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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.
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.
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