Choosing a School Site Near a Fault
A growing town must choose a site for a new middle school. Site A is 3 km from an active fault, on loose river sediment, with groundwater about 2 m below the surface. Site B is 12 km from the same fault, on solid bedrock, with groundwater about 15 m below the surface. Historical earthquakes in the region have reached magnitude 5.5. Site A is $2 million cheaper and closer to existing roads. A parent group argues that Site B is too far from town and that “the fault is old, so it will not move again.” A geologist notes that the sediment at Site A can amplify shaking and that saturated loose sediment can behave like a liquid during strong shaking. The school board must decide which site is safer and what information is still missing before construction.
1. Which site is safer with respect to earthquake hazards? Use the concepts of seismic wave energy, sediment amplification, and liquefaction in your reasoning.
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Site B is safer. Seismic waves lose energy as they travel away from the fault, so at 12 km the shaking is generally weaker than at 3 km. Site A’s loose river sediment can amplify ground shaking because the sediment is less rigid than bedrock; the same earthquake can produce stronger surface motion there. Site A also has groundwater only 2 m down, so the sediment is saturated. During strong shaking, water pressure in the pores can push grains apart and the sediment can behave like a liquid, a process called liquefaction. Buildings can sink, tilt, or lose foundation support. Site B’s bedrock is more stable and its deeper water table lowers liquefaction risk. The $2 million savings at Site A does not outweigh the higher risk to students, especially because the region has had magnitude 5.5 earthquakes.
2. The parent group says the fault is old and will not move again. How would you evaluate that claim, and what evidence would the board need before accepting it?
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The claim is not supported by the information given. “Old” is not the same as inactive. A fault is active if it has moved recently in geologic time and is capable of moving again; the setup states it is an active fault and that the region has had magnitude 5.5 earthquakes. To evaluate the claim, the board would need evidence such as the age of the most recent movement on the fault, the rate of slip, records of small earthquakes along the fault, and maps of offset rock layers or young sediments. The board should also get a site-specific seismic hazard assessment, including soil testing at each site and groundwater measurements. Without that evidence, the board should not assume the fault is harmless. The absence of a large earthquake in the last few decades is not proof that one will not occur.
3. Suppose the board selects Site B. Describe two engineering design features that could reduce earthquake damage to the school, and explain how each feature works.
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One feature is a reinforced concrete and steel frame with shear walls. Steel reinforcing bars and cross-bracing resist sideways forces, and shear walls stiffen the building so it sways less during shaking. Another feature is base isolation, in which flexible bearings separate the building from its foundation. The bearings absorb some of the ground motion and allow the building to move more slowly than the ground, reducing the forces transmitted to the structure. A third option is anchoring nonstructural elements such as ceilings, shelves, and water heaters so they do not fall. The board should also test the soil at Site B and design the foundation for the actual site conditions. These features do not make the school earthquake-proof, but they reduce the chance of collapse and injury during strong shaking.