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A fascinating subculture in Seattle and, to a lesser extent, in Portland, comprises people who live on the water in houseboats, with the largest number on Lake Union in Seattle. This is not inexpensive living; recent real-estate listings were from half a million to nearly a million dollars for a floating home. There are enough houseboaters to have their own neighborhood community council called the Floating Homes Association. People who live on dry land are known as “uplanders".
On a sunny Sunday afternoon, November 3, 2002, Ed Waddington was on the second floor of his floating home on Lake Union reading the newspaper when his boat began to move and rock. The usual reason a houseboat starts to rock is a passing boat exceeding the speed limit of seven knots, but the rocking motion continued for at least five minutes, too long for a boat wake. Log rafts on which houseboats are built were bashing into one another and into piers, and chains were snapping taut. Waddington walked to the end of his pier, where he and several of his neighbors flagged down a police boat. The police officer told him that he had been dispatched by the Harbor Patrol base on Northlake Way to look for speeders. But there were none.
Waddington turned on his radio and heard a report of an earthquake in central Alaska, the Denali Earthquake, of magnitude 7.9. As a professor in the Department of Earth and Space Sciences at the University of Washington, he put two and two together and recognized that the houseboats in Lake Union were feeling the Denali Earthquake thousands of miles away. Surface waves from this earthquake with a period of about twenty seconds were strong enough to cause the sloping lake bottom to move so that the water sloshed and produced damaging surface waves, but these surface waves were too slow to be felt by the “uplanders.” At least twenty houseboats were damaged.
Sloshing water was reported elsewhere, including a five-foot wave at Lake Wenatchee, and high waves on Puget Sound, Lake Washington and on Henry Hagg Lake, Oregon. Both Ross Lake and Lake Chelan in Washington were affected. Water sloshed out of swimming pools. According to Aggeliki Barberopoulou, then of the University of Washington, the concentration of damage at Lake Union and Portage Bay was due to the focusing of seismic waves by the thick Seattle sedimentary basin underlying Lake Union, in addition to the large number of houseboats around the lake.
Barberopoulou’s conclusion is supported by reports of a seiche on Lake Union after the 1964 Alaska Earthquake. About 7:45 in the evening of March 27, 1964, houseboats broke away from their moorings, and water pipes were broken. The north mooring line of the Four Winds Restaurant pulled a piling from the lake bottom, and fifty-five patrons had to be evacuated. Bartender Paul Farris reported a lot of broken glasses. At Aberdeen, on the Washington coast, water sloshed out of the city reservoir and carried gravel into a nearby neighborhood.
Suggestions for Further Reading
Atwater, B.F., et al.,2005. The orphan tsunami of 1700 — Japanese clues to a parent earthquake in North America. U.S. Geological Survey Professional Paper 1707, 133 p. http://pubs.usgs.gov/pp/pp1707/
Atwater, B. F., V. M. Cisternas, J. Bourgeois, W. C. Dudley, J. W. Hadley, and P. H. Stauffer, compilers. 1999. Surviving a Tsunami—Lessons Learned from Chile, Hawaii, and Japan. U.S. Geological Survey Circular 1187. 18p.
Barberopoulou, A., A. Qamar, T. Pratt, K. Creager, and W. Steele. 2003. Local amplification of seismic waves from the MW 7.9 Alaska earthquake and a damaging seiche in Lake Union, Seattle, Washington. Geophysical Research Letters, DOI: 10.1029/2003GL018569.
Benson, B., K. A. Grimm, and J. J. Clague. 1997. Tsunami deposits beneath tidal marshes on northwestern Vancouver Island, British Columbia. Quaternary Research, v. 48, p. 192-204.
Bernard, E. N., et al. 1991. Tsunami Hazard: A Practical Guide for Tsunami Hazard Reduction. Dordrecht, The Netherlands: Kluwer Academic Publishers.
Clague, J. J., A. Munro, and T. Murty. 2003. Tsunami hazard and risk in Canada. Natural Hazards, v. 28, p. 433-61.
Darienzo, M. 2003. The National Tsunami Hazard Mitigation Program. Earthquake Quarterly, Summer 2003, published by the Western States Seismic Policy Council, p. 4-7, 17.
Dudley, W. C., and M. Lee. 1988. Tsunami! Honolulu: University of Hawaii Press.
Governor’s Office of Emergency Services. 1996. Tsunami! How to survive the hazard on California’s coast. Free pamphlet available from OES.
Hawaii State Civil Defense. 2002. Tsunami: Waves of Destruction. 30-min. and 15-min. videos.
Henderson, B., 2014, The next tsunami: Living on a restless coast: OSU Press, 322 p.
Humboldt Earthquake Education Center. 1999. Living on shaky ground: How to survive earthquakes and tsunamis on the north coast.
Nance, J. J. 1988. On Shaky Ground. New York: William Morrow & Co., 416p. Description of the 1964 tsunami in Alaska.
National Tsunami Hazard Mitigation Program (NTHMP). 2001. Designing for tsunamis: background papers. 122 p. , 60 p. Seven principles for planning and designing for tsunami hazards.
Oregon Department of Geology and Mineral Industries. 2001. Tsunami warning systems and procedures: Guidelines for local officials. Special Paper 35, 41 p.
Preuss, J., and G. T. Hebenstreit, G.T. 1998. Integrated tsunami-hazard assessment for a coastal community, Grays Harbor, Washington. USGS Professional Paper 1560, p. 517-36.
Toppozada, T., G. Borchardt, W. Haydon, M. Petersen, R. Olson, H. Lagorio, and T. Anvik. T. 1995. Planning scenario in Humboldt and Del Norte counties, California, for a great earthquake on the Cascadia Subduction Zone. California Division of Mines and Geology, Special Publ. 115. 157p.
Walsh, T. J., C. G. Caruthers, A. C. Heinitz, E. P. Myers, III, A. M. Baptista, G. B. Erdakos, and R. A. Kamphaus. 2000. Tsunami hazard map of the southern Washington coast: modeled tsunami inundation from a Cascadia Subduction Zone earthquake. Washington Division of Geology and Earth Resources Geologic Map GM-49, booklet, 12 p.