Abstract

Objective

The goal of this project is to map earthquake activity and relate it to the positions of the Earth's tectonic plates.

Introduction

Today it is widely accepted that the Earth's crust consists of a series of huge plates that slowly move. The low parts of the plates are beneath the world's oceans, and the high parts of the plates are landmasses. New plate material is generated at deep sea ocean ridges in a process called sea-floor spreading. Material from plates is also recycled at trenches, where dense, oceanic crust dives back (subducts) underneath an adjacent plate towards the upper mantle. Figure 1 (below) shows a map of the major tectonic plates.

The theory of plate tectonics revolutionized geology in the 1960s. Before this, geology had been a descriptive science. Mechanisms for large-scale processes such as the formation of mountain ranges were put down to vague "earth forces." Plate tectonics changed that. A series of scientific papers by Harry Hess, Robert Dietz, Fred Vine, Drummond Matthews, and others brought together a growing body of evidence that massive pieces of the earth's surface were constantly on the move. Subduction of one plate beneath another could provide the massive force to produce uplift of mountain ranges. Fifty years earlier, in 1912, Alfred Wegener had proposed his theory of continental drift, and was widely ridiculed. Wegener, like others before him, had noticed that the continents on either side of the Atlantic Ocean had complementary shapes, suggesting that they might have originated much earlier from the same landmass. He had also noted similarities in rock formations on opposite sides of the ocean, and similarities in both living and fossil animals. Wegener did not have a good explanation for how vast chunks of the earth's surface could move relative to one another, and the community of geologists was not ready to accept his ideas (McPhee, 1981–1998; WGBH, 1998).

Figure 1. Map of major tectonic plates of the earth. (Tilling, date unknown).

Today we still do not know the mechanism for the motion of the plates, although it is thought that convection of heat from the earth's interior is somehow involved. The evidence that clinched the case for plate tectonics came from detailed mapping studies of paleomagnetism. Rocks containing magnetic material reveal the history of when and where they were formed. As the molten rock cooled, the magnetic particles aligned themselves with the earth's magnetic field. Although the positions of the earth's magnetic poles have changed over the billions years of earth's history, geologists have been able to recreate the time line of those changes. Armed with that information, geologists have been able to map the dates of origin of the oceanic crust, and to confirm that sea-floor spreading at suboceanic ridges and subduction at trenches is a constant process.

How are earthquakes related to tectonic plates? The following paragraph from Annals of the Former World by John McPhee, summarizes the connection quite well (McPhee, 1981–1998, 121):

Almost all earthquakes are movements of the boundaries of plates—shallow earthquakes at the trailing edges, where the plates are separating and new material is coming in, shallow earthquakes along the sides, where one plate is ruggedly sliding past another (the San Andreas Fault), and earthquakes of any depth down to four hundred miles below and beyond the trenches where the plates are consumed (Japan, 1923; Chile, 1960; Alaska, 1964; Mexico, 1985).

The goal of this project is to collect historic earthquake data and map it. If the plate tectonics theory of earthquake activity is true, then the great majority of earthquake activity should occur at or near boundaries between tectonic plates. Do you think that historic earthquake data will support McPhee's statement?

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:

• Latitude
• Longitude
• Earthquakes
• Earthquake magnitude
• Layers of the Earth
• Tectonic plate
• Subduction

Questions

• How are tectonic plates related to earthquakes?

Bibliography

• For information on plate tectonics, see:
  • WGBH, 1998. "A Science Odyssey: You Try It: Plate Tectonics," WGBH, Boston, and PBSOnline [accessed March 29, 2007] http://www.pbs.org/wgbh/aso/tryit/tectonics/intro.html.
  • Kious, W.J. and R.I. Tilling, 1996. "This Dynamic Earth: The Story of Plate Tectonics," U.S. Geological Survey [accessed March 29, 2007] http://pubs.usgs.gov/gip/dynamic/dynamic.html.
  • USGS, 2003. "Plate Tectonics and Sea-Floor Spreading," Cascades Volcano Observatory, U.S. Geological Survey [accessed March 29, 2007] http://vulcan.wr.usgs.gov/Glossary/PlateTectonics/description_plate_tectonics.html.
  • Tilling, R.I., 1998. "Volcanoes," U.S. Geological Survey (online version of U.S. Government Printing Office publication) [accessed March 29, 2007] http://pubs.usgs.gov/gip/volc/intro.html.
• This is 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 January 30, 2007] http://pubs.usgs.gov/gip/interior/.
• 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 March 29, 2006] http://earthquake.usgs.gov/regional/world/historical.php.
• Here is an Excel tutorial to get you started using a spreadsheet program:
  James, B., date unknown. "Excel 101," University of South Dakota, [accessed March 29, 2007] http://www.usd.edu/trio/tut/excel/.
• Further reading. What could be more boring than reading about rocks? If you pick the right book, the geology of the Earth and the people who study it are downright fascinating. Here is an excellent and ambitious book on geology for the general reader by a masterful nonficiton writer, John McPhee (his set piece on plate tectonics runs from pages 115–131):
  McPhee, J., 1981–1998. Annals of the Former World. New York, NY: Farrar, Strauss and Giroux.

Materials and Equipment

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

• World map or regional map with lines of latitude and longitude
• Transparency material at least as large as your map
• Computer with Internet access
• Perseverance and attention to detail

Experimental Procedure

1. Gather data on worldwide earthquakes from the archive of Historic Worldwide Earthquakes (USGS, 2007).
   1. For each event, keep track of the following information in a table (or in a spreadsheet file):
      • Latitude
      • Longitude
      • Magnitude
      • Depth
   2. The more events from which you collect data, the better.
2. Let's walk through an example so that you can see where to find the data you'll need.
   1. The screenshot below shows a portion of the list at the Historic Worldwide Earthquakes webpage. To get more information about a particular earthquake, click on the link. We're going to be looking at data from a magnitude 6.3 temblor with epicenter on Kyushu Island, Japan that occurred on June 11, 2006, at 20:01:29 UTC.

   2. Clicking on the link for this quake brings us to a page with the information shown in the screenshot below.

   3. Copy the information you need for each earthquake (highlighted in yellow rectangles, above).
   4. Notice that there are additional tabs (Summary, Maps, Scientific & Technical, highlighted in green rectangle, above) with additional information about each quake. You'll probably be interested in checking out the Summary and Maps tabs for each quake you study.
   5. By default, the quakes on the Historic Worldwide Earthquakes page are sorted by date. At the top right of the page there are additional sorting options, as shown in the screenshot below.

3. Using transparencies, map the collected earthquake data over the world map.
   1. Think of ways to encode the earthquake data with your mapping symbols. For example, you could encode earthquake magnitude with the size of the symbols, and earthquake depth with the color of symbols.
   2. Compare your earthquake data map with maps showing the boundaries of tectonic plates.
   3. Remember, correlation does not prove causation. A correlation of earthquake activity with known plate boundaries would provide supportive evidence for the plate tectonic theory. A lack of correlation would be significant evidence against the plate tectonic theory.

Variations

• Questions to consider:
  • What magnitude quakes are included?
  • How representative are the data you have collected?
  • Is there a correlation between earthquake depth and relative motion of the adjacent plates?
• Use the Google Earth program to map your data and make illustrations for your display board. You can download the program from: http://earth.google.com/download-earth.html.
• Add volcanic activity to your map. See the Science Buddies project Ring of Fire 1: What Volcanoes Tell Us About Plate Tectonics.

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

Credits

Andrew Olson, Ph.D., Science Buddies

Last edit date: 2007-06-21 15:00:00