Introduction to Moment Magnitude

John Jasbinsek
,
Cal Poly, San Luis Obispo
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Initial Publication Date: October 26, 2011 | Reviewed: November 1, 2012

Summary

This activity reviews body-wave magnitude, and takes a closer look at its merits. Then moment magnitude is defined and contrasted with body-wave magnitude. The 2004 Parkfield earthquake is used to illustrate moment magnitude.
The figures are adapted from other sources.

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Context

Audience

Undergraduate, lower-division general education level course on earthquakes. No geology prerequisite.

Skills and concepts that students must have mastered

Fault types, elastic rebound theory, local magnitude, duration magnitude, body-wave magnitude

How the activity is situated in the course

Usually the activity is done in a Discussion meeting, which is a 1-hr meeting of half the class size at a time. Students work in small groups and answers by each group are summarized by the instructor and discussed towards the end of the meeting.

Goals

Content/concepts goals for this activity

Goal: to understand that different magnitude scales use different parts of the seismogram, and that only seismic moment measures energy and connects the earthquake to the faulting instead of the seismogram

Higher order thinking skills goals for this activity

data analysis
synthesis of magnitude scales for comparison
understanding the effect of different parameter values on magnitude

Other skills goals for this activity

Working in groups.
Clear written answers to questions using appropriate terminology developed in the class.

Description of the activity/assignment

To prepare for this assignment students will have already done magnitude calculations, particularly body-wave magnitude where the concept of reading amplitude and P-wave period will have been practiced. Prior to this activity the students will have had a quiz on calculating body-wave magnitude given a seismogram.
The primary outcome of this activity is to understand the difference in what body-wave magnitude measures, compared to what moment magnitude measures and the merits of each.

Determining whether students have met the goals

Answers by each group are presented to the class and the instructor leads a discussion of the merits and/or inaccuracies of these answers. A "good" answer to each question is agreed upon and each student then has a record of these for their future review.

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