Studying Earth's Interior

7/31/2026

There's something a little wild about the fact that we've never drilled anywhere close to the center of the Earth — the deepest hole humans have ever dug barely scratches the crust — and yet we can tell you with real confidence how big the core is, what it's made of, and where the boundary between the mantle and the outer core sits. We don't get there by digging. We get there by listening to earthquakes.

Studying the Earth's Interior with Quakes (pdf)
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This activity is adopted from an awesome EarthScope Lesson (and their new online version) and students use real, live earthquake data to track how seismic waves travel through the planet, notice where they behave strangely, and use that strange behavior to map out the Earth's interior for themselves. This iteration of the investigation follows the main procedure of the original EarthScope activity but simplifies the calculations to make the task more accessible for a 9th grade earth science class.

Set Up

This task requires students to access an activity sheet and have the ability to access on online seismogram viewer. In my classroom, we are 1:1 iPad so I provide the activity as a downloadable pdf and they use their device to complete the activity.

In addition to the digital portion of the investigation, students will also need a printed "Earth Circles" page, scissors, and something to write with (markers work well). All of the necessary files are included at the bottom of this post.

Activity

Select an Earthquake to Investigate

Using www.earthscope.org/app/gsv/, students click on a circle to access data from a recent earthquake. (The red circles represent the most recent quakes)

Mark Each Seismogram Location

Each colorful line represents a seismogram measuring the vibrations of the earthquake from a different location on Earth. These positions are measured as an angular distance in degrees (°).

Students will click on each seismogram to determine the position of the station ( ____ degs away) and mark the locations with a dot on the Circle #1 of the "Earth Circles" printout to represent each of the stations measuring the quake. If they have access to colored markers or pencils, students like to match the star colors to the seismogram color so they don't lose their place but a single color will also function just fine :)

Mark Arrival Time for Each Station

Notice that the vibrations reach the different stations at different times. After taking a screenshot of the seismogram data, students will draw a dot on each graph to represent the first indication of the earthquake waves at that station. An example is shown below…

Using the “Time Since the Earthquake” on the y-axis, students will then determine the time delay to measure the earthquake waves at the closest and farthest stations. This step is to help draw their attention to the values on the y-axis and recognize that the farther away the station is, the longer it takes to register a signal from an earthquake.

Locate the shift in the trend

Students will notice that, in general, the farther away from the earthquake the station is, the longer the delay before the earthquake vibrations are detected. On their graph, they will also notice that for the last 4-6 stations, the pattern seems to shift and the waves arrive slightly later than expected suggesting that these waves were interrupted by something on their way through the earth

Using the graph, students should estimate the angle at which the waves start to appear slower than expected. In the example above, it is clear that it is somewhere between 100° and 115°, so this student might choose an angle somewhere in the middle like 107°.

Draw lines and cut out Earth Circle #1

Students draw lines on Circle #1 from 0° to the approximate angle (on top and bottom) where the waves noticeably slow down. They will then use scissors to cut out this shape. (This is the only cutting involved so they can probably share scissors)

Based on the data, there must be something in this area of the earth slowing down the waves. The example shown matches the 107° angle that was estimated in the last step.

Trace Cutout onto Circle #2

Once cut out, students will overlay the “pizza slice” cutout on Circle #2 with the point lined up with 0° and trace the straight cut-out lines.

Simulate more earthquakes

To better narrow down the area responsible for slowing down the waves, students can simulate other earthquakes by rotating and tracing the cutout with the point at intervals of 45°. In all, this should trace the shape 8 separate times and the result should start to resemble a star shape across the circle.

This can also be achieved by students swapping shapes and tracing from random "starting points" to make it even more like they are tracking different earthquakes. This was my original plan but it made the step a little more complicated and time dependent for students that were at very different steps of the activity so I changed it to be tracing the same shape multiple times.

Locate the "slow down"

You will notice that regardless of where you place the point of the cutout when you trace it, there will always be one area that never receives any lines. This "mysterious slowing" of the waves must occur in this area. With multiple tracings, students can trace a circle representing the blank area inside the earth. This circle represents the area in the earth’s interior that is interrupting the earthquake waves.

Calculate the Size of the Core

Students will measure the size of the interior circle that they drew (core) compared to the overall circle #2 size and use these measurements to calculate the % of the Earth's diameter taken up by its core.

Is it the Inner or Outer Core?

Using the diagram shown, students will determine the accepted value for the percentage of the Earth’s diameter taken by Earth’s inner and outer core.

Looking back at the earthquake data, and measured percentage of the core, is the inner or outer core most responsible for the unexpected slowing of the seismic waves?

Reflection

Overall, I found that this activity did a nice job of guiding students through the analysis of the seismogram data and identifying the "unexpected slow down". There was a little confusion as I anticipated when it came to the step of tracing the cutout multiple times and using this to identify the size of the core. I made sure to wander the room and assist students by modeling one tracing for them when the time came and they usually picked it up quickly after that.

Acknowledgements

This activity is an iteration on a wonderful investigation by EarthScope. They also have a great web-based version that is worth checking out. The tool to access real earthquake data is what makes this activity so great and I truly appreciate all of the work that has gone into making this data so accessible.