Episodic tremor and slip: The Case of the Mystery Earthquakes | Lessons on Plate Tectonics

Roger Groom (Mt. Tabor Middle School), Shelley Olds (EarthScope Consortium), Herb Dragert (Geological Survey of Canada), Bob Butler (U. Portland), Jenda Johnson (IRIS), Nancy West (Quarter Dome Consulting) & David Thesenga, (Alexander Dawson School)
EarthScope Consortium logo. Concentric circles in red grading to purple.
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Initial Publication Date: February 27, 2022 | Reviewed: August 4, 2022

Summary

Earthquakes in western Washington and Oregon are to be expected—the region lies in the Cascadia Subduction Zone. Offshore, the Juan de Fuca tectonic plate subducts under the North American plate, from northern California to British Columbia. The region, however, also experiences exotic seismicity— Episodic Tremor and Slip (ETS).

In this lesson, your students study seismic and GPS data from the region to recognize a pattern in which unusual tremors--with no surface earthquakes--coincide with jumps of GPS stations. This is ETS. Students model ductile and brittle behavior of the crust with lasagna noodles to understand how properties of materials depend on physical conditions. Finally, they assemble their knowledge of the data and models into an understanding of ETS in subduction zones and its relevance to the millions of residents in Cascadia.

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Context

Audience

This lesson was developed for high school and middle school students, grades 6 - 12. However, its focus on data makes it adaptable for introductory college courses. It is expected that this lesson will take two class sessions (45-55 minutes).

Skills and concepts that students must have mastered

Students must be able to read a graph. If they flounder, the activity Introduction to graphing GPS data is designed for novice graphers.

How the activity is situated in the course

This lesson can stand alone or with other lessons about plate tectonics and convergent boundaries. In a sequence of lessons about plate tectonics, it fits best during activities about convergence boundaries.

INTRODUCTORY LESSON: Measuring Plate Motion with GPS: Iceland

Before starting this lesson, the Introductory lesson: Measuring plate motion with GPS, Part 1 is suggested to learn the basics on how GPS works.

RELATED LESSON: Detecting Cascadia's Changing Shape with GPS

In this module, students use data, hands-on physical models, and computer simulations to understand subduction zone tectonics, plate tectonics, earthquakes, tsunamis, faulting, and folding.

PREQUEL ACTIVITY:Introduction to Graphing GPS Data

This lesson has an optional prequel, "Pure and simple graphing GPS data," designed for students who cannot yet graph earth science data skillfully or confidently. Its first two parts teach students to graph position vs. time, and its last part dovetails with this lesson. It teaches about velocity vectors by graphing position data over five years.

It is expected to take two class sessions (45-55 minutes).

Goals

Content/concepts goals for this activity

Students will be able to:

  • Describe the tectonic setting of Cascadia
  • Interpret GPS time series plots (position vs. time) qualitatively
  • Identify and describe patterns of ETS in seismic and GPS data

Higher order thinking skills goals for this activity

  • Model ductile and brittle behavior
  • Summarize geological and societal implications of ETS.

Other skills goals for this activity

None

Description and Teaching Materials

Organization
This lesson consists of four principle parts:

  1. Introducing or reviewing the Cascadia Subduction Zone.
  2. Analyzing unusual seismic signals coincident with unexpected GPS data (after students learn to interpret GPS data). Exploration of physical models complements data analysis.
  3. Making sense of the data and models in the context of a subduction zone.
  4. Recognizing implications of ETS for people in Cascadia and around the Pacific Rim.

This lesson has an optional prequel, Introduction to graphing GPS data, designed for students who cannot yet graph earth science data skillfully or confidently. Its first two parts teach students to graph position vs. time, and its last part dovetails with this lesson. It teaches about velocity vectors by graphing position data over five years.

Teaching Notes and Tips

  1. Because ETS is unusual, you'll want to stage this lesson after students have learned about typical earthquakes and have a background in plate tectonics.
  2. You'll also benefit from assessing how much your students know about GPS. Ask them, for example, how do they use GPS? How do they think geologists use GPS? The PowerPoint presentation "Episodic Tremor and Slip: The Case of the Mystery Earthquakes" explains briefly how to interpret GPS data. However, if you have the time and inclination, the activity "Measuring plate motion with GPS" teaches in greater depth how a GPS works and how to interpret GPS data.
  3. Begin the lesson with the PowerPoint presentation "Episodic Tremor and Slip: The Case of the Mystery Earthquakes." The lesson goes hand-in-hand with a student worksheet.
  4. Work through the presentation with your students. Notes below the slides both provide background geology and additional teaching tips.

Assessment

Assess what your students have learned by doing this activity. You could ask them to draw a concept map for ETS that brings in seismicity, GPS, and plate tectonics. Or, they could write a few paragraphs about ETS. They could draw a cartoon cross-section that compares and contrasts coastal Cascadia with the inland urban corridor and continental interior in terms of seismicity and GPS data. You could, of course, simply grade their worksheets

References and Resources