Plate Tectonic - Earthquakes (6)
Post Lab 

   
OBJECTIVES:
  • Constructing a model of earthquake movement.
  • Observing fault movement.
VOCABULARY:
  • fault
  • focus
MATERIALS:
  • worksheets
  • tape or glue

Students construct a paper model showing fault motion in the 1989 Loma Prieta earthquake

BACKGROUND:

On October 17, 1989, a Magnitude 7.1 earthquake occurred south of San Francisco, California. The resulting disaster was not the largest in the history of the United States, in terms of either money or lives. In this Post Lab, students will construct a paper model of the 1989 fault rupture. This model will help students visualize the process of fault movement during earthquakes. By examining this model they can better visualize the complexity of faulting during an earthquake. This paper model is from U.S. Geological Survey Open File Report (89-640 B) by T. R. Alpha, J. C. Lahr, and L. F. Wagner. The version below includes the model and instructions; the Report also includes background information. If you wish to order your own copy of the Report, it is available through the Internet at:
http://geology.er.usgs.gov/eastern/models. html  

The first place along a fault to break during an earthquake is called the focus. The focus is always located within the Earth. The point on the Earth’s surface directly above the focus is the earthquake’s epicenter. During an earthquake, movement proceeds very rapidly from the focus along the fault. The total area that moves is often called the rupture. The size of the rupture is controlled by many factors, such as the amount of energy stored along the fault, the strengths of the rocks involved, and the orientation of the fault. Generally, the larger the rupture, the bigger the earthquake. Ruptures do not always break through to the Earth’s surface. In this situation, the surface above the fault is torn or bend by fault movement. This is what happened in most locations during the 1989 Loma Prieta earthquake.

Here is a detailed description of the Loma Prieta earthquake and associated surface movements, from the Open-File Report by Alpha, Lahr, and Wagner:

"The northern California earthquake of October 17, 1989 occurred in the Santa Cruz mountains between San Francisco Bay and Monterey Bay. It was named after Loma Prieta, a nearby mountain peak. The focus, the point where movement began within the Earth, was on the San Andreas fault at of depth of 18 kilometers. The break expanded rapidly along the fault plane, moving at approximately 12,000 km/hr (7,000 mi/hr). It extended upward to within 6 km of the surface and along the fault for 25 km to the northwest and 25 km to the southeast. With respect to the North American plate, the Pacific plate moved northwest 1.6 meters and upward 1.1 meter at an angle of 70 degrees during the event.

Because the main fault rupture, or break, did not continue all the way to the surface the upper layers of the earth were bent rather than broken. An area of fissures, or deep cuts in the earth, mapped near the north end of the rupture may have been a result of this bending. In the paper model a series of cuts, which form a stepped or "en echelon" pattern are placed directly above the fault rupture. These cuts were caused by the motion on the fault below them . They illustrate how surface layers might be deformed by underground fault movement. The actual pattern of bending and fissures, however is more complicated and is not portrayed by the paper model."

PROCEDURE:
  1. Explain fault movement and surface rupture to the class. Give them background information on the 1989 Loma Prieta Earthquake.
      
  2. Have the students cut out and construct the paper model. When they have finished, have them simulate the earthquake displacement by shifting the "Pacific Plate" box upward and to the north. Have the students closely observe the surface of the model; they should see how the fissure buckles in an en echelon pattern.
      
    MAKING A PAPER MODEL OF THE 1989 LOMA PRIETA EARTHQUAKE

INSTRUCTIONS:

  1. Cut out all three pieces of the model (see following pages for instructions).
  2. Fold the two large pieces into box shapes, as illustrated. Make sure you fold all the areas before you start gluing.
  3. Glue the tabs of the two boxes and assemble them.
  4. Glue the surface layer to the tops of the two boxes. Be careful that the area of the fissures remains glue free.
  5. To simulate the earthquake displacement, shift the "Pacific plate" box upward and to the north. Notice how the fissure buckles.



Finished Model

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