Comparing Ground Motion from Earthquakes

Students use data provided in REV (Rapid Earthquake Viewer) to compare the amplitude of the seismic waves recorded as a result of ground shaking from recent earthquakes. They explore the concept of a logarithmic scale, and create a graph using a logarithmic axis. By graphing and comparing data for earthquakes of different magnitudes recorded at similar distances from the epicenter, students discover that the amount of ground motion recorded by a seismometer is a measure of magnitude.

Concepts and learning outcomes	Students will understand that: The magnitude of an earthquake represents the amount of energy the earthquake releases. The measure of magnitude is based on a logarithmic scale, with the Richter scale being an example.
Appropriate for	Grades 9-12, or advanced middle school students
Time requirements	One 50-minute class period
Vocabulary	Richter Scale amplitude nanometer micron (micrometer) millimeter logarithmic scale magnitude
Prerequisites	This activity is geared toward high school or advanced middle school students who have a thorough understanding of earthquakes, magnitude, amplitude, and unit conversions. You may need to review powers of 10 and how to convert between them (as shown in the student worksheet). Students can check their answers using the Conversion of Units online conversion tool.
Background for teachers	The following resources discuss earthquake magnitude, including the Richter Scale: The Severity of an Earthquake What is Richter Magnitude? The amplitude of a seismogram is a measure of the amount of ground motion. Amplitude is often measured in nanometers/second, microns/second or millimeters/second. When comparing seismograms of ground motion recorded at different locations, it is important to use the same unit of measure, the typical one being microns/sec. It is important that students be aware of this and compute or look up conversions if the data they use in this activity are not all in the same units. Note: These are all units of velocity because modern seismometers typically record velocity (the speed at which the ground is moving) and not actually how much it has moved. It's actually quite hard to construct an instrument that can measure ground displacement accurately, but velocity and acceleration are easier to measure. In order of decreasing size: Millimeter = 10^ -3 Micron = 10^ -6 Nanometer = 10^ -9 Earthquake magnitudes are estimated using shaking amplitudes that are corrected for distance the waves traveled, but magnitude itself is designed to represent the logarithm of energy release. Advanced information for teachers: Earthquakes do not radiate energy uniformly in all directions. In detail, the distribution of shaking depends on the orientation of the fault that slips. This pattern is different for P waves than for S waves. This activity has students ignore this important aspect, which means that the result is correct enough for a classroom demonstration but not good enough for a research scientist. An improved calculation of the magnitude-amplitude relationship would correct the shaking amplitude for the radiation pattern or make sure that all earthquakes in the comparison had almost the same epicenter and depth.

Materials / Preparation

Computers with internet access
Graph paper or access to Microsoft Excel or similar graphing tool.
Adobe Acrobat Reader
Student Instructions and Data sheet

For the teacher: Answer key (.pdf file) for the student worksheet

Grouping

Groups of two.

Teacher tips

The computer graphing portion of this activity is appropriate for high school students. Middle school students can gather the data from REV but might benefit from creating the graph as a group on a computer with a projector.

Procedures

Students access information about earthquakes of varying magnitudes using REV. The worksheet, Comparing Ground Motion, will guide students though the task of gathering information about earthquake magnitude and seismogram amplitude for five earthquakes. A list of earthquakes and stations is included on the worksheet. The stations listed on the worksheet are all located approximately 20 degrees from the epicenter of their respective earthquake.

Alternate step 1: Students can find their own earthquakes and stations. If students do this, it is important that they choose earthquakes with a range of magnitudes and that they find stations that are a similar distance from that earthquake for comparison, (for example 20 degrees). This can be time consuming and is only recommended for very high level students.
Students record the magnitude of the earthquake and information about the maximum amplitude of ground motion from the seismograms on REV for at least five earthquakes, converting the amplitude data into common units (microns per second) if necessary. For this activity, use the seismogram labeled vertical; however as an extension you can have students note the maximum amplitude of all three seismograms and discuss why the maximum amplitude might be different with respect to time and measure in the different components. See Extensions for more information on the three components of motion that are recorded as seismograms and other variables that can affect the maximum amplitude recorded.
Using the information they have gathered, students create a graph comparing earthquake magnitude (x axis) and amplitude of ground motion (y axis).
Using a graphing tool such as Excel, the y axis can be converted to a logarithmic scale (directions included in the worksheet) and observe that the relationship between the magnitude and amplitude becomes linear. This means that there is an exponential relationship between magnitude and shaking amplitude. In plain English, it means that a slightly larger magnitude indicates an earthquake with much more intense shaking.
Have students draw conclusions about how the magnitude of an earthquake affects the amount of ground motion. The greater the magnitude, the more the ground shakes. As a class, have them identify the constants in their gathering of data that support this conclusion. (The distance from the earthquake was kept constant.)
Students hypothesize about what other factors could influence the amount of shaking one feels in an earthquake. Possible responses include the type of ground material, such as bedrock vs. sandy or loose soil. Discuss students' response to the questions with the entire class.

Homework

Discuss with your parents/guardians what it would mean to you personally or your family, if an earthquake were to occur.

How can you prepare yourself and your house for an earthquake?
ABAG Home Quake Safety Toolkit
Putting Down Roots in Earthquake Country
How can I make my home safer?
What are some examples of human-caused earthquakes and what might be the effects of them?

Assessment

Final graph and questions.

Extension

This exercise uses the vertical component seismogram, however REV shows three seismograms for each station. In an earthquake, the ground can shake all over the place and in every direction. The seismogram labeled "vertical" describes how much the station shook up and down. Where the line is above 0, the station moved upwards. Negative values mean it dropped down. Similarly, the seismograms for East-West and North-South indicate movement in those directions.

This link goes to an animation that will help you visualize the movement. The first sequence shows just the North-South component of shaking. Notice how the house only moves back in forth in a single direction. The second shows only the East-West component. Again, it moves only in a single direction along a straight line. The third part shows a more realistic scenario with all three components of shaking at once. The purple square in this third sequence stays stationary at the original location of the house. It's there to help you recognize that the house is moving all over the place in every direction. The seismometers in the three different directions record this motion, and you need all three of them to reconstruct the motion completely.

The maximum amplitude at a station is also related the depth of the earthquake, and the geology of the Earth the waves had to pass through to reach the station.

An extension to this activity might have students examine more earthquakes and noting the depth of the earthquake, when the maximum amplitude occurs on the seismogram (time since earthquake) and which type of wave it is associated with (P, S or surface) and research into the regional geology associated with the seismograph station. Other questions to consider: How does the maximum amplitude differ for two stations the same distance from the same earthquake? Why? How does maximum amplitude vary for stations that are located very far away from an earthquake? Is the range of maximum amplitude measures similar to that for stations located closer to an earthquake?