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Project Summary

Difficulty	9
Time required	Long (a couple of weeks)
Prerequisites	This project uses the Global Earthquake Explorer program to download and analyze data from a global seismic network. In order to do this project you will need to be comfortable installing and working with a new program on your computer. This project requires a computer with high speed Internet access. You will also need to understand some basic trigonometry. You should be comfortable with determining the lengths of the sides of right triangles when given an angle and the length of one side.
Material Availability	Readily available
Cost	Very Low (under $20)
Safety	No issues

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Abstract

Objective

The goal of this project is to estimate the diameter of the Earth's core by measuring seismic waves around the globe.

Introduction

The shock waves spreading out from an earthquake are called seismic waves (from the Greek word for earthquake). There are two general types of seismic waves: body waves and surface waves.

1. Body waves travel through the Earth's interior.
2. Surface waves, which are analogous to water waves, travel just beneath the Earth's surface.

There are two types of body waves, P waves and S waves. P waves (also called primary waves) are compression waves. Like sound waves, they consist of compressions and rarefactions of the material through which they travel. The compressions and rarefactions are in the same direction that the wave is traveling. S waves (also called secondary waves) are transverse (or shear) waves, meaning that the ground moves perpendicularly to the direction of travel. S waves have much higher amplitude than P waves, but travel more slowly. They carry more destructive force than P waves. Another difference between P waves and S waves is that S waves cannot travel through the Earth's liquid core, while P waves can. S waves can therefore be detected by seismometers near the epicenter of an earthquake, but not by more distant seismometers. P waves can be detected by both local and distant seismometers.

Figure 2 (Robertson, date unknown) is a cross-section of the Earth, showing how P waves and S waves travel through the various layers. Because of the varying density of the layers, the waves are refracted as they pass through the different layers. This is analogous to the refraction of light waves when they pass from air to water, for example.

Figure 2. "Cross section of the whole Earth, showing the complexity of paths of earthquake waves. The paths curve because the different rock types found at different depths change the speed at which the waves travel. Solid lines marked P are compressional waves; dashed lines marked S are shear waves. S waves do not travel through the core but may be converted to compressional waves (marked K) on entering the core (PKP, SKS). Waves may be reflected at the surface (PP, PPP, SS)." (Robertson, date unknown).

Figure 3 is another cross-section of the earth, showing the seismic shadow created by the Earth's core. "Seismologists noticed that records from an earthquake made around the world changed radically once the event was more than a certain distance away, about 105 degrees in terms of the angle between the earthquake and the seismograph as measured at the center of the earth. After 105 degrees the direct P- and S- waves disappeared almost completely, but slow surface waves and waves taking other paths would arrive from over the horizon. The area beyond 105 degrees distance forms a shadow zone. At larger distances, some P waves that travel through the liquid core (path K on the figure above) would arrive, but still no S waves. The Earth has to have a molten, fluid core to explain the lack of S waves in the shadow zone, and the bending of P waves to form their shadow zone." (Louie, 1996b)

Figure 3. The Earth's liquid core creates a seismic shadow distant from the location of a quake. See text above for details. (Louie, 1996b)

Figure 4 shows how you can use the S-wave seismic shadow to estimate the diameter of the Earth's core. If you assume that the S-wave travels in a straight line through the earth, you can use trigonometry to determine the radius of the liquid core. In the diagram, 105° is assumed to be the angle where the seismic shadow begins to take effect. In this project, you will use global seismometer data from earthquakes to measure the actual extent of the seismic shadow, and use it to estimate the diameter of the Earth's core.

Figure 4. Using the seismic shadow of S-waves to estimate the diameter of the Earth. In the diagram, the angle of the seismic shadow is assumed to be 105°. Given that the Earth's radius is about 6370 km, you can use trigonmetry to figure out the diameter of the Earth's outer core. (Louie, 1996b)

Figure 5 shows how seismic waves appear at stations that are progressively more distant from the earthquake. The distance of each seismometer station from the earthquake source is shown on the y-axis, and elapsed time since the earthquake is plotted on the x-axis. By connecting the seismogram components (e.g., P-wave, S-wave, surface wave) in each seismogram, you can construct a travel-time curve, showing the speed of each component as it travels through the Earth. You can also use this type of diagram to see when the S-wave disappears from its expected position because of the core's seismic shadow.

Figure 5. Travel time curves for P-waves, S-waves and surface waves. The y-axis shows the distance from the earthquake event, and the x-axis shows the elapsed time since the event. (USGS, 2006)

In this project, you'll use the Global Earthquake Explorer program to examine worldwide earthquake data. You'll use this data to estimate the starting point for the "seismic shadow" of the primary S-wave. Finally, you'll use the seismic shadow measurements to estimate the diameter of the earth's core.

Terms, Concepts and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

• Plate tectonics
• Earthquake
• Temblor
• Epicenter
• Seismometer
• Coordinated Universal Time (UTC)
• Seismic waves:
  • Body waves, including P-waves and S-waves (also called P and S phases)
  • Surface waves
• Earth layers:
  • Crust
  • Upper mantle
  • Lower mantle
  • Outer core
  • Inner core
• Travel time
• Seismic shadow

Questions

• Can you explain the differences between P-waves and S-waves?
• What causes earthquakes?
• Where do earthquakes occur most frequently?

Bibliography

• This project uses Global Earthquake Explorer (GEE) software, a Java-based program with versions for all three major flavors of personal computer (Windows, Mac, Linux). You can find download the program and user manual from:
  GEE, 2006. "The Global Earthquake Explorer," Department of Geological Sciences, University of South Carolina and the IRIS Consortium [accessed May 25, 2007] http://www.seis.sc.edu/gee/about.html.
• Here are two good articles on seismic waves:
  • Wikipedia contributors, 2007. "Seismic Wave," Wikipedia, The Free Encyclopedia [accessed May 25, 2007] http://en.wikipedia.org/w/index.php?title=Seismic_wave&oldid=102751635.
  • Louie, J., 1996a. "Seismic Waves," Nevada Seismological Laboratory, University of Nevada, Reno [accessed May 25, 2007] http://www.seismo.unr.edu/ftp/pub/louie/class/100/seismic-waves.html.
• This references are a good, brief introduction to the structure of the Earth:
  • Robertson, E.C., date unknown. "The Interior of the Earth," United States Geological Survey, General Interest Report [accessed May 25, 2007] http://pubs.usgs.gov/gip/interior/.
  • Lerner, K.L. and B.W. Lerner, eds., 2003. "Earth, Interior Structure," from World of Earth Science Farmington Hills, MI: Thomson Gale, available online via eNotes.com [accessed May 25, 2007] http://science.enotes.com/earth-science/earth-interior-structure.
• This webpage shows how the earth's liquid core blocks direct passage of S-waves, creating seismic shadows distant from the quake origin:
  Louie, J., 1996b. "Earth's Interior," Nevada Seismologyical Laboratory, University of Nevada, Reno [accessed May 25, 2007] http://www.seismo.unr.edu/ftp/pub/louie/class/100/interior.html.
• This webpage has an illustration showing how travel time curves are constructed from seismic data from multiple stations:
  USGS, 2006. "Seismographs: Keeping Track of Earthquakes," United States Geological Survey [accessed May 25, 2007] http://earthquake.usgs.gov/learning/topics/seismology/keeping_track.php.
• To find earthquake information (date, time, location, magnitude), see this webpage:
  USGS, 2007. "Historic Worldwide Earthquakes," United States Geological Survey, Department of the Interior [accessed May 25, 2007] http://earthquake.usgs.gov/regional/world/historical.php.
• These four lessons are a good general introduction to waves, at the high school physics level:
  Henderson, T., 2004. "Waves," The Physics Classroom [accessed May 25, 2007] http://www.physicsclassroom.com/Class/waves/wavestoc.html.

Materials and Equipment

To do this experiment you will need the following materials and equipment:

• Computer with high-speed Internet access and printer
• Pencil
• Protractor
• Metric ruler

Experimental Procedure

Downloading and Installing the Software

1. This project uses Global Earthquake Explorer (GEE) software, a Java-based program with versions for all three major flavors of personal computer (Windows, Mac, Linux). You can find download the program and user manual from:
   GEE, 2006. "The Global Earthquake Explorer," Department of Geological Sciences, University of South Carolina and the IRIS Consortium [accessed May 16, 2007] http://www.seis.sc.edu/gee/about.html.
   1. Click the appropriate link for your type of computer, and follow the instructions to download the program and install it on your computer.
   2. Note that the program requires high-speed Internet access in order to work properly.
   3. The program requires Java, but the installer should automatically take care of this for you if the Java Runtime Environment is not already installed on your computer.
   4. If you have problems with the installation, you can find complete documentation for the program (in pdf format, requires Adobe Acrobat) at: http://www.seis.sc.edu/gee/docu.html.
2. Before running the program, take some time to read through the user's manual that comes with it so that you are familiar with how the program works.

Obtainng and Analyzing Earthquake Data

1. Start the Global Earthquake Explorer program on your computer.
2. On the startup screen, click on the option, "Explore Recent Earthquakes."

   Global Earthquake Explorer startup screen (GEE, 2006).

3. You will see a world map (similar to the one below) displaying earthquake activity from the past 7 days.
   1. Noteworthy earthquakes are displayed as circles.
   2. The map is automatically centered on the largest earthquake in the time period.
   3. The size of the circles is related to the magnitude of each quake.
   4. The color of the circles is related to the depth of each quake.
   5. When you 'mouse over' the circle for each earthquake, the status line at the bottom of the map displays information about the quake.
   6. The blue triangles show seismic stations with available data. (Be patient, it takes awhile for the program to check on what data is available.)

   Global Earthquake Explorer World Map tab screen shot (GEE, 2006).

4. The controls at the top of the map are fairly self-explanatory.
   The controls allow you to:
   1. Select earthquakes and seismometer stations
   2. Zoom in and out on the map
   3. Pan (i.e., click-and-drag) the map
   4. Restore the map to normal size
   5. De-select all seismometer stations
   6. Get help
   7. Load seismometer data
   For more details, see the Help menu in the program.
5. By default, the World Map shows noteworthy earthquakes from the previous week. You can select data from previous time periods by choosing Edit/Earthquakes/Noteworthy Earthquakes from the program menu.
   1. You'll see a dialog box like the one shown below which you can use for selecting archived earthquake data.

   2. In this example, we've selected earthquakes of magnitude 7 or greater for the previous year.
   3. You can also use the USGS Historic Worldwide Earthquakes webpage (USGS, 2007) to find the dates, times and locations of earthquakes of interest.
6. Here are the steps to get seismometer data from a particular earthquake:
   1. Select an earthquake of interest by clicking on its colored circle. The map will re-center (east-west) on the selected earthquake, and the program will check to see which seismometer stations have data available for the selected quake. (Be patient, it takes awhile for the program to check on what data is available.)
   2. Click to select several blue seismometer stations at increasing distances from the earthquake. Include stations at a progression of distances that are less than 105° from the epicenter as well as stations that are more than 105° from the epicenter. Tip: when the mouse hovers over a seismometer station, the status line at the bottom displays how far away from the event the station is.

   3. Click on the "Load Selected Stations" control button to load seismometer data from the selected stations. The program will retrieve the data and switch to the Seismogram Display tab, as shown in the illustration below.

   4. Each station record is shown in the diagram above. The distances along the y-axis reflect the distance of the station from the earthquake source, in degrees.
   5. To set the display as shown, click on the "Display" control, and set:
      • Plot as Record Section
      • Maximize Amplitude
      • Align on Event Origin Time (as shown in the illustration below)

   6. You can save the seismograms for printing by clicking on the ".PDF" control. Save the graph to a file for printing (requires Adobe Acrobat Reader).
7. Connect travel-time curves for P-wave, S-wave and surface wave components, as shown in Figure 5 in the Introduction. At what distance from the quake (in degrees) does the primary S-wave no longer appear?
8. Repeat for at least ten different large earthquakes. What is the maximum distance at which the primary S-wave appears? Use this distance to estimate the diameter of the Earth's outer core, following the method shown in Figure 4 in the Introduction.

Variations

• For a more basic project on the speed of seismic waves, see the Science Buddies project How Fast Do Seismic Waves Travel?

• For more science project ideas in this area of science, see Geology Project Ideas.

Credits

Andrew Olson, Ph.D., Science Buddies

Last edit date: 2007-09-13 12:00:00

Career Focus

If you like this project, you might enjoy exploring careers in Geology.

	Geoscientist Just as a doctor uses tools and techniques, like x-rays and stethoscopes, to look inside the human body, geoscientists explore deep inside a much bigger patient—planet Earth. Geoscientists seek to better understand our planet, and to discover natural resources, like water, minerals, and petroleum oil, which are used in everything from shoes, fabrics, roads, roofs, and lotions to fertilizers, food packaging, ink, roads, and CD’s. The work of geoscientists affects everyone and everything.			Geographer When you hear the word geography, you might think of maps and names of state capitals, but the work of geographers is much more than creating maps and identifying places. Geographers look at how people, places, and Earth are connected. They study the economy, social conditions, climate, and topography of a region to help answer questions in urban and regional planning, business, agriculture, and medicine.

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