Mysterious motions along the Pacific Northwest Coast

John Taber, IRIS Consortium
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Initial Publication Date: June 7, 2017 | Reviewed: July 17, 2017


Students work in small groups to analyze and interpret Global Positioning System (GPS) and seismic data related to "mysterious ground motions" first along the northern California coastline, and then in British Columbia. This activity emphasizes the analysis and synthesis of multiple types of data and introduces a mode of fault behavior known as Episodic Tremor and Slip (ETS).

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This lab is designed to be used in an intro geoscience level 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 earthquake cycle (storing energy via elastic deformation in rocks, followed by rapid slip on a fault and release of seismic energy, and the repeat).

How the activity is situated in the course

The activity is designed as a stand alone exercise to follow an introduction to earthquakes and the earthquake cycle.


Content/concepts goals for this activity

Students will be able to:
1. Describe the process of Episodic Tremor and Slip and contrast this with the traditional earthquake cycle or Elastic Rebound Theory.
2. Demonstrate how GPS and seismic observations support our understanding of Episodic Tremor and Slip
3. Describe the seismic hazard created as the Juan de Fuca plate subducts beneath the North American plate.

Higher order thinking skills goals for this activity

This collection has been developed to bring examples of frontier research topics into the undergraduate classroom and to engage students in the analysis of real data, comparing and contrasting different models of earthquake and plate boundary deformation. and supporting claims with evidence.

Other skills goals for this activity

Description and Teaching Materials

In this lab, students investigate mysterious ground motions along the Pacific Northwest Coast. The investigation begins as students familiarize themselves with the tectonic setting of the Pacific Northwest coastal region . Next, students explore and explain the movement of the Cape Mendocino, CA region that occurred on April 25th, 1992 using data (vector) from high-precision Global Positioning System (GPS) stations and earthquake location data. This case is contrasted with movement recorded by GPS (time-series) in British Columbia from 2004 to 2011. Finally, students investigate the episodic tremor and slip model to explain the GPS time series data for the British Columbia region.
Instructor guide for ETS activity (Microsoft Word 2007 (.docx) 351kB Jun15 17)
Student worksheet for ETS activity (Microsoft Word 2007 (.docx) 2MB Jun15 17)
Student worksheet KEY for ETS activity (Microsoft Word 2007 (.docx) 2MB Jun15 17)

Teaching Notes and Tips


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

References and Resources

Instructor Background:
Animations specific to the Pacific Northwest:
The use of GPS to monitor plate motion near subduction boundaries:
Episodic Tremor and Slip in cross section recorded on graph.:
Overview of the Pacific Northwest tectonic regime:

Brudzinski, M. (2011). Episodic Tremor and Slip: Potential Clues to the Earthquake Process and How Faults Slip. The Earth Scientist, 27(1), 7-12.

EarthScope. (2011). Episodic Tremor and Slip.

Grand Challenge #1 – How do Faults Slip