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Standing-Room-Only Lecture on "Tsunamis—Lessons and Questions from the Indian Ocean Disaster"
Listeners flocked to a public lecture on tsunamis at the U.S. Geological Survey (USGS) center in Menlo Park, CA, on June 30, their interest sparked by a tsunami warning issued for the U.S. west coast just two weeks earlier (see "Brief Tsunami Warning Startles U.S. West Coast," this issue). Planned months ago as part of the center's monthly public-lecture series, the fortuitously timed event featured three USGS tsunami experts who offered their insights to an overflow crowd of more than 250 people. Eric Geist, Bruce Jaffe, and Brian Atwater described the severe impact of huge waves striking countries around the Indian Ocean during the tsunami of December 26, 2004, and discussed the potential for tsunami-triggering earthquakes on the Cascadia subduction zone off northern California, Oregon, and Washington. Geophysicist Eric Geist (Menlo Park, CA) led off the lecture with a brief introduction to tsunami basics, including an explanation of how earthquakes generate tsunamis, how tsunamis speed through the deep ocean at jet-airliner speeds (500-600 mph), and how they steepen and slow (to 20-30 mph) as they approach the shore. He used striking computer animations to illustrate important aspects of the December 2004 tsunami, including why it hit some areas harder than others and how it eventually reached coastlines all around the globe. With another computer animation, Eric showed what a tsunami generated by a large earthquake on the Cascadia subduction zone might look like. He sprinkled tsunami-safety tips through his talk and directed listeners to Web sites offering tsunami-inundation maps for the San Francisco Bay area (URL http://www.abag.ca.gov/bayarea/eqmaps/tsunami/) and information about the National Weather Service's program to promote tsunami-ready communities (URL http://www.prh.noaa.gov/ptwc/tsunamiready/tsunamiready.htm). Oceanographer Bruce Jaffe (Santa Cruz, CA) showed dramatic photographs of coastal destruction by the December 2004 tsunami in regions visited by USGS scientists on international survey teams: Sri Lanka, the Republic of Maldives, and Indonesia's island of Sumatra. He explained how survey teams collect physical evidence that a tsunami leaves in its wake, to document its wave height and force. A specialist in the study of sediment deposited by tsunamis, Bruce explained how researchers attempt to use data from tsunami deposits to "back-calculate" the height and power of a tsunami. Such analysis could enable the use of ancient tsunami deposits to learn more about the size of the tsunamis that deposited them and about the likely risk of tsunamis in regions where written records of tsunamis are sparse (most everywhere but Japan). Bruce noted that the tsunami waves in Sumatra were higher than expected (up to 30 m high), a finding that may affect planning for tsunamis along U.S. coastlines. Geologist Brian Atwater (Seattle, WA) focused on the U.S. Pacific Northwest, telling a "detective story" about the accumulation of evidence which finally convinced scientists that large, tsunami-generating earthquakes have occurred on the Cascadia subduction zone. The evidence includes "ghost forests" drowned by saltwater tides after earthquake-induced subsidence of coastal land, sheets of sand deposited by ancient tsunamis, and Japanese records of an "orphan tsunami"—a tsunami not associated with any local earthquake there—that struck Japan in early 1700. Brian explained some of the details which made the story so compelling that it transformed the scientific community's perception of earthquake hazards in the Pacific Northwest: scientists who believed in 1980 that the Cascadia subduction zone could not produce earthquakes large enough to generate tsunamis came to realize, by the mid-1990s, that it had produced many such earthquakes, the latest on the evening of January 26, 1700, as calculated from the Japanese tsunami records. (For his research on this topic, Brian was named one of the "Time 100," Time magazine's list of the world's 100 most influential people in 2005; see URL http://www.time.com/time/2005/time100/). Brian pointed out that this improved understanding of the Cascadia subduction zone's ability to produce large earthquakes and tsunamis had led to tsunami-evacuation maps for dozens of Pacific Northwest communities before the December 2004 Indian Ocean tsunami increased concern about Cascadia tsunamis. Eric, Bruce, and Brian gave interviews to reporters before the lecture and fielded numerous questions from the audience afterward. To see an archived videotape of the lecture, including the question-and-answer session, visit the Western Region Evening Public Lecture Series 2005 Video Archive. For more information about Eric and Bruce's tsunami research, visit Tsunami Research at the USGS. For more information about Brian's "detective story," visit Earthquake Research - A Great Detective Story.
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in this issue:
Summer Internship at the USGS National Wetlands Research Center New Web Site About Indian Ocean Tsunami Public Forum About Coral Degradation Hurricanes: Predicting Their Path of Destruction
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U. S. Department of the Interior | U.S. Geological Survey
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