Modeling Asperities with Spaghetti

Nicole LaDue, Northern Illinois University
Josh Schwartz, Northern Illinois University

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Summary

This activity uses a physical model to facilitate students' understanding of elastic deformation of rocks and the episodic nature of motion on a fault, which leads to earthquakes and aftershocks.

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Context

Audience

This activity is used in an introductory, non-major, physical geology course.

Skills and concepts that students must have mastered

Students should know that earthquakes are the result of fault motion and that faults are a crack in the bedrock along which motion occurs. Students should have a definition of elastic, brittle, and ductile deformation.

How the activity is situated in the course

This is a student-driven physical model to facilitate students' observations of a phenomena. The students will have already learned about the types of faults and the components of earthquakes (i.e. epicenter, seismic wave, etc.). This activity is done without introduction to facilitate students in making direct observations about asperities.

Goals

Content/concepts goals for this activity

The goal of this activity is to help students recognize:
- rocks deform in elastic ways
- when significant stress is applied, rocks can undergo brittle deformation
- earthquakes occur when specific sections of a fault fail
- aftershocks are small earthquakes occurring after the primary failure

Higher order thinking skills goals for this activity

In this activity students will perform analogical mapping from this physical model to the earth.

Other skills goals for this activity

Description and Teaching Materials

This activity was developed to facilitate student learning with the Aspertity Model. In fault systems, failure on one part of a fault is preceded by elastic deformation of the rocks on either side of the fault. When a fault fails, there are typically adjustments in other sections of the fault that cause aftershocks. In this activity, students observe that as they apply stress to the model fault, the spaghetti will undergo elastic deformation. As the spaghetti fails, it experiences brittle deformation. This failure does not happen to all of the spaghetti concurrently. There are often aftershocks as the students run the physical model multiple times.

We revised the model that was posted on the IRIS.edu website such that there is a 5 mm groove along the fault boundary that allowed the students to better see the elastic deformation of each spaghetti strand. We also added a metal plate with a tongue-and-groove to the back of the wood blocks to help them stay together while students applied stress.

The challenge question relates this model to the real setting where we can observe GPS stations showing the elastic deformation around a fault. We did not expect many students to be able to answer this question, but offered it as a challenge to those with a stronger science background. Prior to this class, students watched the IRIS video: https://www.youtube.com/watch?v=NMNNr2CyekA about subduction and GPS motion. This particular question is credited to M. Brudzinski of Miami University.

The original model and background text for this activity is credited to the Incorporated Research Institutions for Seismology (IRIS.edu). A video and description of the model can be found here: https://www.iris.edu/hq/inclass/lesson/modeling_asperities_on_a_strikeslip_fault_with_spaghetti
Asperity Worksheet (Microsoft Word 2007 (.docx) 542kB May31 17)

Asperity Description (Acrobat (PDF) 1.4MB May31 17)

Asperity Model Construction (Acrobat (PDF) 589kB May31 17)

Asperity Background (Acrobat (PDF) 153kB May31 17)

Teaching Notes and Tips

This activity works best in groups of two so that they can talk through the model and be able to manipulate it themselves. Circulating around the room is necessary to make sure they have answered the questions at the beginning of the activity. Often students are so excited to run the complete model that they skip ahead and miss the details. Students tend to struggle with how much pressure to apply to the clamp. The first time, all of the spaghetti breaks so quickly that they miss most of the observations they should make. Encourage students to run the model several times. When we ran this activity, we found that the spaghetti broke in place in most cases so there were no safety concerns and clean up was easy.

The attached file includes suggestions for construction of the asperity model. The clamps are relatively expensive at $10 a piece, packaged in sets of 2 clamps. Many hardware stores did not stock a large number of these small clamps, so ordering online was necessary. We used a mixture of whole wheat and regular pasta to accentuate that not all pieces would break at the same time. The pattern of failure was difficult to discern while using one type of pasta.

Assessment

There are two ways we assessed whether students met the goals of this activity. First, before and after the activity, we posed a clicker question about whether rock can bend. This was to evaluate whether students shifted their perspective on elastic deformation of rocks as a result of this activity. Second, students' answers to question 5 on the second page offered insight about whether students correctly mapped the components of the physical model to the real world. This also provides a guide to instructors about which components of the model should be reviewed in class.

References and Resources

The asperity model originated with seismologists working with IRIS. We adapted the model and this activity to reach our learning goals. Information about the original model can be found here: https://www.iris.edu/hq/inclass/lesson/modeling_asperities_on_a_strikeslip_fault_with_spaghetti
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