How do we really know what's inside the Earth? - Imaging Earth's interior with seismic waves

John Taber, IRIS Consortium
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Summary

In this multi-step lab, students explore the concepts of seismic wave propagation through materials with different mechanical properties, and examine seismic evidence from a recent earthquake to infer Earth's internal structure and composition. They calculate the diameter of Earth's core by comparing a model to recorded seismic data, then explore mechanical differences between the lithosphere and the asthenosphere, and then examine models of the boundary between the lithosphere and the asthenosphere. This lab is designed to be done with an instructor present to answer questions and guide students to conclusions.

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Context

Audience

This lab is designed to be used in an intro level geoscience course for either majors or non-majors.

Skills and concepts that students must have mastered

Before undertaking this activity, students should know what earthquakes are, understand the basics of seismic wave propagation, and be able to explain the basic information contained in a seismogram and how a seismogram is recorded.

How the activity is situated in the course

This lab can follow a lecture on whole-Earth structure.

Goals

Content/concepts goals for this activity

Students will be able to:
- Describe Earth's internal structure (layers of different material properties and composition) and summarize how this is inferred through the analysis of seismic data
- Estimate the size of Earth's core using a seismic data from a recent earthquake
- Describe how primary and secondary waves propagate through Earth
- Differentiate between Earth's asthenosphere and lithosphere (layers of different mechanical properties)

Higher order thinking skills goals for this activity

Students have to infer the implications of the differences between synthetic model data and real data.

Other skills goals for this activity

Description and Teaching Materials

In this multi-step lab, students examine seismic evidence used to infer Earth's internal structure and composition. The lab begins with a review of seismic (body) wave propagation in Activity 1. In this review, students "become" solids or liquids to kinesthetically experience how body waves move through materials in each state of mater. In Activity 2, students test the hypothesis that Earth is homogeneously composed of rock. A homogeneous Earth model and predicted arrival times are provided with the lab sheet. Students interpret a seismic data (called a record section) from a recent earthquake to compare with predicted arrival times from the model. Activity 3 enables students to interpret the implications of their analysis of the seismic data in Activity 2. Here, the observed data are transferred to a second scale model of Earth to help visualize the details, measure the diameter of Earth's outer core, and compare findings to accepted measurements. In Activity 4, students apply their understanding of body wave propagation to another seismic record section and ray path model of the Earth to infer the state of matter of these two layers. In Activity 5, students examine a graph of viscosities of common materials to develop the idea that the asthenosphere is a solid and that it deforms more easily than the lithosphere. Finally, in Activity 6, students examine results of seismic imaging to determine that the lithosphere-asthenosphere boundary varies with depth and can be a broad transition rather than a stark change as commonly indicated in textbook drawings.

This IRIS (Incorporated Research Institutions for Seismology) activity is part of a collection of activities based on questions that identify promising research directions on the frontiers of seismology as outlined in the Seismological Grand Challenges in Understanding Earth's Dynamic System. The collection has been developed to engage students in the analysis of real data and to bring examples of frontier research topics into the undergraduate classroom. Further information about the activity including links to related resources is available on the IRIS website at http://www.iris.edu/hq/inclass/lesson/imaging_earths_interior_with_seismic_waves

Instructor guide for "How do we really know what's inside the Earth? - Imaging Earth's interior with seismic waves" (Microsoft Word 2007 (.docx) 245kB Jun15 17)

Student worksheet KEY "How do we know what is inside the Earth" (Microsoft Word 3.9MB Jun15 17)

Student worksheet "How do we know what is inside the Earth" (Microsoft Word 3.9MB Jun15 17)

Haiti record section (Acrobat (PDF) 167kB Jun5 17)

Earth scale model (Acrobat (PDF) 57kB Jun5 17)

Instructor slides for "How do we know what is inside the Earth" (PowerPoint 5.9MB Jun5 17)

Teaching Notes and Tips

Assessment

An activity key is provided which can be used to assess whether students have met the goals of the activity

References and Resources

Lay, T., Aster, R. C., Forsyth, D. W. and the Seismological Grand Challenges Writing Group (2009). Seismological Grand Challenges in Understanding Earth's Dynamic System, http://www.iris.edu/hq/lrsps/seis_plan_final.pdf.

Nettles, M., and A.M. Dziewonski, Radially anisotropic shear velocity structure of the upper mantle globally and beneath North America, J. Geophys. Res. 113, B02303, 2008.

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