Plate Tectonic - Earthquakes (4)
Post Lab 

  • Describing a seismogram.
  • Comparing a seismogram with a seismograph.
  • primary wave
  • secondary wave
  • seismogram
  • seismograph
  • worksheet
  • Internet

Students learn to read seismograms.


Earthquakes are caused by the sudden movement and fracturing of rock masses along preexisting faults. A fault is a broken surface within the Earth’s crust. The point on the fault at which the displacement begins is called the focus of the earthquake. The point on the surface of the earth directly above the focus is the epicenter. An earthquake happens in rocks that have been stressed. This stress is stored until the strength of the rock is exceeded. The actual break (the earthquake) then releases the energy. Again, this energy travels in the form of waves.

Seismograms record the energy emitted by seismic or earthquake waves. Seismograms are created by machines called seismographs. Seismograms are used by seismologists (scientists who study earthquakes) to mathematically describe the magnitude of an earthquake. Seismologists have established a worldwide network of seismographic stations. They are especially concentrated in areas of active earthquakes.

When an earthquake occurs, data from the seismographs feeds into a major research center, where the seismograms are analyzed. This analysis determines where the earthquake occurred, and determines its magnitude. Major earthquake data centers in the United States include Menlo Park and Los Angeles, both in California, and Boulder, Colorado.

Earthquakes generate many different types of seismic waves. Two major types are P (push/pull; compressional, or primary) and S (shear or secondary). P-waves are faster than S-waves, hence their names. Seismograms will record P-wave arrivals before S-wave arrivals.

  1. The worksheet shows a copy of six real seismograms recorded at different seismographs near Mammoth Lake, California. Notice that different locations did not record the seismic waves at the same time, because they were different distances from the epicenter. The first arrivals of P- and S-waves are labeled with a small "P" or "S" respectively on the diagram. The lowest two lines on the seismogram indicate time. This is used by seismologists to determine the distance of the seismograph from the earthquake’s epicenter.
  2. Explain P-waves and S-waves to the class. Emphasize that P-waves are faster than S-waves, and will be recorded first on a seismogram.
    Answers:1) 6; 2) look on seismogram above; 3) time.
  3. Instruct the students to find other seismograms on the Internet. Seismologists make extensive use of the Internet to post data derived from earthquakes. The U. S. Geological Survey maintains an excellent website that has real-time earthquake data for the whole United States, at . It also has good links to other earthquake information.

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