Plate Tectonic - Earthquakes (5)

  • Interpreting seismograms and their associated damage.
  • Predicting damage by looking at a seismogram.
  • epicenter
  • P wave
  • S wave
  • seismogram
  • seismic waves

Students analyze seismograms.

Alaska, 1964


Damage caused by earthquakes varies depending on many factors. In order to predict what the damage may occur, many factors must be considered. These include:

  1. Intensity of the earthquake
  2. Distance from the focus
  3. Type of soil/rock buildings are built on
  4. Type of material the building is composed of
  5. Probability of a landslides if in a mountainous area
  6. Probability of tsunami (large ocean waves) if near a coastline

If a city is close to the epicenter of a large earthquake (6.0 or above on the Richter scale) it may have significant damage. The damage may be substantial if the buildings are not reinforced. Brick buildings are more prone to "fall" apart. In addition, if the city is situated on unconsolidated soil or landfill, damage will likely be greater, because these materials settle and/or liquify during earthquakes.

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. The energy released by earthquakes travels through the Earth as seismic waves.

Scientists have developed many scales to measure the intensity of earthquakes. Two common ones are the Richter Scale and Modified Mercalli Scale. The Richter Scale measures the size of the waves produced by the earthquake, hence the energy it releases. The Richter Scale is absolute, so the same Richter magnitude wherever they measure the earthquake. The Modified Mercalli Scale describes what a person feels during an earthquake. This scale is relative because it changes depending on how far you are away from the epicenter, and how much damage occurs around you.

P-waves and S-waves are easily distinguishable on a seismogram. P-waves are faster than S-waves, so the first shaking recorded by the seismogram is always due to P-waves. When S-waves arrive, their energy combines with the P-waves to amplify the shaking. This makes the waves bigger, and the shaking stronger. The first arrival of S-waves is thus marked by a distinct increase in the size of the waves on the seismogram.

  1. Review and discuss the concept that volcanoes and earthquakes provide data for understanding the movement of the lithosphere, as explained by the theory of plate tectonics.
  2. Show the class images of earthquake damage. You can use slides or transparencies, or the presentation below. We recommend showing the students the pictures before they complete the lab, so they can understanding the damage earthquakes can cause. Explain that damage in urban areas depends on many factors.
    If you wish to customize or create your own presentation, here are several websites that have good earthquake damage information:
    Earthquake damage in Idaho. Good, simple descriptions of damage and location.
    Damage from the San Francisco earthquake in 1906. A link to photographs is towards the bottom of the each page.
    An outstanding but huge collection of images of the effects of natural hazards, including earthquakes, volcanoes and tsunami.
  3. Review the Richter Scale and the Modified Mercalli Scale with the class.
  4. Introduce the assignment to the students. Explain that interpreting the seismograms and describing potential earthquake damage both require imagination. The point is not to get the "right" answers, but to speculate on what might happen. This is a prediction, after all!
    As a class, you may want to have the students find the geographic locations of the seismograms on the worksheet. Use the United States placemats. Try to get the students to locate cities near the epicenters that might be effected by the earthquakes.
  5. Select a few students to read one or more of their answers to the rest of the class. The class should conclude that according to the seismogram, the San Fernando Valley earthquake appears to be the most intense and would probably cause the most damage.
    Here is an example of a student response:
    Seismogram 1. The earthquake was probably a sharp quake. Maybe a 5 or 6 on the Richter scale. However, conditions in the ocean created a large tsunami (or tidal wave) and it caused major destruction along the coast. Downtown Anchorage was not affected, except for small items thrown from shelves.

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