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

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.

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.

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.