6.1.5: Earthquake Hazard Maps of Metropolitan Areas
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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. The hazards measured are the amount of seismic wave amplification, the potential for liquefaction, and the tendency of hillslopes to fail in landslides.
The maps divide the area underlain by Quaternary sediment into three (for Portland) to five (for Salem) hazard categories of ground-shaking amplification based on sediment thickness and S-wave velocity. Areas underlain by bedrock do not amplify seismic waves. Similarly, there are three to five categories of liquefaction potential of surficial sediment, with no liquefaction potential for areas underlain by bedrock. Classification of slope stability is based on the steepness of slope ranging from no hazard where the land is flat to a high hazard where the slope exceeds twenty-two degrees, with a special category for hillsides already marked by landslides.
Maps of individual hazards (seismic shaking, liquefaction, and slope stability against landslides) are combined, using a computer model, to subdivide each area into four earthquake hazard zones, with A marking the highest hazard zone and D the lowest. An A ranking generally means that the area has ranked high in at least two of the three hazards described (seismic shaking, liquefaction, slope stability). An area could rank very high in one category and low in all others and receive a B ranking. The map can be used to state that a broad area such as Portland International Airport has a particular level of hazard (Zone B). The Oregon State Capitol and Willamette University are ranked Zone C. The maps are detailed enough that you could get an idea of the earthquake hazard category for your own home if you live in one of the areas covered by the maps.
The maps are designed for general planning purposes for designing earthquake hazard mitigation programs for Oregon’s major cities. Damage estimates for lifeline services and disaster-response planning could effectively be based on these maps. However, they are not a substitute for site-specific evaluations of a building site based on borings and trenches, although they could be used for feasibility studies and for design. Furthermore, no state law requires that these maps be used in land-use policy. However, they could affect earthquake insurance rates.
Although there is no province-wide program for earthquake hazard maps in British Columbia, a demonstration project for the city of Victoria has been completed, in part funded by the city itself. The City of Seattle has produced a set of Sensitive Area Maps showing slopes greater than fifteen degrees that might have a greater potential for landslides. Similar maps are being constructed by the California Geological Survey for urban areas in southern California. The Seismic Hazard Mapping Act, passed by the California legislature in 1990, requires the State Geologist to identify and map the most prominent earthquake hazards from liquefaction and landslides. Unlike states in the Northwest, developers and local government are required to consult these maps in land-use decisions.
In Washington, Steve Palmer and his colleagues with the Division of Geology and Earth Resources prepared maps showing liquefaction potential in lowland areas of the Seattle and Olympia urban areas because of the extensive liquefaction accompanying the earthquakes in 1949 and 1965. These maps were tested by the Nisqually Earthquake of 2001. Liquefaction and lateral spreading were concentrated in those areas Palmer and his associates had predicted would be hazardous. The Olympia map is shown in Figure 8-16.
The Nisqually experience showed clearly that these maps can predict successfully those areas where damage will be concentrated in an urban earthquake. However, they have only been earthquake-tested in Washington.
Suggestions for Further Reading
Burns, S. 1998. Landslide hazards in Oregon, in Burns, S., ed., Environmental, Groundwater and Engineering Geology Applications from Oregon. Association of Engineering Geologists Special Pub. 11, Star Publishing Co., 940 Emmett Ave., Belmont, CA 94002, p. 303-15.
Burns, S. 1998. Landslides in the Portland area resulting from the storm of February 1996, in Burns, S., ed., Environmental, Groundwater and Engineering Geology Applications from Oregon. Association of Engineering Geologists Special Pub. 11, Star Publishing Co., 940 Emmett Ave., Belmont, CA 94002, p. 353-65.
Burns, S., and L. Palmer. 1996. Homeowner’s landslide guide. Oregon Emergency Management, Federal Emergency Management Agency Region 10, and Oregon Department of Geology and Mineral Industries, 10p. (free).
Dragovich, J. D., and P. T. Pringle. 1995. Liquefaction susceptibility map of the Sumner 7.5-minute quadrangle, Washington, with a section on liquefaction analysis by Palmer, S.P. Washington Division of Geology and Earth Resources Geologic Map GM-44, 1 sheet, 1:24,000, text 26 p.
Gerstel, W. J., M. J. Brunengo, W. S. Lingley, Jr., R. L. Logan, H. Shipman, and T. J. Walsh. 1997. Puget Sound bluffs: The where, why, and when of landslides following the holiday 1996/97 storms. Washington Geology, v. 25, no. 1, p. 17-31.
Jibson, R. W. 1996. Using landslides for paleoseismic analysis, in McCalpin, J. P., ed., Paleoseismology. San Diego, CA: Academic Press, p. 397-438.
Keefer, D. K. 1984. Landslides caused by earthquakes. Geological Society of America Bulletin, v. 95, p. 406-71.
Keller, E. A. 1988. Environmental Geology, Fifth Edition. Columbus, OH: Merrill Publishing Co., 540 p.
Kramer, S. L. 1996. Geotechnical Earthquake Engineering. Englewood Cliffs, N.J.: Prentice-Hall.
Monahan, P. A., V. M.. Levson, E. J. McQuarrie, S. M. Bean,P. Henderson, and A. Sy. 2000. Relative earthquake hazard map of Greater Victoria showing areas susceptible to amplification of ground motion, liquefaction and earthquake-induced slope instability. British Columbia Geological Survey Map GMOO-1.
Monahan, P. A., V. M. Levson, P. Henderson, and A. Sy. 2000. Relative liquefaction hazard map of Greater Victoria (sheet 3A); relative amplification of ground motion hazard map (sheet 3B); seismic slope stability map (sheet 3C) and accompanying report. British Columbia Geological Survey Maps GMOO-3. Muir, J. 1912. The Yosemite. The Century Company, republished by Doubleday and Co., Inc., New York.
Obermeier, S. F. 1996. Using liquefaction-induced features for paleoseismic analysis, in McCalpin, J. P., ed., Paleoseismology. San Diego, CA: Academic Press, p. 331-96.
Oregon Department of Geology and Mineral Industries. n.d. Landslides in Oregon. Free circular.
Oregon Department of Geology and Mineral Industries. 1991-1996. Earthquake hazards maps of Portland and Salem metropolitan areas. GMS 79, 89-92, 104-5.
Oregon Department of Geology and Mineral Industries. 1997. Relative earthquake hazard map of the Portland Metro Region, Clackamas, Multnomah, and Washington Counties, Oregon. Interpretive Map Series IMS-1.
Palmer, S .P., H. W. Schasse, and D. K. Norman. 1994. Liquefaction susceptibility for the Des Moines and Renton 7.5-minute quadrangles, Washington. Washington Division of Geology and Earth Resources Geologic Map GM-41, 2 sheets, scale 1:24,000, text 15 p.
Palmer, S. P., T. J. Walsh, R. L. Logan, and W. J. Gerstel. 1995. Liquefaction susceptibility for the Auburn and Poverty Bay 7.5-minute quadrangles, Washington. Washington Division of Geology and Earth Resources Geologic Map GM-43, 2 sheets, scale 1:24,000, text 15 p. Palmer, S .P., T. J.
Walsh, and W. J. Gerstel. 1999. Geologic folio of the Olympia-Lacey-Tumwater urban area, Washington: Liquefaction susceptibility map. Washington Division of Geology and Earth Resources Geologic Map GM-47, text, 16 p.