Episodic tremor and slip: The Case of the Mystery Earthquakes | Lessons on Plate Tectonics
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.
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).
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
Description and Teaching Materials
This lesson consists of four principle parts:
- Introducing or reviewing the Cascadia Subduction Zone.
- Analyzing unusual seismic signals coincident with unexpected GPS data (after students learn to interpret GPS data). Exploration of physical models complements data analysis.
- Making sense of the data and models in the context of a subduction zone.
- 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.
- Episodic tremor and slip - All Files (Zip Archive 53.9MB Oct17 21)
- Episodic tremor and slip - Teacher Guide (Acrobat (PDF) 851kB Oct17 21)
- Episodic tremor and slip - Student Worksheets (Microsoft Word 2007 (.docx) 858kB Oct17 21)
- Episodic tremor and slip - Presentation (PowerPoint 2007 (.pptx) 33.2MB Oct17 21)
- Episodic tremor and slip - Student Worksheets PDF (Acrobat (PDF) 894kB Oct17 21)
Teaching Notes and Tips
- 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.
- 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.
- 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.
- Work through the presentation with your students. Notes below the slides both provide background geology and additional teaching tips.
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