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Group Aims to Distinguish Tsunami Deposits from Large-Storm Deposits in the Geologic Record By Guy Gelfenbaum October 2002 in this issue: next story Bob Morton (left) and James Goff examine sediment in a push core from Okarito Lagoon and react to finding a possible ancient-tsunami deposit. Bob Morton (St. Petersburg, FL) recently teamed up with Bruce Jaffe (Santa Cruz, CA), Bruce Richmond (Santa Cruz, CA), and Guy Gelfenbaum (Menlo Park, CA) to tackle the problem of trying to distinguish tsunami deposits from storm deposits in the geologic record. Both tsunamis and large storms, particularly hurricanes, are capable of inundating coastal regions and depositing sandy sediment over broad areas landward of the beach. Correctly identifying a sandy bed in the geologic record as either a tsunami or storm deposit is important for an accurate assessment of the hazard frequency. Where historical records of past storm or tsunami events are short or nonexistent, interpreting the geologic record may be the only way to identify recurrence interval, or some statistical measure of the probability of future events. The group met recently in Santa Cruz, CA, to compare observations and to establish sedimentologic and stratigraphic criteria for field-testing the differences between depositional records of the two types of events. Despite some differences between the flow conditions of a tsunami and a hurricane, the deposits from these events display many similar features. The researchers are identifying and cataloguing these features by examining the deposits from recent tsunamis in Papua New Guinea (1998) and Peru (2001) and by comparing them with deposits in the Gulf Coast region from Hurricanes Carla (1961), Camille (1969), and Alicia (1983) and with deposits from the Ash Wednesday northeaster (1962) that devastated much of the east coast of the United States. The group's preliminary research was presented last July in a paper titled "Distinguishing Tsunami and Hurricane Overwash Deposits" in a special session, "Tsunami, Storm Surge, Relative Sea Level, and Coastal Change" at the 2002 Western Pacific Geophysics Meeting in Wellington, New Zealand. After the meeting, Guy and Bob examined beaches near Christchurch before joining James Goff in the field to look at coastal overwash deposits. Goff is a coastal geologist from Christchurch who has published several papers on tsunami hazards in New Zealand. Okarito Lagoon, on the west coast of the South Island of New Zealand. The three traveled from Christchurch across the Southern Alps (and the Alpine Fault) to the west coast of the South Island, where they stayed in the former gold-mining village of Okarito. Most of the next several days were spent examining push cores and digging small trenches in and around Okarito Lagoon. The lagoon is separated from the Pacific Ocean by a narrow sandy barrier with a maximum elevation of only 3 to 4 m above sea level. Previous work in the area by Goff and others identified upward-fining deposits believed to be from a large local tsunami that occurred more than 100 years ago. As can be attested to firsthand, however, distinguishing tsunami deposits from storm deposits in the geologic record can be especially difficult, and lively discussions ensued about the scientific interpretations and their implications. Future plans call for conducting field studies in places, such as Puerto Rico, where both tsunami and storm deposits are preserved, and for collecting additional field data after any major modern tsunami or hurricane. USGS Tsunami Workshop Charts Course for Future Research March 2000 Tsunamis and Earthquakes U.S. Geological Survey (USGS) in this issue: next story

October 2002 Mailing List: Sign up to receive monthly email updates: print this issue in this issue: cover story: Tsunami Deposits in the Geologic Record Monterey Bay National Marine Sanctuary Tenth Anniversary Earth Science Week - Woods Hole Chinese Scientists Visit Gas-Hydrate Labs Blacks In Government Briefing Congress on Coastal Hazards Distinguished Service Award - Bill Dillon Distinguished Service Award - Bill Normark Superior Service Award - Marlene Noble Superior Service Award - Florence Wong Environmental Champion Award - Sue Hunt Safety Award - Ellen Mecray 30th Annual Falmouth Road Race Barth Joins Menlo Park Team Walk for Multiple Sclerosis Twichell to University of Nevada Woods Hole Softball October Publications List

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