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Explaining Seismic Hazard Probabilities

Mary Savina, Carleton College, based on an orginal activity by Kendra Murray, Carleton College
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This material was developed as part of the Carleton Teaching Activity Collection and is replicated on a number of sites as part of the SERC Pedagogic Service Project


Students will compose a memo to one of a group of audiences, explaining the Modified Mercalli Intensity scale, its relation to seismic hazards, and the concept of probability, in time and space, of earthquake occurence and seismic hazard. They will include a specific example in their memo and accompany the memo with appropriate maps and graphs.

Learning Goals

Content Goals:
Types of Seismic Hazard, their relationship to distance from fault and earthquake epicenter, topography and soil condition
Modified Mercalli Intensity Scale (and comparison to other measurements of earthquake strength)
Recurrence interval and probability, as applied to earthquakes and seismic hazards

Higher Order Thinking Skills:
Writing for different audiences
Explaining numbers (in this case, Modified Mercalli intensities, earthquake probabilities, peak ground acceleration, and seismic hazard)
Evaluating material written for different audiences

Professional Skills:
Reading and interpreting maps

Context for Use

This exercise is designed for an introductory geology course, but might also be useful in courses on natural hazards, environmental geoscience, GIS, structural geology and others. It was designed specifically for a first-year seminar on Geology and Human Health, cross-listed between Geology and Environmental and Technology Studies at Carleton.

Although a stand-alone exercise, it is also intended as an introduction to a larger project, in which students use census data and hazard maps in GIS to determine populations at risk.

This exercise is adaptable to a variety of settings. It can be started during a class period or lab session and completed as a homework assignment (either for individuals or groups). It could also be started and completed during a longer lab period.

In the course, this activity is part of a unit on earthquakes and seismic hazards, related to discussions of tectonic plate boundaries.

Before starting this activity, students will find it useful to have done some preliminary reading on ways of measuring earthquakes, on the Modified Mercalli Scale (in particular), and on the types of seismic hazards (fault rupture, shaking, liquefaction, landslides, etc.). Such information is readily available in environmental geology and natural hazards texts and on-line. Some suggestions are given below.

The most difficult concepts to understand (and therefore to explain in writing) are those centered around probability and recurrence. In a course where quantitative skills and concepts are introduced and mastered sequentially, you might want students already to be comfortable explaining data mapped on logarithmic scales (compared to linear scale), basic map reading (including distance measurements), and some basic statistics (measures of central tendency and variance).

The examples in this activity refer specifically to the Wasatch Front in Utah. This activity can be adapted using materials from other seismically active areas, such as New Madrid (Memphis, TN and St. Louis, MO), California (SF Bay Area, LA area and others), the Pacific Northwest (e.g. Seattle), and others, including areas overseas, such as Italy, Greece and New Aealand (as information permits). GIS-based maps and data may not be available for all overseas sites.

Description and Teaching Materials

The materials below include assignment handouts for both the "minimal" assignment described in this example and a sample handout for the larger GIS-based project.

The handouts include references to USGS and other sites with basic information. These sites are listed in the "Resources" section of this page, along with basic references on writing with numbers and natural hazards.

Teaching Notes and Tips

Material on earthquake hazards and measures of earthquake strength, even those written for a general audience, can be confusing. Because understanding the quanitative concepts is the main point of this exercise, you will probably want to check this understanding by asking students to explain concepts orally or in writing at several intermediate points.


References and Resources

Online resources:
(last updated in 2004, accessed June 2, 2006)This is a short general interest USGS publication describing earthquake magnitude (Richter magnitude) and intensity (Modified Mercalli Intensity Scale).
This is a one-page general comparison of the Modified Mercalli Intensity (and damage) expected by earthquakes of different Richter magnitudes.
(from William Spence, Stuart A. Sipkin, and George L. Choy
Earthquakes and Volcanoes, Volume 21, Number 1, 1989; url accessed on June 2, 2006)
This is a more technical description of different kinds of magnitude calculations.