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6.1: Solid Rock and Bowls of Jello

  • Page ID
    6036
    • 6.1.1: Introduction
      We live in earthquake country, but we don’t want to leave the Pacific Northwest. Fortunately, we know how to improve our chances for survival simply by making intelligent decisions about where we live or work and how we build. The technology is at hand to evaluate the geologic setting of a building site with respect to earthquake hazard. Three different earthquake problems are associated with surface sites.
    • 6.1.2: Amplification of Seismic Waves by Soft Surficial Deposits
      It is a short stroll from Fort Mason, west of Ghirardelli Square and Fisherman’s Wharf in San Francisco, to the fashionable townhouses of the upscale Marina District, yet the intensity of ground motion of these two areas during the earthquake of October 17, 1989, was dramatically different. The Marina District experienced intensities as high as IX, higher even than at the epicenter itself, more than sixty miles away. Fort Mason and Fisherman’s Wharf experienced intensities of only VII.
    • 6.1.3: When the Earth Turns to Soup (Liquefaction)
      Liquefaction is defined as “the act or process transforming any substance into a liquid.” If you have the misfortune of building a house on liquefiable sediment, and an earthquake strikes, your house might sink into the ground at a crazy angle as the sediment liquefies and turns into quicksand. Liquefaction is especially common in clean, loose sand, or gravelly sand saturated with water. Most sand layers with liquefaction potential are Holocene in age and are unconsolidated.
    • 6.1.4: Landslides Generated by Earthquakes
      On July 10, 1958, as reported by George Plafker of USGS, an earthquake of M 7.9 on the Fairweather Fault, Alaska, triggered a landslide on the side of a mountain overlooking Lituya Bay. The slide created a huge water wave 100 feet high that swept seaward, carrying three fishing boats at the mouth of the bay into the ocean. An earthquake of M 7.6 on August 18, 1959, in Montana, just north of Yellowstone National Park, triggered a landslide that swept down a mountainside and through a campground.
    • 6.1.5: Earthquake Hazard Maps of Metropolitan Areas
      The Oregon Department of Geology and Mineral Industries has prepared maps of the Portland, Salem, and Eugene metropolitan areas that classify the urbanized areas into earthquake hazard zones. The information discussed earlier in this chapter has been used to make the maps: the bedrock geology, the thickness, density, and seismic shear-wave (S-wave) velocity of near-surface sediment, the steepness of slopes in hillside areas, and the degree of susceptibility of those slopes to landslides.