principles and geometry, students examine seismic data to calculate the seismic wave velocities in several rock layers and then use this information to determine the depth to the boundaries, and estimate what rock type makes up each layer.

#### Objectives:

Students will be able to:

• Identify the following elements of a seismic experiment: Angle of reflection, Angle of refraction, Headwave, Direct wave, Normal, Critical angle, Snell’s Law
• Determine the depth of bedrock by analyzing a simple seismic section.

## Related Animations

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

The Earth has 3 main layers based on chemical composition: crust, mantle, and core. Other layers are defined by physical characteristics due to pressure and temperature changes. This animation tells how the layers were discovered, what the layers are, and a bit about how the crust differs from the tectonic (lithospheric) plates, a distinction confused by many.

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
Animation Intermediate

## Related Lessons

Learning occurs as 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

## 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