Mysterious motions along the Pacific Northwest Coast

John Taber, IRIS Consortium
Author Profile

This activity was selected for the On the Cutting Edge Exemplary Teaching Collection

Resources in this top level collection a) must have scored Exemplary or Very Good in all five review categories, and must also rate as “Exemplary” in at least three of the five categories. The five categories included in the peer review process are

  • Scientific Accuracy
  • Alignment of Learning Goals, Activities, and Assessments
  • Pedagogic Effectiveness
  • Robustness (usability and dependability of all components)
  • Completeness of the ActivitySheet web page

For more information about the peer review process itself, please see

This page first made public: Jun 7, 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) that was discovered less than 20 years ago and has significant implications for understanding megathrust earthquakes in subduction zones.



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, and understand the earthquake cycle (storing energy via elastic deformation in rocks, followed by rapid slip on a fault and release of seismic energy, and then a repeat of the process).

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

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.

This IRIS (Incorporated Research Institutions for Seismology) activity is part of a collection of activities based on questions that identify promising research directions on the frontiers of seismology as outlined in the Seismological Grand Challenges in Understanding Earth's Dynamic System. The collection has been developed to engage students in the analysis of real data and to bring examples of frontier research topics into the undergraduate classroom. Further information about the activity including links to related resources is available on the IRIS website at

Instructor guide for ETS activity (Microsoft Word 2007 (.docx) 351kB Jun1 17)
Student worksheet for ETS activity (Microsoft Word 2007 (.docx) 2.2MB Jun1 17)
Student worksheet KEY for ETS activity (Microsoft Word 2007 (.docx) 2.2MB Jun1 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