Unit 5: 2014 South Napa Earthquake and GPS strain
The 2014 South Napa earthquake was the first large earthquake (Mag 6) to occur within the Plate Boundary Observatory GPS network since installation. It provides an excellent example for studying crustal strain associated with the earthquake cycle of a strike-slip fault with clear societal relevance. The largest earthquake in the California Bay Area in twenty-five years, the South Napa earthquake caused hundreds of injuries and more than $400 million in damages. This activity uses a single triangle of GPS stations (P198, P200, SVIN), located to the west of the earthquake epicenter, to estimate both the interseismic strain rate and coseismic strain. By the end of the exercise, the students also have direct evidence that considering the recurrence interval on a single fault, which is part of a larger system, is not reasonable. An extension option gives the opportunity to discuss earthquake early warning systems.
Unit 5 Learning Outcomes
- Students are able to access and analyze GPS data in order to calculate and interpret interseismic and coseismic strain in the region between three neighboring GPS stations.
- Students are able to evaluate how a calculated recurrence interval relates to the regional strain and fault system.
Unit 5 Teaching Objectives
- Affective: Encourage reflection and analysis of societal impacts of earthquakes.
- Cognitive: Facilitate students' ability to compare interseismic strain with coseismic displacements.
Context for Use
This module was designed for structural geology courses but can also be successfully used in geophysics, tectonics, or geohazards courses or possibly even a physics or engineering course seeking practical applications. Unit 5 assumes that students have already learned to use and analyze results from a GPS Strain Calculator and thus must come after Unit 3: Getting started with GPS data and Unit 4: GPS and infinitesimal strain analysis. In fact, Unit 5 could even be done after Unit 6: Applying strain and earthquake hazard analyses to different regions, which is the summative assessment for the rest of the module (Units 1–4). We have found the 2014 South Napa earthquake example can be a great way to return to and solidify these concepts later in the course and to bring up again the critical importance of understanding ongoing to strain in preparing our society for earthquake hazards.
Description and Teaching Materials
This exercise helps foster analysis and discussion of the Mw 6.0 August 24, 2014, South Napa earthquake using data from permanent GPS arrays. The information provided could be presented as: (1) an instructor-led discussion or (2) part of a student-centered activity building on the GPS strain analysis process. Use the full resources provided to do the student-centered activity. For the discussion-only option, have the students still do the short homework assignment to learn about the 2014 South Napa earthquake, but then primarily use the presentation to guide a class discussion of the analysis.
We suggest leading this exercise with a short homework assignment in which the students are asked to come to class having gathered data about the 2014 South Napa earthquake. This short pre-assignment is also on slide #2 of the presentation.
- In preparation for the next lab period, each student should select an aspect of the 2014 South Napa earthquake for which to do some research: seismology, faulting, or damage (pass around a signup sheet with three columns and only enough spaces so that you get an even distribution of topics in the class)
- Come to class ready to serve as the "expert" on your select topic for your three-person team
- READ the short background document that was emailed to you
Once in class, run a quick jigsaw activity in which the class is divided into groups of three, with one of each type of "expert." Each student is responsible for bringing the entire team up to speed on the topic she or he researched.
Below are the teaching materials for the unit:
- Instructor preparatory materials
- Unit 5 2014 South Napa earthquake instructor notes (Microsoft Word 2007 (.docx) 197kB Dec19 17)
- Unit 5 2014 South Napa earthquake presentation (PowerPoint 2007 (.pptx) 29.3MB Dec8 16)
- Student materials
- Unit 5 2014 South Napa earthquake student pre-exercise reading (Microsoft Word 2007 (.docx) 777kB Dec19 17)
Unit 5 2014 South Napa earthquake student pre-exercise reading PDF (Acrobat (PDF) 741kB Dec19 17)
- Unit 5 2014 South Napa earthquake student primary exercise (Microsoft Word 2007 (.docx) 1.7MB Dec19 17)
Unit 5 2014 South Napa earthquake student primary exercise PDF (Acrobat (PDF) 1.7MB Dec19 17)
- Unit 5 2014 South Napa earthquake student vector map exercise PDF (Acrobat (PDF) 404kB Dec8 16)
Unit 5 2014 South Napa earthquake student vector map exercise PDF (Acrobat (PDF) 404kB Dec8 16)
- Unit 5 2014 South Napa earthquake student GPS data tables handout (Microsoft Word 2007 (.docx) 1.4MB Dec9 16)
Unit 5 2014 South Napa earthquake student GPS data tables handout PDF (Acrobat (PDF) 1.3MB Dec9 16)
- Answer keys
- GPS strain calculators and supporting document—these are the same ones as used in Unit 4: GPS and Infinitesimal Strain Analysis
- GPS Strain Calculator Excel (Excel 55kB Nov18 16)
- GPS Strain Calculator MatLab (Zip Archive 4kB Sep24 15)
- Explanation of GPS Strain Calculator output (Microsoft Word 2007 (.docx) 1.1MB Dec28 17)
- Strain ellipse visualization—this is the same as used in Unit 4: GPS and Infinitesimal Strain Analysis. There is a simple strain ellipse visualizer within the Excel GPS Strain Calculator. However, if you wish your students to interact more with exaggerated ellipses, this viewer can help students gain more intuitive understanding of how the numbers relate to specific examples of strain.
- Have students download the Wolfram CDF Player to use the Visualizer (unless your school has Mathematica licenses).
- Strain ellipse visualization tool (Zip Archive 297kB Sep24 15)—Inside the zipped file is the CDF file for the Strain Ellipse Visualizer. Have the Wolfram CDF Player open and then open the Visualizer from inside it. This will prevent your computer getting confused about which software to use. It might default to a text editor.
Extension Option—Earthquake Early Warning System
As of 2015, Japan and Mexico have public-alert earthquake early warning systems that can sense initial shaking in one region, identify that an earthquake is underway, and send notices within tens of seconds to people and infrastructure further away that imminent shaking can be expected. The United States does not currently have a fully functional earthquake early warning system, but a prototype has been developed for the Bay Area, called ShakeAlert. It was in operation during the 2014 South Napa earthquake and functioned correctly to send a warning, for example, to the BART (Bay Area Rapid Transit) trains. In happened that no trains were moving because the earthquake occurred in the middle of the night, but had it been daytime, trains would have been slowed or stopped by the time shaking arrived in San Francisco. You can ask students, what types of earthquake risk could be reduced by having 10–100 seconds of warning? After a short discussion, you could then show them the short animation about the ShakeAlert system.
Youtube: ShakeAlert—Earthquake Early Warning. How does it work?
MP4 file: ShakeAlert—Earthquake Early Warning. How does it work? (MP4 Video 26MB Oct7 15)
Teaching Notes and Tips
- The pre-activity homework provides excellent motivation for the activity, but instructors may need to limit discussion to complete the activity in a single class meeting.
- Instructors wishing to complete the activity in a single class session may want to have students plot the velocity/offset and strain rate / strain data in advance or provide the plotted data to students so that they can focus on interpretation of the data.
- The conclusion of the activity is an assessment of earthquake recurrence using GPS observations of interseismic and coseismic deformation.
- Instructors may want to revisit the Tohoku earthquake using slide 7 from Unit 4 Example interseismic data from Japan (PowerPoint 2007 (.pptx) 11.8MB Dec7 16) to help students develop their understanding of elastic rebound.
- Physical analogs such as spring-slider models (for example: http://serc.carleton.edu/introgeo/demonstrations/examples/earthquake.html) may also be helpful in developing student intuition.
- Unit 5 2014 South Napa earthquake presentation (PowerPoint 2007 (.pptx) 29.3MB Dec8 16) slides 22–30 are provided to foster discussion of the earthquake cycle using data from the South Napa earthquake.
- The simple model used in the activity results in an unrealistically short recurrence interval. Unit 5 2014 South Napa earthquake presentation (PowerPoint 2007 (.pptx) 29.3MB Dec8 16) slides 31–32 are provided to guide discussion of two major shortcomings of the model. Instructors may want to couch this discussion as an example of how we can learn from the failure of models. State-of-the-art earthquake rupture forecasts use GPS data and elastic rebound theory to estimate earthquake probabilities, but incorporate more realistic fault geometries and other underlying assumptions into their models.
- We provide supporting math materials in Unit 2 for instructors interested in having student take the quantitative analysis to a deeper level. These include vector, matrices, and infinitesimal strain analysis background.
Observation of student activity and conversations, individual questioning, and group discussion are excellent ways to conduct formative assessment as the students complete this exercise.
The student exercise is the summative assessment for this unit. Many of the questions have definite right or wrong answers. To receive full credit, students must show their work, where appropriate. Where an open-ended answer is required, students are assessed based on a simple 2-point system.
2 points = correct answer with thorough supporting evidence and/or complete description
1 point = answer not completely correct or lacking thorough supporting evidence or description
0 points = incorrect answer
References and Resources
- Earthquake Engineering Research Institute EERI Special Earthquake Report on the South Napa Earthquake
- USGS 2014 South Napa earthquake page
- ShakeAlert is developing and testing an earthquake early warning (EEW) system for the west coast of the United States.
- Other earthquake early warming system animations