Obtain a small plastic or paper cup. Fill it three-quarters full with a dry sand. Place several coins upright in the sediment so they resemble vertical walls of buildings constructed on a substrate of uncompacted sediment. This is Model 1. Observe what happens to Model 1 when you simulate an earthquake by tapping the cup on a table top while you also rotate it counterclockwise.
- 1. What happened to the vertically positioned coins in the uncompacted sediment of Model 1 when you simulated an earthquake?
- 2. Now make Model 2. Remove the coins from Model 1, and add a small bit of water to the sediment in the cup so that it is moist (but not soupy). Press down on the sediment in the cup so that it is well compacted, and then place the coins into this compacted sediment just as you placed them in Model 1 earlier. Simulate an earthquake as you did for Model 1. What happened to the vertically positioned coins in the compacted sediment of Model 2 when you simulated an earthquake?
- 3. Based on your experimental Models 1 and 2 above, which kind of Earth material is more hazardous to build on in earthquake-prone regions: compacted sediment or uncompacted sediment? (Justify your answer by citing evidence from your experimental models.)
- 4. Consider the moist, compacted sediment in Model 2. Do you think this material would become more hazardous to build on, or less hazardous to build on, if it became totally saturated with water during a rainy season? To find out, design and conduct another experimental model of your own. Call it Model 3, describe what you did, and tell what you learned.
- 5. REFLECT & DISCUSS
Write a statement that summarizes how water in a sandy substrate beneath a home can be beneficial or hazardous. Justify your reasoning with reference to your experimental models.
San Francisco, California is located in a tectonically active region, so it occasionally experiences strong earthquakes. Figure 16.2 is a map showing the kinds of Earth materials upon which buildings have been constructed in a portion of San Francisco. These materials include hard compact Franciscan Sandstone, uncompacted beach and dune sands, river gravel, and artificial fill. The artificial fill is mostly debris from buildings destroyed in the great 1906 earthquake that reduced large portions of the city to blocks of rubble. Imagine that you have been hired by an insurance company to assess what risk there may be in buying newly constructed apartment buildings located at X, Y, and Z on Figure 16.2. Your job is to infer whether the risk of property damage during strong earthquakes is low (little or no damage expected) or high (damage can be expected). All that you have as a basis for reasoning is Figure 16.2 and knowledge of your experiments with models in Part A of this activity.
- Is the risk at location X low or high? Why?
- Is the risk at location Y low or high? Why?
- Is the risk at location Z low or high? Why?
On October 17, 1989, just as Game 3 of the World Series was about to start in San Francisco, a strong earthquake occurred at Loma Prieta, California, and shook the entire San Francisco Bay area. Seismographs at locations X, Y, and Z (see Figure 16.2) recorded the shaking, and the resulting seismograms are shown in Figure 16.3. Earthquakes are recorded on the seismograms as deviations (vertical zigzags) from a flat, horizontal line. Thus, notice that much more shaking occurred at locations Y and Z than at location X.
- The Loma Prieta earthquake caused no significant damage at location X, but there was moderate damage to buildings at location Y and severe damage at location Z. Explain how this damage report compares to your above predictions of risk (Part B).
- The Loma Prieta earthquake shook all of the San Francisco Bay region. Yet Figure 16.3 is evidence that the earthquake had very different effects on properties located only 600 m apart. Explain how the kind of substrate (uncompacted vs. firm and compacted) on which buildings are constructed influences how much the buildings are shaken and damaged in an earthquake.