2. Exploring What Your Seismograph Records

Background and Introduction:

Located in the display case on the second floor of BC’s Devlin Hall is an “EQ1” educational seismograph, which records earthquakes from around the world. Similar seismographs are located in K-12 schools in the Boston area as part of the Boston College Educational Seismology Project (BC-ESP), as well as in other parts of the U.S. as part of the IRIS Seismographs in Schools program. The purpose of this activity is for you to familiarize yourself with the seismograph and what it records, and to learn how to recognize when it has recorded an earthquake.

When you look at the seismograph screen on the day of a major well-recorded earthquake, it is hard to miss the fact that an earthquake was recorded. For example, anybody who looked at a seismograph screen on the day of the great magnitude 9.0 Sumatra earthquake of December 26, 2004 could not miss the fact that an earthquake was recorded (Figure 1). However, most of the time the seismograph screen shows much less dramatic vibrations, such as people walking near the seismograph, people slamming doors, large trucks passing by the building, and natural “non-earthquake” vibrations, such as wind. Furthermore, most earthquakes recorded by the seismographs will not be as obvious as the great Sumatra earthquake, and some will be quite subtle (Figure 1). Nonetheless, by observing the seismograph screen on a regular basis, you can eventually learn to recognize when an earthquake has been recorded.

As you progress through these lab exercises this semester, you will learn about the various types of waves that are generated by earthquakes and you will become more adept at recognizing the “fingerprints” of an earthquake on your seismograph screen. But just by watching your seismograph screen on a regular basis, you can learn a lot about the different types of earthquake signals that you will be recording as this curriculum progresses. To help you with this inquiry, you will be shown examples of earthquake signals recorded on EQ1 and AS1 seismographs (which are very similar, but not identical seismographs). The examples will range from very dramatic (and therefore hard to miss) recordings of earthquakes to very subtle ones that are hard to identify. It will help you to get a sense of the difference between small, nearby disturbances, and large, distant earthquakes by creating local disturbances (such as jumping up-and-down and slamming a door near the seismograph), and watching what the signals look like on the screen. You will also be shown seismograms from times when there is a lot of background noise from students’ footsteps, traffic, winds and stormy weather, and from times when there is very little background noise. This will help you to learn how to identify earthquake signals during quiet versus noisy times.

With these examples as a guide, try to figure out from your own observations and your own reasoning how to tell if what you are seeing on the seismograph screen is an earthquake or some other “non-earthquake” signal. Also, watch the screen on a regular basis to see if you have recorded an earthquake. As you proceed through this course, you will be given more traditional instruction on what is recorded on a seismograph so that you can formalize the knowledge that you gain through this preliminary, inquiry-based look at how to read a seismogram.

A Note on Magnitudes:

Throughout these lab exercises, we will be using the concept of earthquake “magnitude”. Magnitude is a complicated concept, and we will be learning about the intricacies of this concept in this course. For the purpose of these early exercises, a “Magnitude Primer” will be provided to give the students enough information about magnitude to do these exercises. Note that for each unit on the magnitude scale, the seismic waves recorded on a seismograph increase by about a factor of 10. For example, a magnitude 7.0 earthquake generates seismic waves recorded on your seismograph that are about 10 times larger than those of a magnitude 6.0 earthquake.

The Exercise:

Start by repeating the demonstration from Exercise #1, where we began the class with the students jumping up and down at various distances from the seismograph and then looking at the resulting seismogram. This is a good way for the students to reconnect with the Build Your Own Seismograph exercise. (It is no longer necessary to cover the AS1 seismograph like you did for the Build Your Own Seismograph exercise.)

Study the examples of seismogram screen shots, and try to figure out how you would distinguish between an earthquake and non-earthquake vibrations. When you think that what you are seeing on the screen might be an earthquake, how can you check your hypothesis? Can you figure out where the earthquake is? Can you estimate its magnitude? If you see something on the screen that you think might be an earthquake, check the Internet to see if an earthquake big enough to be recorded on your AS1 seismograph occurred at that time somewhere in the world. Develop hypotheses that explain why seismograms of certain earthquakes appear unusually large or small, long or short, when compared with seismograms of other earthquakes.

Follow-up Activity:

Now that you are familiar with what is recorded on the EQ1 and AS1 seismograms, watch the seismograph on a daily basis, and monitor the Earth for earthquake activity. Report to the class what was recorded each week. If no earthquakes were recorded on a given week, can you explain why?

Teaching Challenges:

Although it is exciting for students to walk into their classroom on the day of a well-recorded earthquake and see it recorded at their own school, sometimes they might have to go for long periods of time before they record any earthquakes. How might you maintain the students’ interest and enthusiasm during this long period of time? Also, how would you get them to appreciate the concept of monitoring the Earth for seismic activity if they have to wait a long time before they record their first earthquake?

Figure 1: (Top Left) The great Sumatra earthquake of December 26, 2004 (magnitude 9.0) recorded by an AS1 seismograph operating in a classroom at Garfield Elementary School in Brighton, MA. (Top Right) The magnitude 7.6 earthquake in Pakistan that occurred on October 8, 2005 recorded by an AS1 seismograph operating at the “Living Lab” (a science program for K-5 students operated by the Westford, MA Public Schools). (Bottom) The magnitude 6.7 earthquake that occurred in Chile on April 30, 2006 recorded by an AS1 seismograph operating at Sea Lab (a science education center of the New Bedford, MA Public Schools). Although the signal from this earthquake is weak, it is definitely observable on the seismogram (dark arrow). Click on figure for larger view.

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