Teach the Earth > Rates and Time > Activity Posters > Determining Earthquake Recurrence Intervals from Trench Logs

# Determining Earthquake Recurrence Intervals from Trench Logs: A Multi-faceted, Thought-Provoking Exercise for Students at All Levels

Patricia H. Cashman
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#### Summary

Trench logs of the San Andreas Fault at Pallett Creek, CA are the data base for a lab or homework assignment that teaches about relative dating, radiometric dating, fault recurrence intervals and the reasons for uncertainty in predicting geologic phenomena. Students are given a trench log that includes several fault strands and dated stratigraphic horizons. They estimate the times of faulting based on bracketing ages of faulted and unfaulted strata. They compile a table with the faulting events from the trench log and additional events recognized in nearby trenches, then calculate maximum, minimum and average earthquake recurrence intervals for the San Andreas Fault in this area. They conclude by making their own prediction for the timing of the next earthquake.

## Context

Audience:

Skills and concepts that students must have mastered:

(exposed to) superposition, cross-cutting relationships

How the activity is situated in the course:

I developed this exercise for use near the beginning of an introductory non-majors' Environmental Geology/Geologic Hazards course, to teach relative dating. The societal relevance is obvious, and the students usually enjoy the exercise. This exercise can be used with younger students (I've run it for an elementary school class) or the general public; it can also be adapted for more advanced undergraduate geology majors.

## Goals

Content/concepts goals for this activity:

earthquake recurrence interval

Geologic skills:

determining geologic history from cross-cutting relationships

Higher order thinking skills goals for this activity:

drawing conclusions from data; dealing with uncertainties in data (here, in isotopic dates); reasons for uncertainty in predicting geologic phenomena

Other skills goals for this activity:

presenting data in tabular form

## Description of the activity/assignment

Trench logs of the San Andreas Fault at Pallett Creek, CA are the data base for a lab or homework assignment that teaches about relative dating, radiometric dating, fault recurrence intervals and the reasons for uncertainty in predicting geologic phenomena. Students are given a trench log that includes several fault strands and dated stratigraphic horizons. They estimate the times of faulting based on bracketing ages of faulted and unfaulted strata. They compile a table with the faulting events from the trench log and additional events recognized in nearby trenches, then calculate maximum, minimum and average earthquake recurrence intervals for the San Andreas Fault in this area. They conclude by making their own prediction for the timing of the next earthquake. While basically an exercise in determining relative ages of geologic horizons and events, this assignment includes radiometric dates, recurrence intervals, and an obvious societal significance that has been well received by students.

With minor modifications, this exercise has been used successfully with elementary school students through university undergraduate geology majors. Less experienced students can work in groups, with each group determining the age of a single fault strand; combining the results from different groups and calculating recurrence intervals can then be done as a class activity. University students in an introductory geology course for non-majors can add their data from the trench log to an existing table with other faulting events already provided. The exercise can be made more challenging for advanced students by using logs from several different trenches, requiring students to design the table themselves, and giving students the uncertainties for the radiometric dates rather than simple ages for the strata. Most students—at all levels—are initially frustrated by their inability to determine an exact date of faulting from the available data. They gain a new appreciation for the task of the geoscientist who attempts to relate geologic phenomena to the human, rather than geologic, time scale.

## Evaluation

The course meets UNR's core curriculum requirements for a natural science course; one of these is a writing requirement, met by lab reports and written reports on homework assignments like this one. Evaluation in our course is therefore based in part on "the answers" (ages of different faulting events; minimum, maximum and average recurrence intervals, etc.) and part on the report (distinguishing data from interpretation, etc.)

## Logistics

No special tools or training are required. Materials consist of simplified versions of several of the Pallett Creek trench logs (different ones assigned different semesters, to minimize copying from previous students), lists of additional faulting events not found in each trench, and data tables for inexperienced students to fill in as they constrain the age of each event.