Determining and Measuring Earth's Layered Interior


In this instructional sequence, students examine seismic evidence to determine that the Earth must have a layered internal structure and to estimate the size of Earth's core. Using an inquiry approach, students are divided into two teams (theoreticians and seismologists) to test the simplest hypothesis for what is inside of Earth; the Earth is homogeneous throughout. Theoreticians use a paper and pencil, scale model of a homogeneous Earth combined with an average seismic wave velocity to make predictions about when seismic waves should arrive at various points around Earth (predicted). Meanwhile, seismologists interpret seismic data from a recent earthquake to determine when seismic waves actually arrive at various points around Earth (observations). After comparing and discussing the fit of the predicted data with the observed data, students use a second scale model to further interpret these results. Ultimately, students measure the diameter of Earth's outer core based on their data and can compare it to widely accepted measurements. Before undertaking this activity, students should know what earthquakes are, understand the basics of seismic waves and their prorogation, and be able to explain how seismograms are created.


  • Students will be able to: - Demonstrate that Earth can’t be homogenous by comparing a seismic record section to predicted arrivals from a homogeneous Earth model.
  • Explain how the internal structure of Earth (concentric layers of different density and composition) is inferred through the analysis of seismic data.
  • Explain the role models play in the scientific process, especially when used in combination with observational data.  
  • Explain how models are refined through the collection of additional data
  • Discuss how working in a team to make data-gathering and procedural decisions provides an efficient means for completing tasks, provides peer support to check work and to develop conceptual understanding.
Part 1/8 Introduction and Opening
In this video, we introduce the activity as a structured inquiry. Next, we reviews the learning outcomes for the activity so you know where the lesson is headed. Finally, we open the instruction and access learners prior knowledge of Earth's interior. The focus is not only what the learner knows, but the evidence they have for what they know. This will serve as the hypothesis we will test in the activity. 
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Part 2/8 The Role of Observed Data 
In this video, we introduce the role the seismologists will play in the investigation and how seismic data from a real earthquake will contribute to our exploration of Earth's interior.   
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Part 3/8 Interpreting Seismic Data
In this video we introduce a record section, or a plot of multiple seismograms from the same earthquake. Using a record section from a recent earthquake we demonstrate how to pick the first seismic arrivals on the seismograms. 
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Part 4/8 Developing a Model 
In this video we introduce the role of the theoretician will play in the investigation. Specifically, we explore how to set up a scale model of Earth that will be used to generate predictions about when seismic waves should arrive at various points around Earth.   
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Part 5/8 Comparing Observed Data to the Model
In this video we explore how closely the observed arrivals of seismic waves from seismograms compares to the arrivals predicted from the model we have developed. 
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Part 6/8 Interpreting Results 
In this video we introduce a second, smaller scale model of Earth to help us make sense of our results. 
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Part 7/8 Interpreting Results Continued
In this video we continued interpreting our results by adding more and more data to the scale model. Through this process we reveal Earth's core and can use the scale model to estimate the radius of Earth's core within a small % error. 
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Part 8/8 Wrapping It Up
In this video we summarize the activity and why we think it's a great addition to any Earth Science classroom.
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Related Fact-Sheets

Earthquakes create seismic waves that travel through the Earth. By analyzing these seismic waves, seismologists can explore the Earth's deep interior. This fact sheet uses data from the 1994 magnitude 6.9 earthquake near Northridge, California to illustrate both this process and Earth's interior structure.

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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.

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

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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.

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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.

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A travel time curve is a graph of the time that it takes for seismic waves to travel from the epicenter of an earthquake to the hundreds of seismograph stations around the world. The arrival times of P, S, and surface waves are shown to be predictable. This animates an IRIS poster linked with the animation.

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Related Videos

Conceptual model of the relative thicknesses of the Lithosphere relative to the diameter of the Earth uses a hard-boiled egg to gain understanding about the scale of the lithospheric plates.

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Related Software-Web-Apps

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.

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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.
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Related Posters

Seismic waves from earthquakes ricochet throughout Earth's interior and are recorded at geophysical observatories around the world. The paths of some of those seismic waves and the ground motion that they caused are used by seismologists to illuminate Earth's deep interior.

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