Using Earthquakes to Map Plate Boundaries
In this lab, you will analyze data from several earthquakes to determine how the plates in the northwestern United States are moving.
Problem How can you use earthquake data to infer the movement of tectonic plates?
Materials
graph paper
Skills Inferring, Analyzing Data
Procedure
Examine the map of Oregon and Washington states. Earthquakes occur in the western parts of these states as a result of the movements of two plates—the Juan de Fuca Plate, which lies under the Pacific Ocean, and the North American plate.
Examine the table of earthquake data. Record any patterns that you observe.
Draw a diagram showing how the edges of the Juan de Fuca and North American plates would move if they formed a convergent boundary. Use Figure 30 on page 682 to help you draw this diagram.
Draw a diagram showing how the edges of the two plates would move if they formed a transform boundary. Use Figure 30 again to help you draw this diagram.
Use the information in the data table to construct a graph showing the location and depth of earthquakes. Plot the distance from the coast on the horizontal axis and the depth of the focus on the vertical axis. Label the vertical axis with zero at the top and maximum depth at the bottom.
Draw a curve as close as possible to all the points you plotted on your graph. The curve shows the shape and position of the boundary between the Juan de Fuca and North American plates. Compare your graph to the two diagrams you drew in Steps 3 and 4.
Analyze and Conclude
Using Graphs What kind of boundary do the Juan de Fuca and North American plates form? Explain how your graph supports your answer.
Inferring What does your graph suggest about the direction in which the Juan de Fuca and North American plates are moving?
Predicting In California, the Pacific plate and the North American plate meet along a transform boundary called the San Andreas fault. Would you expect to observe a similar curve in a graph of the earthquake data from the San Andreas fault? Explain.
Drawing Conclusions Based on your data, how is earthquake depth related to distance from a convergent plate boundary? Explain.
Go Further
Use resources in the library or on the Internet to learn how geologists estimate the depth of an earthquake's focus. Report your findings to the class.