# SeisMac Seismogram Modeling

Michael Wysession
,
Washington University
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#### Summary

Use the freely-downloaded "SeisMac" program on a Mac laptop to demonstrated the differences between P, S, Rayleigh and Love waves. SeisMac is a free program that instantly turns a Mac laptop into a 3-component seismograph using the accelerometers built into the laptop. The activity works as a demo for large classes or a hands-on activity for small classes, and involves learning how to shake the laptop in different ways to recreate actual 3-component seismograms.

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## Context

#### Audience

This can be used in a graduate seismology class, an upper-level undergraduate course in geophysics, or a general physical geology course. Designed for a geophysics course Integrates geophysics into a core course in geology Designed for an introductory geology course

#### Skills and concepts that students must have mastered

Students need to understand (2) that earthquakes cause seismic waves that travel through Earth and are recorded on seismographs, and (2) that these waves travel in different forms (P, S, Love, Rayleigh) that have differing modes of oscillation, differing wave shapes and durations, and appear on different components (radial, transverse, vertical).

#### How the activity is situated in the course

It is a stand-alone exercise that accompanies a discussion of the differences between P, S, Love and Rayleigh seismic waves.

## Goals

#### Content/concepts goals for this activity

Understanding seismic wave propagation.

#### Higher order thinking skills goals for this activity

Forward modeling: predicting what kinds of physical motions will replicate real seismic waves.

## Description of the activity/assignment

1. Teacher will demonstrate the SeisMac program.
• Shake the laptop in the three different directions, showing how the different directions are recorded on the three different components.
• Adjust both the vertical and horizontal scales to show how the same motions can be represented differently.
• Setting the vertical scales on -1 to +1, tilt the laptop 90 deg in each of the three directions to demonstrate that the seismograms are records of acceleration (accelerograms). When the laptop is oriented normally, the baseline of the vertical component is 1 g and the two horizontal components have baselines of 0 g. However, when the laptop is tilted the vertical component goes to 0 g and one of the horizontal components will go to 1 g.
2. Teacher will present real 3-component seismograms for actual data, and discuss the major seismic phases (P, S, Love, Rayleigh), showing the components they arrive on: P is primarily on vertical, S is primarily on radial and transverse, Love is primarily on the transverse, and the Rayleigh wave is primarily on both vertical and radial components.
3. Students will take turns shaking the seismometer to try to directly replicate the actual seismograms. If two sets of seismograms are used, the teacher can talk about the differences between seismograms at stations that are nearer or further from the seismometer. Has minimal/no quantitative component Addresses student misconceptions

## Determining whether students have met the goals

Their forward-modeling efforts are preserved in the window.