geology lab materils (4)

€ The San Francisco Bay area of California is located in a tectonically active region, so it can be subjected to strong earthquakes. Figure 11.3A is a map showing the kinds of geologic materials upon which buildings have been constructed in a portion of Oakland at the west end of the Oakland Bay Bridge. These materials include hard compact Pliocene and older rock, Quaternary alluvial sand and gravel in coastal terrace deposits, a sandy alluvial formation called the Merritt Sand, and an uncompacted artificial fill that is mostly mud. The artificial fill was used in the late 19th and early 20th centuries to create a seaport in an area that was originally dominated by tidal flats and creeks. Imagine that you have been hired by an insurance company to assess the geologic risks in buying newly constructed apartment buildings located at X, Y, and Z in Fig. 11.3A. Your job is to rank the risk of property damage during strong earthquakes at the three sites, ranging from /ow (little or no damage expected) to high (damage can be expected). The only thing that you have as a basis for reasoning is Fig. 11.3A and knowledge of your experiments with models in part A of this activity. 1. Which site has the highest risk of damage during a future earthquake? Why? 2. Which site has the lowest risk of damage during a future earthquake? Why? D On October 17, 1989, a strong earthquake occurred at Loma Prieta, California, and shook the entire San Francisco Bay area. Seismographs temporarily placed at locations S1, S2, and S3 (Fig. 11.3A) after the main shock recorded the shaking of a significant aftershock, and the resulting seismograms that show north—south horizontal motion are given in Fig. 11.3B. More intense shaking is indicated by the larger amplitude of the seismic trace, which diverges more from the midline of the seismogram. The records in Fig. 11.3B show that the intensity of shaking was less at location S1 than at locations S2 or S3. 1. The Loma Prieta earthquake caused little significant damage at location X, but there was moderate damage to buildings at location Y and severe damage at location Z. For example, the double-deck portion of I-880 collapsed between points C1 and C2, causing many deaths (Figs. 11.2 and 11.3A), and was damaged but did not collapse between C2 and C3. Explain how this damage report compares to your risk assessment in part C. 2. The Loma Prieta earthquake shook the entire San Francisco Bay region. Yet Fig. 11.3 provides evidence that the earthquake had very different effects on structures located quite near to each other. Explain how the properties of geologic material on which buildings are constructed (for example, strong versus weak or compacted versus uncompacted) influence how much the buildings are shaken in an earthquake. E REFLECT & DISCUSS Imagine that you are an elected member of the city council in one of the cities around San Francisco Bay. Name two actions that you could propose to decrease the damaging effects of future earthquakes in your community, such as the damage that occurred at locations Y and Z in the Loma Prieta earthquake. 300