Page 430 - Laboratory Manual in Physical Geology

B. 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.
1. Is the risk at location X low or high ? Why?

2. Is the risk at location Y low or high? Why?

3. Is the risk at location Z low or high? Why?

C. 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.
1. 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).

2. 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.

D. REFLECT & DISCUSS Imagine that you are a member of the San Francisco City Council. Name two actions that you
could propose to mitigate (decrease the probability of) future earthquake hazards such as the damage that occurred at
locations Y and Z in the Loma Prieta earthquake.

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