Unit 2 Risk at Transform Plate Boundaries
This unit builds on what students have learned about transform fault hazards to introduce the idea of risk. Students examine earthquake risk along the San Andreas Fault in San Francisco by examining public school sites around the city. Students calculate relative risk (risk = hazard probability x vulnerability x value) due to earthquake hazards at five sites, analyze different seismic hazard scenarios, communicate risks to stakeholders, and evaluate possible responses to seismic hazards. Students conclude with a summative assessment in which they reflect on the value of earthquake forecasts and warnings in mitigating risk.
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 2 Learning Objectives (and areas in the unit where objectives are addressed)
- Students will relate earthquake damage to rock/soil type and distance from epicenter.
- Students will identify factors that contribute to building damage associated with earthquakes, and they will describe potential strategies to mitigate seismic damage to buildings.
- Students will evaluate information about seismic hazard, building construction, and population to quantify risk on the Pacific/North America plate boundary near San Francisco.
Context for Use
Unit 2 builds on ideas from Unit 1 (Hazards at Transform Plate Boundaries), adding the ideas of risk and risk mitigation to seismic hazards associated with transform plate boundaries. Students are expected to come to the activity with the following background:
- Familiarity with the basic tenets of plate tectonics and 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.
- Completion of the prework assignment (which could be incorporated into class time if a longer time is available).
This unit is designed for introductory-level geology courses, including physical geology and geologic hazards, but is also appropriate for any course studying plate tectonics.
Unit 2 is designed for a 50-minute class period, with additional pre-class work designed to help students visualize patterns of damage from the 1906 great earthquake. In class, students are expected to work in large groups (for activity 1) and pairs or small groups (for activity 2), and can access the data necessary for the activities in one of two ways:
- Using printed maps and data sheets (provided).
- Using a computer equipped with Google Earth and a web browser. Necessary files can be downloaded from the links below.
This unit is suitable for most class sizes if enough students are available for at least 4 or 5 student groups for activity 2.
If used for a longer period (e.g. 75-minute lab), the prework and in-class work can all be completed in one session. Internet access is required for prework.
Description and Teaching Materials
USGS data sources for some Google Earth layers are currently not working due to changes at the USGS web site. While they are being repaired, please use the paper maps version of the activity.
If Google Earth is available:
- Students explore San Francisco 1906 earthquake information (KMZ File 747kB Feb20 20) and fill out the following sheet:
- Unit 2 prework sheet (Microsoft Word 2007 (.docx) 25kB Sep20 14)
- PDF version of worksheet (Acrobat (PDF) 66kB Sep22 14)
If Google Earth is not available:
- Paper maps Word version (Microsoft Word 2007 (.docx) 6.4MB Sep22 14)
- PDF of paper maps (Acrobat (PDF) 9MB Sep22 14)
Outline of In-Class WorkBefore the class begins, the instructor will assign students to groups of three to five individuals for the day's activities. Although these groups are only strictly necessary for tasks 3, 4 and 5, it will save time if students are already placed in groups at the beginning of class.
- [~10 min] The instructor solicits student responses to question 3 from the prework ("Suppose that local fire and rescue crews had to prioritize which of the areas in the "Strong Shaking Case Studies" layer to search first for earthquake survivors and casualties. Which area should get priority? Why?"). The instructor then briefly introduces the concept of calculated seismic risk as a combination of seismic hazard, vulnerability to damage, and value. In the case of the activities described here, the value term is assessed in terms of human casualties (or, alternatively, the number of human lives protected from harm by a reduction in risk). This is an ideal opportunity to highlight the reasons that geologists are continually updating their assessments of natural hazards: more and more people are living near plate boundary zones with earthquake potential of trends in the number of people affected by seismic activity through time).
- [~10 min, optional] Students work as a class to hypothesize what the effects of an earthquake similar to the 1906 San Francisco earthquake would be if that earthquake struck San Francisco today. Specifically, what hazards would have greater effects, and what would have smaller effects in the present-day city? Potential examples could include damage to bridges and freeway overpasses (as seen in the Loma Prieta earthquake) that were not present in 1906; improved fire control and building codes since 1906; damage to communication infrastructure that is much more important to San Francisco than it was in 1906; denser population than in 1906. Possible discussion prompts include:
- "Suppose that an earthquake similar to the 1906 earthquake were to strike San Francisco today. In what ways might a city like San Francisco be better equipped to save lives and property after an earthquake, compared to 1906? What about modern cities could make earthquake response efforts more difficult?"
- "Imagine that you lived in San Francisco now, and that an earthquake like the 1906 earthquake had just happened. Your house has not been destroyed, but your life has been affected by the earthquake in other ways. How might your everyday life—your commute to work, your ability to get food and help to your friends, your lines of communication with the outside world—be affected by a major earthquake?"
- [~15 min] Certain factors leave some buildings vulnerable to damage. After a brief instructor-led summary of earthquake-safe design, students examine a set of red background images from this PowerPoint (PowerPoint 2007 (.pptx) 5.8MB Apr2 15) to assess the possible factors that can increase or decrease risk of property damage. Slides with black backgrounds are intended to help students identify ways in which seismic, geological, and construction parameters can influence a building's earthquake safety. The images with red backgrounds show pairs of buildings damaged by earthquakes. These are intended to illustrate differences in each of the major factors that affect a building's response to an earthquake. Construction, ground type, and topography, in particular, are important. The slideshow is designed so that the instructor can choose a few (2-3) of the red background images for a think-pair-share activity, in which students first try individually to identify the factors responsible for the earthquake damage in the photograph. Students then discuss their analyses within their small groups, so that each group reaches a consensus. The instructor then polls the class about their analyses (a show of hands will work, although this would be relatively easy to adapt as a clicker question).
- [~ 15 min] The class now applies its knowledge of earthquake hazard probability (Unit 1) and building vulnerability factors (previous activity) by using Google Earth to look at the relative risk at five school campuses. Small groups of students evaluate the seismic risk at one of five school campuses in San Francisco using Google Earth. Directions are included on a worksheet. An alternative version of this activity uses maps and photographs instead of Google Earth. Student teams will elect a note-taker, a Google Earth navigator, and a spokesperson.
- Students use hazard maps to assess strong shaking (%g) and liquefaction potential (high/med/low) at the school sites.
- Students assess landslide risk (high/med/low) based on LIDAR-derived topography around the schools.
- We have created a scaled-down version of the FEMA Rapid Visual Screening protocol for assessing seismic safety. The basic assessment categories include construction type (unreinforced masonry) and presence of "soft" stories. Assessment is based on Google Street View photos.
- The value of each school is based on the number of students in each school.
- Risk scores are calculated as hazard x vulnerability x value. Additional detail on the calculation is given in the worksheet.
- San Francisco Earthquake Hazards (KMZ File 3.8MB Feb20 20) Google Earth KMZ (Note: schools labeled A-E are intended for this activity. Two alternates are labeled F and G. These are intended for use in large classes where additional groups could examine different schools.)
- Paper maps (Acrobat (PDF) 16.2MB Jan23 15) and demographic statistics (Excel 2007 (.xlsx) 41kB Jan23 15) (PDF version (Acrobat (PDF) 35kB Jan23 15)) for the non-Google-Earth version of the activity
- Risk Factors worksheet for students: Google Earth version (Microsoft Word 2007 (.docx) 27kB Oct17 16) (PDF (Acrobat (PDF) 236kB Sep22 14)) and paper maps version (Microsoft Word 2007 (.docx) 26kB Sep22 14) (PDF (Acrobat (PDF) 229kB Sep22 14))
- Risk Factors spreadsheet (Excel 2007 (.xlsx) 13kB Sep22 14) (or pdf version (Acrobat (PDF) 36kB Sep22 14)) for students. This activity can be done in Excel or on paper with printouts of the spreadsheet.
- Which two schools should receive $10 million for seismic retrofitting? (Because this may involve some debate, it may be better done as small groups, a random sample of which is called on to report back to the class.)
- What measures would you recommend to mitigate seismic risk at each school campus? At some schools, other measures may be more effective at mitigating risk than seismic retrofitting. Possible measures aside from retrofitting include: abandonment and increased earthquake drills/training.
- As a result of learning about calculated risk, you may have changed the way you think about prioritizing emergency services in the event of an earthquake. Would you change your answer from the prework? If so, what would you change, and why? If not, what convinced you that it was correct?
Teaching Notes and Tips
This activity is designed for a 50-minute class period. It can be modified for a longer class period (e.g. 75 minutes) by:
- Including the optional discussion of effects of a major earthquake on San Francisco today. This may be followed, at the end of the activity, by a second discussion of earthquake effects to illustrate ways in which seismic hazard mitigation could be important in earthquake relief and recovery.
- Having student groups assess and compare risk at all of the school sites listed in the activity. This has the advantage of allowing students to compare their rankings and recommendations with those of other groups.
The PowerPoint on earthquake-safe design provides additional slides beyond those necessary for a 50-minute class period. In the prework, students have already observed the effects of epicenter distance and ground type on seismic safety, so these sections of the PowerPoint may be condensed or skipped. In a 50-minute period, students will likely only have time to analyze one red background image or image set. The instructor may choose which image to show. It is important, however, to make sure that students have seen examples of factors that make a building vulnerable to strong shaking, and that they are aware of possible ways to improve the seismic safety of buildings. In one pilot test of this unit, the instructor (Selkin) drastically condensed the final report discussion due to time constraints. This led to less nuanced analyses on the module summative assessment than may have been the case if the class had discussed the school retrofits in detail.
To combine this Unit 2 with Unit 1 (Hazards 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 is needed to the content of the prework, although the references to page numbers in the Unit 2 prework would need to be checked for consistency. In the lab session itself, the Unit 2 classroom activity would then directly follow the Unit 1 classroom activity.
Although Google Earth is not necessary for this unit, it is helpful. If possible, students should be encouraged to bring laptops to class for the school risk assessment activity. Note that the paper map version of the school risk assessment activity requires packets to be set up beforehand, and that the Google Earth version requires certain files to be downloaded. The materials required for this activity are outlined in the
The risk calculation can be done either in Excel or on paper. Excel spreadsheets are sometimes stumbling blocks for students. Although the risk calculation is fairly straightforward, some Excel preparation may be helpful. Alternatively, the instructor may set up a central spreadsheet (perhaps even as a shared document in Google Docs) on which students can enter data.
Not all of the schools in the risk assessment exercise are equally visible in Google Earth and StreetView. This makes the analysis of their construction subject to some uncertainty. In reality, engineers would visit the buildings and walk around and possibly inside them to identify construction type and features indicative of seismic risk. The students should do their best to analyze the seismic safety of the buildings and realize that they are modeling a real-world process, not exactly reproducing it. Some subjectivity is unavoidable, and the instructor may need to point that out as students attempt to reconcile different recommendations at the end of the activity.
Also note that the StreetView imagery was better than LIDAR hillshade maps for assessing slope at several of the school sites. See in particular pp. 38-40 for Garfield Middle School, 45-46 and 49 for Herbert Hoover Middle School, 56 for Guadalupe Elementary School (which looks like it is at the foot of a hill in LIDAR, but is on flat land), and 64-68 for Sunset Ridge Elementary School.
Understanding strong shaking potential is subject to some misinterpretation. While not exactly a misconception, predicted peak ground acceleration, which is given as a percent or fraction of free-fall acceleration, may be confused with the earthquake probability (also given as a fraction or percent) from the previous unit.
Also note that the strong shaking potential mapped in this exercise is not just due to an earthquake along the San Andreas Fault, although that is a likely scenario (and one school sits directly on the fault). See the Association of Bay Area Governments explanation of a similar map for details. Note that they are explaining a peak ground velocity map with a 10% 50-year probability, not a peak ground acceleration map with a 2% 50-year probability.
In the discussion in at least one of the classes (Selkin's), students tended to focus on the numerical result of the risk calculation rather than the specific factors that led some schools to be deemed "riskier" than others (i.e. what about hazard probability, vulnerability, and value led to the students' rankings). Be careful to guide the conversation away from the simplistic answer ("schools A and B have the highest risk, so they should be retrofitted") to more nuanced answers that take the specific factors as well as possible mitigation measures into account. For example, what would students suggest doing for a school that has a high risk because it is built on a slope? Is it more worthwhile to fund seismic upgrades for that school than for a school that has high risk because it has a large open space on the ground floor of a multilevel building? Also, make sure that students are aware of possible measures the schools could take to make their buildings seismically safe.
Students (and people in general) may suggest abandonment as a mitigation measure without fully considering its consequences to the school system or the students. Schools are expensive to build, and land is expensive, especially in places with limited space like San Francisco. It is difficult to move an entire school of students to a new site. Students need to be made aware of the potential consequences of their recommendations in the final discussion. In addition, in the final discussion, students tend to focus on the schools with the highest calculated risk without evaluating whether retrofitting procedures would benefit the school, and without identifying specific retrofitting procedures that might be effective for that building/campus. This is a particular problem if the assessment for this unit is assigned as a writing assignment without being introduced in an in-class discussion. In an in-class discussion, the instructor can push the students to consider factors beyond calculated risk (e.g. effectiveness of retrofits, potential scale of construction, social justice issues). Such a discussion does take time, but is a valuable component of the unit.
It may be useful to note near the beginning that not all buildings in earthquake-prone areas—and not even all public buildings —are built to the modern standards of seismic safety (building code). Old buildings in particular may be "grandfathered in," and do not necessarily have the seismic safety features required of new construction. In California, the Field Act requires schools to upgrade their seismic safety measures to the current building code, though it is typically implemented when modifications are made to the school.
Within the activity, there are two embedded opportunities for formative assessment:
In the seismic safety analysis PowerPoint, students discuss images of earthquake damage to analyze which factors are primarily responsible for differences among the buildings in a photo. They report results of their think-pair-share to the class with a show of hands, allowing the instructor to quickly diagnose and to give feedback on (or solicit feedback about) the students' ability to assess a building's seismic safety.
In the school risk calculation activity, the students are encouraged to think aloud about the information they are using to make their decisions, and about the risk analysis procedure. In small to medium-sized classes, the instructor can walk around the room, listen to conversations, and (if warranted) provide feedback.
Students' mastery of the unit objectives can be assessed using a writing prompt, which can be given as a homework assignment or test question after completing this unit, or can be done less formally as part of the report at the end of the unit. We recommend using this as a writing assignment, administered as homework.
Based on your risk assessment of the five schools in this activity, make the case for funding upgrades to buildings at two schools. Prepare a set of bullet points to be presented to the City of San Francisco that uses data from your analysis to support your recommendations. If additional measures are necessary to mitigate risk at other schools, outline them and support them as well.
We recommend using the following rubric to score the summative assessment. A printable version is here (Microsoft Word 2007 (.docx) 24kB Oct30 14) (PDF (Acrobat (PDF) 40kB Oct30 14) version). The three requirements correspond approximately to the three unit learning goals. Each requirement of this question should be scored on a numerical scale of 0-3, with 0 representing an answer that does not meet standards and a 3 an answer that indicates mastery. The question is therefore worth 9 points.
Requirement 1: The student explains how specific geological characteristics of the school site (strong shaking potential, which includes rock/soil type and distance from potentially active faults; liquefaction potential; landslide potential) contribute to seismic hazard. (Aligned with unit goal 1)
Requirement 2: The student explains how construction at each school site could be upgraded to enhance seismic safety. (Aligned with unit goal 2)
3 Points: The response correctly identifies the degree to which the school sites are exposed to seismic hazard, and explains the factors contributing to the seismic hazard at each specific site. Complete explanations include a discussion of:
- proximity to active faults
- rock/soil type and consolidation (liquefaction potential)
- proximity to steep slope
2 Points: The response correctly identifies the degree to which the school sites are exposed to seismic hazard, but incompletely explains the factors contributing to the seismic hazard at each specific site (i.e. not all of the points listed above under "complete explanations" are included).1 Point: The response incorrectly identifies the degree to which the school sites are exposed to seismic hazard, but explains at least one factor contributing to the seismic hazard at each specific site.
0 Points: The response incorrectly identifies the degree to which the school sites are exposed to seismic hazard, and does not explain any of the factors contributing to the seismic hazard at a specific site.
Requirement 3: The student appropriately calculates risk as a combination of hazard, vulnerability, and value, with value in this case referring to the potential number of lives saved. (Aligned with unit goal 3)
3 Points: The response correctly identifies specific seismic hazard mitigation options and explains why they would be useful at the two chosen school sites. The mitigation options are relevant to the hazards outlined in the response. These may include:
- To mitigate loose soil/ liquefaction effects: add deep piles to foundation, dewater or densify sediment
- To mitigate effects of shaking: reinforce soft stories, add devices to resist vibration
2 Points: The response identifies specific seismic hazard mitigation options and explains why they would be useful at the two chosen school sites. The mitigation options are not all relevant to the hazards outlined in the response.
1 Point: The response identifies seismic hazard mitigation options but does not explain why they would be useful at the two chosen school sites. The mitigation options are not necessarily relevant to the hazards outlined in the response.
0 Points: The response does not identify seismic hazard mitigation options.
3 Points: The response takes into account all risk factors when prioritizing seismic retrofits.1.5 Points: The response is based only on one or two risk factors.
0 Points: The response does not refer to calculated risk.
A successful case will:
- Refer specifically to seismic hazard, construction, and student population information from the activity.
- Identify schools that could be better served by abandonment than retrofitting.
- Use a logical argument, supported by data, to communicate seismic risk.
References and Resources
Catalysts for discussion in Activity 1 (What if the San Francisco Earthquake were to happen today?):
- Daily Mail article speculating on what the 1906 San Francisco earthquake would have done to today's city.
- National Geographic article on the same topic.
- Information about the evolution of building codes and about the Field Act, which sets standards for seismic safety of California schools.
- Earthquake damage image collections: USGS, NOAA, Karl V. Seinbrugge Collection, Berkeley.
- Earthquake sounds from the Seismological Society of America.