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Unit 1 Hazards at Transform Plate Boundaries

Laurel Goodell (Princeton University)
Peter Selkin (University of Washington, Tacoma)
Rachel Teasdale (California State University, Chico)

Summary

This unit uses scientific data to quantify the geologic hazard that earthquakes represent along transform plate boundaries. Students will document the characteristics of the Pacific/North American plate boundary in California, analyze information about historic earthquakes, calculate probabilities for earthquakes in the Los Angeles and San Francisco areas, and assess the regional earthquake probability map.

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Learning Goals

This unit addresses several overarching goals of the InTeGrate program including analyzing geoscience-related grand challenges facing society (impact of natural hazards), developing students' ability to address interdisciplinary problems and use authentic geoscience data, and improving students' geoscientific thinking skills (interpretation of multiple data sets).

Unit 1 Learning Objectives

  1. Students will be able to describe characteristics of transform boundaries.
  2. Students will be able to analyze information about significant historic earthquakes along the North American/Pacific transform plate boundary in California and the risks of those earthquakes to population centers.
  3. Students will be able to use data to determine the conditional probabilities of earthquakes of various magnitudes in the San Francisco and Los Angeles areas over the next year, and over the next 30 years.
  4. Students will compare their results to the regional earthquake probability map for California and assess the regional earthquake hazard along this plate boundary.

Context for Use

This unit is designed to be completed in a 50-minute class period, with suggestions for extension if a longer time period is available (for instance, a lab period). Students are expected to come to the activity with the following background:

  • Familiarity with the basic tenets of plate tectonics, as well as the general characteristics of plate boundaries. One suggested activity is Using Google Earth to Explore Plate Tectonics , by Laurel Goodell.
  • Familiarity with the concept of earthquake magnitude and what is considered a "major" earthquake (magnitude 6 or greater).
  • Completion of the prework assignment (which could be incorporated into class time if a longer time is available).
  • Ability to use a scientific calculator, and ability to plot data on graphs with logarithmic scales. See resources for teaching logarithms .

Description and Teaching Materials

Prework assignment

(15 min) As preparation for this unit, students complete the Unit 1 prework assignment. This first asks them to characterize the transform plate boundary between the Pacific and North American plates in California, using a map which shows:

  • Plate labels and arrows depicting the relative motion between the Pacific and North American plates.
  • Epicenters of earthquakes with magnitudes >=5.0 from 1994-2023.
  • Locations of Quaternary faults in the study area, with the San Andreas fault in red and other fault traces in yellow

Students are then asked questions about nine historic earthquakes in the study area, significant either because of their magnitude or because of the amount of deaths/damage caused. They base their responses on:

  • A map showing the epicenters of the nine earthquakes and the population density of California, from the 2010 Census.
  • A chart listing information about the nine earthquakes including year, fault(s) involved, magnitude, and deaths/damage caused.

Finally, students are shown the earthquake probability map for California and asked to speculate about how they think scientists calculate probabilities for events that have not yet happened.

Prework files:

For instructors only:

Classwork

1. (5 min) Instructor starts with a class discussion, eliciting earthquake experience stories from students. (Where were they? What did they experience? What do/did they know about the characteristics of the quake? How well-prepared were they - did they know what to do? What were the effects of the earthquake - damage, casualties?) If time permits, students can break up into small groups to review and compare answers from the preparation assignment.

2. (10 min) Instructor leads a class discussion which reviews and clarifies the prework and sets up the activity, covering the following:

  • The general characteristics of transform plate boundaries (right-lateral or left-lateral relative motion between plates, shallow earthquakes on multiple faults along linear trends marking the plate boundary, general absence of volcanism). If time permits, these characteristics could also be compared and contrasted with those of convergent and divergent boundaries.
  • Recognition of the complexity and width of this particular plate boundary, as well as plate boundaries in general (especially compared to maps in which plate boundaries are represented by single lines).
  • Recognition that the damage and casualties resulting from a particular earthquake are not just dependent on earthquake size, but also the amount of property and people in the vicinity.
  • The value of quantifying the probability of earthquakes of significant size in populated areas. For example, the San Francisco area has not experienced an earthquake of magnitude greater than 6.9 in the last 30 years. Yet we know they do happen (e.g. studies estimate the magnitude of the 1906 earthquake is estimated to be 7.7 or 7.9 (note: the link to the Wald et al. article on this site is broken; the reference is Wald, et al., Source Study of the 1906 San Francisco Earthquake, Bulletin of the Seismological Society of America, Vol. 83, No. 4, pp. 981-1019.)
    So how do we determine the probability of these large, infrequent but potentially devastating events?
  • A quick look at the California Earthquake Probability map, posing the question about the basis on which such maps are constructed.

3. (25 min) Students fill out worksheets that allow them to calculate for the San Francisco area (33.5-35.5°N / 116.75-119.75°W), the probability of various-sized earthquakes occurring in these regions over the next year, and over the next 30 years. Step-by-step instructions are given in the student handout and in the PowerPoint guide. Students can work through the exercise on their own or in small groups as the instructor walks around, or the whole class can go through the exercise together as the instructor presents the PowerPoint.

Notes on the exercise:

This is a modification and expansion of Eric Baer's exercise: Determining Earthquake Probability and Recurrence from Past Seismic Events which focuses on determining recurrence intervals (return periods). Here we add calculation of probabilities. To make this manageable in the allotted time, the geographic regions are already defined for the San Francisco (and Los Angeles) area on the student worksheets, and the numbers of earthquakes in each magnitude range are given. Also, certain data are filled in, so as they work, students can confirm that they are on the right track.

We focus on calculating annual probabilities and 30-year probabilities. We note that 30 years is a time period of interest to humans (it is a good chunk of a typical human life span, the length of a traditional 30-year mortgage, etc.)

This approach assumes that the occurrence of an earthquake of a particular size in a particular area does not have causal or anti-causal effects on the occurrence of other earthquakes (e.g. if you roll a die and 4 comes up 10 times in a row, the probability of 4 coming up on the 11th role is still 1/6). This is an important assumption - exceptions would include foreshocks or aftershocks of major earthquakes, "locked" segments of faults (e.g. the Parkfield experiment) where stress may be building up over time, or "creeping" fault segments. It is also based on counting earthquakes in a large enough area and for a long enough time period to have a reasonable statistical basis.

4. (10 min) The end of the exercise includes three important formative assessment questions and two optional ones. See "Assessment" section below.

Activity files:

For instructors only:

Teaching Notes and Tips

Handouts, instructor notes, class PowerPoint files, and keys are suitable for a 50- or 75-minute class.

To modify this unit in combination with Unit 2 (Risk at Transform Plate Boundaries) for a lab period, we recommend combining both the Unit 1 and Unit 2 prework assignments into one pre-lab assignment, and then in the lab session, doing the Unit 1 classroom activity followed by the Unit 2 classroom activity. Selected discussion/assessment questions could be assigned as a post-lab report.

To combine this Unit 1 with Unit 2 (Risk at Transform Plate Boundaries) for use in a single two- to three-hour lab period, we recommend combining both the Unit 1 and Unit 2 prework assignments into one pre-lab assignment. Little modification to the prework content is needed, although the references to page numbers in the Unit 2 prework should be checked for consistency. In the lab session itself, the Unit 2 classroom activity would directly follow the Unit 1 classroom activity.

Modifications on this activity from the community...

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Contributed by Linda Reinin, Pomona College

I love the Living on the Edge assignment. I've modified it: (1) we do San Francisco in-class with LA as homework; (2) question comparing SF and LA also includes questions about the personal preparations that residents can make to minimize the impact of the inevitable earthquake; (3) I finish with a question that asks the students to use their results (and additional information) to propose a plan for allocation of the state's finite emergency resources - what factors should/could be considered (e.g. eq probability, population distribution, potential for related hazard like landslides, tsunami, etc., existing infrastructure) and how do you rank/weight each of these factors? Basically I'm trying to get students to make the connection between earthquake science and public policy. (Contributed by a community member from southern California using the materials in a small and medium introductory geology course that emphasize natural hazards.)


Assessment

Formative Assessment

Within the activity, formative assessment is ongoing. Students progressively complete their probability worksheets (stopping at each "stop" sign in the PowerPoint activity guide) and compare their results to the appropriate answers (presented after each "stop" sign) before going on to the next step. Students thus progressively check their own work; the instructor does not need to manually grade it, although he or she may want students to hand it in as documentation of the activity's completion.

To complete the formative assessment, students should answer the questions listed below (and included in the PowerPoint guide and the student handout). Depending on time constraints and/or interest, the questions could be incorporated into small group discussions or given as individual or group homework assignments.

  1. No earthquake with magnitude 7.0-7.9 has occurred in the San Francisco area over the 30-year study period.
    a. What is the probability of an earthquake of magnitude 7.0-7.9 occurring in the San Francisco in the next 30 years? Show your work.
    b. Do you think this probability is high enough to warrant concern? Why or why not?
  2. Suppose that a particular area has an MRI of 30 years for earthquakes of M = 6.0-6.9. Suppose a M=6.7 earthquake occurs in that area this year. How does this affect the probability of such an earthquake occurring next year?
  3. The statewide probability map suggests that overall, there is a 99% chance of a damaging M=(6.7 or greater) earthquake occurring somewhere in the state in the next 30 years. Should resources for earthquake preparedness be spread evenly across the state? Support your position with information from this unit.

Summative Assessment

Student mastery of the unit objectives can be assessed by having them repeat the analysis for:

  • The Los Angeles area (33.5-35.5°N / 116.75-119.75°W), using the spreadsheet provided below which includes numbers of earthquakes for each magnitude range filled in.
  • Other areas (their hometown areas, for example). They would need guidance in choosing an appropriately-sized area for sufficient statistics, and would have to do get their own earthquake counts via the databases mentioned under Resources and References. Microsoft Word, pdf, and Excel versions of the blank worksheet are provided below.
  1. What is the probability of a magnitude 7.0-7.9 earthquake in the Los Angeles area in the next 30 years? How does this compare to what you calculated for a magnitude 7.0-7.9 earthquake in the San Francisco area?

Additional assessment questions appropriate for exams might include questions such as:

An M=5.0 earthquake has a recurrence interval (return period) of 10 years.

  1. What is the probability of that earthquake occurring this year?
  2. Suppose such an earthquake occurs this year. How does that affect the probability of such an earthquake occurring next year?
  3. What is the probability of that earthquake occurring in the next 10 years?

Worksheet files:

For instructors only:

References and Resources

Earthquake Hazards 101 - the Basics

USGS primer on how earthquake shake hazard maps are developed. Note, however, that this unit focuses on earthquake probability (Steps 1 and 2 of the 5 steps discussed on this website), not shaking hazard. But this site includes much useful information on how hazard maps are developed and used.

Excellent primer on earthquake frequency and probability by Paul Denton of the British Geological Survey

United States Geologic Survey searchable earthquake database

Allows custom searches for earthquake information using a variety of parameters. Caution: this database underrepresents earthquakes of low magnitudes, especially <2.0. This can be a useful teaching point when demonstrating/deriving the Gutenberg-Richter relationship. Also note that, particularly when searching for earthquakes in a small area or over a short span of time, earthquake swarms (say those associated with a major quake) may distort the Gutenberg-Richter relationship—which, of course, could be another useful teaching point.

Southern California Earthquake Center searchable earthquake database

Allows custom searches for earthquake information using a variety of parameters, but specifically for southern California. See cautions above, although this database gives more complete counts in the <2.0 magnitude range.

USGS fact sheet on "Forecasting California

"...the chance of having one or more magnitude 6.7 or larger earthquakes in the California area over the next 30 years is greater than 99%. Such quakes can be deadly, as shown by the 1989 magnitude 6.9 Loma Prieta and the 1994 magnitude 6.7 Northridge earthquakes. The likelihood of at least one even more powerful quake of magnitude 7.5 or greater in the next 30 years is 46%—such a quake is most likely to occur in the southern half of the State. Building codes, earthquake insurance, and emergency planning will be affected by these new results, which highlight the urgency to prepare now for the powerful quakes that are inevitable in California's future."

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These materials are part of a collection of classroom-tested modules and courses developed by InTeGrate. The materials engage students in understanding the earth system as it intertwines with key societal issues. The collection is freely available and ready to be adapted by undergraduate educators across a range of courses including: general education or majors courses in Earth-focused disciplines such as geoscience or environmental science, social science, engineering, and other sciences, as well as courses for interdisciplinary programs.
Explore the Collection »